# What has the speed of light got to do with time travel?

1. Aug 29, 2008

### sammy8761

Just wondering what the connection is between them, I understand that it relative to what we see but time in general? why not the speed of sound as its only another sence right? Just interested to know why light is so special?

thanks guys,

Sorry messed up title should be speed of light! doh!

Last edited: Aug 29, 2008
2. Aug 29, 2008

### Staff: Mentor

Re: What has the speed of sound got to do with time travel?

Light is special because it does not require a medium in which to propagate.

3. Aug 29, 2008

### sammy8761

Re: What has the speed of sound got to do with time travel?

I apologise but im afraid i dont understand.

4. Aug 29, 2008

### Janus

Staff Emeritus
Re: What has the speed of sound got to do with time travel?

The speed of light is the only speed that everyone measures as having the same value relative to themselves.

So what does this have to do with time?

Consider this scenerio:

You have a two people, one riding a train and one standing on the embankment. Lightning strikes at two points along the track at equal distances from the second observer. The light from these strikes arives at this person at the same moment the person riding the train passes him. Thus both people see the lightning strikes at the same time.

Each person is also able to measure the speed of that light relative to himself. when the embankment observer does this he determines that the strikes occured at the same instant, as the strikes were at equal distances from him and the light from each strike traveled at the same speed relative to him.

The person on the train will also measure that the both flashes of light traveled at the same speed relative to himself. But he also knows that he is in motion relative to the points at which the strikes took place. Thus for him to be at a point an equal distance from the strikes when he sees them, he had to be closer to one strike than other when they took place, abd thus for him to be able to see the light from both strikes at the same time, the strikes themselves would have to have taken place at different times (one strike would have had to take place before the other.)

Thus we have two observers seeing the same lightning flashes but obe concluding tthat the strikes took place simultaneously, and the other concluding that they did not. All because both measure the same speed for the speed of light.

5. Aug 29, 2008

### epkid08

Define time travel

6. Aug 29, 2008

### peter0302

The speed of light has nothing to do with time travel except in science fiction stories.

There is no reason to believe that "exceeding the speed of light" would result in backwards-in-time travel. Any such idea is mere speculation. The equations say that your clock would be mvoing in imaginary units - multiples of 'i' - which has no meaningful physical interpretation.

The only even remotely credible possibility of travelling backwards in time involves certain paths in general relativity involving extremely high mass-energies and gravitational fields. It has nothing to do with the speed of light (except to the extent that the GR equations have c as a parameter).

7. Aug 29, 2008

### JesseM

Janus gave an explanation of why, if the speed of light is the same in each observer's frame of reference, that means different observers must disagree on whether events at different locations, like the two lightning strikes, happened simultaneously or at different moments. You can also watch a youtube video here which gives a nice illustration of the train thought-experiment. But I want to add a little to this to make the connection to "time travel" more clear. One of the basic principles of relativity is that each inertial (non-accelerating) observer measures the same value for the speed of light in a vacuum, but another principle is that all the laws of physics appear the same in each observer's own rest frame. This means that if I am in a sealed windowless spaceship moving inertially, and you are in a different sealed windowless spaceship which is in motion relative to my ship (and also moving inertially), then if I do some experiment on board my ship and you perform an identical experiment on yours, we should always get the same result. Now, imagine we both had some device which could transmit information faster than light--"instantaneously", let's say.

Now let's repeat the idea of the train thought-experiment, and suppose the observer on the side of the tracks has an FTL transmitter at the location of the strike at the back of the train, and at the moment the strike happens this transmitter sends a signal to a receiver at the location of the strike at the front of the train. Since both strikes happen simultaneously in the frame of the observer on the side of the tracks, and since the transmitter transmits information instantaneously in the frame of the observer on the side of the tracks, that means the receiver will get the message about the strike at the back of the train at the same moment that the lightning is striking right next to its own location at the front of the train (of course the train is moving relative to the receiver, so its location only coincides with the front of the train for a moment). But now remember that in the frame of the observer on board the train, the lightning actually hit the back of the train after it hit the front of the train. So in this frame, the receiver is actually receiving information about an event that "hasn't happened yet"!

This wouldn't be so bad if we just imagined that one frame's definition of simultaneity was the "correct" one and the other wasn't. But if the laws of physics work exactly the same in every frame, that must apply to whatever laws of physics govern the FTL transmitter too...so, if it's possible to build a transmitter which sends information back in time according to the train-observer's definition of simultaneity, it must also be possible to build a transmitter which sends information back in time according to the track-observer's definition of simultaneity. It works out so that if you are moving away from me at some significant fraction of light speed, and we each had FTL transmitters of this kind, then I'd be able to send a message to you which traveled "instantaneously" in my frame but "backwards in time" in your frame, and then you could immediately send a reply which traveled "instantaneously" in your frame but "backwards in time" in my frame, and the result would be that I'd actually receive your reply before I sent the original message! In this case every frame would agree that causality had been violated and that information had traveled back in time. Of course, this is a pretty good argument for suspecting that FTL communication is forbidden by the laws of physics...

8. Aug 29, 2008

### JesseM

9. Aug 30, 2008

### peter0302

I don't, and didn't, equate "time travel" with backwards-in-time signaling.

10. Aug 30, 2008

### JesseM

Well, normally I think physicists would interpret time travel as causality violation, and certainly sending information backwards in time would be a violation of causality. And if you can send information back in time I think you should be able to do quantum teleportation backwards in time too.

11. Aug 30, 2008

### peter0302

If you define quantum teleportation as "travel" ok... since the source object is destroyed I don't think that's what the OP has in mind.

