 Now Stephen Hawking wrote about this, and he said that, well, he doubted that time travel was possible, because if you go backwards in time, you could then go back a second time, and a third time, and a fourth time, and create a loop, an infinite loop. Energy builds up in the loop, and the thing blows up. So he said that time travel is probably not possible, because of all the loops that are formed when you go backwards in time, causing the universe to blow up.