Please keep doing sessions on this topic. I don't get it yet, but I think it is a threshold concept to understanding everything else you teach. Thank you. Fabulous lessons.
An inertial frame of reference is the perspective of an observer, whose motion is such that other objects with no net forces acting on them appear to not accelerate. In other words, a perspective from which Newton's first law is valid. When someone is in a non-inertial frame of reference, fictitious forces need to be introduced to account for the accelerations of other things from their perspective. Imagine you are in deep space and your twin goes past you in a spacecraft at some high speed. From the perspective of the spacecraft, you go past the spacecraft at the same speed. Now the only way for both of you to meet up again is if either you or your twin (or both) reverses your initial direction. In other words, relative to your original frame of reference, you need to acquire a velocity in the direction of the spacecraft (i.e accelerate till you are moving fast enough so you can catch up with the spacecraft) or else your twin has to accelerate relative to his original reference frame so he can catch up with you. If we assume that you remain in the same inertial reference frame throughout the journey of the spacecraft whereas your twin in the spacecraft is the only one who experiences acceleration at some point between his first encounter with you and his second encounter with you, then your twin will have aged much less than you. From your perspective, the spacecraft goes past you in one direction, then when the spacecraft is some distance away, it decelerates and then accelerates towards you and finally he goes past you a second time in the opposite direction. From the perspective of the spacecraft's original inertial reference frame, the spacecraft simply accelerates towards you and thus end up shifting from one inertial frame to another inertial frame in which you are moving towards the spacecraft instead of away. From the perspective of a third observer who is stationary with respect to the spacecraft's original inertial reference frame, you go past the spacecraft first and then keep going in the same direction forever. After you have gained some distance, the stationary spacecraft starts to accelerate towards you and then the spacecraft is travelling at a constant speed, greater than your speed, and the spacecraft will overtake you eventually. There is no inertial frame in which you change direction or speed at any point. Your twin in the spacecraft is in a non-inertial reference frame during the period that he experiences acceleration and he will observe you slowing down and moving towards him. However, the spacecraft's acceleration is of a different nature to yours. You do not feel any of the effects of acceleration whereas your twin does. There is a physical phenomena associated with the spacecraft's acceleration; rocket thrusters have to fire in order for the spacecraft to accelerate towards you. When someone is in a non-inertial frame of reference, fictitious forces need to be introduced to account for the apparent accelerations of things from their perspective. From the perspective of a fourth observer who is stationary with respect to the spacecraft's final inertial frame, both you and the spacecraft moves towards this observer at constant velocities. First, the spacecraft overtakes you since it's moving faster towards this observer. Afterwards the spacecraft decelerates to a stop. Eventually you go past the stationary spacecraft. You maintain the same velocity always. The magnitude and direction of acceleration of any body is relative. However, the nature of the acceleration of a body that accelerates with respect to a single inertial frame is different from the nature of the acceleration of a body that only accelerates with respect to a non-inertial frame. It's the difference in the nature of the two accelerations that breaks the symmetry between the twins. Instruments carried by both can record a difference. Associated with any pair of events which are 'causally connected' (i.e. there exists a cause and effect relationship between the pair of events), there is an inertial reference frame in which the time experienced between the pair of events is a maximum among all the reference frames in which both events happen locally and at the same location. Since you remain in the same inertial reference frame throughout the journey of the spacecraft, then you are in this inertial frame in which the time experienced between the two encounters with your twin is a maximum. Any reference frame whose state of motion deviates from yours will experience less time than you, between the two encounters. The greater the deviation from your state of motion, the greater the difference in time experienced. If your twin travels very fast and very far then he will age a lot less compared to if he travels slow and nearby (from your point of view); even if both these journeys take the same amount of time for you. The difference in time experienced is not due to acceleration per se, instead it depends on the extent to which your twin's state of motion differs from yours.
I am a PhD astrophysicist, and I have tried for 10 years as an undergrad and during my PhD to obtain an explanation for this. Nice video (as always with Dr Lincoln), but I STILL can't see why C can't be assumed to be stationary, and that A and B (and the Earth and Sun) are moving toward C. It still looks like it can be completely symmetric. There is no reason here why we can't assume one frame for C and two frames for A and B. You start be saying we can treat A as stationary, but that is true for C too. In the scenario with C in the rest frame, the clocks on A and B would move slower (when we compare all clocks after we bring them all into contact at a later time). Unless we choose some absolute rest frame (maybe the CMB, which does kinda offer an absolute frame) I simply do not see ANY scenario that can not be flipped through symmetry in a purely special relativistic case.
@Leo Nunes The problem is that, if the twins start out in different locations and/or different velocities, they are not properly synchronized. That is, they are the furthest things from "twins" possible. Any valid explanation of the twin paradox must start with twins which, by definition, must be co-located, co-moving, and fully synchronized. Otherwise, it's blowing smoke violating initial condition assumptions. The real explanation is that once they are properly synchronized, and one twin experiences any kind of acceleration (gravitational or otherwise), his clock rate will change. This clock drift will persist until they are co-moving, at which point a clock offset may remain but the clock drift will not. Relative velocities have nothing to do with it the fundamental cause of desynchronization of local oscillators (clocks), but if you integrate the measurements, you can, of course, mathematically extract the drift result to ascertain the effect of acceleration. This can be done purely by considering relative velocities when masses are not present, but it does not mean velocity has anything to do with the fundamental cause of proper clock drift--change in velocity does.
You are correct. Treating the Earth twin as the stationary observer is an arbitrary choice. It seems natural because the Earth is essentially an immovable object compared to a spaceship, even if you allow for recoil. One can also imagine that the traveller experiences forces and the bystander does not, but none of that factors into the equations. SR simply can't get started until you identify the stationary reference frame and that's what the pontificators of Twin Paradox solutions are arguing about. The Doc makes a good point about acceleration being a red herring, but his 3rd observer is just another candidate for the stationary frame, as you say. What we need is an impartial observer who is inarguably stationary with respect to the departure and arrival events. That would be the centre of mass in a closed system, but it's not clear if that's a legitimate assumption for this scenario. It ultimately boils down to a reference frame in which the net momentum is zero. SR will produce an unambiguous answer if you know the velocity of each twin from that point of view and remember to apply the velocity addition formula.
Everyone tries to explain the twin paradox by starting out with a complete and total violation of the initial assumptions. The twins must start together and be fully synchronized.
@@onehitpick9758 Well, that's an exaggeration. I'm not aware of any proposed solution where the twins don't start and end together. And togetherness implies synchronization. The Doc's 3-body solution is the exception of course, but it can be reduced to a 2-body solution if the travellers exchange identities as they pass one another. The question which eludes us is, which twin is stationary with respect to those events or, more accurately, which reference frame has zero net momentum? Earth twin is stationary if the rocket expels propellant into free space, but it's not clear if that is indeed the case. If it is, then the solution must include the age of the propellant as well as the twins. A simpler formulation is to launch the rocket with a catapult and pull it back with a bungee cord. In either case, there are 3 reference frames to consider and the velocities are functions of mass.
I think there is one thing that can help understand the paradox, that I have never seen being evoked: If we "freeze" the world midway into the paradox, when Ron reach the star, as if he never intended to return earth, then by no mean we can distinguish the travel of Don and Ron. It is perfectly symmetrical from the start. Thus at this moment of the paradox, what is preventing us from saying that Ron is younger than Don from the earth reference, but also the reverse, that is Don is younger that Ron from the spaceship reference? Well actually nothing. We CAN say that. Ron can "see" (from far away) that Don is younger, and Don can "see" that Ron is younger. One have to remember that simultaneity is relative to the reference frame (look "Relativity of simultaneity" on wikipedia). So Ron and Don are really older than each other on their own reference frame. Hope this piece of information can help anyone understand better the paradox.
Hum, this paradox is really a tough one, after all these years I am still not sure of anything :) Ok, in that case: If we say the simultaneity is not involved, then does that mean we cannot say "each one is older that the other one"? Or on the contrary, can we indeed say that, but the explanation is somehow not related to simultaneity? If you tell me that we cannot say "each one is older that the other one", then, do you know where does the asymmetry come from, regarding two reference frames moving but having no acceleration? Because for me the main point is: no asymmetry means that mathematically, no one can grow older, universally more than the other. Actually I would very much like a video about this :) I will search about the video you wrote about
You are looking at it based on their own Reference Frames but we know the rate of time for the observer that IS moving will be less than the one who is not. The only way their two clocks would measure the same elapsed time is if they were both moving away from each other at the same velocity.
For subjects that are counterintuitive (such as time dilation) I consider that a top-bottom approach fits best; first, viewers must understand what is important (time frames) and then they can easily access the mathematical approach as now the reasoning for each step of the methematical approach is easier to understand.
This video did make it a lot clearer. The apparent paradox of the problem comes from missing the fact that the traveling twin has to change reference frames when turning around. I think driving this point home is really the crux of understanding the problem, an the first video, despite having observers B and C really didn't stop to emphasize that fact enough. For those of us who really don't understand the physics fully, it's very easy to just gloss over the importance of this subtlety. In fact, it might have been helpful to show the perceived ages each of the three observers have of each other at the beginning, when B&C cross, and at the end, when A&C cross.
Reading your comment, is the first time I have understood what he is saying. Not that I believe in time dilation, but at least I see what their explanation for the paradox is
To add to my previous comment, I have to say however that for time dilation theory to work, there has to be such as thing as absolute velocity, such as being at rest, which would contradict relativity. Because if all motion is still relative, it does not matter whether you put in acceleration or reference frames explanation, because otherwise happened is the same as saying Ron was still in his spaceship, Don moved away with the earth, therefore DON CHANGED FRAMES and then returned into the same frame as Ron... then we are back to the same place, which one is older
@@acasualviewer5861 the point the gent missed is that just because there's no absolute velocity doesn't mean there's no absolute acceleration. There absolutely (heh) is. Inertial reference frames (that is, non accelerating ones. Stationary, constant velocity, free falling, etc) are indistinguishable. If you are accelerating however, there are many ways to tell.
The twin paradox is rigorously solved in this tutorial, with a realistic acceleration, from both points of view of the traveler and the stay-at-home twin: ruclips.net/video/QFWF90bch3c/видео.html
to make it simple, the twin paradox has only become a paradox when we only used a special relativity that works only for inertial reference frames. However at this case, since one twin made a travel back to earth, the situation involves not only inertial frame but also an accelerated frame. What happens in time at accelerated frames can be explained by General Relativity.
@@matthewtalbot-paine7977 special relativity works for not accelerating spaces(your room and you) General relativity works for accelerating space(We used this here)
But of course "acceleration" is just changing reference frames. Acceleration allows one object to experience 2+ reference frames. In the other case, two different objects experience the reference frames and compare with each other. I think what really trips people up is when you have just two objects that speed away from each other and NEVER come back together. Each experiences just one reference frame. They fundamentally disagree over who experiences more time.
This is unfortunately not correct Don. You are changing the entire scenario by inteiducing a third twin,second spaceship! Also, you are removing acceleration completely. How can you? Not possible! Have you read Einstein's solutiin to the Twin Paradox as mentioned in his 1918 documents? He explicitly states that acceleration of the travelli ng twin causes non- reciprocal time dilation which makes the travell8ng twin to be younger. Easy! Also, you can easily deduce this from the worldlines of the stay at home twin and the travelling twin. Whose wordline is shorter? The travellung twin's worldline. So, he remains younger. Why are you complicating it Mr. FERMILAB? 😊
Perfect explanation. Another way I like to debunk the bogus acceleration explanation is to note that the twin at home is also experiencing 1G due to earths gravity. And 1G is plenty sufficient propulsion to zip around the galaxy and do twin paradox experiments (indeed relativity is not as much of a buzzkill for interstellar travel as most people think, but that's another topic). With a ship capable of accelerating at 1G, you can accomplish a round trip visit to alpha centauri in about 10 years ship time, while about 15 years pass on Earth (accelerate to halfway point, decelerate, arrive and repeat to come home). The twin at home also experiences 1G in the form of gravity on the surface of earth, so both twins are experiencing the same acceleration effect (aside from the launch, of course). So it's not the acceleration. Incidentally, the general relativity dialation effect at 1G is negligible for both twins: it comes to about 3 milliseconds per year, or 0.03 seconds for the traveller or 0.045 seconds for the twin at home, which doesn't make much of a dent in the 5 year difference. So, again, it's not acceleration!
Their acceleration is not symmetric. If the outgoing twin accelerates for 1G over 10 years he will need to undergo a massive mount of de-acceleration and re-acceleration at some point in order to come back. The Earth bound twin needs no such acceleration.
The two frame explanation is the only explanation that I have ever seen or heard that ALMOST makes sense to me. Thank you. I can not help but think that there is a closed integral hiding in there somewhere to describe the condition that the two frames sum to get back to the starting point. BUT, why not make two moving earth frames from the rocket's perspective? It seems to me that brings back the paradox.
The twin paradox is rigorously solved in this tutorial, with a realistic acceleration, from both points of view of the traveler and the stay-at-home twin: ruclips.net/video/QFWF90bch3c/видео.html
I like math. But I think we need logic first. Algebra is just a way to express logic and allows us to quantify. I think it has been a great idea to give two points of views of the same problem. It is only possible for great scientists like you that makes complicate things easy to understand. I can say that I can understand the solution of the twin paradox very well for the first time. Thank you very much and congratulations for this video and for the channel. Great job!!!
This is unfortunately not correct! You are changing the entire scenario by inteiducing a third twin,second spaceship! Also, you are removing acceleration completely. How can you? Not possible! Have you read Einstein's solutiin to the Twin Paradox as mentioned in his 1918 documents? He explicitly states that acceleration of the travelli ng twin causes non- reciprocal time dilation which makes the travell8ng twin to be younger. Easy! Also, you can easily deduce this from the worldlines of the stay at home twin and the travelling twin. Whose wordline is shorter? The travellung twin's worldline. So, he remains younger. Why are you complicating it Mr. FERMILAB? 😊
It is unfortunate that a great channel like FermiLab would give such a highly misleading explanation (twice), and then label it the “Real” explanation. You are only talking about Special Relativity. Relative acceleration does play a role in General Relativity, where a single non-inertial reference frame is allowed for the spaceship’s entire journey. The person in the spaceship can believe that he is standing still by believing that there is a gravitational field present throughout all of space, and that gravitational time dilation is what causes him to be older than his twin.
Of course, we cannot ignore other factors, but "Special" relativity is really just one part of entire story -- unfortunately it is symmetrical. With Special Relativity, both will see other younger by exactly the same amount. Since you are my favorite Physics video producer, it is certainly no where near your level of 100% visually perfect explanation, and I need your help! Because you already made fantastic relativity videos, but can you do it with non-accelerating "Special Relativity" Let us examine it in narrower context -- factor out time just to begin with -- compute only through events! Years are earth revolutions around Sun, and distances are light years! Imagine: only 10 light year away, we discover an identical planet E’ (spinning like our Earth E) orbiting around its sun S’. An Alien of exactly my age would look 10 years younger? Paradox? Imagine we both communicate with our Zero-Latency quantum entangled devices, find one another with our classical telescopes (10 year latency)! We would endlessly argue- Same D.O.B. - how can you look 10 YEAR YOUNGER THAN ME! PARADOX! EINSTEIN ( and Dr. Don) IS WRONG!! Of course, we always see past/younger images of one another even when we both soar towards each other at half the speed of light. After 10 years (exactly ten revolutions of E and E’ around S and S’)-- crossing halfway (5 light years), when we look back through our telescopes and synchronize with our planets with their their 5 year older image!!! We will shout - No! can’t believe it -- how can 10 years ABSOLUTE be 5 years RELATIVE! Even E and E’ (we saw with our own eyes) have revolved only 5 times - that is time dilation! A beautiful Paradox!! Earth feels 100% stationary while riding on it (no jolts), time feels 100% absolute until we look from another FRAME!
Time is a Idea that man created. Such as "Cold" , a description of something that does not exist. Cold defines the lack of something, called heat. You can not fill a jar with cold. It is not something you can measure. You can only measure how much heat that object has. As with "Time", you can not measure within the universe something that isn't there. Time doesn't work with Relativity because it is not a physical constant that you can quantify throughout space. There is no way to know or estimate the true answer without knowing the construct of the entire known universe. Please stop "Educating" people on theories as if they are the truth.
I agree! Periods are just repetition of rotational position, and hence are deterministicaily measurable. Time observance is at the mercy of speed of light, and hence gets skewed at velocities that are comparable to speed of light. I propose this: factor out time just to begin with -- compute only through events! Years are earth revolutions around Sun, and distances are light years! Imagine: only 10 light year away, we discover an identical planet E’ (spinning like our Earth E) orbiting around its sun S’. An Alien of exactly my age would look 10 years younger? Paradox? Imagine we both communicate with our Zero-Latency quantum entangled devices, find one another with our classical telescopes (10 year latency)! We would endlessly argue- Same D.O.B. - how can you look 10 YEAR YOUNGER THAN ME! PARADOX! EINSTEIN IS WRONG? Of course, we always see past/younger images of one another even when we both soar towards each other at half the speed of light. After 10 years (exactly ten revolutions of E and E’ around S and S’)-- crossing halfway (5 light years), when we look back through our telescopes and synchronize with our planets with their their 5 year older image!!! We will shout - No! can’t believe it -- how can 10 years ABSOLUTE be 5 years RELATIVE! Even E and E’ (we saw with our own eyes) have revolved only 5 times - that is time dilation! A beautiful Paradox!!
I wish I could like this comment multiple times! It is so sad that a reputable particle physicist would slip up like Dr Lincoln has with these two videos. Perhaps it has just been too long since he was dealing with first principles? :-(
I'm baffled at this failure to explain this (twice). and like.. I'm sorry, but doesn't time dilation have to do with space time? Forget relativity for a moment. The faster you travel through space the slower time passes. Just like the closer you are to a large mass, the slower time passes. Isn't that how this works? Doesn't that make the paradox not a paradox?
Don't understand the debunking for the acceleration resolution of the paradox. I don't think that acceleration causes time dilation, it is because speed as you stated. But acceleration breaks the symmetry and therefore you can tell which one is older, the one that is not experiencing acceleration, as the original paradox point out that they meet again at the end and compare they ages. So in my opinion, acceleration still resolves the paradox (in the way it is originally presented) although do not explain the time dilation.
This video does not remove the paradox. It keeps it in place instead! It's the reference frame which is the beef of the story. Or to be more tangible: the simultaneity lines for the traveling person is entirely different and even changes on his/her way back. That's how the traveler has two different simultaneity lines (reference frames), both differ from that of the stationary person. The gentleman either ran out of time or he was scared too few would understand it, because without understanding how to use the Lorentz Transforms to create world lines and simultaneity lines, one is not going anywhere, so to speak.
Assume for a minute that the universe has a closed spherical topology, it is small enough to traverse in a finite amount of time, and one twin is moving close to C. That should allow the traveling twin to return to their origin without frame jumping. How would the paradox be resolved?
I was thinking curved in terms of topology, not geometry. Consider a video game like asteroids. Parallel lines don't converge or diverge, so the geometry is flat, but the topology is curved, and you come back to your starting point.
so you are basically asking why the paradox doesn't hold up in an imaginary universe with different rules regarding space than our current universe? I would suggest that the paradox doesn't hold up in that case because it is totally invented and not real.
Acceleration determines who changed the reference frame when there's only two objects. Another way to explain it is when two twin brother travel away from each other in empty space and come back to the same spot, in a case when they do exactly the same thing in opposite direction they have aged the same amount when reunited, but when one goes slightly faster, both have changed reference frames, both have felt acceleration, but one is younger, since he felt more acceleration?
Basically, yeah. The question of who is older is meaningless (subjective, based on who is making that determination) until they reunite. I think it's a fair observation in the video, that it's not really about acceleration: time dilation is a phenomenon of relative velocity. But to re-establish a common time-base to measure who has experienced more passage of time, you need to re-unite the twins. You can choose any non-accelerating frame of reference to measure the velocities of the twins. From this vantage point, some accelerations will increase the apparent velocity of the twin you're observing, making their time slower relative to that vantage point, and some accelerations will decrease the apparent velocity of that twin, making their time proceed more normally, relative to that vantage point. Whatever vantage point you choose, the end result (the difference in time experienced by the two) will be the same.
I think I got it: At 5:29, when Observer B and Observer C get to the distant star, they hold up a sign that displays If A is moving away or it's getting closer respectively. If B's sign displays: "A is moving away" and C's sign displays: "A is getting closer", Since both cannot be true, B and C conclude that A is stationary. That's why you need more than Two observers, or what is the same, Two reference frames in addition to the Stationary Reference Frame, to distinguish which observer is really the stationary. In real life we always have more than "two observers" (all stars, planets, atoms etc) and that's why there is no paradox in real life, We all can distinguish who really is moving and who is stationary. The over simplistic case of only two observers in the universe doesn't exist in real life and and adding to that, in real life we do not have purely "Inertial Frames", we always have at least one "Non Inertial frame" therefore the symmetry is broken
C might see B as moving, but B doesn't see himself as moving, and therefore B's clock will be faster than A's. So the time C copies will be the time B sees, so faster than A's. C also sees A as moving so his clock will also be faster than A's. But if his time = t/γ.it would be slower.
My undergrad modern physics teacher explained that you can't understand an electron by thinking of a particle, wave, or both, but must intuitively understand (grok, if you will) Schrödinger's equation. You can't "see" it as something that relates to your corporeal understanding.. To truly understand Relativity (Special and General), you must see it through the math, and use that understanding. I eventually did that, and found it was the only way to truly understand imaginary numbers and how the complex number plane worked relative to impedance in my Electrical Engineering classes. I had to "see" it though the math and stop trying to force it to fit into my day to day experience. Kudos to Dr. Lincoln for his outstanding explanation that avoids the math and brings it closer to a layman's understanding. However, to truly grok this subject you must indeed live in a world where the math is your basis for reality.
I agree that your example tells us that we don't need acceleration to calculate the time lag difference between A and B. But to acceleration or who's reference frame is stationary still does matter in this "paradox". Similar to your strategy , I can choose B to be stationary, and let A and all the planets (Actually all the matters in the universe except B) move to the left at constant velocity. Imagine another observer C moving to the right at constant velocity, and passes A and sync his clock with A at the moment when the distant planet passes B. C can still moving towards B at constant velocity, but the all the planets have to turn around and move with C so that when C meets B, the planets return to there original positions. Since at the end C's clock should be much older than B's clock. It means the acceleration of all the planets have a huge effect. But I'm not sure how this effect change the clocks for B and C.
I still don't get it. Isn't it equivalent to acceleration to switch between measuring a clock going +v to a clock going -v? That seems like an instantaneous jump of -2v in my mind and thus not constant velocity and thus an acceleration. The fact that the same theoretical person isn't physically holding the clock in both directions seems rather irrelevant. I mean obviously Dr.Don is way smarter than me so I'll trust that he's right, but this explanation doesn't really explain much.. it just introduces some additional variables that seems to mask the paradox more than resolve it. Even in the previous video where he did that math, he was always explaining from A's perspective (he put up the equations for B's and C's perspectives but didn't really get into why B/C think they're younger than A at the end of the trip and I'm neither smart enough nor motivated enough to figure it out for myself.. I learn my physics from soundbite RUclips videos for a reason!)
1. Who measures the clock? Everybody in their own reference - no switching, no acceleration. 2. Who compares the measured values? A, by reading the sign (which, btw, will be length-contracted). P.S.: I believe your idea of frame-switching-acceleration comes from the fact, that two observes may not agree that two distant events happend at the same time. Observer A will agree with observer B that observer B measured the time when reaching the star, because B was at the same place at that time and every observer must agree to that. Observer B may not agree that A measured the time B reached the star, becase A and B (A = measurement, B = event) are not at the same place.
I can't help for most of this - I have a masters in physics and Dr Don still keeps correcting me on everything! But I can point out one flaw in your comment. Acceleration is calculated as Δv/t, and in your instantaneous frame-jump, that's 2v/0, or ∞. Clearly that's an impossible acceleration, so you can't use it in any calculation, and therefore has nothing to do with the physics involved.
It is true that the space traveler experiences acceleration. But acceleration itself does not make him younger than the person on Earth. The traveler becomes younger because he is in two inertial reference frames. So we cannot use time dilation formula from his perspective.
I think that if you take acceleration out of the Lorenz transformation of the moving ship, ie. It instantly changes its reference frame, which I think would be impossible, then the time difference is simply lost to the younger twin as that information would have to be moving faster than the speed of light. Minute physics has a couple of good videos on this using Lorenz graphs instead of formulas which make it a little easier.
Hmmmm... A bit obsessed with debunking the acceleration argument. The problematic question is: why is there no symmetry? If we took the Ron's reference frame as stationary and tried to repeat the argument about Don changing reference frames, where does it all break down?
ScienceNinjaDude OK, let me spell it out a bit more: what if we had Ron's frame as stationary and checked clocks in his location? i.e. what's stopping us from replacing"Ron" with "Don" in argument, except having a minus velocity?
Now, ScienceNinjaDude, you're talking about an accelerating Ron (inbound and outbound), which is not within the original context of special relativity. Special relativity is very simple, and while it can be extended to analyze scenarios with acceleration by using calculus, the original postulates of the theory did not assumed no acceleration (inertial frames).
Hey Dr. Don... Both of these were awesome videos (i have been watching everything in your series... quite fascinating, thanks so much!) So your explanation here of the frame change and how it solves the so-called "paradox" makes perfect sense to me. Having said that, I have a thought experiment of my own that I'm hoping you can help me work out. This experiment has to with reconciling what I experience in a moving frame with what I observe to be happening in a stationary frame. Let me set the stage for you... Peter, Paul, and Mary are physics enthusiasts who decide to engage in an experiment. Peter and Paul are on their home planet Earth, where Peter will leave and travel at some relativistic speed (say .75C) toward a star "Massive 1", approximately 1 light year away, make some observations, and then turn around and head back home at .75C. Mary is on a distant planet (the three enjoy communicating by ham radio). Mary's planet is situated such that she will observe Peter's journey from the side, as her planet is at a point equidistant between Earth and Massive 1 (the apex of a triangle). Peter, Paul, and Mary will, in addition to keeping time on their own clocks, observe their favorite pulsar, P, which is also equidistant from Earth, Massive 1, and Mary's planet (the apex of another triangle, which ultimately forms a pyramid). The departure date for their journey is Earth date January 1, 2020. I'm making up the numbers because I don't know all the math and the details are not so important here as the concept. As Peter heads away from Earth, it's my understanding that he'll experience a few things: 1) The apparent distance between him and Massive 1 will appear to contract (he will be quite surprised to see that it doesn't seem to be as far away as he thought). Massive 1 will also appear heavily blue-shifted 2) His view of the Earth will appear red-shifted, and the space behind him should appear elongated, thus making it appear as though he has traveled farther than he ought to have (balancing out the contracted space he sees before him I suppose). 3) Upon arriving at Massive 1, Peter will determine, based on his own clock, that he has been traveling for less time than what Paul and Mary have measured. 4) It is further my understanding that, although he is traveling at .75C as measured by Paul and Mary, due to time dilation, Peter will experience some amount of time that is less than the expected 1.3 years that Paul and Mary would observe. That specific amount is probably not of importance here, but we'll call it six months. Having laid that all out... Have I made any faulty assumptions so far? Here are my questions... 1) Peter, as a consequence of time dilation, should at some point observe that time is moving faster for Paul back on Earth. Traveling at .75C, Paul will observe Peter leaving Earth on January 1, 2020, traveling 1 light-year, and thus arriving on May 1, 2021. Peter will experience less time, yet will simultaneously observe Paul being red-shifted, but at some point his clock also moving faster. When Peter arrives at Massive 1, He will observe Paul's clock to read May 1, 2020 (four months after he left), because the image is one year out of date due to the distance. As Peter experienced say six months himself, Peter sees four month's elapse on Paul's red-shifted,, time delayed image, but where does the missing time go? Is this simply made up for in the return trip, when Peter would observe Paul's (now blue-shifted) clock to be be moving faster? 2) In keeping with #1, since Peter and Paul both observe the same thing with regard to one another's clocks (each sees the other first receding and red-shifted and then coming closer, blue-shifted), aat what point do the observations vary? Obviously Peter ends up experiencing less time overall. How is the accounted for in what they observe? 3) I included the pulsar and Mary here as outside factors that are marginally affected at best by Peter's travel, because of their angular position with respect to his line of motion, and the fact that a pulsar pulses at a highly regular and predictable interval. Since it is perpendicular to Peter's trajectory, should he and the others not observe it to be pulsing at essentially the same interval? Wouldn't that cause a problem reconciling reality when he reaches Massive 1? There's obviously an answer here... I'm just not sure where my logic is flawed. Thanks.
Makes sense. Take t_moving to be less than t_stationary due to length contraction. In earths perspective, the spaceship travels t_stationary+t_stationary, since the earth and star are at a fixed distance. In the spaceships perspective, first the star is moving towards it, then on the return trip, the earth is moving towards it. So the spaceship experiences t_moving + t_moving. Since t_moving
Thank you! My father is a theoretical physicist, he and a colleague (astrophysicist) explained why "Physics Girl" is wrong, she has a video making claims about acceleration that you warned us about in your videos. They said her explanation is fundamentally flawed. Of course they declined my request in posting a video refuting her video. Glad you posted this.
I don't know why you're congratulating Fermilab! The Twin Paradox, *_by definition,_* requires A & B to come back together to compare their times and get a mutual agreement of difference. In Fermilab's commentary above A and B keep moving forever apart, which means his example is *_NOT_* addressing The Twin Paradox... just the pre-amble to it.
Thanks for great explanation! But I'm still confused by few things. First of all, we are adding times in two _different_ frames of reference.. Is that even legal? The second thing I'm filling confused is the statement "all observers agree...". But B does not have any information on A's clock. Maybe, if I add some observer to carry information from A to B, it would happen that both B and A will think that the other one stayed younger. And the third thing I can't understand, is the picture from B's and C's point of view. If we apply relativistic addition of velocities, would they agree they met at L? Or, approaching A, C would tell "I met B at L/3", for example? I'll be grateful if anyone explains this to me.
Thanks for the videos. I enjoy watching them a lot, especially the ones that debunk wrong explanations. Really cool stuff. I've binge watched on a few occasions!
