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The Many Worlds of Hugh Everett

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The Many Worlds of Hugh Everett HISTORY OF PHYSICS The Many Worlds of Hugh Everett After his now celebrated theory of multiple universes met scorn, Hugh Everett abandoned the world of academic physics. He turned to top-secret military research and led a tragic private life • • • BY PETER BYRNE ugh Everett III was a brilliant mathema- gene Shikhovtsev, myself and others and inter- KEY CONCEPTS tician, an iconoclastic quantum theorist views I conducted with the late scientist’s col- ■ Fifty years ago Hugh H and, later, a successful defense contractor leagues and friends, as well as with his rock-mu- Everett devised the many- with access to the nation’s most sensitive mili- sician son, unveil the story of a radiant intelligence worlds interpretation of tary secrets. He introduced a new conception of extinguished all too soon by personal demons. quantum mechanics, in reality to physics and influenced the course of which quantum effects world history at a time when nuclear Armaged- Ridiculous Things spawn countless branches don loomed large. To science-fiction aficiona- Everett’s scientific journey began one night in of the universe with differ- dos, he remains a folk hero: the man who invent- 1954, he recounted two decades later, “after a ent events occurring ed a quantum theory of multiple universes. To slosh or two of sherry.” He and his Princeton in each. his children, he was someone else again: an classmate Charles Misner and a visitor named ■ The theory sounds like a emotionally unavailable father; “a lump of fur- Aage Petersen (then an assistant to Niels Bohr) bizarre hypothesis, but in niture sitting at the dining room table,” ciga- were thinking up “ridiculous things about the fact Everett inferred it rette in hand. He was also a chain-smoking implications of quantum mechanics.” During from the fundamental alcoholic who died prematurely. this session Everett had the basic idea behind the mathematics of quantum At least that is how his history played out in many-worlds theory, and in the weeks that fol- mechanics. Nevertheless, most physicists of the time our fork of the universe. If the many-worlds the- lowed he began developing it into a dissertation. dimissed it, and he had to ory that Everett developed when he was a stu- The core of the idea was to interpret what abridge his Ph.D. thesis on dent at Princeton University in the mid-1950s is the equations of quantum mechanics represent the topic to make it seem correct, his life took many other turns in an un- in the real world by having the mathematics of less controversial. fathomable number of branching universes. the theory itself show the way instead of by ap- Everett’s revolutionary analysis broke apart pending interpretational hypotheses to the ■ Discouraged, Everett left physics and worked on a theoretical logjam in interpreting the how of math. In this way, the young man challenged military and industrial quantum mechanics. Although the many- the physics establishment of the day to recon- mathematics and comput- worlds idea is by no means universally accepted sider its foundational notion of what consti- ing. Personally, he was even today, his methods in devising the theory tutes physical reality. emotionally withdrawn presaged the concept of quantum decoherence— In pursuing this endeavor, Everett boldly and a heavy drinker. a modern explanation of why the probabilistic tackled the notorious measurement problem in ■ He died when he was just weirdness of quantum mechanics resolves itself quantum mechanics, which had bedeviled phys- 51, not living to see the into the concrete world of our experience. icists since the 1920s. In a nutshell, the problem recent respect accorded Everett’s work is well known in physics and arises from a contradiction between how ele- his ideas by physicists. philosophical circles, but the tale of its discovery mentary particles (such as electrons and pho- —The Editors and of the rest of his life is known by relatively tons) interact at the microscopic, quantum level few. Archival research by Russian historian Eu- of reality and what happens when the particles 98 SCIENTIFIC AMERICAN December 20 07 Hugh Everett are measured from the macroscopic, classical ) level. In the quantum world, an elementary par- ticle, or a collection of such particles, can exist ); in a superposition of two or more possible states handwriting verett of being. An electron, for example, can be in a adultE superposition of different locations, velocities and orientations of its spin. Yet anytime scien- tists measure one of these properties with preci- sion, they see a definite result—just one of the elements of the superposition, not a combina- tion of them. Nor do we ever see macroscopic objects in superpositions. The measurement problem boils down to this question: How and why does the unique world of our experience emerge from the multiplicities of alternatives available in the superposed quantum