Useless (youngwilliam) wrote,

Merry-Go-Round Broke Down

A little thing that crops up now and again in speculative fiction stories is what I like to call circular causality. The actual term(s) for it can be Predestination Paradox and/or Ontological Paradox.

To blatantly steal and slightly modify an example from a recent bit of popular televised fiction:

* You have Bob and Time Travelling Ted.
* Ted gets kidnapped.
* Bob's wondering what to do and where Ted is, when suddenly... *poof* ...Ted appears!
* Ted gives Bob a map to find him and then vanishes off again.
* Bob follows the map and finds Ted, who asks Bob where he got the map from.
* Bob explains, so Ted says, "Oh! Ok, hold on a sec", takes the map from Bob, draws up a copy of it, then pops back in time to hand the copy to Bob before popping back.
* That done, Bob and Ted go on their merry way.

For some strange reason, I've never had a problem with this. Alas, it's really tricky trying to explain it to folks who don't get it.

Recently when trying to explain it to someone, I got a flea in my ear about trying to work up a good physical analogy. The best I could come up with at the time was this:

Imagine a path that runs right up alongside a merry-go-ground. You walk along the path, hop onto the merry-go-ground, take it for a spin, then hop off back onto the path to continue your way along.

Trouble is, that's not perfect since one has to keep in mind that you've effectively always been on the merry-go-ground, always will be on it, and always are on it, while you're also continuing along the path. I thought about it at work, and came up with this one which seems a bit better:

This time, it's a walkway that goes next to an upright waterwheel. You're walking along the path with a soda-pop bottle (full) in your hand, and someone following you who also has a bottle (empty). The water-wheel itself has a number of soda-pop bottles attached to the spokes. When you start to get past the water-wheel, you pour the contents of your bottle into the nearest wheel-bottle. This carries it around until it gets back to where it 'first' crosses the path relative to a walker, and your pal with the empty bottle gets there in time to have their bottle filled.

When they get to the far side of the wheel, they empty their bottle into a wheel's bottle, just like you did, continuing the cycle for the next person along.

The second one is better, but eh... I'm having a hard time working up a good clear "object lesson" analogy.
Tags: geek-chat
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