The idea of travelling forward into the future or back into the past has always fascinated science fiction writers. The 'grandfather paradox' is the argument many people use to suggest that time travel is impossible. What if you went back in time and prevented your grandfather from meeting your grandmother so that your mother was never born? Then you would never have been born . . . and so on. Until very recently such arguments led most scientists to believe that time travel could never exist outside science fiction. But amazingly, some interpretations of the weirdness of the quantum world now suggest that time travel is possible - at least in theory.