*Photo: Kelly Sikkema/Unsplash.*

Quantum physics is strange. At least, it is strange to us, because the rules of the quantum world, which govern the way the world works at the level of atoms and subatomic particles (the behaviour of light and matter, as the renowned physicist Richard Feynman put it), are not the rules that we are familiar with — the rules of what we call “common sense.”

The quantum rules, which were mostly established by the end of the 1920s, seem to be telling us that a cat can be both alive and dead at the same time, while a particle can be in two places at once. But to the great distress of many physicists, let alone ordinary mortals, nobody (then or since) has been able to come up with a common-sense explanation of what is going on. More thoughtful physicists have sought solace in other ways, to be sure, namely coming up with a variety of more or less desperate remedies to “explain” what is going on in the quantum world.

These remedies, the quanta of solace, are called “interpretations.” At the level of the equations, none of these interpretations is better than any other, although the interpreters and their followers will each tell you that their own favored interpretation is the one true faith, and all those who follow other faiths are heretics. On the other hand, none of the interpretations is worse than any of the others, mathematically speaking. Most probably, this means that we are missing something. One day, a glorious new description of the world may be discovered that makes all the same predictions as present-day quantum theory, but also makes sense. Well, at least we can hope.

Meanwhile, I thought I might provide an agnostic overview of one of the more colorful of the hypotheses, the many-worlds, or multiple universes, theory. For overviews of the other five leading interpretations, I point you to my book, “Six Impossible Things.” I think you’ll find that all of them are crazy, compared with common sense, and some are more crazy than others. But in this world, crazy does not necessarily mean wrong, and being more crazy does not necessarily mean more wrong.

If you have heard of the Many Worlds Interpretation (MWI), the chances are you think that it was invented by the American Hugh Everett in the mid-1950s. In a way that’s true. He did come up with the idea all by himself. But he was unaware that essentially the same idea had occurred to Erwin Schrödinger half a decade earlier. Everett’s version is more mathematical, Schrödinger’s more philosophical, but the essential point is that both of them were motivated by a wish to get rid of the idea of the “collapse of the wave function,” and both of them succeeded.

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As Schrödinger used to point out to anyone who would listen, there is nothing in the equations (including his famous wave equation) about collapse. That was something that Bohr bolted on to the theory to “explain” why we only see one outcome of an experiment — a dead cat or a live cat — not a mixture, a superposition of states. But because we only detect one outcome — one solution to the wave function — that need not mean that the alternative solutions do not exist. In a paper he published in 1952, Schrödinger pointed out the ridiculousness of expecting a quantum superposition to collapse just because we look at it. It was, he wrote, “patently absurd” that the wave function should “be controlled in two entirely different ways, at times by the wave equation, but occasionally by direct interference of the observer, not controlled by the wave equation.”

Although Schrödinger himself did not apply his idea to the famous cat, it neatly resolves that puzzle. Updating his terminology, there are two parallel universes, or worlds, in one of which the cat lives, and in one of which it dies. When the box is opened in one universe, a dead cat is revealed. In the other universe, there is a live cat. But there always were two worlds that had been identical to one another until the moment when the diabolical device determined the fate of the cat(s). There is no collapse of the wave function. Schrödinger anticipated the reaction of his colleagues in a talk he gave in Dublin, where he was then based, in 1952. After stressing that when his eponymous equation seems to describe different possibilities (they are “not alternatives but all really happen simultaneously”), he said:

Nearly every result [the quantum theorist] pronounces is about the probability of this or that or that … happening — with usually a great many alternatives. The idea that they may not be alternatives but all really happen simultaneously seems lunatic to him, just impossible. He thinks that if the laws of nature took this form for, let me say, a quarter of an hour, we should find our surroundings rapidly turning into a quagmire, or sort of a featureless jelly or plasma, all contours becoming blurred, we ourselves probably becoming jelly fish. It is strange that he should believe this. For I understand he grants that unobserved nature does behave this way—namely according to the wave equation. The aforesaid alternatives come into play only when we make an observation — which need, of course, not be a scientific observation. Still it would seem that, according to the quantum theorist, nature is prevented from rapid jellification only by our perceiving or observing it … it is a strange decision.

In fact, nobody responded to Schrödinger’s idea. It was ignored and forgotten, regarded as impossible. So Everett developed his own version of the MWI entirely independently, only for it to be almost as completely ignored. But it was Everett who introduced the idea of the Universe “splitting” into different versions of itself when faced with quantum choices, muddying the waters for decades.

It was Hugh Everett who introduced the idea of the Universe “splitting” into different versions of itself when faced with quantum choices, muddying the waters for decades.

Everett came up with the idea in 1955, when he was a PhD student at Princeton. In the original version of his idea, developed in a draft of his thesis, which was not published at the time, he compared the situation with an amoeba that splits into two daughter cells. If amoebas had brains, each daughter would remember an identical history up until the point of splitting, then have its own personal memories. In the familiar cat analogy, we have one universe, and one cat, before the diabolical device is triggered, then two universes, each with its own cat, and so on. Everett’s PhD supervisor, John Wheeler, encouraged him to develop a mathematical description of his idea for his thesis, and for a paper published in the Reviews of Modern Physics in 1957, but along the way, the amoeba analogy was dropped and did not appear in print until later. But Everett did point out that since no observer would ever be aware of the existence of the other worlds, to claim that they cannot be there because we cannot see them is no more valid than claiming that the Earth cannot be orbiting around the Sun because we cannot feel the movement.

