Re: nonsensical FTL without time travel?

b_crowell67@xxxxxxxxxxx wrote:
The structure of special relativity seems to imply that any method of
FTL travel would also be a method of time travel. (If events A and B
are separated by a spacelike interval in spacetime, then different
observers don't agree on which came first in time.) For this reason,
I've always considered it scientifically wrong when SF posits FTL
travel without any time travel involved. (That doesn't mean it can't
be entertaining.)

However, can anyone form a really sharp paradox that demonstrates why
FTL without time travel doesn't make sense?

No.

Why not?

Special Relativity does indeed show that from the viewpoint of
observers in two different inertial frames, events A and B, separated
by a spacelike interval, may occur either in the order of A preceding
B, or A following B.

This means that any method of travel or communication that allows one
to leave at A and arrive at B, *if that method is bound by the premise
of Special Relativity that the laws of physics are the same in all
inertial frames* would also allow one to leave at B and arrive at A.

So an FTL spaceship leaves Earth, travels to a star 100 light years
away in a week. A week later, it launches from a *moving STL platform*
with suitable velocity, and arrives back on Earth perhaps 99 years
before it left.

There's only one problem.

The postulate of Special Relativity that the laws of physics are the
same in all reference frames is simply a postulate. It does rest on
observations. *But we've never observed a working FTL spacecraft!*

The Minkowski diagram divides spacetime into three regions. The
unambiguous past, the unambiguous future, and the inaccessible
spacelike. The viewpoints of different inertial frames divide up the
spacelike into past and future in different ways. *Each of those
viewpoints is self-consistent.*

Therefore, _no paradox is created_ if we assume that FTL travel,
impossible using the forces of nature we are familiar with, which
experiment has confirmed behave the same in all reference frames, is
powered by Force X, which behaves in a noticeably *different* way
depending on what inertial reference frame you're experimenting with
Force X in, so that *all FTL spaceships* make journeys that seem to be
going forwards in time *from the viewpoint* of privileged reference
frame S, the inertial reference frame of the pan-dimensional
continuum!

And, indeed, if a faster-than-light spaceship moved through the fourth
dimension into a plane where the speed of light was infinite
(protecting itself by surrounding itself with a bubble of normal
space) such a plane, if associated at each point with our space, would
define a privileged reference frame.

John Savard

.

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