A spaceship capable of travelling at some significant fraction of the speed of light *is* a time machine, since after a trip in space the traveler returns to an earth that has advanced through time farther than his body has. If his ship was fast enough and his trip long enough he would return to a world in which all his friends were dead. That's time travel. Some say the trick is getting back to the same time-frame from which one left-- but who in their right mind would want to do that? Beam me up, Scotty!