Why do faster than light signals break spacetime?
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- Опубликовано: 9 май 2024
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Let's explore why faster than light signals reverse time and break causality. Why they can make effects occur before cause, causing time paradoxes.
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first
Sir,unable to get 30 days free trail even after clicking the link.
Space time diagrames use 45° angle for light only as CONVENTION. If you decide angle for light is 0°, light particles would not travel back in time but travel in present and wouldn't experience time. Misconception of this video is it suggests faster than light particles would travel back in time. Retarded action is the reason why physics cannot make progress, since relativity suggests every particle travelling faster than light travel into the past. But the truth is faster than light particles travel into the present from the perspective of the source, they don't actually travel back in time. Relativity of simultaneity opens a possibility for faster than light propagation, since synchronization convention prevents you from measuring one way speed of light.
@TriTr-qd2bd If there was no speed of light, universe would look the same but eveything would happen all at once. Speed of light is actually speed of causality, which suggest c is round trip distance divided by time.
nah. this episode was way too forced. in real life faster than light travel dont cause time travel.. it just shortens time and the people you used in the example dont know about FTL and cant do proper calculations.
"What will it be?" bartender asks. Tachyon walks into a bar.
Causality has left the room
@Mahesh_Shenoy bomb moves backwards from bomb or the event occurs chronologically in reverse?
What’s the difference between the two?
Entropy
Well not really, just seems like it to the near speed of light observer
If you don't understand the shirt in calculus if we consider x to be position and t to be time then the rate of change of the position over time is called dx/dt. The rate of change of velocity is acceleration and so it's d^2x/dt^2. The rate of change of acceleration is d^3x/dt^3 and the name for that is jerk. So the shirt says "Don't be a jerk".
And bonus fun fact for cereal fans: the 4th, 5th and 6th derivatives are called snap, crackle and pop.
Thanks
bro i legit came here to explain the same thing but u beat me to it. kudos!
Relativity is seriously a amazing topic to talk to the people's who likes it
Yeah, its great if you have friends that share same intrest
it is interesting their finding that, technically its possible to look back in time. but the idea is nothing like back to the future movie or anything... i mean deterioration of the universe, rotting, aging, ( whatever you call it.. ) - could go slightly backwards just walking around. but in our eyes this would be like 1 in 1000th of a second, i mean you wouldnt even notice it. you couldnt even do the dejavu cat from the matrix. and its a 1 in billion possibility in every day life...
Our observation of reality, and reality aren't the same thing. Models need to remember that perception and reality are not the same thing.
@@chrisoakey9841 objective reality don't exist tough, atleast we can't see.
@@malto_only we cant see, but in general dont need to as the stuff that affects us enough to make a difference are seeable. we dont worry about the pull of gravity from proxima because it is insignificant. but models like general relativity are fine until we extrapolate concepts like space compression etc because of taking the model of our observation and suggesting that we therefore know... which results in idiotic things like the expanding universe, dark matter and dark energy and twin paradoxes etc.
I wish you SHOWED us at 22:14 how causality is broken when the fast observer send FTL message to stop the bomb. That was the most important part of the entire visualization of events
Missed that too
Yup simply observation will not do anything
Even if he had some instantaneous transmission device, his trigger would be the reception of the light signal. This, by nature, would have taken a year (relative to the ship) so the signal would arrive at the exact moment the launch signal arrived at the second ship. Still not breaking causality.
The blue ships trigger is the explosion. The blue ship is right next to explosion when it happens, so the time it takes for the light signal of the explosion to reach the blue ship is negligible. If they send an instant/ftl/faster than missile signal to the destroyer, that’s the paradox. Rewatch starting at 20:00, with key points at 22:00
@@nickwalden6425 It would seem so, but it isn't, is it. No matter when you send the signal and how fast you send it, the missile was already fired from the firing ship's perspective and the moon has already been blown up by the missile before any signal reaches the blue ship.
Here's my idea of what actually happens:
Ship a fires an FTL missile, the missile blows up the moon, ship b sees the moon blown up, ship b sends an FTL signal to ship a, ship a can receive this signal anywhen between the moment that the explosion was seen by ship b and infinite time from now in the future, depending on which direction and speed ship a has in relation to ship b's message signal.
From ship b's perspective their message will reach ship a in the future, after they see the moon explode and the ship firing the missile, and not in the past. The light from ship a at the time when it receives the missile will reach ship b waaaaaaaay after it has all happened.
There is no breach of causality. It's only that some observers will not be able to calculate the correct order of events without additional maths that corrects for the incorrectly observed time that passes for each event from their perspective. Some events, like the path the missile traverses, will seem to go backwards, but in reality they don't, they just go really really fast.
To actually break causality you'd have to find a way to send the signal to a time before the missile was fired and that didn't happen, and wouldn't, no matter the speed. Even at infinite speed you'd still be stuck with the present. We have to remember that we can see the past because light takes time to get here. Things in the past have already happened, regardless of the time or speed with which our information is updated with the events.
i have to admit your are really smooth with the promos
Haha, thanks!
@@Mahesh_Shenoy breaking of causality is not a paradox but an usual phenomenon.rocket's light will travel slower than rocket therefore we will see that rocket hasn't hit anything but in reality rocket would already have smashed into the object and the light of the moment when rocket hit the object will take time to reach us therefore we will see destroyed object first then we will see rocket smashing into object.
@@pwinsider007 What you've described was the first scenario, with the astronaut right next to the planet - there's an illusion that makes it look like it happened in reverse, but it actually didn't. The second scenario, with the near-light-speed space ship passing the planet at the time of impact shows that for some observers, the events *actually* happen in reverse, it's not just an illusion that makes it look that way.
How is the causality broken in any of the cases discussed? As you said, what you see is not what is happening. So, even if the explosion is observed to happen before the missile is being launched. In reality, the effect is still following the cause. For example, we see lightning before the thunder. But anyhow the thunder occurred before the lightning. So, even if we are seeing the causality to break just because of seeing light signals in wrong order, that does not mean that the events have also occurred in wrong order. So the causality should not be broken even if the missile is travelling faster than light. Consequently, the argument that causality will break if an object travels faster than light should not stand. As an analogy, a supersonic aircraft travels faster than the speed of sound resulting in different effects without breaking the causality.
You explain well, in a very simple and entertaining way. Thank you, for sharing. Keep educating us.
The lighting example dont work because one event is not causing the other. The sound and the light comes from the same event, but is not one that is leading to the other.
And examples using sound waves also dont work because sound uses air as a medium. Most experients bases itself in a vacuum. But sound not travel in a vacuum.
