I've just watched a fun documentary on Channel 4: 'The World's First Time Machine'. It was rather dumbed down, but this — as it turned out — was just as well, because this branch of physics can explode the brains of the unwary (it nearly did with mine, anyway). They had some of the clearest explanations (no doubt very simplified) of the 'great-grandfather paradox', parallel universes, and how Superman could save Lois Lane that I've ever heard. They featured a physicist called Ronald Mallett, who is actually trying to build a time machine — rather touchingly inspired by a wish to go back and meet his father who died young.
Describing his experiment, he said that it was quite possible that when he switched it on, he might see a particle that (brace yourself for this) was the result of an experiment that he would do some weeks or even years in the future. That really made me go, "Wha?"