Martha Jones: It's like in those films: if you step on a butterfly, you change the future of the human race. The Doctor: Then don't step on any butterflies. What have butterflies ever done to you? Science fiction writers can't seem to agree on the rules of time travel. Sometimes, as in Doctor Who (above), characters can travel in time and affect small events without appearing to alter the grand course of history. In other stories, such as Back To The Future, even the tiniest of the time travellers' actions in the past produce major ripples that unpredictably change...