Why is it often said that travelling faster than the speed of light would cause you to travel back in time? I don’t understand the reasoning behind this at all. I would think that travelling from point A to point B at FTL speeds would simply get you to point B faster than sublight speeds, much as a car is simply faster than walking… it shouldn’t get you there before you left point A.
Try to leave out the usual stuff about the impossibility of actually breaking c because of infinite mass/energy, please. (Besides, people said the same thing about the sound barrier.) I’m just interested in what happens once you skip that barrier.
I’ll take a crack at this as a layman, then one of the big hitters (hello Q.E.D., Ring, Achernar…) can come and set the record straight.
The universal speed limit is not the speed of light in a vacuum.
The universal speed limit is the speed of “information” from point A to point B. Light just happens to travel at that speed, since it’s composed of massless photons. (I believe massless particles must travel at c but I don’t understand why.)
In a sense, c is the speed of propagation of ‘reality’. If you could outrun c, you could “get a jump on reality” and so travel back in time. This also has interesting implications for causality - i.e. cause would no longer need to preceed effect if FTL travel were possible.
The reason they say that time would reverse for you if you were to exceed c has to do with how time acts at relativistic velocities. As you approach c time appears to slow down, and should you actually reach c, time would slow to zero. By extension, if you exceed c, time must begin to move in the opposite direction, from ytour perspective. Of course, as you point out, since one cannot travle faster than c (or even at it), this isn’t possible.
OK, I’ll take a stab at this, as an astronomer. Think about an “event” at P which causes a reaction at “R”, at a speed U>c, with respect to an intertial frame S. If we choose coordinates in S so that the time like and space like seperations (dt and dx respectively) are both greater than zero.
Now, if we transfer to some arbitrary frame S1, travelling at a speed v relative to S, then dt1 (the time interval in the new frame) is
dt1= gamma * dt * [1 - (v*U)/c[sup]2[/sup]]
but, v must lie between c[sup]2[/sup]/U and c.
However, this gives dt1<0. So there would exist a frame where R takes place before P, and so effect happens before cause.
Thinking about it from a practical point of view, if this could happen, and R was the breaking of a glass, we would see the glass break first in S1, without seeing what caused it to break.
From what I’ve read, in the initial moments of the Big Bang, the universe expanded faster than c. If this is true, I’m sure this didn’t cause it to precede its own origin, and it would seem to create a scientific inconsistency with the notion that travelling faster than c would cause you to go back in time. Then there’s the mess about tachyons, but I don’t know how much of that is real and how much is just Treknobabble…
Really, in a Newtonian world, it would be easy to travel faster than light without going back in time.
It is the addition of Relativity into the calculations that makes it impossible to travel faster than light without creating frames of reference that allow information, signals, to go back in time, and if you tried hard enough, you could go with them.
Sorry if that post sounded snarky. However, there is absolutely no way that at a speed which is an appreciable fraction of light, you can ignore relativity, its like saying that you can ignore the effects of the sun’s gravity on us.
Furthermore, if there was to be a theory which superceeded Einstein’s, then, it would have to incorporate Einstein’s theory of general relativity into it. That is to say, any such future theory, would have to hold with the principle that information cannot usefully travel faster than light, unless we get a unified quantum gravity theory, which I believe would be the only way of getting faster than light information exchange.
Well, Relativity does not rule out FTL travel, in the same way that classical physics does not rule out travelling back in time. All Relativity does is say that FTL travel is a causality violation. Now, causality is very well-accepted, to be sure, but there’s no fundamental reason that it has to hold universally.
You interest me strangely. So there might be a theory of Quantum Gravity which allows FTL information exchange. What effects would that have on causality?
No, perhaps not; in fact there is supposed to be a symmetry for time just as there is for charge and parity;
however in the universe as we percieve it has a direction of time dictated by thermodymamics and entropy.
If time travel was possible at all, then some person, human or alien, now or in the past or future, will decide to make extensive use of it; they would be foolish not to, as it is a useful tool for shaping the universe to suit yourself.
As i have attemped to point out on several threads, extensive use of time travel would cause events to change seemingly at random, destabilise the universe and affect its mass balance- so reversal of causality is not a good thing in the universe * as we percieve it*.
Nobody else answered this, so let me take a stab at it.
The simple answer is that the speed of light is a property of mass, but not of space. Space itself can expand at any speed. This causes no inconsistencies the way faster than light travel of a massed object would.
As long as we’re on the subject… are wormholes, tesseracts, and the like for real, or are they more Star Trek bs? Can you really fold space in such a way as to allow an object on the macroscopic scale (i.e., grander than an electron teleporting between shell states), to go from point A to point B without passing through all the points in between?
Faster than light and traveling back in time are equivalent in the following sense: If can construct devices that travel faster than light, you can use them to construct devices that can travel back in time, and if you have devices that can travel back in time, you can use them to travel faster than light.
As for the universe expanding faster than light, I don’t understand what that could mean. Expansion is not a velocity. The recession velocity is proportional to the distance. Expansion is measured in units of velocity/distance. If you get far enough out, the recession velocity will be greater than light.
Well, if you look at it from a purely mathematical standpoint, then the embedding equations of a black hole suggest that we only see half of it, and that space continues out to the other side and opens up again. So there is a justification for it. However, without any sort of evidence to support it, we can’t assume that this mathematical embedding structure describes reality.
Always remember, though that black holes and wormholes are three dimensional objects, and don’t look like whirlpools; a black hole is actually a black star and is spherical, and a wormhole if expanded with strings of negative energy would perhaps look like this.
Yes, but, for an advanced mathematical understanding you should think of spacetime as a four-dimensional surface embedded in a five-dimensional space, its appearance to our physical eyes notwithstanding. The whirlpool appearance of a two-dimensional surface embedded in a three-dimensional space is a means of visualizing this.
Acutally, a black hole is a singularity, a point if you will. It has no shape.
I’m not entirely certain about a QM Theory of Gravity which allows FTL information exchange, I think it has something to do with quantum entanglement, but that really is not my field, and I do not understand it at all. In fact, I’m fairly certain that such a thing could be light speed, but not FTL, simply because of causality.
b]Actually, a black hole is a singularity, a point if you will. It has no shape.**
oops- that’s right- the event horizon is the spherical (or spheroidal ) bit.
mea culpa.
But it does mean that you can approach a black hole and get pulled in from any angle; great fun.
(Besides, people said the same thing about the sound barrier.) I’m just interested in what happens once you skip that barrier.
