The 2 years didn't go anywhere, Bob traveled them... but as SPACE instead of TIME. I know that sounds crazy but if you do a little pythagoras: Bob traveled 6 light years and 8 years time. Time and space are the same, but orthogonal. So to add them you take the square root of the squares. What's the square root of 6^2 and 8^2? 10. He traveled all ten years, the same distance as Alice.
The way I try to intuitively understand it is to think of a motor you can't shut off that only goes one speed, attached to 2 gears, space gear and time gear. If the space gear isn't moving or moving very slowly the time gear gets all the power, as the space gear gets faster it takes power away from the time gear. I know it's a crappy analogy, but it's the only way I can wrap my head around it. Not the gear analogy specifically but just the general concept that everything has the same energy, but some things put all that energy into the time dimension. If they experience more resistance going through space that energy has to go somewhere. Doesn't really explain contraction or expansion of space. I'm sure that also has to do with the seasaw of spacetime but I probably need some mushrooms to figure out where to go from there. Fortunately trying to understand this is just a way I torture myself and has no real bearing on my life. Thank-you for yet another excellent video.
What happens if Bob races away on a spacecraft for x years and then goes through a wormhole that puts him back at his starting point? What happens to the twins paradox then? Or maybe if the universe is finite and loops back on itself (like the surface of the earth)?
So I just watched the movie Lightyear and they mentioned space dilation and it sent me into a tailspin. For years, I have watched superhero shoes where the hero moves so fast that time "slows down" and he can complete tasks that normally take a person minutes to in mere seconds. So when I here that going super fast into space makes time on earth "Move faster" than time for you, I struggled to wrap my head around it. I have spent the last 4 hours trying to get a baseline understanding of time dilation and nothing was clicking for me until I came across your video. Thank you so much for being so patient and engaging and such a great educator on this subject.
Think of it like this. Consider your driving North at 100mph with the cruise control set. That means your moving 0 mph in the east direction. Then you start turning east. Your still moving at a constant 100mph but some of your momentum is north and the rest is east. To an observer that can only see your northern movement you are decelerating. For eastern observer your accelerating. Once your going Northeast your moving 50mph in North direction and 50 mph in the East direction. Both observer will agree your speed is 50mph. For you your speed is 100mph. This is normal relative motion in classical physics. Spacetime has 4 Dimensions (1 temporal and 3 spacial). Let’s call the Temporal Dimension North and Spacial Dimensions East. In spacetime your never at rest and move at a constant speed which is the Speed of Light (C). In everyday experience we only move at a fraction of C in any spacial direction. The rest of our momentum is in time. In spacetime we are mostly moving North (time) and slightly east (space). The more we move east (accelerate) in spacetime the slower we move North. So in 4d spacetime we are moving slower in time. In spacetime the 3 spacial Dimension and 1 temporal dimension depend on the frame of reference your in. So observer A will see Observer B moving slower in time. Observer B will see time as normal but space is contracting. There no magic to any of this It’s just how we perceive things.
Hi Paul, great video! I think this : the two twins (TWIN A and TWIN B) occupy the same position at instants of time t = t '= 0 , and then move away. ( They both move with uniform linear speed v wrt. each other ) I believe that the distance between the twins in both perspectives is always the same. For example, if the distance between the twins is equal to d: 1) from the perspective of TWIN A , t = d / v and t '= d / (gamma * v) 2) from the perspective of TWIN B , t '= d / v and t = d / (gamma * v) If the Earth-star distance is d in the Earth frame (and if the spaceship reaches the star in the Earth frame), then t = d / v and t '= d / (gamma * v). In this case the ship clock slows down compared to the Earth clock. If event A occurs: 1) twin B reaches the star in the Earth frame 2) and the star reaches twin B in the ship frame, then the ship twin is " younger ". But the ship twin's perspective is not false. The perspective of the ship twin is true, but it is relative to the distance d / gamma. If the distance between the twins is equal to d / gamma: 1) from the perspective of TWIN A , t = d / (gamma * v) and t '= d / (gamma * gamma * v) 2) from the perspective of TWIN B , t ' = d / (gamma * v) and t = d / (gamma * gamma * v) If the star reaches the spaceship in the ship frame, it makes no sense to consider the perspective of the ship twin. (because it is a perspective relative to a distance other than d) 2) from the perspective of TWIN B , t ' = d / (gamma * v) and t = d / (gamma * gamma * v) The ship clock slows down as the spaceship reaches a star (or any point on the frame of the Earth) AND Earth's clock slows down as the twin remained on Earth reaches any point on the ship frame. In my opinion it is not important that the twins meet again, and accelerations are not important either.
Well, Paul that isn't the best reason since Bob can spend just one full day in acceleration, another two days for changing direction, and one more extra day for deceleration when reaching Earth. Total 4 days in a noninertial frame and the rest of the 10 years at an inertial frame where Lorentz's gamma factor is applicable. A better way to see it is to understand that Lorentz's equation deals with 3 physical magnitudes, space, time, and mass. From Bob's view, Alice just acquires apparent kinetic energy at the expense of losing mass-energy; total energy (mass + kinetic) is conserved. Alice doesn't increment its energy, so she doesn't get Lorentz's time dilation. Meanwhile, Bob does, he is the one that acquires kinetic energy from the impulse that the rocket gives; so, he is the only twin that get a time dilation... no more paradox. Regards
9:11 I disagree too ;-) When Alice and Bob disagree, that not like everday situations when men and women disagree ;-). They have clocks, and the disagreement is real. 2. You can abstract from Bobs U-turn and acceleration, the longer the journey is, the less the accelartion plays a role, but the more the dilation is. You can also imagine, that Bob ist moving along and they share information with a probe always flying exactly between them. 3. Spacetime doesn't solf the paradox, makes it even more complicated. After his Travel his rocket has its normal size again, but his age has changed. But above all, great work!
Frame hopping is not the resolution of the paradox - this should be clear from the length contraction explanation. When Bob and the star have a co-location event (in Bob's POV the star comes to him,) the star has only moved 2.4 ly - and this would be true whether or not Bob accelerates to head back to Earth. Bob's mission might be a fast fly-by of the star, after all. Bob would still be 4 years older to Alice's 5 years at the co-location event. And in fact, we could postulate a mid-way space station that Bob flys-by, and at the mid-way co-location event Bob will have aged 2 years to Alice's 2.5 years BECAUSE BOB perceives he has traveled only 1.2 ly not 1.5ly. In truth, the real paradox is that AT NO TIME DO BOB AND ALICE DISAGREE ABOUT WHO IS OLDER.
No. Your explanation attributes all of the (Bob - Alice) clock discrepancy to Bob's acceleration when he slows down to turn back. Ok, do this, keep Bob's speed the same and extend his trip so he's gone 100 years instead of 10 (from Alice's perspective). When Bob turns around, his acceleration is exactly the same as before. He comes home to Alice. Is he still 2 years younger? No. Paradox not explained. Also, you never accounted for Alice's being on earth's surface accelerating at 9.8 m/s/s the entire time of Bob's trip.
Great video dude. Just wondering…if you could catch up to the speed of light, and stay there, would you age at all? And, to prove that theory would you need a twin? What happens to our internal biological clocks, how do they know whether we are at the speed of light or, not?
You would still age normally, but you'd instantly be alone in the heat death of the universe or whatever end awaits us. From our point of view, you wouldn't age.
