If time travel is possible,which seems more and more likely every day, then a good many people would try and change the past. Maybe not just their own past, but the worlds as well. Fortunately, this is not possible, for nature can not allow that to happen. For example, say I wanted to go back and prevent President Lincoln from dying at the hands of John Wilkes Booth. I go back in time, and try to stop John Wilkes Booth from shooting Lincoln. The universe will make sure that no matter what, Lincoln will be killed in the same spot. The only thing that could possibly change is the shooter. The entire reason for going back in time, would have been to save Lincoln from dying. If all my life I had learned that Lincoln had been shot, then i would attempt to go back and save him. If i had learned that Lincoln had only survived an assasination attempt, then i wouldn't have needed to go back in time to save lincoln, because he wouldn't have needed saving. Basically, I can't go back and save lincoln because if i do, i wouldn't have been able to go back and save him in the first place, because he was already saved. Just a small but answering paradox.