Notes of a Fringe-Watcher

Multiverses and Blackberries

Martin Gardner

It all began seriously with an approach to quantum mechanics (QM) called "The Many Worlds Interpretation" (MWI). In this view, widely defended by such eminent physicists as Murray Gell-Mann, Stephen Hawking, and Steven Weinberg, at every instant when a quantum measurement is made that has more than one possible outcome, the number specified by what is called the Schrödinger equation, the universe splits into two or more universes, each corresponding to a possible future. Everything that can happen at each juncture happens. Time is no longer linear. It is a rapidly branching tree. Obviously the number of separate universes increases at a prodigious rate.

If all these countless billions of parallel universes are taken as no more than abstract mathematical entities-worlds that could have formed but didn't-then the only "real" world is the one we are in. In this interpretation of the MWI the theory becomes little more than a new and whimsical language for talking about QM. It has the same mathematical formalism, makes the same predictions. This is how Hawking and many others who favor the MWI interpret it. They prefer it because they believe it is a language that simplifies QM talk, and also sidesteps many of its paradoxes.

There is, however, a more bizarre way to interpret the MWI. Those holding what I call the realist view actually believe that the endlessly sprouting new universes are "out there," in some sort of vast super-space-time, just as "real" as the universe we know! Of course at every instant a split occurs each of us becomes one or more close duplicates, each traveling a new universe. We have no awareness of this happening because the many universes are not causally connected. We simply travel along the endless branches of time's monstrous tree in a series of universes, never aware that billions upon billions of our replicas are springing into existence somewhere out there. "When you come to a fork in the road," Yogi Berra once said, "take it."

It is true that the MWI, in this realist form, avoids some of the paradoxes of QM. The so-called "measurement problem," for example, is no longer a problem because whenever a measurement occurs, there is no "collapse of the wave function" (or rotation of the state vector in a different terminology). All possible outcomes take place. Schrödinger's notorious cat is never in a mixed state of alive and dead. It lives in one universe, dies in another. But what a fantastic price is paid for these seeming simplicities! It is hard to imagine a more radical violation of Occam's razor, the law of parsimony which urges scientists to keep entities to a minimum.

The MWI was first put forth by Hugh Everett III in a Princeton doctoral thesis written for John Wheeler in 1956. It was soon taken up and elaborated by Bryce DeWitt. For several years John Wheeler defended his student's theory, but finally decided it was "on the wrong track," no more than a bizarre language for QM and one that carried "too much metaphysical baggage." However, recent polls show that about half of all QM experts now favor the theory, though it is seldom clear whether they think the other worlds are physically real or just abstractions such as numbers and triangles. Apparently both Everett and DeWitt took the realist approach. Roger Penrose is among many famous physicists who find the MWI appalling. The late Irish physicist John S. Bell called the MWI "grotesque" and just plain "silly." Most working physicists simply ignore the theory as nonsense.

In an article on "Quantum Mechanics and Reality" (in Physics Today, September 1970), DeWitt wrote with vast understatement about his first reaction to Everett's thesis: "I still recall vividly the shock I experienced on first encountering the multiworld concept. The idea of 10¹⁰⁰⁺ slightly imperfect copies of oneself all constantly splitting into further copies, which ultimately become unrecognizable, is not easy to reconcile with common sense. This is schizophrenia with a vengeance!"

In the MWI, most of its defenders agree, there is no room for free will. The multiverse, the universe of all universes, develops strictly along determinist lines, always obeying the deterministically evolving Schrödinger equation. This equation is a monstrous wave function which never collapses unless it is observed and collapsed by an intelligence outside the multiverse, namely God.

In recent years David Deutsch, a quantum physicist at Oxford University, has become the top booster of the MWI in its realist form. He believes that quantum computers, using atoms or photons and operating in parallel with computers in nearby parallel worlds, can be trillions of times faster than today's computers. He is convinced that many famous QM paradoxes, such as the double slit experiment and a similar one involving two half-silvered mirrors, are best explained by assuming an interaction with twin particles in a parallel world almost identical with our own. For example, in the double slit experiment, when both slits are open, our particle goes through one slit while its twin from the other world goes through the other slit to produce the interference pattern on the screen.

Deutsch calls our particle the "tangible" one, and the particle coming from the other world a "shadow" particle. Of course in the adjacent universe our particle is the shadow of their tangible particle. Because communication between universes is impossible, it is hard to imagine why a particle would bother to jump from one universe to another just to produce interference.

Deutsch believes that the results of calculating simultaneously in parallel worlds can somehow be brought back here to coalesce. Critics argue that QM paradoxes, as well as quantum computers, are just as easily explained by conventional theory or by such rivals as the pilot wave theory of David Bohm. In any case, Deutsch's 1997 book The Fabric of Reality: The Science of Parallel Universes-and Its Implications is the most vigorous defense yet of a realistic MWI.

Deutsch is fully aware that the MWI forces him to accept the reality of endless copies of himself out there in the infinity of other worlds. "I may feel subjectively," he writes (p. 53), "that I am distinguished among the copies as the 'tangible' one, because I can directly perceive myself and not the others, but I must come to terms with the fact that all the others feel the same about themselves. Many of those Davids are at this moment writing these very words. Some are putting it better. Others have gone for a cup of tea." And he is puzzled by the fact that so few physicists are as enthralled as he about the MWI!

