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

Much of SU (3)’s power came from the way it embodied a concept increasingly central to the high-energy theorist’s way of working: the concept of inexact symmetry, almost symmetry, near symmetry, or—the term that won out

— broken symmetry . The particle world was ful of near misses in its symmetries, a dangerous problem, since it seemed to permit an ad hoc escape route whenever an expected relationship failed to match. Broken symmetry implied a process, a change in status. A symmetry in water is broken when it freezes, for now the system does not look the same from every direction. A magnet embodies symmetry breaking, since it has made a kind of choice of orientation. Many of the broken symmetries of particle physics came to seem like choices the universe made when it condensed from a hot chaos into cooler matter, spiked as it is with so many hard-edged, asymmetrical contingencies.

Once again Gel -Mann trusted his scheme enough to predict, as a consequence of broken symmetry, a specific hitherto-unseen particle. This, the omega minus, duly turned

up in 1964—a thirty-three-experimenter team had to canvass more than one mil ion feet of photographs—and Gel -Mann’s Nobel Prize fol owed five years later.

His next, most famous invention came in an effort to add explanatory understanding to the descriptive success of the Eightfold Way. SU (3) should have had, along with its various eight-member and ten-member and other families, a most-basic three-member family. This seemed a strange omission. Yet the rules of the group would have required this threesome to carry fractional electric charges: ? and –

?. Since no particle had ever turned up with anything but unit charge, this seemed implausible even by modern standards. Nevertheless, in 1963 Gel -Mann and, independently, a younger Caltech theorist, George Zweig, proposed it anyway. Zweig cal ed his particles aces . Gel -

Mann won the linguistic battle once again: his choice, a croaking nonsense word, was quark . (After the fact, he was able to tack on a literary antecedent when he found the phrase “Three quarks for Muster Mark” in Finnegans Wake , but the physicist’s quark was pronounced from the beginning to rhyme with “cork.”)

It took years for Gel -Mann and other theorists to generate al the contrivances needed to make quarks work.

One contrivance was a new property cal ed color —purely artificial, with no connection to everyday color. Another was flavor : Gel -Mann decided that the flavors of quarks would be cal ed up , down ,

and strange . There had to be

antiquarks and anticolors. A new mediating particle cal ed

t he gluon would have to carry color from one quark to another. Al this encouraged skepticism among physicists.

Julian Schwinger wrote that he supposed such particles would be detected by “their palpitant piping, chirrup, croak, and quark.” Zweig, far more vulnerable than Gel -Mann, felt that his career was damaged. The quark theorists had to wrestle with the fact that their particles never appeared anywhere, though people did begin a dedicated search in particle accelerators and supposed cosmic-ray deposits in undersea mud.

There was a reality problem, distinctly more intense than the problem posed by more familiar entities such as electrons. Zweig had a concrete, dynamical view of quarks

—too mechanistic for a community that had learned as far back as Heisenberg to pay attention only to observables .

Gel -Mann’s comment to Zweig was, “The concrete quark model—that’s for blockheads.” Gel -Mann was wary of the philosophical as wel as the sociological problem created by any assertion one way or the other about quarks being real. For him quarks were at first a way of making a simple toy field theory: he would investigate the theory’s properties, abstract the appropriate general principles, and then throw away the theory. “It is fun to speculate about the way quarks would behave if they were physical particles of finite mass (instead of purely mathematical entities as they would be in the limit of infinite mass),” he wrote. As if they were physical particles; then again, as if they were conveniences of mathematics. He encouraged “a search

for stable quarks”—but added with one more twist that it

“would help reassure us of the nonexistence of real quarks.”

His initial caveats were quoted by commentators again and again in the years that fol owed. One physicist’s typical y uncharitable interpretation: “I always considered that to be a coded message. It seemed to say, ‘If quarks are not found, remember I never said they would be; if they are found, remember I thought of them first.’” For Gel -Mann this became a permanent source of bitterness.

Feynman, meanwhile, had disregarded so much of the decade’s high-energy physics that he had to make a long-term project of catching up. He tried to pay more attention to experimental data than to the methods and language of theorists. He tried, as always, to read papers only until he understood the issue and then to work out the problem for himself. “I’ve always taken an attitude that I have only to explain the regularities of nature—I don’t have to explain the methods of my friends,” he told a historian during these years. He did manage to avoid some passing fashions.

Stil , he was turning back to a community after having drifted outside, and he had to learn its shared methods after al . It was no longer possible to approach these increasingly formidable, specialized problems as an outsider. He had stopped teaching high-energy physics; in the late sixties he began again. At first his syl abus contained no quarks.

By the late sixties and early seventies a new accelerator embedded in the rol ing hil s near Stanford University in northern California had taken the dominant role in the

strong-interaction experiments that were so central to the search for quarks. The Stanford Linear Accelerator Center (SLAC) made a straight two-mile cut in the grassy landscape. Aboveground, cows grazed and young physicists in jeans and shirts—nearly a hundred of them—

sat at picnic tables or walked in and out of the center’s many buildings. Below, inside a knife-straight evacuated copper tube, a beam of electrons streamed toward targets of protons. The electrons achieved energies far greater than theorists had ever had to manage. They struck their targets inside an end station like a giant airplane hangar and then, with luck, entered a detector inside a concrete blockhouse, lined with lead bricks, riding on railroad tracks and angled upward toward the ceiling. Sometimes high-speed motion-picture cameras recorded the results, and elsewhere in the laboratory teams of human scanners guided an automatic digitizer that could read the particle tracks from—for a given monthlong experiment—hundreds of mil ions of filmed images. A single bubble chamber at the end of the particle beam, in its five-and-a-half-year useful lifetime, saw the discovery of seventeen new particles.

It was a tool for exploring the strong force—so cal ed because, at the very short distances in the domain of the nucleus, it must dominate the force of electromagnetic repulsion to bind protons and neutrons ( hadron was now the general term for particles that felt the strong force).

Feynman had been thinking about how to understand the working of the strong force in col isions of hadrons with

other hadrons. These were complex: at the high energies now available for studying short distances, hadron-hadron col isions produced gloriously messy sprays of detritus. The hadrons themselves were neither simple nor pointlike. They had size, and they seemed to have internal constituents—a whole swarming zoo of them. As Feynman said, the hadron-hadron work was like trying to figure out a pocket watch by smashing two of them together and watching the pieces fly out. He began visiting SLAC regularly in the summer of 1968, however, and saw how much simpler was the interaction offered by electron-proton col isions, the electron tearing through the proton like a bul et.