Time travel is one of those things i love to talk about (along with black holes and dark/anti-matter). A Multitude of things could happen. The first, and most likely, is that you would create your own alternate dimension when you travel back or forward. Whether or not you would go back to your own dimension when you travel is not likely but not impossible (like in Deja Vu). Another thing that could happen is that you wouldn't change a thing. Or, to reword it, you would act out history as it was supposed to happen to begin with (as in Timeline).