Just for consideration... Einstein introduced this scenario using clocks in his 1905 paper on SR. One clock stays on Earth while a second clock makes a round trip in space. In 1911, Langevin introduced a similar scenario, but using people in place of clocks, which is the origin of the twin version. Neither Einstein nor Langevin questioned the validity of the solution nor was it called a paradox. Indeed, if there was a paradox, it wasn't in how the problem was solved, it was in the result. It was incomprehensible to people (including physicists) of the day that one of the two twins essentially travelled into the future. And keep in mind, this was decades before any real experimental proof. There were critics with many arguments against SR. Some claimed that it couldn’t be true because it would require clock springs to become less springy or changes in biological chemistry. And some claimed that the math was wrong. That since velocity is relative, either twin should see the same thing. But these were criticisms, not paradoxes. And Einstein and others addressed them from time to time. It was only later that this twin scenario became the twin paradox. What was a simple and elegant example of time dilation and its consequences became a paradox to solve or explain. If velocity is relative, then we should be able to say that the twin on the spaceship is stationary and everything else is making the trip, and then it will be the earth twin who will age less! And then countless people with good intentions came up with countless reasons why you couldn’t do that. The space twin accelerated, not the earth twin. Acceleration has nothing to do with it, you don't need GR, the space twin changed reference frames, not the earth twin. The difference is that the space twin turned around, not the earth twin and you can see that the difference occurred there. If we do the math look there, poof, that is where the time went. Etc. But let’s be honest, even the best “explanations” have always seemed an inch short, even to us doing the explanation. I mean, this is a thought experiment, so why can’t we just say that it is the earth and the star doing all of the moving and the spaceship is stationary? Well it turns out that we absolutely can turn this problem around and consider that the spaceship is stationary and the earth and star are doing the moving. And we get the same result. The spaceship twin ages less than the earth twin. Say what? I thought that would have lead to the earth twin aging less? Follow… Two twins, one takes a spaceship at 0.6c from earth to a star 3 light years away and back. We will say that the spaceship moves to the right at 0.6c till it reaches the star, and then moves to the left at 0.6c till it reaches earth. You do the math and to the earth twin the trip took 10 years, while to the traveling twin it only took 8 years. Ok, but let’s turn it around so that the spaceship is stationary and the earth twin and star are doing the moving. Thus, the earth and star will move to the left at 0.6c till the star reaches the spaceship, and then move to the right at 0.6c till the earth reaches the spaceship. Sounds kind of crazy, but it’s not really that crazy. Instead of the earth and star, think of a rod 3 light years long, representing the distance between the earth and the star. Then the two scenarios look like this… Scenario 1: The rod is stationary and the spaceship is moving to the right at 0.6c. How long does it take the spaceship to traverse the rod from each perspective? Scenario 2: The spaceship is stationary and the rod is moving to the left at 0.6c. How long does it take the spaceship to traverse the rod from each perspective? Result Scenario 1: From the perspective of the stationary rod, it is 3 ly long, the ship is moving at 0.6c, it takes 5 years. From the perspective of the moving ship, the rod is 2.4 ly long (length contraction), the speed is 0.6c, it takes 4 years. Result Scenario 2: From the perspective of the moving rod, the rod is 3 ly long, the ship is moving at 0.6c, it takes 5 years. From the perspective of the stationary ship, the rod is 2.4 ly long (length contraction), the speed is 0.6c, it takes 4 years. Just double those numbers for a round trip, but you see, it doesn't matter who is moving and who is stationary, it is always the twin on the ship who ages less. At this point, you probably have two questions… 1. Why are there 62,343 explanations of why you can’t have the spaceship be the stationary actor, when it appears that you can? 2. Why did we get the same result in both scenarios? I thought that when two observers, A and B, pass each other, A will see B’s clock as running slow, and B will see A’s clock as running slow. So how come in the twin paradox, regardless of who is moving, the earth always sees the ship's clock being slow and the ship always sees the earth's clock as being fast? I will answer the second question. It has to do with what we mean by “see”. Imagine that A and B are passing each other. A pulls out a ruler and times how long it takes B to traverse it from end to end. B also times how long it takes to traverse A’s ruler from end to end. A’s time will be longer than B’s time, or put another way, A will see B’s clock running slower. Now imagine that B takes out a ruler and measures how long it takes A to traverse it from end to end. And A also times how long it takes to traverse B’s ruler from end to end. Now B’s time will be longer than A’s time. Thus, it is true that if A measures time with their ruler, then B’s clocks will appear slow. And likewise, if B measures time with their ruler, then A’s clock will appear slow. But you cannot compare these two scenarios because you are using different rulers. You have to choose which ruler you are going to use and stay with that. If A and B measure time with A's ruler, then A and B will agree that A's clocks are faster than B's clocks. If A and B measure time using B's ruler, then A and B will agree that B's clocks are faster than A's clocks. Now knowing this, think again about the twin paradox. Which ruler did we use in both scenarios? We used the distance from the earth to the star (the rod) which is always in earth’s (and the twin on earth) frame of reference. That is why you get the same result regardless of who you pick as stationary and who you pick as moving. Ultimately, this has to do with the relativity of simultaneity, but you really don't need to go there to see it in this example. As to question 1… This is obviously a very simple problem to solve simply, as I have shown, and it doesn’t depend on which twin is considered stationary and which is moving, and thus we don’t need to further complicated it by who is accelerating or who is changing frames or who turned around or when did the time difference occur, etc. We don't even need space diagrams. At some point, the name of the problem became “twin PARADOX” and at that point all of the focus went to explaining a paradox that doesn’t even exist. We have always been able to do the problem with the spaceship stationary. I just don’t think a lot of critics tried. They just assumed that this would cause the earth twin to age less. And some people unfortunately believed them (lol, including me) and set about to show why it MUST be the spaceship changing frames. Btw, this dawned on me while I was reviewing the barn pole paradox. In that paradox you basically have two lengths passing each other. Then I realized that in the twin paradox, you have a length (earth to star) passing a point (spaceship).
In your analysis of the Scenarios, you don't actually calculate how much time each twin thinks passes for the other twin: Result Scenario 1: From the perspective of the stationary rod, it is 3 ly long, the ship is moving at 0.6c, it takes 5 years. The ship twin is traveling at 0.6c during that time, therefore 4 years (0.8 * 5) have passed for the spaceship twin. Result Scenario 2: From the perspective of the stationary ship, the rod is 2.4 ly long (length contraction), the speed is 0.6c, it takes 4 years. The other twin is traveling at 0.6c during that time, therefore according to the stationary ship twin, 3.2 years (0.8 * 4) have passed for the other twin (NOT 5 years) They don't agree with each other.
So, as I understand it, we suspect the universe is flat. However, I don't think we know for sure? Assuming the universe is curved, and that if one theoretically travelled in one direction long enough to return back to your starting point, would this explanation still be valid? Would the giant cosmologically-scaled loop constitute a change in your inertial frame of reference?
1) Special theory of relativity is a local theory. Twin paradox works even in looped universe, as long as you travel back roughly along the same path. 2) Looped universe can be flat, and curved universe can be infinite. These two aspects are often confused but not really related. 3) Traveling along the loop, you'd arrive at the same place at different time, but it's not really the "twin paradox". 4) If there is a cosmological loop in the universe, it defines a preferred reference frame - the one where you age the same along the loop as without looping. Special relativity mostly still holds, but one of its premises is not true any more.
How do we actually know how slowing down processes for the fast moving one will lead to a state comparable physically to anyone who was aging at lower speed? Spacetime is no satisfactory answer
Dr. Sutter Thanks for interesting videos. kept my attention the entire time. Please do more examples of time dilation. Your the 3rd physicist I've seen on RUclips covering this. All 3 brilliant men having big fun with their elegant math. I questioned the other two guys and they don't respond, but you always ask us to request content. So please please do more on proving time dilation. So far I've found that either all of the examples used are wrong or there is simply no time dilation at all. So I really enjoy hearing all the examples from as many highly qualified scientist as I can. Today you broke the twin paradox by exclaiming it so well. Sissy slowed down on the way out? Sped up on the way back home? Out n back are equal distance & speed (equal time) so the effect is equal, leaving you with no paradox. Since no time dilation actually occurred this is as expected, no paradox. I missed that portion before, always thought the stop n turn around was just a way to claim one was right, not that it was also the explanation of why Bro. was 2 years younger than Sissy. This one doesn't really explain how or why time dilates anyhow, just assumes it's already understood and plays with a situations that might-could occur in that case. 😅
Wasn't this something theorized if something was traveling away from the sun at a percentage of light speed that time would only seem to slow down simply because your going so fast that it takes light longer to catch up with you but the second you start stopping the light will then catch you and equal which would leave you in the same time as your twin. It's like an illusion
I'm OK with the accelerating frame of reference causing the time dilation but the lost years being ascribed to Bob travelling a shorter distance? A distance to another place does not change does it?
I know this was about time. it does make me wonder, if there was a distant galactic body that was moving at much faster rates then here on earth, would are perspective on are distance to them be different. if we were say 20 light years away, could they in theory only be a few feet from us, from there perspective?
The claim that it is the acceleration that is the effective cause of the asymmetry is simply incorrect. The fundamental reason is because of effective Lorentz contraction. The Earth and Star has a length. The twin on earth always sees this distance as L, whether he considers that he is at rest or not. The traveller twin always sees this length as l/g where g is the Lorentz factor, whether he considers that he is at rest or not. Times synchronised to events for these lengths will always show that the traveller experiences the less proper time. The full calculation is on my General Relativity site. Unfortunately RUclips deletes posts with links in them. My about profile has the links. It’s the one that ends with /gr. The page is named “Twins Paradox” The claim that everything all slows down so no one notices, is also false, according to Special Relativity. SR functions analogous to Dr. Who and his TARDIS. He travels into the future at a different rate through time (space-time), say at 100 sec/sec, compared to another.
Ok lets apply this paradox to entangled particles. One is on a ship that accelerates close to light speed while the other remains on earth. The space ship travels 100 years in the future with its entangled particle on board. Would the particles remain entangled? Would the entangled particle of the future communicate with its entangled partner back on earth such that a change in spin affects the spin of the other? Would the entangled particle on the ship communicate backwards in time to its entangled partner on earth?
@@Kid_Naps That blows my mind then. That means that entangled particles can communicate both backward in time and in the future. WTAF? In fact imagine using the entangled particles as communication devices. The spin up and spin down nature could be used to communicate in binary code between someone in the future to someone in the past and vice versa.
@@sinebar You can't use entanglement to communicate. It could be used to generate the same random bit on 2 distant places, but that's not really all that useful.
_"Would the particles remain entangled?"_ - there is no theoretical reason why they would not stay entangled. _"Would the entangled particle ... communicate with its entangled partner back on earth such that a change in spin affects the spin of the other?"_ - Their spins would be correlated. _"Would the entangled particle on the ship communicate backwards in time to its entangled partner on earth?"_ - it's not possible to tell which communicates with which, just that their spins are correlated.
Entangled particles don't communicate at all. They just inform on each other. That's another nice feature about the Many Worlds interpretation. No backwards in time stuff, no non-local stuff.
How does that resolution work if both Alice and Bob are in two separate spaceships that are 10 light years apart with a star that's exactly at the halfway mark between Alice and Bob. Now when that star goes nova Alice and Bob begins accelerating towards one another at a fraction of the speed of light. When Alice and Bob meet and stop at the center of the distance (where the star exploded) who will be younger?
Paul, When you talk about time, how long is NOW? Is it 1 planck time? Does physical mass only exist in the NOW? You are looking at an apple on a table and lets say it is now 0 planck time. At +1 planck time the apple is still there but the apple at 0 planck time has moved into the past to -1 planck time. What happened to the physical mass when it moved from 0 planck time to -1 planck time? If mass disappears in the past then there is no past and traveling to the past you will see nothing (if time travel to the past became possible).
So If A&B both leave earth and accelerate away from eachother, they'll still be in the same inertial reference frame? When they return, everyone else is now older but A&B are still the same age right?
_"they'll still be in the same inertial reference frame?"_ - they would each be in their own reference frame. _"When they return, everyone else is now older but A&B are still the same age right?"_ - if they experience identical accelerations at identical times during their trips, then yes.
How would this play out in a closed universe, where Bob can remain in the same reference frame throughout? Can we conduct an experiment using relativity to rule out a closed universe?
So basically when physicists found contradictions in the Newtonian physics, Einstein explained them away by "lets just agree to disagree". But seriously tanks for the explanation now I finally understand the concept. Up until now all I understood was that it has something to do with acceleration but I didn't know that acceleration is not as relative as just speed (velocity?). I thought from Bob's perspective it's the Alice who's accelerating so that leaves us in the same place: why it's Bob that has to be younger?
Is that where the space-time bubble of the Alcubierre drive makes it possible to go faster than the speed of light with regard to the universe's frame of reference? Is there some constraint in relativity that will not allow Bob to arrive before he left in so doing, or does this require a new kind of physics?
Bob also expends energy to accelerate to 3/5ths the speed of light at the beginning of the journey. Doesn't this also contribute to the asymmetry (not just the turn around)?
