THESIS Everything is Particles

The mathematical formulation of quantum theory had ever managed more than an Eventually, Feynman abandoned that idea, theory based on path integration occupies a ad hoc description of the reactive force on the convinced by the Lamb shift in hydrogen central place in modern physics. Textbooks particle created as a result. that electrons do interact with themselves, in relativistic quantum theory cover it in Seeking a better route to quantum presumably through the intermediary of the part because of its analytical convenience electrodynamics, Wheeler and Feynman electromagnetic field. for certain problems, but also in view of its hoped to resolve this issue of classical field Wheeler apparently gave up on the idea elegance and conceptual appeal. Invented theory in one bold stroke, by postulating that only later. “Until the early 1950s,” he later by Richard Feynman in his classic 1948 accelerated particles don’t radiate, and that recalled, “I was in the grip of the idea that paper, “A space-time approach to quantum there are no fields. They also supposed that Everything is Particles.” mechanics”, the technique expresses the particles interact only with other particles, Yet the ideas of science do go dynamics of quantum systems in terms and never with themselves. The resulting round and round, and the notion that of a surprisingly simple, almost intuitive, theory based on action-at-a-distance puts Everything is Particles hasn’t died. To principle of least action. all the physics into particles and their direct take one example, researchers exploring In Feynman’s terms, the evolution of interactions through advanced or retarded alternative interpretations of quantum any quantum system follows from a sum influences. The radiation reaction, in their theory — especially those based on the ideas of probability amplitudes over all possible view, reflects not a particle giving energy to a of David Bohm — argue that theories of trajectories, each amplitude being simply this type, which have been elaborated for exp(i/ħS), where S is the classical action for essentially any quantum field theory, are in the trajectory. This suggests, in effect, that fact simplest when based on a purely particle a quantum system differs from its classical ontology; that is, when they assert that only analogue because it explores all possible particles, and not fields, really exist. trajectories in space-time at once. One Proponents of this view also argue, with cannot, as usual, say more about which some reason, that while physicists talk a lot trajectory the system actually follows, about fields and field theories, nothing in except in the classical limit, where classical physics, strictly speaking, ever reflects the trajectories emerge as stationary trajectories, measurement of a field. Experiments measure for which the action S varies slowly, causing particles passing through detectors, or record the amplitudes for adjacent paths to add spot-like marks on a screen. Fields may be coherently, rather than cancelling out. Nothing in physics, strictly seemingly unavoidable theoretical elements, It’s undeniably beautiful. How did yet the evidence points only to particles. Feynman come up with it? The answer speaking, ever reflects the Feynman, of course, had a practical is certainly ninety percent Feynman’s measurement of a field. approach to physics, and pushed individual brilliance, but the historical ‘philosophical’ questions to the side. record also suggests some timely guidance His interest in avoiding fields was from the greater luminaries of twentieth- field (and effectively acting on itself through expedient — to get a better theory of quantum century physics. that intermediary), but an interaction electrodynamics, which he did eventually help In his original paper, Feynman attributes between the charge and all other charged to do, though not exactly in this way. his initial interest in this line of thinking to matter in the Universe. As Sauer relates, the audience for a section in Paul Dirac’s famous textbook All this enters into the Wheeler– Feynman’s first seminar as a young graduate on quantum mechanics. In a recent Feynman theory through a principle of least student at Princeton, at which he presented exploration of the history of the era, however, action involving particles, but no fields. And his and Wheeler’s ideas, included several Tilman Sauer of the Einstein Papers Project Sauer suggests that they were led to their guests that Eugene Wigner had invited points out that Feynman’s approach traces at mathematical formalism, at least in part, by specially — the great mathematician least some of its motivation back to his earlier conversations with Einstein. They apparently John von Neumann, the eminent physicist efforts with John Wheeler to build a theory met for a long discussion at Einstein’s home Wolfgang Pauli and, though he rarely of electrodynamics that would abandon in Princeton, during which he alerted them to attended such seminars, Albert Einstein. the concept of fields, and posit a universe earlier work by Hans Tetrode and Walter Ritz, Feynman later recalled turning “a of only particles — an idea that both also who had also developed similar formulations yellowish green, or something”, before he discussed in some depth with Albert Einstein, of electrodynamics involving particles only. began talking physics and relaxed. among others. Science goes round and round, though it Einstein, with characteristically Wheeler’s and Feynman’s efforts were never quite returns to the same place. penetrating logic, commented after the motivated by a belief that the conceptual These ideas, as Sauer argues, left clear lecture that the ideas were inconsistent with difficulties of quantum electrodynamics traces in Feynman’s space–time formulation the principles of general relativity. But that, might reflect problems present in classical of quantum theory, also based on a least- he said, wasn’t necessarily so bad. “After all, electrodynamics, and have little to do with action principle. Indeed, Feynman stated in general relativity is not so well established as quantum mechanics per se. In particular, the abstract that part of his intention was to electrodynamics,” Feynman recalls Einstein while it was clear in classical theory that an explore applications that would “eliminate saying, “maybe we can develop a new way of accelerated particle must radiate energy, the coordinates of the field oscillators from doing the gravitational interaction too.” giving it to the electromagnetic field, no the equations of quantum electrodynamics”. Mark Buchanan nature physics | VOL 4 | APRIL 2008 | www.nature.com/naturephysics 259 © 2008 Nature Publishing Group