= Faster-than-light
{title2=FTL}
{wiki}
In <special relativity>, it is impossible to travel faster than light.
One argument of why, is that if you could travel faster than light, then you could send a message to a point in <Spacetime> that is <spacelike-separated> from the present. But then since the target is spacelike separated, there exists a <inertial frame of reference> in which that event happens before the present, which would be hard to make sense of.
Even worse, it would be possible to travel back in time: