Presence

My good friend Dan pointed out the apparent paradox in the Time Turner sequence at the end of Harry Potter and the Prisoner of Azkaban; namely that if Harry hadn't been saved from the Dementors, he would have died, in which case, there would have been no future Harry to go back and save Harry in the first place. This is a version of what is commonly referred to as a predestination paradox. A classic example (supposing time travel possible): I decide to murder my direct ancestor, in an effort to prevent my own birth (or to start a chain reaction that would unravel the very fabric of the space time continuum, and destroy the entire universe! Granted, that's a worse case scenario. The destruction might in fact be very localized, limited to our own galaxy. Yes, quite a relief). The potential of this paradox has led some to speculate that time travel is in fact impossible, or so I've been told. The Harry Potter case is basically the opposite, but essentially the same (we will not deal here with the ingenius use of time travel employed by Bill S. Preston, Esq. and Ted "Theodore" Logan to obtain the keys; suffice it to say, that might be the single most briliant application of time travel, ever).

I recently came across an article from BBC News detailing a new model for time travel that avoids the problems created by a predestination paradox, and is somewhat reminiscent of Schrödinger's cat. Basically, at the point in time at which an individual leaves his timeline, wherever he ends up, he will be unable to alter anything that was known to be true at the time at which he left. As an example, let's say I want to travel back in time and kill my grandfather, so as to prevent myself from ever having existed. If at the time that I step into my time machine, I know my grandfather is alive, then there is no way that I can possibly kill him, since the probability of him being alive when I left was 1. It would follow that a person would never be able to prevent himself from being born, since the probability of their being alive later on is also 1. Try as I might, I will be unable to change the past, since I know it not to have come true. If, however, at the time of travel an individual's status is unknown to me, it is possible that, once in the past, it turns out I was the one who killed them.

What I find interesting about this approach is that it basically says you can't alter the past, you can only learn more about it. Whatever has happened must happen, and you might just find, you had a hand in it. Marty McFly would have had nothing to worry about back in 1955 according to this theory, since try as he might, his existence acted as knowledge that his parents ended up together (the theory, however, would still permit us the fun in seeing how Marty played a role in his own creation). Accordingly, anyone of us could have shot JFK. The other interesting idea that is suggested by this theory is that time is not linear, but is happening "all at once." Our perception of it is linear, for whatever reason, but the model presents things as happening "all at once" (I would venture to say that it would be impossible to accurately describe this model; its probably beyond the reaches of our language).

With this in mind, let's reconsider the Harry Potter scenario. One minute Harry is lying on the ground, overwhelmed by the Dementors; the next, he is in the Hospital wing, about to go back through time in an attempt to set things right. We don't know how Harry was saved from the Dementors, Harry doesn't even know; he just knows that somehow, he lived. As such, at the moment before Harry heads into the past, the probability of his being alive is 1. Fast forward back to the scene by the lake; Harry #1 lays dying on the banks, while Harry #2 waits expectantly for his past savior to (re)appear. Only then does Harry realize that he was the one who saved himself originally, and so he does it (again?). Nothing has changed, only that Harry #2 is able to complete his knowledge of the events of the past, and in doing so, stepped in and played a part.

A wise man once said, "Time flies like the wind. Fruit flies like bananas." So true.