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OT: Relativity (Was: OT: Helen Keller & Whorf-Sapir)

From:Caleb <cph9fa@...>
Date:Tuesday, August 17, 2004, 3:25
> Perhaps you can comment on something for me.
I'll try to.
> Einstein basically said that if one could move > faster than light (ignoring the problems with that for > now) then one would catch up with light-cones that had > already escaped one's reference frame. But is this > time-travel? Because the reflected light signals and > what-not are NOT the object(s) which reflected them.
So far, you are correct. This by itself is not time travel, but I don't think scientists would claim this is time travel either. AFAIK, if you were to move faster than light from point A to point B, and then look backwards (through a telescope), you would see two things. First of all, you would see an image of yourself during the journey, but in reverse, as all the light waves you emmitted finally caught up with you. Strictly speaking, this is _not_ a hologram. It is actual lightrays from the actual you in the past (just like looking at distant galaxies is looking into the past). Secondly, you would see an image of yourself at your origin (from before you left). Eventually, the two images would merge when they met each other. You cannot directly interact with these images anyway, because they are far away. Here's where things get sticky. As long as you are travelling slower than c in one reference frame, the Lorenz transformation guarantees that you are moving slower than c in all possible reference frames. But once you break the light barrier in any reference frame, the same equations guarantee that you will not only be going faster than c in all possible reference frames, but also that there will be reference frames in which your trajectory goes backwards in time. This has to do with the fact that different reference frames can't agree on the definition of simulataneousness. This is _much_ easier to understand when you see a picture! Let me see if I can give an example that I read once. Its a twist on the old twin paradox. This example is slightly different than the one above, because it uses wormholes, but the principles remain the same. Lets say we have twin A and twin B. Twin A stays at home while twin B travels to a nearby star and back (slower than light). Lets say that the relativistic effects are such that B's trip lasts 5 years in B's frame, but 25 years in A's frame. So when B gets back to Earth, he is 5 years older and his brother is 25 years older. Now for the twist. Lets say that B carries one end of a mini-wormhole with him, and A keeps the other end at home. For sake of argument, lets say they hold hands through the wormhole. This is only to show that time passes at the same rate for each through the wormhole. That is to say, B's hand does not grow old five times faster than A's. Put another way, if B stepped through the wormhole, he would not step into a spaceship where things were accelerated 5 times faster than normal. Just at the end of five years (in both frames) B tells A that he is getting home. Of course, to A, this is ridiculous, since in his reference frame the ship isn't even halfway to the turnaround point. B isn't due to arive home for another 20 years still. But B assures A that he really is home. Now we have a wormhole on earth in which one mouth is stationed 20 years in the past of the other one. If B stepped through the wormhole, he would reach A 20 years in the past, and would be able to look through the telescope and see his ship not even half away to the turnaround point. He could even spend the next 20 years constructing a giant laser, and shoot his ship just before it returns home. Or, he could wait for the ship, and then meet a 20 year younger version of himself (who then steps through the wormhole to become himself 20 years in the past). Does your head hurt yet? :-S This same type of paradox could also be acheived via 'normal' FTL travel, without the help of wormholes. Scientists are seriously studying wormholes, though, to see if there is any theoretical way to allow FTL travel without causing a time paradox (which most scientists feel are forbidden by physics). AFAIK, there has not been any progress in this regard yet. In fact, so far as I can tell, once you admit FTL, time-travel becomes almost trivially easy. This is unfortunate, because I believe that time-travel is impossible, and yet I want to create believable sci-fi with FTL. :-( ~Caleb


Mark P. Line <mark@...>