Re: Ten Things That Annoy Me in Science Fiction

On Sep 13, 7:58 pm, wdst...@xxxxxxxxx (William December Starr) wrote:

Speaking of which -- the FTL part -- can someone point me at a good,
clear explanation, probably with pictures with circles and arrows
on them, of the way that if you've got FTL then you've got time
travel, i.e., that there'll always be (if I understand it) a
two-step path out of and then back into your light cone that'll land
you in your past?

I don't know of one on the web with pictures. But it isn't *quite*
true that if you've got FTL, you _must_ have time travel.

There's an FTL FAQ which says "pick any two" of the following three
things:

- causality (no time travel to the past),
- relativity (all the laws of physics are the same for all states of
constant motion - "inertial reference frames")
- FTL travel and/or communications

If you have an FTL drive that takes your ship into an alternate
dimension which corresponds to our space-time continuum in such a way
that...

all faster-than-light journeys which are possible are those which
correspond to the start of the journey being earlier than the end of
the journey

as viewed from the special inertial reference frame S*, which
corresponds to the equilibrium point between our space-time continuum
and the FTL dimension, then there's no time travel.

If you have relativity, though, so that the observed phenomenon of all
physical laws being true in all inertial reference frames carries
over, and applies to the force or phenomenon that allows FTL travel,
then there is a handy standard proof (usually used as a proof that FTL
travel is _impossible_) in relativity textbooks of what you're
seeking.

Basically, assume that from reference frame S, it is possible to
travel in either direction at some faster-than-light speed, even one
only slightly faster - say 1.01c.

If you travel in the right direction relative to reference frame S, at
a conventional slower-than-light speed that is close enough to the
speed of light, a ship starting from frame S and travelling at 1.01c
will seem to be going backwards in time.

If FTL travel is relative, you can travel in any direction at 1.01c
using your new reference frame, let's call it S', as the jumping-off
point.

So a spacechip leaves Earth, heads at 1.01c in the direction of the
Andromeda Galaxy, then, 1,000 years later, drops out of FTL drive,
accellerates to 99.999% of the speed of light (in the right
direction), and *then* engages its FTL drive to head back to Earth...
and it gets back to Earth before it left. The maneuver bears some
similarity to how a sailing ship might tack against the wind.

.... tacking against the winds of time,
on a voyage to yesteryear

John Savard
.

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