starship-design: Many things...

[Steve VanDevender <stevev@efn.org>]

Kyle R. Mcallister writes:
 > Why when you are travelling FTL, does causality violate in only some
 > reference frames? 

Isaac has shown that you can construct a situation in which causality is
violated in _all_ reference frames via a round trip FTL message; all
observers will agree that the incoming FTL message was received before
the outgoing message was sent in that situation.

When I was attempting to explain the fundamental behavior that leads to
this situation, I was using a somewhat looser definition of causality.
However, if you claim a direct cause-and-effect relationship between two
events via FTL communication, the frame measurements of those events
will show one time order for observers below a certain velocity and the
other time order for observers above a certain velocity, making the
notion of which is cause and which is effect completely frame-dependent.
For LETL (less-than-or-equal-to-light) relationships, cause and effect
are frame-independent.

This is all a simple consequence of the Lorentz transformation.  If you
haven't read your copy of _Spacetime Physics_ yet, get cracking.  Digest
the information yourself, rather than expecting us to spoon-feed it to
you.

 > Why would a preffered reference frame eliminate this problem? 

A preferred reference frame (one in which observations were "more real"
than in any other) would allow you to claim that there really was a
consistent explanation of FTL-connected events accessible to all
observers.  Relativity, however, does not have any preferred reference
frames, and there is no evidence that any exist.  No observer has
observations that are "more real" than any other observer's.

 > Have we measured Time dilation directly? Speed (.01C+) 

Experiments using highly accurate clocks have measured time dilation at
speeds much less than 0.01 c (one of the frequently cited experiments
involved flying atomic clocks on aircraft).

Subatomic particles have been accelerated to within extremely tiny
fractions of c, and have been observed to have decay times perfectly
consistent with the predicted time dilation effects.

Don't even bother trying to claim that a macroscopic object could behave
differently at relativistic speeds than subatomic particles do.  We
already have an excellent idea of the relationship between the behavior
of the microscopic and the macroscopic, and your claim would have
observable consequences if it were true, which it's not.

 > What if you carried your light/lightcone with you at FTL, in a way that
 > eliminates TD and Lorentz contraction? 

Then you have somehow figured out how to leave our universe, because you
are in a region of completely different physical laws.

 > What causes Centrifugal force EXACTLY?

Inertia, and nothing else.  To move an object in a circular path you
must continuously exert acceleration on the object.  For someone sitting
on the object this acceleration is perceived as centrifugal force.

 > What causes inertia EXACTLY?

I've never seen anyone claim to know, but it's even more observationally
supported than relativity.

 > Is it at all possible that there is something we have yet to find which
 > would provide a loophole for FTL? (not by 2050)

Is it at all possible that the sun will go nova by 2050?  Maybe, but
there's absolutely no evidence to support it.

 > Here's a dreadful thought: what if someday at .99999+C we find out that
 > Einstein was wrong there? Bad news for timetravel.

We've already seen things go at 0.99999+ c.  Einstein hasn't been wrong
yet.