Джеймс Глик - Genius - The Life and Science of Richard Feynman

He stayed with his sister; she had moved to the Stanford area to work for a research laboratory, and her house was just across Sand Hil Road from the accelerator center. The physicists who would gather on the outdoor patio to listen to his stories that summer would see him slamming his open hands together in a boisterous il ustration of a new idea he had. He was talking about “pancakes”—flat particle pancakes with hard objects embedded in them.

The Caltech connection was important to experimenters at SLAC, and by the late sixties the connection meant Gel -

Mann far more than Feynman. Gel -Mann had created the scientific subculture of current algebra, the mathematical framework surrounding his quarks, and SLAC theorists thought of themselves as trying to generalize these tools to smal er distances, higher energies. At accelerators like SLAC, most of the thinking focused on the simplest reactions—two particles in, two particles out—although

most of the actual col isions produced enormous flashes of many more particles. Experimenters wanted the most precise possible data, and precision was impossible in these bursts of detritus. Feynman chose a different point of view. He introduced a formalism in which one could look at the distributions of twenty or fifty or more particles. One did not have to be able to measure the momentum of each particle; in effect one could sum over al the possibilities. A Stanford theorist, James D. Bjorken, had been thinking along similar lines. An electron hits a proton; an electron comes out, along with a burst of immeasurable fragments.

The emerging electron was a common factor. Bjorken decided to set aside the miscel aneous spray and simply plot the distribution of the energies and angles of the emerging electrons, averaged over many col isions.

He isolated a remarkable regularity in the data, a phenomenon he cal ed “scaling”—the data looked the same at different energy scales. He did not know just how to interpret this. He had a variety of guesses, most framed in the language of current algebra. When Feynman arrived, Bjorken happened to be away; Feynman saw the graphed data without hearing a clear explanation of its origin. He suddenly recognized it, however, and he calculated long into the evening. It could be viewed as a graph of his pancake theory, the theory he had been toying with al summer on his own.

He had decided to cut through the incalculable swarming muddle of proton pieces by positing a mysterious new constituent that he cal ed a parton , a name based

inelegantly on the word part . (Final y he had an entry of his own in the Oxford English Dictionary .) Feynman made almost no assumptions about his partons except two: they were pointlike, and they did not interact meaningful y with one another but floated freely about inside the proton. They were an abstraction—just the kind of unobservable entity that physicists hoped not to have to fal back on—yet they were tantalizingly visual in spirit. They were pegs on which to hang a field theory of the old, manageable sort, with wave functions and calculable probability amplitudes. By analogy, quantum electrodynamics had its partons, too: the bare electrons and photons.

Feynman showed that col isions with these hard nuggets inside the proton would produce the scaling relations in a natural manner, unlike col isions with the puffy whole proton.

He chose not to decide what quantum numbers they did or did not carry, and he most emphatical y decided not to worry one way or the other about whether his partons were the fractional y charged quarks of Gel -Mann and Zweig.

By the time Bjorken returned, he found the theory group awash in partons. Feynman buttonholed him. He had idolized Feynman ever since taking an old-fashioned, historical y organized quantum electrodynamics course at Stanford. “When Feynman diagrams arrived,” he said, “it was the sun breaking through the clouds, complete with rainbow and pot of gold. Bril iant! Physical and profound!”

Now here was Feynman in the flesh, explaining Bjorken’s own theory to him with a new language and a new visual image. As he could instantly see, Feynman’s essential

image. As he could instantly see, Feynman’s essential insight was to place himself once again in the electron, to see what the electron would see at light speed. He would see the protons flashing toward him—and they were therefore flattened relativistical y into pancakes. Relativity also slowed their internal clocks, in effect, and, from the electron’s point of view, froze the partons into immobility.

His scheme reduced the messy interaction of an electron with a fog of different particles to a much simpler interaction of an electron with a single pointlike parton emerging from the fog. Bjorken’s scaling pattern flowed directly from the physics of this picture. The experimenters grasped it instantly.

The parton model was oversimplified. It explained nothing that Bjorken could not explain, although Bjorken’s explanation seemed less fundamental. Partons required considerable hand-waving. Yet physicists clutched at them like a lifeboat. Three years passed before Feynman published a formal paper and many more before his partons final y and definitively blended with quarks in the understanding of physicists.

Zweig’s aces, Gel -Mann’s quarks, and Feynman’s partons became three paths to the same destination.

These constituents of matter served as the quanta of a new field, final y making possible a field theory of the strong force. Quarks had not been seen or detected in the direct fashion of more venerable particles. They became real nonetheless. Feynman took on a project in 1970 with two students, assembling a vast catalog of particle data in an

effort to make a judgment about whether a simple quark model could underlie it al . He chose an unconventional model once again, using data that let him think in terms of the electromagnetic field theory of the last generation, instead of the hadron-col ision data that interested most theorists. For whatever reason, he was persuaded—

converted into a quarkerian, as he said—although he continued to stress the tentativeness of any one model. “A quark picture may ultimately pervade the entire field of hadron physics,” this paper concluded. “About the paradoxes of the quark model we have nothing to add, except perhaps to make these paradoxes more poignant by exhibiting the mysteriously good fit of a peculiar model.”

Younger theorists learned how to explain confinement—the quark’s inability to appear as free particles—in terms of a force that grew rapidly with distance, in strange contrast to forces such as gravity and electromagnetism. Quarks became real not only because ingenious experiments gave an indirect look at them, but because it became harder and harder for theorists to construct a coherent model in which they did not figure. They became so real that Gel -Mann, their inventor, had to endure the after-the-fact criticism that he had not ful y believed in them. He never understood why Feynman had created his own alternative quark and maintained a distinction that faded in the end. He missed no opportunity to cal Feynman’s particles “put-ons.” Like Schwinger years before, he disliked the fanfare over a picture that he thought was oversimplified—anyone could use it.

Quarks were real , at least to physicists of the last years of this century. Partons were not, in the end. What is real?

Feynman tried to keep this question from disappearing into the background. In a book assembled from his lectures, Photon-Hadron Interactions , he concluded: We have built a very tal house of cards making so many weakly based conjectures one upon the other… .

Even if our house of cards survives and proves to be right we have not thereby proved the existence of partons… . On the other hand, the partons would have been a useful psychological guide … and if they continued to serve this way to produce other valid expectations they would of course begin to become