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*To*: starship-design@lists.uoregon.edu*Subject*: Re: starship-design: FTL and time travel*From*: "Kyle R. Mcallister" <stk@sunherald.infi.net>*Date*: Thu, 28 Aug 1997 15:58:53 -0700*References*: <9708282030.AA17235@bit.csc.lsu.edu>*Reply-To*: "Kyle R. Mcallister" <stk@sunherald.infi.net>*Sender*: owner-starship-design

Isaac Kuo wrote: > > Kyle R. Mcallister wrote: > >I think its safe to say this: > > >1) FTL probably won't be possible within the next 200 years. As far as > >the distant future, I'm convinced that we will someday find a way to do > >it right. Right now, we can't even speculate on the tech necessary > >though. > > Well, the good news is that even if FTL travel turns out to be > impossible, there will always be countless humans who will be > convinced it must somehow be possible, so it won't be because > we stop trying. (Assuming we don't go extinct, of course.) > > And of course, I'm assuming that those humans who try to figure > out how to go FTL aren't idiots. Seeing as the likes of Stephen > Hawking won't rule out the possibility of FTL, I think that's a > safe assumption. Of course, he doesn't rule out the possibility > of time travel either. > > >2) FTL may one day be performed in such a way that the time travel part > >is nullified. Since we don't know what happens at FTL (we've never been > >there) we can't make any assumtions. There may be some obscure phenomena > >(like Hawking's time protection system) that would allow FTL, but cancel > >timetravel. > > But we can make deductions about how FTL would work given our current > understanding of physics. However, if you're going to play that game > you've got to first learn our current understanding of physics. > > One tool which really is useful is the Lorentz transformation, because > it is a _geometrical_ transformation of space-time (it doesn't require > or directly use any information about the velocity of the points being > transformed, only their positions in space-time). This is critical > because you can use it for anything--FTL or not, "continuosly existing" > or "teleporting". > > >3) We may find a way of generating gravity fields without matter, so > >that we can build alcubierre type ships, that have no causal problems. > > Alcubierre type ships probably _do_ have causal problems. Alcubierre > himself noted that it wouldn't be difficult to slightly modify his > metric to produce a closed-time causality loop. > > >4) Maybe we can make hyper-jumps across space to avoid FTL, but avoid > >traversing the space between points. (bring two sides of a paper > >together, cross the edge, and unfold. No causal problems. > > Yes, there are causal problems. The typical example, of which I gave > one, doesn't require anything about the FTL communication signal other > than the fact that it arrives at its destination at a particular > speed faster than light would have. It's irrelevant what happenned > to the signal "in the meantime". It could have been doing loop-de-loops > or it could have disappeared from the universe altogether. > > >A few other things: > > >Wouldn't wormholes be causality violating? > > In a flat/convex space, yes. However, it looks like in order to > make them, you need space to be hyperbolic (average energy density > is negative). > > >What is a preffered reference frame, and why would it maybe allow FTL? > > A prefered reference frame is one which is inherently different from > others. One of the easiest to understand methods of FTL without > time travel is to assign some particular reference frame to be THE > prefered reference frame. Somehow, FTL travel is allowed, but only > at speeds where it doesn't go backwards in time in this particular > reference frame. > > This means Mach's principle of relativity is wrong, because you could > tell one reference frame from another by trying to go FTL and > determining at which velocities you couldn't move. > > >Why can't light (as in the light clock) be carried with you at FTL > >speed? FTL+C may work at FTL. > > The light clock is a geometrical construction, which assumes that > the speed of light is constant. This assumption is reasonable given > the amount of experimental evidence in its favor. Actually, the speed of light is not constant in a vacuum, but can be sped up...its too in depth to post, but it happens in a casimir cavity. Ask Steve, he knows more. > > >Unanswered question: If I travel 1600 light years in 2 years earth time, > >how far back in time do I travel upon return? > > It depends upon how much you accelerate away (sublight) from Earth > before returning. Lets say I navigate around the star system 1600 lightyears away for, say 5 years, and return to earth in 2 years earthtime. Is there an equation for this? Question: How do we know time runs backwards in FTL? Then again, how do we know time runs forward here... Kyle Mcallister

**Follow-Ups**:**Re: starship-design: FTL and time travel***From:*Steve VanDevender <stevev@efn.org>

**Re: starship-design: FTL and time travel***From:*kuo@bit.csc.lsu.edu (Isaac Kuo)

**References**:**Re: starship-design: FTL and time travel***From:*kuo@bit.csc.lsu.edu (Isaac Kuo)

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