Yes but he could travel a bit away from Earth, turn around, accelerate quickly to 3/5c and as he passes Earth, compare his age with Alice. He'd be pretty much still the same age. Also, during return, they can compare ages without slowing down, as long as they are close enough in space. The turning point is really the critical part of Twin paradox.
Ahhhh Time Dilation, Paul you are such a clever bugger in answering these topics, what did the robot in the book By Adams say, " I have a brain the size of a planet"
What would happen if instead of flying away bob would get into an orbit around the sun, and slow down (but maintain the same distance from the sun, pushing outwards against the suns gravity) until the earth overtakes him again. When he lands, who would be older? He changed the frame of reference and feeled the constant acceleration, but alice also constantly accelerated towards the sun just to stay in orbit. I guess alice would still be older because she had less acceleration to do, but I'm not sure.
You are explaining the paradox without answering the fundamental question. Leaving aside all acceleration, deceleration, twins etc, The actual question we should be asking here is, "Between two objects that are in constant relative motion, which one sees time dilation and length contraction". If we can't do an experiment to determine who is moving as per the theory, then the time dilation for only one of the objects disproves this very statement, as we can simply compare the clocks and see who moved !!
Acceleration is absolute. Nothing can travel faster than the speed of light making it an absolute reference marker. Clocks are instruments that measure motion in space. A slower running clock means it has experienced more space as the photon travels a greater distance from emiiter to receiver. The Hafel-Keating experiment proved that there is a preferred frame of reference. That motion is absolute. That time, being a measurement of acceleration, is absolute. The real irony is that relativists are the real flat Earthers. Unable to discard disproven theories when reality goes against their religious beliefs.
Acceleration, deceleration does not explain really why we need to adjust the clocks of GPS satellites every day - they do not speed up or slow down. Do not forget Earth has 1G acceleration with Alice on it and Bob can also fly in big circle with 1G acceleration. Bob will be still younger, why?
That's the thing they did "speed" up. Those satellites were built on earth and then launched into space. They didn't come into being at their orbital velocity those clocks were manufactured at earth speed (Ie the baseline, at least for earthly things like Alice and Bob). Also, a clock on earth adjusts relative to earth speed making it unnoticeable. Remember that term- RELATIVE. But a clock on A salellite is adjusting to the satellites speed. This difference (earth clock vs gps satellites clock) is very minor. The only reason it's noticable is because of how precise the calculations for gps need to be. And gravity effects the time dilation more that time (kinetic). For kinetic dilation general relativity predicts that gos clocks run 6 micro seconds (not milliseconds) slower a day how ever; Gravitational Time dilation predicts they gain 45 micro seconds a day (time goes slower in gravitational fields and we are closer to earth that gps clocks). That equals 38 micro seconds gained every day. Considering that a mechanical clock can lose anywhere from 1- 10 seconds a day the effect is quite small. So quite frankly even if you compared one of these atomic clocks to your wrist watch the gps satellite's clock would probably be more accurate. The only reason they adjust them so frequently it because gps requires extreme amounts of accuracy.
I'm still not convinced. When Bob flies away from Alice at high speed, the distance he travels seems shorter than it does to Alice. OK, but why couldn't we argue the equivalent for Alice? If she is seen to be travelling away from /Bob/ at that high speed, why shouldn't /she/ experience it as a shorter journey?? I don't see the asymmetry there. It still needs to be explained.
@silverrahul Yes, thank you. I am familiar with that explanation. I don't understand /why/ it should be an explanation, but I do understand what it says. In response, I could discuss how calculations proceed in the case of two travellers who cross paths out in space, as there is no acceleration there, but there is at least a jump from one inertial frame to another. But that doesn't help to explain why Alice couldn't be construed as the traveller throughout, switching from one frame to another at the end of the outward journey. BUT - let's reconstruct the experiment, so that Bob just takes the outward journey, at velocity v, and at some point along the way he radios back to Alice to say what time his watch reads, and how far he has travelled, measured in his frame. Knowing the speed of transmission, c, Alice can work out that he transmitted his message at s/c ago, and that since the distance s=t/v, has been travelling for the time s/v. On the other hand, Bob measures the time of travel as only t', which is less than t. That's the time dilation, which Alice is able to observe via these calculations. Each of the participants remained in their own single non-inertial frame throughout. And yet what we have just seen Bob go through, from Alice's frame, cannot be equally correct regarding what Alice has gone through from Bob's frame. There is supposed to be REAL time-dilation, with Bob getting to Alpha Centauri in much less time than Alice can measure and calculate. Is the claim that we can flip this around and say that from Bob's frame it is equally true that Alice experiences a shorter duration for the same journey? Actually, let's firm up the story a bit, by saying that Bob travels past Alpha Centauri, and sends his message at the moment he passes it. The physical journey is then fixed for both. Would Bob calculate, from his t' and the value of c, that Alice endured /less/ time than t' while the journey took place? I don't see that as being symmetrical.
@silverrahul thank you for that. You said "yes, it is not 100% symmetrical because one thing that is not symmetric is that , alpha centauri is at rest relative to Alice, but it is in motion relative to Bob." Let's ditch Alpha Centauri, then, and just regard Bob and Alice as uniformly moving away from each other, with no external reference points to worry about. Symmetry demands that we treat them equivalently. But how can this be, if Bob's calculation of Alice's time duration is less than her calculation of his? If you flip that around, you get exactly the opposite set of figures. How do you resolve this 'contradiction'?
@silverrahul I think I know the answer. It doesn't get resolved? Just as with metre rulers passing each other in space, each seeing the other one as shorter, the same reduction works in both frames. Correct?
@silverrahul Understood. I was just citing the supposed 'contradiction', that constitutes an alleged paradox, and then acknowledging that there is nothing to resolve. As you say, the measurment of time and length is truly relative to frame. I get it. No, I was using 'see' in a sloppy way, and I do understand what you are saying there. Thanks.
@silverrahul I did the work myself. You helped by pointing out that it was correct to describe Bob's measure of Alice's time interval as being even shorter than his own. I eventually realised that the asymmetry that implied was not to be resolved, because it was what relativity entails. In Alice's frame, Bob's time really is shorter than hers. In Bob's frame, Alice's time really is shorter than his. That is all internally consistent, and just exemplifies what relativity is all about.
OK, so Alice aged 10 years, Bob 8 years. What I really want to know is why did I age 20 years watching this? But seriously, other physicists have (supposedly, as I am incapable of this judgment) proven that neither general relativity nor acceleration are necessary to explain the effect. So I remain as baffled as I was 20 years ago.
Space and time are seperate frames of reference. Clocks are instruments that acceleration in Space. The slower running clock is the one that experiences more space. Both experience the same amount time. Unless the observer is in cryostasis, there is no relevant age difference.
One additional comment... The twin paradox is probably the most cited example of the consequences of the special theory of relativity. The story starts with two twins. One twin travels on a spaceship to a distant star and when he returns, he finds that he has aged less than the twin who stayed on earth. The age difference is attributed to the time dilation resulting from the fact that the travelling twin was going at some velocity to and from the star, while the twin on earth was stationary. A suspicious student will undoubtedly ask “But since velocity is relative, why can’t it be the twin on earth who was moving, and the travelling twin was stationary? Then the twin on earth will age less, correct?” To avoid this contradiction, the teacher will offer an explanation along the lines that the twin on the spaceship is different because he has changed inertial reference frames and thus the problem is asymmetric, and if you make the traveling twin stationary, it is a different problem. And these explanations always fall an inch short of satisfying the student, or in many cases, of even satisfying the teacher, and so a crusade has occurred that has spawned countless explanations involving acceleration, more spaceships, lines of simultaneity, space diagrams, etc. And they all fall an inch short. Plus, they have spawned many odd and wrong interpretations of where the difference in time went or when it occurred. I came across this situation and asked myself “What the heck is going on?” Keep in mind, that at the time, my own explanation was along the lines that since the traveling twin was making the moves it only made sense that he was special and that the time dilation only applied to him. But after weeks of reviewing countless explanations and interpretations, I couldn’t find one that explains why this scenario is asymmetric and yet the explanation not come up an inch short. Remember, this is a thought experiment, we have a lot of leeway. Then I realized that the problem is not asymmetric, it is symmetric, and if let the traveling twin be stationary and have the earth and star do all the moves, we get the same result. The traveling twin ages less. As I explained in the other comment, this is because the distance used, from the earth to the star, is always in the earth’s frame of reference, and because this is what we are using to measure time, regardless of whether it is the earth and star moving or the spaceship moving, we get the same result. The earth twin ages more, the traveling twin ages less. I don't know if this twin paradox will ever go back to the simple and elegant example of SR it started as. Where the only paradox is the amazing result. But It has for me. It does amaze me how much we have been conditioned that we must explain that this problem is asymmetric, when in fact it was always symmetric. We were so afraid of a paradox, that didn't even exist. That also explains why explanations that this problem is asymmetric, even the good ones, always come up an inch short. Because it isn't.
The only problem with you analysis is that it is completely wrong. Newton's first law of motion states that an object at rest remains at rest unless acted upon by an outside force. The spaceship twin is at rest within the ship just as much as the Earthbound twin. Therefore, they both age the same amount. This has been proven by many NASA experiments showing similar heart rates for stationary, takeoff and in-flight segments. This whole issue with time-dilation started because Einstone era scientists did not understand basic electrical and wave properties. Since physicists are unable to easily solve this simple problem, its time to throw out their physics books and go get a degree in applied science first. Then they will see just how wrong they have been about everything.
I am not sure how you are reasoning. Let's formulate the twin's paradox as a completely symmetric one. There are two identical spaceships, one with Alice in it, and one with Bob in it. There is nothing else in the universe, no Earth, no stars, nothing. The two spaceships have a high relative speed wrt. one another. At some point of time, the pass each other, at which point each resets their clock to zero. Then they travel away from each other. After a while, they pass each other again, which gives them the opportunity to compare their clocks. They had a high relative speed (almost) the whole time. Therefore SR says that according to Alice, less time will have passed on Bob's clock, and according to Bob, less time will have passed on Alice's clock. They cannot be both correct when they meet up again. So, who is correct? When the situation is symmetric, it is not possible to tell which is the "stay-at-home" twin, and which is the "travelling" twin. The only thing that breaks the symmetry, and therefore resolves the paradox, is the acceleration. And that is exactly how SR resolves the paradox: during Bob's acceleration, Alice's clock appears to run faster to him, not slower.
