There are better, more technical answers, but I prefer more common-sense ones. So one problem is that a person who travels backward in time must exist in two places. Suppose you traveled back to see what you looked like as a little kid. There you would be, as an adult looking at a kid, AND as the kid. There would be two of you. . . A second problem is that you could easily change history. What if you traveled back to when your *father* was a kid, and you killed him. What does that mean for your own existence? . . There are lots of other problems like this, but you can see how the issues develop.. . Good luck!!