I just can't escape the feeling that you created the assimetry in the thought experiment. I mean you intended to exclude acceleration of "Ron" by separating him into two frames (B, and C). But you didn't do it with A, so the only reason why Don (A) is only in one frame is because you did not separate him into two frames. (I know you didnt say your thought experiment was "related" to Don and Ron, but the connection seems obious.) I just cannot grasp what you mean by "A is only in one reference frame" and "B and C are in two". Maybe its a language barrier, but I can't wrap my head around what it means. Also, you said that the measurements are mada at A's location. So how would the thought experiment looked like if it were measured at B"s or C's location? Would the result be the same (as Don aged more), even measured at different places? Sigh.. :/
Have you found any answers to this? Been looking for years. I haven't found one video that doesn't lead me to the conclusion that being stationary or having an absolute velocity with respect to the universe are a thing.... and I always learned they are not.
Indeed! Dr Don uses sleight of hand in this video. As you say, *_he_* creates the asymmetry (rather than it being a physical asymmetry in his experiment) by _choosing_ to look at a particular passing observer C and ignoring all others (such as a symmetrical observer D, who happens to be passing A the way C is passing B). So long as A and B continue moving apart at a constant speed there is no symmetry break. The Twin Paradox _requires_ B and/or A to accelerate to break the symmetry and cause a mutually agreeable difference in age.
@@UncertaintyPopsicle , you are sort of right in a way: _that being stationary or having an absolute velocity with respect to the universe are a thing_ Consider it in these few steps... *(1) **_that being stationary with respect to the universe..._* Observation 1 If you look around yourself you will measure that light travels at the same speed _c_ (300,000 km/sec) in all directions. Conclusion 1 You are obviously stationary with respect to space (whatever that is). *(2) **_having an absolute velocity with respect to the universe..._* Observation 2 You observe that all moving objects (all the way down to atomic level) become shortened in their direction of movement and have a similar slowing down of their time processes. The closer to the speed _c_ they get, the flatter and slower they get. Conclusion 2 Not only are you, who are neither flattened nor slowed, clearly stationary with respect to space, but anything that moves is affected by moving through space and so clearly has an absolute velocity with respect to it. *(3) **_a weird fun house mirror_* Observation 3 If an observer (eg, something capable of observing and measuring its surroundings, such as a human or a space probe) is moving then it will become distorted (squished, slowed, etc). A severe distortion (such as hitting a brick wall at 200 km/hour) can total wreck an observer's view point, but the distortion we've observed above is nice and uniform so that a moving person will still be able to see, and think, and measure. Conclusion 3 Because they are squished, time-slowed, and moving with respect to space through which light moves at a constant speed, a moving person will observe the universe in a *_different_* way than we do. All of their observations and measurements will be *_distorted._* *(4) **_through another person's eyes..._* Observation 4 Since we know the formula for how a moving person is distorted, we can work out how the world looks to them. (a) How does time look to them? We can see how their slowed watches act. (b) How long are things (such as the length of their spaceship)? We can see how their shrunken measuring sticks measure. (c) How fast do they measure the speed of light? We can see how their slowed clocks, shrunken measuring sticks, and motion relative to space alters their perception of the speed of light. Conclusion 4 They think they are stationary and normal!!! (a) They think their time is normal! That's no surprise since they are using the slowed clocks they are carrying with them to measure their slowed selves. (b) They think their length is normal! Again no surprise... they use the shrunken measuring sticks they are carrying with them to measure their shrunken selves. (c) They think the speed of light is the same in all directions and is 300,000 km/sec! _THIS_ is not at all obvious! This isn't "light that they carry with them"... light moves at the same speed relative to space regardless of the speed of the person that is emitting and/or measuring it. So why do we think they see it as uniform? We set up an experiment to measure the speed of light ourselves, and then we calculate the speed that a distorted moving observer would measure using their distorted moving device. The standard device is to have a long stick (of known length) with a mirror on the end. Light is shone at the mirror and a clock is used to measure the time it takes to do the round trip. Round trip distance divided by the round trip time is the speed of light. A moving observer has a shortened stick, and a slowed clock. But they are also moving with respect to space. In one direction the light will take extra time because it is chasing a stick end that is moving away from it, while in the other direction it will takes less time as it is chasing a stick end that is moving towards it. When you work out that time, the shrunken stick and the slowed clock you get... 300,000 km/sec in all directions! Weird! :-) *(5) **_looking at them looking at us..._* Observation 5 We now have measured that the moving observer views themselves, through their _speed distorted eyes_ and their _speed distorted measuring devices,_ as being normal in size and time flow, and as being stationary in space with respect to the speed of light. But how do they see us??? We know *_we_* are actually the stationary and normal ones... but we know they see things distorted. So what weird distorted view do they have of us??? Conclusion 5 When we do the measurements we conclude that they think we are: moving at the speed that they actually are but in the opposite direction and that we are shrunken in that direction of travel and time slowed! *(6) **_spOOky..._* Observation 6 But... hang on. We know that WE are stationary and that THEY are moving and hence distorted. BUT... if that distortion means that their distorted measurements and observations means they see themselves as stationary and US as moving and distorted, perhaps WE are the distorted ones? =:-O Or a third observer moving at a different speed might be the REAL stationary person. Or a fourth? Or a fifth? ARRGGGHHH! Conclusion 6 How CAN we tell which one of us is the one that is REALLY stationary with respect to real space (which is the real Sparticus?)?? The answer is... we can't. :-( Aside from light moving at a fixed speed through it, and objects moving through it being distorted, we don't have any OTHER property of SPACE that we can measure. And because that distortion just happens to make every constant speed observer see themselves as stationary, we have no absolute way of measuring which observer is the real stationary one. So the answer is: any of them, or none of them, or all of them. Some people will say that Einstein showed that there was no Aether (ie, absolute space through which light travelled). But others will tell you that Special Relativity does not preclude an Aether... it just makes it irrelevant since you can never actually measure it in an absolute way. Footnote: If sometime in the future space DID show an additional absolute property that could be measured, then that would change things. But at this stage nobody really expects that to happen.
@@corwin-7365 So you explain the paradox perfectly. But what is the resolution? We know that the twin the flew away and came back ends up younger w/ respect to the twin on earth.
I believe there is significance to the fact that an observer who remains in an inertial frame experiences the change of inertial reference frames of the twin with a delay, whereas the twin who changes inertial reference frames can immediately observe the change. Here, observing the change refers to watching how fast a clock runs for the other twin, say with a telescope (which is equivalent to observing with what frequency a periodic signal of constant frequency arrives from the other twin).
Thank you for your videos and explanations. Loved the pair of docs pun. As a physician my understanding of this level of physics is somewhat limited. But I do have my own biological take on this. The biochemical reactions inside the human body, indeed inside any organic life form, also take place at a quantum level. Electrons and protons jumping around from compound to compound and stuff. This jumping around of particles takes place with a certain speed and is facilitated by enzymes. But these particles still have to "get caught" in the enzymes in order to partake in the reactions. Enter travelling twin at the speed of light. Assuming that the particles have their own inertia, they have to overcome this speed deficit in order to be able to partake in the biochemical reactions. This slows the reactions down and as a macroscopic effect, the biological clock of the travelling twin slows down. While the distances to be travelled in the particular case of biochemical reactions inside an organic cell are minute, over time this adds up (especially on a roundtrip to a distant star). So please Dr. Don take a look at these questions. Does this make any kind of sense to you as a physicist? Are any of my assumptions wrong on a theoretical level? Does this explanation also rely on acceleration to work (and is therefore wrong)? I would very much appreciate an answer.
Hello Breabanm, six years have passed: did you get your answer? In all these 'explanations', like in this video, there is way too much emphasis on the math, acceleration, time slowing down etc., and with it comes a lot of confusion that is hardly ever addressed. You are absolutely right: this whole thing with relativity and time dilation is about nothing else but the relative rate of aging of matter at different speeds (i.e. elementary particles having to bridge more space to interact). After all, at the bottom line and away from the hocus-pocus, it is/must be a tangible process! Also, it is not about tíme slowing down. Time slowing down does not make any sense. It's meaningless! Time, if it exists at all, is the temporal 'space' that elementary particles need to interact, or: time is a function of particles interacting. For me, and probably for you too, this is the fascinating part of it all, and suddenly everything starts to make sense! Yet, it is as if this real explanation is anxiously avoided by knowledgeable people like dr. Lincoln. As for 'time' and the relativity of simultaneity, Lawrence Krauss told me the following a few years ago (I am paraphrasing): the relativity of simultaneity simply states that we can never know, in any frame of reference, the true, absolute sequence of events that underpin the universal 'flow of time'. Still, this absolute sequence of events (that happen only once, and only at one moment and place in the cosmos) is real (in the Minkowski diagram these are all events going up relative to horizontal lines). The benchmark for the absolute sequence of events is the gravitationally unbound space in the universe. An imaginary object in that empty space is said to be truly at rest with the expansion of the universe (in the Minkowsky diagram this would be the vertical time line going up). Only this gravitationally unbound space 'knows' the true sequence of events (ánd the true speeds of A, B and C in this video!). Gravity and different speeds in different reference frames make you inevitably loose track of this true sequence. But still, the cosmos is 13.8 billion years old...everywhere!
The 2 observer example,as he said Don(earth) was in 1 reference frame, Ron(spaceship) was in 2 reference frames. I don't understand, if you look from Ron's(spaceship) point of view, Don(earth) is in 2 frames of reference and Ron(spaceship) is in 1. 3 observers example is more complex, so I will just have to leave it for some other time.
A only uses his one watch (his own) to measure the time, while the other duration is measured using two different watches (B's and C's). That's the one vs two reference frame.
I completely agree with Dr. Don - it is just about FRAMES (places)! Acceleration is irrelevant! Let us simplify -- compute only through events! Years are earth revolutions around Sun, and distances are light years! Imagine: only 10 light year away, we discover an identical planet E’ (spinning like our Earth E) orbiting around its sun S’. An Alien of exactly my age would look 10 years younger? Paradox? Imagine we both communicate with our Zero-Latency quantum entangled devices, find one another with our classical telescopes (10 year latency)! We would endlessly argue- Same D.O.B. - how can you look 10 YEAR YOUNGER THAN ME! PARADOX! EINSTEIN ( and Dr. Don) IS WRONG!! Of course, we always see past/younger images of one another even when we both soar towards each other at half the speed of light. After 10 years (exactly ten revolutions of E and E’ around S and S’)-- crossing halfway (5 light years), when we look back through our telescopes and synchronize with our planets with their their 5 year older image!!! We will shout - No! can’t believe it -- how can 10 years ABSOLUTE be 5 years RELATIVE! Even E and E’ (we saw with our own eyes) have revolved only 5 times - that is time dilation! A beautiful Paradox!! Earth feels 100% stationary while riding on it (no jolts), time feels 100% absolute until we look from another FRAME!
Yep, once he mentioned that acceleration = frame switching, then I got what he was trying to say. He's still not being entirely accurate in saying "acceleration isn't the answer" though. It's better to say "acceleration and frame switching are the same thing." One big thing that still confuses me though, from the perspective of the twin on the spaceship, doesn't he see the twin on earth accelerate away from him, stop, and then accelerate back towards him? I still don't quite grasp why these two perspectives aren't equivalent...
I don't think 2 frame or acceleration can explain twin paradox. The very moment B get out of earth, his time has already been slower than A. No need acceleration or 2 frame for that to happen.
I think many worlds theory can explain twin paradox well. Imagine A & B are near a point in space, they then travel far away each other near the speed of light; after 100 years, they travel back to the original point and meat each other. According to the relative principle, both A and B can assume they are stationary so that the remain fellow is younger. It is still valid no matter you apply '2 frame' or not. So, only many worlds theory can explain that since A and B can not be younger at the same time. (It is logically wrong; If A is younger, B has to be older and vice versa). Many worlds theory can dictate that: there is one world in which A is younger, and another world in which B is younger.
@@mattkilgore7323 , you are correctly identifying the critical difference between what is considered "stationary" and what is considered "moving". These videos do not provide that explanation. Adding kinetic energy to a mass is the fundamental difference in explaining the twins-paradox. In essence, adding kinetic energy to a mass makes the reference frames non-arbitrary. This video does a terrible job and just confuses people IMO.
In the A, B, C example, B never needs to return to A. All that is required is that B's clock time is transferred to C and when C gets even with A that A and C compare clock times. The clock time that was carried forward from B will show less time having passed than A's clock time. No twins required. Not even humans required. Not even any clocks required! Only physical processes occurring in time. I think the proper take on this, this lack of symmetry, is that the distance from A to the "goal" (space station or star) is different for the A, B, and C observers. Since A and the goal are traveling at the same speed (more or less) there is no Lorentz contraction involved. A sees the "proper" length. But since B and C are moving with respect to A and the goal they see the distance between A and the goal as shortened. A, B, and C all agree on the speed that B and C are moving with respect to A and the goal. This must mean that B and C will take less time to travel the shorter distance than A experiences back on Earth. It's that simple.
Acceleration is not needed to solve this paradox (i.e. the A,B,C example in video), but the paradox scenario (A & B only) requires acceleration to put B in two different reference frames. It is how you determine 'who is moving', and how to compare information (i.e. event 1 before A and B separate and event 2 when they meet again to compare clocks). Got it.
This is from the one perspective the one on the ground What about the observation from the perspective of one of the vehicles? Do they see A experience more or less time?
Fuseteam this is my problem with this paradox. I’ve NEVER see anyone address this even though it completely breaks their theory. If we change the observation to the one on the rocket it would seem like the one on Earth is younger
@@Maribro4 i'm starting think the takeaway is that *special* relativity tell us that both will _expect_ the other to be younger based on their observations, while the one earth _will_ be younger according to *general* relativity at least according to encyclopedia britanica's website
@@Maribro4 My basic understanding is this: The moving observer sees the effects of something called Relativity of Simultaneity (together with length contraction and time dilation), which basically means that events that are simultaneous in the earth-bound reference frame will not appear simultaneous to the moving observer. As an example, imagine that before rocket man sets off, the earth-bound observer arranges for a clock to be placed at the half-way point of the journey, and he synchronises the clocks (they tick at a simultaneous rate, in other words). According to RoS, as soon as the rocket starts moving at a constant velocity relative to the earth-bound reference frame, the clock at the half-way point becomes out of sync with the earth clock (from rocket man's perspective) and jumps forward by some increment of time that's dependant partly on the distance between the two events (the events being the clocks striking a certain time in this case, but it could be anything, such as two lightning strikes). Crucially, when the moving observer changes reference frames at the halfway point, this jump in time occurs once more, but this time on earth (remember, I'm talking about this happening from rocket man's perspective. At no point does time become out of sync in the earth/outside environment frame). Now it turns out that this clock jumping, time dilation and length contration makes it so that when the observer arrives back at earth (or even just flys past), he will see the earth clock just as the earth-bound observer sees it. This is why all these videos explain that the changing reference frame is the important aspect of this thought experiment, and why acceleration, although not insignificant, is not key to understanding why they both end up agreeing who is younger.
I've seen both of Don's twin paradox videos and I think I like the one with the math better. The math really isn't that difficult, just grab a pencil and paper, pause the video as you work through the algebra. Math-again-beautifully and simply explains this phenomenon.
But the math itself is acting as an abstraction for the explanation. which is just lazy. There's a Motherboard (I think) series where someone is asked to explain a concept at 4 different levels of understanding. Maths serves as one one the levels, but so does thought experiments, analogy, etc. Imagine going to a doctor and instead of explaining your condition, they simply showed the labs results and sent you on your way. I mean, all the same information is there. Many scientist come off the same. When in doubt, do it in more than way.
+quintessenceSL _"But the math itself is acting as an abstraction for the explanation. which is just lazy."_ You have that backwards: the verbal "explanation" is just an approximation of the math. _"Imagine going to a doctor and instead of explaining your condition, they simply showed the labs results and sent you on your way."_ If you had a medical degree or equivalent knowledge, the results would be all you would need (unless the particular tests required specialized knowledge to interpret). Likewise with SR: anyone with a high-school education should be able to follow the math. What most people don't have is the knowledge of how to apply the basic math they (should) know to the problem at hand. The way to gain that knowledge, and to really understand SR, is not to ignore the math, but to internalize it.
Michael Sommers What do you mean? To explain say, why things with mass cannot move the speed of light, you COULD just use lorentz transformations and call it a day. That doesn’t do much to understand the logic itself. I’d say it’d be better to understand the logic first, then the math-the easier stuff-afterward.
Dr D i know you're going to read comments to see how this explanation went, I was one of the critics of the previous video. Thank you for another bite of the cherry. I understand everything you say up until 7:50 the bit where you say "A is in one reference frame and B and C are in two". What does this mean? What's a reference frame? You confuse me by then going on to describe other people's examples and not immediately explaining this point. 9:36 seems to be the critical nexus of this video and it's glossed over for observer D stuff. I'm still lost 😔
I know what a reference frame is, long story short it effectively means how everything else looks to that observer, and for me it still doesn't explain it. The explanation seems to be that A is stationary while the other 2 are not. But what is stationary? Does stationary exist? In true relativity, everything is only relative to everything else. If one thing is capable of being stationary, then that suggests there is some kind of an additional field determining what is stationary and what isn't.
I've been trying to find an answer to this for years. Have you found any explanations? Every time I post it on r/askscience it gets deleted by mods. But I still see no way to resolve the paradox without accepting that "stationary" or "absolute velocity" are a thing.
If you still have any interest on it at all, i can give you a not so brief explanation (which may not be very satisfactory, but it is in fact the answer). When you work with special relativity, you use lorentz transformations. Those transformations applies only to inertial reference frames (those that are not accelerated) and you can only talk about time dilation and stuff if you stick to one inertial reference frame until you are done, you cant use one in a time and some other reference frame in another time and try to compare anything, it wont work and it doesnt even make any sense. So let's go: When you start this problem, you have two inertial reference frames, earth's reference frame and rocket's reference frame. During the trip, everyone's point of view is right: you are right when you are in the rocket and you say that earth's observer is moving and therefore has a clock slower than yours on the rocket and earth's observer is right when he says you (in the rocket) are moving and therefore your clock is slower than his clock. There is nothing wrong with that, because you guys are in different places calculating clock's rate of the other in different times, this is special relativity's first postulate, no one is wrong here. However, you want to back to earth and you must decelerate to do it. Yes, you can do the same thing as Don did (using a frame on the rocket to go away from earth and another frame on another rocket to back to earth), but in the end the result is the same as if you were decelerating back to earth in the same rocket. By 'same result', i mean that if you have two situations: (A): where you use one rocket and deceleration's reasoning, the interval measured by your clock on your deceleration will have a impact such that it will increase earth's time the right amount so that as you are seeing earth's clock slower than yours, earth's time will still be higher when you reach it, because in your deceleration period, you increased the amount necessary for it. Now, if you use (B): Don's situation, if you could 'jump' to this other rocket coming back to earth (therefore being not decelerated), then the moment that you jumped, earth's observer clock time would instantly go to a higher value than it had just before you jumped to this other rocket, so that even if you try to use the frame in that rocket, you could still apply the same reasoning as before and say earth is moving back to you, so earth's observer clock time is slower than yours (since there is a 'jump' of earth's time when you go to the other rocket, the things themselves compensates and earth's time will have a higher value than your clock when you go back to earth, just like the deceleration situation). If you accept that, you can continue on my reasoning. So when Don says the problem with this 'paradox' is because you are using two different frames, he is talking of course about situation B, since you are using a frame to go away from earth and another frame to go back to earth (two different rockets, therefore two different frames). Now, remember what i said at the beginning about lorentz transformations: "those transformations applies only to inertial reference frames (those that are not accelerated) and you can only talk about time dilation and stuff if you stick to one inertial reference frame until you are done, you cant use one in a time and some other reference frame in another time and try to compare anything". What i mean by this is that once you pick up one reference frame and you start doing all this stuff, you cant simply change to another frame and get any conclusions about the time on the former's frame. For example, if you get three reference frames X, Y and Z, use Y to compare with X for some time and then Z to compare with X and try to conclude anything about X's and Y's times, you are doing it wrong, because you changed from Y to Z. In twin's paradox, you are doing just that: using X (Earth) and Y (Rocket going away from Earth) and then you suddenly change Y for Z (Rocket coming back to Earth), that's why it doenst make any sense, you changed one reference frame. However, when you do it on earth's X frame, everything is fine, because you didn't change X, just Y and Z, which works just fine. There is some other way where you can indeed use a frame that coincides with the outgoing rocket as your reference frame, but as always, you must keep using this frame after your rocket starts to go back to earth (or if you just jumps to another rocket, it doenst matter): it means that in this frame, the outgoing rocket is at rest, but the incoming rocket is with a velocity -2v (since from earth's perspective, both outgoing rocket and incoming rocket are with the same speed v, but with opposite directions) and when you go from the outgoing rocket to the incoming rocket, you will still be using the frame i talked before, which will notice your 'new' clock's rate running even slower than earth's, so that after all, there is some absolute compensation to the earth's clock and it will be marking a higher time when you reach earth.
I studied a bit of this in meteorology school, and somehow understood it then. That goes along with all the other math required. One of my mentors/professors pointed out that if you do not think about and/or practice these experiments/equations on a nearly daily basis, the thought patterns fade. It's true, at least in my case. Meteorologists rely on computer models, observations and some intuition to forecast weather now. We rarely do any kind of equations. That's why I enjoy this channel so much. Occasionally, I have to shake up my brain like a snow globe and get out of a mental rut. Thanks, Doc!
The information (passage of time synchronized via a timestamp) is being accelerated exactly when switching reference frames from B to C. Acceleration here does not require that a physical body is being accelerated.
I'm late to the party, but another way to think about it is that in Ron's frames (both of them), he can consider himself stationary, but he will see the distance to Alpha Centauri as much shorter than Don sees it because of the relativistic length contraction. Even though he can consider Alpha Centauri coming to him at near the speed of light, he only sees it as 4 light months away, rather than 4 light years away. In Don's frame, he always sees Alpha Centauri as 4 light years away.
From Ron's point of view, Don is still moving away and then coming back. That is a shorter distance than the "real" distance between Alpha Centauri and Earth, but Don is still moving. Because Don is moving according to Ron, Don's clocks should run slower, both on the outbound and the return trips. That is, length contraction does not resolve the paradox. If you think that some factor is important for the solution of a problem, then it is always good to consider how removing that factor affects the problem. Suppose there is no Earth and Alpha Centauri. Ron and Don are each in their own spaceship drifting past each other somewhere in deep space, far away from any stars or planets. Ron sees Don move away and come back, Don sees the same for Ron. Which one ages less?
I think I understand what you are saying. Yes, Don's clock "appears" to run slower than Ron's in the same way, that Don's appears slower to Ron. If you do the whole process carefully, though, you find that after Ron changes reference frames for the return trip, he misses a big chunk of Don's clock time. It appears to him that Don's clock suddenly went forward about 4 years worth of time. I know it sounds crazy, but isn't the whole thing crazy? Either way, according to Ron, he only went 4 light months out away from Don, whether he went to Alpha Centauri or not.
@@stevenriley7055 _“If you do the whole process carefully, though, you find that after Ron changes reference frames for the return trip, he misses a big chunk of Don's clock time.”_ - fully correct. According to Ron, Don’s clock suddenly jumps forward during the reference frame change. Or, said in other words, B and C disagree over the age of A. It's too bad that Lincoln does not mention that in the video, because it is the key detail that resolves the paradox. But the main question is: “why does Ron need to change reference frames and not Don?”, that is, “what makes the situation asymmetric?” There is only one answer to that question: Ron experiences acceleration; he is not in an inertial reference frame the whole trip. It’s the acceleration that creates the asymmetry. Another key detail that is left out by Lincoln. If you analyse the original paradox (without reference frame switch), then the end result needs to be the same as the analysis with reference frame switch. Therefore, the acceleration that Ron experiences must cause a (perceived) speed-up of Don’s clock, in order to account for the jump forward of his clock. General Relativity describes that the speed-up is dependent on both that amount of acceleration and the distance between Ron and Don.
@@renedekker9806 Yeah, there are a lot of tricky details in this paradox, which to be fair to Don Lincoln, are very difficult to explain in a short video with limited math. I think I agree with you on all points, except that you don't really need to invoke General Relativity to get the right answer. You just need to invoke Special Rel including accelerated frames. It requires some calculus, but if you do that, you get an effect that is very similar to General Rel, with the speeding up of remote clocks.
Okay, you start off talking about Doctor Don being stationary, and Doctor Ron moving. But, this is a poor way to describe it. Doctor Don is stationary with respect to the earth’s frame, because Doctor Don is moving with a relative velocity of v = 0 with respect to the clock in the earth’s frame of reference. But, there is nothing special about this, because Doctor Ron is moving with a relative velocity of v = 0 with respect to the clock in the ship’s frame. So, we can literally say that each observer is equally stationary with respect to their own frame and with respect to the clock inside their own frame. As for saying Doctor Ron is moving, it is clear that he is not moving with respect to his own frame, but rather, his motion is taken to be some velocity v > 0 with respect to the earth’s frame. But, if motion is relative inside Special Relativity, and no frame is nailed down as absolutely fixed, then we must also recognize that Doctor Don, in the earth’s frame of reference, is moving with a velocity v > 0 with respect to the ship’s frame of reference. Since both frames of reference are separating away from each other at the same velocity v > 0, we say that the motion is relative motion and hence the velocity is relative velocity. Consequently, on the outbound trip, each observer applying gamma(v) to determine what is going on with time in the other observer’s frame will conclude that time is slowing down in the other observers frame with respect to the clock in the frame of the observer doing the math. And hence the paradox is preserved along every point of the outbound trip. So, when you say Doctor Don is stationary and Doctor Ron is traveling, such references are not a description of relative motion based in what the two observers are actually observing from their respective frames of reference, and hence have nothing to do with Special Relativity. This will be further confirmed by the fact that it would be wrong for Doctor Ron to plug v > 0 in for gamma(v) when calculating for what is happening to time in Doctor Don’s frame (in the earth’s frame), if you are insisting Doctor Don is stationary (moving with a velocity v = 0). Doctor Ron does not see Doctor Don as being stationary like that. Doctor Ron sees Doctor Don moving relative to Doctor Ron’s frame with a velocity v > 0, not v = 0. So, your description here blatantly deviates from what Doctor Ron is actually observing happening to Doctor Don’s frame of reference. To work the problem the way you have described it initially, we then must conclude that Doctor Ron’s observational data, even though he sees Doctor Don moving relative to Doctor Ron’s frame of reference with a velocity v > 0, not a stationary velocity of v = 0, is then an unreliable observational perspective within your description of Special Relativity here. Consequently, it then follows that the notion that velocities are relative inside Special Relativity is a flawed premise, if you can absolutely fix Doctor Don’s frame of reference to pretend there is no paradox in play here. But, I think if you go back and look at this from Doctor Ron’s frame of reference, you will see that Doctor Don is changing coordinates (moving a distance) inside Doctor Ron’s coordinate system, and that it is Doctor Ron who is fixed inside Doctor Ron’s coordinate system over the time interval traveled. Consequently, Doctor Ron is seeing Doctor Don experience a change in distance over a change in time with respect to Doctor Ron’s coordinate system which implies that Doctor Don and the earth’s frame are moving with a velocity v > 0 inside Doctor Ron’s coordinate system, not stationary. So, I really do not know how you can say the earth’s frame is stationary without admitting that you are preferring one frame of reference over the other. Now, to your credit, you do point out that trying to explain away the paradox with the excuse of acceleration to allow for the turn around trip has issues. It definitely does, because the paradox would have already been in play at every point along the outbound trip before any engagement to turn Doctor Ron’s ship around, and to use acceleration to break the symmetry of the problem would at most only clear the paradox out of situations involving General Relativity, but not ensure in any way that situations strictly involving only relative velocities in Special Relativity was free of the paradox. Next, you attempt to ensure that the paradox does not occur in Special Relativity either, but you opened yourself up to some vulnerabilities in the attempt. So, let’s look at what happened. First you tell us we are going to try a thought experiment using three observers in what you say is “constant motion” where none of the observers will experience acceleration, but then you turn right around and say observer A sits stationary on the earth. Well, this is fine, so far as observer A is in fact stationary with respect to the earth’s frame of reference, but this in no way ensures that observer A is stationary with other frames of reference moving relative to observer A’s (the earth’s) frame of reference. Yet, you arbitrarily treat the earth’s frame as fixed no matter what, and the other two frames for observers B and C as moving. However, explaining it that way opens for you an even bigger can of worms, because if observers B and C are moving relative to each other as they head towards each other with the same relative velocity v > 0, we can by no means say that either of thse observers is stationary, right? And so when they do the math, observer B will calculate observer C’s time to be, T_c = T_b*gamma(v) and observer C will calculate the time for observer B to be T_b = T_c*gamma(v), where both observers plug the same value V > 0 into the gamma function. And wouldn’t you know it, the paradox pops right into place, because each observer in this case sees the time on their own clock run normal, but calculates that the time on the other moving ship’s clock to be slowing down by comparison. So, the way you set the problem up actually preserves the paradox between observers B and C even if we let you insist that the earth’s frame where observer A is happens to be stationary. That is the danger of using three observers where two of them are moving towards each other with a relative velocity v > 0.
I find it fascinating that you're able to articulate the twin paradox so precisely yet are completely confounded by the explanation which is perfectly sound. FYI it sounds to me like you've confused the idea of velocity and acceleration and that you're trying to think through it through two reference frames simultaneously instead of focusing on one.