@@renedekker9806 It has nothing to do with acceleration, and there are scenarios that prove this using 3 ships all traveling at a constant velocity, passing each other and comparing clocks. It is all based on the relativity of simultaneity. The error most people make with your example of bob and alice going in opposite directions and then bob turns around and catches back up with alice is with one simple piece. At what time in alice's frame did he turn around? Most people assume that the time at which bob turns around is the same for both bob and alice, but it is not. They are in two different frames of reference. If the time difference was due to acceleration, then whether bob goes for 1 light year and turns around or for 20 light years and turns around, the time difference would be the same. But it will obviously be 20 times more in the second example because he has travelled at constant speed for 20 times longer.
@@roberthansen9876 _"At what time in alice's frame did he turn around?"_ - Did I say Bob turned around in the symmetric scenario? Let me check... no I didn't. Why does it matter which of them turns around? What makes the turn-around different from the normal travel? Why does Alice's clock appears to jump forward during Bob's turn-around? Why does Bob's clock not appear to jump forward according to Alice? _"it will obviously be 20 times more in the second example because he has travelled at constant speed for 20 times longer"_ - and therefore he is 20 times further away. The time difference depends both on the amount of acceleration and the distance between them. Acceleration in SR is analogous to rotation in 3D. When you are sitting on a merry-go-round and see the world revolve around you. From your perspective, the trees that are closest to the merry-go-round move slowly around you. The trees that are further away move faster. The apparent speed of the trees depends both on your rotation speed and on how far away the trees around. Acceleration in SR is similar. A rotation, not in space, but in time. The apparent rate of other clocks depends both on the size of your acceleration and on how far away the clocks are.
@@renedekker9806 You are getting closer, the relativity of simultaneity depends on time and distance and those two are related by velocity, not acceleration. If Alice laid out a long series of synchronized clocks in her reference frame for bob to pass, as he passes each one, he will see that they are getting further and further behind his own. GPS satellites exhibit the same thing. No one has to accelerate at all, the time dilation occurs in a continuous fashion. Consider the alice on earth example. Bob takes off in the space ship, travels for a year and the returns. He ages less than alice. You say it occurs during the acceleration. But say bob doesn't land back on earth and just flies by and glances at alice's clock as he passes. It will show the same difference as it would have if he decelerated and landed.
@silverrahul Special Relativity don't have direction factor! It speaks of speed than velocity. In light clocks, the measurement of speed of light when it travel in both directions! No velocity of light in one direction done up to now. Therefore Relativity speaks only of speed of light!
You make the assumption that acceleration is absolute . Bob could easily claim that Alice is the one accelerating. You also assume that acceleration can be detected that requires Bob and Alice to have prior knowledge of physics and not rely on observations alone and the theory clearly says observers. As for the experiments that can be done : lets take Einstein s own experiment he used , that of the 2 clocks . One at the equator and one at the pole. He stated the equator clock as the one being slower. He picked the equator clock as the one moving because he knew about earths rotation, not by observation and thus violating his own theory. If sub clocks with observers , neither can observe or detect If they are accelerating or even moving. He even said to support his theory that the 2 statements “ the earth is moving and the sun is stationary or the sun is moving and the earth is stationary are equally valid” So what experiment can be done to show that its the equator clock thats moving?
Synchronized clocks measure relative differences in motion and/or gravity. Whichever clock is slower is in a higher gravity field or is traveling faster.
@@stewiesaidthat the problem is determining which clock is the one moving slower . Both can claim its the other. So from their perspective the others clock that moves slow and theirs is the faster. But a clock cannot be running slower and faster at the same time. Hence the twin paradox.
@@nickmerix2900 from a single point of view, an observer cannot tell which is running slower but all the twins have to do is compare the readouts on their clocks.
@@stewiesaidthat thats circular logic . You are using the supposed outcome of a theory to prove the theory. You assume the twins will agree about their clocks . I have a broken clock in my house but i cant claim that time stoped . One can say the twins compare their clocks and they read the same. What then ? By your reasoning if i have a clock that runs slow in one room and after a year i notice that it is slower than the one in another room i can then claim that the first clock was moving thats why its slower.
Has science fallen this far down the rabbit hole that they can't even solve a simple problem? If you don't understand why clocks in motion run slower, you will never ever figure out the real solution.
The only, place where time has both no meaning & stationary simultaneously infinitely very quick invisible too? WOW, what could it be, a bird a plane a pulse a beat a throb or a plain old pain in the neck? Gravity has its' own rhythm: it's been roundly about somewhat longer than human understanding, I think! {;}
5:53 _"If you're in an airplane & you're crusing along at attitude & you toss a ball...You can't tell that you're moving because you throw the ball & u catch it in ur hand....How do u know that ur staying still & the Earth's not rotating underneath you? You don't! There's no physical experiement that you can perform. You're relying on your intuition (that the earth is large & probably isn't moving) to tell you that you're moving. But there's no physics reason why that has to be true."_ WRONG! If we steer our car towards the setting sun & accelerate then, from our frame of reference, we're stationary & Sun accelerates towards us. Thus our foot on the accelerator pedal has moved a star. (& the galaxies behind Sun) This is ABSURD. Our foot cannot move galaxies
This is why the heliocentric theory took so long to be accepted. The sun obviously moves through the sky. There's no obvious sign that the observer is on the surface of a spinning sphere that is orbiting the sun, but there are experiments and observations you can do to confirm it. There are experiments and observations that confirm time dilation.
@@e.claytonrowe4394 Ok but then how do you explain our feet moving galaxies? We put our foot down & the setting sun moves towards us? It can't possibly be true.
@@alwaysdisputin9930 he's not wrong. His statement was for a constant speed (IE cruising). Not accelerating or decelerating. So your foot wouldn't be on the peddle and the car would be in cruise control (or at the very least maintaining pressure on the peddle for a constant speed). Yes we can feel acceleration and test for it as well as deceleration but there is no way to prove/test that we are the ones moving vs the earth when at a constant speed. If I accelerate 0-60 in 5 seconds I will feel my body slam back into the seat, if i have a glass of water it will spill, if i toss a ball up it will fly into the back seat, but if I've been going 95 for an hour the only thing that tells me I'm moving is the logic. Let's say you're driving your car to the sun but this time you have a passenger. This passenger is an amnesiac of sorts. They can remember how to talk and are intelligent but have no frame of reference for anything. They don't remember starting the trip and as far as they are concerned the sun is coming towards them. You tell this passenger that they are wrong but they demand proof for your assertion because to them, they feel stationary. Now remember Paul said that you can't test for movement at an inertial frame of reference (constant speed) specifically. So for him to be wrong you would have to prove to this amnesiac passenger without decelerating or accelerating. This means no moving the pedal up or down. How do you do it? How do you prove you're moving. You can know something to be true but that's a whole lot different than proving it. You moving the peddles takes you out of an inertial frame of reference. Which invalided your statement because Paul specifically states that you can do physical tests with in an inertial frame of reference. Also you second comment isn't really relevant. Like he said you can logically know that we aren't the ones moving the stars but that's a whole lot different from physically proving it in a test. That was his point.
Standing still on Earth is not standing still in space. The Earth undergoes a 25,000 mile rotation every 24 hours while orbiting the Sun at 1.5 million miles per planetary rotation and riding the turn of the galaxy at 1 million miles per planetary rotation.
He acknowledges this at the beginning and then says to ignore it. This is reasonable since the acceleration is small compared to the acceleration of the space ship.
@@johnbennett1465 At the beginning he says that standing still, you are moving at the speed of light. He does NOT then say that standing still on Earth is not standing still in space. You are mistaken.
@@cajunqueen5125 Timestamp? I could just watch it again and try to guess just exactly where you think Paul says the Earth is moving through space so you have to step off the Earth to experience a stationary position, or you could just tell me when to look.
@@ZeroOskul true he doesn't say it right there, but he does say it a little later. The speed that Earth is traveling is fast by human standards, but is small by comparison to the speed of light. If he added a caveat every time he talked about "stationary" it would add 10-20% to the run time without adding anything to people's understanding.
I love how a 30 year old gets grey and crows feet but a 28 year old body slams a table in half 🙃 haha
this was awesome, the best explanation i've ever seen.
The end of this, and the "where did the two years go" part.....TRULY makes my head hurt. :) Great video, as always, Dr Sutter!
The 2 years didn't go anywhere, Bob traveled them... but as SPACE instead of TIME. I know that sounds crazy but if you do a little pythagoras: Bob traveled 6 light years and 8 years time. Time and space are the same, but orthogonal. So to add them you take the square root of the squares. What's the square root of 6^2 and 8^2? 10. He traveled all ten years, the same distance as Alice.
11:02 I think when you drop something when you are in a spaceship in space, it doesn't fall at all, it just floats.
😅😁😅
Alice is in fast-forward from Bob's perspective when he's coming back
but
How Bob looks to Alice in that period of time?
I subscribed to this channel based on this presentation. Looking forward to following.
The way I try to intuitively understand it is to think of a motor you can't shut off that only goes one speed, attached to 2 gears, space gear and time gear. If the space gear isn't moving or moving very slowly the time gear gets all the power, as the space gear gets faster it takes power away from the time gear. I know it's a crappy analogy, but it's the only way I can wrap my head around it. Not the gear analogy specifically but just the general concept that everything has the same energy, but some things put all that energy into the time dimension. If they experience more resistance going through space that energy has to go somewhere. Doesn't really explain contraction or expansion of space. I'm sure that also has to do with the seasaw of spacetime but I probably need some mushrooms to figure out where to go from there. Fortunately trying to understand this is just a way I torture myself and has no real bearing on my life.
Thank-you for yet another excellent video.
What happens if Bob races away on a spacecraft for x years and then goes through a wormhole that puts him back at his starting point? What happens to the twins paradox then? Or maybe if the universe is finite and loops back on itself (like the surface of the earth)?
Hey, that's a pachinko game back there! Much respect!
Is this why Alice chases a rabbit with a pocket watch? I don't remember a Bob though.