@life-my9tl I was wondering the same thing! I wrote a comment wondering if you redid the thought experiment, but with a supersonic missile and observers LISTENING for these events, would the causality also be broken? And @sonofcronos7831 I think it’s ok to just add air to the thought experiment so that sound can propagate, or assume sound is also an EM wave for the sake of thought experiment
It's not a real explanation, but I think a good way to think about it is like this: You are always constantly travelling at the speed of light. But that speed is distributed between time and space. The faster you move through space, the slower you move through time and vice versa. But no matter what the distribution is, both speeds must sum to lightspeed. If you then want to travel through space faster than light, while the sum stays fixed, you have to have a negative speed through time.
In the spaceship’s frame, the explosion did happen before launching the FTL missile. Check that section of the video again :)
Supersonic travel doesn’t contract space, so it’s not an appropriate analogy. This weird concept is happening because traveling at the speed of light is doing a weird effect on spacetime. Any other speed can’t be used as an analogy
Before this video I didn't understand what the problem with seeing things backwards is, now thanks to the faster than light signal "don't shoot!" I understand. Bravo, as usual! Event circles is also a good depiction
I wanted to see how the faster than light "don't shoot" signal traveling, he said it will arrive before the light of the moment they "shot" the bomb, but how though? I wish he showed us instead of just saying it does
@@abdulqader1829 You can imagine it going arbitrarily quickly, or even instantly, after the "boom" detection as the animation plays.
Closed timelike loops
@@abdulqader1829On a regular 1D+1D Minkowski spacetime diagram, two inertial observers at physically distant locations in space, usually get drawn as parallel vertical lines... But... The "same time" for each of those observers are connected with 45° diagonal lines. (It's not a horizontal displacement on the graph.)
To shift from one observer's coordinate system to the other, you slide the parallel lines up and down (in time) so that points intersecting on the same 45° diagonal line, will be moved to match on the diagonal line perpendicular to the first one (i.e. -45° or 135°)
Faster than light signals will intersect with the "past" of each observer's vertical line after transforming to the other observer's coordinate system.
(This happens _both ways_ symmetrically.)
If the boom is the triggering event to send a signal to stop the boom, then it is irrelevant because by virtue of the boom happening, the firing had already happened.
Does sending a signal back to the destroyer to tell them to stop firing erases the boom from happening? Of course not.
Therefore, causality is maintained.
Many videos ago, you said 'speed of light is actually speed of causality'.
With every fresh video, that is becoming clearer and clearer.
Thanks!
I would say it is the speed of PERCEIVED causality.
Hands down the best relativity physics content on RUclips. Your approach of leading the audience to discover the meaning of each concept for themselves with the help of animations and Socratic dialogue is wonderful. A superb teacher.
Thank you! I've been waiting soo long to find video explaining this in simple way and since I found your channel I was hoping that one day you will touch this subject.
Big thanks!
Amazing video sir ! each and every video is getting even better!
waiting for the next one !
There's a couple things about this sort of thing I've found fascinating for a while now.
First is that if there was a stationary observer sitting somewhere between the destroyer and the moon, when the FTL missile passed them they would get the optical equivalent of a sonic boom. They would see the image of the missile appear out of nowhere at the point of its closest approach, then *split in two.* One image would race forwards towards the moon, the other backwards towards the destroyer. Like the astronaut they could do the math later and work out the order of events, but I still find it neat.
Second is that there's a relationship between the speeds of the spaceship and the FTL missile in order for causality to break. If the ship isn't traveling close enough to lightspeed, it won't see causality break. Similarly, if the missile isn't travel as far above lightspeed -- let's say, only two or three times lightspeed instead of four -- the spaceship won't see causality break. As demonstrated, at exactly the right combination of speeds the spaceship sees it all happen simultaneously.
I don't know the math well enough to figure this out exactly, but I have a hunch it's something close to an inverse relationship between the speed of the missile and the time dilation/length contraction observed by the spaceship. It's not the raw speed of the spaceship because the relativistic effects don't scale linearly; you don't get 50% time dilation/length contraction at 50% of lightspeed, you get it at about 86.6% of lightspeed.
So for a spaceship observing 10% time dilation/length contraction (41.7% lightspeed), you would only start to see causality break from things traveling more than ten times faster than light.
At 20% TD/LC (55% lightspeed) you'd see it break for things above five times faster.
At 50% TD/LC it would break for anything above two times lightspeed.
You can break causality with any signal velocity greater than light and a much clearer demonstration of this is to do a round-trip journey from "Location A" to "Location B" and then back to "Location A" again. If the trip is done faster than light [FTL] it will arrive at its destination "Location A" *_BEFORE_* it departed from "Location A". I was hoping that this video would demonstrate this case, but it didn't.
The math is a line, y = Mx + b, so you can do it. For a launch at t,X = 0,0 in years, light years, and an impact at (1/v, 1) where v is the missile speed. For a rocket ship going u and launch it 0,0. The hit occurs at t’ = gamma(1/v - u), so Lorentz contraction and time dilation are irrelevant, but the break point is indeed inverse u > c^2 / v
@@juliavixen176 it’s from pov of B. From pov A sequence is normal. You can’t see spacecraft coming at point b from pov b, but once it arrives, images of it’s travel will appear like moving backward, and then you’ll see launch from point A. And if before that spacecraft launches from B to A, from perspective of B, that didn’t see launching yet, it will seems like spacecraft will return before it was launched. But when light reaches B all sequences will be in order. From Pov B the’ll see two spacecrafts flight towards A. One of them moving backwards, and another moving forward. But they reach A with same delay as between arrival at B and departure. And as far as I understand, we don’t really understand what means (-dt)^(1/2) (result of v > c). Maybe it’s just limit of theory, or maybe time travel in some way. If it’s later, than causality can be broken, but it’s likely former.
It can be considered as time travel in a way. Imagine B observing caveman on A in far system, and suddenly those ”caveman” arrive to B on FTL spacecraft.
@vichav3167 Location A and Location B _are both in each other's past_ symmetrically. The FTL object/signal arrives in the past of the other location _each way_
A round trip puts the FTL thing in *everyone's past* including the original location where it started.
In Special Relativity, time *is* space. Every location in space is a location in time, and every location in space is in the past of every other location in space. (The use of " _i_ " on the time coordinate is a mathematical way to deal with this.) When you look with your eyes, in a straight line from the tip of your nose out into distant space, what you are currently seeing _right now_ is the past.
The straight line distance away from you in space is the 45° line on a Minkowski spacetime diagram. Everything you see and interact with *_right now_* is on this 4D light cone. Anything not on this light cone is not happening to you _right now_
That's time; time is the radial distance in a "straight line" away from you.