@@ultimateman55 Gief: @Corey Bray He didn't fail to realize. You misunderstood. Well, I am sure we will find out soon enough. So, let’s walk through your logic below and see what you conjured up for me to deal with here. Gief: They're moving towards each other, yes, Certainly! but he never says they're moving towards each other with speed v. He doesn’t have to say it, Gief! If they are moving towards each other, as you claim earlier they must be moving towards each other with some relative speed v, such that their motion resolves being to within the range 0 < v < c to guarantee that they are not violating the universal speed limit of Relativity. Gief: B moves away from EARTH with speed v and C moves towards EARTH with speed v. So, if I am guilty of an abuse of notation here, we can simply realize that since you said they are moving towards each other, their actual mutual relative speed is 2v between B and C, unless that speed 2v > c, in which case we will need a Relativistic approach to adding velocities to fix the violation of superluminal relative speed here. But, this is really just a minor point, it doesn’t change the unfortunate symmetric outcome of the problem here going on Relativistically between observers B and C. Gief: (The ships' relative velocities could be calculated easily and sum to less than c.) Sure, Special Relativity has an adding velocities work around for that even if the Euclidean sum of the linear advance of both observers towards each other is v => c between any two observers. Gief: His point is that what resolves the paradox is the Earth always remains in the same reference frame, This is because he falsely assumes that the paradox can only show up in the place he anticipated it might in his explanation. His explanation does not resolve the paradox that is occuring between B and C. We do not need the earth’s perspective anymore to consider their case, because B doing the math and C doing the math will symmetrically guarantee that each observer calculates that the other observers clock is slowing down at the same time by the same amount with respect to the clock on their own ship, and that would immediately be a direct violation of Algebraic Trichotomy through every point in time of their closing in on each other, and so the symmetry problem of the Twin Paradox just pops right out of the problem. I honestly cannot believe the author failed to see it. I mean, it is so obvious. Gief: where as the ships count as TWO reference frames. But, you must ask what is happening between these two ships relativistically. You must always be aware that with any two observers in uniform relative motion, relativistic effects can occur as predicted by the Lorentz Transformation between them. Gief: This is true whether or not you use the A B C two ship example or the single ship A B example. The acceleration of the ship in the A B example is incidental, not specifically relevant. Well, we need not talk about any acceleration, because it has no bearing on a paradox that is strictly a problem in Special Relativity which is why a breaking of symmetry in the classic explanation of resolving the paradox with a noninertial frame in the outbound twin’s motion is kind of meaningless. In any case, I applied Gamma between B and C and conversely between C and B to show the same old symmetry problem arises that has always plagued Special Relativity, only this time around the observers are heading towards each other, so there is no need for introducing a noninertial turn around excuse as in other explanations to cloud the issue. Gief: The correct question to be asking after this video is "Why does comparing the one reference frame of Earth to the two of the ship necessitate that the ship experiences less time?" No, the far more important question is what happens with the clocks on B and C’s ships when their paths intersect, compared to what is predicted by the equations of Relativity? Because each observer B and C will calculate at every point of their closing in that the other observer’s clock is slowing down with respect to their own ship’s clock, and this is where reality and Relativity are going to have a nasty collision where Relativity gets shattered into pieces. Because both clocks cannot physically be slowing down more than each other at the same time as the Lorentz Transform being employed by each observer will inevitably predict. which is probably quite difficult to understand without the math Is it?
Gief: I find it fascinating that you're able to articulate the twin paradox so precisely Well, I have spent years watching people convolute and complicate it to their own confusion, and it’s really a very simple idea ultimately when you strip away all the nonsense and take the time to grasp what the math is really saying. yet are completely confounded by the explanation which is perfectly sound. No, the explanation is not perfectly sound. The author did not guard against the paradox popping up between the inertial reference frames B and C. Gief: FYI it sounds to me like you've confused the idea of velocity and acceleration I am not sure what has given you that idea. But, given any position function y = f(x): Velocity is given by y’ = f’(x), and Acceleration is given by y’’ = f’’(x). Such that, velocity is the measure of uniform constant motion and is the subject matter of Special Relativity along a flat Menkovski metric, and acceleration is nonuniform motion which increases or decreases, and is dealt with using curved menkovski metrics in General Relativity, such as in the case of curved space/time in the case of discussions about gravity. Gief: and that you're trying to think through it through two reference frames simultaneously Sure, because the whole point of a transformation like the Lorentz Transform is to allow you to compare what is going on in another observer’s relatively moving inertial frame with respect to what is going on in your own local inertial rest frame as concerns how time and space are being measured. And, simultaneously, the other observer can do the same type of math from their frame. That’s normal to the dynamic model of Relativistic mechanics. Not sure why you would think that is any kind of real problem here, unless you are not clear why symmetry became a problem in Relativity to begin with. Gief: instead of focusing on one. Okay, you seem to be confused here. If you are the observer who applies the Lorentz Transformation, focusing on one and only one reference frame, namely your own frame, then you will get measurements, M’ = M*Gamma(0) = M. So, we learn nothing more with this approach than that to apply the Lorentz Transformation to one and only one frame, namely the observer’s own frame of reference, gives us back measurements that are consistent with the observer’s own frame of reference. Why does this occur, because you being the observer are at rest with respect to time and space inside your own local frame of reference. But, this is good to know, because it explains to us that when you apply the Lorentz Transfermation to your own frame of reference, you are getting something completely consistent with local observations. So, while this is good to know, motion in Relativity is, well, Relative, and the real point of the Lorentz Transform is to use it on another observer’s frame of reference, but when the other observer does the same, SIMULTANEOUSLY, we get a symmetric paradox where each observer sees the clock in the other observer’s frame is slowing down with respect to the clock in their own local rest frame. Various attempts have been made to resolve this paradox like having the outbound Twin turn around and break symmetry with a noninertial frame of reference, but this really does not work for the fact that it fails to account for the fact that the Paradox is preserved along every moment of the outbound leg of the trip. So, the usual explanation around the paradox is just smoke and mirrors mostly. Others try to deal with the problem by arguing that the universe splits into multiple universes which gets even more ridiculous still. Of course, this is really the least of our worries with Relativity. According to Relativity, if you are moving relative to me, space around you contracts from my perspective. So much so that there is a famous Relativity problem that says a 40-foot poll can be distance contracted down to fit inside a 20 foot barn door opening. Let’s think about the physics of this for just a moment. So, the faster your speed, the more that poll shrinks, such that if you could possibly achieve v = c, the poll would shrink down to length 0. Of course, that would be ridiculous, but long before that poll shrunk down to 0, if you went fast enough, you and the poll would shrink down below your shwartzchild radius and you would be stuck inside a black hole due to no other reason than Relativity says you were traveling at a velocity fast enough to physically shrink you down to such a size. The fact that objects in our universe moving near the speed of light do not form black holes should tell us immediately that there is something extremely funny with many of the claims being made by folks about Relativistic mechanics. At the most, any space contraction could only be an optical illusion, else lights motion alone would doom us all to living in a compact singularity as light does move at v = c with respect to all observers. Ooooops!!!
Except if you watch other experts in relativity they say that relativistic time dilation effects are from a combination of Acceleration and Velocity. So there is another paradox....both experts can't both be right.
Dr Lincoln’s muddle becomes even greater in this video - which is designed to counter the criticisms. In order to ‘prove’ that “acceleration has nothing to do with time-dilation”, Dr Lincoln now has triplets, A, B and C. Somehow, B and C weren’t accelerated in order to get them to move, according to Dr Lincoln. The particular daftness of this video is that Dr Lincoln claims that the existence of C somehow determines which object experiences time-dilation. But removing C won’t affect B’s time-rate, and the distance L is irrelevant; it doesn’t matter whether B has travelled ½ L or L, B has the time-rate that it already has. The object that experiences time-dilation in this example is the object that Dr Lincoln defined as ‘moving’. The object’s that are moving were accelerated to get them to move. (P.S. - If we let B carry on it will eventually have travel the full distance L. Let’s call the distance L a light-second (i.e. 299,792,458 meters). When B reaches L, B could send a flash of light back to A and A can write down the time showing on its clock (minus 1 second) that B reached L. Later, when the two times written down are compared, - if B ‘lost time’ compared to A, (time-dilation) - then that is because Dr Lincoln started out with the assumption that B is ‘moving’. He defined B as a ‘moving’ frame. ) The distance L is irrelevant to the calculation. All that matters is - which object is moving and its relative speed.
Perhaps a visual will help. There are 2 possible cases in the 2-body scenario if we are to presume that one of the twins is indeed stationary: Case 1: /| Case 2: |\ / | | \ / | | \ \ | | / \ | | / \| |/ Vertical lines are bystander worldlines. Zigzags are traveller worldlines. The traveller is younger in both cases, but which twin is that? SR can't tell (nor can GR) so you have to figure it out by other means. There are 2 ways I know of: 1) Ask the twins who felt the force. 2) Count the number of redshifted light rays each twin receives from the other. The bystander receives more.
minutephysics really clears this up at ruclips.net/video/0iJZ_QGMLD0/видео.html - In the classic twin paradox, during both the outbound and inbound trips, BOTH twins see the other as aging slower. However, during twin B's frame change, he see's twin A's time jump forward. (While decelerating and accelerating, he would see twin A's time speed up.) By using two travelers (B and C), Dr. Don avoids the acceleration issue, but now he is comparing apples (B) and oranges (C). He never mentions that B and C do not agree on what time A is living in. During their entire trips, C is seeing A further in the future than B is. If they exchange notes as they pass each other, B might say A was living in 2019 while C would say A is living in 2020. (And they would also disagree about each other's starting times.)
This would seem to suggest that said time dilation and the difference in ages between the 2 doctors would only occur IF Ron returns to earth? Maybe I’m misunderstanding but you seem to be saying that the act of turning around and returning to earth creates the second frame of reference that is required for this measurement. But wouldn’t that suggest that simply accelerating out in one direction at 99.99% of the speed of light would result in no age difference?
The very problem is, that two observes can only agree that two events happen at the same time if they happen at the same place, but may not agree that two distant events happen at the same time. Therefore, you need the meet-point to compare the values, because the two may not agree at what time the turn-around happen, or in other words, the one in rocket may not agree that the one on earth stopped the watch at the time he/she reached the star ;)
LocutOs the idea, and it is a crazy idea for sure, is that Ron is experiencing time as if it was the same but only when he returns to Don where time is being measured from the same viewpoint of Don do they see that their times have diverged. So imagine: Ron left earth and circled around in the universe for 5 minutes at a very high speed, just for fun, he returns home immediately. Don saw him leave in 2018, but it is now 2060 when Ron returns and Don is 42 years older, 42 years he waited for Ron to come back. Ron is 5 mins older and only experienced 5 minutes. The problem demonstrated in this paradox is that the theory of relativity says all motion is relative, there is no such thing as absolute motion, literal rest etc. This means that it is exactly correct to say that Don and the earth LEFT Ron who sat in the same place inside a stationary spaceship (THEY accelerated away from him, THEY changed frames and returned to his frame would also be correct to say if you think about it) for 5 minutes and when they reunite, Don is 5 mins older while Ron is the one who is 42 years older. Well, which is it? Who is older? Well, I have heard all the answers and no one has an answer because the theory is completely flawed and yet science refuses to let go.
Hi Dr. Don .. I am sorry this video still didn't quite satisfy everything (at least for me) .. because if you add another observer D at a distance 2L on the left of the starting position (where A and B coincided) .. moving to the right towards B with the same speed A is moving to the left .. and you did the exact same analysis while fixing the screen frame to B (the earth will be moving with A or in other words A is the earth .. you can forget about the star since it will only add confusion and it is not necessary for anything) .. then you will reach that for B the time experienced by A should be less and B should be Older. which is coming back to the essence of the Paradox. Appreciating your thoughts on that and thanks for the great Effort. PS: I was pretty satisfied with the explanation to the paradox provided by Eugene ( ruclips.net/video/bjHLboK2M1g/видео.html ) .. this is why i am particularly very interested to understand your argument that says acceleration is not the culprit. Thanks.
i mean i also feel it has to be the acceleration because you can forget about the earth also .. these are 2 points in space that separated and then combined again together .. it is a very hard symmetry which can only be broken by someone doing an action that the other doesn't do. somehow i don't feel your explanation breaks this symmetry
remember the important part - gamma. While as I move away from the earth, I can say that the earth is moving away from me from my perceptive - it is I who has gamma increased and not the earth.
BINARYGOD why is it you who has the gamma increased and not the earth? Is it because u moved? In relativity there is no absolute rest position and gamma is a property of the relative motion between 2 observers .. so both think the other has dilated time.
@Ayman Mahmoud I was also still asking the same question as you do now. I finally got it after 2 days of pondering over this problem. The solution is much easier than I thought and I understand what it means to be in one or two reference frames. I think Don's explanation added some complexity but I understand he wanted to explain the problem differently. Didn't quite help me :P
If I understand correctly, time dilation is necessary to reconcile motion through space with the fact that all matter moves through spacetime at a constant velocity: the speed of light. That reconciliation is accomplished mathematically by gamma, and it allows observers in separate frames of reference to measure both time and the speed of light *within their respective frames* the same as they always do.
Good job, most people that attempt to explain it do not remove confounding variables to show it cant be all acceleration. They say there is no paradox because one twin had experienced acceleration on the turn around journey which implies moving near the speed of light had zero impact. You can reach light speed at 1 G in less than a year (if you had the fuel) which would also keep the frames the same (earth is 1G) in terms of acceleration.
If you are actually spinning around your centre of gravity then your head and your feed will age the slowest - and whatever is in the middle will age the fastest. It's your choice how you do it of course.
I'm not a math guy, but I still found the other video more helpful. What was problematic for me then, though, is still problematic for me here. I am persuaded, and I see why, acceleration doesn't solve the paradox, and I can acknowledge that the number of frames of reference is a difference. I can concede based on your word that it is the *only* difference. What I don't know is *why* different numbers of frames of reference would make the difference. Why would Dr Ron definitely age less being in two frames of reference whereas Dr Don age more being in one. I see that Dr Ron ages less because he is the one in motion. But what is it about the multiple frames of reference that gives that motion the meaning and so has the impact on aging? If you were to draw their worldlines out on a space-time diagram could you see the connection more clearly?
ScienceNinjaDude can you point me the timestamp where the math one showed it. I just rewatched it, and it seemed what he did there was mathematically demonstrate that it's not the acceleration that causes the time dilation but rather the motion itself. I understand that. But then he just points out the only remaining difference we can appeal to is the difference in frames of reference. Then he just asserts that solves the paradox. But why does that solve it? That's a logical argument sure in the form of Either A or B, ~A, :. B, but how does showing it is NOT acceleration--which I agree with--demonstrate that it is frames of reference? That's what I'm asking for. At bottom, I don't yet see how these videos solve the paradox but rather how they disprove one proposed solution. So where does he explain how and where the number of frames of reference makes the difference? I take your word for it that he did, but I'm not seeing it yet.
I think he's trying to address my point around the nine minute mark in this video. I'll need to watch that and the associated videos a bit because I am missing something. There is onviouslyvsomje reason that B or C (or Dr Ron) cannot claim to be stationary relative to A (or Don) per the classical relativity claim. The multiple frames of reference is the purported answer. I'm not seeing how that is yet.
I think he's trying to address my point around the nine minute mark in this video. I'll need to watch that and the associated videos a bit because I am missing something. There is onviouslyvsomje reason that B or C (or Dr Ron) cannot claim to be stationary relative to A (or Don) per the classical relativity claim. The multiple frames of reference is the purported answer. I'm not seeing how that is yet.
Perhaps acceleration causes the change of reference frames. Or the choice to move through space means one moves less through time. Could it be that easy?
Can you make a different twin paradox example using A, B, and C experiencing different gravity? Maybe all three are stationary to all observers. Then a black hole moves through the single frame of reference, with A, B, and C at different distances from the black hole. The gravity of the black hole attracts the three observers A, B, and C. Normally this would make the observers closest to the black hole appear to move faster, relative to a fourth observer. But remember in this example all three appear stationary to all observers. So for example, if A is closest to the black hole, A must accelerate away from the black just enough to have no apparent motion relative to itself or any other observers. The same for B and C, although they will require less acceleration to appear stationary because they are farther away from the black hole. After the black hole passes, all observers agree to have observed no motion. However, observer A will have experienced the less time than B and C.
Suppose B "jumps" between spaceships at midpoint L. By changing time frames, a portion of A's time segment (and associated events) is completely cut off from B's point of view when B "jumps" between spaceships at midpoint. It's like B goes from A's past to A's future through changing moving directions. Even though from B's point of view, A ages slower during the two half-trips, the change of time-coordinate cuts off A's time MORE THAN compensates for the shorter time A experiences during the half trips. From B's point of view, the cut-off time has to be added back to A's age when calculating A's time duration. Thus, only as a net effect, B sees A age faster when they unite.
Wait! ... Just do I make sure I got things right, When and Where did B and C exchange information? According to what frame of reference? Because I think they will disagree with A about that, am I right? What if we did this whole calculations based on the timing of C as stationary, don't we fall again in the Paradox that A is the one that aged less???? I am so confused and I don't trust my own math, but I do logically claim that acceleration is the factor here.
You need to think about what B and C would observe from their stationary frames. In the frame where B is stationary, they see: >A receding from them at -V >The star moving toward them at +V >C moving toward them at +2V In the frame where C is stationary, they see: >A moving toward them at +V >The star moving toward them at +V >B moving toward them at +2V B and C will agree on the elapsed time at which they are both at the star. They see the relative B-star-C motion as the same, and will compute the same clock time as having elapsed when B, star, and C are all in the same place. They will disagree on where this happens, since B will say "I didn't move. Instead the star and C came to me." C will disagree, and say "I didn't move. Instead, B and the star came to me." But they won't disagree on the time elapsed (which they will each say is L/V).
Déjà vu Still the paradox is not explained. Ron see Don's clock moving slower, Don see Ron's clock moving slower, but when Ron returns from his journey obviously his clock lags behind Don's clock. Since that could not happen instantly, at what time did Ron see Don's moving faster than his?
I was able to understand the explanation better after this video combined with the one minutephysics did and a graph I found on the Internet. This is the video by minutephysics: ruclips.net/video/0iJZ_QGMLD0/видео.html And here's the one simple graph I found on the net. It shows that thing about the ship being in two reference framesfrom the perspective of observer A: physics.stackexchange.com/questions/202679/the-twin-paradox-using-reference-frame-following-the-ship? Even still - I am not really sure if I understand it correctly and I need to rewatch those videos. Maybe it will be more clear to me why does B and C see A being in one reference frame all the time.
Thanks szpinak222, but minutephysics's video is bogus to say the least. *"the important bit sine you're moving"* that we know is not a valid argument. For "me" it's him that changed speed, with the one exception, "I" felt the acceleration and "he" did not, but Dr. Don insists this is not the reason. Less I care about mathematical or even scientific proofs. Ron must see Don's clock ticking faster at some point, all I want to know is when and why.
Well if you're talking about what is "seen", as in real-time observation of light signals, then both twins observe the other's clock moving faster during the inbound leg of the journey, due to the Doppler effect.If you have one twin flying to a location 3ly away and back at 0.6c, his total journey takes 10 years from Earth's perspective, but only 8 from his. The Lorentz factor here is 1.25, so clocks at one location appear to tick at 80% the rate as observed from the other. However, the Doppler effect will actually make each side appear to be ticking by at 50% the normal rate (62.5% the rate due to Doppler, plus 80% the rate due to time dilation). For the traveler, after reaching his destination 4 years in, he will only see that 2 years have passed on Earth, since light from his idea of "now" on Earth has only just started to head his way. On Earth, they won't see the traveler reach the destination until 8 years in (5 years for him to get there + 3 years for the light to reach Earth). Because of the 50% rate, they'll see 4 years have passed on the ship at this time. As the traveler heads back, the Doppler effect works in the other direction. In this scenario, it works out that each side sees the other's clock moving twice as fast as a result of the effect (2.5 times as fast due to Doppler, but 1.25 slower due to time dilation). So, for the traveler, as he leaves he's observed 2 years pass on Earth, and during his 4 year journey back, he observes 8 years pass on Earth during this time (for a total of 10 as he arrives). From Earth, they see the traveler begin to come back after 8 years in, only 4 years having passed for them. This trip will appear to take 2 years from Earth's perspective, during which they observe another 4 years pass on the ship. Another interesting effect you'll notice is that Earth will observe the ship cover 3 light years in a period of only 2 years, seeming to travel faster than light. This is only an illusion caused by the Doppler effect. When you account for it, you're able to see that the actual speed is less than light. But, due to Doppler, anything approaching you at greater than 50% the speed of light will appear to be moving faster than light, until you account for this effect. Similarly, something moving away from you can never appear to be moving at or above 50% the speed of light, due to Doppler.
Dr Deuteron It's Don that changes direction not Ron (at least that's what Ron think,) and nothing in *our* world changes "suddenly" everything takes time and neither of them has missing time gaps.
Arkalius80 "For the traveler, after reaching his destination 4 years in"? 4 years is earth view of his clock at 3ly distance not his, and his view of earth clock is the same.
Time to simplify. First, a point. Who is younger or older or the same age is only a question about how time passes. It is related to what events are simultaneous. It is a question of time simultaneity. PART A. This is the problem....... Rocket A and Z pass each other in space. 1.To rocket A..... it is Z that is moving and so it's clocks will run slower. Z
Wes: Time to simplify. First, a point. Who is younger or older or the same age is only a question about how time passes. It is related to what events are simultaneous. It is a question of time simultaneity. PART A. This is the problem....... Rocket A and Z pass each other in space. So, before they pass each other, they will each do the math and predict that the clock in the other ship is slowing down. At the instant they pass each other, when their relative velocity v jumps to 0 for an instance, they will see the equations of Special Relativity made a bad prediction. 1.To rocket A..... it is Z that is moving and so it's clocks will run slower. Z
You are still wrong about the acceleration. What he is showing here is that even without acceleration (by using 3 frames, instead of one moving away, stopping then accelerating to return) everything still works the same. Acceleration is not the cause.
@@SolidSiren Cori: You are still wrong about the acceleration. Acceleration should never even be an issue in attempting to resolve a paradox in Special Relativity like the Twin Paradox. Acceleration is a topic in General Relativity, the Twin Paradox is a Special Relativity problem ultimately. Cori: What he is showing here is that even without acceleration (by using 3 frames, instead of one moving away, stopping then accelerating to return) everything still works the same. You are missing the meat of the paradox raised by this reworking of the problem entirely! First of all, observer A is just a distraction. The real meat this time around is what is happening between observers B and C that really deserves our attention. The distance from observer B to the distant star is l and the distance from the star to observer C is l. So, as both observers B and C close the gap of 2l towards the same distant star, they end up having a relative velocity of v > 0 between them by default of the fact that he insisted these two observers were the traveling/moving observers, ooops! So, there is no stationary observer to appeal to this time around to convolute the issue with, and no accelerated turn around trip either to serve as a distraction, just a pure twin paradox all the way as they close the 2l gap between them to that distant star. Each observer will calculate that their own clock is running normally in their own frame, because their relative velocity with respect to their own clock is v = 0. And gamma(0) = 1. Hence, we get, T_B = T_B*gamma(0), and T_c = T_C*gamma(0) Or, in other words, T_B = T_B and T_C = T_C But, when they try to calculate how time is passing in the other observer’s frame, they will get the symmetrical paradox of For a positive relative velocity 0 < v < c, between B and C, we have, T_C = T_B*gamma(v), and T_B = T_C*gamma(v) So, since gamma is less than 1 when the relative velocity v between observers is greater than 0, each observer sees that the other observer’s clock will slow down with respect to their own clock and this means algebraic trichotomy is being violated along every point of the trip to that distant star as predicted by the use of gamma in the equations; hence, the symmetric paradox is perfectly preserved here. So, as a rule of thumb, it is never a good idea to convolute the Twin Paradox by adding more frames into the soup, unless you want to run the risk of the paradox popping up in more and more places in your model. Such that, if you have n total observers distributed across n frames that are moving with positive relative velocities with respect to each other of v > 0, then the number of potential symmetric time dilation paradoxes you will be inviting into your model are the number of combinations of n choose 2, or n! / (2!)*(n-2)! So, for n = 100 observers, this would appear to potentially give rise to 99*50 = 4950 different symmetric paradoxes going on in your model. By the way, even if this guy had produced a simple case where the paradox does not show up, such an isolated example would never constitute a logically sufficient treatment of the matter to guarantee that Special Relativity is free of the Twin Paradox occuring. Such a demonstration will never be found in these lazy laymen explanations. Rather, it is far easier to produce counter examples to such a treatment as emerged between B and C, because the more frames you try to account for, the more possibilities the paradox will emerge becomes the real problem to contend with here. Plus, it is just as trivial to show that any clock in Special Relativity is slowing down with respect to itself if you use the Lorentz Transform to frame hop around a circle of frames. So, the question of how reliable clocks are in Relativity is itself a very overlooked problem. To see this, let’s imagine three observers moving away from a star. Such that the star resides at the center of an equilateral triangle, and the ships moving away from the star are oriented to head in the directions of the three verticies of this growing equilateral triangle with some relative velocity v away from the sun and a greater relative velocity V between any two observers. So, the outward path of each observer is 120-degrees away from each other observer’s path of motion in the plane they are moving within. We could have used four observers and the verticies of a square, or five and the verticies of a pentagon and so on. But, the equilateral triangle is a simple case to consider. So, A calculates that B’s clock slows down, B calculates that C’s clock slows down, and C calculates that A’s clock slows down, but that means A’s clock is slower than C’s clock is slower than B’s clock is slower than A’s clock, so A’s clock is paradoxically slower than itself in Relativity, and this is simply an unfortunate situation that Einstein never thought to plan for; yet, it pops right out of the math if you want to carry out the calculations for yourself, and building paradoxes from this point becomes a simple matter of adding more observers to layer the paradoxes upon any clock you target. Okay, I really do need to get a life, but hey, math is fun. What can I say.
It’s not acceleration, because in the thought experiment described, nothing accelerates. But one measurement involves measurement of a time interval in a single inertial frame, while the other involves summing measurements of time intervals in two different inertial frames. Different paths through space time result in different measurements of elapsed time. The measurement in a single inertial frame will result in the longest measured time.
@@zzubra Zubra: It’s not acceleration, because in the thought experiment described, nothing accelerates. But, the thought experiment does not get rid of the paradox. The paradox is clearly guaranteed between observers B and C who are heading towards each other on the way out to the distant star with a relative velocity v > 0 with respect to each other ultimately. So, each observer B and C will pop gamma(v) into their calculations and the transform will guarantee that the other person’s clock is slowing down by that same amount from each observer’s perspective relative to the clock in their own frame of reference. So, the way the problem was set up betrays the effort to resolve the paradox. Zubra: But one measurement involves measurement of a time interval in a single inertial frame, while the other involves summing measurements of time intervals in two different inertial frames. And all that is an irrelevant distraction from the real issue here. Forget a turn around involving acceleration or splitting the trip into two different inertial frames to try and avoid acceleration’s influence, just consider the fact that A is on earth and B is moving away from A with a relative velocity of v > 0. If A and B calculate the time of the clock in the other observers frame at any point t on this outbound trip, using the same velocity v > 0 in gamma(v), they will each calculate, at every point on the outbound trip, that the other observer’s clock is running slower than their own clock. At every point t on this outbound trip, algebraic trichotomy is being violated and the paradox is guaranteed. So, people who think the turn around trip or using multiple frames saves the day clearly do not grasp what is really going on with the math behind the transform itself and what the math is actually saying during the outbound trip where the real problem resides. And if you know how to set the problem up right, you can show that any clock is not properly keeping time with itself at any time t in any relativity problem, so the entire idea of Special Relativity is an unstable mess. And for those who buy into space contraction being a real phenomenon, then we need to ask what happens to a sun as you move towards it with a relative velocity of v > 0, such that v is close enough to c to get the mass of the sun in the direction of motion to flatten out below its schwartzchild radius, something Einstein clearly did not even think to anticipate. Long before you cause the mass of the sun to collapse into one or more black hole states, the star should exhibit an instability due to a breakdown in hydrostatic equilibrium as it is considerably flattening out in the direction of its motion and not maintaining a near perfect spherical shape anymore. Sense we do not observe objects moving at great speeds relative to stars producing these kinds of physical effects, it tends to suggest that space contraction is also a rather dubious idea in special Relativity as well.
Ok, here is what I understand, we know that the moving person will experience the time dilation. In this thought experiment B and C is actually moving, not A. Because A is experiencing constant velocity from B and C, where B will experience a constant velocity with A but a faster velocity with C as they were coming towards each other. Their velocity will add up. Same situation for C. So, we can correctly say that in this reference frame where A, B, C are all together, A is stationary, B and C are actually moving. And a moving one will experience the time dilation. 1 vs 1 reference frame creates the paradox, so we needed C, a third reference frame to understand. I do not know if I am right, but that is what I understood.
Here's my take at an explanation: First we must give up the assumption of a perfect observer, one that can instantly know how the clocks are ticking for both twins. How much time has elapsed for both twins is a nonsensical question when the twins haven't communicated their results to the other, only local time makes sense for each observer. So now comes the question we are trying to answer, and the explicit question in this case should be "If the twin on Earth is considered a stationary point, how does their elapsed time compare to the time of their traveling sibling, if the traveling twin sends a signal to Earth" (sorry for the possibly confusing redaction). The traveling twin could send the message to the Earth twin in a multitude of ways. They could turn back after a traveled distance and compare clocks directly once on Earth; they could synchronize their clocks with a third observer (a messenger) that travels back to Earth instead of the twin (like observer C in the video), or they could send a message at light speed, to name a few examples. When the message reaches Earth, and the clocks are compared ON EARTH, the traveling twin will be inevitably younger. That is because information does not travel instantly, it has a finite speed and a trajectory through time and space. The message itself has a frame of reference. So no matter what the traveling twin did, either turn back to Earth, send a message, etc., the Earth twin would measure their siblings elapsed time to be shorter. Every possible way of communication, every possible experiment, includes the traveling twin's clock going through 2 or more reference frames FROM THE POINT OF VIEW OF THE EARTH TWIN. Now let's change the question, which will of course change the answer. Let's say we want the traveling twin to be an inertial frame of reference, and we want to know how they would perceive their sibling's age if a message came from Earth to the rocket. Maybe the Earth twin decided to send a messenger going faster than the traveling twin to catch up, or a laser signal, doesn't matter. Now, the traveling twin sees their sibling's clock as changing its frame of reference, from the Earth to the message. So when the signal catches up, and the traveling twin compares the clocks, their Earth twin will be younger. In short, we cannot compare the twins' age at widely different points in space. And when we decide WHERE to measure and compare their ages, we are influencing the result. That is why relativity is so relative, that is why time, speed and position matter so much. If you ask a question, but don't specify one of those parameters, your question doesn't make sense anymore, because it assumes a universal frame of reference (a universal clock or grid) that we can use to instantly compare the state of both twins light-years apart.
A small add-on: reading of “Close to the Speed of Light”: Dispersing Various Twin Paradox Related Confusions" by M. Arsenijevic, where the meaning of the misleading word "relativity" is physically addressed: the word relativity is actually a pitfall and nowadays used and abused like it was for the word incompleteness of the incompleteness theorem of the mathematics. Contrary to popular belief an absolute entity still exists even in relativity ... read and discover the clue to any apparent paradox. I find that reading self-explaining, althouhg not "absolutely" easy, a bit phylosophical, at the end of the day it's crystal clear
It is indeed the acceleration. In Don's example, there is also acceleration. The worldline whos length is measured by the two clocks is indeed not straight, so has acceleration.
