There was a good article in a sciam about time travel. It had all of the major theories about time travel. It did give a theory about travel to the past. If you could create a wormhole, and rotate it somehow to slow time down in and around it, when you entered that wormhole later you be taken back to the time when the wormhole was created (well, plus a little bit, because you can't slow down time completely).
Of course, this could all be rubbish, because even though wormholes are a consequence of Einsteinian physics, they could just not exist. Same with the turtle that is carrying the disk-shaped earth on its' back.