# Ok...there seems to be Physics Gurus here - Faster than light = time travel question

I’m pretty new here (though I lurked many years ago for a bit)

When I was younger I liked to read about astronomy/speed of light things like that.

Recently, for some unknown reason, I got into a debate with my FIL about faster-than light.

IF faster than light is possible then I remember that would be the same as time travel. Does anyone have an example to explain?

I was trying something like ‘instantaneous travel (FTL)’ Go instantly to a light year from Earth and you could receive a message from your future because your future self (ahead half a year) knows you did this so he went HALF a light year minus a few minutes away and signals outward. When you arive you will hear your future self’s signal.

Is this legit? There has to be a better example…

I don’t think so.
If you could travel faster than light, you could in some sense, look into the PAST, because that information would not have reached your present position yet. I don’t think you could actually GO to the past, though.

My example…trying to think it through…
Let’s say you invent a instantaneous telelportation device which allows you to teleport up 1 light year away. I then do so…and come back.

6 months later, I find that stock XYZ went up 10000000% in value and wish I would have bought some. I then decide to teleport .5 light years away minus 5 minutes. I then broadcast out . My past self will be 1 light year away. Therefore, my signal should reach him 5 minutes after he arrives telling him (me) to buy XYZ stock.

My brain hurts.

I don’t understand this. Your past self teleported home 6 months ago, he’s not still there to get the message.

What past self? Didn’t you say you came back?
I don’t think that just traveling faster than light is going to equate to time travel.
Say I teleport to another galaxy and teleport back - I’ve just traveled a long ways way faster than light could do it, but I haven’t gone either forward or backward in time.

Ok…you have me doubting myself…

My ‘past self’ will be 1 lightyear away at time zero. At time 6 months, I know this…so I go out half a light year minus 5 minutes. When I send a signal…it will arrive at the one light year mark when my past self is there.

Am I totally off base on this?

The wikipedia article on time travel is pretty good:

Now you have ME confused…

When you say " I go out half a light year minus 5 minutes", what do you mean. A light-year is a measure of distance, 5 minutes is a measure of time.

Yes.
I’m guessing the “FTL=Time Travel” bit stems from a misinterpretation of a relativity thought experiment, in which a man hops in a rocket, flies through space at near-light speeds and returns to Earth to find himself physically much younger than his twin brother, having experienced time dilation and “felt” only a few months while his twin “felt” several decades. Since the dilation effects increase as one approaches c, it’s assumed, I guess, that if c is acheived, the rocket traveler’s local time stops (relative to his Earth-bound twin) and if he exceeds c, it moves backward.

A light-year is a measure of distance, not time.

Rewrite your question and substitute “5,865,696,000,000 miles”, for “light-year”, and then see if the question makes sense.

Assuming that special relativity is correct, you could, in fact, go into the past.

The short version of the explanation relies on the fact that the concept of “simultaneity” does not exist in relativity; or rather, it depends on which reference frame one chooses (all of which are equally valid). As an example, suppose we have two stars which are dying, and which are at rest relative to each other. Further suppose that, to an observer at rest relative to the two stars, they both go supernova at the same time. If that’s the case, then to an observer flying from star A to star B, star B exploded before star A. Meanwhile, to an observer flying in the other direction, star A exploded before star B. If you take an observer, and that observer measures the positions and times of both supernovae, he can calculate the value u = (x[sub]B[/sub] - x[sub]A[/sub]) / (t[sub]B[/sub] - t[sub]A[/sub]). You’ll notice that this value has units of a speed, and if you do the math, this speed will always be greater than the speed of light (if you’re in the frame at rest relative to the two stars, it’ll be infinite, since in that frame, t[sub]B[/sub] = t[sub]A[/sub]). Note, by the way, that this isn’t actually the speed of anything; it’s just the result of a calculation. But even though this speed u is always greater than c, it can be as close to c as you like. As your observer’s speed relative to the stars approaches c from below, this calculated speed approaches c from above. In fact, it turns out that u = c[sup]2[/sup]/v , where v is the speed you’re going relative to the stars.

So, now suppose you’ve developed a sufficiently advanced technology which lets you travel at an effective speed of (say) 1.002c. It doesn’t matter how this technology works, whether it be a wormhole, a warp drive, or a magic carpet: All that matters is that it can get you from point A to point B, and arrive there before a photon travelling through vacuum could get there. Let’s now bring this FTL device on board an ordinary rocket. Since all reference frames are equally valid, we can, if we want, start off in a reference frame travelling at .999c, in the direction from star A to star B (we can get into this reference frame using our non-warp drive rocket engine, if we’re not in it already). So we fly at this velocity right past star A, and we time our trip so that, just at the moment we pass the star, it goes supernova. We’ll call this position t = 0, x = 0 for simplicity. We know, then, that after a time t = t[sub]B[/sub], and at a distance x = x[sub]B[/sub] from us, star B explodes, and from our previous calculation, we know that u = x[sub]B[/sub] / t[sub]B[/sub] = 1.001c.

Well, now suppose that right at (0,0), as we’re passing the supernova of A, we aim our warp drive at B, and turn it on. Our warp drive has an effective speed of 1.002c, which is faster than u. So our warp drive actually arrives at star B a little bit before B explodes.

But wait a moment, here: We already said that, in the reference frame where the stars are at rest, both supernovae happen at the same time. The problem is completely symmetrical. So we can have a traveller going the opposite direction at high speed, who catches our warp drive at star B, and sends it back to us as soon as they catch it. By the same reasoning above, if it leaves star B a little before B explodes, then it’ll come back to A before A explodes.

But the last time the warp drive was at star A, it was right at the moment of explosion. Now here it is, having travelled some ways, and returning to A before A explodes. In short, it’s gone back in time.

Is there any way that Relativity could ultimately turn out to be wrong (in the same sense that Newtonian mechanics was “wrong”) that: (a) wouldn’t contradict our observations so far and (b) would allow for faster than light travel?

Man, what if, like, “C-A-T” spelled “dog” ?
[sub]That’s the only line from that movie I remember[/sub]

I imagine if there was, it would replace the current model of physics.

I read an article a long time ago by, I think, the late, great, Isaac Asimov. One of the points he made is that relativity did not invalidate Newtonian physics, it just…uh…“further refined” it in the special case of very high speeds and such.

In other words, his point was that for everyday use, Newtonian physics is just fine, but for certain special situations, relativity provided a more accurate solution.

Following that, I think it likely that a newer and better theory (if found) would also just further refine relativity for some special cases.

As to what it might mean for something to prove that FTL travel, time travel, etc., was allowable and provable in some limited fashion, I simply don’t know; but I do seriously doubt that it would invalidate everything else (relativity and Newtonian physics).

Disclaimer: I don’t pretend to be any sort of expert on this. I barely made it through calculus in college–don’t even mention tensor equations in my presence!!

I seem to recall that tachyons (that are hypothetical subatomic FTL particles) were suggested as a way to inteact with the past. Not time travel, but across-time communication.

On the other hand, wiki seems to indicate that it was ruled out.

My appologies…
Replace “suggested” with “speculated” in the above post.

If you read this as “I go out half a light year, minus 5 light minutes.” i.e. 2.9x10[sup]12[/sup] miles minus 5.59x10[sup]7[/sup] miles, it makes more sense.

The OP made me think of an Arthur C Clarke book, The Light of Other Days. Wormholes used to see across vast distances are used to see back in time when a friend of the inventor accidentally stumbles across the same sort of idea that BlinkingDuck had.

Can you explain the bolded portion? I thought division by zero was undefined. Did you mean to say that it’ll approach infinity?