The Simulation Argument.
Ahhh my favorite scientific theory. In 50 years its speculated that computers will have the computing power of a human brain. In 500 years who is to say computers can't simulate entire Universes? Considering we're on the brink of being able to do this, who is to say this isn't happening now? Why couldn't it be happening now? If humans can refrain from killing each other between now and 1000 years from now (which is unlikely), we WILL be able to simulate entire Universes, so rather its happening now or not is irrelevant, because it will happen then. So, its entirely possible that 1000 years from now, humans figure out that backwards time travel is impossible, so they simulate the past, build computer replicas of their brains, insert their computers into a simulation, and now live 1000 years in the past in the year 2011.