I don't know much about physics, but I was wondering the other day whether what I was thinking was a popular theory. If time is like a one dimensional line and it starts at the orgin and it grows like a vector along the line as time progresses, then wouldn't it only be possible to travel back in time since that portion of the vector has already been created. And if you went back in time, the all of the vector after the time you went back to erases. So once you go back in time you can't go back to he original time. Does that make sense?