Did anyone watch the "multiple universes" episode of THE UNIVERSE on the History Channel last week? It's fascinating to realize that many physicists believe alternate universes actually exist. As far as I could understand the exposition (and I admit I wasn't paying rigorously close attention), there are theoretically several ways parallel or alternate universes may come into existence.
During the inflationary phase following the Big Bang, self-contained "bubbles" may have formed, each one a separate space-time continuum having no contact with the others. Or the quantum uncertainty principle may be invoked to imply that every event that can possibly happen has happened, each tiny change generating another universe. Infinite numbers of variants of ourselves and everything around us may exist in the "same" place and time, imperceptible to us. By extension, some alternate universes would be identical to ours except for one detail, others so different as to be incomprehensible.
Then there's the possibility that other universes may have radically different systems of natural laws—different elements, different physical constants, etc.
Of course, we'd never be able to make contact with those other realities, so there would be no way to prove they exist, much less travel to them, as I understand the situation anyway. Except by magic, perhaps? Through the wardrobe or down the rabbit hole?
By the way, I don't completely approve of the term "multiple universes." By definition, "universe" means the oneness of everything that exists, so "multiple universes" is grammatically equivalent to "most unique." I suppose you could argue, though, that if every space-time continuum is inaccessible to the others, for practical purposes each one is the only one and therefore can be called a universe. Anyway, SF seems to have permanently embraced the terminology of "universe" and "multiverse."
If you could travel to an alternate universe and meet "yourself," would he or she be the "same" person as you? If you believe in the soul, does each counterpart have a separate soul? More intriguing for SF romance purposes, suppose you crossed over to a neighboring space-time continuum and met the counterpart of your primary-world lover. Would the two of you inevitably fall in love in that world? There was an episode of LOIS AND CLARK when Lois and Clark were sent to a parallel world in which that-world Clark was in love with Lana Lang and didn't use his powers. This-world Lois and Clark persuaded him to become that world's Superman. He and primary-world Clark were definitely not only two separate individuals but different in personality in some respects. If you saw the newest Shrek movie, you remember Shrek had a tough time getting the alternate-world version of his wife to give him a second look. But she did finally fall in love with him all over again, suggesting that in their core selves, they were the same people in any version of reality. The cast of the TV show EUREKA is currently coping with a slightly altered present-day world they accidentally produced by traveling into the past to the time of the town's founding. Several characters' roles and relationships have changed, but at least one person also discovers that his other self in this altered timeline had a radically different personality from his. (So when they came back to the present, what happened to the "other selves" they displaced? The show doesn't raise that question, and I hadn't thought of it until this minute.) "The Dark Tower," an unfinished novel attributed to C. S. Lewis, postulates the invention of a window into an "Othertime" that appears to be an offshoot of our world but strikingly different in many ways. A character in the story involuntarily changes places with his double in Othertime. There the young traveler meets the Othertime counterpart of his fiancee. She turns out to be a nicer person than his "real" fiancee but recognizably a variation on the same woman. He, in turn, is definitely nicer than his Othertime counterpart. Unfortunately, the fragment breaks off just when the plot and worldbuilding get exciting.
If you've read Robert Heinlein's NUMBER OF THE BEAST, you'll remember its premise: In the infinite number of universes that make up the multiverse, anything that can possibly happen has happened somewhere. That means each person has an infinite number of counterparts, presumably with small or large variations depending on the degree of deviation. Moreover, "anything that can happen" includes anything that can be imagined. So all fictional worlds ever created exist out there somewhere among the parallel universes. "Pantheistic multipersonal solipsism," as Heinlein's characters call it, implies that if we had a suitable transport device (like the delightfully luxurious transmogrified spaceship in the novel), we could visit Oz, Burroughs's or Bradbury's Mars, Narnia, or any other imaginary world we wished.
Margaret L. Carter
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