A person named Eilidh Macdougall is flossing her teeth and notices what she takes for a gnat that flitted somewhere in her field of vision. Then she notices, on a horizontal part of the tap of her washbasin, a thin bar of shine, which is actually a small reflection of the sky outside the window, and she realizes that the tiny movement had been, or probably had been, the reflection of a gull flying past, perhaps thirty-five yards away.
How about that for a trivial incident?
The multiversist holds that this exact incident – this exact incident – happened also in another universe. Indeed, in an infinity of other universes.
Our own universe is inconceivably vast. There may be millions of inhabited planets in our galaxy. And there are billions of galaxies. Cosmologists speculate that besides this universe – the only universe we can observe, or rather of which we can observe a part – there may be other universes. (Where are these other universes? Not in our space; space came into existence with the Big Bang that started our universe, and there is no space around it; so perhaps other universes exist somehow in parallel with ours.)
Other scientists say that the subject of other universes is not a subject for science, since they can never be observed, only speculated about.
Those who speculate, however, go on to speculate that universes pop into existence out of black holes in our universe; our universe itself popped in such a way out of another. Or that there are other universes matching all of the ways in which microscopic events could be randomly different on the quantum level. That there is an infinite number of other universes; and that this implies there must be at least one similar to ours, indeed identical with it; indeed, an infinity of universes identical to ours. If you could travel from universe to universe (which you certainly cannot), you would travel through billions of universes and eventually come to one identical to our own; and through billions more to another; and so on to infinity.
Some people on hearing of the idea of another universe similar or identical to ours, in which copies of themselves exist, react that it is “Rather nice!” I find it sickening, and I also suspect it to be logically impossible. We need someone trained in the discipline of logic to determine whether a proof can be formulated; also, to tell us whether logic and mathematics are independent of the physics of universes and valid in all of them (I would think they are).
Consider what identical means. It’s vanishingly unlikely for two pinheads to be identical. Consider what identical means when you’re talking about universes. I’ve read discussions that say things like “In an alternative universe, you decide not to turn in to Starbucks for your coffee break after all, so you miss meeting that executive and getting that job and moving to Spokane.” That’s such a feebly imagined difference! It’s so gigantic a difference between universes!
There is not only that second universe, the replica-universe, that is identical to ours down to Eilidh’s name being spelled with two l’s and a Mac instead of Mc, and her being at the flossing stage (usually she flosses before brushing, but on this Tuesday she’s doing it the other way around), and wearing only one slipper because she lost the other on the stairs, and flossing not while standing up but while sitting on the commode so as to multitask, and happening, despite her Scottish name, to be a trans-gender person and to be living at this time as an exchange student at Murcia in Spain, where seagulls are uncommon – and she is thinking the identical thought – and though using, unnecessarily, the hot tap, she has turned it off – but not quite tightly, so that small drips are falling from it at six-second intervals – and they happen at this moment to become cooler because two minutes ago the air pressure switch in the boiler downstairs failed – and her mother’s father’s mother’s father’s mother’s father’s father’s mother’s mother’s father was the Puritan named Man-Is-Born-To-Evil-As-The-Sparks-Fly-Upward Jobson – and elsewhere in her surroundings are several other bits of gleam that are reflections of the window, hence of the sky and the passing gull, but she doesn’t notice them – and there’s a slight blur in another part of her field of vision because a bit of soap got into her eye – and the gull is a juvenile Larus argentatus and has lost one left wing feather and has only fifteen fleas – and a certain cottonwood leaf is falling at this instant in Wyoming – those, too, are all grossly huge details. There is a third universe that is identical with all of that – all of that, and all of everything else – except for one atom, and only one atom, being in a different excited state in a particle of silicon, not in the gravel of a path through the municipal gardens at Hyderabad in India but in a layer of magma 1435 kilometers below the surface of a planet in the galaxy called (in that universe as well as in ours) Abell 4132.
And there are other universes identical to both of those except that she did not realize that the reflection was a gull, or did not notice it, or in which she was mistaken, it was not a reflection but a gnat, or a floater in her eye; or in which she turned the tap very slightly tighter, so that the drips are falling at 5.1-second intervals. Or that the incident happened 3.3 microseconds earlier. Or eighteen million years later. And at all other times. An infinity of those other universes. Each with its infinity of variants.
Look around you. You can see an almost-infinite number of details, whether you’re in a forest, a library, or a prison cell, and can be aware of an almost infinite number of micro-positions for your body and an almost-infinite number of states inside your own mind. (Eilidh was at the same moment trying to find a rhyme for “chastity” for a jokey Bar Mitzvah card she had half-drawn – no, 75.1 percent drawn – for a friend of a friend. And simultaneously was noticing, for the second time, a small itch under her third left rib.) For all that, and for the whole earth, galaxy, and universe consisting of such details, there is another universe in which all those details are identical; an infinite number of such universe, in some of which a certain atomic detail is either different or not.
And consider what it means to believe that you, identically, exist elsewhere. It means that the whole same chain of causation that ended in you, with all your cells and habits and your momentary movements, reaches back identically through your present and past trains of thought, your experiences, your family tree, the evolution of animals, the placing of individual molecules in the cloud from which the solar system formed – not a single kink in that path of trillionfold minute causes can be different.
If two universes are identical they must have been identical throughout the billions of years since they began. The slightest difference in a molecule would have led in time to huge differences. Every train of causation must have continued in parallel. Every ant must have turned left or right at the same instant. (Therefore it is not possible that the Eilidh in the other universe blinked a microsecond earlier, or a million years later.)
What I am trying to say, in this multiplicatio ad absurdum, is that, between any two universes that differ in only one detail, there is – according to the extreme-multiversist – another possibility; between any two possibilities there is another, or an infinity of others. And that is not maintainable.
The logical disproof might take a form something like this:
Is it true that a certain number (unknown, and ranging from one to infinity) of universes exists? Yes. We can see one.
That being true, can it also be true that a certain different number (ranging from two to a different infinity) of universes exists? No.
(Can there be “different infinities”? Yes, even though they have the same infinite size. Suppose there is an infinite number of particles, or people, and then each divides into two. The second infinity is different from the first, because it contains particles or people that the first did not. For every particle in Universe A there are two particles in Universe B, and yet the universes have the same size as each other and as everything else that is infinite!
Therefore, despite infinity, there are at least some conceivable universes that do not exist. It remains logically possible that there could be an infinite number of universes, but not logically possible that every conceivable universe – including the one or ones identical or even merely similar to our own – must exist. You can easily conceive of other universes identical or similar to ours, but even infinity doesn’t mean that they have to exist.
If there are other universes, they could all be different. They could, though they don’t have to be, greatly different. Or they could all not exist.
And I’m guessing that theory will in due course settle back on the position that none of them do.
Parallel universes of the imagination, yes. You can have your infinity of those.