Velocity is just the conversion factor between two observers of how much of spacetime to label "space" and how much to label "time" for each other... because all inertial observers are at rest with respect to themselves and their clock always ticks at one second per second.
I love the energy you have while explaining things!
Well done. I really liked the nested cause and effect circles. Great way of looking at this.
The circles are fine, but if you've invented ftl then you have sped up the cause circle. Pretending that the speed of light circle is the cause circle doesn't get to the core paradox, the claim that you could get a signal back to the cause before it happened given you have observed the effect.
A simple way to rework this is to imagine the default refresh rate of the universe is C (Light speed) so if something could move faster than the speed of light it wouldn't be drawn properly . It might look something like a laterally, directionally stretched object that flashes in time/space cycles as it moves through large areas of space and if you were to cut out all the gaps when it wasn't visible it would seem to be moving at C , but when you add the dark gaps in it's illumination you can deduce how much faster than C it's going . If time stands still at the speed of light then moving closer and closer to C would be like reducing the frame-rate until it's approaching zero frames a second which would be invisible . A simple way to think of it is how cameras make wheels going a certain speed start to appear to turn backwards . If you had an infinately powerful camera and you wanted to reduces the movement of light to a completely still image when reduced back to 24 frames a second , the best you could ever acheive is smaller and smaller fractions of a frame , which is why it would take infinite power to acheive 1c . But if you could go from 0c to 1c without accelerating , then you should be able to go over C . But it's just possible that going C+ looks like a ghostly still image beaming in and out of space in such a flash you might not see it if was right infront of your computer screen . Anything visible would be reduced to the same laws as seeing something move at lightspeed because it would be visual abberations of C speed photons being disturbed by a partially drawn mass . Maybe it would look more like a streched out collection of flickering entangled point particles . Maybe faster than light travel has an embedded quantum probability mechanic . Not really something I've given a lot of thought . Fun to imagine though . Anyway , just because you see an effect before a cause doesn't mean it actually happened that way . Could be little difference between that and using different speed communication devices to hear an answer before a question - it doesn't mean you have the ability from your perspective to get an answer before asking a question .
That's How I think about, and that also, maybe things move at discrete steps (yet really small ones) like a Planck's length. Because of light speed is limited and a field can't transmit information to all particles simultaniously (even though, entanglement effects could happen between bunch of particles, that wouldn't change the overall perspective for a macroscopic observation, so we could ignore it, if things go at speeds lesser than C).
Intuitively understood so easily by the end; marvelous, thank you!
yay new video! i look forward to them all the time!
The missile knows when it is in all reference frames. It knows this because it knows when it isn’t.
Someone was a missileer
i love how you make everything so understandable keep doing what others don't i love it!!
Sabine Hossenfelder stated that FTL doesn't actually result in backwards causation because the paradox is due to only solving for SR, not GR, and the paradox goes away in the case of the latter.
The more I learn about light speed, relativity, FTL, etc, the more intuitive my understanding becomes. I followed this video easily!
I'm also reading _Faster than Light_ by Robert Nemiroff, which is also helping a lot.
Your shirt! “Don’t be a jerk!” 😂
I was really proud to have understood that too 😅
The transformations on the cause and effect loop, the length contractions are being made according to special relativity, which assumes the speed of light to be the limit. So using special relativity to say that faster than light travel doesn't exist while using it on a case where faster than speed of light travel occurs doesn't make sense to me.
No. Is exactly because faster than light breaks casuality that we know that nothing can travel faster than light, because one of the laws of physics is the law of casuality.
There's a much better demonstration, that wasn't covered in this video, of taking a round-trip voyage faster than light and arriving at where you started *BEFORE* you left. I was hoping that this would be in the video.
You missed a critical step. We MUST change the frame of reference back to the destroyer's frame of reference when the message not to shoot is received. When we do that, there is no paradox. In your example, the moving ship observes the order of events as its message arrived before the missile was fired, but the destroyer still doesn't get the message until after from their frame of reference.
Here's an analogy sonic experiment to demonstrate why.
We have a gun pointed at a target down range. The gun has a light sensor that will lock the gun when the sensor activates. The target has a laser aimed at the gun's light sensor and will activate when a bullet hits the target. With the precision of our setup, we'll have an observer safely positioned near the target, ready to witness the sequence of events.
1) The gun fires.
2) The observer hears the bullet hit the target (I know this because I've been in this scenario).
2a) The laser fires.
3) The laser hits the gun's light sensor and locks the gun.
4) The observer hears the gunshot *BANG!
Even though the observer sees the gun lock before hearing it fire, we know the gun fired before the bullet hit the target.
I know folks may say, "But this is sonic, not the speed of light. They're different." Yes, but apply the same logic to the ships. If we keep the destroyer's frame of reference, they will never receive the message not to fire before they fire. You can even use instantaneous communication, like hypothetical portals, and there is no way to create a paradox.
I really enjoy your videos... I am learning a lot from them :)
the only problem i have with this is that to me it means therhetically we can still travel ftl. All we need to do is create a dilation contraction bubble.
if an object or signal travels faster than light whilst interacting with the universe, it breaks causality sequence of events.
but isn't it theoretically then possible, that if we can create a bubble that surrounds the missile, that pulls it out from time dilation and length contraction affects of the universe.
we could fire a missile. once it would hit ftl speed. it would disappear like travelling into a wormhole and then only reappear once it drops below light speed and hits the moon.
basically ftl travel is akin to teleporation, or similar to travelling through wormholes, or similar to how 5th dimensional objects come into out dimension and dissappear from our dimension.
from the ships perspective, all they would see would be a missile suddenly pop into existence and hit the moon. they won't know what happened until months later when they'd see a missile being fired and disappearing from existence. so they wouldn't be able to send a message to the cruiser to not fire the missile.
so causality would not be broken. and we could still have ftl travel.
all it would mean is that ftl travel involves a type of teleporation / 5th dimensional movement of going out of our 4 dimensions and then coming back in.
So that causality can never be broken through the time dilation length contraction of other observers.
Excellent explanation- thank you 👍
I'm thinking though, for the FTL missile: even if we see the missile's explosion's first and then the missile going backwards to the ship, if we knew the missile was FTL... can we incorporate that knowledge into our thinking and deduce that we saw the events in reverse?
I was thinking the same thing. Shouldn't the spaceship people have accounted for the fact they were themselves travelling in the same direction at relativistic speeds when they back-calculated where the missile came from? Wouldn't that account for the disparity in their view of cause and effect events?
I was left with the impression that there was a missing coordinate frame transformation there.
There's a much better demonstration, that wasn't covered in this video, of taking a round-trip voyage faster than light and arriving at where you started *BEFORE* you left. I was hoping that this would be in the video.
