Re: starship-design: FTL idea - Steve

[Steve VanDevender <stevev@efn.org>]

Isaac Kuo writes:
 > Steve VanDevender wrote:
 > >You can't have another "rest" frame.  If you're going to declare a
 > >particular frame to be the "rest" frame, you need to work the problem
 > >consistently with reference to that.
 > 
 > >The point, which you now seem to understand better, is that if events
 > >are connected by a spacelike (FTL) worldline, then not all observers
 > >agree on their time ordering, as opposed to the case of events connected
 > >by timelike or lightlike worldlines where all observers do agree on a
 > >time ordering.  I'm not sure what you mean by "the event reversal
 > >happens in all frames".  For any spacelike relation of events there are
 > >always some observers who see one time ordering and some who see the
 > >other time ordering.
 > 
 > Absolutely wrong.  Any string of events where any pair of events is
 > within the light cones of each other will have a time ordering which
 > _all_ (sublight) observers will agree upon.  In particular, the time
 > ordering of events happenning to any sublight moving entity will be
 > unambiguous.
 > 
 > Now, with FTL communication, you can typically set up a signal which
 > is sent to yourself earlier in time.  Thus, the event of receiving
 > the signal in your past is within the light cone of the event of
 > your sending the signal.  This results in a signal sent backwards
 > in time, and _all_ (sublight) observers will agree upon the fact
 > that it went backwards in time.

This does somewhat clarify the situation for me.  However, I was really
trying to make the point that FTL travel implies ambiguity of time
ordering for the FTL-connected events, and that ambiguity of time
ordering can then be used to produce causality paradoxes, although by
itself ambiguity of time ordering is not violation of causality.  Your
clarification that being able to produce FTL-connected events allows one
to produce observer-independent paradoxes is important and a more subtle
issue than I had thought of.

 > Note I'm talking about "sublight" observers.  What about FTL observers?
 > Don't even talk about FTL frames of reference.

This is another excellent point, and I'm at least familiar enough with
the mathematics to have an idea why FTL frames of reference are
problematic.