Theoretical and experimental work on quantum computers is now a complex, controversial, rapidly growing field with Deutsch as its pioneer and leading theoretician. You can keep up with this research by clicking on Oxford's Centre for Quantum Computation's Web site www.Qubit.org.

The MWI should not be confused with a more recent concept of a multiverse proposed by Andrei Linde, a Russian physicist now at Stanford University, as well as by a few other cosmologists such as England's Martin Rees. This multiverse is essentially a response to the anthropic argument that there must be a Creator because our universe has so many basic physical constants so finely tuned that, if any one deviated by a tiny fraction, stars and planets could not form-let alone life appear on a planet. The implication is that such fine tuning implies an intelligent tuner.

Linde's multiverse goes like this. Every now and then, whatever that means, a quantum fluctuation precipitates a Big Bang. A universe with its own space-time springs into existence with randomly selected values for its constants. In most of these universes those values will not permit the formation of stars and life. They simply drift aimlessly down their rivers of time. However, in a very small set of universes the constants will be just right to allow creatures like you and me to evolve. We are here not because of any overhead intelligent planning but simply because we happen by chance to be one of the universes properly tuned to allow life to get started.

We come now to a third kind of multiverse, by far the wildest of the three. It has been set forth not by a scientist but by a peculiar philosopher, now at Princeton University, named David Lewis. In his best-known book, The Plurality of Worlds (Oxford, 1986), and other writings, Lewis seriously maintains that every logically possible universe-that is, one with no logical contradictions such as square circles-is somewhere out there. The notion of logical possible worlds, by the way, goes back to Leibniz's Theodicy. He speculated that God considered all logically possible worlds, then created the one He deemed best for His purposes.

Both the MWI and Lewis's possible worlds allow time travel into the past. You need never encounter the paradox of killing yourself, yet you are still alive, because as soon as you enter your past the universe splits into a new one in which you and your duplicate coexist.

Most of Lewis's worlds do not contain any replicas of you, but if they do they can be as weird as you please. You can't, of course, simultaneously have five fingers on each hand and seven on each hand because that would be logically contradictory. But you could have a hundred fingers, and a dozen arms, or seven heads. Any world you can think of without contradiction is real. Can pigs fly? Certainly. There is nothing contradictory about pigs with wings. In an infinity of possible worlds there are lands of Oz, Greek gods on Mount Olympus, anything you can imagine. Every novel is a possible world. Somewhere millions of Ahabs are chasing whales. Somewhere millions of Huckleberry Finns are floating down rivers. Every kind of universe exists if it is logically consistent.

David Lewis's mad multiverse was anticipated by hordes of science-fiction writers long before the MWI of QM came from Everett's brain. More recent examples include Larry Nivens's 1969 story "All the Myriad Ways" and Frederick Pohl's 1986 novel The Coming of the Quantum Cats. Jorge Luis Borges played with the theme in his story "The Garden of Forking Paths." There is a quotation from this tale at the front of The Many Worlds Interpretation of Quantum Mechanics (1973), a standard reference by DeWitt and Neill Graham. For other examples of multiverses in science fiction and fantasy see the entry on "Parallel Worlds" in The Encyclopedia of Science Fiction (1995) by John Clute and Peter Nichols.

Fredric Brown, in What Mad Universe (1950), described Lewis's multiverse this way:

There are, then, an infinite number of coexistent universes.

"They include this one and the one you came from. They are equally real, and equally true. But do you conceive what an infinity of universes means, Keith Winton?"

"Well-yes and no."

"It means that, out of infinity, all conceivable universes exist.

"There is, for instance, a universe in which this exact scene is being repeated except that you-or the equivalent of you-are wearing brown shoes instead of black ones.

"There are an infinite number of permutations of that variation, such as one in which you have a slight scratch on your left forefinger and one in which you have purple horns and-"

"But are they all me?"

Mekky said, "No, none of them is you-any more than the Keith Winton in this universe is you. I should not have used that pronoun. They are separate individual entities. As the Keith Winton here is; in this particular variation, there is a wide physical difference-no resemblance, in fact."

---

Keith said thoughtfully, "If there are infinite universes, then all possible combinations must exist. Then, somewhere, everything must be true."

---

"And there are an infinite number of universes, of course, in which we don't exist at all-that is, no creatures similar to us exist at all. In which the human race doesn't exist at all. There are an infinite number of universes, for instance, in which flowers are the predominant form of life-or in which no form of life has ever developed or will develop.

"And infinite universes in which the states of existence are such that we would have no words or thoughts to describe them or to imagine them."

I find it hard to believe that so many academics take Lewis's possible worlds seriously. As poet Armand T. Ringer has put it in a clerihew:

David Lewis
Is a philosopher who is
Crazy enough to insist
That all logically possible worlds actually exist.

The stark truth is that there is not the slightest shred of reliable evidence that there is any universe other than the one we are in. No multiverse theory has so far provided a prediction that can be tested. In my layman's opinion they are all frivolous fantasies. As far as we can tell, universes are not as plentiful as even two blackberries. Surely the conjecture that there is just one universe and its Creator is infinitely simpler and easier to believe than that there are countless billions upon billions of worlds, constantly increasing in number and created by nobody. I can only marvel at the low state to which today's philosophy of science has fallen.

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