I’m teaching a freshman seminar on time travel at Brown this year, so I’ve been watching a lot of time travel movies as ‘preparation’. I always knew that many time travel movies don’t make a lot of sense on a bit of reflection. What surprised me on recent re-watchings was that some seemed unintelligible even on relatively generous assumptions.
Philosophers normally break time-travel stories into two categories: those that do make sense within a ‘one-dimensional’ view of time and those that don’t.
The ones that make sense on a ‘one-dimensional’ view never have it the case that at a particular time something both is and isn’t the case. They don’t require that the direction of causation always goes from past to future, that would stop them from being time travel stories after all, but they require that there be a single complete and coherent story that can be told of the history of the world. Some philosophers are known to reserve the label ‘consistent’ for these stories, but that’s probably a bit harsh.
Some stories keep to this constraint, even when they are under a lot of pressure to break it. The first Terminator does, the second Terminator might (though it’s normally interpreted as violating it), and both 12 Monkeys and it’s inspiration _La Jetée_ display quite a bit of ingenuity in telling an involved time-travel story that has a coherent one-dimensional history.
But obviously this kind of constraint is not a universal norm among time-travel stories. For example, the whole point of the Back to the Future movies is that what time-travellers do can change the course of future history. (If you need, or even want, a refresher on what happens in the movie, one is available here, though be warned that site launches a very annoying MIDI file unless your browser is configured to block that kind of thing.)
In Back to the Future in 1985 the first time around George works for Biff, and the second time around, after Marty has changed the past, Biff works for George. So this is a violation of the one-dimensionality principle. I had always assumed that the movie could be made sense of on a ‘branching time’ model. Indeed in the second movie that’s exactly the kind of model they say they are using.
The idea is that the history we are familiar with is only one branch of the tree of time. This isn’t a wholly unknown picture. I’ve been told that Aristotle believed something similar, and (if you believe everything you read on the web) a few quantum mechanics specialists also hold a similar view. (Personally I think it’s about as plausible as the world-rests-on-a-giant-turtle theory, but the history of philosophers making speculations about physics is not great, so I’ll be a little restrained here.) On this picture the other branches exist, and the only thing that’s special about our branch is that we’re in it. Before a branch point it isn’t determined which branch we will end up on. The full story of the world includes a whole array of things totally unlike anything we know – our history is the story of a particular climb up the tree of time, a climb that could have turned out very very differently to how it actually did.
It should be easy to fit Back to the Future style time travel into this picture. When Marty goes back into 1955 it isn’t pre-determined whether he will stay in the branch from whence he came. And he changes his world enough that he more or less has to move into another branch – ultimately a branch in which his parents are much more successful than they actually are. (Or were. Or something. Ordinary tense words don’t handle this kind of situation very well, as Douglas Adams pointed out somewhere.)
So far so good. Now obviously one part of the movie isn’t compatible with this picture. If Marty is safely and soundly in his new branch, there’s no reason to think he will ‘fade away’ if in that branch his parents don’t meet and marry and conceive etc. He’s there and that’s all there is to it. So a major plot line of the movie becomes a little incomprehensible. But apart from that, I thought it was going to be possible to make sense of it all.
What surprised me on re-watching the movie  was that even granting them a branching time universe, and ignoring the lack of reason for Marty to ‘fade away’, the story in the movie still didn’t make sense. Here’s why. In the new branch that Marty moves onto, his parents meet, he is conceived, born and grows up in a successful family, rather unlike the family he remembers growing up in. Marty also travels forward in time in that branch from 1955 to 1985. The Marty that got to new 1985 by time travel is around at the end of the movie – we see his surprise at how different new 1985 is. But the Marty that was born, raised etc is not. On the branching time model, there should be two Martys around now, but the movie only gives us one.
Maybe the movie could make sense on an even stranger metaphysics than regular branching time. What we need is a metaphysics with not only branching time, but also some cross-branch relations that determine who (in one branch) is the same person as whom (in a different branch). And we need those relations to have enough causal force that when a person is in a branch they shouldn’t be in, or are too often in a branch they shouldn’t be that often in, the relations somehow make the world fix things. But even this doesn’t explain why new 1985 Marty should not remember growing up in a successful household. It’s really all a mess, even granting a really wild metaphysical picture. What amazes me is how it seems to work under its own logic while one is watching it. Some enterprising grad student should work out just what that logic is – they could probably justify anything whatsoever using it.
There are several interesting aesthetics questions related to this distinction. For instance, is it a vice in a time-travel story that it does not make sense on a one-dimensional view of time? I used to think the answer was yes, then I decided that was much too snobby. But after my recent bout of time travel movieing, I’m drifting back to my former position. At the very least, it’s a virtue of those stories that do keep to one-dimensional time, just because one-dimensional time-travel stories are so pretty when done well. The plot devices in the last two Harry Potter stories may have been fairly awful, but the time travel story at the end of The Prisoner of Azkaban is rather good for just this reason. That story gets bonus degree of difficulty points for having the characters interact with themselves (admittedly at a distance) in a more-or-less psychologically plausible way.
I think that stories that violate this constraint too frequently rely on our assumption that causality always moves forward in movie (or book) time. I’d be surprised if someone could tell a decent time-travel story in a movie where the order of scenes didn’t match up with what happened in real time or in any character’s personal time. (Think Pulp Fiction meets Back to the Future.) I imagine that the result would be incomprehensible. I’ve seen some people argue that the final scenes of Tim Burton’s Planet of the Apes should be understood this way, but since those scenes are incomprehensible, that doesn’t really hurt my point. On the other hand, I imagine that with some ingenuity one could chop up a good ‘one-dimensional’ movie like 12 Monkeys into all kinds of rearranged scenes and it still be tolerably coherent.
Well, not the only thing. As has been noted here previously, the 80s were a really strange time. The ‘fashions’ are … well the less said the better. But the thing I’d totally blacked out was that in the movie they try and make Marty look cool by having him play in a Huey Lewis cover band. It’s hard to comprehend what they were thinking. I was rather shocked to hear a Huey Lewis song on a ‘classical rock’ station in Seattle, but the idea that at one time associating with his music was a way to impress pretty 17 year olds is just wild.
On the other hand, I shouldn’t play up the fact that I remember much of this time at all. Many of the students in my course won’t have been born when Back to the Future was released. Hopefully that means they won’t ask too many hard questions about why the plot doesn’t seem incomprehensible on first viewing.