I love these videos. But this one, although it did better than most, still failed for me. It did show how you don't need acceleration to calculate. That was great. But for me, it failed to describe "what broke symmetry". Why? In the video, he used 3 objects, and that is his way of eliminating acceleration from the solution, as well as shows 1 vs. 2 frames of reference. But the twin paradox only has 2 objects (if you don't count the earth and background of starts, etc) So what happens if there is nothing in the universe except these two twins? There is no preferred frame of reference, acceleration is not relevant, yet 1 twin does fire his rocket and then turns around and fires it again in the reverse direction and the 2 twins come to rest once again next to each other. Who aged more? Who moved? Who was stationary? What "location" is where the time duration is experienced? The only reason you can say that there is a definite location in Dr. Don's example, is that there is all of this background mass in the same reference frame. But when there is no background mass, what is a location?
isn't each twin their own frame? and the return trip is the 3rd frame so A is the earth dude in only one frame while B the other twin is moving away which is a frame and than coming back which is another frame aka perspective difference of 1 frame and 2 frames?
I completely agree with Dr. Don - it is just about FRAMES (places)! Acceleration is irrelevant! Let us simplify -- compute only through events! Years are earth revolutions around Sun, and distances are light years! Imagine: only 10 light year away, we discover an identical planet E’ (spinning like our Earth E) orbiting around its sun S’. An Alien of exactly my age would look 10 years younger? Paradox? Imagine we both communicate with our Zero-Latency quantum entangled devices, find one another with our classical telescopes (10 year latency)! We would endlessly argue- Same D.O.B. - how can you look 10 YEAR YOUNGER THAN ME! PARADOX! EINSTEIN ( and Dr. Don) IS WRONG!! Of course, we always see past/younger images of one another even when we both soar towards each other at half the speed of light. After 10 years (exactly ten revolutions of E and E’ around S and S’)-- crossing halfway (5 light years), when we look back through our telescopes and synchronize with our planets with their their 5 year older image!!! We will shout - No! can’t believe it -- how can 10 years ABSOLUTE be 5 years RELATIVE! Even E and E’ (we saw with our own eyes) have revolved only 5 times - that is time dilation! A beautiful Paradox!! Earth feels 100% stationary while riding on it (no jolts), time feels 100% absolute until we look from another FRAME!
You are suggesting that there is no time dilation and that time flows that same for all observers and that there is only a perceived difference from viewing events at great distance. That means the final measurements will be the same. That means when all observers reunite, all their clocks will match and nobody will be older or younger. I'm fine with that, but you are contradicting this video and the idea of time dilation. Everyone is saying that the observers will reunite to find their clocks don't match and their aging has diverged. Are you saying that is false?
@@fakherhalim I couldn't make any sense out of your comment. The whole theory is crazy for this reason both distance and speed are said to be relative, no absolutes, and yet time is different based on speed, space is different based on distance etc. The only way to make this work will be to add more insanity to it, for example Ron is in a parallel universe where Don is older when they reunite, but Don is a parallel universe where Ron is older when they reunite. Because as long as motion is strictly relative, either one of them can be said to have been the one who changed frame and then returned to previous frame, or the one who experienced acceleration, or the one who moved at a greater speed. It's science fiction
From A's reference frame, B is aging slower but that also means C is aging faster. This means the original twin paradox doesn't work because Ron would have to go both directions (away and towards Don) in order to return to Earth, and so he would end up the same age as Don again, all accelerations negated.
The confusion caused by this demonstration seems to be because it's missing other elements that the Lorentz transforms bear out. Firstly that from A's perspective, the point at which B&C pass each other is a different distance away from A than it is from B&C's point of view due to length contraction, and secondly the fact that considering things from a different perspective breaks simultaneity. The example shown from A's perspective shows B leaving A and C starting it's journey towards A at the same time. However from B's perspective C actually sets off first which explains how C can seem to have slower time to B and yet have covered what B believes to be the same distance.
Good try. You added some much needed explanation of what ISN'T happening but it could use a little more on the topic of what IS happening. You dance around it a bit and refer back to the previous video which didn't give enough attention to it, either. And for the record, the "frame jumping" proponents have an obvious comeback: it's not the observer but the observations that are important and that is what is "frame jumping." Information is changing acceleration... three times, in fact. That's me playing devil's advocate and maybe taking an oblique swipe (meow) at Copenhagen believers at the same time.
"claiming the TP comes exclusively from frame-jumping as depicted, an astronaut that travels away and back to Earth in a curvilinear trajectory shouldn't age, or not?" That's _me_ playing devil's advocate
What if the universe had positive curvature and B flies away at the speed of light long enough to come back to where he started from the other direction? ;).. There's a lot of devils to advocate for here!
@@altrag You can orbitate around the sun faster than your brother so that you can outdrow him in the future laps. The same you should accelarate from him in the beginning, but If your masses where equal you could think about two satellites orbitating clockwise and anticlockwise around the earth. They should be initially toghether, than a spring has shoot them in opposite directions. What if their masses where not equal? The havier will be the slower and older?
You're still making it way too complicated. Much simpler is to explain it by the Lorenz contraction of the distance. For the non-moving observer, the rockets gets flatter when going fast. But that does not reduce the travel time for a given speed. For the moving observer, the guy in the rocket, the whole universe moves and contracts in the direction of travel. So he gets to his destination faster. No need for 2 reference frames. Or am I missing something?
ScienceNinjaDude Not at all. Both see length contraction of the other. But the guy in the rocket sees the universe contract, while for the one who stays on earth only the rocket contracts. No paradox. No dilemma.
See this makes more sense to me. When I try to understand the video I always end up thinking about the role of contraction. I think a key thing to remember is the Earth-stationary observer sees not just the rocket but also the star moving at a certain speed, and the rocket sees not just the Earth receding at -V but also the star approaching at V, which the Earthbound observer does not. Since the speed of light is constant but length is not, it seems logical that the difference measured by the observers would be there. I don't get why three observers are needed for the thought experiment.
but this is because you refer to other reference points like the universe. but if this happened in a blank space with nothing other than a planet with Don and a ship with Ron? Can you explain the answer in that scenario please? Thanks!
Great explanation, Luc. I _hope_ it's correct, because by golly I think I've got it! I'm guessing that the answer to Davemaster's question is that even if there is no planet, there's still a point in space where he turns round.
You can say that there are an acceleration, but that in this example it's infinte high (witch is nonsence of cause). But this is more a question of Simultaneity
I hardly dare to say this, but though I am dyscalculative, I do understand your explanation without maths. Your philosofical approache works quite well for me. Thank you for this way of explaining.
The following explanation should remove any ambiguousness about this problem (Only if doesn't happen to be wrong :D As I am not a physicist. But it sounds logical to me): After one of the twins starts traveling with the spacecraft, about any of the twins we can say that he is younger than the other one (he is aging more slowly than the other one), and it can be correct. It all depends on what our reference time (clock) is. If we are taking the time passing on the earth as the reference, then the twin in the spacecraft is younger. But if we take the time passing in the spacecraft as the reference, then the twin on the earth is younger. But the somehow hidden assumption in this experiment relies under that part that we assume that the twin in the spacecraft returns back to earth and then we compare them together. In fact, by this assumption, we are meaning that at the end we are taking the time of the earth as the reference time. So, in order to make everything symmetric, we can even think about the dual of this scenario and see how we will get the dual expected result: After the twin which has left the earth with space craft has traveled to the space for some distance, instead of him returning back, the other twin on the earth takes another spacecraft and catches his twin in the space. Let's assume that the speed of both spacecrafts are constant. So it means that in order for the twin who has left later to be able to catch his brother, he should have a speed faster than him. For example if the first spacecraft has the speed of V, the second one will have the speed of 2V. So, at some point the second spacecraft reaches the first spacecraft. Let's imagine that somehow at that point the twin in the second spacecraft jumps into the spacecraft of his brother. Now if they compare themselves to each other they will find that the brother who left the earth later is younger. It is because in this scenario we are taking the time in the first spacecraft as our reference.
"No math" - there were math, just not as prominent. I do not find this intuitive at all. I think I can understand it better with the equations. Please do not take it mean equations is always better. It is still hard to grasp. Honestly, would have preferred a different representation. Perhaps space-time diagrams would help (in particular if they are animated. Yes, I know that doubles the representation of time, it helps). I just re-watched the old video minutephysic did on the topic (which I can understand better now) where it uses space-time diagrams, sadly it does not present the alternative version that presents the paradox without acceleration. Moreover, due to that it failed in shaking the idea of the importance of acceleration, at least for me.
Right! Dr. Don used math in the video. His explanation for time dilation came down to a 'gamma' variance in stationary time vs moving time. But he didn't explain why gamma exists for a moving person, or why A's 'one' frame elicits a 'gamma' from B&C's 'two' frames. All I know to corroborate Dr. Don's explanation is that scientists put an atomic clock on a plane in the "Hafele-Keating experiment" and then flew the plane around the world, and upon returning the moving plane had experienced 'less time' than the stationary clock. Dang it.
_ GPS satellites are the perfect way to explain relativity, because everyone can relate to it. On Earth, the satellite's signal is BlueShifted by gravity and RedShifted by its velocity, relative to earth's core. _
Except that GPS doesn't use relativity. That's a myth. There is no need to synchronise clocks in the satellites with clocks on earth. Your iPhone or car doesn't have an atomic clock in it. Change the time on your iPhone and see if your GPS changes your location. Of course it won't. The clocks in the satellites only need to be synchronised with each other, and they are all travelling at roughly the same speeds.
@@RileyGHunter here's the actual interface specification for GPS: www.gps.gov/technical/icwg/IS-GPS-200E.pdf Just search it for "relativistic effects", and you can see the various ways that GPS is corrected for relativistic effects--both on the satellites, and in the receiver. Anyone who tells you that GPS doesn't require corrections for both special and general relativity is misleading you, or doesn't really understand how the system works.
I find it easier to view this through length contraction, in the same way we describe the muon arrival at Earth when generated from a cosmic ray. From the detectors pov time is passing slower for the muon, whereas from the pov of the muon the distance to the Earth is short. In the twin paradox, the Earth and star are at a certain distance from the Earth pov and a much shorter distance from the pov of the traveler. This applies to travel in both directions.
Hmm. I've come across this argument before but I've never been very sold on the idea that 'acceleration isn't really the explanation' for the twin paradox. After all, what is all this 'jumping between different inertial frames'? It's just how we describe a non-inertial frame (i.e. an accelerating one). What we have here is just two different pathways through spacetime, connecting a single pair of spacetime events: Event A is the spaceship leaving Earth and event B is the return of the spaceship. The earthbound observer (ignoring gravity) is in an inertial frame the whole time. That's what it means when we say we only need one frame to describe this experience. In relativity, the proper time between two events is maximised for the inertial observer (i.e. the earthbound twin). The other observer's proper time can be obtained by a path integral, adding up the invariant spacetime intervals along the path (in any frame of reference, e.g. the earthbound frame). It will always be greater because this path goes through more than one inertial frame of reference (it's not a 'geodesic' in spacetime). That's basically the same as your explanation but my point is that when you start amalgamating different inertial frames of reference, this is literally what is meant by a 'non-inertial frame', i.e. an accelerating frame of reference. Of course, the math works out, whether or not you associate the non-inertial frame with the movement of a single body or not but I would say it has no physical meaning unless you do. Well, that's philosophy I suppose but I'm sticking to the claim that 'acceleration really is the explanation for the twin paradox' because acceleration = switching between inertial frames. :) Actually, perhaps the even deeper explanation is that spacetime interval (dx^2 - c^2dt^2 in Minkowski space) is invariant, whilst proper time is simply the interval measured along a world line. Everything just follows from these two axioms.
Sorry, Don, still doesn't do it. You say that the secret is that A is in one reference frame and B and C are in two reference frames, to which I say, "So what?" You didn't go one step further and say WHY two reference frames make a difference. More importantly, though, you don't need C at all. Even without C, presumably B's clock looks to A like it's running slower. So goodbye to the two-reference-frame explanation. Plus, you totally ignored the problem you mentioned at the outset -- to B, it looks like A is moving and he is stationary, so B should see A's clock running slower. And, of course, they can't both be running slower. So, as far as I can see, we're back to square one.
Exactly, none of the explanations make sense. Scientists are smart but they are people too, dogmatic and all. Clearly this is all nonsense unless they end up in different parallel universes. In Ron's parallel universe he is younger, but in Don's Don is younger. Maybe someone will advocate that next. If B is accelerating to A, the A is accelerating to B. If B changed frames and returned to A, then A changed frames and returned to B. So unless there is absolute motion and location, which contradicts the relativity theory, then this paradox has no answer, and if it has no answer than time dilation is false
@@RME76048 see, I'm a firm believer in not that... However I'm having trouble with this one. I know it. I get it. I've done the math, but trying to explain this one without Math is hard. To the others; no really, it really does make provable logical sense. But you have to see the whole picture first, and unless you've worked at it for a while, that picture can be pretty fuzzy.
Actually he did explain this, though not totally clearly. You have to consider at what point you want to say "Alright, this is the point from which we will measure all the times and distances etc." When you remember this, and then calculate everything from one point "so either A's position or B's position or C's position," you will find that everybody agrees that A has aged more than B has. C is meant to be a second reference frame that represents B's return journey, which in reality would just be when B switched direction and comes back. Thus acceleration still plays a key role in breaking up the symmetry of the problem, but here's an even easier way: Take a spacetime diagram (space x axis, time y axis). Now draw it from the perspective of A. A's worldline (path on spacetime diagram) will just go straight up because from A's perspective, he is stationary. But B's worldline will look like some diagonal line coming out from the origin (Earth, same starting point for A and B) until at some point the line flips around and heads back towards the time axis (at some later time, obviously). Now, consider that the Lorentz transform is a specific type of rotation/stretch which allows us to correctly swap between reference frames. Now, also consider that (from A's perspective) B's worldline is some combination of two lines joined by either a sharp corner or some curved but tight corner which represents the acceleration and turning around. There is no way you can Lorentz transform B's entire worldline to make it "straight up" like A's in order to represent B's stationary frame, because during B's journey his frame CHANGES. Mathematically, and graphically, you would have to separately Lorentz transform each "straight line" making up B's worldline, which introduces multiple different line segments, which correspond to different frames of reference. By observing the spacetime diagram in this way, you will see that B reaches A as younger. It may seem symmetrical, but the Lorentz transform and this whole idea of being able to flawlessly swap between reference frames and call the situation symmetrical only works when considering inertial frames. But the B twin is not in an inertial frame. Inherently, then, A can't make any claims about also not being in an inertial frame because the situation is NOT symmetrical.
Problem (paradox)....if one of two twins in a single frame chooses to move through space (accelerate), if they should recombine into that initial frame, which would be youngest. Answer... Each and every time, the youngest will be the twin that accelerated more. Necessary and sufficient. Occam. Why necessary? Because acceleration causes changes in relative motion. The paradox starts in a frame without relative motions. Your claim acceleration is not fundamental assumes an initial state already having relative motions. This is untenable. And rather disrespectful of the Equivalence Principle.
I found both of Dr. Lincoln’s clips on this subject helpful. His thought experiment is not quite the same as the traveling twins thought experiment, but his experiment is successful in its own right in showing that acceleration is not key to resolving the twins paradox. For me, the essential point is this. The experiences of the two twins are not equivalent, because one involves motion of the twin with respect to his cosmic (space time) background, and the other does not. You can argue that, from his point of view, the spaceship twin has remained motionless while the Earth, and indeed the entire cosmos surrounding the Earth, have rushed away and back again. But in that scenario, from the point of view of the spaceship twin, the earthbound twin is moving along with his cosmic background; he is not moving WITHIN it. In contrast, from the point of view of the earthbound twin, the spaceship twin is moving within his cosmic background, with corresponding time dilation effects. That, I think, is the essential point. The seeming “paradox” arises because it is difficult for us to set up and explain the different frames of reference involved and how they relate to each other.
I understood it after your explanation here. The problem with this video's explanation, which aimed to be less math-oriented and more intuitive, is his reliance on the ambiguous word "frame"; I didn't know exactly what he meant by "frame", but it was the most elementary component of the explanation. Having watched some of his other videos helps me understand your version, which is that the "stationary" observer is stationary only within his relative location in space. Another helpful component of your explanation goes off his other videos as well: objects further than about 14 billion light years from us are moving "faster than the speed of light" relative to our location in spacetime, so why wouldn't they experience zero or negative time passing relative to our own? It's because space itself is expanding, not the objects themselves moving within their own space. When the younger twin who goes to the distant star is travelling, it is within his own space; his movement is not because space itself is expanding or moving in any way, which is what's implied if we consider the older twin and star to be the ones who are "moving".
Hi Bill, you are right. If we consider the Earth and a spaceship moving relative to each other at constant speed, there are two possible scenarios: Scenario 1 If you consider the Earth stationary, the spaceship moves from a starting point A to reach an end point B and the spaceship's clock slows down relative to the Earth's clocks. Scenario 2 If you consider the spaceship stationary, the Earth moves from the starting point A to reach an end point C (C is a point on the spaceship frame), and the Earth's clock slows down relative to the spaceship's clock. If you consider Scenario 1 (the spaceship heading towards a star), then you have to forget Scenario 2. If you consider Scenario 2 (the Earth heading towards a point on the spaceship frame), then you have to forget Scenario 1. Both scenarios are correct. (but they cannot be considered both) If you consider Scenario 1, Scenario 2 is no longer valid, and if you consider Scenario 2, Scenario 1 is no longer valid. And there is no contradiction, because the frame of the Earth and the frame of the spaceship are two different and independent frames. (the Earth and the spaceship keep moving away) Acceleration is not the key to resolving the twin paradox, even in my opinion.
There is only one set of fixed stars. Absolute speed is all that counts. Every analysis is dependent on our fixed star frame of reference. Einstein (and Mach) despised this truth and spent their lives trying to eliminate the fixed star frame of reference because it involved action at a distance that created the forces of inertia. The Special Theory of Relativity was incomplete because it failed to eliminate the fixed star reference frame. Ultimately, the problem was never solved.
The answer. When a spacecraft accelerates, it gains mass. If it accelerated to the speed of light, it would effectively become a black hole (infinite mass - which is why you can't go faster than the speed of light). The additional mass (tacked on as a result of velocity) slows the clock down. Mass creates gravity (spacetime curvature). The greater the gravity, the slower the clock ticks. The unaccelerated twin does not experience this increase in mass. Thus, their clock does not tick more slowly. So, the reason is in fact the acceleration. RE-DO THIS VIDEO.
Seems like the best explanation so far. Mass appears to be the thing that slows down time and not their speed or reference plane. Objects in a heavier gravity well will experience time slower than objects in a lighter gravity well, even when they are not moving away from each other.
Dr. Linclon already made his case there is no such thing as relative mass, and it does not increase with velocity: ruclips.net/video/LTJauaefTZM/видео.html
I think I understand this confusion about acceleration (correct me if I'm wrong). It's because someone has to accelerate to get to the other's reference frame, and the one that has to accelerate is always going to be the "younger" one. If A speeds up to catch B then A will be younger, and if B turns around and goes back to A then B will be younger. Is this correct? Of course, in all our thought experiments we want to compare so we always include an acceleration.
In the example considered you have to take into account that A, B, and C disagree on which events are simultaneous when those events are separated in space. To compare time intervals on clocks which are moving relative to each other, the spacetime events when the clocks start and when they stop must be simultaneous as seen by all. That is why in the standard twin paradox B's clock is measured as it leaves and then returns to Earth; both observers agree that those events are simulaneous. In the example, in A's coordinate system, A's clock reading 0 is simulaneous with C starting out at 2L. But in C's coordinate system C starting out at 2L is simulaneous with A's clock reading some time T' > 0. This is why C will find that A's clock showed less ellapsed time than C's when C arrives at A's position. Simple explanation of the (standard) twin paradox: In Euclidean space a straight line is the shortest distance between two points. In relativity, objects move in a 4-dimensional spacetime along a 'world line', (even if they are 'at rest', as A is). Since time is different from space, the corresponding statement is that a straight world line corresponds to the longest time, as measured by an observer moving from one spacetime event to another. In the twin paradox the Earth-bound twin moves along a straight world line while the world line of the twin on the spaceship is bent. Hence the Earth twin experiences a longer time than the spaceship twin. Another way of looking at the standard twin paradox, where B goes out, turns around and returns to A: B will see A's clock running slower than B's on the trip out and on the trip in, since in both cases B is using a different spacetime coordinate system from A to assign coordinates to spacetime events. But B's CS on the way out is also different from B's CS on the way in. During the 'acceleration period' when B is turning around, B is shifting from one CS to another. During this shift, B will see A's clock running FASTER. The net effect from B's perspective (A's clock running slower on the trip out, running faster during the shift, then running slower again on the trip in) is that A's clock will show a longer elasped time than B's. A's coordinate system, on the other hand, did not change during B's trip out and trip in. In this CS, B's clock ran slower than A's on both legs. So both A and B will agree that A's clock showed a longer elasped time than B's clock.
No, this video is way more complicated than it needed to be. Pretty much a waste of time for everyone. Watch a show called The Universe, or How The Universe Works. On the science channel, or its app.
Not better explanation than the previous one. If you want to explain your own paradox then name it appropriately like "triplet brother paradox" but not "twin paradox". You are compering time of A with time of B on its way to the star plus time of C on its way from the star. I agree that the time of A would be greater than time of B+C. But B and C are not the same entities so you can not say A is more aged than B nor A is more aged than C. And saying A is more aged than B and C together does not make any sense. You seems to pretend that B changes its identity to C in the middle of experiment but that pretending hides an intrinsic acceleration. In this case infinite acceleration because the velocity of B changes into velocity of C instantly. So it is not like you do not need any acceleration to explain twin paradox in fact in your experiment you needed an infinite acceleration although you tried hard to persuade us you did not needed any.
The time experienced for all 3 observers is the same. You left out the time that passes for B and C before and after A begins measurement. When B passes A and starts stopwatch C must also start a stopwatch since time is also passing for him. Also when C passes star, B must continue to measure time, Therefore when C reaches earth all stopwatches measure the same time.
The twin paradox still holds, you’ve simply changed the question from who’s accelerating to who’s in one or two frames Why can’t someone that first went on B then on C when the two met claim that they have only one frame and it is the earth and the rest of the universe that had 2 frames?
Fog: Paradox unresolved. 😣 Yeah, the guy failed to realize that if he says observers B and C are moving towards each other with velocities +v and -v, that we can then eliminate the earth as a stationary frame altogether, and the paradox is preserved between the remaining two ship’s frames he accidentally admits are clearly moving towards each other. Ooops!
@@coreybray9834 He didn't fail to realize. You misunderstood. They're moving towards each other, yes, but he never says they're moving towards each other with speed v. B moves away from EARTH with speed v and C moves towards EARTH with speed v. (The ships' relative velocities could be calculated easily and sum to less than c.) His point is that what resolves the paradox is the Earth always remains in the same reference frame, where as the ships count as TWO reference frames. This is true whether or not you use the A B C two ship example or the single ship A B example. The acceleration of the ship in the A B example is incidental, not specifically relevant. The correct question to be asking after this video is "Why does comparing the one reference frame of Earth to the two of the ship necessitate that the ship experiences less time?" which is probably quite difficult to understand without the math.
@@coreybray9834 Corey Bray You are misunderstanding his entire point. You can remove b or c and all the facts remain. The point of the entire video was to show that acceleration is irrelevant by giving an example that doesn't include any. The third frame was added for clarity, not necessity.
@@SolidSiren Cori: The third frame was added for clarity, But, adding that frame convolutes the issue by forcing the paradox to emerge between observers B and C. It does not matter if he can get the result he wants with respect to C and A, because he is playing around with asking you to accept that A is stationary and C is moving, so that he can suggest a difference in clock speeds on that basis when push comes to shove, but he cannot play that game with B and C, because he insists both those observers are moving towards each other, and there is no acceleration in that case either, so the symmetry problem becomes a genuine problem for him there that he overlooked when he added this third frame into the soup. So, his attempt to save Special Relativity from the Twin Paradox really did nothing but cement it in place between observers B and C here.
I have a question... Is it possible to be absolutely stationary in space? I mean like totally stationary? No rotation in this galaxy no movement through space. I guess you would need to apply a ridiculous amount in off thrust in a reallllly specific angle to counter the massive movement of us relative to the sun, our solar system relative to galaxy and galaxy relative to galaxy rotation.. I'm so confused. so many questions. Reference frames are one thing but what about ABSOLUTE stationary..... Due to our moving through space... What is our time differential as standard observers on earth in our movement compared to a genuinely stationary object????
I guess due to expansion even a stationary object has "velocity" from any given point of reference... But even with this factored in.. I still wonder what the time differential would be.. :/
ScienceNinjaDude this is what I mean. Due to expansion space is moving apart. I get this doesn't really affect us on the scale of you to me. But could an object be genuinely stationary?
+rundata The definition of an inertial reference frame is a little tricky. Most of the time you can gloss over the trickiness, but ultimately it has to be dealt with. One of the tricky bits is that IRFs must be limited in extent; you can't put the entire universe, or even a hefty chunk of it, in one IRF. That's why expansion doesn't matter. _"But could an object be genuinely stationary?"_ That was already answered: No. The phrase "genuinely stationary" is meaningless. Assuming you are not accelerating, you are at rest in your own rest frame, but there is nothing special about that frame.
The best you could do is to be stationary with respect to the CMB. It's usually filtered out of the pictures you see but it's slightly bluer in one direction and redder in the other. You'd want to head towards the redder part at about 370 km/s. There appear to be large scale flows that likely predate the last scattering so there's a decent chance if several people did that in different parts of the universe, they'd still be moving relative to each other even after subtracting out expansion. That's not to say that there couldn't possibly be an absolute zero velocity, we just don't know how to determine what it might be and GR doesn't need one. There's some math on the QM side that makes me think you might be able to follow the minimum achievable temperature but that's pure conjecture on my part.
OK... I'm going to point out some possible points of confusion. Your presentation, however, is very appreciated and well understood. First of all, special relativity as well as the Lorentz transforms were not originally based on the postulate than any observer can claim they are stationary. The assumptions are originally based on inertial frames. This applies only to non-accelerating observers, and observers far away from any mass. So a twin that departs and returns cannot claim stationarity within special relativity. Second of all, the experimental definition begins with a distance "L", and then immediately starts talking about observer B. Distance "L" is only distance "L" to the co-moving Earth-star frame and to co-moving observer A. The distance between Earth and star is not L to observers B nor C, and is even less so for the B-C frame. It might be helpful to incorporate length contraction into the discussion, but perhaps this will confuse things. Starting out with the most important 3 frames (and gammas) here might elucidate things. There is the "origin" frame of A with a gamma of 1. There is a related A-B and A-C frame with a gamma of 1/sqrt(1-v^2/c^2). There is a B-C frame with a gamma of 1/sqrt(1-4v^2/c^2). In the end, only two gammas matter for this discussion and you can disregard the B-C frame. I think it is important to note that these gammas dilate time, but also contract space. These two effects cancel out for an observer, so an observer cover more _original_ distance vs time without any limits by merely applying thrust and/or acclerating. There is an acceleration in this scenario that I'm struggling with -- the acceleration of information. Information has entropy, and even a time measurement should have some very nominal "mass". Thus the information exchanges at the clock measurement hand-offs represent some sort of acceleration. I'm not trying to debunk your argument here, I'm just trying to resolve this very subtle point. Finally, it is evident that true twins that start from the same point and experience identical R-dot-dot (radial acceleration with respect to the origin) will have identical clocks when they reunite. Twins that depart and reunite while experiencing different radial accelerations with respect to the starting point will have different clock readings. We should carefully define causality, but this is a causal relationship between acceleration and time measurement differential according to many definitions.
I don't consider myself being particularly competent in math, but I felt like I understood you less watching this video, Dr. Don. It seemed more abstract. I think I took your meaning far more easily when you broke it down mathematically.
My 55 Yr old wife claims she is 25 as she's been moving faster than me all over the house and around me. I can't disagree.
Please keep doing sessions on this topic. I don't get it yet, but I think it is a threshold concept to understanding everything else you teach. Thank you. Fabulous lessons.
An inertial frame of reference is the perspective of an observer, whose motion is such that other objects with no net forces acting on them appear to not accelerate. In other words, a perspective from which Newton's first law is valid. When someone is in a non-inertial frame of reference, fictitious forces need to be introduced to account for the accelerations of other things from their perspective.
Imagine you are in deep space and your twin goes past you in a spacecraft at some high speed. From the perspective of the spacecraft, you go past the spacecraft at the same speed. Now the only way for both of you to meet up again is if either you or your twin (or both) reverses your initial direction. In other words, relative to your original frame of reference, you need to acquire a velocity in the direction of the spacecraft (i.e accelerate till you are moving fast enough so you can catch up with the spacecraft) or else your twin has to accelerate relative to his original reference frame so he can catch up with you. If we assume that you remain in the same inertial reference frame throughout the journey of the spacecraft whereas your twin in the spacecraft is the only one who experiences acceleration at some point between his first encounter with you and his second encounter with you, then your twin will have aged much less than you. From your perspective, the spacecraft goes past you in one direction, then when the spacecraft is some distance away, it decelerates and then accelerates towards you and finally he goes past you a second time in the opposite direction. From the perspective of the spacecraft's original inertial reference frame, the spacecraft simply accelerates towards you and thus end up shifting from one inertial frame to another inertial frame in which you are moving towards the spacecraft instead of away.
From the perspective of a third observer who is stationary with respect to the spacecraft's original inertial reference frame, you go past the spacecraft first and then keep going in the same direction forever. After you have gained some distance, the stationary spacecraft starts to accelerate towards you and then the spacecraft is travelling at a constant speed, greater than your speed, and the spacecraft will overtake you eventually. There is no inertial frame in which you change direction or speed at any point. Your twin in the spacecraft is in a non-inertial reference frame during the period that he experiences acceleration and he will observe you slowing down and moving towards him. However, the spacecraft's acceleration is of a different nature to yours. You do not feel any of the effects of acceleration whereas your twin does. There is a physical phenomena associated with the spacecraft's acceleration; rocket thrusters have to fire in order for the spacecraft to accelerate towards you. When someone is in a non-inertial frame of reference, fictitious forces need to be introduced to account for the apparent accelerations of things from their perspective.
From the perspective of a fourth observer who is stationary with respect to the spacecraft's final inertial frame, both you and the spacecraft moves towards this observer at constant velocities. First, the spacecraft overtakes you since it's moving faster towards this observer. Afterwards the spacecraft decelerates to a stop. Eventually you go past the stationary spacecraft. You maintain the same velocity always.
The magnitude and direction of acceleration of any body is relative. However, the nature of the acceleration of a body that accelerates with respect to a single inertial frame is different from the nature of the acceleration of a body that only accelerates with respect to a non-inertial frame. It's the difference in the nature of the two accelerations that breaks the symmetry between the twins. Instruments carried by both can record a difference.
