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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. Still, he was turning back to a community after having drifted outside, and he had to learn its shared methods after all. 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 syllabus contained no quarks.

By the late sixties and early seventies a new accelerator embedded in the rolling hills 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 millions 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 called 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 collisions of hadrons with other hadrons. These were complex: at the high energies now available for studying short distances, hadron-hadron collisions 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 collisions, the electron tearing through the proton like a bullet.

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 Hill 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 illustration 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 Gell-Mann far more than Feynman. Gell-Mann had created the scientific subculture of current algebra,