I often (really often) wish I could go back into the past. I wish I could go back and live with Australopithecus africanus. That would be the shit. I just wish I could so I would better know, ya know?

I wonder if time travel is even theoretically possible within the parameters of causal determinism. It seems like it's quite possible; it merely requires the reversal of universe expansion. But that doesn't sit well with me, it seems that one's own presence in the midst of that reversal necessitates an insincere reversal, and a change. If time travel is defined as returning to the unadulterated past, exactly as it was, then I am inclined to think it is not possible, simply because one's own presence makes it a novel situation. Surely, all other facets of the situation could have existed exactly as before, but because one's self is included, it is the past and one's self, and unless one's self is nothing, which is impossible, then the past plus one's self is not the past, and time travel in the sense that we talk about is rendered impossible.

If we acquiesce to the change, the future will necessarily be altered. A host of problems arise. What if you went back into the past and shot your great-grandfather? Then how do you even exist? Even if one argued that this was possible by allowing repeat cycles that included singular changes, this would still fall prey to the argument outlined above. Traveling back through vast quantities of time (space) could occur, but this would still happen in the same direction of the space-time continuum, and thus, be part of the future. This is analogous to turning around 180 degrees while walking on a train that is moving in the same direction as you were initially walking, but now backpedaling. Everything would look like the past but you are still moving in the same direction. Furthermore, contemporary events would cease to exist - this is easier understood after considering a less self-centered and perhaps more convincing argument.

If one were to travel through time, surely one would not bring contemporary events with them; that is, the occurence of events would not depend on a singular presence. Therefore, the past, future, and, indeed, all points in time would have to be occuring ALL the time. To arrive in another point in time would necessitate that point in time existing at that moment. This reality would require an infinite array of parallel universes, each experiencing a different point in time of this same world, ALL the time, simultaneously. Because this would necessitate distinction in space, and time and space are the same thing, this reality must be impossible.

It seems to me that time travel cannot occur even theoretically, and, interestingly, seems to put an ultimate limit on what can happen in this universe.