This is one of the more interesting and better explained theories of the twin paradox. Thank you.
So I just watched the movie Lightyear and they mentioned space dilation and it sent me into a tailspin.
For years, I have watched superhero shoes where the hero moves so fast that time "slows down" and he can complete tasks that normally take a person minutes to in mere seconds.
So when I here that going super fast into space makes time on earth "Move faster" than time for you, I struggled to wrap my head around it. I have spent the last 4 hours trying to get a baseline understanding of time dilation and nothing was clicking for me until I came across your video. Thank you so much for being so patient and engaging and such a great educator on this subject.
Think of it like this. Consider your driving North at 100mph with the cruise control set. That means your moving 0 mph in the east direction. Then you start turning east. Your still moving at a constant 100mph but some of your momentum is north and the rest is east. To an observer that can only see your northern movement you are decelerating. For eastern observer your accelerating. Once your going Northeast your moving 50mph in North direction and 50 mph in the East direction. Both observer will agree your speed is 50mph. For you your speed is 100mph. This is normal relative motion in classical physics.
Spacetime has 4 Dimensions (1 temporal and 3 spacial). Let’s call the Temporal Dimension North and Spacial Dimensions East. In spacetime your never at rest and move at a constant speed which is the Speed of Light (C).
In everyday experience we only move at a fraction of C in any spacial direction. The rest of our momentum is in time. In spacetime we are mostly moving North (time) and slightly east (space). The more we move east (accelerate) in spacetime the slower we move North. So in 4d spacetime we are moving slower in time.
In spacetime the 3 spacial Dimension and 1 temporal dimension depend on the frame of reference your in. So observer A will see Observer B moving slower in time. Observer B will see time as normal but space is contracting.
There no magic to any of this It’s just how we perceive things.
Hi Paul, great video!
I think this :
the two twins (TWIN A and TWIN B) occupy the same position at instants of time
t = t '= 0 , and then move away.
( They both move with uniform linear speed v wrt. each other )
I believe that the distance between the twins in both perspectives is always the same.
For example, if the distance between the twins is equal to d:
1) from the perspective of TWIN A , t = d / v and t '= d / (gamma * v)
2) from the perspective of TWIN B , t '= d / v and t = d / (gamma * v)
If the Earth-star distance is d in the Earth frame (and if the spaceship reaches the star in the Earth frame),
then t = d / v and t '= d / (gamma * v).
In this case the ship clock slows down compared to the Earth clock.
If event A occurs:
1) twin B reaches the star in the Earth frame
2) and the star reaches twin B in the ship frame,
then the ship twin is " younger ".
But the ship twin's perspective is not false.
The perspective of the ship twin is true, but it is relative to the distance d / gamma.
If the distance between the twins is equal to d / gamma:
1) from the perspective of TWIN A ,
t = d / (gamma * v)
and t '= d / (gamma * gamma * v)
2) from the perspective of TWIN B ,
t ' = d / (gamma * v)
and t = d / (gamma * gamma * v)
If the star reaches the spaceship in the ship frame, it makes no sense to consider the perspective of the ship twin. (because it is a perspective relative to a distance other than d)
2) from the perspective of TWIN B ,
t ' = d / (gamma * v)
and t = d / (gamma * gamma * v)
The ship clock slows down as the spaceship reaches a star (or any point on the frame of the Earth)
AND
Earth's clock slows down as the twin remained on Earth reaches any point on the ship frame.
In my opinion it is not important that the twins meet again, and accelerations are not important either.
Well, Paul that isn't the best reason since Bob can spend just one full day in acceleration, another two days for changing direction, and one more extra day for deceleration when reaching Earth. Total 4 days in a noninertial frame and the rest of the 10 years at an inertial frame where Lorentz's gamma factor is applicable. A better way to see it is to understand that Lorentz's equation deals with 3 physical magnitudes, space, time, and mass. From Bob's view, Alice just acquires apparent kinetic energy at the expense of losing mass-energy; total energy (mass + kinetic) is conserved. Alice doesn't increment its energy, so she doesn't get Lorentz's time dilation. Meanwhile, Bob does, he is the one that acquires kinetic energy from the impulse that the rocket gives; so, he is the only twin that get a time dilation... no more paradox. Regards
Yes, that was total bullshit. ;-)
9:11 I disagree too ;-) When Alice and Bob disagree, that not like everday situations when men and women disagree ;-). They have clocks, and the disagreement is real.
2. You can abstract from Bobs U-turn and acceleration, the longer the journey is, the less the accelartion plays a role, but the more the dilation is. You can also imagine, that Bob ist moving along and they share information with a probe always flying exactly between them.
3. Spacetime doesn't solf the paradox, makes it even more complicated. After his Travel his rocket has its normal size again, but his age has changed.
But above all, great work!
Frame hopping is not the resolution of the paradox - this should be clear from the length contraction explanation. When Bob and the star have a co-location event (in Bob's POV the star comes to him,) the star has only moved 2.4 ly - and this would be true whether or not Bob accelerates to head back to Earth. Bob's mission might be a fast fly-by of the star, after all. Bob would still be 4 years older to Alice's 5 years at the co-location event. And in fact, we could postulate a mid-way space station that Bob flys-by, and at the mid-way co-location event Bob will have aged 2 years to Alice's 2.5 years BECAUSE BOB perceives he has traveled only 1.2 ly not 1.5ly. In truth, the real paradox is that AT NO TIME DO BOB AND ALICE DISAGREE ABOUT WHO IS OLDER.
No. Your explanation attributes all of the (Bob - Alice) clock discrepancy to Bob's acceleration when he slows down to turn back. Ok, do this, keep Bob's speed the same and extend his trip so he's gone 100 years instead of 10 (from Alice's perspective). When Bob turns around, his acceleration is exactly the same as before. He comes home to Alice. Is he still 2 years younger? No. Paradox not explained. Also, you never accounted for Alice's being on earth's surface accelerating at 9.8 m/s/s the entire time of Bob's trip.
Great video dude. Just wondering…if you could catch up to the speed of light, and stay there, would you age at all? And, to prove that theory would you need a twin? What happens to our internal biological clocks, how do they know whether we are at the speed of light or, not?
You would still age normally, but you'd instantly be alone in the heat death of the universe or whatever end awaits us. From our point of view, you wouldn't age.
Awesome; I listened to the podcast episode 3x, and then loved this video version too.
Thank you Dr.Paul.
Just for consideration...
Einstein introduced this scenario using clocks in his 1905 paper on SR. One clock stays on Earth while a second clock makes a round trip in space. In 1911, Langevin introduced a similar scenario, but using people in place of clocks, which is the origin of the twin version. Neither Einstein nor Langevin questioned the validity of the solution nor was it called a paradox. Indeed, if there was a paradox, it wasn't in how the problem was solved, it was in the result. It was incomprehensible to people (including physicists) of the day that one of the two twins essentially travelled into the future. And keep in mind, this was decades before any real experimental proof. There were critics with many arguments against SR. Some claimed that it couldn’t be true because it would require clock springs to become less springy or changes in biological chemistry. And some claimed that the math was wrong. That since velocity is relative, either twin should see the same thing. But these were criticisms, not paradoxes. And Einstein and others addressed them from time to time.
It was only later that this twin scenario became the twin paradox. What was a simple and elegant example of time dilation and its consequences became a paradox to solve or explain. If velocity is relative, then we should be able to say that the twin on the spaceship is stationary and everything else is making the trip, and then it will be the earth twin who will age less! And then countless people with good intentions came up with countless reasons why you couldn’t do that. The space twin accelerated, not the earth twin. Acceleration has nothing to do with it, you don't need GR, the space twin changed reference frames, not the earth twin. The difference is that the space twin turned around, not the earth twin and you can see that the difference occurred there. If we do the math look there, poof, that is where the time went. Etc.
But let’s be honest, even the best “explanations” have always seemed an inch short, even to us doing the explanation. I mean, this is a thought experiment, so why can’t we just say that it is the earth and the star doing all of the moving and the spaceship is stationary? Well it turns out that we absolutely can turn this problem around and consider that the spaceship is stationary and the earth and star are doing the moving. And we get the same result. The spaceship twin ages less than the earth twin. Say what? I thought that would have lead to the earth twin aging less?
Follow…
Two twins, one takes a spaceship at 0.6c from earth to a star 3 light years away and back. We will say that the spaceship moves to the right at 0.6c till it reaches the star, and then moves to the left at 0.6c till it reaches earth. You do the math and to the earth twin the trip took 10 years, while to the traveling twin it only took 8 years. Ok, but let’s turn it around so that the spaceship is stationary and the earth twin and star are doing the moving. Thus, the earth and star will move to the left at 0.6c till the star reaches the spaceship, and then move to the right at 0.6c till the earth reaches the spaceship. Sounds kind of crazy, but it’s not really that crazy. Instead of the earth and star, think of a rod 3 light years long, representing the distance between the earth and the star. Then the two scenarios look like this…
Scenario 1: The rod is stationary and the spaceship is moving to the right at 0.6c. How long does it take the spaceship to traverse the rod from each perspective?
Scenario 2: The spaceship is stationary and the rod is moving to the left at 0.6c. How long does it take the spaceship to traverse the rod from each perspective?
Result Scenario 1: From the perspective of the stationary rod, it is 3 ly long, the ship is moving at 0.6c, it takes 5 years. From the perspective of the moving ship, the rod is 2.4 ly long (length contraction), the speed is 0.6c, it takes 4 years.
Result Scenario 2: From the perspective of the moving rod, the rod is 3 ly long, the ship is moving at 0.6c, it takes 5 years. From the perspective of the stationary ship, the rod is 2.4 ly long (length contraction), the speed is 0.6c, it takes 4 years.
Just double those numbers for a round trip, but you see, it doesn't matter who is moving and who is stationary, it is always the twin on the ship who ages less.
At this point, you probably have two questions…
1. Why are there 62,343 explanations of why you can’t have the spaceship be the stationary actor, when it appears that you can?
2. Why did we get the same result in both scenarios? I thought that when two observers, A and B, pass each other, A will see B’s clock as running slow, and B will see A’s clock as running slow. So how come in the twin paradox, regardless of who is moving, the earth always sees the ship's clock being slow and the ship always sees the earth's clock as being fast?
