That's not supposed to make any sense?
Faster than light travel is time travel.
The following is known as the Tachyon pistol duel thought experiment. It demonstrates why FTL travel results in time paradoxes. For simplicity of calculation's sake, FTL is considered to have infinite speed, striking its target instantly. The same effects apply with limited FTL velocities, but they are a bitch to do the number crunching with.
Man A and Man B meet at a point in space, and agree to turn around, move away from each other, count down 8 seconds, then turn and fire their weapons at each other. We will follow from man A's perspective.
Both parties exit and travel away from each other at 0.866 C. Man A counts down to 0, turns and then fires. Due to time dilation, when Man A counts to zero, Man B has only counted down to 4, thus his shot flies past Man B at 4 seconds.
Enraged that Man A has fired before he has hit 0, he fires at 4 seconds, and due to time dilation, man A has only counted down to 6! It strikes man A and kills him dead after he has counted down only 2 seconds, a full 6 seconds before he fired his initial shot. A classic grandfather paradox. It gets made worse by the fact that if you started from man B's perspective instead of man A, the exact inverse happens, with man B dead 6 seconds before he fires his shot instead.
So we're not only left with a grandfather paradox, but 2 events that both should have happened, but are mutually exclusive.
The light-speed-limit on information transfer is the ONLY thing that enforces causality. Without causality, the entire foundation of physics crumbles down onto itself.
[Note: This is not mine, all credit goes to Adamgrifs over at gametrailers.]
Anyone else find this completely mind numbing? Pretty crazy stuff. The whole explanation is extremely long, but really interesting. PM me if you want to know all the other stuff.
FTL = Faster than Light
(The original page with the paradox is now one of those "what you want, when you need it" squatter websites, so take this forum post saying the paradox.)