hello, mcfly!


On the implications of massive contigency for time travel

As we've all seen on the Simpsons , it's good advice to would-be time travelers into the past to avoid killing things. Imagine if you went back in time and killed your grandfather before he met your grandmother - you wouldn't exist! And Marty McFly showed us a different danger: if one of your ancestors fell in love with you they may never show the same favor to the one they "did" end up conceiving you or another ancestor of yours with. Of course we then get the familiar paradox: if you don't exist, you couldn't have gone back to mess things up, so things didn't get messed up, so you do exist after all... well, I'll talk more about that later.
But, you might think, as long as nothing so untoward took place, it would be quite nice to go back and talk to your grandparents as young people. Well, too bad, Sunny Jim! The slightest interactions with not only your ancestors but with anyone or anything in the remote past will wipe you out and cause the same paradox. Why?
Consider one of the the defining events of your life: the moment when the sperm that provided half your genes met with the egg that provided the other half. At that moment, as Richard Dawkins has pointed out, the odds against your existence dropped from the truly astronomical to the readily -- conceivable. Out of the hundreds of millions of sperm swimming their way to the egg, only one succeeds; any other one succeeding would have given rise to an individual who would not have been you. So anything that affects this process can wipe you out, if it acts early enough. What can affect it? Almost anything: a few seconds of delay, a few millimeters of shift in position. And the effects of tampering get greater as time goes on, i.e., the earlier the tampering occurs. A time traveller's displacement of air molecules on the other side of the globe will change local wind patterns, which will eventually result in a difference in weather or temperature, which will change people's dispositions to do all sorts of things. The famous "butterfly effect" will be your undoing.
What are some of the upshots of this.. well, think of all the terrible events that happened in the remote past, from the loss of ancient knowledge the genocides of the 20th century. We all in some way wish these events had never happened. But wishing for this is the same as wishing we had never been born. Even if you are not a direct descendent of one of humanity's scourges, anyone who goes back in time and prevents their existence is also "preventing" you and everyone you know.

I find it interesting to imagine what time travel would be like in a cellular automaton like the Game of Life , or a one-dimensional cellular automaton.

Side note: of all the possible time travel scenarios, which have been depicted in stories or film?
We have 6 possibilities. I'll fill in the ones I know. "The present" means the present of the story.
Someone from the past comes to the present. (Bill and Ted's Excellent Adventure)
Somone from the past goes to the future. (?)
Someone from the present goes to the past. (A Connecticut Yankee in King Arthur's Court)
Someone from the present goes to the future. (The Time Machine)
Someone from the future comes to the present. (Star Trek IV: The Voyage Home)
Someone from the future goes to the past. (?)

I don't know of any depictions of the "skip-the-present" scenario, but such a story might be interesting...

Posted: Sat - August 21, 2004 at 01:09 AM | | | |

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