Associated with any pair of events which are 'causally connected' (i.e. there exists a cause and effect relationship between the pair of events), there is an inertial reference frame in which the time experienced between the pair of events is a maximum among all the reference frames in which both events happen locally and at the same location. Since you remain in the same inertial reference frame throughout the journey of the spacecraft, then you are in this inertial frame in which the time experienced between the two encounters with your twin is a maximum. Any reference frame whose state of motion deviates from yours will experience less time than you, between the two encounters. The greater the deviation from your state of motion, the greater the difference in time experienced. If your twin travels very fast and very far then he will age a lot less compared to if he travels slow and nearby (from your point of view); even if both these journeys take the same amount of time for you. The difference in time experienced is not due to acceleration per se, instead it depends on the extent to which your twin's state of motion differs from yours.
I am a PhD astrophysicist, and I have tried for 10 years as an undergrad and during my PhD to obtain an explanation for this.
Nice video (as always with Dr Lincoln), but I STILL can't see why C can't be assumed to be stationary, and that A and B (and the Earth and Sun) are moving toward C. It still looks like it can be completely symmetric. There is no reason here why we can't assume one frame for C and two frames for A and B. You start be saying we can treat A as stationary, but that is true for C too. In the scenario with C in the rest frame, the clocks on A and B would move slower (when we compare all clocks after we bring them all into contact at a later time). Unless we choose some absolute rest frame (maybe the CMB, which does kinda offer an absolute frame) I simply do not see ANY scenario that can not be flipped through symmetry in a purely special relativistic case.
@Leo Nunes The problem is that, if the twins start out in different locations and/or different velocities, they are not properly synchronized. That is, they are the furthest things from "twins" possible. Any valid explanation of the twin paradox must start with twins which, by definition, must be co-located, co-moving, and fully synchronized. Otherwise, it's blowing smoke violating initial condition assumptions.
The real explanation is that once they are properly synchronized, and one twin experiences any kind of acceleration (gravitational or otherwise), his clock rate will change. This clock drift will persist until they are co-moving, at which point a clock offset may remain but the clock drift will not. Relative velocities have nothing to do with it the fundamental cause of desynchronization of local oscillators (clocks), but if you integrate the measurements, you can, of course, mathematically extract the drift result to ascertain the effect of acceleration. This can be done purely by considering relative velocities when masses are not present, but it does not mean velocity has anything to do with the fundamental cause of proper clock drift--change in velocity does.
c can claim to be stationary but his distance measure will be different from a.
You are correct. Treating the Earth twin as the stationary observer is an arbitrary choice. It seems natural because the Earth is essentially an immovable object compared to a spaceship, even if you allow for recoil. One can also imagine that the traveller experiences forces and the bystander does not, but none of that factors into the equations. SR simply can't get started until you identify the stationary reference frame and that's what the pontificators of Twin Paradox solutions are arguing about. The Doc makes a good point about acceleration being a red herring, but his 3rd observer is just another candidate for the stationary frame, as you say. What we need is an impartial observer who is inarguably stationary with respect to the departure and arrival events. That would be the centre of mass in a closed system, but it's not clear if that's a legitimate assumption for this scenario. It ultimately boils down to a reference frame in which the net momentum is zero. SR will produce an unambiguous answer if you know the velocity of each twin from that point of view and remember to apply the velocity addition formula.
Everyone tries to explain the twin paradox by starting out with a complete and total violation of the initial assumptions. The twins must start together and be fully synchronized.
@@onehitpick9758 Well, that's an exaggeration. I'm not aware of any proposed solution where the twins don't start and end together. And togetherness implies synchronization. The Doc's 3-body solution is the exception of course, but it can be reduced to a 2-body solution if the travellers exchange identities as they pass one another. The question which eludes us is, which twin is stationary with respect to those events or, more accurately, which reference frame has zero net momentum? Earth twin is stationary if the rocket expels propellant into free space, but it's not clear if that is indeed the case. If it is, then the solution must include the age of the propellant as well as the twins. A simpler formulation is to launch the rocket with a catapult and pull it back with a bungee cord. In either case, there are 3 reference frames to consider and the velocities are functions of mass.
I think there is one thing that can help understand the paradox, that I have never seen being evoked:
If we "freeze" the world midway into the paradox, when Ron reach the star, as if he never intended to return earth, then by no mean we can distinguish the travel of Don and Ron. It is perfectly symmetrical from the start.
Thus at this moment of the paradox, what is preventing us from saying that Ron is younger than Don from the earth reference, but also the reverse, that is Don is younger that Ron from the spaceship reference?
Well actually nothing. We CAN say that. Ron can "see" (from far away) that Don is younger, and Don can "see" that Ron is younger. One have to remember that simultaneity is relative to the reference frame (look "Relativity of simultaneity" on wikipedia). So Ron and Don are really older than each other on their own reference frame.
Hope this piece of information can help anyone understand better the paradox.
Hum, this paradox is really a tough one, after all these years I am still not sure of anything :)
Ok, in that case:
If we say the simultaneity is not involved, then does that mean we cannot say "each one is older that the other one"? Or on the contrary, can we indeed say that, but the explanation is somehow not related to simultaneity?
If you tell me that we cannot say "each one is older that the other one", then, do you know where does the asymmetry come from, regarding two reference frames moving but having no acceleration?
Because for me the main point is: no asymmetry means that mathematically, no one can grow older, universally more than the other.
Actually I would very much like a video about this :) I will search about the video you wrote about
You are looking at it based on their own Reference Frames but we know the rate of time for the observer that IS moving will be less than the one who is not. The only way their two clocks would measure the same elapsed time is if they were both moving away from each other at the same velocity.
I laughed way too hard at the "pair o' docs" joke.
literally spit out my water lol
Me too - so funny
You are just saying this bcz you know he reads comments
A pair of docs!!!!!!!!!!!!!!!!!!!!!!!!!!!
A pair of docs, Coral!
Badum-tiss!
Double puns are double punny.
For subjects that are counterintuitive (such as time dilation) I consider that a top-bottom approach fits best; first, viewers must understand what is important (time frames) and then they can easily access the mathematical approach as now the reasoning for each step of the methematical approach is easier to understand.
We can say these videos shows the same topic from two different reference frames
1:16 Every single time my teacher makes a joke about physics.
This video did make it a lot clearer. The apparent paradox of the problem comes from missing the fact that the traveling twin has to change reference frames when turning around. I think driving this point home is really the crux of understanding the problem, an the first video, despite having observers B and C really didn't stop to emphasize that fact enough. For those of us who really don't understand the physics fully, it's very easy to just gloss over the importance of this subtlety. In fact, it might have been helpful to show the perceived ages each of the three observers have of each other at the beginning, when B&C cross, and at the end, when A&C cross.
Reading your comment, is the first time I have understood what he is saying. Not that I believe in time dilation, but at least I see what their explanation for the paradox is
To add to my previous comment, I have to say however that for time dilation theory to work, there has to be such as thing as absolute velocity, such as being at rest, which would contradict relativity. Because if all motion is still relative, it does not matter whether you put in acceleration or reference frames explanation, because otherwise happened is the same as saying Ron was still in his spaceship, Don moved away with the earth, therefore DON CHANGED FRAMES and then returned into the same frame as Ron... then we are back to the same place, which one is older
@@okebaram I agree.. I still don't get it
@@acasualviewer5861 the point the gent missed is that just because there's no absolute velocity doesn't mean there's no absolute acceleration. There absolutely (heh) is.
Inertial reference frames (that is, non accelerating ones. Stationary, constant velocity, free falling, etc) are indistinguishable. If you are accelerating however, there are many ways to tell.
The twin paradox is rigorously solved in this tutorial, with a realistic acceleration, from both points of view of the traveler and the stay-at-home twin: ruclips.net/video/QFWF90bch3c/видео.html
I think I'm going to slowly back away from relativity now and accept my simple understanding of it.
I keep watching these videos in an attempt to grasp time-dilation ,but I still have no clue what do ever. ^^
to make it simple, the twin paradox has only become a paradox when we only used a special relativity that works only for inertial reference frames. However at this case, since one twin made a travel back to earth, the situation involves not only inertial frame but also an accelerated frame. What happens in time at accelerated frames can be explained by General Relativity.
@@bryanfuentes1452 Yeah so that's the simple explanation, okay I'm not going to get it.
@@matthewtalbot-paine7977 special relativity works for not accelerating spaces(your room and you)
General relativity works for accelerating space(We used this here)
I am going nuts
But of course "acceleration" is just changing reference frames. Acceleration allows one object to experience 2+ reference frames. In the other case, two different objects experience the reference frames and compare with each other.
I think what really trips people up is when you have just two objects that speed away from each other and NEVER come back together. Each experiences just one reference frame. They fundamentally disagree over who experiences more time.
Thanks for expanding on the previous video! I appreciate both explanations, the mathematical and the conceptual.
This is unfortunately not correct Don. You are changing the entire scenario by inteiducing a third twin,second spaceship! Also, you are removing acceleration completely. How can you?
Not possible!
Have you read Einstein's solutiin to the Twin Paradox as mentioned in his 1918 documents?
He explicitly states that acceleration of the travelli
ng twin causes non- reciprocal time dilation which makes the travell8ng twin to be younger. Easy!
Also, you can easily deduce this from the worldlines of the stay at home twin and the travelling twin.
Whose wordline is shorter? The travellung twin's worldline. So, he remains younger.
Why are you complicating it Mr. FERMILAB? 😊
Perfect explanation. Another way I like to debunk the bogus acceleration explanation is to note that the twin at home is also experiencing 1G due to earths gravity. And 1G is plenty sufficient propulsion to zip around the galaxy and do twin paradox experiments (indeed relativity is not as much of a buzzkill for interstellar travel as most people think, but that's another topic).
With a ship capable of accelerating at 1G, you can accomplish a round trip visit to alpha centauri in about 10 years ship time, while about 15 years pass on Earth (accelerate to halfway point, decelerate, arrive and repeat to come home). The twin at home also experiences 1G in the form of gravity on the surface of earth, so both twins are experiencing the same acceleration effect (aside from the launch, of course). So it's not the acceleration.
Incidentally, the general relativity dialation effect at 1G is negligible for both twins: it comes to about 3 milliseconds per year, or 0.03 seconds for the traveller or 0.045 seconds for the twin at home, which doesn't make much of a dent in the 5 year difference. So, again, it's not acceleration!
Their acceleration is not symmetric. If the outgoing twin accelerates for 1G over 10 years he will need to undergo a massive mount of de-acceleration and re-acceleration at some point in order to come back. The Earth bound twin needs no such acceleration.
The two frame explanation is the only explanation that I have ever seen or heard that ALMOST makes sense to me. Thank you. I can not help but think that there is a closed integral hiding in there somewhere to describe the condition that the two frames sum to get back to the starting point. BUT, why not make two moving earth frames from the rocket's perspective? It seems to me that brings back the paradox.
Yes my thoughts exactly. I have spent so long trying to understand the twin paradox and once again am stumped.
The twin paradox is rigorously solved in this tutorial, with a realistic acceleration, from both points of view of the traveler and the stay-at-home twin: ruclips.net/video/QFWF90bch3c/видео.html
I like math. But I think we need logic first. Algebra is just a way to express logic and allows us to quantify. I think it has been a great idea to give two points of views of the same problem. It is only possible for great scientists like you that makes complicate things easy to understand. I can say that I can understand the solution of the twin paradox very well for the first time. Thank you very much and congratulations for this video and for the channel. Great job!!!
This is unfortunately not correct! You are changing the entire scenario by inteiducing a third twin,second spaceship! Also, you are removing acceleration completely. How can you?
Not possible!
Have you read Einstein's solutiin to the Twin Paradox as mentioned in his 1918 documents?
He explicitly states that acceleration of the travelli
ng twin causes non- reciprocal time dilation which makes the travell8ng twin to be younger. Easy!
Also, you can easily deduce this from the worldlines of the stay at home twin and the travelling twin.
Whose wordline is shorter? The travellung twin's worldline. So, he remains younger.
Why are you complicating it Mr. FERMILAB? 😊
It is unfortunate that a great channel like FermiLab would give such a highly misleading explanation (twice), and then label it the “Real” explanation. You are only talking about Special Relativity. Relative acceleration does play a role in General Relativity, where a single non-inertial reference frame is allowed for the spaceship’s entire journey. The person in the spaceship can believe that he is standing still by believing that there is a gravitational field present throughout all of space, and that gravitational time dilation is what causes him to be older than his twin.
Of course, we cannot ignore other factors, but "Special" relativity is really just one part of entire story -- unfortunately it is symmetrical. With Special Relativity, both will see other younger by exactly the same amount. Since you are my favorite Physics video producer, it is certainly no where near your level of 100% visually perfect explanation, and I need your help! Because you already made fantastic relativity videos, but can you do it with non-accelerating "Special Relativity"
Let us examine it in narrower context -- factor out time just to begin with -- compute only through events! Years are earth revolutions around Sun, and distances are light years!
Imagine: only 10 light year away, we discover an identical planet E’ (spinning like our Earth E) orbiting around its sun S’.
An Alien of exactly my age would look 10 years younger? Paradox? Imagine we both communicate with our Zero-Latency quantum entangled devices, find one another with our classical telescopes (10 year latency)!
We would endlessly argue- Same D.O.B. - how can you look 10 YEAR YOUNGER THAN ME! PARADOX! EINSTEIN ( and Dr. Don) IS WRONG!!
Of course, we always see past/younger images of one another even when we both soar towards each other at half the speed of light.
After 10 years (exactly ten revolutions of E and E’ around S and S’)-- crossing halfway (5 light years), when we look back through our telescopes and synchronize with our planets with their their 5 year older image!!!
We will shout - No! can’t believe it -- how can 10 years ABSOLUTE be 5 years RELATIVE! Even E and E’ (we saw with our own eyes) have revolved only 5 times - that is time dilation! A beautiful Paradox!!
Earth feels 100% stationary while riding on it (no jolts), time feels 100% absolute until we look from another FRAME!
Time is a Idea that man created. Such as "Cold" , a description of something that does not exist. Cold defines the lack of something, called heat. You can not fill a jar with cold. It is not something you can measure. You can only measure how much heat that object has. As with "Time", you can not measure within the universe something that isn't there. Time doesn't work with Relativity because it is not a physical constant that you can quantify throughout space. There is no way to know or estimate the true answer without knowing the construct of the entire known universe. Please stop "Educating" people on theories as if they are the truth.
I agree! Periods are just repetition of rotational position, and hence are deterministicaily measurable. Time observance is at the mercy of speed of light, and hence gets skewed at velocities that are comparable to speed of light. I propose this: factor out time just to begin with -- compute only through events! Years are earth revolutions around Sun, and distances are light years!
Imagine: only 10 light year away, we discover an identical planet E’ (spinning like our Earth E) orbiting around its sun S’.
An Alien of exactly my age would look 10 years younger? Paradox? Imagine we both communicate with our Zero-Latency quantum entangled devices, find one another with our classical telescopes (10 year latency)!
We would endlessly argue- Same D.O.B. - how can you look 10 YEAR YOUNGER THAN ME! PARADOX! EINSTEIN IS WRONG?
Of course, we always see past/younger images of one another even when we both soar towards each other at half the speed of light.
After 10 years (exactly ten revolutions of E and E’ around S and S’)-- crossing halfway (5 light years), when we look back through our telescopes and synchronize with our planets with their their 5 year older image!!!
We will shout - No! can’t believe it -- how can 10 years ABSOLUTE be 5 years RELATIVE! Even E and E’ (we saw with our own eyes) have revolved only 5 times - that is time dilation! A beautiful Paradox!!
I wish I could like this comment multiple times! It is so sad that a reputable particle physicist would slip up like Dr Lincoln has with these two videos. Perhaps it has just been too long since he was dealing with first principles? :-(
I'm baffled at this failure to explain this (twice). and like.. I'm sorry, but doesn't time dilation have to do with space time? Forget relativity for a moment. The faster you travel through space the slower time passes. Just like the closer you are to a large mass, the slower time passes. Isn't that how this works? Doesn't that make the paradox not a paradox?
Don't understand the debunking for the acceleration resolution of the paradox. I don't think that acceleration causes time dilation, it is because speed as you stated. But acceleration breaks the symmetry and therefore you can tell which one is older, the one that is not experiencing acceleration, as the original paradox point out that they meet again at the end and compare they ages. So in my opinion, acceleration still resolves the paradox (in the way it is originally presented) although do not explain the time dilation.
This video does not remove the paradox. It keeps it in place instead!
It's the reference frame which is the beef of the story. Or to be more tangible: the simultaneity lines for the traveling person is entirely different and even changes on his/her way back. That's how the traveler has two different simultaneity lines (reference frames), both differ from that of the stationary person.
The gentleman either ran out of time or he was scared too few would understand it, because without understanding how to use the Lorentz Transforms to create world lines and simultaneity lines, one is not going anywhere, so to speak.
Assume for a minute that the universe has a closed spherical topology, it is small enough to traverse in a finite amount of time, and one twin is moving close to C. That should allow the traveling twin to return to their origin without frame jumping. How would the paradox be resolved?
I was thinking curved in terms of topology, not geometry. Consider a video game like asteroids. Parallel lines don't converge or diverge, so the geometry is flat, but the topology is curved, and you come back to your starting point.
im curious as well.
In that case, I think a toroidal universe might be a better example than a spherical one, since there is no direction change.
so you are basically asking why the paradox doesn't hold up in an imaginary universe with different rules regarding space than our current universe? I would suggest that the paradox doesn't hold up in that case because it is totally invented and not real.
@@marshalldestro : You don't know that we *don't* live in a (very large) toroidal universe.
Thank you for the additional explanation regarding the 2 frames vs 1 frame. That information seemed a little rushed passed in the previous video.
Acceleration determines who changed the reference frame when there's only two objects. Another way to explain it is when two twin brother travel away from each other in empty space and come back to the same spot, in a case when they do exactly the same thing in opposite direction they have aged the same amount when reunited, but when one goes slightly faster, both have changed reference frames, both have felt acceleration, but one is younger, since he felt more acceleration?
Basically, yeah. The question of who is older is meaningless (subjective, based on who is making that determination) until they reunite.
I think it's a fair observation in the video, that it's not really about acceleration: time dilation is a phenomenon of relative velocity. But to re-establish a common time-base to measure who has experienced more passage of time, you need to re-unite the twins. You can choose any non-accelerating frame of reference to measure the velocities of the twins. From this vantage point, some accelerations will increase the apparent velocity of the twin you're observing, making their time slower relative to that vantage point, and some accelerations will decrease the apparent velocity of that twin, making their time proceed more normally, relative to that vantage point. Whatever vantage point you choose, the end result (the difference in time experienced by the two) will be the same.
I think I got it: At 5:29, when Observer B and Observer C get to the distant star, they hold up a sign that displays If A is moving away or it's getting closer respectively. If B's sign displays: "A is moving away" and C's sign displays: "A is getting closer", Since both cannot be true, B and C conclude that A is stationary.
That's why you need more than Two observers, or what is the same, Two reference frames in addition to the Stationary Reference Frame, to distinguish which observer is really the stationary. In real life we always have more than "two observers" (all stars, planets, atoms etc) and that's why there is no paradox in real life, We all can distinguish who really is moving and who is stationary.
The over simplistic case of only two observers in the universe doesn't exist in real life and and adding to that, in real life we do not have purely "Inertial Frames", we always have at least one "Non Inertial frame" therefore the symmetry is broken
C might see B as moving, but B doesn't see himself as moving, and therefore B's clock will be faster than A's. So the time C copies will be the time B sees, so faster than A's. C also sees A as moving so his clock will also be faster than A's. But if his time = t/γ.it would be slower.
My undergrad modern physics teacher explained that you can't understand an electron by thinking of a particle, wave, or both, but must intuitively understand (grok, if you will) Schrödinger's equation. You can't "see" it as something that relates to your corporeal understanding.. To truly understand Relativity (Special and General), you must see it through the math, and use that understanding. I eventually did that, and found it was the only way to truly understand imaginary numbers and how the complex number plane worked relative to impedance in my Electrical Engineering classes. I had to "see" it though the math and stop trying to force it to fit into my day to day experience.
Kudos to Dr. Lincoln for his outstanding explanation that avoids the math and brings it closer to a layman's understanding. However, to truly grok this subject you must indeed live in a world where the math is your basis for reality.
I agree that your example tells us that we don't need acceleration to calculate the time lag difference between A and B. But to acceleration or who's reference frame is stationary still does matter in this "paradox".
Similar to your strategy , I can choose B to be stationary, and let A and all the planets (Actually all the matters in the universe except B) move to the left at constant velocity. Imagine another observer C moving to the right at constant velocity, and passes A and sync his clock with A at the moment when the distant planet passes B. C can still moving towards B at constant velocity, but the all the planets have to turn around and move with C so that when C meets B, the planets return to there original positions.
Since at the end C's clock should be much older than B's clock. It means the acceleration of all the planets have a huge effect. But I'm not sure how this effect change the clocks for B and C.
I still don't get it. Isn't it equivalent to acceleration to switch between measuring a clock going +v to a clock going -v? That seems like an instantaneous jump of -2v in my mind and thus not constant velocity and thus an acceleration. The fact that the same theoretical person isn't physically holding the clock in both directions seems rather irrelevant. I mean obviously Dr.Don is way smarter than me so I'll trust that he's right, but this explanation doesn't really explain much.. it just introduces some additional variables that seems to mask the paradox more than resolve it. Even in the previous video where he did that math, he was always explaining from A's perspective (he put up the equations for B's and C's perspectives but didn't really get into why B/C think they're younger than A at the end of the trip and I'm neither smart enough nor motivated enough to figure it out for myself.. I learn my physics from soundbite RUclips videos for a reason!)
1. Who measures the clock? Everybody in their own reference - no switching, no acceleration.
2. Who compares the measured values? A, by reading the sign (which, btw, will be length-contracted).
P.S.: I believe your idea of frame-switching-acceleration comes from the fact, that two observes may not agree that two distant events happend at the same time. Observer A will agree with observer B that observer B measured the time when reaching the star, because B was at the same place at that time and every observer must agree to that. Observer B may not agree that A measured the time B reached the star, becase A and B (A = measurement, B = event) are not at the same place.
I can't help for most of this - I have a masters in physics and Dr Don still keeps correcting me on everything! But I can point out one flaw in your comment. Acceleration is calculated as Δv/t, and in your instantaneous frame-jump, that's 2v/0, or ∞. Clearly that's an impossible acceleration, so you can't use it in any calculation, and therefore has nothing to do with the physics involved.
It is true that the space traveler experiences acceleration. But acceleration itself does not make him younger than the person on Earth. The traveler becomes younger because he is in two inertial reference frames. So we cannot use time dilation formula from his perspective.
C just got some information by looking at B (get got B's clock reading). Nobody experienced acceleration.
I think that if you take acceleration out of the Lorenz transformation of the moving ship, ie. It instantly changes its reference frame, which I think would be impossible, then the time difference is simply lost to the younger twin as that information would have to be moving faster than the speed of light. Minute physics has a couple of good videos on this using Lorenz graphs instead of formulas which make it a little easier.
Hmmmm... A bit obsessed with debunking the acceleration argument.
The problematic question is: why is there no symmetry?
If we took the Ron's reference frame as stationary and tried to repeat the argument about Don changing reference frames, where does it all break down?
ScienceNinjaDude OK, let me spell it out a bit more: what if we had Ron's frame as stationary and checked clocks in his location?
i.e. what's stopping us from replacing"Ron" with "Don" in argument, except having a minus velocity?
You're saying that Ron thinks he switched from one stationary ref frame to another stationary ref frame? Doesn't sound right.
Now, ScienceNinjaDude, you're talking about an accelerating Ron (inbound and outbound), which is not within the original context of special relativity. Special relativity is very simple, and while it can be extended to analyze scenarios with acceleration by using calculus, the original postulates of the theory did not assumed no acceleration (inertial frames).
Mr Batweed B and C are perfectly symmetric with respect to the remote start in this scenario, but A and C are not.
Agreed. I still don’t get it. What would be the difference if we took B as the frame of reference? Shouldn’t A in that case age slower?
Hey Dr. Don... Both of these were awesome videos (i have been watching everything in your series... quite fascinating, thanks so much!) So your explanation here of the frame change and how it solves the so-called "paradox" makes perfect sense to me. Having said that, I have a thought experiment of my own that I'm hoping you can help me work out. This experiment has to with reconciling what I experience in a moving frame with what I observe to be happening in a stationary frame. Let me set the stage for you...
Peter, Paul, and Mary are physics enthusiasts who decide to engage in an experiment. Peter and Paul are on their home planet Earth, where Peter will leave and travel at some relativistic speed (say .75C) toward a star "Massive 1", approximately 1 light year away, make some observations, and then turn around and head back home at .75C. Mary is on a distant planet (the three enjoy communicating by ham radio). Mary's planet is situated such that she will observe Peter's journey from the side, as her planet is at a point equidistant between Earth and Massive 1 (the apex of a triangle). Peter, Paul, and Mary will, in addition to keeping time on their own clocks, observe their favorite pulsar, P, which is also equidistant from Earth, Massive 1, and Mary's planet (the apex of another triangle, which ultimately forms a pyramid). The departure date for their journey is Earth date January 1, 2020.
I'm making up the numbers because I don't know all the math and the details are not so important here as the concept. As Peter heads away from Earth, it's my understanding that he'll experience a few things:
1) The apparent distance between him and Massive 1 will appear to contract (he will be quite surprised to see that it doesn't seem to be as far away as he thought). Massive 1 will also appear heavily blue-shifted
2) His view of the Earth will appear red-shifted, and the space behind him should appear elongated, thus making it appear as though he has traveled farther than he ought to have (balancing out the contracted space he sees before him I suppose).
3) Upon arriving at Massive 1, Peter will determine, based on his own clock, that he has been traveling for less time than what Paul and Mary have measured.
4) It is further my understanding that, although he is traveling at .75C as measured by Paul and Mary, due to time dilation, Peter will experience some amount of time that is less than the expected 1.3 years that Paul and Mary would observe. That specific amount is probably not of importance here, but we'll call it six months.
Having laid that all out... Have I made any faulty assumptions so far?
Here are my questions...
1) Peter, as a consequence of time dilation, should at some point observe that time is moving faster for Paul back on Earth. Traveling at .75C, Paul will observe Peter leaving Earth on January 1, 2020, traveling 1 light-year, and thus arriving on May 1, 2021. Peter will experience less time, yet will simultaneously observe Paul being red-shifted, but at some point his clock also moving faster. When Peter arrives at Massive 1, He will observe Paul's clock to read May 1, 2020 (four months after he left), because the image is one year out of date due to the distance. As Peter experienced say six months himself, Peter sees four month's elapse on Paul's red-shifted,, time delayed image, but where does the missing time go? Is this simply made up for in the return trip, when Peter would observe Paul's (now blue-shifted) clock to be be moving faster?
2) In keeping with #1, since Peter and Paul both observe the same thing with regard to one another's clocks (each sees the other first receding and red-shifted and then coming closer, blue-shifted), aat what point do the observations vary? Obviously Peter ends up experiencing less time overall. How is the accounted for in what they observe?
3) I included the pulsar and Mary here as outside factors that are marginally affected at best by Peter's travel, because of their angular position with respect to his line of motion, and the fact that a pulsar pulses at a highly regular and predictable interval. Since it is perpendicular to Peter's trajectory, should he and the others not observe it to be pulsing at essentially the same interval? Wouldn't that cause a problem reconciling reality when he reaches Massive 1?
There's obviously an answer here... I'm just not sure where my logic is flawed. Thanks.
Makes sense. Take t_moving to be less than t_stationary due to length contraction. In earths perspective, the spaceship travels t_stationary+t_stationary, since the earth and star are at a fixed distance.
In the spaceships perspective, first the star is moving towards it, then on the return trip, the earth is moving towards it. So the spaceship experiences t_moving + t_moving.
Since t_moving
Thank you! My father is a theoretical physicist, he and a colleague (astrophysicist) explained why "Physics Girl" is wrong, she has a video making claims about acceleration that you warned us about in your videos. They said her explanation is fundamentally flawed. Of course they declined my request in posting a video refuting her video. Glad you posted this.
I don't know why you're congratulating Fermilab! The Twin Paradox, *_by definition,_* requires A & B to come back together to compare their times and get a mutual agreement of difference. In Fermilab's commentary above A and B keep moving forever apart, which means his example is *_NOT_* addressing The Twin Paradox... just the pre-amble to it.
Thanks for great explanation! But I'm still confused by few things. First of all, we are adding times in two _different_ frames of reference.. Is that even legal? The second thing I'm filling confused is the statement "all observers agree...". But B does not have any information on A's clock. Maybe, if I add some observer to carry information from A to B, it would happen that both B and A will think that the other one stayed younger. And the third thing I can't understand, is the picture from B's and C's point of view. If we apply relativistic addition of velocities, would they agree they met at L? Or, approaching A, C would tell "I met B at L/3", for example? I'll be grateful if anyone explains this to me.
Thanks for the videos. I enjoy watching them a lot, especially the ones that debunk wrong explanations. Really cool stuff. I've binge watched on a few occasions!
I just can't escape the feeling that you created the assimetry in the thought experiment. I mean you intended to exclude acceleration of "Ron" by separating him into two frames (B, and C). But you didn't do it with A, so the only reason why Don (A) is only in one frame is because you did not separate him into two frames. (I know you didnt say your thought experiment was "related" to Don and Ron, but the connection seems obious.)
I just cannot grasp what you mean by "A is only in one reference frame" and "B and C are in two". Maybe its a language barrier, but I can't wrap my head around what it means.
Also, you said that the measurements are mada at A's location. So how would the thought experiment looked like if it were measured at B"s or C's location? Would the result be the same (as Don aged more), even measured at different places? Sigh.. :/
Have you found any answers to this? Been looking for years. I haven't found one video that doesn't lead me to the conclusion that being stationary or having an absolute velocity with respect to the universe are a thing.... and I always learned they are not.
How can A be separated into two time frames?
Indeed! Dr Don uses sleight of hand in this video. As you say, *_he_* creates the asymmetry (rather than it being a physical asymmetry in his experiment) by _choosing_ to look at a particular passing observer C and ignoring all others (such as a symmetrical observer D, who happens to be passing A the way C is passing B).
So long as A and B continue moving apart at a constant speed there is no symmetry break.
The Twin Paradox _requires_ B and/or A to accelerate to break the symmetry and cause a mutually agreeable difference in age.
