A couple of questions around faster than light travel. 1. I don't understand why this implies travelling back in time (as is sometimes suggested). For example : A tachyon travelling at 150% c travels between Point A and Point B and back again. While it will arrive at Point B before it is observed from there leaving Point A, but not before it actually left. Similarly it arrives back at Point A some time after it left (not before). So no (backward) time travel is involved. 2. But what would an observer looking at the tachyon's clock see ? I understand that at c (e.g. for a photon) time will appear to have stopped from the point of view of the observer), but at faster than c the equations as I understand it mean that time doesn't become negative, but rather becomes imaginary (due to the √ impact). What meaning can be applied to this ?