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Everett himself never promoted the idea of the MWI. Even before he completed his PhD, he had accepted the offer of a job at the Pentagon working in the Weapons Systems Evaluation Group on the application of mathematical techniques (the innocently titled game theory) to secret Cold War problems (some of his work was so secret that it is still classified) and essentially disappeared from the academic radar. It wasn’t until the late 1960s that the idea gained some momentum when it was taken up and enthusiastically promoted by Bryce DeWitt, of the University of North Carolina, who wrote: “every quantum transition taking place in every star, in every galaxy, in every remote corner of the universe is splitting our local world on Earth into myriad copies of itself.” This became too much for Wheeler, who backtracked from his original endorsement of the MWI, and in the 1970s, said: “I have reluctantly had to give up my support of that point of view in the end — because I am afraid it carries too great a load of metaphysical baggage.” Ironically, just at that moment, the idea was being revived and transformed through applications in cosmology and quantum computing.

“Every quantum transition taking place in every star, in every galaxy, in every remote corner of the universe is splitting our local world on Earth into myriad copies of itself.”

The power of the interpretation began to be appreciated even by people reluctant to endorse it fully. John Bell noted that “persons of course multiply with the world, and those in any particular branch would experience only what happens in that branch,” and grudgingly admitted that there might be something in it:

The “many worlds interpretation” seems to me an extravagant, and above all an extravagantly vague, hypothesis. I could almost dismiss it as silly. And yet … It may have something distinctive to say in connection with the “Einstein Podolsky Rosen puzzle,” and it would be worthwhile, I think, to formulate some precise version of it to see if this is really so. And the existence of all possible worlds may make us more comfortable about the existence of our own world … which seems to be in some ways a highly improbable one.

The precise version of the MWI came from David Deutsch, in Oxford, and in effect put Schrödinger’s version of the idea on a secure footing, although when he formulated his interpretation, Deutsch was unaware of Schrödinger’s version. Deutsch worked with DeWitt in the 1970s, and in 1977, he met Everett at a conference organized by DeWitt — the only time Everett ever presented his ideas to a large audience. Convinced that the MWI was the right way to understand the quantum world, Deutsch became a pioneer in the field of quantum computing, not through any interest in computers as such, but because of his belief that the existence of a working quantum computer would prove the reality of the MWI.

This is where we get back to a version of Schrödinger’s idea. In the Everett version of the cat puzzle, there is a single cat up to the point where the device is triggered. Then the entire Universe splits in two. Similarly, as DeWitt pointed out, an electron in a distant galaxy confronted with a choice of two (or more) quantum paths causes the entire Universe, including ourselves, to split. In the Deutsch–Schrödinger version, there is an infinite variety of universes (a Multiverse) corresponding to all possible solutions to the quantum wave function. As far as the cat experiment is concerned, there are many identical universes in which identical experimenters construct identical diabolical devices. These universes are identical up to the point where the device is triggered. Then, in some universes the cat dies, in some it lives, and the subsequent histories are correspondingly different. But the parallel worlds can never communicate with one another. Or can they?

Deutsch argues that when two or more previously identical universes are forced by quantum processes to become distinct, as in the experiment with two holes, there is a temporary interference between the universes, which becomes suppressed as they evolve. It is this interaction that causes the observed results of those experiments. His dream is to see the construction of an intelligent quantum machine — a computer — that would monitor some quantum phenomenon involving interference going on within its “brain.” Using a rather subtle argument, Deutsch claims that an intelligent quantum computer would be able to remember the experience of temporarily existing in parallel realities. This is far from being a practical experiment. But Deutsch also has a much simpler “proof” of the existence of the Multiverse.

What makes a quantum computer qualitatively different from a conventional computer is that the “switches” inside it exist in a superposition of states. A conventional computer is built up from a collection of switches (units in electrical circuits) that can be either on or off, corresponding to the digits 1 or 0. This makes it possible to carry out calculations by manipulating strings of numbers in binary code. Each switch is known as a bit, and the more bits there are, the more powerful the computer is. Eight bits make a byte, and computer memory today is measured in terms of billions of bytes — gigabytes, or Gb. Strictly speaking, since we are dealing in binary, a gigabyte is 230 bytes, but that is usually taken as read. Each switch in a quantum computer, however, is an entity that can be in a superposition of states. These are usually atoms, but you can think of them as being electrons that are either spin up or spin down. The difference is that in the superposition, they are both spin up and spin down at the same time — 0 and 1. Each switch is called a qbit, pronounced “cubit.”

Using a rather subtle argument, Deutsch claims that an intelligent quantum computer would be able to remember the experience of temporarily existing in parallel realities.