To me time travel is the idea of moving a person either backwards in time or forwards in time faster than normal. And the common myth in sci-fi and I think in the general public is that travelling faster than light would allow you to do this, just like in Superman reversing the rotation of the earth sends you back in time. There isn't even a theoretical basis to believe that you would be travelling backwards in time if you exceeded the speed of light. Your lorentz factor would be imaginary, not negative, implying it's a nonsensical scenario.

I do agree with you that the only thing standing in the way of FTL or B-I-T signaling is causality problems, which itself is a dubious concept. We all know in QM traditional causality gets thrown out the window. Maybe there will be a way to learn information about the future via a signal from it; but that doesn't mean anyone suddenly found themselves hundreds of years in the past. That will never happen.

12. Aug 30, 2008

### JesseM

According to quantum indistinguishability I think it's basically meaningless to ask whether the teleported system is really the "same" as the original system or whether it's really a "copy", for the same sort of reasons that it's meaningless to ask which slit a photon in the double-slit experiment "really" went through in cases where you see an interference pattern (under certain interpretations of QM these questions might have a true answer, but an answer that would be impossible in principle to determine experimentally).
There's no theoretical basis for saying that the Lorentz transformation would allow you to figure out how things look in the "frame" of an FTL object, no--the principle of relativity would be violated by such a frame. But if tachyons existed, and if they could interact, one might be able to design some type of "clock" out of tachyonic matter even if its rate of ticking at various speeds could not be inferred from the Lorentz transform. And it is true that if such a tachyonic clock could exist, then if it was ticking forward in some frames it would necessarily be ticking backwards in others.
"Causality" in physics is not really the same as vague philosophical notions of "cause and effect" (which are dubious even in classical physics because classical laws are time-symmetric and thus there's no basis for saying the present state of the universe is determined by its past state but not by its future state). In quantum field theory it is still provably impossible to use measurements at one location to gain information about any events outside the light cone of that point, so quantum theory is still said to obey causality.
In terms of causal paradoxes, being able to send information into the past is every bit as problematic as sending matter into the past (you could send a message to your younger self trying to get them to change history, or even upload your mind into a computer simulation and send your mind back in time), so I don't see why you'd be so confident that the latter will never happen if you think the first is possible. General relativity does open up the possibility of material objects going back in time and interacting with earlier points on their own worldline, though many physicists would probably bet on quantum gravity eliminating this possibility.

13. Aug 31, 2008

### peter0302

Ok, I take it you'll be first in line to volunteer for the quantum transporter?

Two reasons: 1) signaling is not travel; 2) MWI avoids causality paradoxes.

14. Aug 31, 2008

### cristo

Staff Emeritus
As stated above, "signalling" is sending information into the past. How do you propose this information is sent?

15. Aug 31, 2008

### Antenna Guy

You don't seem to realize that logic falls with the domain of philosophy.

Regards,

Bill

16. Aug 31, 2008

### JesseM

Quantum transportation of macroscopic objects is pretty ridiculously futuristic, but if some form of transporter had been invented (even a non-quantum one that was able to measure the position and momentum of every atom in an object up to the limits of the uncertainty principle) and had been tested on other brainy organisms with no evidence of long-term problems...well, I still probably wouldn't volunteer to be the first human to try it (their might be some kind of subtle errors in reproducing the brain that just weren't apparent in animals), but my objections wouldn't be on the philosophical grounds that it'd just be a "copy" and not really "me" (after all, the matter which makes up my brain is almost completely replaced by new matter every two months or so according to this article, but I'm not worried that I'm going to be replaced by a 'copy' in two months just because my brain won't be made of the same atoms).
You can't have it both ways! If the MWI avoids the dangers of paradoxes for sending information back in time, then it would obviously do the same for sending physical objects back in time (which as I said might be possible according to general relativity). The point is that there is absolutely no good reason to think that sending objects back in time is any more physically problematic than sending information back in time.

17. Aug 31, 2008

### JesseM

What's your point? Just because logic is a part of philosophy and is also used in science, that doesn't mean all philosophy (including notions of 'cause and effect' which are not part of logic) is part of science too. Just think of it as a Venn diagram with "logic" in the overlap between the circle marked "philosophy" and the circle marked "concepts essential to science".

18. Aug 31, 2008

### peter0302

Wow, ok, a very interesting (and consistent) viewpoint.

Well you didn't my earlier post where I mentioned GR opens up possibilities for time travel. But if you do go into the past - or send a signal into the past - it will be a different past than yours and won't affect yours. You can't do the Back to the Future thing where you change the past and come back to a different future. I think mainstream physicists are unanimous that that is not possible under any theoretical model. Anyway, that's "time travel" to me. If the OP meant something different, he should clarify.

Oh and Cristo, I don't know what your point is. Signaling and travel are two different things because, if for no other reason, one can happen at the speed of light, and the other cannot.

19. Aug 31, 2008

### Antenna Guy

That you don't seem to understand philosophy.

"cause and effect" and "if this then that" are different ways of saying the same thing. Consider: "if these are your initial conditions (cause), then this is what results (effect)". Causality is not the ambiguous concept you make it out to be.

Regards,

Bill

20. Aug 31, 2008

### JesseM

Perhaps you could point out what specific aspect I don't understand.
Not according to any philosopher I've ever read. For example, we could say that "if X is a human, then X must have a mother", but you wouldn't say the person "caused" their mother. You can read the article The Metaphysics of Causation from the Stanford Encyclopedia of Philosophy to see a discussion of causation in philosophy (along with related articles like Causal Processes and Counterfactual Theories of Causation and Probabilistic Causation and Backward Causation), it's a lot more complicated then just logical implications of the form "if X, then Y" (if you think there are any professional philosophers who say causation is nothing more than logical implication, please name them).

Last edited: Aug 31, 2008