@@UncertaintyPopsicle , you are sort of right in a way: _that being stationary or having an absolute velocity with respect to the universe are a thing_
Consider it in these few steps...
*(1) **_that being stationary with respect to the universe..._*
Observation 1
If you look around yourself you will measure that light travels at the same speed _c_ (300,000 km/sec) in all directions.
Conclusion 1
You are obviously stationary with respect to space (whatever that is).
*(2) **_having an absolute velocity with respect to the universe..._*
Observation 2
You observe that all moving objects (all the way down to atomic level) become shortened in their direction of movement and have a similar slowing down of their time processes. The closer to the speed _c_ they get, the flatter and slower they get.
Conclusion 2
Not only are you, who are neither flattened nor slowed, clearly stationary with respect to space, but anything that moves is affected by moving through space and so clearly has an absolute velocity with respect to it.
*(3) **_a weird fun house mirror_*
Observation 3
If an observer (eg, something capable of observing and measuring its surroundings, such as a human or a space probe) is moving then it will become distorted (squished, slowed, etc).
A severe distortion (such as hitting a brick wall at 200 km/hour) can total wreck an observer's view point, but the distortion we've observed above is nice and uniform so that a moving person will still be able to see, and think, and measure.
Conclusion 3
Because they are squished, time-slowed, and moving with respect to space through which light moves at a constant speed, a moving person will observe the universe in a *_different_* way than we do. All of their observations and measurements will be *_distorted._*
*(4) **_through another person's eyes..._*
Observation 4
Since we know the formula for how a moving person is distorted, we can work out how the world looks to them.
(a) How does time look to them? We can see how their slowed watches act.
(b) How long are things (such as the length of their spaceship)? We can see how their shrunken measuring sticks measure.
(c) How fast do they measure the speed of light? We can see how their slowed clocks, shrunken measuring sticks, and motion relative to space alters their perception of the speed of light.
Conclusion 4
They think they are stationary and normal!!!
(a) They think their time is normal! That's no surprise since they are using the slowed clocks they are carrying with them to measure their slowed selves.
(b) They think their length is normal! Again no surprise... they use the shrunken measuring sticks they are carrying with them to measure their shrunken selves.
(c) They think the speed of light is the same in all directions and is 300,000 km/sec! _THIS_ is not at all obvious! This isn't "light that they carry with them"... light moves at the same speed relative to space regardless of the speed of the person that is emitting and/or measuring it.
So why do we think they see it as uniform? We set up an experiment to measure the speed of light ourselves, and then we calculate the speed that a distorted moving observer would measure using their distorted moving device.
The standard device is to have a long stick (of known length) with a mirror on the end. Light is shone at the mirror and a clock is used to measure the time it takes to do the round trip. Round trip distance divided by the round trip time is the speed of light.
A moving observer has a shortened stick, and a slowed clock. But they are also moving with respect to space. In one direction the light will take extra time because it is chasing a stick end that is moving away from it, while in the other direction it will takes less time as it is chasing a stick end that is moving towards it. When you work out that time, the shrunken stick and the slowed clock you get... 300,000 km/sec in all directions! Weird! :-)
*(5) **_looking at them looking at us..._*
Observation 5
We now have measured that the moving observer views themselves, through their _speed distorted eyes_ and their _speed distorted measuring devices,_ as being normal in size and time flow, and as being stationary in space with respect to the speed of light.
But how do they see us??? We know *_we_* are actually the stationary and normal ones... but we know they see things distorted. So what weird distorted view do they have of us???
Conclusion 5
When we do the measurements we conclude that they think we are: moving at the speed that they actually are but in the opposite direction and that we are shrunken in that direction of travel and time slowed!
*(6) **_spOOky..._*
Observation 6
But... hang on. We know that WE are stationary and that THEY are moving and hence distorted. BUT... if that distortion means that their distorted measurements and observations means they see themselves as stationary and US as moving and distorted, perhaps WE are the distorted ones? =:-O
Or a third observer moving at a different speed might be the REAL stationary person. Or a fourth? Or a fifth? ARRGGGHHH!
Conclusion 6
How CAN we tell which one of us is the one that is REALLY stationary with respect to real space (which is the real Sparticus?)?? The answer is... we can't. :-(
Aside from light moving at a fixed speed through it, and objects moving through it being distorted, we don't have any OTHER property of SPACE that we can measure. And because that distortion just happens to make every constant speed observer see themselves as stationary, we have no absolute way of measuring which observer is the real stationary one.
So the answer is: any of them, or none of them, or all of them.
Some people will say that Einstein showed that there was no Aether (ie, absolute space through which light travelled). But others will tell you that Special Relativity does not preclude an Aether... it just makes it irrelevant since you can never actually measure it in an absolute way.
Footnote: If sometime in the future space DID show an additional absolute property that could be measured, then that would change things. But at this stage nobody really expects that to happen.
@@corwin-7365 So you explain the paradox perfectly. But what is the resolution? We know that the twin the flew away and came back ends up younger w/ respect to the twin on earth.
I believe there is significance to the fact that an observer who remains in an inertial frame experiences the change of inertial reference frames of the twin with a delay, whereas the twin who changes inertial reference frames can immediately observe the change. Here, observing the change refers to watching how fast a clock runs for the other twin, say with a telescope (which is equivalent to observing with what frequency a periodic signal of constant frequency arrives from the other twin).
I'm the 3rd observer watching don and Ron. To me, their age doesn't change.
Thank you for your videos and explanations. Loved the pair of docs pun.
As a physician my understanding of this level of physics is somewhat limited. But I do have my own biological take on this.
The biochemical reactions inside the human body, indeed inside any organic life form, also take place at a quantum level. Electrons and protons jumping around from compound to compound and stuff. This jumping around of particles takes place with a certain speed and is facilitated by enzymes. But these particles still have to "get caught" in the enzymes in order to partake in the reactions. Enter travelling twin at the speed of light. Assuming that the particles have their own inertia, they have to overcome this speed deficit in order to be able to partake in the biochemical reactions. This slows the reactions down and as a macroscopic effect, the biological clock of the travelling twin slows down. While the distances to be travelled in the particular case of biochemical reactions inside an organic cell are minute, over time this adds up (especially on a roundtrip to a distant star).
So please Dr. Don take a look at these questions.
Does this make any kind of sense to you as a physicist? Are any of my assumptions wrong on a theoretical level? Does this explanation also rely on acceleration to work (and is therefore wrong)? I would very much appreciate an answer.
Hello Breabanm, six years have passed: did you get your answer? In all these 'explanations', like in this video, there is way too much emphasis on the math, acceleration, time slowing down etc., and with it comes a lot of confusion that is hardly ever addressed. You are absolutely right: this whole thing with relativity and time dilation is about nothing else but the relative rate of aging of matter at different speeds (i.e. elementary particles having to bridge more space to interact). After all, at the bottom line and away from the hocus-pocus, it is/must be a tangible process! Also, it is not about tíme slowing down. Time slowing down does not make any sense. It's meaningless! Time, if it exists at all, is the temporal 'space' that elementary particles need to interact, or: time is a function of particles interacting. For me, and probably for you too, this is the fascinating part of it all, and suddenly everything starts to make sense! Yet, it is as if this real explanation is anxiously avoided by knowledgeable people like dr. Lincoln. As for 'time' and the relativity of simultaneity, Lawrence Krauss told me the following a few years ago (I am paraphrasing): the relativity of simultaneity simply states that we can never know, in any frame of reference, the true, absolute sequence of events that underpin the universal 'flow of time'. Still, this absolute sequence of events (that happen only once, and only at one moment and place in the cosmos) is real (in the Minkowski diagram these are all events going up relative to horizontal lines). The benchmark for the absolute sequence of events is the gravitationally unbound space in the universe. An imaginary object in that empty space is said to be truly at rest with the expansion of the universe (in the Minkowsky diagram this would be the vertical time line going up). Only this gravitationally unbound space 'knows' the true sequence of events (ánd the true speeds of A, B and C in this video!). Gravity and different speeds in different reference frames make you inevitably loose track of this true sequence. But still, the cosmos is 13.8 billion years old...everywhere!
The 2 observer example,as he said Don(earth) was in 1 reference frame, Ron(spaceship) was in 2 reference frames.
I don't understand, if you look from Ron's(spaceship) point of view, Don(earth) is in 2 frames of reference and Ron(spaceship) is in 1.
3 observers example is more complex, so I will just have to leave it for some other time.
A only uses his one watch (his own) to measure the time, while the other duration is measured using two different watches (B's and C's). That's the one vs two reference frame.
Thanks for the better explanation of frame change it helps a lot.
I completely agree with Dr. Don - it is just about FRAMES (places)! Acceleration is irrelevant!
Let us simplify -- compute only through events! Years are earth revolutions around Sun, and distances are light years!
Imagine: only 10 light year away, we discover an identical planet E’ (spinning like our Earth E) orbiting around its sun S’.
An Alien of exactly my age would look 10 years younger? Paradox? Imagine we both communicate with our Zero-Latency quantum entangled devices, find one another with our classical telescopes (10 year latency)!
We would endlessly argue- Same D.O.B. - how can you look 10 YEAR YOUNGER THAN ME! PARADOX! EINSTEIN ( and Dr. Don) IS WRONG!!
Of course, we always see past/younger images of one another even when we both soar towards each other at half the speed of light.
After 10 years (exactly ten revolutions of E and E’ around S and S’)-- crossing halfway (5 light years), when we look back through our telescopes and synchronize with our planets with their their 5 year older image!!!
We will shout - No! can’t believe it -- how can 10 years ABSOLUTE be 5 years RELATIVE! Even E and E’ (we saw with our own eyes) have revolved only 5 times - that is time dilation! A beautiful Paradox!!
Earth feels 100% stationary while riding on it (no jolts), time feels 100% absolute until we look from another FRAME!
Yep, once he mentioned that acceleration = frame switching, then I got what he was trying to say. He's still not being entirely accurate in saying "acceleration isn't the answer" though. It's better to say "acceleration and frame switching are the same thing."
One big thing that still confuses me though, from the perspective of the twin on the spaceship, doesn't he see the twin on earth accelerate away from him, stop, and then accelerate back towards him? I still don't quite grasp why these two perspectives aren't equivalent...
I don't think 2 frame or acceleration can explain twin paradox. The very moment B get out of earth, his time has already been slower than A. No need acceleration or 2 frame for that to happen.
I think many worlds theory can explain twin paradox well. Imagine A & B are near a point in space, they then travel far away each other near the speed of light; after 100 years, they travel back to the original point and meat each other. According to the relative principle, both A and B can assume they are stationary so that the remain fellow is younger. It is still valid no matter you apply '2 frame' or not.
So, only many worlds theory can explain that since A and B can not be younger at the same time. (It is logically wrong; If A is younger, B has to be older and vice versa).
Many worlds theory can dictate that: there is one world in which A is younger, and another world in which B is younger.
@@mattkilgore7323 , you are correctly identifying the critical difference between what is considered "stationary" and what is considered "moving".
These videos do not provide that explanation. Adding kinetic energy to a mass is the fundamental difference in explaining the twins-paradox.
In essence, adding kinetic energy to a mass makes the reference frames non-arbitrary. This video does a terrible job and just confuses people IMO.
In the A, B, C example, B never needs to return to A. All that is required is that B's clock time is transferred to C and when C gets even with A that A and C compare clock times. The clock time that was carried forward from B will show less time having passed than A's clock time. No twins required. Not even humans required. Not even any clocks required! Only physical processes occurring in time.
I think the proper take on this, this lack of symmetry, is that the distance from A to the "goal" (space station or star) is different for the A, B, and C observers.
Since A and the goal are traveling at the same speed (more or less) there is no Lorentz contraction involved. A sees the "proper" length.
But since B and C are moving with respect to A and the goal they see the distance between A and the goal as shortened. A, B, and C all agree on the speed that B and C are moving with respect to A and the goal. This must mean that B and C will take less time to travel the shorter distance than A experiences back on Earth. It's that simple.
Acceleration is not needed to solve this paradox (i.e. the A,B,C example in video), but the paradox scenario (A & B only) requires acceleration to put B in two different reference frames. It is how you determine 'who is moving', and how to compare information (i.e. event 1 before A and B separate and event 2 when they meet again to compare clocks). Got it.
This is from the one perspective the one on the ground
What about the observation from the perspective of one of the vehicles?
Do they see A experience more or less time?
Fuseteam this is my problem with this paradox. I’ve NEVER see anyone address this even though it completely breaks their theory. If we change the observation to the one on the rocket it would seem like the one on Earth is younger
@@Maribro4 i'm starting think the takeaway is that *special* relativity tell us that both will _expect_ the other to be younger based on their observations, while the one earth _will_ be younger according to *general* relativity
at least according to encyclopedia britanica's website
@aDBo'Ch 1 thinking about it all he's saying is that a change of reference frame does not require acceleration
@aDBo'Ch 1 yeah true i for one couldn't completely get his point untill now
@@Maribro4 My basic understanding is this: The moving observer sees the effects of something called Relativity of Simultaneity (together with length contraction and time dilation), which basically means that events that are simultaneous in the earth-bound reference frame will not appear simultaneous to the moving observer.
As an example, imagine that before rocket man sets off, the earth-bound observer arranges for a clock to be placed at the half-way point of the journey, and he synchronises the clocks (they tick at a simultaneous rate, in other words). According to RoS, as soon as the rocket starts moving at a constant velocity relative to the earth-bound reference frame, the clock at the half-way point becomes out of sync with the earth clock (from rocket man's perspective) and jumps forward by some increment of time that's dependant partly on the distance between the two events (the events being the clocks striking a certain time in this case, but it could be anything, such as two lightning strikes).
Crucially, when the moving observer changes reference frames at the halfway point, this jump in time occurs once more, but this time on earth (remember, I'm talking about this happening from rocket man's perspective. At no point does time become out of sync in the earth/outside environment frame). Now it turns out that this clock jumping, time dilation and length contration makes it so that when the observer arrives back at earth (or even just flys past), he will see the earth clock just as the earth-bound observer sees it.
This is why all these videos explain that the changing reference frame is the important aspect of this thought experiment, and why acceleration, although not insignificant, is not key to understanding why they both end up agreeing who is younger.
I've seen both of Don's twin paradox videos and I think I like the one with the math better. The math really isn't that difficult, just grab a pencil and paper, pause the video as you work through the algebra. Math-again-beautifully and simply explains this phenomenon.
But the math itself is acting as an abstraction for the explanation. which is just lazy.
There's a Motherboard (I think) series where someone is asked to explain a concept at 4 different levels of understanding. Maths serves as one one the levels, but so does thought experiments, analogy, etc.
Imagine going to a doctor and instead of explaining your condition, they simply showed the labs results and sent you on your way. I mean, all the same information is there.
Many scientist come off the same.
When in doubt, do it in more than way.
The math helps but only if you first understand the explanation. At least from my frame of reference.
+quintessenceSL _"But the math itself is acting as an abstraction for the explanation. which is just lazy."_
You have that backwards: the verbal "explanation" is just an approximation of the math.
_"Imagine going to a doctor and instead of explaining your condition, they simply showed the labs results and sent you on your way."_
If you had a medical degree or equivalent knowledge, the results would be all you would need (unless the particular tests required specialized knowledge to interpret). Likewise with SR: anyone with a high-school education should be able to follow the math. What most people don't have is the knowledge of how to apply the basic math they (should) know to the problem at hand. The way to gain that knowledge, and to really understand SR, is not to ignore the math, but to internalize it.
+szpinak222 _"The math helps but only if you first understand the explanation."_
But without the math, you can't truly understand the "explanation".
Michael Sommers What do you mean? To explain say, why things with mass cannot move the speed of light, you COULD just use lorentz transformations and call it a day. That doesn’t do much to understand the logic itself. I’d say it’d be better to understand the logic first, then the math-the easier stuff-afterward.
Dr D i know you're going to read comments to see how this explanation went, I was one of the critics of the previous video. Thank you for another bite of the cherry.
I understand everything you say up until 7:50 the bit where you say "A is in one reference frame and B and C are in two".
What does this mean? What's a reference frame? You confuse me by then going on to describe other people's examples and not immediately explaining this point.
9:36 seems to be the critical nexus of this video and it's glossed over for observer D stuff.
I'm still lost 😔
ScienceNinjaDude why didn't we look at A+B, or A+C?
There's something fundamental in the video's intent I'm failing to grasp.
I know what a reference frame is, long story short it effectively means how everything else looks to that observer, and for me it still doesn't explain it. The explanation seems to be that A is stationary while the other 2 are not. But what is stationary? Does stationary exist? In true relativity, everything is only relative to everything else. If one thing is capable of being stationary, then that suggests there is some kind of an additional field determining what is stationary and what isn't.
I've been trying to find an answer to this for years. Have you found any explanations? Every time I post it on r/askscience it gets deleted by mods. But I still see no way to resolve the paradox without accepting that "stationary" or "absolute velocity" are a thing.
If you still have any interest on it at all, i can give you a not so brief explanation (which may not be very satisfactory, but it is in fact the answer). When you work with special relativity, you use lorentz transformations. Those transformations applies only to inertial reference frames (those that are not accelerated) and you can only talk about time dilation and stuff if you stick to one inertial reference frame until you are done, you cant use one in a time and some other reference frame in another time and try to compare anything, it wont work and it doesnt even make any sense. So let's go:
When you start this problem, you have two inertial reference frames, earth's reference frame and rocket's reference frame. During the trip, everyone's point of view is right: you are right when you are in the rocket and you say that earth's observer is moving and therefore has a clock slower than yours on the rocket and earth's observer is right when he says you (in the rocket) are moving and therefore your clock is slower than his clock. There is nothing wrong with that, because you guys are in different places calculating clock's rate of the other in different times, this is special relativity's first postulate, no one is wrong here. However, you want to back to earth and you must decelerate to do it. Yes, you can do the same thing as Don did (using a frame on the rocket to go away from earth and another frame on another rocket to back to earth), but in the end the result is the same as if you were decelerating back to earth in the same rocket. By 'same result', i mean that if you have two situations:
(A): where you use one rocket and deceleration's reasoning, the interval measured by your clock on your deceleration will have a impact such that it will increase earth's time the right amount so that as you are seeing earth's clock slower than yours, earth's time will still be higher when you reach it, because in your deceleration period, you increased the amount necessary for it.
Now, if you use
(B): Don's situation, if you could 'jump' to this other rocket coming back to earth (therefore being not decelerated), then the moment that you jumped, earth's observer clock time would instantly go to a higher value than it had just before you jumped to this other rocket, so that even if you try to use the frame in that rocket, you could still apply the same reasoning as before and say earth is moving back to you, so earth's observer clock time is slower than yours (since there is a 'jump' of earth's time when you go to the other rocket, the things themselves compensates and earth's time will have a higher value than your clock when you go back to earth, just like the deceleration situation).
If you accept that, you can continue on my reasoning. So when Don says the problem with this 'paradox' is because you are using two different frames, he is talking of course about situation B, since you are using a frame to go away from earth and another frame to go back to earth (two different rockets, therefore two different frames). Now, remember what i said at the beginning about lorentz transformations: "those transformations applies only to inertial reference frames (those that are not accelerated) and you can only talk about time dilation and stuff if you stick to one inertial reference frame until you are done, you cant use one in a time and some other reference frame in another time and try to compare anything". What i mean by this is that once you pick up one reference frame and you start doing all this stuff, you cant simply change to another frame and get any conclusions about the time on the former's frame. For example, if you get three reference frames X, Y and Z, use Y to compare with X for some time and then Z to compare with X and try to conclude anything about X's and Y's times, you are doing it wrong, because you changed from Y to Z. In twin's paradox, you are doing just that: using X (Earth) and Y (Rocket going away from Earth) and then you suddenly change Y for Z (Rocket coming back to Earth), that's why it doenst make any sense, you changed one reference frame. However, when you do it on earth's X frame, everything is fine, because you didn't change X, just Y and Z, which works just fine. There is some other way where you can indeed use a frame that coincides with the outgoing rocket as your reference frame, but as always, you must keep using this frame after your rocket starts to go back to earth (or if you just jumps to another rocket, it doenst matter): it means that in this frame, the outgoing rocket is at rest, but the incoming rocket is with a velocity -2v (since from earth's perspective, both outgoing rocket and incoming rocket are with the same speed v, but with opposite directions) and when you go from the outgoing rocket to the incoming rocket, you will still be using the frame i talked before, which will notice your 'new' clock's rate running even slower than earth's, so that after all, there is some absolute compensation to the earth's clock and it will be marking a higher time when you reach earth.
@@alkaholic4848 A stationary frame is a frame that isn't moving. That's it. The 'A' frame is not moving, so it's stationary.
I studied a bit of this in meteorology school, and somehow understood it then. That goes along with all the other math required. One of my mentors/professors pointed out that if you do not think about and/or practice these experiments/equations on a nearly daily basis, the thought patterns fade. It's true, at least in my case. Meteorologists rely on computer models, observations and some intuition to forecast weather now. We rarely do any kind of equations. That's why I enjoy this channel so much. Occasionally, I have to shake up my brain like a snow globe and get out of a mental rut. Thanks, Doc!
The information (passage of time synchronized via a timestamp) is being accelerated exactly when switching reference frames from B to C. Acceleration here does not require that a physical body is being accelerated.
I'm late to the party, but another way to think about it is that in Ron's frames (both of them), he can consider himself stationary, but he will see the distance to Alpha Centauri as much shorter than Don sees it because of the relativistic length contraction. Even though he can consider Alpha Centauri coming to him at near the speed of light, he only sees it as 4 light months away, rather than 4 light years away. In Don's frame, he always sees Alpha Centauri as 4 light years away.
From Ron's point of view, Don is still moving away and then coming back. That is a shorter distance than the "real" distance between Alpha Centauri and Earth, but Don is still moving. Because Don is moving according to Ron, Don's clocks should run slower, both on the outbound and the return trips. That is, length contraction does not resolve the paradox.
If you think that some factor is important for the solution of a problem, then it is always good to consider how removing that factor affects the problem. Suppose there is no Earth and Alpha Centauri. Ron and Don are each in their own spaceship drifting past each other somewhere in deep space, far away from any stars or planets. Ron sees Don move away and come back, Don sees the same for Ron. Which one ages less?
I think I understand what you are saying. Yes, Don's clock "appears" to run slower than Ron's in the same way, that Don's appears slower to Ron. If you do the whole process carefully, though, you find that after Ron changes reference frames for the return trip, he misses a big chunk of Don's clock time. It appears to him that Don's clock suddenly went forward about 4 years worth of time. I know it sounds crazy, but isn't the whole thing crazy? Either way, according to Ron, he only went 4 light months out away from Don, whether he went to Alpha Centauri or not.
@@stevenriley7055 _“If you do the whole process carefully, though, you find that after Ron changes reference frames for the return trip, he misses a big chunk of Don's clock time.”_ - fully correct. According to Ron, Don’s clock suddenly jumps forward during the reference frame change. Or, said in other words, B and C disagree over the age of A. It's too bad that Lincoln does not mention that in the video, because it is the key detail that resolves the paradox.
But the main question is: “why does Ron need to change reference frames and not Don?”, that is, “what makes the situation asymmetric?” There is only one answer to that question: Ron experiences acceleration; he is not in an inertial reference frame the whole trip. It’s the acceleration that creates the asymmetry. Another key detail that is left out by Lincoln.
If you analyse the original paradox (without reference frame switch), then the end result needs to be the same as the analysis with reference frame switch. Therefore, the acceleration that Ron experiences must cause a (perceived) speed-up of Don’s clock, in order to account for the jump forward of his clock. General Relativity describes that the speed-up is dependent on both that amount of acceleration and the distance between Ron and Don.
@@renedekker9806 Yeah, there are a lot of tricky details in this paradox, which to be fair to Don Lincoln, are very difficult to explain in a short video with limited math. I think I agree with you on all points, except that you don't really need to invoke General Relativity to get the right answer. You just need to invoke Special Rel including accelerated frames. It requires some calculus, but if you do that, you get an effect that is very similar to General Rel, with the speeding up of remote clocks.
Okay, you start off talking about Doctor Don being stationary, and Doctor Ron moving. But, this is a poor way to describe it. Doctor Don is stationary with respect to the earth’s frame, because Doctor Don is moving with a relative velocity of v = 0 with respect to the clock in the earth’s frame of reference. But, there is nothing special about this, because Doctor Ron is moving with a relative velocity of v = 0 with respect to the clock in the ship’s frame. So, we can literally say that each observer is equally stationary with respect to their own frame and with respect to the clock inside their own frame.
As for saying Doctor Ron is moving, it is clear that he is not moving with respect to his own frame, but rather, his motion is taken to be some velocity v > 0 with respect to the earth’s frame. But, if motion is relative inside Special Relativity, and no frame is nailed down as absolutely fixed, then we must also recognize that Doctor Don, in the earth’s frame of reference, is moving with a velocity v > 0 with respect to the ship’s frame of reference. Since both frames of reference are separating away from each other at the same velocity v > 0, we say that the motion is relative motion and hence the velocity is relative velocity. Consequently, on the outbound trip, each observer applying gamma(v) to determine what is going on with time in the other observer’s frame will conclude that time is slowing down in the other observers frame with respect to the clock in the frame of the observer doing the math. And hence the paradox is preserved along every point of the outbound trip.
So, when you say Doctor Don is stationary and Doctor Ron is traveling, such references are not a description of relative motion based in what the two observers are actually observing from their respective frames of reference, and hence have nothing to do with Special Relativity. This will be further confirmed by the fact that it would be wrong for Doctor Ron to plug v > 0 in for gamma(v) when calculating for what is happening to time in Doctor Don’s frame (in the earth’s frame), if you are insisting Doctor Don is stationary (moving with a velocity v = 0). Doctor Ron does not see Doctor Don as being stationary like that. Doctor Ron sees Doctor Don moving relative to Doctor Ron’s frame with a velocity v > 0, not v = 0. So, your description here blatantly deviates from what Doctor Ron is actually observing happening to Doctor Don’s frame of reference. To work the problem the way you have described it initially, we then must conclude that Doctor Ron’s observational data, even though he sees Doctor Don moving relative to Doctor Ron’s frame of reference with a velocity v > 0, not a stationary velocity of v = 0, is then an unreliable observational perspective within your description of Special Relativity here. Consequently, it then follows that the notion that velocities are relative inside Special Relativity is a flawed premise, if you can absolutely fix Doctor Don’s frame of reference to pretend there is no paradox in play here. But, I think if you go back and look at this from Doctor Ron’s frame of reference, you will see that Doctor Don is changing coordinates (moving a distance) inside Doctor Ron’s coordinate system, and that it is Doctor Ron who is fixed inside Doctor Ron’s coordinate system over the time interval traveled. Consequently, Doctor Ron is seeing Doctor Don experience a change in distance over a change in time with respect to Doctor Ron’s coordinate system which implies that Doctor Don and the earth’s frame are moving with a velocity v > 0 inside Doctor Ron’s coordinate system, not stationary. So, I really do not know how you can say the earth’s frame is stationary without admitting that you are preferring one frame of reference over the other.
Now, to your credit, you do point out that trying to explain away the paradox with the excuse of acceleration to allow for the turn around trip has issues. It definitely does, because the paradox would have already been in play at every point along the outbound trip before any engagement to turn Doctor Ron’s ship around, and to use acceleration to break the symmetry of the problem would at most only clear the paradox out of situations involving General Relativity, but not ensure in any way that situations strictly involving only relative velocities in Special Relativity was free of the paradox.
Next, you attempt to ensure that the paradox does not occur in Special Relativity either, but you opened yourself up to some vulnerabilities in the attempt. So, let’s look at what happened. First you tell us we are going to try a thought experiment using three observers in what you say is “constant motion” where none of the observers will experience acceleration, but then you turn right around and say observer A sits stationary on the earth. Well, this is fine, so far as observer A is in fact stationary with respect to the earth’s frame of reference, but this in no way ensures that observer A is stationary with other frames of reference moving relative to observer A’s (the earth’s) frame of reference. Yet, you arbitrarily treat the earth’s frame as fixed no matter what, and the other two frames for observers B and C as moving. However, explaining it that way opens for you an even bigger can of worms, because if observers B and C are moving relative to each other as they head towards each other with the same relative velocity v > 0, we can by no means say that either of thse observers is stationary, right? And so when they do the math, observer B will calculate observer C’s time to be, T_c = T_b*gamma(v) and observer C will calculate the time for observer B to be T_b = T_c*gamma(v), where both observers plug the same value V > 0 into the gamma function. And wouldn’t you know it, the paradox pops right into place, because each observer in this case sees the time on their own clock run normal, but calculates that the time on the other moving ship’s clock to be slowing down by comparison. So, the way you set the problem up actually preserves the paradox between observers B and C even if we let you insist that the earth’s frame where observer A is happens to be stationary. That is the danger of using three observers where two of them are moving towards each other with a relative velocity v > 0.
your answer lies in the difference between velocity and rapidity. i think this is where trying to dumb everything down broke everything
At least now I understand why this video explanation didn't make sense to me
I find it fascinating that you're able to articulate the twin paradox so precisely yet are completely confounded by the explanation which is perfectly sound. FYI it sounds to me like you've confused the idea of velocity and acceleration and that you're trying to think through it through two reference frames simultaneously instead of focusing on one.
@@ultimateman55
Gief: @Corey Bray He didn't fail to realize. You misunderstood.
Well, I am sure we will find out soon enough. So, let’s walk through your logic below and see what you conjured up for me to deal with here.
Gief: They're moving towards each other, yes,
Certainly!
but he never says they're moving towards each other with speed v.
He doesn’t have to say it, Gief! If they are moving towards each other, as you claim earlier they must be moving towards each other with some relative speed v, such that their motion resolves being to within the range 0 < v < c to guarantee that they are not violating the universal speed limit of Relativity.
Gief: B moves away from EARTH with speed v and C moves towards EARTH with speed v.
So, if I am guilty of an abuse of notation here, we can simply realize that since you said they are moving towards each other, their actual mutual relative speed is 2v between B and C, unless that speed 2v > c, in which case we will need a Relativistic approach to adding velocities to fix the violation of superluminal relative speed here. But, this is really just a minor point, it doesn’t change the unfortunate symmetric outcome of the problem here going on Relativistically between observers B and C.
Gief: (The ships' relative velocities could be calculated easily and sum to less than c.)
Sure, Special Relativity has an adding velocities work around for that even if the Euclidean sum of the linear advance of both observers towards each other is v => c between any two observers.
