The reason time travel cannot happen...

...is because time, once it has passed, no longer exists. And the future, does not exist. There is only the present. Imagine a pencil tracing a line. And imagine that this pencil, once it traces the line, the mark behind where it currently is at no longer exists...so you would have nowhere to go back to. And you can't go forward on the line, because there is no spot to go forward to, because the pencil has not gotten that far yet. Time is that line. It exists now. You're reading this now. You can come back and read it again in the future, but you can't go back to the point where you first read it, because that point does not exist. There is only the present.