I will answer the second question.
It has to do with what we mean by “see”. Imagine that A and B are passing each other. A pulls out a ruler and times how long it takes B to traverse it from end to end. B also times how long it takes to traverse A’s ruler from end to end. A’s time will be longer than B’s time, or put another way, A will see B’s clock running slower. Now imagine that B takes out a ruler and measures how long it takes A to traverse it from end to end. And A also times how long it takes to traverse B’s ruler from end to end. Now B’s time will be longer than A’s time.
Thus, it is true that if A measures time with their ruler, then B’s clocks will appear slow. And likewise, if B measures time with their ruler, then A’s clock will appear slow. But you cannot compare these two scenarios because you are using different rulers. You have to choose which ruler you are going to use and stay with that. If A and B measure time with A's ruler, then A and B will agree that A's clocks are faster than B's clocks. If A and B measure time using B's ruler, then A and B will agree that B's clocks are faster than A's clocks.
Now knowing this, think again about the twin paradox. Which ruler did we use in both scenarios? We used the distance from the earth to the star (the rod) which is always in earth’s (and the twin on earth) frame of reference. That is why you get the same result regardless of who you pick as stationary and who you pick as moving. Ultimately, this has to do with the relativity of simultaneity, but you really don't need to go there to see it in this example.
As to question 1…
This is obviously a very simple problem to solve simply, as I have shown, and it doesn’t depend on which twin is considered stationary and which is moving, and thus we don’t need to further complicated it by who is accelerating or who is changing frames or who turned around or when did the time difference occur, etc. We don't even need space diagrams.
At some point, the name of the problem became “twin PARADOX” and at that point all of the focus went to explaining a paradox that doesn’t even exist. We have always been able to do the problem with the spaceship stationary. I just don’t think a lot of critics tried. They just assumed that this would cause the earth twin to age less. And some people unfortunately believed them (lol, including me) and set about to show why it MUST be the spaceship changing frames.
Btw, this dawned on me while I was reviewing the barn pole paradox. In that paradox you basically have two lengths passing each other. Then I realized that in the twin paradox, you have a length (earth to star) passing a point (spaceship).
In your analysis of the Scenarios, you don't actually calculate how much time each twin thinks passes for the other twin:
Result Scenario 1: From the perspective of the stationary rod, it is 3 ly long, the ship is moving at 0.6c, it takes 5 years. The ship twin is traveling at 0.6c during that time, therefore 4 years (0.8 * 5) have passed for the spaceship twin.
Result Scenario 2: From the perspective of the stationary ship, the rod is 2.4 ly long (length contraction), the speed is 0.6c, it takes 4 years. The other twin is traveling at 0.6c during that time, therefore according to the stationary ship twin, 3.2 years (0.8 * 4) have passed for the other twin (NOT 5 years)
They don't agree with each other.
This was the plot to a Heinlein novel, Time for the Stars, written in 1956
The Universe is a really complicated when trying to understand more in depth.
So, as I understand it, we suspect the universe is flat. However, I don't think we know for sure? Assuming the universe is curved, and that if one theoretically travelled in one direction long enough to return back to your starting point, would this explanation still be valid? Would the giant cosmologically-scaled loop constitute a change in your inertial frame of reference?
1) Special theory of relativity is a local theory. Twin paradox works even in looped universe, as long as you travel back roughly along the same path.
2) Looped universe can be flat, and curved universe can be infinite. These two aspects are often confused but not really related.
3) Traveling along the loop, you'd arrive at the same place at different time, but it's not really the "twin paradox".
4) If there is a cosmological loop in the universe, it defines a preferred reference frame - the one where you age the same along the loop as without looping. Special relativity mostly still holds, but one of its premises is not true any more.
Thank you. Many pop science explanations forget to mention Bob's journey is shortened by distance.
Time is a perception. It is the distance in which an action takes to occur.
How do we actually know how slowing down processes for the fast moving one will lead to a state comparable physically to anyone who was aging at lower speed? Spacetime is no satisfactory answer
Yes, the best explanation for me so far...Great job, saying hi from Czech rep.
So, it's Alice and the earth that have "traveled into the future" while Bob has been lagging behind...in his own frame of reference.
Right?
Dr. Sutter
Thanks for interesting videos. kept my attention the entire time.
Please do more examples of time dilation.
Your the 3rd physicist I've seen on RUclips covering this. All 3 brilliant men having big fun with their elegant math. I questioned the other two guys and they don't respond, but you always ask us to request content. So please please do more on proving time dilation.
So far I've found that either all of the examples used are wrong or there is simply no time dilation at all. So I really enjoy hearing all the examples from as many highly qualified scientist as I can.
Today you broke the twin paradox by exclaiming it so well. Sissy slowed down on the way out? Sped up on the way back home?
Out n back are equal distance & speed (equal time) so the effect is equal, leaving you with no paradox. Since no time dilation actually occurred this is as expected, no paradox.
I missed that portion before, always thought the stop n turn around was just a way to claim one was right, not that it was also the explanation of why Bro. was 2 years younger than Sissy. This one doesn't really explain how or why time dilates anyhow, just assumes it's already understood and plays with a situations that might-could occur in that case. 😅
What if instead of travelling on an out and back journey he travels in a circular trajectory what happens then?
Then he's accelerating all the way... sideways. I'm 99% sure the result is the same time dilation as in the standard case.
Wasn't this something theorized if something was traveling away from the sun at a percentage of light speed that time would only seem to slow down simply because your going so fast that it takes light longer to catch up with you but the second you start stopping the light will then catch you and equal which would leave you in the same time as your twin. It's like an illusion
I'm OK with the accelerating frame of reference causing the time dilation but the lost years being ascribed to Bob travelling a shorter distance? A distance to another place does not change does it?
CORRECTION : Got it ! my mistake - it's a shorter distance in curved space time
Just disovered your videos, love them and hilarious usage of stock video!
I know this was about time. it does make me wonder, if there was a distant galactic body that was moving at much faster rates then here on earth, would are perspective on are distance to them be different.
if we were say 20 light years away, could they in theory only be a few feet from us, from there perspective?
The claim that it is the acceleration that is the effective cause of the asymmetry is simply incorrect. The fundamental reason is because of effective Lorentz contraction. The Earth and Star has a length. The twin on earth always sees this distance as L, whether he considers that he is at rest or not. The traveller twin always sees this length as l/g where g is the Lorentz factor, whether he considers that he is at rest or not. Times synchronised to events for these lengths will always show that the traveller experiences the less proper time.
The full calculation is on my General Relativity site. Unfortunately RUclips deletes posts with links in them. My about profile has the links. It’s the one that ends with /gr. The page is named “Twins Paradox”
The claim that everything all slows down so no one notices, is also false, according to Special Relativity. SR functions analogous to Dr. Who and his TARDIS. He travels into the future at a different rate through time (space-time), say at 100 sec/sec, compared to another.
Ok lets apply this paradox to entangled particles. One is on a ship that accelerates close to light speed while the other remains on earth. The space ship travels 100 years in the future with its entangled particle on board. Would the particles remain entangled? Would the entangled particle of the future communicate with its entangled partner back on earth such that a change in spin affects the spin of the other? Would the entangled particle on the ship communicate backwards in time to its entangled partner on earth?
The change would be instant regardless of that thats why its so crazy
@@Kid_Naps That blows my mind then. That means that entangled particles can communicate both backward in time and in the future. WTAF? In fact imagine using the entangled particles as communication devices. The spin up and spin down nature could be used to communicate in binary code between someone in the future to someone in the past and vice versa.
@@sinebar You can't use entanglement to communicate. It could be used to generate the same random bit on 2 distant places, but that's not really all that useful.
_"Would the particles remain entangled?"_ - there is no theoretical reason why they would not stay entangled.
_"Would the entangled particle ... communicate with its entangled partner back on earth such that a change in spin affects the spin of the other?"_ - Their spins would be correlated.
_"Would the entangled particle on the ship communicate backwards in time to its entangled partner on earth?"_ - it's not possible to tell which communicates with which, just that their spins are correlated.
Entangled particles don't communicate at all. They just inform on each other.
That's another nice feature about the Many Worlds interpretation. No backwards in time stuff, no non-local stuff.
How does that resolution work if both Alice and Bob are in two separate spaceships that are 10 light years apart with a star that's exactly at the halfway mark between Alice and Bob. Now when that star goes nova Alice and Bob begins accelerating towards one another at a fraction of the speed of light. When Alice and Bob meet and stop at the center of the distance (where the star exploded) who will be younger?
The time that we follow is earth time, spacetime and earthtime differs.
Paul, When you talk about time, how long is NOW? Is it 1 planck time? Does physical mass only exist in the NOW? You are looking at an apple on a table and lets say it is now 0 planck time. At +1 planck time the apple is still there but the apple at 0 planck time has moved into the past to -1 planck time. What happened to the physical mass when it moved from 0 planck time to -1 planck time? If mass disappears in the past then there is no past and traveling to the past you will see nothing (if time travel to the past became possible).
So If A&B both leave earth and accelerate away from eachother, they'll still be in the same inertial reference frame?
When they return, everyone else is now older but A&B are still the same age right?
_"they'll still be in the same inertial reference frame?"_ - they would each be in their own reference frame.
_"When they return, everyone else is now older but A&B are still the same age right?"_ - if they experience identical accelerations at identical times during their trips, then yes.
How would this play out in a closed universe, where Bob can remain in the same reference frame throughout? Can we conduct an experiment using relativity to rule out a closed universe?
So basically when physicists found contradictions in the Newtonian physics, Einstein explained them away by "lets just agree to disagree".
But seriously tanks for the explanation now I finally understand the concept. Up until now all I understood was that it has something to do with acceleration but I didn't know that acceleration is not as relative as just speed (velocity?). I thought from Bob's perspective it's the Alice who's accelerating so that leaves us in the same place: why it's Bob that has to be younger?
Is that where the space-time bubble of the Alcubierre drive makes it possible to go faster than the speed of light with regard to the universe's frame of reference? Is there some constraint in relativity that will not allow Bob to arrive before he left in so doing, or does this require a new kind of physics?
Bob also expends energy to accelerate to 3/5ths the speed of light at the beginning of the journey. Doesn't this also contribute to the asymmetry (not just the turn around)?
