Parallel universes pop up everywhere. My first encounter was a short report about a professor building a machine to move between universes in order to prove such things existed. There was of course a disbelieving world, his non-disbelieving daughter, and…well, it’s a long time since I read it. Written by John Wyndham, was my hazy recollection. A search of our bookshelves failed to find my copy—perhaps it was a victim of our buy one, throw one out rule for managing finite (bookshelf) space. No doubt, in one of the infinite number of universes, we have more bookshelf space—with a copy still there. In the end I resorted to Google to find it—and lo.  Now, after the professor’s daughter crossed from one universe into another, her discussions with her there-but-not-here husband could pin-point more or less when the two universes had parted ways, but not why. And that is my headache: what causes a universe to split?
A significant event—that seems to be the consensus, with the duplicated universes taking the different possible outcomes. The typical example cited is the Naz—oops, that was close, if I’d used that word I’d have lost the argument—a war having a different outcome. The problem with the significant event theory is that the universe is so vast that there would be a universe-splitting event occurring somewhere all the time. We wouldn’t have time for the paint to dry before we split again.
This might not be a big problem if the split occurred simultaneously, but the speed-of-light limit seems to rule that option out. Perhaps the split is synchronised by a note being passed around: ‘we split at noon, on a day quite a few years away’. But by then the effect of whatever event caused the split would be firmly entrenched and splitting would be rather pointless. That suggests a split must ripple across the universe, though that also leads to difficulties. What happens when the ripple from one split meets the ripple from a different split? Do we get Siamese universes, four half-sized universes, or perhaps gross distortions? That’s all too hard—let’s go back to the why question.
How significant must an event be to trigger a split? Completing an article for CRANK? Creating 8,145,060 duplicate universes after each Tattslotto draw would seem to put a considerable strain on the universe splitting infrastructure, however well-oiled. Is the problem left to a committee to decide? Maybe, but setting up such a committee would be a highly significant event, and so there must be a universe in which it wasn’t done and has been undergoing unregulated splitting ever since. The likelihood that splitting is event-based is fading fast.
To counter this doubt, some philosophers have suggested that splitting is localised, say planet by planet. If that were the case, which of the duplicate Jupiters do we Earthlings see, a not insignificant question given that Jupiter turned into a small star in 2010 in some universes.  Hence local must be something larger. But however big, this argument can be repeated again and again until we must conclude that universe splitting is all or nothing.
Looking at how the theory of evolution evolved suggests an answer to the why question. We went from a deterministic theory to a serendipitous theory: the idea giraffes actively grew longer necks to reach higher leaves was replaced by an extended lottery, where those giraffes that by chance had long-necks had an advantage and were more likely to survive. That suggests to me that, rather than being triggered by a specific event, the universe splits in two at some random point in time, a bit like a bacterium or an amoeba. This animate analogy isn’t so far-fetched in light of Professor Challenger’s successful but odoriferous demonstration that planets were alive, like giant sea urchins, floating through space. 
Because the ripple from of a split was delayed by the Senate, the author’s clone was able to send a profuse apology for the above which, he assures us, is very much better in his universe.
- John Wyndham, 'Opposite number' in The seeds of Time, 1956
- Arthur C Clark, 2010: Odyssey Two, 1982.
- Arthur Conan Doyle; When the world screamed, 1928.
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