@@akaHarvesteR They did. What they "see" is different than what they calculated, and they still reached the conclusion that it happened backwards. The only way for them to conclude that the cause happened first would be to assume they are not valid observers, or that the astronaut POV is more valid (remember, from their perspective she is the one traveling backwards at relativistic speeds). That also goes against relativity, because all inertial observers are valid regardless of velocity (and they all observe the speed of light to be the same).
See the astronaut, she also saw the explosion first, but she could calculate it backwards and realize that the missile is FTL and was launched before the explosion without assuming she isn't a valid observer. The ship did the same and reached a different conclusion.
You could do the same experiment with the destroyer and the target both traveling close to the speed of light and the ship being "stationary". They would still find that the explosion happened first (the target would still see it happening first but conclude it happened afterwards).
EDIT: Just to be clear, the ship will be able to conclude that the missile was shot first from the reference frame of the destroyer or the astronaut. But since they are also a valid reference frame, you can't just do that and call the other reference frame "more correct". There is nothing that makes the ship a less valid reference frame.
I always had this question ,thanks 🙏
I love this idea that the cause signal remains contained within the effect signal even under transformation for signals less than or at the speed of light. That is a fantastic way to arrive at the relationship between reference frames without calling on any maths. I'm going to watch this a few times to really bed down this representation.
also watch some animations of Minkowski diagrams (where the expanding circle here is repented by the light cones' X....it never moves, while the (t, x) axises flip flop around, that is: all references frame agree the effect envelope is a sphere expanding at the speed of light.
Great video, thanks!
I have a suggestion for your next t-shirt: a graphic with three cartoon characters eating a puffed rice breakfast cereal, each character labeled d^4x/dt^4, d^5x/dt^5, and of course d^6x/dt^6
Took me a moment.
@@astrokevin92 glad someone did 😉
snap, crackle and pop .....xD
You got a new subscriber!
More! More! I want more mind bending videos like this!
It took me a minute to get the joke on your shirt. Very clever.
At first I thought the shirt was saying “don’t be an accelerationist” (a sentiment I agree with!) but acceleration is second order, not third, so I was confused and stopped thinking about it. When I saw your comment I thought about it again, and remembered that third order force (jolt) is sometimes called “jerk” thus “don’t be (a) jerk”. Very good indeed!
Next he needs a shirt with three characters eating breakfast cereal, each character labeled d^4x/dt^4, d^5x/dt^5, and of course d^6x/dt^6
@@jpe1 Jerk is better than jolt because when you jerk something around you're changing the acceleration but when you jolt something you're probably throwing lightning around.
@@abebuckingham8198 I don’t disagree. When I learned physics in high school (_many_ years ago) it was jolt, but it seems jerk is now the more common term. Like, back then, my dad would have said (describing my mom’s driving) “don’t jolt the transmission” but now I think the more common phrase would be “don’t jerk the car around”
Thank you sir
When we are in the original scenario, we looked at the length contraction from the missile POV. However, when looking at the FTL missile, you completely skip that step. I understand that the length contraction would make the distance imaginary, but it still seems like an important part of why things break at FTL.
I love this video and your channel. Thank you so much for sharing knowledge in such an entertaining way.
I wanted to ask: why is light the determining factor in causality? Is it because it’s constant speed? Given that there are indeed particles and phenomena that travel faster than light (like the expansion of space) isn’t it a matter of choosing an entity whose speed is faster than speed of light as a determinant of causality? Just throwing random questions from my shallow understanding of the matter. Again thank you so much for the videos, I enjoyed a lot.
Insane.... Your explanations are traveling FTSL... I feel the effects even before you start explaining...
There's a Sabine Hossenfelder video about ftl not necessarily breaking causality, what do you think of it?
I think that's somewhat clickbait, but at the same time, because there is not a combined theorem of GR and QM. We can't be *absolutely* certain.
She is a failed physicist but she is funny.
I love your video, you expalin physics so well
22:10 Intuitively I'd say, the problem is _transitioning_ a signal between slower-than-light and faster-than-light speeds. As long as the stl-"world" and ftl-world stay cleanly separated, causality doesn't break. Goes with what I heard, that the problem is _crossing_ the speed of light, not if you are below or above.
I have to think about that.
However: Reeeealy great video! Me like 😁
How about if the object or light in to blackhole, someone said that they moving faster than light. Could you explain it?
at 11:50 from the spaceship perspective the missile launcher was traveling left(let in negative direction) then the missile has to first overcome that negative velocity(due to inertia) to hit the moon and this will slower it and finally take 1 light year only.Can anyone please answer this question.
Hi, I love your explanation !
But I have a problem to understand why we care about someone perceive something?
Light is a wave, so the sound is, so when a lightning strike it appends before we hear it, doesn't make a mater.
And the animation would be the same with someone shoot a missile faster than sound (mach4 for example), at a distance from the strike, and a plane moving near the speed of sound...
Because an observer perceive something earlier doesn't make a mater from traveling speeder than sound or light ?
These are some of the best physics explanations there are. Period.
Also, what does the shirt mean?
I don’t know about shattering spacetime, but photons do exactly that every time. So if create a mind experiment, in which photon released from Point A which lead to destruction of Point B, then spacecraft must see it in reverse too.
It’s impossible to see “rocket” at speed of light approaching. But light reflected of rocket still should exist. I think, that light reflected from rocket while it’s travelling must be taken in consideration, and shown as separated circle expanding at speed of light. Or maybe it’s effect of sqrt(-dt).
I'm still just impressed the astronaut can see a tiny ship and missile a light year away.
It seems to me that, all the emphasis is placed on what different observers will see. Regardless of differing perceptions due to proximity and speed, the cause, in and of itself, always occurs before the effect. A person cannot be shot and wounded by a bullet before the trigger is pulled.
Scenarios to the contrary, defy the logical linearity of observed reality which underpins our understanding. Could it even be possible to explain how an effect came into existence before its cause?
Wow thank you
22:21 But the problem is that this reverse phenomenon is only seen in the spaceship's perspective, no? Even if he sends a signal back, in the perspective of the shooter, the rocket has already launched. It is because of the limit of light's speed that the rocket is only seen after the boom occured.
Sir I know it's totally irrelevant but why Tension across a string is constant in a pulley and what exactly is this tension if it's internal force then how does it pulls a body attached to it?Thank you sir for reading my doubt
The demonstration was mind-bending until the very end when you explained the "venn diagrams" of casualty, combined with stretching and shrinking of space within a reference frame. Causality really is nature's ultimate master.
How do you explain quantum effects like quantum tunneling where there seems to be some communication that happens faster than light. Is that a right way to think?
