Consider this. It is impossible for matter to travel at the speed of light, yes? However, imagine a cannon on a train travelling (speed_of_light - 10 MPH). The cannon shoots a cannonball at 210 MPH. So when the cannon fires, you'd expect the cannonball to go (speed_of_light + 200 MPH), right? Well, no... matter cannot exceed the speed of light. What actually happens (afaik) is time itself is altered as one approaches the speed of light. So the closer you are to the speed of light, the more time is altered.

I don't know any of the details though.