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

You told me that Block might be right. And you don’t do a damn thing about it. You should write it up, for crying out loud, when you have something like this. She also reminded him that he had mentioned an idea for a universal

theory of weak interactions—tying together beta decay and the strange-particle decays based on the weak force—and urged him, final y, to see where it would lead.

In its classic form, beta decay turns a neutron into a proton, throwing off an electron and another particle, a neutrino—massless, chargeless, and hard to detect.

Charge is conserved: the neutron has none; the proton carries + 1 and the electron – 1. Analogously, in the meson family, a pion could decay into a muon (like a heavy electron) and a neutrino. A good theory would predict the rates of decay in such processes, as wel as the energies of the outgoing particles. There were complications. The spins of the particles had to be reconciled, and for the massless neutrinos, especial y, problems of handedness arose in calculating the appropriate spins. So the new understanding of parity violation immediately changed the weak-interaction landscape—for Feynman, for Gel -Mann, and for others.

In sorting the various kinds of particle interactions, theorists had created a classification scheme with five distinct transformations of one wave function into another. In one sense it was a classification of the characteristic algebraic techniques; in another, it was a classification of the types of virtual particles that arose in the interactions, according to their possible spins and parities. As shorthand, physicists used the labels S , T , V , A , and P , for scalar , tensor , vector , axial vector , and pseudoscalar . The different kinds of weak interaction had evident similarities, but this classification scheme posed a problem. As Lee pointed out at the 1957 Rochester meeting, most experiments on beta decay had demonstrated S and T

interactions, while the new parity-violation experiments tended to suggest that meson decay involved V and A .

Under the circumstances, the same physical laws could hardly be at work.

In reading Lee and Yang’s preprint for the meeting—

In reading Lee and Yang’s preprint for the meeting—

Joan had ordered him, for once, to sit down like a student and go through it step by step—Feynman saw an alternative way of formulating the violation of parity. Lee and Yang described a restriction on the spin of the neutrino.

He liked the idea enough to mention it from the audience during five minutes cadged from another speaker. He went far back into the origins of quantum mechanics—back not only to the Dirac equation itself but beyond, to the Klein-Gordon equation that he and Welton had manufactured when they were MIT undergraduates. Using path integrals, he moved forward again, deriving—or “discovering”—an equation slightly different from Dirac’s. It was simpler: a two-component equation, where Dirac’s had four components. “Now I asked this question,” Feynman said: Suppose that historical y [my equation] had been discovered before the Dirac equation? It has absolutely the same consequences as the Dirac equation. It can be used with diagrams the same way.

The diagrams for beta decay, of course, added a neutrino field interacting with the electron field. When Feynman made the necessary change to his equation, he found: Of course I can’t do that because this term is parity unsymmetri c. But ——beta decay is not parity symmetric, so it’s now possible.

There were two difficulties. One was that he came out with the opposite sign for the spin: his neutrino would have to spin in the opposite direction from Lee and Yang’s prediction. The other was that the coupling in his formulation would have to be V and A , instead of the S and T that everyone knew was correct.

Gel -Mann, meanwhile, had also thought about the

problem of creating a theory for weak interactions. Nor were Feynman and Gel -Mann alone: Robert Marshak, who had put forward the original two-meson idea at the Shelter Island conference in 1947, was also leaning toward V and A with a younger physicist, E. C. G. Sudarshan. That summer, with Feynman traveling in Brazil, Marshak and Sudarshan met with Gel -Mann in California and described their approach.

Feynman returned at the end of the summer determined, for once, to catch up with the experimental situation and fol ow his weak-interaction idea through to the end. He visited Wu’s laboratory at Columbia, and he asked Caltech experimenters to bring him up to date. The data seemed a shambles—contradictions everywhere. One of the Caltech physicists said that Gel -Mann even thought the crucial coupling could be V rather than S . That, as Feynman often recal ed afterward, released a trigger in his mind.

I flew out of the chair at that moment and said,

“Then I understand everything. I understand everything and I’l explain it to you tomorrow morning.”

They thought when I said that, I’m making a joke… .

But I didn’t make a joke. The release from the tyranny of thinking it was S and T was al I needed, because I had a theory in which if V and A were possible, V and A were right, because it was a neat thing and it was pretty.

Within days he had drafted a paper. Gel -Mann, however, decided that he should write a paper, too. As he saw it, he had his own reasons for focusing on V and A . He wanted the theory to be universal. Electromagnetism depended on vector coupling, and the strange particles favored V and A .

He was unhappy that Feynman seemed to be thoughtlessly dismissing his ideas.

Before the tension between them rose higher, their

department head, Robert Bacher, stepped in and asked them to write a joint paper. He preferred not to see rival versions of the same discovery coming out of Caltech’s physics group. Col eagues strained to overhear Feynman and Gel -Mann in the corridors or at a cafeteria table, engrossed in their oral col aboration. They stimulated each other despite the characteristic differences in their language: Feynman offering what sounded like you take this and it zaps through here and you come out and pull this together like that , Gel -Mann responding with you substitute there and there and integrate like so… . Their article reached the Physical Review in September, days before Marshak presented his and Sudarshan’s similar theory at a conference in Padua, Italy. Feynman and Gel -

Mann’s theory went further in several influential respects. It proposed a bold extension of the underlying principles beyond beta decay to other classes of particle interactions; it would be years before experiment ful y caught up, showing how prescient the two men had been. It also introduced the idea that a new kind of current—analogous to electrical current, a measure of the flow of charge—

should be conserved; new extensions of the concept of current became a central tool of high-energy physics.

Feynman tended to recal that they had written the paper together. Gel -Mann sometimes disdained it, complaining particularly about the two-component formalism—a ghastly notation, he felt. It did bear Feynman’s stamp. He was applying a formulation of quantum electrodynamics that went back to his first paper on path integrals in 1948; Gel -

Mann al owed him to remark fondly, “One of the authors has always had a predilection for this equation.” Yet it could hardly have been Feynman who wrote that their approach to parity violation “has a certain amount of theoretical raison d’être .” Evident, too, was Gel -Mann’s drive to make the theory as unifying and forward-looking as possible. The