From what I've seen, most speculations of time travel seem to say that when you go back in time, everything else around you returns to an earlier state that it used to be. For example, if you stand outside your house and go back in time, the house will "un-build" itself and there will be nothing there. However, what it seems like is that everything else AROUND you is going back in time, not you. If it were just you that went back in time, you would simply get younger. But here's what I don't get if this is true. Let's say you went back one year. You would supposedly get one year younger. However, during that year, you interacted with the outside universe in a number of ways; you breathed and modified air molecules, you ate food, and you manipulated objects (like driving a car, picking up things, etc.). In addition, in order for the universe and you to exist in two separate states of time, the matter that you are composed of would have to be put on "hold", because if everything else around you went back in time, a certain portion of matter would disappear. If it were just you that went back in time, all those ways in which you interacted with the outside world would have to be undone, but this would cause the entire universe and all it's contents to shift back in time in equal amounts, thus creating no net time travel effect at all. If this is true, then any attempt to go back in time will require an almost infinite amount of energy (since you are modifying the state of everything) and it will appear that absolutely nothing has happened. I don't see how time travel is even remotely possible.