NEWS & VIEWS NATURE|Vol 453|1 May 2008

OBITUARY The experiment has since been done, and it starkly displays the situation. In Wheeler’s words, “We have a strange inversion of the John Wheeler (1911–2008) normal order of time. We, now, by moving Theoretical physicist, inspired and inspiring teacher. the mirror in or out, have a right to change what we say about the already past history of that photon.” As a logical extension, The fertile imagination of went on to ask whether our present Wheeler, who died on 13 April aged 96, experiments influence what we can say of roamed from the properties of atomic nuclei the Universe in the distant past. Does the to the physics of nuclear explosions; from observer even in some way participate in the the quantum mechanics of exotic atoms to birth of the Universe? the philosophy of quantum measurements; Wheeler had no fear of such ‘hard’ THE WHEELER FAMILY and from the nature of black holes to questions. He infected others with his own the properties of space-time. The thread intellectual courage, emboldening Hugh connecting these disparate areas was Everett, another of his doctoral students, to Wheeler’s love of acquiring and imparting present his thesis positing the ‘many worlds’ knowledge. Nowhere was that more evident interpretation of quantum mechanics, than with the science of , which despite his, Wheeler’s, own philosophical Wheeler and his students — many of them discomfort with it. themselves future leaders — took from a John Wheeler was an exuberant person. He theoretical backwater to a vigorous area of happened to be in the office of one of us, Jim scientific enquiry. Peebles, when a student, Dan, came in with Wheeler loved ‘what if’ questions. What his new dissertation. happens if electrons and their antiparticles, John: What’s it about? positrons, bubble up from the vacuum Dan: A search for correlations of position and cause light to be scattered by light? In angles of galaxies. 1934, just two years after the positron was John: Gödel would be interested. discovered, Wheeler and Gregory Breit Dan: Who’s Gödel? worked out the answer. Their work is still John: To say Gödel is the greatest logician used to describe the scattering of energetic since Aristotle would be to slight Gödel. photons from distant galaxies. What if you John picked up the phone, got Gödel on the replace the proton in a hydrogen atom by line, and handed the phone to Dan. We don’t a positron, or the electron in an atom by a Gravitation, written with two former students know what Gödel made of this call (he had muon, its heavier cousin? Wheeler found of his, Charles Misner and Kip Thorne. First discovered a rotating-universe model), but that out in the 1940s. What if electrons published in 1973, it has not been out of John had got to do what he loved best: inspire and positrons are in fact the same particle, print since. a young person. travelling in opposite directions in time? He One problem emphasized by the book The rotation of galaxies. What Gödel’s put this question to his graduate student, was that of singularities. When a massive incompleteness theorem had to do with Richard Feynman; it led to Feynman’s star exhausts its supply of nuclear fuel, it can physics. The end of space-time. John powerful diagrammatic approach to collapse first to a ‘’ — as Wheeler Wheeler’s eclectic passions are remote from quantum electrodynamics. named it — and within that into a singularity the practical problems of society. But to him, In 1939, nuclear fission was discovered. where space and time cease to exist. As fundamental scientific research was both a Wheeler, then at the start of a tenure of Gravitation was being published, another pleasure and a duty. His attitude was summed almost 40 years at , of Wheeler’s students, Jacob Bekenstein, up in the dedication of Gravitation: collaborated with Niels Bohr to deliver an argued that black holes have thermodynamic underlying theory. Soon after that, war broke properties such as energy and . Three To our fellow citizens out. Wheeler willingly went to work first on years later, showed that Who, for love of truth, the American atomic bomb, and later the they have a temperature, emit radiation at Take from their own wants hydrogen bomb. He was a complex person: that temperature, and so can evaporate. The By taxes and gifts, a patriot, he was offended by criticism of US strong observational evidence for black holes And now and then send forth military policy, for example in Vietnam. But keeps Wheeler’s fundamental questions One of themselves he also encouraged one of us, Bill Unruh, to alive — what is the meaning of a singularity; As a dedicated servant, go to the University of California, Berkeley — what is left after a black hole evaporates; what To forward the search he said that the many protests there at the happens to information that is swallowed by Into the mysteries and marvelous time made it a stimulating place to work. a black hole. simplicities In 1952, Wheeler turned his attention to Besides gravity, quantum mechanics was Of this strange and beautiful universe, gravity. Could the malleable space-time of Wheeler’s passion: “Why the quantum?” Our home. Einstein’s general theory of relativity explain was another favourite question. He was the characteristics of elementary particles? especially fascinated by the idea that whether, P. James E. Peebles and William G. Unruh Wheeler’s mantras, “charge without charge”, looking at light, you see wave or particle P. James E. Peebles is in the Department of “ without mass” and “topology is too behaviour depends on what experiment you Physics, Princeton University, Princeton, New important to be left to the mathematicians”, perform. In a famous thought experiment, Jersey 08544, USA. William G. Unruh is in the shocked and inspired others to draw out Wheeler considered what happened if the Department of Physics and Astronomy, University the physical implications of the theory. The choice of experiment could be delayed until, of British Columbia, Vancouver, Canada. rich harvest of physics and mathematics assuming causality, it was too late for the e-mails: [email protected]; unruh@physics. was displayed in the monumental treatise decision to affect the way the light behaves. ubc.ca

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