This is not one of my normal columns, but I think of what kind of letter to the editor I would get if I published this in Scroll. Some of my columns border on irrelevance, but this one ... well, just read it.
There are three ways that time travel works in books and movies. I'm not talking about how you actually travel back, but I could devote another column to the methods I've seen — sorry, heard of — used, including black hole, flux capacitators, temporal displacement and the like. I'm talking about the logistics of what would happen when you travel back in time and inevitably change the past.
I've actually been thinking about this pretty deeply. To illustrate each, let's say I went back in time and killed my grandfather before he could ever meet my grandmother. In each scenario, let's look at what might play out next.
Way #1: The predestination paradox model. Basically, everything that you did while going back in time was supposed to happen and has already been taken into account in the current timeline. This is the time-travel model used in the TV show "LOST." If I went back in time and killed my grandfather, somehow that's how things were SUPPOSED to happen. Maybe the guy I thought was my grandpa wasn't — maybe he was a guy who would have killed my real grandpa had I not killed him. It's also used in the "Terminator" movies — a robot from the future was left in a factory and the factory workers used that robot to build more robots, which resulted eventually in a robot from the future being send back in time. ...
And then, in "Harry Potter and the Prisoner of Azkaban," Harry's interaction with the past didn't change anything in the past. He simply used a Patronus to save his past self the way things were supposed to happen. In other words, the events originating from your supposed interference in the past were already in place in the established timeline before you traveled back in time. Daniel Faraday, from "LOST," says it best: "Whatever happened, happened."
Way #2: The one universe model. According to this model, whatever you go back and change will be reflected when you return to your own time. Though this is the model used in the "Back to the Future" movies, it's impossible and creates and entire host of universe-shattering paradoxes. Let's say I were to go back and kill my grandfather before he could have a chance to meet my grandmother. This would make it so I was never born, right? So if I had never been born, nobody would have gone back and killed my grandfather. If nobody had killed my grandfather, I would have been born. Then, I would have gone back and killed my grandfather. Then I would have never been born and ... See the mess that this creates? "Back to the Future" simplifies this concept.
Way #3: The multiple universes model. This is used in the new "Star Trek" movie and is the least complicated of the three models of time travel. If you go back and change the past, any change you make will only create a parallel universe — your own universe will be totally unaffected. If I were to go back and kill my grandfather before he could meet my grandmother, it would only create an alternate universe where none of my grandfather's descendants were alive.
Stay tuned for more ramblings, assuming your head hasn't yet exploded.