# Faster than light time travel

1. Jun 15, 2011

### abledpilot

Hello, i'm a high school senior interested in studying physics in college. I was listening to a podcast by Neil deGrasse Tyson in which he stated that if one could travel faster than the speed of light, it might be possible to travel back in time. He didn't really expand on that so I was wondering if someone here could explain this to me.

2. Jun 15, 2011

### bcrowell

Staff Emeritus
Either you misunderstood him or he made a mistake. Material objects can only travel at less than the speed of light.

3. Jun 15, 2011

### abledpilot

I think he was speaking theoretically, and that if you plug in a time faster than the speed of light into Einstein's equations time starts to flow backwards.

4. Jun 15, 2011

### nitsuj

Things that have happened cannot be undone. no matter the frame.

Simularly, I can add numbers to 299,792,458m/s, however that doesn't impact the reality. What point would it be to see SR/GR equations compute time reversal at FTL speeds, it's beyond the predictability of those equations I'm sure (let alone reality).

I don't know in what context Neil deGrasse Tyson was speaking from. It seems a very odd statement for SR/GR.

Last edited: Jun 15, 2011
5. Jun 15, 2011

### bcrowell

Staff Emeritus
No, that's incorrect, so either he made a mistake or you misunderstood him.

-Ben

6. Jun 15, 2011

### AndyUrquijo

Well, as you tend to the limit of traveling at the speed of light time dilations increases. That is, time appears to 'slow down' (on the outside of your spaceship). If you extrapolate this tendency just for the fun of it you would say that time stays 'still' at the speed of light, and taking this to a further step would lead you to say that time goes 'backwards' at higher speeds.
So its not crazy talk, just a baseless or "hypotetical" speculation. You can't even travel at the speed of light, much less at a higer speed*.

*at least as our observations appear to confirm us so far

7. Jun 15, 2011

### bcrowell

Staff Emeritus
No, that's incorrect. If you plug v>c into the equation $\gamma=1/\sqrt{1-v^2/c^2}$, you don't get a negative result, you get an imaginary one.

-Ben

8. Jun 15, 2011

### BruceW

In special relativity, causality is limited to one direction because of the speed limit of c.
(It is possible to reverse the order of two events, by looking at them through a different reference frame, but this requires that the two events must be separated such that one event could not have caused the other).
But, if there was a wormhole between the two events, then you could in principle reverse the order of two causally connected events.
But of course, a wormhole doesn't mean travelling faster than c. Nothing can travel faster than c. So you'd be more correct to say that in certain situations, the curvature of spacetime could lead to the reversal of the cause-and-effect of two events.

9. Jun 15, 2011

### AndyUrquijo

Yeah, my bad. I didn't mean to say that was correct. I just think this was probably the (naive) reasoning that went on to make such claim. You are right of course. And you get an undefined result when you use v=c. Which is irrelevant since you can't get to that speed to begin with (if you have non-zero mass).

10. Jun 15, 2011

### DrGreg

It's possible he was referring to the tachyon, a hypothetical particle which, if it existed, would always travel faster than light and would never go slower. There are many good reasons for believing that tachyons don't exist, but nevertheless we can consider how they might behave if they did.

If a tachyon is travelling faster than light forwards in time relative to one inertial observer, it is travelling backwards in time relative to some other inertial observers. If you could relay a message via at least two tachyons, you could send a message into your own past. See Tachyonic_antitelephone for details. That would give rise to all sorts of grandfather paradox-like problems, one reason that we suspect tachyons don't exist -- there are other reasons, too.

11. Jul 15, 2011

### byron178

Why could virtual particles travel faster than light but yet i don't hear them violating causality.

12. Jul 21, 2011

### kamenjar

Last edited by a moderator: May 5, 2017
13. Jul 21, 2011

### Aimless

If, by FTL, we mean the ability to travel between two spacelike-separated points, then in that sense there is no distinction between faster-than-light travel and time travel, because one can easily draw a spacelike curve which ends within the past lightcone of its origin. (Assuming Lorentz invariance; if there's a preferred foliation which restricts such trajectories, then that's another matter.)