Sir I have very very much questions, fastly is that the question of why everything falls at a same rate which is 9.8 m per second square in that question I think that the g value of the earth is 9.8 m per second square so everything should fall at 9.8 m per second square and this is right or wrong and like Einstein said we can't reach the speed of light but how does speed of light is being measured and in water why it is 2.25 into 10 to the power 8 metre per second that I had asked you some month ago. You said you will make a video. And the last question what the heck is gravity? "Defination"
Time dilation in this video????????
There is a video by ScienceClic, "Visualising General Relativity", the best video I've seen. Everything "falls" down at 9.8m/s² because they don't, they are stationary but the ground us accelerating outwards which seems to the people who are accelerating with the ground(us) as if the objects were falling down. Space is being contracted continuously because mass bends Spacetime, so the accelerating ground seems to be in the same place because where it moves, space itself moves back.
There are some good videos recently posted here on RUclips which explain how light propagates through a transparent material, and how it "slows down". The full explanation is really long and complicated. I'll try to summarize, but this is the incomplete explanation. The atoms (or molecules) of the transparent medium are displaced by the electric field oscillation of the light, and then returned to their original equilibrium location by the other atoms/molecules of the medium. Because the atoms/etc. are accelerating, they now radiate their own light at the same frequency (usually) as the incoming light wave, but with a phase delay. So now the original light that wasn't absorbed, and the new radiated light from the material itself are constructively and destructively interfering with each other. Anyway, if you do the math, when the light leaves the material it will be delayed... there's more stuff going on I didn't mention here.
I am so happy to figure out your shirt (a friend coincidently discussed it too) Hint: it's a third derivative of distance over time
where do you get these type of t shirts bro?
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Kitni baar jeetoge
Par yeh dil maange more…Ahaaa!
If you are between the missle launch and explosion, wouldn't you see it appear wherever your distance to the path it's traveling is closest and then see it split, one going forward and one backward?
Yes. There's some videos on RUclips here of someone who wrote a raytracer for rendering 3D models moving at relativistic and superluminal speeds.
Wonderful
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wouldn't the time slow down for observers in space ship ? won't it also affect their calculations?
This might be a silly question on my part, but how can we use the results from relativity here, when there are objects moving faster than light? Wouldn't that be a more appropriate reason that things break?
Edit : To clarify my doubt, in such a circumstance how can we assume that light has a constant speed, given that anything can move faster than light, light ahould be able to do it too. Or maybe it moves slower? How can we tell?
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When an object travels faster than the speed of light (or the speed of causality) it surpasses photons, thus after reaching at the destination, the effect would get hit by those photons (which were lagged behind, due to faster travel), thus revealing the effect first than the cause. An observer would see a "delayed" future of an object travelling at the speed of light (or causality). That's my take on the faster than light travel.
PS: I haven't yet watched the video, this is my initial understanding over this topic. However, I will be watching the video, for my future.
How every observer observes some event does not change how an event happened. Using the faster than light weapons you mentioned, objectively, the missile would be launched, before the impact...regardless of how other observers perceived it.
What you just shown does not mean causality is broken, and thus it meaning faster than light is possible....
If, hypothetically the spaceship were to detect such an event, we can conclusively proof that faster than light travel is 100% possible.
The final example about a signal being sent back to the destroyer to tell them not to fire, from the destroyer's perspective....the signal should be received after they fire.
Reality is reality. Something causes, something happens. Just because one sees it differently doesn't makes impossible.
Even if the hypothetical weapons is an instantaneous weapons with zero travel time (infinity speed), the moment the weapon is launched, it already hit. Even if the observer spaceship is travelling at the speed of light at the target, and saw the boom, from their perspective, the boom happened, then the light of the launch arrives...so, whatever fancy reconstruction of the event from their perspective is irrelevant. The spaceship's signal to the destroyer would've been red shifted to heck.
Exactly... I've been arguing this here ad nauseum... good to see that there are at least a few people left here that can think logically!
I noticed the careful wording of the title: _"Why do faster than light _*_signals_*_ reverse time?"_ [emphasis mine] We know that-due to the expansion of space itself-there are objects right now that are receding from us faster than the speed of light. However, this particular type of FTL doesn't break causality. Am I right? Is it because the expansion of space causes objects (and signals) to move *away* from each other; they can never move *towards* each other FTL?
That space is expanding faster than light can cross that distance, which means that the light will never reach the far side... at all, ever. There's an "event horizon" where very distant locations will never have any cause and effect relationship with each other.
(I guess you could flip the coordinate transformation around and say that light is slowing down and stopping.)
That raises a really interesting point... in the obsolete idea that the universe might expand and then contract again back to a singuarity, I wonder if (for the sake of argument, assume the theory was true) there would be a point at which spacetime collapses inwards faster than the speed of light, and the utterly bizarre way the universe would behave from that point on.
@h14hc124 You wouldn't be able to see the collapse coming until it was 'too late'... The exact details depend on... well, the exact details of this situation, but everything in the shrinking volume of space will hit you all at the same instant from your point of view. It may not be meaningful to talk about time or space (or you) existing after that happens... but like I said, it depends on the exact details of what is collaping, where and how much, and for how long.
Wouldn’t this also be possible with very large distances at typically non relativistic speeds because of the expansion of the universe? Does that imply that what is a relativistic speed has a proportional relationship to distance and scale relative to the expansion of the universe? I guess what I’m asking is do relativistic effects happen at low speed on large scale and high speed on small scale?
This "effect before cause" situation will always happen with faster than light signals, under _all_ circumstances. Ask me and I'll explain.
@@juliavixen176 ok, why? Wouldn’t we see ripples of time running forward and backwards between some multiple of the speed of light?
Questions:
Why do we ignore the light that is being emitted as the missile travels through space?
I understand that the observer sees the missile being fired and the moon being destroyed at the same time, but wouldn't they also see the missile destroying the moon since the light being emitted from the missile reaches the observer at the same time it's being destroyed since the missile is at the moon at the time of it being destroyed?
Wouldn't this phenomenon make it look like the missile was being stretched across space instead of seeing it in one place while another thing happens because of it?
For example, if the light at the time of the missile being fired and the light when it hits the moon arrived at the same time from your perspective, wouldn't it look like the missile was stretched across the entire lightyear of space in an instant?
Wouldn't it then look like the missile reached the moon instantaneously from the moon's perspective by stretching across the entire lightyear of space?
Speculation & Thought Experiment:
I'm thinking, what if space contraction happens because of the fact that the information you observe is more frequent in the direction you are traveling and less frequent in the direction you are not?
What if light moves at a constant speed no matter the perspective because light is just the speed of information for any interaction in the universe and not the speed limit of information itself in the universe?
