Time travel has been a favourite science fiction theme ever since it was first used in H.G. Wells's trailblazing novel "The Time Machine". But not everything it describes is science fiction: travelling forward in time, for example, is a proven fact. Einstein's theory of relativity predicts that an observer moving relative to Earth can leap into Earth's future, and the effect has been confirmed using atomic clocks. Dramatic time warps require speeds close to that of light, which is possible in principle but would take a major feat of engineering, not to mention a lot of money. Going back in time is far more problematic. Relativity does not rule out an observer being able to make a journey through space-time and return to the past. But all scenarios so far discussed require exotic circumstances.