carroll@uiucdcsb.CS.UIUC.EDU (11/13/85)
Not strictly true. You can travel at the speed of light, if you have no "rest mass", e.g. a photon. As to traveling faster than light, your time dilation becomes imaginary, and no one knows what that means. The main argument against faster than light (FTL) is causality violations. If you could get from place to place faster than light, you could send messages back in time, because what is "now" is different for people who are moving at different speeds. It is easy to create a situation with three people, one saying "A happened before B", "B happened before A", and "A and B were simultaneous".