Because of this quantum property, each qbit is equivalent to two bits. This doesn’t look impressive at first sight, but it is. If you have three qbits, for example, they can be arranged in eight ways: 000, 001, 010, 011, 100, 101, 110, 111. The superposition embraces all these possibilities. So three qbits are not equivalent to six bits (2 x 3), but to eight bits (2 raised to the power of 3). The equivalent number of bits is always 2 raised to the power of the number of qbits. Just 10 qbits would be equivalent to 210 bits, actually 1,024, but usually referred to as a kilobit. Exponentials like this rapidly run away with themselves. A computer with just 300 qbits would be equivalent to a conventional computer with more bits than there are atoms in the observable Universe. How could such a computer carry out calculations? The question is more pressing since simple quantum computers, incorporating a few qbits, have already been constructed and shown to work as expected. They really are more powerful than conventional computers with the same number of bits.

Deutsch’s answer is that the calculation is carried out simultaneously on identical computers in each of the parallel universes corresponding to the superpositions. For a three-qbit computer, that means eight superpositions of computer scientists working on the same problem using identical computers to get an answer. It is no surprise that they should “collaborate” in this way, since the experimenters are identical, with identical reasons for tackling the same problem. That isn’t too difficult to visualize. But when we build a 300-qbit machine—which will surely happen—we will, if Deutsch is right, be involving a “collaboration” between more universes than there are atoms in our visible Universe. It is a matter of choice whether you think that is too great a load of metaphysical baggage. But if you do, you will need some other way to explain why quantum computers work.

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Most quantum computer scientists prefer not to think about these implications. But there is one group of scientists who are used to thinking of even more than six impossible things before breakfast — the cosmologists. Some of them have espoused the Many Worlds Interpretation as the best way to explain the existence of the Universe itself.

Their jumping-off point is the fact, noted by Schrödinger, that there is nothing in the equations referring to a collapse of the wave function. And they do mean thewave function; just one, which describes the entire world as a superposition of states — a Multiverse made up of a superposition of universes.

Some cosmologists have espoused the Many Worlds Interpretation as the best way to explain the existence of the Universe itself.

The first version of Everett’s PhD thesis (later modified and shortened on the advice of Wheeler) was actually titled “The Theory of the Universal Wave Function.” And by “universal” he meant literally that, saying:

Since the universal validity of the state function description is asserted, one can regard the state functions themselves as the fundamental entities, and one can even consider the state function of the whole universe. In this sense this theory can be called the theory of the “universal wave function,” since all of physics is presumed to follow from this function alone.

… where for the present purpose “state function” is another name for “wave function.” “All of physics” means everything, including us — the “observers” in physics jargon. Cosmologists are excited by this, not because they are included in the wave function, but because this idea of a single, uncollapsed wave function is the only way in which the entire Universe can be described in quantum mechanical terms while still being compatible with the general theory of relativity. In the short version of his thesis published in 1957, Everett concluded that his formulation of quantum mechanics “may therefore prove a fruitful framework for the quantization of general relativity.” Although that dream has not yet been fulfilled, it has encouraged a great deal of work by cosmologists since the mid-1980s, when they latched on to the idea. But it does bring with it a lot of baggage.

The universal wave function describes the position of every particle in the Universe at a particular moment in time. But it also describes every possible location of those particles at that instant. And it also describes every possible location of every particle at any other instant of time, although the number of possibilities is restricted by the quantum graininess of space and time. Out of this myriad of possible universes, there will be many versions in which stable stars and planets, and people to live on those planets, cannot exist. But there will be at least some universes resembling our own, more or less accurately, in the way often portrayed in science fiction stories. Or, indeed, in other fiction. Deutsch has pointed out that according to the MWI, any world described in a work of fiction, provided it obeys the laws of physics, really does exist somewhere in the Multiverse. There really is, for example, a “Wuthering Heights” world (but not a “Harry Potter” world).

That isn’t the end of it. The single wave function describes all possible universes at all possible times. But it doesn’t say anything about changing from one state to another. Time does not flow. Sticking close to home, Everett’s parameter, called a state vector, includes a description of a world in which we exist, and all the records of that world’s history, from our memories, to fossils, to light reaching us from distant galaxies, exist. There will also be another universe exactly the same except that the “time step” has been advanced by, say, one second (or one hour, or one year).

But there is no suggestion that any universe moves along from one time step to another. There will be a “me” in this second universe, described by the universal wave function, who has all the memories I have at the first instant, plus those corresponding to a further second (or hour, or year, or whatever). But it is impossible to say that these versions of “me” are the same person. Different time states can be ordered in terms of the events they describe, defining the difference between past and future, but they do not change from one state to another. All the states just exist. Time, in the way we are used to thinking of it, does not “flow” in Everett’s MWI.

*John Gribbin is a Visiting Fellow in Astronomy at the University of Sussex, UK and the author of *In Search of Schrödinger’s Cat*, *The Universe: A Biography* and *Six Impossible Things* from which this article is excerpted.*

*This article has been republished from *The MIT Press Reader*.*