Thursday, May 20, 2010

Backward In Time

According to relativity theory, it is actually possible for someone to travel forward in time faster than one's peers. You can do it either by traveling close to the speed of light (special relativity) or hanging out within a very strong gravity field like you might find near a black hole (general relativity). In both cases, you need enormous amounts of energy to accomplish the task, making it still quite out of reach, but nonetheless possible.

But even relativity does not allow for traveling backward in time. In fact, without even considering any physical limitations, there are philosophical problems. The whole idea of changing history creates logical paradoxes that cannot be easily resolved. Fortunately, most writers who venture into this territory are aware of this and usually spend a good deal of time (no pun intended) explaining things from within the story. One way to solve some paradoxes is to admit that duplicate histories of the world exist in parallel, but this is rather unsatisfying because both versions of history can claim equal status, and that grates against our sense of personal continuity. Most authors either pick the new history or the original one as the "real" one and place us there in the end, hoping that we will believe the other history has somehow ceased to exist. Alright, I'll go with that if you tell me there's some special rule that prefers one version of reality over the other. But this is not the only obstacle to overcome.

Another issue is the law of conservation of mass and energy. If you physically travel back in time, the matter that comprises your own body has essentially been substracted from the universe at the time you leave and is added to the universe at the time you arrive. This isn't a problem going forward in time as long as you still exist somewhere as you travel forward, but that is not possible in the backward direction. I suppose you can conjecture that the conservation laws might be extended over the space-time manifold, so that they could be violated in time, but not in space-time. But that would mean there is only one space-time history with the potential of several copies of the same matter existing at one time. That's not conservation of matter, that's duplication. But it could conceivably work.

And there are other difficulties with the single history paradigm that must be handled carefully. If anyone in the future ever travelled to our time, we would encounter them today. If in fact we've never encountered future travelers, then backward time travel must be rare and not within the realm of human control. There is also the problem of the relativity of space-time coordinates. To illustrate, think of the earth moving in orbit around the sun. If I travel back in time to yesterday, where should I be relative to the sun? If I remain in the same relative position, I will end up materializing in space since yesterday the earth was in a different orbital position than it is today. In short, a relative time coordinate has no meaning without an associated relative spatial coordinate. So it is essential that any time machine not only include a way to specify the time but also the place that one wishes to travel to, whether it be to the past or future.

By far, the most difficult problem with the single history timeline is the foreknowledge issue. Any time you know what is going to happen in the future, you can take action to change it, and that brings us back to the paradox of duplicate histories again, the ultimate being that of your own existence. Can you go back in time and kill yourself? Or can you change something that ultimately prevents you from being born. A writer can stay safe by avoiding such possible conflicts, but there is no way to get around this on a fundamental level without using multiple time histories.

So although current physics, and general philosophy, doesn't support it, there are ways to handle the paradoxes gracefully if a writer is careful. I find myself very forgiving in those cases simply because the intellectual puzzles that can be woven in time are some of the most fun and intellectually satisfying narratives that I've encountered. I am going to list some of my favorite ones below:

The only time I have ever seen an author deal with the conservation of matter paradox was in a short essay by C.S. Lewis called The Dark Tower. Here, it is explained that time travel in the body is impossible because of this law, but is possible in the mind. Thus, it is possible to create a time viewer that allows one to see into the past or future, and it is also possible for two people to swap respective consciousness' between two time periods. That is what happens in the story, although it is a parallel world that is seen on the viewer and one of the students swaps with a creature on the other side. No exchange of matter, just minds. It's brilliant, but alas, Lewis only wrote a few chapters and never finished the story.

A wonderful movie that comes to mind that deals with backward time travel is a little independent film called Primer. These guys (the film's creators are also in the lead roles) create a home movie or documentary type feel and then meticulously attempt to consistently represent the paradox of time travel as discovered by some young entrepeneurs in their garage, who stumble upon it by accident while working on other things. This was the best treatment I know of for the relative coordinate problem. The time machine here requires you to turn it on at the point you wish to travel back to and then wait to enter it later. This in effect creates the perfect way to specify the arrival point, namely, by being there already in advance. Of course, this method does not allow you to travel back in time very far, but it was very well thought out. The circumstances and reactions of the characters to their find are scripted to look as convincing as possible, making it easy to let yourself be strung along. And if that's not enough, the author weaves a storyline so complex that it's almost impossible to follow, yet still makes sense if you do. For anyone who likes time travel puzzles, this is the Mona Lisa.

Another backward time travel movie that I thought was well put together was based on a book by Michael Crichton called Timeline. It's about a group of scientists that travel back to Medieval times and get themselves involved in a centuries long mystery and adventure. Although it's a little more light-hearted, it does a pretty good job of setting up time puzzles that make you say - "Man, why didn't I see that?". I've read that the movie departs from the book quite a lot and given that I like Crichton's work, I'm sure I'd find the book even more interesting. I'll have to put it on my future reading list.

This one is a little dark, and at the same time adeptly humorous. TimeCrimes traces out a few hours of an evening when a man travels back in time and ends up having to interact with several versions of himself as he tries to undo his own mistakes. The time puzzle is ingeniously complex and revealed in pieces as the same scenes are replayed from different angles. It is a Spanish film and I think a little hidden gem, although I wouldn't recommend it to everyone.

And few more that have that certain special paradoxical something:
12 Monkeys

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