Faster than light time travel

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.

bcrowell

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

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.

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.

bcrowell

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

-Ben

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

bcrowell

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

-Ben

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.

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).

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.

byron178

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

kamenjar

Oh ok, so FTL message transfer is paradoxic because formula says so :)))) I don't mean to offend - But do you realize how hilarious that sounds to someone who doesn't rely on formula doctrine? In just another thread on this forum ("A photon's time") it is agreed that photon's frame can not be expressed with Lorentz transformations which are used in the Tachyonic Antitelephone example. It's the same as saying that GR is wrong because newtonian formulas give wrong results at relativistic speeds.

Does anyone have a better thought experiment that doesn't involve formulas that are likely not accounting for FTL? I'm sure there has to be something.

And FYI, I am not saying that FTL is possible... All I am saying is that formulas are not useful answers to reply to thoughts outside of things that we can experiment with.

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.)

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.

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

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.

nkadambi

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.