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The Many Worlds Interpretation “Ts’ui Pen did not believe in a uniform, absolute time. He believed in an infinite series of times, in a growing dizzying net of divergent, convergent and parallel times. This network of times which approached one another, forked, broke off, or were unaware of one another for centuries, embraces all possibilities of time. We do not exist in the majority of these times; in some you exist, and not I; in others I, and not you; in others, both of us.” Jorge Luis Borges, Garden of the Forking Path This picture of time as a forking path envisaged by Borges in 1941 reappeared in 1957 as a serious scientific theory proposed by physicist Hugh Everett, although there is no evidence that he was influenced by Borges. It was called ‘The Many Worlds Interpretation of Quantum Mechanics’ and it forms the basis of Nick Payne’s play Constellations. To understand what quantum mechanics is and why it lends itself to such a weird view of reality, we need to talk a bit about theoretical physics. How did the universe begin? What are its fundamental constituents? What are the laws of nature that govern these constituents? As we look back over the last century we can identify two pillars on which our theories rest: quantum mechanics and Einstein’s general theory of relativity. Quantum theory deals with the very small: atoms, subatomic particles and the forces between them. General relativity deals with the very large: stars, galaxies and gravity, the driving force of the cosmos as a whole. Quantum mechanics was a radical departure from the deterministic clockwork universe of Isaac Newton. In the classical Newtonian view, if we knew the position and speed of every particle in the universe we could predict with certainty its future evolution, at least in principle. In any event, fate was predetermined. However, the traditional interpretation of quantum theory, developed in Copenhagen by Niels Bohr in the 1920s, asserts that we cannot predict with certainty the outcome of any experiment; the best we can do is to assign it a probability. For example, suppose an experiment has two possible outcomes, A and B. We are able to tell the likelihood of each, say 70% A and 30% B. If we repeat the experiment 100 times then A will happen roughly 70 times and B roughly 30 times, but we cannot predict an individual outcome. This does not reflect any inadequacy on our part or any defect in our experimental apparatus; it is a fact of nature. Moreover, until we perform a measurement on the quantum system both outcomes A and B coexist; it is the very act of measuring that brings about one reality rather than another. This was encapsulated by the paradox of Erwin Schrödinger’s cat-in-a-box, which could be both 70% dead and 30% alive until we open the box and observe it to be either one or the other. Everett was dissatisfied with this indeterminacy and proposed that the act of measurement produces a fork in the road: in one universe the outcome is A but there is another universe where the outcome is B. The two universes continue to coexist but go their separate ways and never communicate with one another. As the cat might have said to Elvis Presley: “We do not exist in the majority of these times; in some you exist, and not I; in others I, and not you; in others, both of us.” Repeating this with millions of measurements on millions of systems and you are not far from the “infinite series of times, in a growing dizzying net of divergent, convergent and parallel times” of Ts’ui Pen. Not everybody liked the Everett interpretation. In fact the biggest objection was that that was all it was: an interpretation. To a physicist, unless there is some experimental way of discriminating between the Copenhagen and Many Worlds descriptions then there is no point in trying to choose between them. This argument continues to divide the physics community. Some say we will never tell them apart; others that a genuine difference will emerge when we apply quantum theory to the origin of the universe itself, the big bang. However, this would require incorporating Einstein’s general relativity, and the dilemma theoretical physics faces at the beginning of the 21st century is that its two 20th-century pillars are mutually incompatible. On the microscopic scale, Einstein’s theory fails to comply with the quantum rules that govern the behaviour of the subatomic particles, while on the macroscopic scale black holes are threatening the very foundations of quantum mechanics. Something big has to give. This augurs a new scientific revolution. Many physicists believe that this revolution is already underway with the theory of superstrings. As their name suggests, superstrings are one-dimensional string-like objects. Just like violin strings, they can vibrate and each mode of vibration, each note if you like, corresponds to a different subatomic particle. This note is an electron, this one a quark, that one a Higgs boson and so on. One strange feature of superstrings is that they live in a universe with nine space dimensions and one time dimension. Since the world around us seems to have only three space dimensions, the extra six would have to be curled up to an unobservably small size (or else rendered invisible in some other way) if the theory is to be at all realistic. Fortunately, the equations admit solutions where this actually happens. The main reason why theorists are so enamoured with string theory is that it seems at last to provide the longdreamed-of consistent quantum theory of gravity and holds promise, incorporating and extending the standard models of particle physics and cosmology. String theorists are the first to admit that the theory is by no means complete but is constantly undergoing improvement in the light of new discoveries. For example, one of the problems with the form of the theory developed in the 1980s was that there was not one but five mathematically consistent superstrings. If one is looking for a unique theory of everything, five theories of everything seems like an embarrassment of riches. In 1995, the theory underwent a revolution when it was realised that these five strings were not after all different theories but just five corners of a deeper and more profound new theory, called ‘M-theory’. M-theory involves membrane-like extended objects, which themselves live in a universe with 11 dimensions (10 space and 1 time). String and M-theory continue to make remarkable theoretical progress, for example by providing the first microscopic derivation of the quantum black hole formula first proposed by Stephen Hawking in the mid-1970s. Solving long-outstanding theoretical problems such as this indicates that we are on the right track. But, as often happens in science, M-theory presented new problems of its own, not least of which is that its equations admit even more ways of curling up the extra dimensions than string theory does, and at the moment we have no idea which one, if any, nature should pick to describe our universe. Theorists are divided on this issue. Some think that when we understand the theory better, we will understand why one unique universe will be singled out, thus answering in the negative Einstein’s question: “Did God have any choice in creating the universe?” Others think that there are indeed many, possibly infinitely many, different universes and we just happen to be living in one of them. For example, Leonard Susskind at Stanford University regards this ‘multiverse’ as a virtue to be exploited, as it fits in with ideas advocated by astronomer royal Lord Martin Rees and others, in which there was not one big bang but many, possibly infinitely many, stretching back into the infinite past. This version of ‘many worlds’ should not be confused with Everett’s interpretation of quantum mechanics; at least that is what I intended to tell the cast and crew of Constellations when Nick Payne invited me to address them on the subject last November. Nick’s script seemed in places to conflate the two. But I decided to check on the latest developments before giving my talk. I was astonished to find that Susskind and his colleague Raphael Bousso had recently written a paper called ‘The Multiverse Interpretation of Quantum Mechanics’ with the opening sentence: “We argue that the many worlds of quantum mechanics and the many worlds of the multiverse are the same thing, and that the multiverse is necessary to give exact operational meaning to probabilistic predictions from quantum mechanics.” Another case of life imitating art. Professor Michael Duff holds the Abdus Salam Chair of Theoretical Physics at Imperial College London. He is a fellow of the Royal Society. Further reading: Theory of Everything, Michael Duff, New Scientist, 02 June 2011.