Re: starship-design: FTL and time travel

["Kyle R. Mcallister" <stk@sunherald.infi.net>]

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