world? );COURTESYEUGENESHIKHOVTSEV OF ANDKENNETHFORD( W. Physicists use mathematical entities called verett wave functions to represent quantum states. A E wave function can be thought of as a list of all young the possible configurations of a superposed );COURTESYAMERICANTHE OF INSTITUTE PHYSICS,OF NIELS BOHR LIBRARY ANDARCHIVES ( quantum system, along with numbers that give the probability of each configuration’s being the one, seemingly selected at random, that we will mushroomcloud detect if we measure the system. The wave func- tion treats each element of the superposition as equally real, if not necessarily equally probable from our point of view. The Schrödinger equation delineates how a quantum system’s wave function will change CABE;COURTESYPRINCETON OF UNIVERSITY LIBRARY ( C through time, an evolution that it predicts will be smooth and deterministic (that is, with no randomness). But that elegant mathematics seems to contradict what happens when hu- ILLUSTRATIONSEANM BY U.S. DEPARTMENTU.S. ENERGY/PHOTOOF RESEARCHERS, ( INC. mans observe a quantum system, such as an www.SciAm.com SCIENTIFIC AMERICAN 99 [QUANTUM MEASUREMENT] “The Copenhagen electron, with a scientific instrument (which it- THE PROBLEM self may be regarded as a quantum-mechanical Interpretation system). For at the moment of measurement, the An unresolved question in quantum is hopelessly wave function describing the superposition of mechanics is to understand fully how the alternatives appears to collapse into one mem- quantum states of particles relate to the incomplete ... ber of the superposition, thereby interrupting classical world we see around us. as well as the smooth evolution of the wave function and a philosophic introducing discontinuity. A single measure- Quantum mechanics represents the states of par- ticles by mathematical entities called wave func- monstrosity ...” ment outcome emerges, banishing all the other tions. For example, a wave function representing —Hugh Everett possibilities from classically described reality. a particle at a definite location A (such as an Which alternative is produced at the moment of electron in a nanoscopic trap) will have a peak at A and be zero everywhere else. measurement appears to be arbitrary; its selec- tion does not evolve logically from the informa- tion-packed wave function of the electron be- fore measurement. Nor does the mathematics of collapse emerge from the seamless flow of the Schrödinger equation. In fact, collapse has to be added as a postulate, as an additional process A B that seems to violate the equation. Many of the founders of quantum mechanics, Much as ordinary waves can combine, so, too, notably Bohr, Werner Heisenberg and John von can wave functions add together to form super- positions. Such wave functions represent parti- Neumann, agreed on an interpretation of quan- cles that are in more than one alternative state tum mechanics—known as the Copenhagen in- at once. The amplitude of each peak relates to terpretation—to deal with the measurement the probability of finding that alternative when a measurement is made. problem. This model of reality postulates that the mechanics of the quantum world reduce to, and only find meaning in terms of, classically observable phenomena—not the reverse. [THE AUTHOR] This approach privileges the external observ- er, placing that observer in a classical realm that Peter Byrne (www.peterbyrne. info) is an investigative journalist is distinct from the quantum realm of the object A B and science writer based in north- observed. Though unable to explain the nature of ern California. He is writing a full- the boundary between the quantum and classi- Another way of thinking of the wave function is length biography of Hugh Everett. cal realms, the Copenhagenists nonetheless used as a list of each alternative and its amplitude. Byrne acknowledges a debt to Eu- quantum mechanics with great technical success. gene Shikhovtsev of Kostromo, Position Amplitude Probability Russia, who was the first historian Entire generations of physicists were taught that A 0.8 64% to study the life of Everett and the equations of quantum mechanics work only B 0.6 36% who generously shared his re- in one part of reality, the microscopic, while ceas- search material; to the American ing to be relevant in another, the macroscopic. It Institute of Physics for financial is all that most physicists ever need. But if an apparatus measures a particle in such a support; to George E. Pugh and superposition, it produces a specific result—A or Kenneth Ford for their assistance; B, seemingly at random—not a combination of both, and the particle ceases to be in the super- and to the physicists who reviewed Universal Wave Function position. Nor do we ever see macroscopic objects the science in this article: Stephen In stark contrast, Everett addressed the mea- such as baseballs in superpositions. Shenker, Leonard Susskind, David surement problem by merging the microscopic Deutsch, Wojciech H. Zurek, James and macroscopic worlds.
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