Stephen King's new novel 11/22/63 involves a tunnel to the past, and a guy who tries to tamper with the Kennedy assassination. Talking to Wired, King explains something about the rules of time travel as he sees them:
1) "The more potential a given event has to change the future, the more difficult that event would be to change. If you wanted to go back and speak to somebody on a street corner so that they were five minutes late to an appointment-that might not be too hard. But if you wanted to stop the assassination of a president, that would be really difficult. The past would try to protect itself."
2) "Every time you go back and change something, you create an alternate timeline. There are these guardians who stand watch over all the time portals, because they understand that whenever you go back, you damage the time-space continuum."
3) "The further back you go, the more precautions you have to take. It would go right to the language-you'd have to be careful about the way you speak; the accents would be different. If you were to return to, say, 1858, you'd really have to prepare ahead of time."
So there you have it — whatever Stephen Hawking says, Stephen King has already sorted out the rules of time travel for us all. The whole interview is well worth checking out. [Wired]