I wonder if everything in the universe is predestined. This would lead to two possibilities (that I can think of): either time-travelling is impossible or you can only be an observer. The latter means that when you travel back, you cannot change anything at all. ie. nobody would be able to see you, hear you, feel you etc.

Or maybe it is not predestined, which means time travelling would cause history to change. I cannot think of what would happen if you change the future. ie. Would the present alter itself to fit the future?

Or maybe only some points in history cannot be changed (partially predestined). Other events can be altered to give a slightly different history. eg. If you go back and kill Hitler, someone else would replace him as Fuhrer and start World War 2. Which means Hitler would be a nobody in history, but World War 2 would still happen.