This would mean that if any particle were to interact with any information that was traveling faster than light (or at any speed at all relative to it) that it would only be effected by what it receives from that information's light speed emissions; making it look like the information was being stretched across space (due to a perceived space contraction from the particle's perspective) while being hit with a different frequency of information (due to the particle receiving information faster than it can interact with anything else in the universe. This includes spacetime, so it also causes the perceived space contraction and time acceleration).
This includes the emissions of information (even at light speed) coming towards you as you approached the speed of light. Information is indeed coming towards you at a faster speed than light, but since you can only observe the information's light speed emissions, and interactions between particles operate at light speed, from the perspective of every object traveling with you, everything would be observed to have more information/energy (as the information received from every field of the universe including spacetime would be observed to be compressed or at a higher frequency when it is received), time would be observed to move faster, and distances would be observed to contract.
I believe this would allow faster than light objects to exist in the universe without breaking causality. If this works, this also gets rid of the "Universe conspires to keep things below light speed" weirdness that special relativity currently has. It would be that way because faster than light speed literally could not be obtained unless you already had some field that could accelerate information to something faster than the speed of light since the speed of interaction in all fields of the universe are capped at light speed. Like black holes for example.
In a mathematical model, I can imagine a thought experiment where the spaceship sends an FTL signal to undo the missile launch after seeing the explosion (and the missile itself), but it seems like they could also “correct” their perception using physics to conclude that entropy cannot decrease and therefore they are the entity moving and must recompute their perspective from a hypothetical observer that is either stationary or moving at any speed at which entropy is not seen as decreasing; can they not in principle?
What if the thing which is faster then speed of light has no refrence frame? Would then the paradox never happen? I mean if the light has no refrence frame,why then the thing with over the speed of light?
Reference frames are not a physically real thing, they're a mathematical tool... for... um, calculating real things... this particular way of doing things makes it _much easier_ than the alternative methods.
Because... what you need to deal with... is that every location in space is also a location in time. And, every location in space is _in the past_ of _every other location in space_ Got that? This Minkowski 3D+1D spacetime allows you deal with this situation with a bit of linear algebra.
Anyway, I was really hoping that this video would mention the case of something taking a round-trip voyage faster than light, because it will arrive at where it started *BEFORE* it originally left!
(Ask me and I'll explain.)
@juliavixen176 after seeing you comment about the round trip a bunch of times, i really need that explanation 😂
A reference frame means all the objects that are moving together with each other. If two objects are moving towards each other, or away from each other, then they're not in the same reference frame, and you need to perform a lorenz transformation to switch between the points of view of those two objects.
@sintaxera Oh! I just saw your reply right now. I'm going to bed at the moment, so I'll write an explanation later. (Although, elsewhere on this video are some other comments of mine where I do explain it.)
Can we make a similar correlation with lets say the speed of sound? Some stands with a large plastic cup of beer on their head at 500 meters away. You shoot the gun and the bullet hits the beer before the person hears the sound. No bring a supersonic fly into the mix . How can we relate this to your example or can we.
Sir, when things travel faster than the speed of light, it's only the effect that is visible and cause is not visible or visibly delayed. But cause effect relationship is not broken . It's maintained. It's just that we see only the effect and not the cause. That's it.
Keep up the excellent work F.H.P.
I don´t understand why it matters that causality is broken to observers as long as its not broken from the cause and effects "reality" as observed by them.
Even if something as in the example is launched 4xFTL in a 1LY distance, the message from an FTL observer to the effect would still reach the cause from the cause point of view after it has acted no matter how fast the message was transmitted.
Can you explain how a universal preferred reference frame would fix this time travel issue?
It doesn't actually... so... I recently got around to reading up on all the old luminiferous aether theories and... to make a long story short, the Lorentz-Larmor-(Poincaré) aether theory has the exact same 'effect before cause' thing happen as Special Relativity does when anything propagates faster than light.
This involves a really _long_ explanation that I don't really want to write in this RUclips comment right now unless someone replies to this and asks me to.
Question about equating the speed of light with the speed of causality, as opposed to the speed of perception: if we take the whole of your setup (rocket + moon + spaceship etc), and put it under water, thus slowing (?) the (apparent?) speed of light, could the lunatic not see the explosion before they see the rocket fired? Does the rest of the argument still apply? Does the reality of Cherenkov radiation invalidate the reasoning that ftl breaks causality?
Any interaction in which things located 299,792,458 meters apart in space, that occurs in less than 1.000000000 seconds (as measured by an inertial observer) will always have the "effect" occur _before_ the "cause".
This video doesn't really give a good explanation of the complete logic of FTL signals, and I'm not going to write an explanation here right now either, but yeah, it's the speed of light and anything else that is massless, in the vacuum, for all inertial reference frames, to make a long story short.
Nice video, as allways. But I have question. If the observer knows that cause is travelling faster that light, even if he sees the efect before the cause, he can conclude that in reality, the cause was befrore than the efect so no casuality broken
Around 21:50 is the causality breaking part. You can observe the event, conclude what actually happened, THEN send a FTL signal to stop the cause.
IMO, FTL signal will only cause the sender of the FTL rocket to see the response before they see the rocket impact (lightspeed), but this will not affect the "boom" or the launch.
If two rockets are travelling 0.9c in opposite directions parallel to each other does it mean from one rocket’s reference frame the other is 1.8c would there be any complications there?
Velocities don’t add up like that!
I would think that to show causality is broken you would show that be RESPONDING with a faster than light anti-missile missile, you could, you could hit the source before it launched.
Sir,we had an equation,which tells us about the relation between the change in inertial mass with change in speed,why does mass increases with increase in speed?
Throw that equation away and forget you ever saw it.
Mass _doesn't_ increase with velocity, *_momentum_* is what increases. This is an important distinction. Inertial mass is *constant*.
Anyway, for velocities greater than the speed of light, the momentum becomes negative and imaginary (square root of negative one).
Assumption 1: nothing can travel through space time that has positive rest mass at speed of light.
Assumption 2: nothing forbids space time itself to expand in faster than light speeds. (They do indeed for very far distances from us)
So if you pack the missile into a warp bubble and that warp bubble is responsible for contracting space in front of missile and expanding behind the missile could be still in its patch of space but the space could be displacing at speeds higher than c. No relativity violation! Than pretty close to the target the bubble breaks apart and the missile becomes subluminal and fires it’s engines and hits the target. So, from the point of view of anyone the missile was launched just after the bubble explodes.
There was a super luminal travel without any violation. No issues about light cone for observers.
No, this still has _the exact same problem_ It doesn't matter *_how_* you technologically accomplish the FTL motion.
And....
