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

Quantum mechanical y, there is no such temperature.

Atomic motion never does cease. That precise a zero would violate the uncertainty principle.

Landau and others had set the stage with a handful of useful conceptions of liquid helium. One powerful idea,

which continued to dominate al kinds of solid-state physics, was the notion of new entities—“quasiparticles” or

“elementary excitations”—col ective motions that traveled through matter and interacted with one another as if they were particles. Quantum sound waves, now cal ed phonons, were one example. Another: liquid helium seemed to contain units of rotational motion christened rotons. Feynman tried to work out the implications of these ideas. He also explored the notion that liquid helium acts as though it were (here, as elsewhere, the old-fashioned is had to be permanently replaced by the provisional acts as though it were ) a mixture of two coexisting substances, a normal liquid and a pure superfluid.

One of the strangest of al the liquid-helium manifestations demonstrated how the mixture would work.

A circular tube like a bicycle tire was packed with powder and then fil ed with liquid helium. It was set spinning and then abruptly halted. The powder would halt the flow of any normal liquid. But the superfluid component of liquid helium would continue to flow, around and around, passing through the microscopic interstices in the powder, in effect ignoring the presence of another, normal liquid. Students could sense the flow by feeling the tire’s resistance to torque, as a spinning gyroscope resists sidelong pressure. And, once set in motion, the superflow would persist as long as the universe itself.

At a meeting in New York of the American Physical Society in 1955 Feynman startled a Yale group, students of Onsager, who described a new experiment they were conducting with rotating buckets. (In the low-temperature business “buckets” tended to be glass tubes the size of a thimble.) Feynman rose and said that a rotating bucket of superfluid would be fil ed with peculiar vortices, whirlpools hanging down like strings. The speakers had no idea what he was talking about. This peculiar image was the essence of his visualization of the atomic behavior of liquid helium.

He had tried to picture how individual atoms would move together within the fluid; he calculated the forces between them as directly as he could, with tools dating back to his undergraduate research with John Slater. He saw that rotational motions would arise, just as Landau had suggested, and he applied the quantum-mechanical restriction that such motion would have to come in indivisible units. For a while he struggled to find the right image for an elementary excitation in a superfluid. He considered an atom in a cage, oscil ating. A pair of atoms revolving one around the other. A tiny rotating ring of atoms.

The chal enge was to drive toward a solution of a many-particle problem in quantum mechanics without being able to begin with a formal, mathematical line of reasoning. It was a chal enge in pure visualization.

He lay awake in bed one night trying to imagine how rotation could arise at al . He imagined a liquid divided by a thin sheet, an imaginary impermeable membrane. On one side the liquid was motionless; on the other side it flowed.

He knew how to write the old-fashioned Schrödinger wave function for both sides. Then he imagined the sheet disappearing. How could he make the wave functions join?

He thought about the different phases combining. He imagined a kind of surface tension, energy proportional to the surface area of his sheet. He considered what would happen when an individual atom moved across the boundary—at what point in the rising and fal ing wave of energy the surface tension would fal to zero and the atom would be able to move freely. He was starting to see a surface divided into strips of glue, where the atoms could not mix, and other narrow strips where atoms would be able to change places. He calculated how little energy it would take to distort the wave function until the atoms would be held back, and realized that the strips of free motion would be no more than the width of a single atom. Then he realized that he was seeing lines, vortex lines around which

the atoms circulated in rings. The rings of atoms were like rings of children waiting to use a playground slide. As each child descended—the wave function changing from plus to minus—another would slip into position at the top. But the fluid version was more than just a two-dimensional ring. It also wound back on itself through the third dimension—like a smoke ring, Feynman concluded, twenty years after he had led an investigation of smoke-ring dynamics in his high school physics club. These quantum smoke rings, or vortex lines, would circle about the tiniest conceivable hole, just one atom’s width across.

In a succession of articles spanning five years he worked out the consequences of his view of the interplay of energy and motion in this quantum fluid. The vortex lines were the fundamental units, the indivisible quanta of the system.

They set limits on the ways in which energy could be exchanged within the fluid. In a thin enough tube, or a slow enough flow, the lines would not be able to form, and the flow would just coast, unchanging, losing no energy, and thus absolutely free of resistance. He showed when vortex lines would arise and when they would vanish. He showed when they would begin to entangle one another and bal up, creating another unexpected phenomenon that no one had yet seen in the laboratory: superfluid turbulence. Caltech hired low-temperature experimental specialists, and Feynman worked with them closely. He learned al the details of the apparatus, vacuum pumps for cooling by lowering the vapor pressure; rubber O-rings for ensuring tight seals. Before long, word was spreading of an experiment that struck physicists as “typical Feynman.” Tiny wings, airfoils, were attached to a thin quartz fiber hanging down through a tube. The superfluid was pul ed through vertical y. A normal fluid would have spun the wings like a tiny propel er, but the superfluid refused to cause twisting.

Instead it slipped frictionlessly past. In their search for lighter and lighter airfoils, the experimenters final y kil ed

some local flies, or so they claimed, and the investigation became known as the flies’-wings experiment.

Physicists who had worked in the area of condensed matter for longer than Feynman—and who would remain there after Feynman had once again departed—were struck by his method as much as by his success. He used none of the technical apparatus for which he was now famous: no Feynman diagrams, no path integrals. Instead he began with mental pictures: this electron pushes that one; this ion rebounds like a bal on a spring. He reminded col eagues of an artist who can capture the image of a human face with three or four minimal and expressive lines.

Yet he did not always succeed. As he worked on superfluidity, he also struggled with superconductivity, and here, for once, he failed. (Yet he came close. At one point, about to leave on a trip, he wrote a single page of notes, beginning, “Possibly I understand the main origin of superconductivity.” He was focusing on a particular kind of phonon interaction and on one of the experimental signatures of superconductivity, a transition in a substance’s specific heat. He could see, as he jotted to himself, that there was “something stil a little haywire,” but he thought he would be able to work out the difficulties. He signed the page: “ In case I don’t return. R. P. Feynman.”) Three younger physicists, intensely aware of Feynman’s competitive presence—John Bardeen, Leon Cooper, and Robert Schrieffer—invented a successful theory in 1957.