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