Yes but he could travel a bit away from Earth, turn around, accelerate quickly to 3/5c and as he passes Earth, compare his age with Alice. He'd be pretty much still the same age.
Also, during return, they can compare ages without slowing down, as long as they are close enough in space.
The turning point is really the critical part of Twin paradox.
Ahhhh Time Dilation, Paul you are such a clever bugger in answering these topics, what did the robot in the book By Adams say, " I have a brain the size of a planet"
What would happen if instead of flying away bob would get into an orbit around the sun, and slow down (but maintain the same distance from the sun, pushing outwards against the suns gravity) until the earth overtakes him again. When he lands, who would be older? He changed the frame of reference and feeled the constant acceleration, but alice also constantly accelerated towards the sun just to stay in orbit. I guess alice would still be older because she had less acceleration to do, but I'm not sure.
And, it's pretty cool too...
So to take the shortcut just go super fast and it will be faster and shorter.
❤ Very good 👍🏼
You are explaining the paradox without answering the fundamental question. Leaving aside all acceleration, deceleration, twins etc, The actual question we should be asking here is, "Between two objects that are in constant relative motion, which one sees time dilation and length contraction". If we can't do an experiment to determine who is moving as per the theory, then the time dilation for only one of the objects disproves this very statement, as we can simply compare the clocks and see who moved !!
If Bob is moving at not full speed of light, how so he can achieve a distance of 5 light years in less time than 5 light years
I understand. :)
What if Bob is traveling through a gravity warped space that returns him to his origin point without a change of his reference frame?
No matter what you do, tomorrow is coming at you at the speed of light.
But that means....Death is coming towards us at the speed of light too ☠
I can get into other people's future faster by accelerating.
I can run around in circles and survive you by millions of years if I do it fast enough.
Alice shoud know drinking alcohol make you look older! But, while drinking the time is flyin faster 🤔
Acceleration is absolute. Nothing can travel faster than the speed of light making it an absolute reference marker. Clocks are instruments that measure motion in space. A slower running clock means it has experienced more space as the photon travels a greater distance from emiiter to receiver.
The Hafel-Keating experiment proved that there is a preferred frame of reference. That motion is absolute. That time, being a measurement of acceleration, is absolute.
The real irony is that relativists are the real flat Earthers. Unable to discard disproven theories when reality goes against their religious beliefs.
Acceleration, deceleration does not explain really why we need to adjust the clocks of GPS satellites every day - they do not speed up or slow down.
Do not forget Earth has 1G acceleration with Alice on it and Bob can also fly in big circle with 1G acceleration. Bob will be still younger, why?
That's the thing they did "speed" up. Those satellites were built on earth and then launched into space. They didn't come into being at their orbital velocity those clocks were manufactured at earth speed (Ie the baseline, at least for earthly things like Alice and Bob). Also, a clock on earth adjusts relative to earth speed making it unnoticeable. Remember that term- RELATIVE. But a clock on A salellite is adjusting to the satellites speed.
This difference (earth clock vs gps satellites clock) is very minor. The only reason it's noticable is because of how precise the calculations for gps need to be. And gravity effects the time dilation more that time (kinetic). For kinetic dilation general relativity predicts that gos clocks run 6 micro seconds (not milliseconds) slower a day how ever; Gravitational Time dilation predicts they gain 45 micro seconds a day (time goes slower in gravitational fields and we are closer to earth that gps clocks). That equals 38 micro seconds gained every day. Considering that a mechanical clock can lose anywhere from 1- 10 seconds a day the effect is quite small. So quite frankly even if you compared one of these atomic clocks to your wrist watch the gps satellite's clock would probably be more accurate. The only reason they adjust them so frequently it because gps requires extreme amounts of accuracy.
They're not in perfect orbits and WE are accelerating (in the sense of general relativity).
I'm still not convinced. When Bob flies away from Alice at high speed, the distance he travels seems shorter than it does to Alice. OK, but why couldn't we argue the equivalent for Alice? If she is seen to be travelling away from /Bob/ at that high speed, why shouldn't /she/ experience it as a shorter journey??
I don't see the asymmetry there. It still needs to be explained.
@silverrahul Yes, thank you. I am familiar with that explanation. I don't understand /why/ it should be an explanation, but I do understand what it says. In response, I could discuss how calculations proceed in the case of two travellers who cross paths out in space, as there is no acceleration there, but there is at least a jump from one inertial frame to another. But that doesn't help to explain why Alice couldn't be construed as the traveller throughout, switching from one frame to another at the end of the outward journey.
BUT - let's reconstruct the experiment, so that Bob just takes the outward journey, at velocity v, and at some point along the way he radios back to Alice to say what time his watch reads, and how far he has travelled, measured in his frame. Knowing the speed of transmission, c, Alice can work out that he transmitted his message at s/c ago, and that since the distance s=t/v, has been travelling for the time s/v.
On the other hand, Bob measures the time of travel as only t', which is less than t. That's the time dilation, which Alice is able to observe via these calculations.
Each of the participants remained in their own single non-inertial frame throughout. And yet what we have just seen Bob go through, from Alice's frame, cannot be equally correct regarding what Alice has gone through from Bob's frame. There is supposed to be REAL time-dilation, with Bob getting to Alpha Centauri in much less time than Alice can measure and calculate.
Is the claim that we can flip this around and say that from Bob's frame it is equally true that Alice experiences a shorter duration for the same journey?
Actually, let's firm up the story a bit, by saying that Bob travels past Alpha Centauri, and sends his message at the moment he passes it. The physical journey is then fixed for both. Would Bob calculate, from his t' and the value of c, that Alice endured /less/ time than t' while the journey took place? I don't see that as being symmetrical.
@silverrahul thank you for that. You said "yes, it is not 100% symmetrical because one thing that is not symmetric is that , alpha centauri is at rest relative to Alice, but it is in motion relative to Bob." Let's ditch Alpha Centauri, then, and just regard Bob and Alice as uniformly moving away from each other, with no external reference points to worry about. Symmetry demands that we treat them equivalently. But how can this be, if Bob's calculation of Alice's time duration is less than her calculation of his? If you flip that around, you get exactly the opposite set of figures. How do you resolve this 'contradiction'?
@silverrahul I think I know the answer. It doesn't get resolved? Just as with metre rulers passing each other in space, each seeing the other one as shorter, the same reduction works in both frames. Correct?
@silverrahul Understood. I was just citing the supposed 'contradiction', that constitutes an alleged paradox, and then acknowledging that there is nothing to resolve. As you say, the measurment of time and length is truly relative to frame. I get it.
No, I was using 'see' in a sloppy way, and I do understand what you are saying there. Thanks.
@silverrahul I did the work myself. You helped by pointing out that it was correct to describe Bob's measure of Alice's time interval as being even shorter than his own. I eventually realised that the asymmetry that implied was not to be resolved, because it was what relativity entails. In Alice's frame, Bob's time really is shorter than hers. In Bob's frame, Alice's time really is shorter than his. That is all internally consistent, and just exemplifies what relativity is all about.
OK, so Alice aged 10 years, Bob 8 years. What I really want to know is why did I age 20 years watching this? But seriously, other physicists have (supposedly, as I am incapable of this judgment) proven that neither general relativity nor acceleration are necessary to explain the effect. So I remain as baffled as I was 20 years ago.
Space and time are seperate frames of reference. Clocks are instruments that acceleration in Space. The slower running clock is the one that experiences more space. Both experience the same amount time. Unless the observer is in cryostasis, there is no relevant age difference.
One additional comment...
The twin paradox is probably the most cited example of the consequences of the special theory of relativity. The story starts with two twins. One twin travels on a spaceship to a distant star and when he returns, he finds that he has aged less than the twin who stayed on earth. The age difference is attributed to the time dilation resulting from the fact that the travelling twin was going at some velocity to and from the star, while the twin on earth was stationary. A suspicious student will undoubtedly ask “But since velocity is relative, why can’t it be the twin on earth who was moving, and the travelling twin was stationary? Then the twin on earth will age less, correct?” To avoid this contradiction, the teacher will offer an explanation along the lines that the twin on the spaceship is different because he has changed inertial reference frames and thus the problem is asymmetric, and if you make the traveling twin stationary, it is a different problem. And these explanations always fall an inch short of satisfying the student, or in many cases, of even satisfying the teacher, and so a crusade has occurred that has spawned countless explanations involving acceleration, more spaceships, lines of simultaneity, space diagrams, etc. And they all fall an inch short. Plus, they have spawned many odd and wrong interpretations of where the difference in time went or when it occurred.
I came across this situation and asked myself “What the heck is going on?” Keep in mind, that at the time, my own explanation was along the lines that since the traveling twin was making the moves it only made sense that he was special and that the time dilation only applied to him. But after weeks of reviewing countless explanations and interpretations, I couldn’t find one that explains why this scenario is asymmetric and yet the explanation not come up an inch short.
Remember, this is a thought experiment, we have a lot of leeway.
Then I realized that the problem is not asymmetric, it is symmetric, and if let the traveling twin be stationary and have the earth and star do all the moves, we get the same result. The traveling twin ages less. As I explained in the other comment, this is because the distance used, from the earth to the star, is always in the earth’s frame of reference, and because this is what we are using to measure time, regardless of whether it is the earth and star moving or the spaceship moving, we get the same result. The earth twin ages more, the traveling twin ages less.
I don't know if this twin paradox will ever go back to the simple and elegant example of SR it started as. Where the only paradox is the amazing result. But It has for me. It does amaze me how much we have been conditioned that we must explain that this problem is asymmetric, when in fact it was always symmetric. We were so afraid of a paradox, that didn't even exist. That also explains why explanations that this problem is asymmetric, even the good ones, always come up an inch short. Because it isn't.
The only problem with you analysis is that it is completely wrong.
Newton's first law of motion states that an object at rest remains at rest unless acted upon by an outside force.
The spaceship twin is at rest within the ship just as much as the Earthbound twin. Therefore, they both age the same amount.
This has been proven by many NASA experiments showing similar heart rates for stationary, takeoff and in-flight segments.
This whole issue with time-dilation started because Einstone era scientists did not understand basic electrical and wave properties.
