This staple of science fiction is sometimes argued to harbour a contradiction. If the time traveller goes back to an earlier time, then he can alter things so that he never comes into existence, in which case he will not be there to begin his time travel. It can be replied that this just shows that he doesn't alter things in that way, but then the question remains whether he can introduce any difference whatsoever into the past world without similar contradiction arising. However, Kurt Gödel produced models of space-time consistent with general relativity, that allow travel into the past, and modern physics can be interpreted so as to countenance the notion. One interpretation, however, is that of the physical unreality of time altogether. See also block universe.