Imagine a spaceship leaving Earth at a large fraction of the speed of light. As we all know, according to relativity, the spaceship experiences 'time dilation'; that is, to observers on Earth, the spaceships clocks seem to run slow, say by a factor of 2-1. So when a year has passed

on Earth, the Earth astronomers will observe that the spaceship's clocks say only six months have passed. Remember, this effect is _real_; is will appear even after correcting for propagation delays caused by the finite speed of light.

But according to relativity, the spaceship's reference frame is just as valid as Earth's. In the ship's frame, the _ship_ is standing still, and Earth is travelling away from them at a large fraction of lightspeed. So the ship observers will observe _Earth_'s clocks as slowed by the same factor of 2-1. Again, this is a _real_ effect (to the extent that the word 'real' has any meaning)_not_ an illusion caused by the finite speed of light.

How can the ships clocks and the Earths clocks run slower than each other? This is one of the paradoxes of relativity, and I can't really explain it myself. But if you do the math, that's the result you get. Due to the finite speed of light, we never get causality violation in this situation. But what if we can communicate, or teleport, _faster_ than the speed of light?

For ease of argument, let's suppose we have an instantaneous communicator, an 'ansible'.

Now, a year after the spaceship leaves Earth, Mission Control fires up their ansible and sends an instantaneous message to the spaceship. 'Hi, how are you?' Due to time dilation, the message arrives after six months have passed on the ship. The ship uses their ansible to reply instantaneously.

'Were fine, how are you?' But according to the ship's observers, only three months have passed on Earth in the six months they've been in space. So the ship's reply arrives three months after the ship leaves Earth.

The reply arrives nine months before the message was sent!