Spacetime in General Relativity... ok, the full explanation is going to take a long time to explain, but what your warp bubble is actually doing _is not moving a bubble of spacetime itself to a different location in spacetime_ What it is doing is "shrinking" the space in front of it, so the actual distance needed to travel is less, and "stretching" the space behind it. Yes, this destroys anything along its path. Also doing this requires several times the mass of the Sun in both positive *AND* "negative energy", which doesn't exist as far as we know. (It's like having inertial mass be a complex number. )
A great movie as always.
And by the way, maybe you have already seen him, but I recommend watching Andrzej Dragan and his book: "Unusually Special Relativity".
If you send this message FTL, from another observer travelling near FTL this could ring true, on your recommendation!
You're an phenomenal teacher! Einstein would be proud of you.
As always, great video! But with this in mind, wouldn't an alcubierre drive also break causality? Mount the rocket on one to get it to the moon in 3 months (but still slower than light, because thats how alcubierre work) and use another one to get the "dont shoot" message to the firing ship... !?
I love your videos
And the reverse is true, too. If a phenomena requires a reversal in time, it breaks the special theory of relativity. This happens in certain double-slit experiments.
I have a theory, based on several other theories I've read, that seems to remove the time paradox. Imagine you are moving through time and your position is (A). (A) is constantly moving through time in a loop at a set "time speed", lets also imagine a prior time (B) moving at the same "time speed". If you were to travel back to (B) any change you made would always stay with (B) as (B) moves through the time loop but would never reach (A) as (A) is also constantly moving at the same "time speed". If, after making changes to (B), you returned to (A) you would not see any effect. Note that however much time you spent in (B) would also pass in (A). So, if 1 unit of time passes you would then be returning to (A+1) from (B+1). However, if you were to remain in (A+1) for a period of time, say 10 units of time or (A+11), then return to (B), which would now be 10 units of time to the future of when you 1st visited, or (B+11), you would see the effect of any changes you made.
I didn't really follow this, because everyone travels through time at exactly one second per second (in inertial motion, i.e. no acceleration (long story)) That's just it... It's always one second per second. Where Relativity kicks in is that every location in space is also a _location in time_ and every location in space is in _the past_ of every other location in space. Yes, both ways symmetrically. Everything is in the past of everything else, and the further distance away something is in space is exactly how far away it is in time (in the past).
The only way two things will ever be "at the same time" is when they are "at the same place".
So, if you have two clocks located at the same place at the same time, you can synchronize them to start counting seconds together, 1, 2, 3, etc.
If you take one (or both) of the clocks and move them around through a different path in space, you also move them a different path through time. The clocks still tick one second per second, but when you bring them back together, they will have counted different quantities of seconds _because they traveled different distances in time_ and space from the first time their paths crossed to this second time their paths crossed. (That's all those "Twin Paradox" setups.)
If the clocks are not at the same location in space, then what is considered "right now" (the same numbered clock tick) is the 45° lines of their "light cones" on a Minkoeski diagram. Also, the -45° or 135° diagonal lines. Both are equally valid and you can't say that either clock is "before" or "after" the other clock as long as they are separated in space. (Seriously, events within the time window duration equal to their distance in space in light-speed units, can *not* be definitively ordered into "cause" and "effect". Any order is valid within that duration of time. (The window is of course zero when the clocks are zero distance apart, hence why you *can* synchronize them then.))
@@juliavixen176 I understand your confusion. I's a hard concept to explain in a format like this. Imagine a film reel every frame on that reel a snapshot in time. Each frame a separate reality, one that has its own past and its own future but following the same path. So, there are an infinite amount of you, each a slice of time existing in their own frame and moving through time at the same speed, so 1 second ago you, you and 1 second in the future you never meet, present you is always present you, moving through time. So, if you were to travel back 1 second in time you could meet your past self without creating a paradox as that self would be its own present moving through time and would never reach "your" present. Any changes made by you would remain in that present and if you were to return to "your" present you would not see any changes that you made nor would you suddenly have a memory of being visited by your future self as it would have never happened for the "present" you. Your "present" however would now have moved into the future by the same amount of time you had spent in the past and were you to return to the same exact time you left that would actually be a different reality, in the past relative to your actual present.
Have experienced this with sound standing next to a rifle target being shot at a huge distance. You hear the bullet hit the target before you hear the gun fire.
In the same way the lightning (effect) arrives before the sound of the thunder (cause)
None of this is an issue if you consider that physical interactions are symmetric over time, so any "effect" is mathematically consistent with being a cause. What the FTL ship sees is debris in space coming together in just the perfect way to form a moon and a missile, which travels backwards, collecting exhaust, until it slides perfectly into the launching tube of a destroyer. Entropy has been reversed, but no other laws of physics have been broken.
Consider if the FTL ship tries to "prevent" the missile launch by firing something like a brick wall into the missile's path. Then from the astronaut's perspective (who's moving through time "normally"), debris from a wall will appear to spontaneously come together from all directions in space to form a missile.
Issues only arise from the assumption that agents have the "free will" to change future events and a desire to say entropy can only increase/decrease one way.
Put even more abstractly, the common sense notion of causality creates an asymmetry in time, where past events can't be changed, but future ones can. But the mathematics of physics have no such asymmetry, only a requirement of continuous change between states. You can tack-on the second law of thermodynamics, but that relies on a sense of probability that's not causal.
I want to rewatch TENET now...
I mean we are talking about an impossible event so if impossible event occurs things happens where human brain can't comprehend
Like we get infinite voltage if the rate of change in current in just a small capacitor increase abruptly like in time period of 0 according calculation hence whole universe will be destroyed because of this which is an impossible event
Thanks! But I still believe that FTL communication doesn't break anything. What gets broken is a subjective illusion of causality in certain observers' frame of reference. But Einstain's bending of time-space reminds me of an equation: if you adjust the left side, then something gets automatically adjusted on the right side, so the balance is saved. I also believe that this space-time bending has nothing to do with time travel.
Mahesh 20:42 why not the man in ship do the physics to conclude cause happens first similar to the astronaut?
Do you remember "breaking" speed of light by sweeping laser pointer across moons surface? Now imagine that missile moving from destroyer to planet is represented by one such swept of laser pointer and ftl signal from blue ship to destroyer (22:14) is represented by second swept of laser pointer. Is there any reference frame where the end of second swept happens before the start of first?
This video is correct about FTL signals reversing cause and effect, although the presentation kinda burries the actual reason in a bunch of descriptive stories.
It's a bit easier to understand the problem with "FTL anything" by analyzing a FTL round-trip journey between two distant locations in space. To make a long story short, observers at each location will see the FTL thingy come from the distant location's own past... _Both_ ways!
If something goes back and forth FTL several times, it will time travel further and further backwards in time each trip.
