## The Theory TodayHalf a century has passed since 27-year-old Hugh Everett III published a version of his Princeton Ph.D. dissertation in a leading physics journal, introducing the scientific world to his radical theory of parallel universes. In what ways did the theory break from existing theories of the day? How has it fared in those five decades, and where does it stand today in the physics community? Journalist Peter Byrne, who is midway through writing an authorized biography of Everett, answers these and other questions in this interview. Despite flunking algebra in high school, as he is the first to admit, Byrne has a remarkable talent for translating quantum mechanics into terms the layman can grasp. ## Granting permission
The quantum world builds the classical world. Everything in the classical, macroscopic world is composed of microscopic particles acting in unison. But in quantum physics, for 50 years, the only way that physicists could interpret or think about the work they were doing was to say that anything that happens in the microscopic world only has meaning in terms of how it is looked at as a large object. We cannot even talk about what happens in the microscopic world, because it is so indeterministic that we can never lay our fingers on what is actually going on. Everett broke with that. He said—and he wrote the mathematics to back it up—that we can look at the entire universe as quantum-mechanical. We do not have to have an arbitrary division between the classical and the quantum, an arbitrary division that exists because people had no other way to explain the results they were getting. For many years before Everett introduced his theory, people had thought about this problem, but Everett was the first to propose a logically consistent way of removing the barrier. Everett's argument that the universe is quantum-mechanical is logically indisputable in and of itself. It's an "interpretation," which is a fundamentally different animal than what physicists call formalism. Formalisms are mathematical devices that show you how to operate experiments but that do not need to pop up any kinds of meanings beyond simply If you do A, you get B. Interpretation tries to tell you
So Everett came up with this universal wave function, which is just [Austrian physicist Erwin] Schrödinger's equation for describing how elementary particles move around writ large—that is, applied to the whole universe! And it makes beautiful mathematical and logical sense. Actually, it's very much in use in physics today. However, it has consequences to it that people were and remain uneasy with, which basically is that ## Dirty little secret
Now, the wave function that describes all of the positions that that electron can be at does not say that any one of those positions is more or less real than any other position. They have an equal reality. That is, before you actually measure the electron, it could show up in any of the places the wave function allows it to. When you measure it, though—when you interact with it, when you observe it—it takes one position. We see only one position. In quantum mechanics, however, there's only a Say the first time you measure it, the quantum-mechanical formulas pop up a 30 percent chance that the particle is at position X. The next time you do the measurement, exactly the same way, you might get a 70 percent chance that the particle is at position Y. That means that there's a 100 percent chance that it is at either X or Y. Not only is it a chance, but according to quantum mechanics, it actually is at both X
Now, according to Everett, you have superpositions in macroscopic objects as well as microscopic, like a gram of carbon or the tip of your pencil. Those classical objects are composed of microscopic systems that are all in superpositions, but we don't see 100 million positions for a gram of carbon or the tip of your pencil, we only see one. The founders of quantum theory, people like Niels Bohr and Werner Heisenberg and Paul Dirac and John von Neumann and others, were faced with this problem back in the 1920s. It was inexplicable using any kind of formula that they could come up with up, so they postulated—that is to say, they decided arbitrarily—that what happens is that the wave function "collapses." ## Becoming jellyfish
Everett, who was around Bohr in 1954 and talking to other people around Bohr—as well as to his own mentor, the famous physicist John Wheeler, who died this summer after a long, glorious career in physics—Everett looked at this and likely thought back to something Schrödinger had said a few years before in Dublin. Schrödinger had said physicists fear that if we don't have the collapse, "We should find our surroundings rapidly turning into a quagmire, or sort of featureless jelly or plasma, all contours becoming blurred, we ourselves probably becoming jellyfish." What he meant by that was that, if we don't collapse it, then all these possibilities are going to start propagating, and there won't be any cause or effect anymore that we can follow from one event to another, and our selves, our physical beings, will start to become duplicated, and every possible position that a human body can be in will suddenly exist in classical reality.
## Splitsville
In order to demonstrate the consequence of this mathematically, Everett came up with a solution showing that the observer, the human being, correlates with every possible state that that gram of carbon, that pencil tip, could be in. So So, in Everett's view, when the human correlates herself—that is, interacts, exchanging energy with the gram of carbon or a clock or whatever—she splits like an amoeba. She splits into copies of herself, one for each element in the superposition. [See Everett's draft of his never-published amoeba analogy.]
But the upshot of the Many Worlds theory is that this universal wave function describes a series of branching universes that make up what [physicist] David Deutsch calls the "multiverse," and that in these branching universes, there are beyond ## Multiple theories
People spotted weaknesses in Everett's argument early on, however. Bryce DeWitt certainly spotted it when he was looking at it carefully in the early 1970s. Here was the problem: If all possible physical events occur, then what happens to probability? We're making a measurement that says that there's a 30 percent chance that this electron is at position X, but if we believe in a many-worlds theory, we believe that actually there's a universe in which it's at every single possible position it could ever be at. So how do we assign a probability value to it? It's a very deep question that philosophers in particular have been struggling with a lot in the last 20 years especially.
