One of the startling discoveries which came with the theory of relativity
was that if we were ever actually able to communicate FTL, we could use
this mechanism to send messages backwards in time. This is part of the
reason, up until recently, I'd been convinced that FTL communication was
impossible.
This is from the Relativity and FTL FAQ at:
http://www.physics.purdue.edu/~hinson/ftl/FTL_StartingPoint.html
A situation which could take place if FTL communication were possible:
>1. As observers O and Op pass by one another (as they are shown in
Diagram 8-1 ) Op uses some method to send out an FTL bullet from his
reference frame. The event "O and Op pass one another" will be called the
"passing event" from here on.
>2. The bullet strikes and kills a victim. This event occurs after the
passing event in Op's frame of reference, but it occurs before the passing
event in O's frame.
>3. A third observer is at the victim's side as he dies and thus he
witnesses the death. This third observer is stationary in O's frame of
reference (i.e. his frame is the same as O's), so the passing event (when
the bullet was fired) occurs after the victims death in this third
observer's frame. Thus, the third observer has witnessed a result which
comes from an event in his future--he has information about a future event
in his frame of reference.
>4. The third observer sends this information about the future to O using
an FTL signal, and in the third observer's frame of reference, O can
receive this information before the passing event occurs (and thus before
the bullet is fired).
>5. O receives the message and learns of the victims death before the
bullet is fired. He thus knows about the bullet being fired--an event in
his own future which will occur at his very location.
>6. O uses this information to prevent Op from firing the bullet, thus
causing a self-inconsistent situation--an unsolvable paradox.
For the purposes of consistency, we may suppose that instead of sending a
FTL bullet, Op actually sent an instant message to an assassin hiding
behind a nearby grassy knoll, and that the FTL communique was his DigiCash
payment. :)
-INSATIABLE-