Re: starship-design: FTL and time travel

[kuo@bit.csc.lsu.edu (Isaac Kuo)]

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.

>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.
-- 
    _____     Isaac Kuo kuo@bit.csc.lsu.edu http://www.csc.lsu.edu/~kuo
 __|_)o(_|__
/___________\ "Mari-san...  Yokatta...
\=\)-----(/=/  ...Yokatta go-buji de..." - Karigari Hiroshi