Well, it's tied to the question of, "If everything exists in superpositions, why do we not see all objects existing in superpositions?" We live in a classical world. It has an arrow of time that goes in a certain direction. It has entropy, which is related to probability and information. And in order to make sense of the Everett theory, you really do need to explain why we think probability exists, at least in "our" branch.
For days they debated this question of probability in Everett, and a related weakness, which is that if you have these branches splitting off constantly every nanosecond all over the universe, going in all different directions, how does one universe, one branch, link itself to all these different states so that a coherent, single branch in which my history, say, Peter Byrne's history and life, which I remember as being singular, I remember not being born as a thousand people but as one person, how does that emerge? These are difficult questions that pertain to philosophical topics that have been discussed for centuries.
So while Everett's is not the only interpretation out there, it was the one featured on the cover of ## Putting to use
This was another huge flaw in the Copenhagen Interpretation and its collapse-of-the-wave-function postulate, because by saying that while we can't explain what happens to the superposition, and we know that in our classical world we have only one measurement result, we have to postulate that in order to get that one measurement result, we stand outside of the quantum object. So what Bohr and the rest of them were saying is that, when you're making a measurement, you the physicist are a classical object, the measurement is a quantum system, and when you make a measurement, the classical world trumps the quantum world. What it predicates is that whenever you make a measurement of the quantum system, you have to be external to it. You have to be outside of it. What Everett did was he showed how you could make measurements and be Now, in cosmology this is really important, because if you want to understand the early state of the universe, the inflation state where quantum mechanics is very orderly, you can't get external to it. You've go to be able to calculate from inside the wave function that describes the entire universe. So one of the greatest uses of Everett's theory today is in cosmology, not just in a technical sense, but in an interpretive sense, because if you're a cosmologist who wants to understand the universe from an understanding also that, as a person, you're inside the universe—because how could you be outside of it?—then this gives you a perspective from which to view that whole universe that you're trying to comprehend.
People who are building quantum computers don't necessarily have to believe that there are multiple universes. But they are faced with working with these quantum qubits that exist in what can easily be described as multiple universes. If they're not that, then nobody has any other way of describing how they're situated. And if you're David Deutsch, who's been one of the founders of the science of quantum computation, you will look at this situation and you'll say that it's proof that Everett's theory is correct. In fact, David Deutsch has said that quantum mechanics itself is proof that there are multiple universes, although he thinks of them in a more sophisticated way than Everett did.
What that means is that, using Everett's analysis as a starting point, physicists like Zurek and Dieter Zeh at the University of Heidelberg, and James Hartle and Murray Gell-Mann and others, have developed a theory of quantum mechanics basically called "decoherence theory." It's not interpretation, but rather a technique that, while not exactly solving the measurement problem, does explain how the classical world can emerge from the quantum universe. Some decoherence theorists think there are multiple universes; some think that there's only one. All of them will tell you that they were inspired to go along the path of developing this theory because of having Everett's universal wave function as a useful tool. ## A difficult birth
However, people didn't attack his theory publicly, because it's very hard to attack Everett's logic. They did attack it privately. For instance, in 1956, before it was published, Wheeler and Everett sent a copy of the dissertation to Bohr in Copenhagen to see if he would agree that it was true. It wasn't likely that he would, because if he did agree he'd have to admit that he'd been wrong for decades about everything else. As it happened, Bohr was pretty polite. He didn't attack it himself, but he assigned his acolytes to attack Everett, and actually for decades they took every opportunity they could to say that Everett was stupid, that his theory didn't work, that Everett didn't understand quantum mechanics, stuff like that.
When quantum mechanics came along, you had this problem where you could only calculate with probabilities, where you could not say that a quantum system existed in a certain position before you look at it, because you can't. All you can say is that it exists in a distribution of possible positions, and so you have the problem that the quantum-mechanical world is indeterministic. Our classical world is largely deterministic, and collapsing quantum indeterminism into classical images is the only way we can describe the quantum world, Bohr said, because we must use what he called "ordinary language." Nonetheless, Bohr said, we have to be honest and admit that indeterminism is a basic force in the universe. We just cannot talk about it, you see, because it's inexplicable. So we have to postulate that the world we see is the only real world. It was John von Neumann who invented the mathematics of wave function collapse in the early 1930s, and Bohr went along with it.
But when the founders of quantum mechanics, including Niels Bohr, were looking at their new, beautiful theory back in the '20s, they realized that getting one world out of many was a problem, but they couldn't explain it any other way except to say what was in front of their eyes, which is In fact, there's nothing wrong with the postulate in terms of hurting the ability of the physicists to do their work. It enables them to do their work. But, as Everett said when he was being attacked privately before his thesis was printed, he said the Copenhagen Interpretation is a "monstrosity," with one reality for the quantum world and another for the classical. Many, many people agreed with him over time, and many people actually agreed with him ## A quantum-mechanical world
To give you an example, in your television you've got a cathode-ray tube that shoots electrons at a screen. According to quantum mechanics, there is a chance that one out of every 137 electrons that you shoot out of that tube will go where you want it to go—which means that 136 of them are just going to be lost. Quantum mechanics shows you how to set up a device so that you can get a coherent picture using only one out of every 137 electrons streaming out of a cathode-ray tube. Billions and billions of them are just streaming out every second, so that's plenty.
