From: [j h k 6] at [namaste.cc.columbia.edu] (John H Kim) Newsgroups: rec.games.frp.advocacy Subject: Re: Realistic FTL? Date: 28 Sep 1993 21:29:17 GMT OK - my apologies for this being not really related to gaming, but it seemed that enough people here were posting with questions about relativity and *why* FTL implies time travel. This is a post about the real-life *theory* of relativity, which may and should be ignored to some degree in a fictional game. ---------------- Now relativity is based on a fundamental assumption: that anyone who is not accelerating (i.e. speeding up, or turning with the Earth, etc.) should see the exact same physics and the *exact same speed of light*. So if you are in a spaceship travelling at 0.5c relative to the Earth, and flip on your headlights - you see the light travelling ahead of you at 1*c = the speed of light. However, an observer on Earth also sees that light going at 1*c relative to _him_, so it is moving ahead of you at only 0.5c. This contradiction is resolved by saying that dilations of time and space occur between these two viewpoints (or 'reference frames'). This makes light different than sound or some other waves - the speed of light is a fundamental constant of the universe. ---------------- Instant teleportation like the stutterwarp or wormhole 'gateways' are just as much a paradox to relativity as pushing past the speed of light. To understand this you need to know a bit about time dilation: relativity says that someone going at speeds close to c relative to a given frame experiences time slower than someone standing still. Let's say you have two spaceships, piloted by twins Andy and Bruno, going at 0.8c relative to each other. Andy sees himself sitting still, and hot-shot Bruno is zooming ahead past him. Bruno, meanwhile, sees himself sitting still, and Andy is flying back past him in reverse. A day later (when he is 0.8 light-days ahead of Andy), Bruno activates a teleport device which pops him back to where Andy has zipped to. Now, from Andy's point of view, Bruno has been going at relativistic speeds, and thus his clock should be slowed down. Andy reads that a day has gone by; but Bruno should read that only 14 hours have gone by. From Bruno's point of view, _Andy_ has been going at relativistic speeds, and _his_ clock should only read 14 hours, while Bruno's reads a full day. In real relativity problems, this is not a contradiction unless Andy and Bruno are in the same place - because time and space are intelinked. The only way they can be in the same place is if one or both of them accelerates. When they accelerate, they no longer have a valid viewpoint in term of relativity, and their timeline can adjust to fit. If you allow *any* sort of FTL travel or communication, then a paradox like this can be constructed - and you have invalidated relativity's solution to why everyone sees light going at the same speed. Relativity relies on a relation between time and space. If you can teleport through space without a shift in time, then relativity is wrong. It can be salvaged, but only by throwing out causality, and thus introducing time travel. ---------------- Gravity *does* have an effect on light propagation, and is an important part of relativity - but it does *not* allow for breaks in relativity. In fact, the effect of gravity on light propagation was one of the first tests of relativity. During a solar eclipse, it is possible to view stars whose light goes right past the sun. These stars have their positions altered with respect to their usual positions: the light was bent by the Sun's gravitational field (an effect correctly predicted by General Relativity). ---------------- My conclusion is that, if you want to satisfy gung-ho science types, you have a few choices: 1) "Shut up and play!" This is actually the most reasonable, in my mind, and should definitely be considered. Relativity may or may not be given lip service as a theory which works - but no one actually deals with the mechanisms by which it is broken. Develop a set of reasonable, likely sounding rules for FTL (using important keywords like 'quantum tunneling' and 'wormholes'), and ignore the consequences to relativity. 2) "Relativity is *wrong*." There actually are preferred frames - just that our observational techniques here on Earth are too limited to notice them. The effects are large ones across space - which are only noticable if you look on scales larger than the solar system. One version of this was my 'sub-space currents' theory which I posted before. This is 'hoaky' in that relativity has made many darn good predictions, some of which must turn out to be just coincidences in order for this to work. But frankly, you're going to have to do worse than this in any other FTL explanation, as well. 3) "We're violating causality, and we don't care." Relativity is true, time travel *is* possible, and the universe really is a wacky, crazy place. I recommend against this. If you want the universe wacky and crazy, throw out relativity and use solution #1. Frankly, no one will understand this universe - which has time travel considerably less sensible than that in _Star Trek_ and much more complicated. --------------------------------------------------------------------------- John Kim | Less well known last words: [j h k 6] at [cunixb.cc.columbia.edu] | "I fireball the fools who dare to hijack Columbia University | the _Hindenburg_."