Since physicists are unable to easily solve this simple problem, its time to throw out their physics books and go get a degree in applied science first. Then they will see just how wrong they have been about everything.
I am not sure how you are reasoning. Let's formulate the twin's paradox as a completely symmetric one.
There are two identical spaceships, one with Alice in it, and one with Bob in it. There is nothing else in the universe, no Earth, no stars, nothing. The two spaceships have a high relative speed wrt. one another. At some point of time, the pass each other, at which point each resets their clock to zero. Then they travel away from each other. After a while, they pass each other again, which gives them the opportunity to compare their clocks.
They had a high relative speed (almost) the whole time. Therefore SR says that according to Alice, less time will have passed on Bob's clock, and according to Bob, less time will have passed on Alice's clock. They cannot be both correct when they meet up again. So, who is correct?
When the situation is symmetric, it is not possible to tell which is the "stay-at-home" twin, and which is the "travelling" twin.
The only thing that breaks the symmetry, and therefore resolves the paradox, is the acceleration. And that is exactly how SR resolves the paradox: during Bob's acceleration, Alice's clock appears to run faster to him, not slower.
@@renedekker9806 It has nothing to do with acceleration, and there are scenarios that prove this using 3 ships all traveling at a constant velocity, passing each other and comparing clocks. It is all based on the relativity of simultaneity. The error most people make with your example of bob and alice going in opposite directions and then bob turns around and catches back up with alice is with one simple piece. At what time in alice's frame did he turn around? Most people assume that the time at which bob turns around is the same for both bob and alice, but it is not. They are in two different frames of reference. If the time difference was due to acceleration, then whether bob goes for 1 light year and turns around or for 20 light years and turns around, the time difference would be the same. But it will obviously be 20 times more in the second example because he has travelled at constant speed for 20 times longer.
@@roberthansen9876 _"At what time in alice's frame did he turn around?"_ - Did I say Bob turned around in the symmetric scenario? Let me check... no I didn't. Why does it matter which of them turns around? What makes the turn-around different from the normal travel? Why does Alice's clock appears to jump forward during Bob's turn-around? Why does Bob's clock not appear to jump forward according to Alice?
_"it will obviously be 20 times more in the second example because he has travelled at constant speed for 20 times longer"_ - and therefore he is 20 times further away. The time difference depends both on the amount of acceleration and the distance between them.
Acceleration in SR is analogous to rotation in 3D. When you are sitting on a merry-go-round and see the world revolve around you. From your perspective, the trees that are closest to the merry-go-round move slowly around you. The trees that are further away move faster. The apparent speed of the trees depends both on your rotation speed and on how far away the trees around.
Acceleration in SR is similar. A rotation, not in space, but in time. The apparent rate of other clocks depends both on the size of your acceleration and on how far away the clocks are.
@@renedekker9806 You are getting closer, the relativity of simultaneity depends on time and distance and those two are related by velocity, not acceleration. If Alice laid out a long series of synchronized clocks in her reference frame for bob to pass, as he passes each one, he will see that they are getting further and further behind his own. GPS satellites exhibit the same thing. No one has to accelerate at all, the time dilation occurs in a continuous fashion. Consider the alice on earth example. Bob takes off in the space ship, travels for a year and the returns. He ages less than alice. You say it occurs during the acceleration. But say bob doesn't land back on earth and just flies by and glances at alice's clock as he passes. It will show the same difference as it would have if he decelerated and landed.
The faster you go the shorter you are? Ah, that totally explains men getting Corvettes.
Special relativity is about two objects. Not a bunch of objects. Earth, stars and the rest.
Now solve it.
What if Alis do the same and moves to Bob, instead of Bob coming back. Everything is symmetrical?
@silverrahul why younger ? It breaks the symmetry?
If Alice moves towards Bob, they'll see each other rapidly getting older and have clocks that agree when they meet.
@silverrahul Special Relativity don't have direction factor! It speaks of speed than velocity. In light clocks, the measurement of speed of light when it travel in both directions! No velocity of light in one direction done up to now. Therefore Relativity speaks only of speed of light!
@silverrahul direction is not only left and right, what about vectors coming in to the explanation?
So does this mean that photons experience the passage of time?
From a biological standpoint can we verify that Alice is 2 years older?
I would stay home with Alice.
You make the assumption that acceleration is absolute . Bob could easily claim that Alice is the one accelerating. You also assume that acceleration can be detected that requires Bob and Alice to have prior knowledge of physics and not rely on observations alone and the theory clearly says observers. As for the experiments that can be done : lets take Einstein s own experiment he used , that of the 2 clocks . One at the equator and one at the pole. He stated the equator clock as the one being slower. He picked the equator clock as the one moving because he knew about earths rotation, not by observation and thus violating his own theory. If sub clocks with observers , neither can observe or detect If they are accelerating or even moving. He even said to support his theory that the 2 statements “ the earth is moving and the sun is stationary or the sun is moving and the earth is stationary are equally valid” So what experiment can be done to show that its the equator clock thats moving?
Synchronized clocks measure relative differences in motion and/or gravity. Whichever clock is slower is in a higher gravity field or is traveling faster.
@@stewiesaidthat the problem is determining which clock is the one moving slower . Both can claim its the other. So from their perspective the others clock that moves slow and theirs is the faster. But a clock cannot be running slower and faster at the same time. Hence the twin paradox.
@@nickmerix2900 from a single point of view, an observer cannot tell which is running slower but all the twins have to do is compare the readouts on their clocks.
@@stewiesaidthat thats circular logic . You are using the supposed outcome of a theory to prove the theory. You assume the twins will agree about their clocks . I have a broken clock in my house but i cant claim that time stoped . One can say the twins compare their clocks and they read the same. What then ? By your reasoning if i have a clock that runs slow in one room and after a year i notice that it is slower than the one in another room i can then claim that the first clock was moving thats why its slower.
@@nickmerix2900 if you understand why one clock is slower than the other, it's easy to figure out who is moving faster through space.
Hoo!
😃
Has science fallen this far down the rabbit hole that they can't even solve a simple problem?
If you don't understand why clocks in motion run slower, you will never ever figure out the real solution.
What you just said is nonsense and he gave the real solution.
Motion is relative, acceleration (in the sense mentioned above) is absolute.
@@MrCmon113
Yes, that was bullshit. ;-)
There is no paradox
The only, place where time has both no meaning & stationary simultaneously infinitely very quick invisible too? WOW, what could it be, a bird a plane a pulse a beat a throb or a plain old pain in the neck? Gravity has its' own rhythm: it's been roundly about somewhat longer than human understanding, I think! {;}
5:53 _"If you're in an airplane & you're crusing along at attitude & you toss a ball...You can't tell that you're moving because you throw the ball & u catch it in ur hand....How do u know that ur staying still & the Earth's not rotating underneath you? You don't! There's no physical experiement that you can perform. You're relying on your intuition (that the earth is large & probably isn't moving) to tell you that you're moving. But there's no physics reason why that has to be true."_ WRONG! If we steer our car towards the setting sun & accelerate then, from our frame of reference, we're stationary & Sun accelerates towards us. Thus our foot on the accelerator pedal has moved a star. (& the galaxies behind Sun) This is ABSURD. Our foot cannot move galaxies
This is why the heliocentric theory took so long to be accepted. The sun obviously moves through the sky. There's no obvious sign that the observer is on the surface of a spinning sphere that is orbiting the sun, but there are experiments and observations you can do to confirm it.
There are experiments and observations that confirm time dilation.
@@e.claytonrowe4394 Ok but then how do you explain our feet moving galaxies? We put our foot down & the setting sun moves towards us? It can't possibly be true.
@@alwaysdisputin9930 he's not wrong. His statement was for a constant speed (IE cruising). Not accelerating or decelerating. So your foot wouldn't be on the peddle and the car would be in cruise control (or at the very least maintaining pressure on the peddle for a constant speed). Yes we can feel acceleration and test for it as well as deceleration but there is no way to prove/test that we are the ones moving vs the earth when at a constant speed. If I accelerate 0-60 in 5 seconds I will feel my body slam back into the seat, if i have a glass of water it will spill, if i toss a ball up it will fly into the back seat, but if I've been going 95 for an hour the only thing that tells me I'm moving is the logic. Let's say you're driving your car to the sun but this time you have a passenger. This passenger is an amnesiac of sorts. They can remember how to talk and are intelligent but have no frame of reference for anything. They don't remember starting the trip and as far as they are concerned the sun is coming towards them. You tell this passenger that they are wrong but they demand proof for your assertion because to them, they feel stationary. Now remember Paul said that you can't test for movement at an inertial frame of reference (constant speed) specifically. So for him to be wrong you would have to prove to this amnesiac passenger without decelerating or accelerating. This means no moving the pedal up or down. How do you do it? How do you prove you're moving. You can know something to be true but that's a whole lot different than proving it. You moving the peddles takes you out of an inertial frame of reference. Which invalided your statement because Paul specifically states that you can do physical tests with in an inertial frame of reference. Also you second comment isn't really relevant. Like he said you can logically know that we aren't the ones moving the stars but that's a whole lot different from physically proving it in a test. That was his point.
"what else are birthdays for", how ironic
You probably need to watch George Carlin's explanation on whats ironic and whats not :)
Standing still on Earth is not standing still in space.
The Earth undergoes a 25,000 mile rotation every 24 hours while orbiting the Sun at 1.5 million miles per planetary rotation and riding the turn of the galaxy at 1 million miles per planetary rotation.
He acknowledges this at the beginning and then says to ignore it. This is reasonable since the acceleration is small compared to the acceleration of the space ship.
@@johnbennett1465 At the beginning he says that standing still, you are moving at the speed of light.
He does NOT then say that standing still on Earth is not standing still in space.
You are mistaken.
@@ZeroOskul nope, John Bennett is correct, you missed it. Re-watch if wanna.
@@cajunqueen5125 Timestamp?
I could just watch it again and try to guess just exactly where you think Paul says the Earth is moving through space so you have to step off the Earth to experience a stationary position, or you could just tell me when to look.
@@ZeroOskul true he doesn't say it right there, but he does say it a little later. The speed that Earth is traveling is fast by human standards, but is small by comparison to the speed of light. If he added a caveat every time he talked about "stationary" it would add 10-20% to the run time without adding anything to people's understanding.