Hey! You Got Science in my Science Fiction (Essays)

The Grandfather Paradox

Having solved the problem of how to build a time machine, we move on to the next important question. Can you go back in time and kill your own grandfather? If you can, then you won't have been born, which means you didn't go back to kill him, which means you are born, which means you eventually go back in time and kill him,....

How does the writer solve this problem? Well, maybe first we can ask a scientist how to solve this problem. Igor Novikov is among the handful of physicists who have seriously considered time travel. He said that the grandfather paradox was too complex a system to consider (at least initially) and we needed to work with the question at a more fundamental level. He wanted to start with nothing but particles and wormholes.

A simplified variant of the grandfather paradox is the following: shoot a particle through one wormhole at just the right trajectory so that it comes out of the other one and knocks itself off course--preventing it from going into the wormhole in the first place. The solution to this problem is that when the particle emerges from the wormhole to knock itself off course, it comes out slightly off course--which means that when it strikes itself, it knocks itself slightly off course--which was why it came out slightly off course. The beauty of this is that there is a self-consistent solution.

What does that mean for the person who tries to kill their own grandfather? Well, maybe the plan goes slightly off course and he survives the apparently fatal wound, only to fall in love with his nurse, who becomes the shooter's grandmother. To add irony to injury, perhaps the grandfather was considering entering the priesthood before the attempted shooting.

Of course, self-consistent time travel is not the only solution available to the writer--though it is, perhaps the highest and noblest route. :) Bill and Ted followed it, to excellent results.

Another possibility is that a new history is created whenever someone travels into the past. Both exist, the one where the shooter was born and travels back to kill their own grandfather, and another in which they weren't. Thus, as methods of suicide go, shooting your own grandfather is ineffective in this multiple history version of time travel. It bugs me, though. Creating a whole new history should be the equivalent of creating a whole new universe, which should require a lot of energy. Where would it come from? The writer probably doesn't need to solve that one, but it still bugs me.

Another possibility is that the time traveler goes sideways in time. That is to say, when the traveler goes into the past, they actually travel to an alternate universe. So when the shooter travels back in time to kill their own grandfather, they discover that instead of being a botanist granddad became a martial arts expert and kills the shooter first! Maybe in that version of history, granddad had twins of you. Maybe the dinosaurs didn't die and the whole world is different.

What other solutions to the grandfather paradox exist? The most common is the least satisfying (to me). If you kill your grandfather, then you disappear after you do it. It's something like the energy conservation I mentioned before. There's only one history can exist, so the other one has to fade out--except they both existed for a while. This version of the paradox is popular with Star Trek, Back to the Future, etc.

My biggest problem with it is that, in this version of things, one can travel back in time and keep trying to change history until the good guys win--which feels like cheating. Often, there's a notion of the way history is supposed to be in such stories, and often a cosmic power trying to make things come out right--which feels even more like cheating.

Did I miss anything? What other answers are there to the grandfather paradox? Write them here, then I'll go back in time and write them first and claim credit. :)