>> FTL travel allows you to set up very nasty paradoxes. Just that something
>> would allow FTL is a good theoretical reason to doubt its existence.
> Apparent paradox has often been associated with interesting advances in
>science; "I can't understand how that could happen" seems not to convince
>the universe, nor does it convince me
I'm not talking about about confusing "paradoxes" like the twin paradox or
the puzzles about observing ship clocks at relativistic speeds. If you can
get to your "absolute elsewhere" - the area of spacetime outside of your
light cone, which would include both FTL and time travel - it's trivial to
set up sequencing disagreements where some observers see one sequence of
events in the same place, and others see others. If some events make others
impossible, you have a big problem. My favorite example would be a spaceship
FTLing into an incipient nova. Some observers will see the ship fly into a
nova and be destroyed. Others see the ship fly through before the nova
happens. So when we have the Party at the End of the Galaxy, is the ship
there or not?
Traditional "paradoxes" show that something (apparently) impossible happens.
What happens is consistent with itself, just perhaps confusing to us. FTL
paradoxes can show that mutually incompatible things must happen together.
It's a much nastier paradox, because it shows internal inconsistency.