Stephen Wolfram has written a few thousand words on the structure of universes which is, as might be expected, quite interesting. My favorite passage is this one:
A good friend of mine has kept on encouraging me not to throw away any even vaguely plausible universes--even if we can show that they're not our universe. He thinks that alternate universes have to be good for something.
I certainly think it'll be an interesting--almost metaphysical--moment if we finally have a simple rule which we can tell is our universe. And we'll be able to know that our particular universe is number such-and-such in the enumeration of all possible universes.
As a science fiction editor, I know what alternate universes are good for!
