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

bounce like a bil iard bal off the surface of a mirror. It seems to travel in straight lines. These paths—the paths of least time—are special because they tend to be where the contributions of nearby paths are most closely in phase and most reinforce one another. Far from the path of least time

—at the distant edge of a mirror, for example—paths tend to cancel one another out. Yet light does take every possible path, Feynman showed. The seemingly irrelevant paths are always lurking in the background, making their contributions, ready to make their presence felt in such phenomena as mirages and diffraction gratings.

Optics students learned alternative explanations for such phenomena in terms of waves like those undulating through water and air. Feynman was—with finality—eliminating the wave viewpoint altogether. Waviness was built into the phases carried by amplitudes, like little clocks. Once, with Wheeler, he had dreamed of eliminating the field itself. That idea had proved fanciful. The field had lodged itself deeply in the consciousness of physicists. It was indispensable and it was multiplying—a new particle, such as the meson, meant a new field, like a new plastic overlay, of which the particle was a quantized manifestation. Stil , Feynman’s theory retained the mark of its original scaffolding, though the scaffolding was long discarded. The actors were, more clearly than ever, particles. That became an attractive feature for physicists seeking help in visualization, in an experimental world dominated more and more by the cloud trails, the nomenclature, the behaviorism of particles.

Schwinger’s Glory

Feynman’s path integrals belonged to a loose kit of ideas and methods, a private physics that he had assembled but not organized. Much relied on guesswork or, as he said,

“semi-empirical shenanigans.” It was al hodgepodge and purpose-driven, and he could barely communicate it, let alone prove it, even to his most sympathetic listeners, Bethe and Dyson. In the fal of 1947 he attended a formal lecture by Bethe on his approach to the Lamb shift. When Bethe concluded by stressing the need for a more reliable way of making the theory finite, a way that would observe the requirements of relativity, Feynman realized that he could compute the necessary correction. He promised Bethe an answer by the next morning.

By morning he realized that he did not know enough about Bethe’s calculation of the electron’s self-energy to translate his correction into the normal language of physics.

They stood together at the blackboard for a while, Bethe explaining his calculation, Feynman trying to translate his technique, and the best answer they could reach diverged not modestly, like Bethe’s, but horrendously. Feynman, thinking about the problem physical y, was sure it should not diverge at al .

In the days that fol owed, he taught himself about self-energy al over again. When he reexpressed his equations in terms of the observed, “dressed” mass of the electron instead of the theoretical, “bare” mass, the correction came

out just as he had thought, converging to a finite answer.

Meanwhile, glowing news of Schwinger’s progress was reaching Ithaca from Cambridge via Weisskopf and Bethe.

When Feynman heard late in the fal that Schwinger had worked out a calculation for the magnetic moment of the electron—another tiny experimental anomaly newly found in Rabi’s

laboratory—he

solved

the

problem,

too.

Schwinger’s elaborate piece of calculating gave leading physicists a conviction that theory was once again on the march. “God is great!” Rabi wrote Bethe with characteristic wryness, and Bethe replied: “It is certainly wonderful how these experiments of yours have given a completely new slant to a theory and how the theory has blossomed out in a relatively short time. It is as exciting as in the early days of quantum mechanics.”

Feynman felt increasingly competitive about Schwinger, and increasingly frustrated. He had his quantum electrodynamics, he believed, and what he now thought of as “the Schwinger-Weisskopf-Bethe camp” had another. In January the American Physical Society met in New York, and Schwinger was the star. His program was not complete, but he had integrated the new idea of renormalization into the standard quantum mechanics in a way that let him demonstrate a series of impressive derivations. He showed how the anomalous magnetic moment, like the Lamb shift, came from the electron’s interaction with its own field. His lecture drew a crowd that packed the hal . Too many physicists were forced to stand out in the corridors to hear the bursts of applause (and the

embarrassed laughter that came when Schwinger final y said, “It is quite clear that …”). Hasty arrangements were made for Schwinger to repeat the lecture later the same day in Columbia’s McMil in Theater. Dyson attended.

Oppenheimer smoked his pipe conspicuously in the front row. Feynman rose during the question period to say that he, too, had reached these results and that, in fact, he could offer a smal correction. Immediately he regretted it. He thought he must have sounded like a little boy piping up with “I did it too, Daddy.” Few people that winter realized the depths of the rivalry he felt, but he made a bitter remark to a girlfriend, who understood the drift of his disappointment if not the exact circumstances.

“I’m so sorry that your long worked-on experiment was more or less stolen by someone else,” she wrote back. “I know it just makes you feel sick. But Dick dear, how could life or things be interesting if there was not competition?”

She wondered, why couldn’t he and his competitor combine their ideas and work together?

Schwinger and Feynman were not alone in trying to produce the calculations—the explanation—required by the immediate experiments on the Lamb shift and the electron’s magnetic moment. Other theorists fol owed the lead provided by Bethe’s back-of-the-envelope approach.

They saw no need to create a monumental new quantum electrodynamics, when they might generate the right numbers

merely

by

patching

the

technique

of

renormalization onto the existing physics. Independently, two pairs of scientists succeeded in this, producing

solutions that went beyond Bethe’s in that they took into account the way masses fattened at relativistic speeds.

Before publishing, one team, Weisskopf and a graduate student, Bruce French, committed a fatal act of indecision by consulting both Schwinger and Feynman. Engrossed in their more ambitious programs, Schwinger and Feynman each warned Weisskopf off, saying that he was in error by a smal factor. Weisskopf decided it was inconceivable that these bril iant upstarts could both be wrong, independently, and delayed his manuscript. Months passed before Feynman cal ed apologetical y to say that Weisskopf’s answer had been correct.

For Feynman’s own developing theory, a breakthrough came when he confronted the ticklish area of antimatter.

The first antiparticle, the negative electron, or positron, had been born less than two decades earlier as a minus sign in Dirac’s equations—a consequence of a symmetry between positive and negative energy. Dirac had been forced to conceive of holes in a sea of energy, noting in 1931 that “a hole, if there were one, would be a new kind of particle, unknown to experimental physics.” Unknown for the next few months—then Carl Anderson, at Caltech, found the trail of one in a cloud chamber built to detect cosmic rays. It looked like an electron, but it swerved up through a magnetic field when it should have swerved down.

The vivid photographs, along with the lively name coined by a journal editor against Anderson’s wil , gave the positron a legitimacy that theorists found hard to ignore.