Phlegmatic has a good post on how dumb it is to use time travel as a literary device. I've also loathed it for ages. In fact, as Phlegm also notes, the only one to get it right was Douglas Adams.
As Ford patiently explains to Arthur on prehistoric earth, "we've gone backwards and forwards through time to end up here. We know the future but we can't change it because we've seen it, it's already happened." The concept of "already happened" is a curious one, we humans tend to misunderstand it because we only look at time in the forward direction. But everything that happens, happens.
This is also why an omniscient God does not invalidate free will. For example, God knows exactly what I am about to do, and might even adjust the Universe accordingly. But at no point did my choice become predestined - any more than Hitler was predestined to slaughter 6 million human beings just because I read about it in my history book. (I've written on this before)
The larger question is whether to take the many worlds theory seriously. You can prove quite simply that it cannot coexist with the idea of time travel (thus rendering every single fictional work about time travel using th emany worlds theory to resolve the inherent paradoxes, a pure fantasy, not science fiction). Like so:
- if there are an infinitely many worlds (ie, realities/timelines etc), and time travel is possible, then there must be an infinite subset of worlds where time travel is invented
- if there are an infinite number of worlds where time travel has been invented, then there must be an infinite subset of which will use it to travel to London Bridge, 1/1/2001.
- the date 1/1/2001 is arbitrary, since there are an infinite number of potential visitors for any date, since all dates have significance.
- the location London Bridge is arbitrary, since there are an infinite number of potential visitors for any location, since all locations have significance.
Therefore, if time travel is possible, at any given instant of time, we will be inundated with an infinite number of travelers from the future arriving at all points on Earth simultaneously, from all the possible futures that branched off from the instant of time in question.
Since we are reasonably certain that this has not happened, either time travel is impossible or the many worlds theory is impossible.
Personally, since my religion mandates absolute free will and also mandates the omniscience of God, I find the many worlds hypothesis to be impossible on theological grounds also. But note that if many worlds is impossible, then time travel becomes neutered (though still possible), in that any action taken by a time traveler becomes part of history itself and can therefore represent no paradox.
It already happened.