Gief: His point is that what resolves the paradox is the Earth always remains in the same reference frame,
This is because he falsely assumes that the paradox can only show up in the place he anticipated it might in his explanation. His explanation does not resolve the paradox that is occuring between B and C. We do not need the earth’s perspective anymore to consider their case, because B doing the math and C doing the math will symmetrically guarantee that each observer calculates that the other observers clock is slowing down at the same time by the same amount with respect to the clock on their own ship, and that would immediately be a direct violation of Algebraic Trichotomy through every point in time of their closing in on each other, and so the symmetry problem of the Twin Paradox just pops right out of the problem. I honestly cannot believe the author failed to see it. I mean, it is so obvious.
Gief: where as the ships count as TWO reference frames.
But, you must ask what is happening between these two ships relativistically. You must always be aware that with any two observers in uniform relative motion, relativistic effects can occur as predicted by the Lorentz Transformation between them.
Gief: This is true whether or not you use the A B C two ship example or the single ship A B example. The acceleration of the ship in the A B example is incidental, not specifically relevant.
Well, we need not talk about any acceleration, because it has no bearing on a paradox that is strictly a problem in Special Relativity which is why a breaking of symmetry in the classic explanation of resolving the paradox with a noninertial frame in the outbound twin’s motion is kind of meaningless. In any case, I applied Gamma between B and C and conversely between C and B to show the same old symmetry problem arises that has always plagued Special Relativity, only this time around the observers are heading towards each other, so there is no need for introducing a noninertial turn around excuse as in other explanations to cloud the issue.
Gief: The correct question to be asking after this video is "Why does comparing the one reference frame of Earth to the two of the ship necessitate that the ship experiences less time?"
No, the far more important question is what happens with the clocks on B and C’s ships when their paths intersect, compared to what is predicted by the equations of Relativity? Because each observer B and C will calculate at every point of their closing in that the other observer’s clock is slowing down with respect to their own ship’s clock, and this is where reality and Relativity are going to have a nasty collision where Relativity gets shattered into pieces. Because both clocks cannot physically be slowing down more than each other at the same time as the Lorentz Transform being employed by each observer will inevitably predict.
which is probably quite difficult to understand without the math
Is it?
Gief: I find it fascinating that you're able to articulate the twin paradox so precisely
Well, I have spent years watching people convolute and complicate it to their own confusion, and it’s really a very simple idea ultimately when you strip away all the nonsense and take the time to grasp what the math is really saying.
yet are completely confounded by the explanation which is perfectly sound.
No, the explanation is not perfectly sound. The author did not guard against the paradox popping up between the inertial reference frames B and C.
Gief: FYI it sounds to me like you've confused the idea of velocity and acceleration
I am not sure what has given you that idea. But, given any position function y = f(x):
Velocity is given by y’ = f’(x),
and
Acceleration is given by y’’ = f’’(x).
Such that, velocity is the measure of uniform constant motion and is the subject matter of Special Relativity along a flat Menkovski metric, and acceleration is nonuniform motion which increases or decreases, and is dealt with using curved menkovski metrics in General Relativity, such as in the case of curved space/time in the case of discussions about gravity.
Gief: and that you're trying to think through it through two reference frames simultaneously
Sure, because the whole point of a transformation like the Lorentz Transform is to allow you to compare what is going on in another observer’s relatively moving inertial frame with respect to what is going on in your own local inertial rest frame as concerns how time and space are being measured. And, simultaneously, the other observer can do the same type of math from their frame. That’s normal to the dynamic model of Relativistic mechanics. Not sure why you would think that is any kind of real problem here, unless you are not clear why symmetry became a problem in Relativity to begin with.
Gief: instead of focusing on one.
Okay, you seem to be confused here. If you are the observer who applies the Lorentz Transformation, focusing on one and only one reference frame, namely your own frame, then you will get measurements, M’ = M*Gamma(0) = M. So, we learn nothing more with this approach than that to apply the Lorentz Transformation to one and only one frame, namely the observer’s own frame of reference, gives us back measurements that are consistent with the observer’s own frame of reference. Why does this occur, because you being the observer are at rest with respect to time and space inside your own local frame of reference. But, this is good to know, because it explains to us that when you apply the Lorentz Transfermation to your own frame of reference, you are getting something completely consistent with local observations. So, while this is good to know, motion in Relativity is, well, Relative, and the real point of the Lorentz Transform is to use it on another observer’s frame of reference, but when the other observer does the same, SIMULTANEOUSLY, we get a symmetric paradox where each observer sees the clock in the other observer’s frame is slowing down with respect to the clock in their own local rest frame. Various attempts have been made to resolve this paradox like having the outbound Twin turn around and break symmetry with a noninertial frame of reference, but this really does not work for the fact that it fails to account for the fact that the Paradox is preserved along every moment of the outbound leg of the trip. So, the usual explanation around the paradox is just smoke and mirrors mostly. Others try to deal with the problem by arguing that the universe splits into multiple universes which gets even more ridiculous still.
Of course, this is really the least of our worries with Relativity. According to Relativity, if you are moving relative to me, space around you contracts from my perspective. So much so that there is a famous Relativity problem that says a 40-foot poll can be distance contracted down to fit inside a 20 foot barn door opening. Let’s think about the physics of this for just a moment. So, the faster your speed, the more that poll shrinks, such that if you could possibly achieve v = c, the poll would shrink down to length 0. Of course, that would be ridiculous, but long before that poll shrunk down to 0, if you went fast enough, you and the poll would shrink down below your shwartzchild radius and you would be stuck inside a black hole due to no other reason than Relativity says you were traveling at a velocity fast enough to physically shrink you down to such a size. The fact that objects in our universe moving near the speed of light do not form black holes should tell us immediately that there is something extremely funny with many of the claims being made by folks about Relativistic mechanics. At the most, any space contraction could only be an optical illusion, else lights motion alone would doom us all to living in a compact singularity as light does move at v = c with respect to all observers. Ooooops!!!
"ACCELERATION ISN'T THE EXPLANATION", ... in large, friendly letters. (Similar to "DON'T PANIC", in large friendly letters.)
Yet falsified by the cover of version 2.0 :(
I seriously doubt that the answer is 42. Much more likely it'll be 101101000110010101100010111.........
Except if you watch other experts in relativity they say that relativistic time dilation effects are from a combination of Acceleration and Velocity. So there is another paradox....both experts can't both be right.
@@michaeljorgensen790 yes, I"m utterly confused!
Dr Lincoln’s muddle becomes even greater in this video - which is designed to counter the criticisms. In order to ‘prove’ that “acceleration has nothing to do with time-dilation”, Dr Lincoln now has triplets, A, B and C. Somehow, B and C weren’t accelerated in order to get them to move, according to Dr Lincoln. The particular daftness of this video is that Dr Lincoln claims that the existence of C somehow determines which object experiences time-dilation. But removing C won’t affect B’s time-rate, and the distance L is irrelevant; it doesn’t matter whether B has travelled ½ L or L, B has the time-rate that it already has. The object that experiences time-dilation in this example is the object that Dr Lincoln defined as ‘moving’. The object’s that are moving were accelerated to get them to move.
(P.S. - If we let B carry on it will eventually have travel the full distance L. Let’s call the distance L a light-second (i.e. 299,792,458 meters). When B reaches L, B could send a flash of light back to A and A can write down the time showing on its clock (minus 1 second) that B reached L. Later, when the two times written down are compared, - if B ‘lost time’ compared to A, (time-dilation) - then that is because Dr Lincoln started out with the assumption that B is ‘moving’. He defined B as a ‘moving’ frame. )
The distance L is irrelevant to the calculation. All that matters is - which object is moving and its relative speed.
Perhaps a visual will help. There are 2 possible cases in the 2-body scenario if we are to presume that one of the twins is indeed stationary:
Case 1: /| Case 2: |\
/ | | \
/ | | \
\ | | /
\ | | /
\| |/
Vertical lines are bystander worldlines. Zigzags are traveller worldlines. The traveller is younger in both cases, but which twin is that? SR can't tell (nor can GR) so you have to figure it out by other means. There are 2 ways I know of:
1) Ask the twins who felt the force.
2) Count the number of redshifted light rays each twin receives from the other. The bystander receives more.
minutephysics really clears this up at ruclips.net/video/0iJZ_QGMLD0/видео.html - In the classic twin paradox, during both the outbound and inbound trips, BOTH twins see the other as aging slower. However, during twin B's frame change, he see's twin A's time jump forward. (While decelerating and accelerating, he would see twin A's time speed up.) By using two travelers (B and C), Dr. Don avoids the acceleration issue, but now he is comparing apples (B) and oranges (C). He never mentions that B and C do not agree on what time A is living in. During their entire trips, C is seeing A further in the future than B is. If they exchange notes as they pass each other, B might say A was living in 2019 while C would say A is living in 2020. (And they would also disagree about each other's starting times.)
Very well said!
Thank you! Finally a video that actually shows what this gut is talking about.
This would seem to suggest that said time dilation and the difference in ages between the 2 doctors would only occur IF Ron returns to earth? Maybe I’m misunderstanding but you seem to be saying that the act of turning around and returning to earth creates the second frame of reference that is required for this measurement. But wouldn’t that suggest that simply accelerating out in one direction at 99.99% of the speed of light would result in no age difference?
The very problem is, that two observes can only agree that two events happen at the same time if they happen at the same place, but may not agree that two distant events happen at the same time. Therefore, you need the meet-point to compare the values, because the two may not agree at what time the turn-around happen, or in other words, the one in rocket may not agree that the one on earth stopped the watch at the time he/she reached the star ;)
LocutOs the idea, and it is a crazy idea for sure, is that Ron is experiencing time as if it was the same but only when he returns to Don where time is being measured from the same viewpoint of Don do they see that their times have diverged. So imagine: Ron left earth and circled around in the universe for 5 minutes at a very high speed, just for fun, he returns home immediately. Don saw him leave in 2018, but it is now 2060 when Ron returns and Don is 42 years older, 42 years he waited for Ron to come back. Ron is 5 mins older and only experienced 5 minutes. The problem demonstrated in this paradox is that the theory of relativity says all motion is relative, there is no such thing as absolute motion, literal rest etc. This means that it is exactly correct to say that Don and the earth LEFT Ron who sat in the same place inside a stationary spaceship (THEY accelerated away from him, THEY changed frames and returned to his frame would also be correct to say if you think about it) for 5 minutes and when they reunite, Don is 5 mins older while Ron is the one who is 42 years older. Well, which is it? Who is older? Well, I have heard all the answers and no one has an answer because the theory is completely flawed and yet science refuses to let go.
Hi Dr. Don .. I am sorry this video still didn't quite satisfy everything (at least for me) .. because if you add another observer D at a distance 2L on the left of the starting position (where A and B coincided) .. moving to the right towards B with the same speed A is moving to the left .. and you did the exact same analysis while fixing the screen frame to B (the earth will be moving with A or in other words A is the earth .. you can forget about the star since it will only add confusion and it is not necessary for anything) .. then you will reach that for B the time experienced by A should be less and B should be Older. which is coming back to the essence of the Paradox. Appreciating your thoughts on that and thanks for the great Effort.
PS: I was pretty satisfied with the explanation to the paradox provided by Eugene ( ruclips.net/video/bjHLboK2M1g/видео.html ) .. this is why i am particularly very interested to understand your argument that says acceleration is not the culprit.
Thanks.
i mean i also feel it has to be the acceleration because you can forget about the earth also .. these are 2 points in space that separated and then combined again together .. it is a very hard symmetry which can only be broken by someone doing an action that the other doesn't do. somehow i don't feel your explanation breaks this symmetry
remember the important part - gamma. While as I move away from the earth, I can say that the earth is moving away from me from my perceptive - it is I who has gamma increased and not the earth.
Ditto. I wrote a screed about it.
BINARYGOD why is it you who has the gamma increased and not the earth? Is it because u moved? In relativity there is no absolute rest position and gamma is a property of the relative motion between 2 observers .. so both think the other has dilated time.
@Ayman Mahmoud
I was also still asking the same question as you do now. I finally got it after 2 days of pondering over this problem. The solution is much easier than I thought and I understand what it means to be in one or two reference frames.
I think Don's explanation added some complexity but I understand he wanted to explain the problem differently. Didn't quite help me :P
If I understand correctly, time dilation is necessary to reconcile motion through space with the fact that all matter moves through spacetime at a constant velocity: the speed of light. That reconciliation is accomplished mathematically by gamma, and it allows observers in separate frames of reference to measure both time and the speed of light *within their respective frames* the same as they always do.
Good job, most people that attempt to explain it do not remove confounding variables to show it cant be all acceleration. They say there is no paradox because one twin had experienced acceleration on the turn around journey which implies moving near the speed of light had zero impact. You can reach light speed at 1 G in less than a year (if you had the fuel) which would also keep the frames the same (earth is 1G) in terms of acceleration.
I'm going to make a revolving bed, so I age slower when I sleep. ( Beauty Sleep ).
If you are actually spinning around your centre of gravity then your head and your feed will age the slowest - and whatever is in the middle will age the fastest. It's your choice how you do it of course.
How time dilation occur moving from one empty space to other empty space
I'm not a math guy, but I still found the other video more helpful. What was problematic for me then, though, is still problematic for me here. I am persuaded, and I see why, acceleration doesn't solve the paradox, and I can acknowledge that the number of frames of reference is a difference. I can concede based on your word that it is the *only* difference. What I don't know is *why* different numbers of frames of reference would make the difference. Why would Dr Ron definitely age less being in two frames of reference whereas Dr Don age more being in one. I see that Dr Ron ages less because he is the one in motion. But what is it about the multiple frames of reference that gives that motion the meaning and so has the impact on aging? If you were to draw their worldlines out on a space-time diagram could you see the connection more clearly?
ScienceNinjaDude can you point me the timestamp where the math one showed it. I just rewatched it, and it seemed what he did there was mathematically demonstrate that it's not the acceleration that causes the time dilation but rather the motion itself. I understand that. But then he just points out the only remaining difference we can appeal to is the difference in frames of reference. Then he just asserts that solves the paradox. But why does that solve it? That's a logical argument sure in the form of Either A or B, ~A, :. B, but how does showing it is NOT acceleration--which I agree with--demonstrate that it is frames of reference? That's what I'm asking for. At bottom, I don't yet see how these videos solve the paradox but rather how they disprove one proposed solution. So where does he explain how and where the number of frames of reference makes the difference? I take your word for it that he did, but I'm not seeing it yet.
I think he's trying to address my point around the nine minute mark in this video. I'll need to watch that and the associated videos a bit because I am missing something. There is onviouslyvsomje reason that B or C (or Dr Ron) cannot claim to be stationary relative to A (or Don) per the classical relativity claim. The multiple frames of reference is the purported answer. I'm not seeing how that is yet.
I think he's trying to address my point around the nine minute mark in this video. I'll need to watch that and the associated videos a bit because I am missing something. There is onviouslyvsomje reason that B or C (or Dr Ron) cannot claim to be stationary relative to A (or Don) per the classical relativity claim. The multiple frames of reference is the purported answer. I'm not seeing how that is yet.
Perhaps acceleration causes the change of reference frames. Or the choice to move through space means one moves less through time. Could it be that easy?
Can you make a different twin paradox example using A, B, and C experiencing different gravity? Maybe all three are stationary to all observers. Then a black hole moves through the single frame of reference, with A, B, and C at different distances from the black hole.
The gravity of the black hole attracts the three observers A, B, and C. Normally this would make the observers closest to the black hole appear to move faster, relative to a fourth observer. But remember in this example all three appear stationary to all observers. So for example, if A is closest to the black hole, A must accelerate away from the black just enough to have no apparent motion relative to itself or any other observers. The same for B and C, although they will require less acceleration to appear stationary because they are farther away from the black hole.
After the black hole passes, all observers agree to have observed no motion. However, observer A will have experienced the less time than B and C.
Suppose B "jumps" between spaceships at midpoint L. By changing time frames, a portion of A's time segment (and associated events) is completely cut off from B's point of view when B "jumps" between spaceships at midpoint. It's like B goes from A's past to A's future through changing moving directions. Even though from B's point of view, A ages slower during the two half-trips, the change of time-coordinate cuts off A's time MORE THAN compensates for the shorter time A experiences during the half trips. From B's point of view, the cut-off time has to be added back to A's age when calculating A's time duration. Thus, only as a net effect, B sees A age faster when they unite.
Wait! ... Just do I make sure I got things right, When and Where did B and C exchange information? According to what frame of reference? Because I think they will disagree with A about that, am I right?
What if we did this whole calculations based on the timing of C as stationary, don't we fall again in the Paradox that A is the one that aged less???? I am so confused and I don't trust my own math, but I do logically claim that acceleration is the factor here.
You need to think about what B and C would observe from their stationary frames.
In the frame where B is stationary, they see:
>A receding from them at -V
>The star moving toward them at +V
>C moving toward them at +2V
In the frame where C is stationary, they see:
>A moving toward them at +V
>The star moving toward them at +V
>B moving toward them at +2V
B and C will agree on the elapsed time at which they are both at the star. They see the relative B-star-C motion as the same, and will compute the same clock time as having elapsed when B, star, and C are all in the same place.
They will disagree on where this happens, since B will say "I didn't move. Instead the star and C came to me." C will disagree, and say "I didn't move. Instead, B and the star came to me." But they won't disagree on the time elapsed (which they will each say is L/V).
@@martinwoolf3861 thanks for the clarification
Déjà vu
Still the paradox is not explained.
Ron see Don's clock moving slower, Don see Ron's clock moving slower, but when Ron returns from his journey obviously his clock lags behind Don's clock.
Since that could not happen instantly, at what time did Ron see Don's moving faster than his?
I was able to understand the explanation better after this video combined with the one minutephysics did and a graph I found on the Internet.
This is the video by minutephysics: ruclips.net/video/0iJZ_QGMLD0/видео.html
And here's the one simple graph I found on the net. It shows that thing about the ship being in two reference framesfrom the perspective of observer A: physics.stackexchange.com/questions/202679/the-twin-paradox-using-reference-frame-following-the-ship?
Even still - I am not really sure if I understand it correctly and I need to rewatch those videos. Maybe it will be more clear to me why does B and C see A being in one reference frame all the time.
Thanks szpinak222, but minutephysics's video is bogus to say the least.
*"the important bit sine you're moving"* that we know is not a valid argument.
For "me" it's him that changed speed, with the one exception, "I" felt the acceleration and "he" did not, but Dr. Don insists this is not the reason.
Less I care about mathematical or even scientific proofs.
Ron must see Don's clock ticking faster at some point, all I want to know is when and why.
Well if you're talking about what is "seen", as in real-time observation of light signals, then both twins observe the other's clock moving faster during the inbound leg of the journey, due to the Doppler effect.If you have one twin flying to a location 3ly away and back at 0.6c, his total journey takes 10 years from Earth's perspective, but only 8 from his. The Lorentz factor here is 1.25, so clocks at one location appear to tick at 80% the rate as observed from the other. However, the Doppler effect will actually make each side appear to be ticking by at 50% the normal rate (62.5% the rate due to Doppler, plus 80% the rate due to time dilation).
For the traveler, after reaching his destination 4 years in, he will only see that 2 years have passed on Earth, since light from his idea of "now" on Earth has only just started to head his way. On Earth, they won't see the traveler reach the destination until 8 years in (5 years for him to get there + 3 years for the light to reach Earth). Because of the 50% rate, they'll see 4 years have passed on the ship at this time.
As the traveler heads back, the Doppler effect works in the other direction. In this scenario, it works out that each side sees the other's clock moving twice as fast as a result of the effect (2.5 times as fast due to Doppler, but 1.25 slower due to time dilation). So, for the traveler, as he leaves he's observed 2 years pass on Earth, and during his 4 year journey back, he observes 8 years pass on Earth during this time (for a total of 10 as he arrives). From Earth, they see the traveler begin to come back after 8 years in, only 4 years having passed for them. This trip will appear to take 2 years from Earth's perspective, during which they observe another 4 years pass on the ship.
Another interesting effect you'll notice is that Earth will observe the ship cover 3 light years in a period of only 2 years, seeming to travel faster than light. This is only an illusion caused by the Doppler effect. When you account for it, you're able to see that the actual speed is less than light. But, due to Doppler, anything approaching you at greater than 50% the speed of light will appear to be moving faster than light, until you account for this effect. Similarly, something moving away from you can never appear to be moving at or above 50% the speed of light, due to Doppler.
Dr Deuteron
It's Don that changes direction not Ron (at least that's what Ron think,) and nothing in *our* world changes "suddenly" everything takes time and neither of them has missing time gaps.
Arkalius80
"For the traveler, after reaching his destination 4 years in"? 4 years is earth view of his clock at 3ly distance not his, and his view of earth clock is the same.
Time to simplify. First, a point. Who is younger or older or the same age is only a question about how time passes. It is related to what events are simultaneous. It is a question of time simultaneity.
PART A. This is the problem....... Rocket A and Z pass each other in space.
1.To rocket A..... it is Z that is moving and so it's clocks will run slower. Z
Wes: Time to simplify. First, a point. Who is younger or older or the same age is only a question about how time passes. It is related to what events are simultaneous. It is a question of time simultaneity.
PART A. This is the problem....... Rocket A and Z pass each other in space.
So, before they pass each other, they will each do the math and predict that the clock in the other ship is slowing down. At the instant they pass each other, when their relative velocity v jumps to 0 for an instance, they will see the equations of Special Relativity made a bad prediction.
1.To rocket A..... it is Z that is moving and so it's clocks will run slower. Z
You are still wrong about the acceleration. What he is showing here is that even without acceleration (by using 3 frames, instead of one moving away, stopping then accelerating to return) everything still works the same.
Acceleration is not the cause.
@@SolidSiren
Cori: You are still wrong about the acceleration.
Acceleration should never even be an issue in attempting to resolve a paradox in Special Relativity like the Twin Paradox. Acceleration is a topic in General Relativity, the Twin Paradox is a Special Relativity problem ultimately.
Cori: What he is showing here is that even without acceleration (by using 3 frames, instead of one moving away, stopping then accelerating to return) everything still works the same.
You are missing the meat of the paradox raised by this reworking of the problem entirely! First of all, observer A is just a distraction. The real meat this time around is what is happening between observers B and C that really deserves our attention. The distance from observer B to the distant star is l and the distance from the star to observer C is l. So, as both observers B and C close the gap of 2l towards the same distant star, they end up having a relative velocity of v > 0 between them by default of the fact that he insisted these two observers were the traveling/moving observers, ooops! So, there is no stationary observer to appeal to this time around to convolute the issue with, and no accelerated turn around trip either to serve as a distraction, just a pure twin paradox all the way as they close the 2l gap between them to that distant star. Each observer will calculate that their own clock is running normally in their own frame, because their relative velocity with respect to their own clock is v = 0. And gamma(0) = 1. Hence, we get,
T_B = T_B*gamma(0), and T_c = T_C*gamma(0)
Or, in other words,
T_B = T_B and T_C = T_C
But, when they try to calculate how time is passing in the other observer’s frame, they will get the symmetrical paradox of
For a positive relative velocity 0 < v < c, between B and C, we have,
T_C = T_B*gamma(v), and T_B = T_C*gamma(v)
So, since gamma is less than 1 when the relative velocity v between observers is greater than 0, each observer sees that the other observer’s clock will slow down with respect to their own clock and this means algebraic trichotomy is being violated along every point of the trip to that distant star as predicted by the use of gamma in the equations; hence, the symmetric paradox is perfectly preserved here.
So, as a rule of thumb, it is never a good idea to convolute the Twin Paradox by adding more frames into the soup, unless you want to run the risk of the paradox popping up in more and more places in your model. Such that, if you have n total observers distributed across n frames that are moving with positive relative velocities with respect to each other of v > 0, then the number of potential symmetric time dilation paradoxes you will be inviting into your model are the number of combinations of n choose 2, or n! / (2!)*(n-2)! So, for n = 100 observers, this would appear to potentially give rise to 99*50 = 4950 different symmetric paradoxes going on in your model.
By the way, even if this guy had produced a simple case where the paradox does not show up, such an isolated example would never constitute a logically sufficient treatment of the matter to guarantee that Special Relativity is free of the Twin Paradox occuring. Such a demonstration will never be found in these lazy laymen explanations. Rather, it is far easier to produce counter examples to such a treatment as emerged between B and C, because the more frames you try to account for, the more possibilities the paradox will emerge becomes the real problem to contend with here. Plus, it is just as trivial to show that any clock in Special Relativity is slowing down with respect to itself if you use the Lorentz Transform to frame hop around a circle of frames. So, the question of how reliable clocks are in Relativity is itself a very overlooked problem.
To see this, let’s imagine three observers moving away from a star. Such that the star resides at the center of an equilateral triangle, and the ships moving away from the star are oriented to head in the directions of the three verticies of this growing equilateral triangle with some relative velocity v away from the sun and a greater relative velocity V between any two observers. So, the outward path of each observer is 120-degrees away from each other observer’s path of motion in the plane they are moving within. We could have used four observers and the verticies of a square, or five and the verticies of a pentagon and so on. But, the equilateral triangle is a simple case to consider. So, A calculates that B’s clock slows down, B calculates that C’s clock slows down, and C calculates that A’s clock slows down, but that means A’s clock is slower than C’s clock is slower than B’s clock is slower than A’s clock, so A’s clock is paradoxically slower than itself in Relativity, and this is simply an unfortunate situation that Einstein never thought to plan for; yet, it pops right out of the math if you want to carry out the calculations for yourself, and building paradoxes from this point becomes a simple matter of adding more observers to layer the paradoxes upon any clock you target. Okay, I really do need to get a life, but hey, math is fun. What can I say.
It’s not acceleration, because in the thought experiment described, nothing accelerates. But one measurement involves measurement of a time interval in a single inertial frame, while the other involves summing measurements of time intervals in two different inertial frames. Different paths through space time result in different measurements of elapsed time. The measurement in a single inertial frame will result in the longest measured time.
@@zzubra
Zubra: It’s not acceleration, because in the thought experiment described, nothing accelerates.
But, the thought experiment does not get rid of the paradox. The paradox is clearly guaranteed between observers B and C who are heading towards each other on the way out to the distant star with a relative velocity v > 0 with respect to each other ultimately. So, each observer B and C will pop gamma(v) into their calculations and the transform will guarantee that the other person’s clock is slowing down by that same amount from each observer’s perspective relative to the clock in their own frame of reference. So, the way the problem was set up betrays the effort to resolve the paradox.
Zubra: But one measurement involves measurement of a time interval in a single inertial frame, while the other involves summing measurements of time intervals in two different inertial frames.
And all that is an irrelevant distraction from the real issue here. Forget a turn around involving acceleration or splitting the trip into two different inertial frames to try and avoid acceleration’s influence, just consider the fact that A is on earth and B is moving away from A with a relative velocity of v > 0. If A and B calculate the time of the clock in the other observers frame at any point t on this outbound trip, using the same velocity v > 0 in gamma(v), they will each calculate, at every point on the outbound trip, that the other observer’s clock is running slower than their own clock. At every point t on this outbound trip, algebraic trichotomy is being violated and the paradox is guaranteed. So, people who think the turn around trip or using multiple frames saves the day clearly do not grasp what is really going on with the math behind the transform itself and what the math is actually saying during the outbound trip where the real problem resides. And if you know how to set the problem up right, you can show that any clock is not properly keeping time with itself at any time t in any relativity problem, so the entire idea of Special Relativity is an unstable mess.
And for those who buy into space contraction being a real phenomenon, then we need to ask what happens to a sun as you move towards it with a relative velocity of v > 0, such that v is close enough to c to get the mass of the sun in the direction of motion to flatten out below its schwartzchild radius, something Einstein clearly did not even think to anticipate. Long before you cause the mass of the sun to collapse into one or more black hole states, the star should exhibit an instability due to a breakdown in hydrostatic equilibrium as it is considerably flattening out in the direction of its motion and not maintaining a near perfect spherical shape anymore. Sense we do not observe objects moving at great speeds relative to stars producing these kinds of physical effects, it tends to suggest that space contraction is also a rather dubious idea in special Relativity as well.
Ok, here is what I understand, we know that the moving person will experience the time dilation. In this thought experiment B and C is actually moving, not A. Because A is experiencing constant velocity from B and C, where B will experience a constant velocity with A but a faster velocity with C as they were coming towards each other. Their velocity will add up. Same situation for C. So, we can correctly say that in this reference frame where A, B, C are all together, A is stationary, B and C are actually moving. And a moving one will experience the time dilation. 1 vs 1 reference frame creates the paradox, so we needed C, a third reference frame to understand. I do not know if I am right, but that is what I understood.
Here's my take at an explanation:
First we must give up the assumption of a perfect observer, one that can instantly know how the clocks are ticking for both twins. How much time has elapsed for both twins is a nonsensical question when the twins haven't communicated their results to the other, only local time makes sense for each observer.
So now comes the question we are trying to answer, and the explicit question in this case should be "If the twin on Earth is considered a stationary point, how does their elapsed time compare to the time of their traveling sibling, if the traveling twin sends a signal to Earth" (sorry for the possibly confusing redaction).
The traveling twin could send the message to the Earth twin in a multitude of ways. They could turn back after a traveled distance and compare clocks directly once on Earth; they could synchronize their clocks with a third observer (a messenger) that travels back to Earth instead of the twin (like observer C in the video), or they could send a message at light speed, to name a few examples.
When the message reaches Earth, and the clocks are compared ON EARTH, the traveling twin will be inevitably younger. That is because information does not travel instantly, it has a finite speed and a trajectory through time and space. The message itself has a frame of reference. So no matter what the traveling twin did, either turn back to Earth, send a message, etc., the Earth twin would measure their siblings elapsed time to be shorter. Every possible way of communication, every possible experiment, includes the traveling twin's clock going through 2 or more reference frames FROM THE POINT OF VIEW OF THE EARTH TWIN.
Now let's change the question, which will of course change the answer. Let's say we want the traveling twin to be an inertial frame of reference, and we want to know how they would perceive their sibling's age if a message came from Earth to the rocket. Maybe the Earth twin decided to send a messenger going faster than the traveling twin to catch up, or a laser signal, doesn't matter. Now, the traveling twin sees their sibling's clock as changing its frame of reference, from the Earth to the message. So when the signal catches up, and the traveling twin compares the clocks, their Earth twin will be younger.
In short, we cannot compare the twins' age at widely different points in space. And when we decide WHERE to measure and compare their ages, we are influencing the result. That is why relativity is so relative, that is why time, speed and position matter so much. If you ask a question, but don't specify one of those parameters, your question doesn't make sense anymore, because it assumes a universal frame of reference (a universal clock or grid) that we can use to instantly compare the state of both twins light-years apart.