14. Jul 21, 2011

### abaio

Actually a few things travel faster than light but are not helpful or useful. For example, the EPR parardox that information between 2 electrons is shared instantly and faster than lightspeed, however it proves no use if the information cannot be controlled and only random.

15. Jul 22, 2011

### GrayGhost

Yes, but I'm supposing we don't really know that the other entangled particle changes state instantly upon the 1st being observed, even though we might casually assume such. There's no proof it's instant, yes?

GrayGhost

16. Jul 22, 2011

### abaio

I believe we do know its instant. Thats why Einstein himself referred to the event as "spooky". Either way, there are still other ways to conquer the speed of light.

17. Jul 22, 2011

Coming from a very pure math background, I see why travelling at or faster than light screws up all the math formulas, but physically why can't an object travel at (or faster) than light?

What fundamental law of the universe could you be breaking? Thanks.

18. Jul 22, 2011

### abaio

Well photons (the particles that make up light) have 0 mass so it is very easy for them to stay at a constant speed through the vacuum. It takes extremely high energies to harness the speed of light, energies that we cannot quite reach yet (suck as the planck energy). However certain theories have been originated where negative energy and exotic matter may propel faster than light travel. But as far as breaking any laws it would just really screw up mathematic equations as you mentioned before. (You should look up a particle called tachyons which may be able to break the light barrier)

19. Jul 23, 2011

### mathfeel

It take infinite work to accelerate any massive particle from subliminal speed to speed of light.

20. Jul 23, 2011

### DaveC426913

That's pretty much it in a nutshell.

There's several ways to look at it.

Mass increases without limit as you approach c. Mass would exceed total mass of universe before you got there.
Energy required to accelerate that mass would exceed total energy of universe.

21. Jul 23, 2011

### CJames

The invariance of the speed of light. Every experiment that has been performed to measure the speed of light has determined that it is always c. This measurement is independent of the "velocity" of the observer, and the velocity of the observer can only be stated relatively speaking.

In short, even if an observer were traveling at 99.9999% the speed of light in comparison with the earth, that observer would still measure the speed of light to be c in comparison to them. In other words, the speed of light is c relative to any observer. It is the only speed that has this unique property, and it is the reason for the "time warps" that occur for observers who are traveling near c.

Obviously you can never travel faster than something which is always 300 million m/s faster than you.

22. Jul 23, 2011

### CJames

I think the standard interpretation of this is that the information is non-local, which is more like saying that the system is in two places at once than saying that information is traveling from one particle to another at faster than the speed of light.

23. Jul 23, 2011

### CJames

Most likely he was referring to "spacelike" trajectories. Suppose you observe two events taking place "simultaneously." There are reference frames in which event A occurred before event B. There are also reference frames in which event B occurred before event A.

The catch here is that event A couldn't possibly have caused event B, or visa versa, because light hasn't had enough time to connect the two events.

For this reason, if somebody could travel from point A to point B, they would be traveling backward in time from the perspective of some reference frames. This means that if they had a method of doing this, they could theoretically travel backward in time to any event that they wished.

Obviously this is all theoretical and there is no reason to expect that it is possible given the current laws of physics. For sure, it is impossible to travel from event A to event B, or visa versa, simply by accelerating faster than the speed of light, unless the laws of physics as we know them break down at extremely high energies.

24. Jul 23, 2011

### GrayGhost

My understanding, is that the mathematical problem arises only at v=c, whereby an anomaly occurs. Physically, no material particle can accelerate to speed c, because it would take infinite energy and the universe is assumed to possess finite energy. If I recall correctly, "faster than c" involves an imaginary spacetime from our perspective. There is a theory (or hypothesis maybe) that any mass traveling at >c might be possible assuming it were never c or less to begin with. It could not attain a relative light speed either. However, there has never been any physical evidence of such in nature to date.

GrayGhost

25. Jul 23, 2011

### BruceW

CJames has written a good explanation of what people are often talking about when people say "exceeding speed of light would mean time travel."
Of course, locally exceeding the speed of light is impossible, which is very explicit in Einstein's relativity. Which is why I think this phrase would be more accurate: "the speed limit of c means that in special relativity, the reversal of causality is not allowed".
In general relativity, you still can't locally exceed c, but due to curvature of spacetime, it is possible to imagine wierdly shaped universes where you could effectively time travel.