On Thu, Mar 28, 2019 at 02:14:27PM +0000, David Shaw wrote:
> I've been thinking - never a good sign. I understand that good old uncle
> Albert's theories state that FTL travel isn't possible, because - and I'm
> paraphrasing from memory here, so I could well be wrong - if you jump from
> A to B, some observer at C could see you arrive before you depart, leading
> to all sorts of nasty time-travel paradoxes.

No, it's more subtle than that.

If relativity accurately describes the universe, then any physical
phenomena that are permitted relative to one inertial reference frame
are also permitted relative to any other.  So if it is possible to
travel 1 parsec in 1 week from one inertial reference frame (say, one
comoving with Sol), then is also possible relative to any other
inertial reference frame.

The trouble is that in other reference frames, the division between
"space" and "time" isn't the same - they're related the the first via
Lorentz transforms that behave somewhat like rotations.  So if it's
physically possible to jump 170 light-weeks (1 parsec) spinward and 1
week futureward, then it's necessarily also possible to jump 170.009
light-weeks spinward and -2 weeks futureward.  This isn't *yet* time
travel, but some explanations confusingly stop here.

Now from there, you can give a message to some other ship that jumps
170 light-weeks antispinward and 1 week futureward, and it ends up at
a point 0.009 light-weeks spinward and -1 week futureward from the
starting point.  A radio signal from there will arrive at the starting
point 0.991 weeks before the first ship left.

This is time travel, and nothing but the suppositions of FTL travel
and relativity were required to derive it.  The means by which FTL was
accomplished were irrelevant.  In Traveller, the second ship could
avoid the final radio signal step with a very slightly longer jump to
the actual starting point, but here I am assuming nothing more about
the FTL than that it can go 1 parsec in about 1 week.

- Tim