Stephen Hawking once tried to prove time travel was impossible by means of a convoluted argument that took into account several factors, and then tried to argue that all of them didn't work, probably in an effort to explain why he considered such things as the "grandfather paradox" to be impossible. Seems to run afoul of Occam's Razor to me (but I wish I remembered the argument). There is another school of thought: that if people were to travel back in time and change history, they'd wind up in an alternative universe or in another timeline (and THAT has spawned an entire science fiction subgenre itself).

Some physicists (such as Michio Kaku) have argued that time travel is possible in theory, via means such as wormholes and heating up things so much that wormholes appear. Hawking contends that wormholes would collapse as soon as someone attempts to send light (or information) back in time through it. (And some s.f. authors, such as Stephen Baxter, have tried to cash in on such subjects - "The Light of Other Days", along with time travel stories).