RE: starship-design: FTL

[Steve VanDevender <stevev@efn.org>]

L. Parker writes:
 > If you can move from here to there instantaneously, or nearly instantaneously, 
 > is causality violated? If you go from here to Tau Ceti in 1 second of real 
 > time, you don't arrive at Tau Ceti 11 years ago you arrive at t+1 sec. There
 > is no FTL communication that I can see and no causality violation.

It's important to remember that the notions of "instantaneous" and
"simultaneous" are completely frame-dependent.  Simultaneous events, in
relativistic terms, _must_ be separated by a spacelike interval, and
since they are their time ordering is completely observer-dependent --
there is only one frame in which they can be claimed to be
simultaneous.

Now, Isaac has been chiding me for being somewhat confused about the
notions of unique time ordering and causality, which I'll admit to
having played too loosely with.  Observer-dependent time ordering isn't
the same as causality violation, and Isaac has explained that difference
pretty well in recent examples.  However, I do think it's accurate to
say that if you claim to be able to link events separated by a spacelike
interval, then you can construct causality violation under the right
conditions.