I'll write out a long explanation of this if anyone here wants to read it.
I'd love to hear the explanation
@mistersadfaceman4257 Woo-hoo! I should have prepared some text beforehand, but I'll try to summarize the important things. (I'm in a hurry at the moment and need to be doing other stuff than writing right now. )
Things to know about Special Relativity:
• Every location in space is also a location in time.
• When you look in a straight line directly in front of your nose to the distant stars, everything you see and feel and can effect you in any way *_right now_* is the 45° surface of hyperspace "light cone" on a 4D Minkowski spacetime diagram. (Like slicing a 3D cone into circles, you are looking at concentric spheres centered on your eyes, and spheres are slices of a 4D hypercone. It's a 45° line on the regular 2D diagram everyone draws in books and videos. )
• What you see *_right now_* is the past of everything everywhere else in the universe.
• Everything everywhere at every location in space is also located in the past of every other location in space. Your feet are six nanoseconds in the past from your head and your head is six nanoseconds in the past from your feet. The Moon is 1.2s in the past of Earth, and Earth is 1.2s in the past of the Moon. The Earth is ~600s in the past from the Sun, and the Sun is ~600s in the past from Earth. The Earth is 2,537,000 years in the past from the Andromeda Galaxy, and the Andromeda Galaxy is 2,537,000 years in the past of Earth...
... _right now_
• If you flip this around, every location in space is also located in the future from every other location in space. This is if you count t=0 as everything you can see *_right now_* which is everyone else's past.
• This is symmetric, both ways.
• The _only_ way for two things to actually happen "at the same time" is by being located "at the same place"
• Syncronizing remote clocks is a bitch.
• There is a gap of time between any two distant locations in space, equal to the amount of time it takes for light to travel between those two locations, during which events do not have a strict cause-and-effect ordering. Event "A" at one location and Event "B" at a distant location can occur "before", "simultaneously", or "after", each other if they both occur during this time period. (It's the diamond shape between two light cones on a Minkowski diagram. ) All arrangements are valid, because who gets to be called "right now" is an arbitrary choice.
• Oh, I should mention: Everyone and everything's own "proper time" clock always ticks at exactly one second per second, _always_ no matter what they do. Time dilation is everyone else's problem.
I think that's most of the basics. So, everyone's current moment of *_right now_* is synchronized with light (or any kind of light speed signals, but light is the most practical.) Everyone arbitrarily chooses whether or not they will align their own current "right now" time to be named the "past" or the "future" on someone else's clock.
Alice and Bob are located on planets or space stations or whatever, four light years apart. Let's pretend that they are standing still with zero relative velocity with each other to keep this simple.
Alice broadcasts a radio message: "At the tone, it will be 00:00:00 January 1, 2000... *BEEP* "
From Alice's reference frame, Bob will receive this signal on New Years Day 2004 _on Alice's own clock_
But Bob sets his clock to match Alice's clock. So the instant when he receives Alice's radio signal, _it _*_IS_*_ Jan 1, 2000 for him_
(Back to Alice for a bit) When Alice was broadcasting that message on New Years 2000... from Alice's reference frame, it was "currently" 1996 at Bob's location.
Ok, got all that? Here's the thing: This is symmetric. Swap the names "Alice" and "Bob" in the text above, and it's exactly the same. It's valid for either one or even both to decide when to set the "zero" time to start counting seconds from. They could even use a third location halfway between them, that doesn't change their timekeeping situation.
So, Bob declares that it's "now" the year 2000-Bob-Time, and so Alice is in 1996-Bob-Time. Alice declares that it's "now" 2000-Alice-Time and so Bob is currently "right now" in 1996-Alice-Time.
You can slide these scales back and forth however you want as long as the offset _is less than four years_ As soon as light can get from Alice to Bob (and the other way) the order of cause and effect becomes frozen into a single reality... because they have both been "at the same time" for each other's "current time right now".
If a Baby is born on Alice's planet in 2001-Alice-Time, that's 1997-BobTime. If a baby is born on Bob's planet in 2002-Bob-Time, that's 1998-Alice-Time. Which Baby was born first? The answer is that it is valid to say that both babies were born before, simultaneously, or after each other.
If a Baby is born on Alice's planet in 2005-Alice-Time, that's 2001-Bob-Time. So, a baby born on Bob's planet in 2000-Bob-Time *IS* born _before_ that baby on Alice's planet. (1996-Alice-Time)
Cool, got all that?
Faster than light signals travel from the future to the past. They outrun the t=0 "right now" present moment synchronization that keeps cause and effect and "the present instant" in order.
When Bob receives Alice's radio signal, he's hearing it live, exactly as it is broadcast "right now". It's not a recording, it's really happening.
If Alice transmits a Faster Than Light [FTL] signal to Bob in 2000-Alice-Time, and Bob receives it in 1998-Alice-Time.... and then Bob immediately replies with his own FTL signal back to Alice. Bob is broadcasting his FTL signal in 1998-Alice-Time... which should be 1994-Bob-Time... which means that Alice will receive Bob's FTL signal in 1996-Alice-Time.... *_Four years before Alice broadcasts the original message in 2000_*
(I did this math in my head, and so if it's off by 2 or 4 years: oops! But the round-trip time is always negative. )
Slower than light round-trip: positive time length
Light-speed Round-trip: zero time length
Faster than light round-trip: negative time length.
I have to go do other stuff. Ask if you have any further questions.
@@juliavixen176
Okay, so I think I understand now. Thanks
Love your vids. You should have a higher sub count. When the missile or anything travels faster than light, mathematically it enters the i coordinate. It does not travel back in time. Nothing can travel faster than the speed of causality, but what if space itself moves faster than light? Then we have black holes.
Sir What is a speed of a time
1 second per second
Can you do one on how transformations of space-time itself can preserve causality in the event of faster than light speed?
If our calculations on the expansion of the universe is correct, space itself can bend faster than the speed of light, right? And concepts like Alcubierre warp are mathematically sound (even if impossible to actually achieve without the presence of very hypothetical negative mass/energy).
The expansion of the universe isn't the boundary of the universe flowing outward, it's that every single spot in the universe is slowly getting bigger simultaneously. Like technically everywhere is the center of the universe due to inflation which is a little odd but it's accurate. So the space itself isn't really moving which is why it can seem to exceed the speed of light.
More like time appears to be moving backwards, much like being inside a moving train.
Now here's a thought experiment, have the train grow or shrink and see how object appear to move both on and off the train.
Some people will turn off when it's about FTL that won't happen but the relativity of simultaneity means similar things can really happen, where A comes before B for one observer and B before A for another
The real paradox occurs only if you can send a signal back in time.