However, we cannot tell you
I've talked to any number of experimental physicists, and these guys are not philosophically oriented. They're very interested in getting results by manipulating elementary particles in certain ways—say, to make a quantum computer—but to do this, they almost have to take a dualistic attitude towards what's going on in the elementary particle world. Don Eigler at IBM told me recently, "When I look at an electron from a distance, it's a particle. When I look it up really close, it's a wave." Electrons and all elementary particles behave dualistically—as waves sometimes and as particles sometimes—depending on the environment that they're in, and your point of view. These are questions that experimental theorists have been essentially taught in school do not concern them. They're issues of philosophy, they're told, and there's a certain wisdom in that, because if people were puzzling over unsolvable problems, nobody would want to go into that line of work. Look at [Nobel Prize-winning mathematician] John Forbes Nash. In her biography of him, ## Goodbye to all that
His mentor there was John Wheeler, who was one of the inventors of the hydrogen bomb—he'd invented it the year before he met Everett—and he was a huge shaper and player in the military-industrial complex. Princeton was a center of military research, and Everett, it turns out, had a bent for doing military work. He was never that excited about working in academia, because military work, especially if you started doing it in the private sector, which he did after working at the Pentagon for a few years, paid a lot better than academic work. Everett also didn't trust academia. Here he had come up with this remarkable idea that, 50 years later, is one of the most powerful ideas in physics, acknowledged by physicists everywhere in publications. And in his day either people attacked it viciously because they had their own kind of dogmatic pursuits to protect, or they didn't want to talk about it. I think the idea of working in academia kind of repulsed him after that.
In the late '60s, when he was working seriously in quantum cosmology, DeWitt was attracted to the universal wave function as an interpretive method of dealing with what was going on, and he started writing about it. In 1970, he published an article in John Wheeler had made Everett cut three quarters of his thesis. Wheeler had had this dream that Bohr was somehow going to approve it, so he made Everett remove his direct attacks on the Copenhagen Interpretation as well as his provocative metaphors about splitting observers and bifurcating cannonballs (and, for some reason, a whole chapter on information and probability theory). So a lot of the explanation of things that people considered to be weaknesses in Everett's theory were cut out of the version that people read. In 1973, DeWitt published the long version, along with the short version and some other papers, including one by himself, in a book called
## Groundbreaker
Everett also wrote one of the classic military game theory papers of all time in his first year as a grad student at Princeton. In fact, it's such a remarkable paper that when one of the founders of game theory, Harold Kuhn, put out a book 10 years ago on the greatest of all game theory papers, he included Everett's paper. Everett's game theory work, his work in logic with the algorithm that he invented, his work in quantum mechanics, his work in developing software—all these things are still impacting science and computation today.
People turned to technocrats like Hugh Everett to design programs that would give them options. So interestingly enough, in his work as a military operations researcher, Everett's specialty was looking at alternatives in different situations. Given that his quantum-mechanical theory said that everything that is physically possible happens, and he believed in it, he also had to live with the fact that there were millions and billions of universes in which the nuclear wars that he was designing took place. No wonder he drank. ## Byrne's take
I think the arguments against Everett hold some water, but they're inconclusive as well, and I see that Everett's theory has had a material and positive affect on the development of science. It would be kind of crazy to say that the universal wave function is true but the rest of it isn't, because you really can't have one without the other. So I just have to say I wouldn't be surprised to find that Everett's theory is true, and I'm not going to say that it's not. |
Hugh Everett in 1964, when he was 34 years old and was working at the Pentagon. He had abandoned quantum physics, to which he would never return.
A half century after he shared his radical idea with the world in the July 1957 issue of a leading physics journal, Everett and his Many Worlds theory finally get their day in the sun. Here, the cover of "Before I started looking into this, I would have thought it was crazy," Peter Byrne says of Everett's theory. "Now I wouldn't be surprised if it's true." "Wild as it sounds—a person splitting into numerous copies of herself—Everett's theory has not been shown to be mathematically incorrect." "If there's a wave function, there's a collapse. They don't want to think about it much further than that." "People didn't attack his theory publicly, because it's very hard to attack Everett's logic." "Quantum mechanics is the most successful physical theory in the history of humankind." "I think that's one of the reasons he went to the Pentagon—they had the best computers." "Everett also had to live with the fact that there were millions and billions of universes in which the nuclear wars that he was designing took place. No wonder he drank." "I wouldn't be surprised to find that Everett's theory is true, and I'm not going to say that it's not." |

Interview conducted on August 29, 2008 and edited by Peter Tyson, editor in chief of NOVA Online Parallel Worlds, Parallel Lives Home | Send Feedback | Image Credits | Support NOVA |
© | Created October 2008 |