Science fiction authors have been speculating about this for hundreds of years. Only recently has it become popular to compare and contrast different ideas to try and formulate a set of ideas that could plausibly work together with internal consistency and at the same time allow for interesting time travel stories of the sort we are familiar with.
Time travel has been a dream of science fiction writers for at least a hundred years. In 1889, Mark Twain wrote A Connecticut Yankee in King Arthur's Court, a story about a contemporary (19th Century) man who, after a blow to the head, finds himself back in the time of legendary Britain at King Arthur's court. H.G. Wells envisioned his Time Machine story in 1895. Since then, uncountable stories and a few theoretical physics professors have speculated about time travel. But just what is time travel?