Discover Magazine's Cosmic Variance blog brings us Rules for Time Travelers, a set of eleven rules that help to explain (or contradict) various time-travel scenarios in pop culture. And believe me, time travel is the new romantic comedy -- with Star Trek, Lost, and Fringe all getting in on some form of time travel/alternate universe action, there's enough grandfather paradoxes and time loops to last you all year. Or...wait...if you started reading the theories now, but then built a time machine, could you...oh, forget it, my math-brain exploded in an electromagnetic incident, thus allowing psionic frogs to destroy my universe.
Here are some snippets from the rules:
6. If something happened, it happened.
What people want to do with time machines is to go into the past and change it. You can't. The past already happened, and it can't un-happen. You might wonder what's to stop you from jumping in your time machine, finding your high-school self, and convincing them that they really shouldn't go to the senior prom after all, thereby saving yourself all sorts of humiliation. But if you really did go to the prom, then that can't happen. The simple way out, of course, is to suppose that travel into the past is simply impossible. But even if it's not, you can't change what already happened; every event in spacetime is characterized by certain things occurring, and those things are fixed once and for all once they happen. If you did manage to go back in time to your years in high school, something would prevent you from dissuading your younger self from doing anything other than what they actually did. Even if you tried really hard.
8. You can't travel back to before the time machine was built.
Right now, at the particular place you are sitting, at the time when you are sitting there, one of two things is true: either there is a closed timelike curve passing through that point in spacetime, or there is not. And that situation will never change -- no matter what clever engineers may do in the future, if they create closed timelike curves they cannot pass through events in spacetime through which closed timelike curves did not pass (corollary of Rule 6). Or in plain English: if you build a time machine where there wasn't one before, it may be possible for future travelers to come back to that time, but nothing can help you go back to times before the machine was built.
9. Unless you go to a parallel universe. ...