Re: starship-design: FTL idea

[Steve VanDevender <stevev@efn.org>]

Timothy van der Linden writes:
 > OK, I catch your drift. But... knowing that you move this fast with respect
 > to the observed phenomenon, you can reconstruct what really(=in a frame at
 > rest) happens and remove the apparent causality reversal.
 > For those few that happen to see everything at once, they are at a loss,
 > they will never be able to reconstruct what happened.

There's your problem.  There is no "frame at rest".  Relativity has no
preferred frames.  In the FTL case, there is no unique time ordering of
events (what if the "frame at rest" is really a "frame in motion"?  You
can't prefer one over the other!) and hence no way to establish
causality.  In the STL (slower-than-light) case, a unique time ordering
of events exists for all possible observers.

 > OK, now for a horizontal bar falling down on a parallel floor. The event
 > that one end touches the floor is called A, the event that the other bar
 > touches the floor is called B.
 > (If you like, assume the bar is 1 lightsecond long)
 > 
 > v=0     A(0,0,0,0)  B( 0.000,1.000,0,0)
 > v=0.1   A(0,0,0,0)  B(-0.101,1.005,0,0)
 > v=0.9   A(0,0,0,0)  B(-2.065,2.294,0,0)
 > v=-0.1  A(0,0,0,0)  B( 0.101,1.005,0,0)
 > 
 > So even for very small velocities without FTL, you can measure a reversal in
 > time ordering. Does my example differ from yours?

Yes, because no one is claiming that there is a FTL link between the
ends of the bar.  It is perfectly accepted that when events are
separated by a spacelike interval (t^2 - x^2 < 0) all observers cannot
agree on the time ordering of those events, and the ends of the bar are
separated by such a spacelike interval.  FTL travel means sending a
particle between events that have spacelike intervals between them.

It may be worth pointing out that in relativity rigid objects cannot be
truly perfectly rigid, because, for example, the disturbance caused by
striking a rigid rod on one end with a hammer must travel at c or less
to the other end of the rod.  In fact, you can derive Lorentz
contraction from this, and these derivations are in _Spacetime Physics_.
If you push on one end of a rigid rod, it must contract because the far
end always lags behind in receiving the force that was applied to the
near end, and as measured in the frame where the rod was initially at
rest the rod gradually shrinks.