I'm still having trouble working out why faster than light speed equates to time travel. I understand that for the traveller time slows down the faster he/she travels and have familiarised myself with the standard examples of Earth man and space man using the Lorentz equations. At the extreme, I also get that at c the observed traveller's clock slows to zero. So for example a photon travelling at c will "experience" no time as it travels say away to, and then immediately, back from a galaxy 100,000 light years from Earth. On Earth 200,000 years will have passed. For the photon, no time will have passed at all. For a scenario of faster than light, say twice the speed of light, 100,000 years will pass on Earth. Presumably the maths show that for the photon (or if you like a neutrino) time went backwards. But what does this mean ?