A small add-on: reading of “Close to the Speed of Light”: Dispersing Various Twin Paradox Related Confusions" by
M. Arsenijevic, where the meaning of the misleading word "relativity" is physically addressed: the word relativity is actually a pitfall and nowadays used and abused like it was for the word incompleteness of the incompleteness theorem of the mathematics. Contrary to popular belief an absolute entity still exists even in relativity ... read and discover the clue to any apparent paradox. I find that reading self-explaining, althouhg not "absolutely" easy, a bit phylosophical, at the end of the day it's crystal clear
If both can be seen as moving from each other perspective, how can we know which one will get older?
Raimar Lunardi they aged at the same rate since both are moving at the same speed , the A observer gets older since he is a stationary
It is indeed the acceleration. In Don's example, there is also acceleration. The worldline whos length is measured by the two clocks is indeed not straight, so has acceleration.
I love these videos. But this one, although it did better than most, still failed for me. It did show how you don't need acceleration to calculate. That was great. But for me, it failed to describe "what broke symmetry". Why?
In the video, he used 3 objects, and that is his way of eliminating acceleration from the solution, as well as shows 1 vs. 2 frames of reference. But the twin paradox only has 2 objects (if you don't count the earth and background of starts, etc) So what happens if there is nothing in the universe except these two twins? There is no preferred frame of reference, acceleration is not relevant, yet 1 twin does fire his rocket and then turns around and fires it again in the reverse direction and the 2 twins come to rest once again next to each other. Who aged more? Who moved? Who was stationary? What "location" is where the time duration is experienced? The only reason you can say that there is a definite location in Dr. Don's example, is that there is all of this background mass in the same reference frame. But when there is no background mass, what is a location?
isn't each twin their own frame? and the return trip is the 3rd frame so A is the earth dude in only one frame while B the other twin is moving away which is a frame and than coming back which is another frame aka perspective difference of 1 frame and 2 frames?
@@lucifiaofthefreecouncil1312 The twin doesn't come back.
Isn't this 'explanation' simply a circular argument? I.e. begging the question as it were.
I completely agree with Dr. Don - it is just about FRAMES (places)! Acceleration is irrelevant!
Let us simplify -- compute only through events! Years are earth revolutions around Sun, and distances are light years!
Imagine: only 10 light year away, we discover an identical planet E’ (spinning like our Earth E) orbiting around its sun S’.
An Alien of exactly my age would look 10 years younger? Paradox? Imagine we both communicate with our Zero-Latency quantum entangled devices, find one another with our classical telescopes (10 year latency)!
We would endlessly argue- Same D.O.B. - how can you look 10 YEAR YOUNGER THAN ME! PARADOX! EINSTEIN ( and Dr. Don) IS WRONG!!
Of course, we always see past/younger images of one another even when we both soar towards each other at half the speed of light.
After 10 years (exactly ten revolutions of E and E’ around S and S’)-- crossing halfway (5 light years), when we look back through our telescopes and synchronize with our planets with their their 5 year older image!!!
We will shout - No! can’t believe it -- how can 10 years ABSOLUTE be 5 years RELATIVE! Even E and E’ (we saw with our own eyes) have revolved only 5 times - that is time dilation! A beautiful Paradox!!
Earth feels 100% stationary while riding on it (no jolts), time feels 100% absolute until we look from another FRAME!
Q.E.D.
You are suggesting that there is no time dilation and that time flows that same for all observers and that there is only a perceived difference from viewing events at great distance. That means the final measurements will be the same. That means when all observers reunite, all their clocks will match and nobody will be older or younger. I'm fine with that, but you are contradicting this video and the idea of time dilation. Everyone is saying that the observers will reunite to find their clocks don't match and their aging has diverged. Are you saying that is false?
@@fakherhalim I couldn't make any sense out of your comment. The whole theory is crazy for this reason both distance and speed are said to be relative, no absolutes, and yet time is different based on speed, space is different based on distance etc. The only way to make this work will be to add more insanity to it, for example Ron is in a parallel universe where Don is older when they reunite, but Don is a parallel universe where Ron is older when they reunite. Because as long as motion is strictly relative, either one of them can be said to have been the one who changed frame and then returned to previous frame, or the one who experienced acceleration, or the one who moved at a greater speed. It's science fiction
From A's reference frame, B is aging slower but that also means C is aging faster. This means the original twin paradox doesn't work because Ron would have to go both directions (away and towards Don) in order to return to Earth, and so he would end up the same age as Don again, all accelerations negated.
The confusion caused by this demonstration seems to be because it's missing other elements that the Lorentz transforms bear out. Firstly that from A's perspective, the point at which B&C pass each other is a different distance away from A than it is from B&C's point of view due to length contraction, and secondly the fact that considering things from a different perspective breaks simultaneity. The example shown from A's perspective shows B leaving A and C starting it's journey towards A at the same time. However from B's perspective C actually sets off first which explains how C can seem to have slower time to B and yet have covered what B believes to be the same distance.
Good try. You added some much needed explanation of what ISN'T happening but it could use a little more on the topic of what IS happening. You dance around it a bit and refer back to the previous video which didn't give enough attention to it, either.
And for the record, the "frame jumping" proponents have an obvious comeback: it's not the observer but the observations that are important and that is what is "frame jumping." Information is changing acceleration... three times, in fact. That's me playing devil's advocate and maybe taking an oblique swipe (meow) at Copenhagen believers at the same time.
"claiming the TP comes exclusively from frame-jumping as depicted, an astronaut that travels away and back to Earth in a curvilinear trajectory shouldn't age, or not?" That's _me_ playing devil's advocate
What if the universe had positive curvature and B flies away at the speed of light long enough to come back to where he started from the other direction? ;).. There's a lot of devils to advocate for here!
@@altrag You can orbitate around the sun faster than your brother so that you can outdrow him in the future laps. The same you should accelarate from him in the beginning, but If your masses where equal you could think about two satellites orbitating clockwise and anticlockwise around the earth. They should be initially toghether, than a spring has shoot them in opposite directions.
What if their masses where not equal? The havier will be the slower and older?
You're still making it way too complicated.
Much simpler is to explain it by the Lorenz contraction of the distance. For the non-moving observer, the rockets gets flatter when going fast. But that does not reduce the travel time for a given speed. For the moving observer, the guy in the rocket, the whole universe moves and contracts in the direction of travel. So he gets to his destination faster. No need for 2 reference frames.
Or am I missing something?
ScienceNinjaDude Not at all. Both see length contraction of the other. But the guy in the rocket sees the universe contract, while for the one who stays on earth only the rocket contracts. No paradox. No dilemma.
See this makes more sense to me. When I try to understand the video I always end up thinking about the role of contraction. I think a key thing to remember is the Earth-stationary observer sees not just the rocket but also the star moving at a certain speed, and the rocket sees not just the Earth receding at -V but also the star approaching at V, which the Earthbound observer does not. Since the speed of light is constant but length is not, it seems logical that the difference measured by the observers would be there.
I don't get why three observers are needed for the thought experiment.
Miki Cerise There is no real need. It's only an extra check.
but this is because you refer to other reference points like the universe. but if this happened in a blank space with nothing other than a planet with Don and a ship with Ron? Can you explain the answer in that scenario please? Thanks!
Great explanation, Luc. I _hope_ it's correct, because by golly I think I've got it!
I'm guessing that the answer to Davemaster's question is that even if there is no planet, there's still a point in space where he turns round.
You can say that there are an acceleration, but that in this example it's infinte high (witch is nonsence of cause).
But this is more a question of Simultaneity
I hardly dare to say this, but though I am dyscalculative, I do understand your explanation without maths. Your philosofical approache works quite well for me. Thank you for this way of explaining.
The following explanation should remove any ambiguousness about this problem (Only if doesn't happen to be wrong :D As I am not a physicist. But it sounds logical to me):
After one of the twins starts traveling with the spacecraft, about any of the twins we can say that he is younger than the other one (he is aging more slowly than the other one), and it can be correct. It all depends on what our reference time (clock) is. If we are taking the time passing on the earth as the reference, then the twin in the spacecraft is younger. But if we take the time passing in the spacecraft as the reference, then the twin on the earth is younger.
But the somehow hidden assumption in this experiment relies under that part that we assume that the twin in the spacecraft returns back to earth and then we compare them together. In fact, by this assumption, we are meaning that at the end we are taking the time of the earth as the reference time.
So, in order to make everything symmetric, we can even think about the dual of this scenario and see how we will get the dual expected result:
After the twin which has left the earth with space craft has traveled to the space for some distance, instead of him returning back, the other twin on the earth takes another spacecraft and catches his twin in the space. Let's assume that the speed of both spacecrafts are constant. So it means that in order for the twin who has left later to be able to catch his brother, he should have a speed faster than him. For example if the first spacecraft has the speed of V, the second one will have the speed of 2V. So, at some point the second spacecraft reaches the first spacecraft. Let's imagine that somehow at that point the twin in the second spacecraft jumps into the spacecraft of his brother. Now if they compare themselves to each other they will find that the brother who left the earth later is younger. It is because in this scenario we are taking the time in the first spacecraft as our reference.
"No math" - there were math, just not as prominent.
I do not find this intuitive at all. I think I can understand it better with the equations. Please do not take it mean equations is always better. It is still hard to grasp.
Honestly, would have preferred a different representation. Perhaps space-time diagrams would help (in particular if they are animated. Yes, I know that doubles the representation of time, it helps). I just re-watched the old video minutephysic did on the topic (which I can understand better now) where it uses space-time diagrams, sadly it does not present the alternative version that presents the paradox without acceleration. Moreover, due to that it failed in shaking the idea of the importance of acceleration, at least for me.
Right! Dr. Don used math in the video. His explanation for time dilation came down to a 'gamma' variance in stationary time vs moving time. But he didn't explain why gamma exists for a moving person, or why A's 'one' frame elicits a 'gamma' from B&C's 'two' frames. All I know to corroborate Dr. Don's explanation is that scientists put an atomic clock on a plane in the "Hafele-Keating experiment" and then flew the plane around the world, and upon returning the moving plane had experienced 'less time' than the stationary clock. Dang it.
Why would you expect to find special relativity intuitive when your ancestors brains' evolved at low speeds?
_
GPS satellites are the perfect way to explain relativity,
because everyone can relate to it.
On Earth, the satellite's signal is BlueShifted by gravity
and RedShifted by its velocity, relative to earth's core.
_
Except that GPS doesn't use relativity. That's a myth. There is no need to synchronise clocks in the satellites with clocks on earth. Your iPhone or car doesn't have an atomic clock in it. Change the time on your iPhone and see if your GPS changes your location. Of course it won't. The clocks in the satellites only need to be synchronised with each other, and they are all travelling at roughly the same speeds.
There's so much information out there debunking the myth that GPS relies on relativity - ease yourself into it
@@RileyGHunter It's the myth that will not die
@@RileyGHunter here's the actual interface specification for GPS: www.gps.gov/technical/icwg/IS-GPS-200E.pdf
Just search it for "relativistic effects", and you can see the various ways that GPS is corrected for relativistic effects--both on the satellites, and in the receiver. Anyone who tells you that GPS doesn't require corrections for both special and general relativity is misleading you, or doesn't really understand how the system works.
That pun was epic!!! "Pair of Docs"
I find it easier to view this through length contraction, in the same way we describe the muon arrival at Earth when generated from a cosmic ray. From the detectors pov time is passing slower for the muon, whereas from the pov of the muon the distance to the Earth is short.
In the twin paradox, the Earth and star are at a certain distance from the Earth pov and a much shorter distance from the pov of the traveler. This applies to travel in both directions.
Hmm. I've come across this argument before but I've never been very sold on the idea that 'acceleration isn't really the explanation' for the twin paradox. After all, what is all this 'jumping between different inertial frames'? It's just how we describe a non-inertial frame (i.e. an accelerating one). What we have here is just two different pathways through spacetime, connecting a single pair of spacetime events: Event A is the spaceship leaving Earth and event B is the return of the spaceship. The earthbound observer (ignoring gravity) is in an inertial frame the whole time. That's what it means when we say we only need one frame to describe this experience. In relativity, the proper time between two events is maximised for the inertial observer (i.e. the earthbound twin). The other observer's proper time can be obtained by a path integral, adding up the invariant spacetime intervals along the path (in any frame of reference, e.g. the earthbound frame). It will always be greater because this path goes through more than one inertial frame of reference (it's not a 'geodesic' in spacetime).
That's basically the same as your explanation but my point is that when you start amalgamating different inertial frames of reference, this is literally what is meant by a 'non-inertial frame', i.e. an accelerating frame of reference. Of course, the math works out, whether or not you associate the non-inertial frame with the movement of a single body or not but I would say it has no physical meaning unless you do. Well, that's philosophy I suppose but I'm sticking to the claim that 'acceleration really is the explanation for the twin paradox' because acceleration = switching between inertial frames. :) Actually, perhaps the even deeper explanation is that spacetime interval (dx^2 - c^2dt^2 in Minkowski space) is invariant, whilst proper time is simply the interval measured along a world line. Everything just follows from these two axioms.
It's not the math, it's the fact that this guy is just a bad explainer.
Sorry, Don, still doesn't do it. You say that the secret is that A is in one reference frame and B and C are in two reference frames, to which I say, "So what?" You didn't go one step further and say WHY two reference frames make a difference. More importantly, though, you don't need C at all. Even without C, presumably B's clock looks to A like it's running slower. So goodbye to the two-reference-frame explanation. Plus, you totally ignored the problem you mentioned at the outset -- to B, it looks like A is moving and he is stationary, so B should see A's clock running slower. And, of course, they can't both be running slower. So, as far as I can see, we're back to square one.
Thank you! This video makes no sense.
Exactly, none of the explanations make sense. Scientists are smart but they are people too, dogmatic and all. Clearly this is all nonsense unless they end up in different parallel universes. In Ron's parallel universe he is younger, but in Don's Don is younger. Maybe someone will advocate that next. If B is accelerating to A, the A is accelerating to B. If B changed frames and returned to A, then A changed frames and returned to B. So unless there is absolute motion and location, which contradicts the relativity theory, then this paradox has no answer, and if it has no answer than time dilation is false
Uhhh.... try doing the math. Then it is crystal clear. If you can't do the math then you're forever consigned to never understanding it. Period.
@@RME76048 see, I'm a firm believer in not that... However I'm having trouble with this one. I know it. I get it. I've done the math, but trying to explain this one without Math is hard. To the others; no really, it really does make provable logical sense. But you have to see the whole picture first, and unless you've worked at it for a while, that picture can be pretty fuzzy.
Actually he did explain this, though not totally clearly.
You have to consider at what point you want to say "Alright, this is the point from which we will measure all the times and distances etc." When you remember this, and then calculate everything from one point "so either A's position or B's position or C's position," you will find that everybody agrees that A has aged more than B has. C is meant to be a second reference frame that represents B's return journey, which in reality would just be when B switched direction and comes back. Thus acceleration still plays a key role in breaking up the symmetry of the problem, but here's an even easier way:
Take a spacetime diagram (space x axis, time y axis). Now draw it from the perspective of A. A's worldline (path on spacetime diagram) will just go straight up because from A's perspective, he is stationary. But B's worldline will look like some diagonal line coming out from the origin (Earth, same starting point for A and B) until at some point the line flips around and heads back towards the time axis (at some later time, obviously). Now, consider that the Lorentz transform is a specific type of rotation/stretch which allows us to correctly swap between reference frames. Now, also consider that (from A's perspective) B's worldline is some combination of two lines joined by either a sharp corner or some curved but tight corner which represents the acceleration and turning around. There is no way you can Lorentz transform B's entire worldline to make it "straight up" like A's in order to represent B's stationary frame, because during B's journey his frame CHANGES. Mathematically, and graphically, you would have to separately Lorentz transform each "straight line" making up B's worldline, which introduces multiple different line segments, which correspond to different frames of reference. By observing the spacetime diagram in this way, you will see that B reaches A as younger.
It may seem symmetrical, but the Lorentz transform and this whole idea of being able to flawlessly swap between reference frames and call the situation symmetrical only works when considering inertial frames. But the B twin is not in an inertial frame. Inherently, then, A can't make any claims about also not being in an inertial frame because the situation is NOT symmetrical.
Problem (paradox)....if one of two twins in a single frame chooses to move through space (accelerate), if they should recombine into that initial frame, which would be youngest. Answer... Each and every time, the youngest will be the twin that accelerated more. Necessary and sufficient. Occam.
Why necessary? Because acceleration causes changes in relative motion. The paradox starts in a frame without relative motions.
Your claim acceleration is not fundamental assumes an initial state already having relative motions. This is untenable. And rather disrespectful of the Equivalence Principle.
Thank you! This video makes no sense.
I couldn't have said this better myself (But already did!)
Nice explanation to disambiguate Acceleration from the Frame-Relevance among Stationary and Moving Reference Frames and the respective Time Dilation.
I found both of Dr. Lincoln’s clips on this subject helpful. His thought experiment is not quite the same as the traveling twins thought experiment, but his experiment is successful in its own right in showing that acceleration is not key to resolving the twins paradox.
For me, the essential point is this. The experiences of the two twins are not equivalent, because one involves motion of the twin with respect to his cosmic (space time) background, and the other does not. You can argue that, from his point of view, the spaceship twin has remained motionless while the Earth, and indeed the entire cosmos surrounding the Earth, have rushed away and back again. But in that scenario, from the point of view of the spaceship twin, the earthbound twin is moving along with his cosmic background; he is not moving WITHIN it. In contrast, from the point of view of the earthbound twin, the spaceship twin is moving within his cosmic background, with corresponding time dilation effects.
That, I think, is the essential point. The seeming “paradox” arises because it is difficult for us to set up and explain the different frames of reference involved and how they relate to each other.
I understood it after your explanation here. The problem with this video's explanation, which aimed to be less math-oriented and more intuitive, is his reliance on the ambiguous word "frame"; I didn't know exactly what he meant by "frame", but it was the most elementary component of the explanation. Having watched some of his other videos helps me understand your version, which is that the "stationary" observer is stationary only within his relative location in space. Another helpful component of your explanation goes off his other videos as well: objects further than about 14 billion light years from us are moving "faster than the speed of light" relative to our location in spacetime, so why wouldn't they experience zero or negative time passing relative to our own? It's because space itself is expanding, not the objects themselves moving within their own space. When the younger twin who goes to the distant star is travelling, it is within his own space; his movement is not because space itself is expanding or moving in any way, which is what's implied if we consider the older twin and star to be the ones who are "moving".
Hi Bill, you are right.
If we consider the Earth and a spaceship moving relative to each other at constant speed, there are two possible scenarios:
Scenario 1
If you consider the Earth stationary, the spaceship moves from a starting point A to reach an end point B and the spaceship's clock slows down relative to the Earth's clocks.
Scenario 2
If you consider the spaceship stationary, the Earth moves from the starting point A to reach an end point C (C is a point on the spaceship frame), and the Earth's clock slows down relative to the spaceship's clock.
If you consider Scenario 1 (the spaceship heading towards a star), then you have to forget Scenario 2.
If you consider Scenario 2 (the Earth heading towards a point on the spaceship frame), then you have to forget Scenario 1.
Both scenarios are correct. (but they cannot be considered both)
If you consider Scenario 1, Scenario 2 is no longer valid,
and if you consider Scenario 2, Scenario 1 is no longer valid.
And there is no contradiction, because the frame of the Earth and the frame of the spaceship are two different and independent frames. (the Earth and the spaceship keep moving away)
Acceleration is not the key to resolving the twin paradox, even in my opinion.
There is only one set of fixed stars. Absolute speed is all that counts. Every analysis is dependent on our fixed star frame of reference. Einstein (and Mach) despised this truth and spent their lives trying to eliminate the fixed star frame of reference because it involved action at a distance that created the forces of inertia. The Special Theory of Relativity was incomplete because it failed to eliminate the fixed star reference frame. Ultimately, the problem was never solved.
I listened but didn't comprehend :/
😊 Sometime soon I'm going back to Don's previous video, with a notepad, and a determination to use the pause/back/replay ability 😉
Now do it again using REAL numbers ;-)
The answer. When a spacecraft accelerates, it gains mass. If it accelerated to the speed of light, it would effectively become a black hole (infinite mass - which is why you can't go faster than the speed of light). The additional mass (tacked on as a result of velocity) slows the clock down. Mass creates gravity (spacetime curvature). The greater the gravity, the slower the clock ticks. The unaccelerated twin does not experience this increase in mass. Thus, their clock does not tick more slowly. So, the reason is in fact the acceleration. RE-DO THIS VIDEO.
Dude. Both explanations are valid. There's nothing wrong with this one. Calm down.
Seems like the best explanation so far. Mass appears to be the thing that slows down time and not their speed or reference plane. Objects in a heavier gravity well will experience time slower than objects in a lighter gravity well, even when they are not moving away from each other.
Dr. Linclon already made his case there is no such thing as relative mass, and it does not increase with velocity:
ruclips.net/video/LTJauaefTZM/видео.html
I think I understand this confusion about acceleration (correct me if I'm wrong). It's because someone has to accelerate to get to the other's reference frame, and the one that has to accelerate is always going to be the "younger" one. If A speeds up to catch B then A will be younger, and if B turns around and goes back to A then B will be younger. Is this correct? Of course, in all our thought experiments we want to compare so we always include an acceleration.
In the example considered you have to take into account that A, B, and C disagree on which events are simultaneous when those events are separated in space. To compare time intervals on clocks which are moving relative to each other, the spacetime events when the clocks start and when they stop must be simultaneous as seen by all. That is why in the standard twin paradox B's clock is measured as it leaves and then returns to Earth; both observers agree that those events are simulaneous.
In the example, in A's coordinate system, A's clock reading 0 is simulaneous with C starting out at 2L. But in C's coordinate system C starting out at 2L is simulaneous with A's clock reading some time T' > 0. This is why C will find that A's clock showed less ellapsed time than C's when C arrives at A's position.
Simple explanation of the (standard) twin paradox: In Euclidean space a straight line is the shortest distance between two points. In relativity, objects move in a 4-dimensional spacetime along a 'world line', (even if they are 'at rest', as A is). Since time is different from space, the corresponding statement is that a straight world line corresponds to the longest time, as measured by an observer moving from one spacetime event to another.
In the twin paradox the Earth-bound twin moves along a straight world line while the world line of the twin on the spaceship is bent. Hence the Earth twin experiences a longer time than the spaceship twin.
Another way of looking at the standard twin paradox, where B goes out, turns around and returns to A: B will see A's clock running slower than B's on the trip out and on the trip in, since in both cases B is using a different spacetime coordinate system from A to assign coordinates to spacetime events. But B's CS on the way out is also different from B's CS on the way in. During the 'acceleration period' when B is turning around, B is shifting from one CS to another. During this shift, B will see A's clock running FASTER.
The net effect from B's perspective (A's clock running slower on the trip out, running faster during the shift, then running slower again on the trip in) is that A's clock will show a longer elasped time than B's.
A's coordinate system, on the other hand, did not change during B's trip out and trip in. In this CS, B's clock ran slower than A's on both legs. So both A and B will agree that A's clock showed a longer elasped time than B's clock.
I like this video. It's the first time I understood that there does not need to be any acceleration for there to be time dilation.
Oh, I get it! ... I don't get it...
Am I too dumb? Do I need more computing power?
I've also been trying to understand this for years and I STILL don't fucking get either.
No, this video is way more complicated than it needed to be. Pretty much a waste of time for everyone. Watch a show called The Universe, or How The Universe Works. On the science channel, or its app.
Not better explanation than the previous one. If you want to explain your own paradox then name it appropriately like "triplet brother paradox" but not "twin paradox". You are compering time of A with time of B on its way to the star plus time of C on its way from the star. I agree that the time of A would be greater than time of B+C. But B and C are not the same entities so you can not say A is more aged than B nor A is more aged than C. And saying A is more aged than B and C together does not make any sense. You seems to pretend that B changes its identity to C in the middle of experiment but that pretending hides an intrinsic acceleration. In this case infinite acceleration because the velocity of B changes into velocity of C instantly. So it is not like you do not need any acceleration to explain twin paradox in fact in your experiment you needed an infinite acceleration although you tried hard to persuade us you did not needed any.
It is more like you do not understand the comment of Azer.
No it's clearly you that didn't understand the comment of ScienceNinjaDude.
Makes sense.
The time experienced for all 3 observers is the same. You left out the time that passes for B and C before and after A begins measurement. When B passes A and starts stopwatch C must also start a stopwatch since time is also passing for him. Also when C passes star, B must continue to measure time, Therefore when C reaches earth all stopwatches measure the same time.
The twin paradox still holds, you’ve simply changed the question from who’s accelerating to who’s in one or two frames
Why can’t someone that first went on B then on C when the two met claim that they have only one frame and it is the earth and the rest of the universe that had 2 frames?
Paradox unresolved. 😣
Fog: Paradox unresolved. 😣
Yeah, the guy failed to realize that if he says observers B and C are moving towards each other with velocities +v and -v, that we can then eliminate the earth as a stationary frame altogether, and the paradox is preserved between the remaining two ship’s frames he accidentally admits are clearly moving towards each other. Ooops!
@@coreybray9834 He didn't fail to realize. You misunderstood. They're moving towards each other, yes, but he never says they're moving towards each other with speed v. B moves away from EARTH with speed v and C moves towards EARTH with speed v. (The ships' relative velocities could be calculated easily and sum to less than c.)
His point is that what resolves the paradox is the Earth always remains in the same reference frame, where as the ships count as TWO reference frames. This is true whether or not you use the A B C two ship example or the single ship A B example. The acceleration of the ship in the A B example is incidental, not specifically relevant.
The correct question to be asking after this video is "Why does comparing the one reference frame of Earth to the two of the ship necessitate that the ship experiences less time?" which is probably quite difficult to understand without the math.
@@coreybray9834 Corey Bray You are misunderstanding his entire point. You can remove b or c and all the facts remain. The point of the entire video was to show that acceleration is irrelevant by giving an example that doesn't include any. The third frame was added for clarity, not necessity.
@@SolidSiren
Cori: The third frame was added for clarity,
But, adding that frame convolutes the issue by forcing the paradox to emerge between observers B and C. It does not matter if he can get the result he wants with respect to C and A, because he is playing around with asking you to accept that A is stationary and C is moving, so that he can suggest a difference in clock speeds on that basis when push comes to shove, but he cannot play that game with B and C, because he insists both those observers are moving towards each other, and there is no acceleration in that case either, so the symmetry problem becomes a genuine problem for him there that he overlooked when he added this third frame into the soup. So, his attempt to save Special Relativity from the Twin Paradox really did nothing but cement it in place between observers B and C here.
Agreed.
1:10 - 😂 😂 💀
I have a question... Is it possible to be absolutely stationary in space? I mean like totally stationary? No rotation in this galaxy no movement through space. I guess you would need to apply a ridiculous amount in off thrust in a reallllly specific angle to counter the massive movement of us relative to the sun, our solar system relative to galaxy and galaxy relative to galaxy rotation..
I'm so confused. so many questions.
Reference frames are one thing but what about ABSOLUTE stationary.....
Due to our moving through space... What is our time differential as standard observers on earth in our movement compared to a genuinely stationary object????
I guess due to expansion even a stationary object has "velocity" from any given point of reference... But even with this factored in.. I still wonder what the time differential would be.. :/
ScienceNinjaDude this is what I mean. Due to expansion space is moving apart. I get this doesn't really affect us on the scale of you to me. But could an object be genuinely stationary?
I think I'm wording this incorrectly lol. But I think I mean without motion. Not just acceleration
+rundata
The definition of an inertial reference frame is a little tricky. Most of the time you can gloss over the trickiness, but ultimately it has to be dealt with.
One of the tricky bits is that IRFs must be limited in extent; you can't put the entire universe, or even a hefty chunk of it, in one IRF. That's why expansion doesn't matter.
_"But could an object be genuinely stationary?"_
That was already answered: No. The phrase "genuinely stationary" is meaningless. Assuming you are not accelerating, you are at rest in your own rest frame, but there is nothing special about that frame.
The best you could do is to be stationary with respect to the CMB. It's usually filtered out of the pictures you see but it's slightly bluer in one direction and redder in the other. You'd want to head towards the redder part at about 370 km/s. There appear to be large scale flows that likely predate the last scattering so there's a decent chance if several people did that in different parts of the universe, they'd still be moving relative to each other even after subtracting out expansion. That's not to say that there couldn't possibly be an absolute zero velocity, we just don't know how to determine what it might be and GR doesn't need one. There's some math on the QM side that makes me think you might be able to follow the minimum achievable temperature but that's pure conjecture on my part.
OK... I'm going to point out some possible points of confusion. Your presentation, however, is very appreciated and well understood.
First of all, special relativity as well as the Lorentz transforms were not originally based on the postulate than any observer can claim they are stationary. The assumptions are originally based on inertial frames. This applies only to non-accelerating observers, and observers far away from any mass. So a twin that departs and returns cannot claim stationarity within special relativity.
Second of all, the experimental definition begins with a distance "L", and then immediately starts talking about observer B. Distance "L" is only distance "L" to the co-moving Earth-star frame and to co-moving observer A. The distance between Earth and star is not L to observers B nor C, and is even less so for the B-C frame. It might be helpful to incorporate length contraction into the discussion, but perhaps this will confuse things.
Starting out with the most important 3 frames (and gammas) here might elucidate things. There is the "origin" frame of A with a gamma of 1. There is a related A-B and A-C frame with a gamma of 1/sqrt(1-v^2/c^2). There is a B-C frame with a gamma of 1/sqrt(1-4v^2/c^2). In the end, only two gammas matter for this discussion and you can disregard the B-C frame. I think it is important to note that these gammas dilate time, but also contract space. These two effects cancel out for an observer, so an observer cover more _original_ distance vs time without any limits by merely applying thrust and/or acclerating.
There is an acceleration in this scenario that I'm struggling with -- the acceleration of information. Information has entropy, and even a time measurement should have some very nominal "mass". Thus the information exchanges at the clock measurement hand-offs represent some sort of acceleration. I'm not trying to debunk your argument here, I'm just trying to resolve this very subtle point.
Finally, it is evident that true twins that start from the same point and experience identical R-dot-dot (radial acceleration with respect to the origin) will have identical clocks when they reunite. Twins that depart and reunite while experiencing different radial accelerations with respect to the starting point will have different clock readings. We should carefully define causality, but this is a causal relationship between acceleration and time measurement differential according to many definitions.
I don't consider myself being particularly competent in math, but I felt like I understood you less watching this video, Dr. Don. It seemed more abstract.
I think I took your meaning far more easily when you broke it down mathematically.