Genius_ The Life and Science of Richard Feynman - James Gleick [177]
A Quantum Liquid
By then science-fiction writers had learned an interesting rule: not to let their imaginations run too freely, too widely. It was often better to be conservative. To create a strange new world, they had only to alter one or two features of the usual reality and let the manifold unexpected implications play themselves out. Nature, too, seemed capable of adjusting a single rule and thereby creating the most bizarre phenomena.
Superfluid helium showed what happens when a liquid can flow with no friction—not just low friction, but zero friction. Resting in a beaker, the liquid spontaneously glides in a thin film up and over the walls, apparently in defiance of gravity. It passes through cracks or holes so microscopically small that even a gas would not fit through. No matter how perfectly a pair of glass plates are polished to a smooth surface, and no matter how hard they are pressed together, superfluid helium will still flow freely between them. The liquid conducts heat far better than any ordinary substance, and no amount of cooling will freeze it into a solid.
When Feynman talked about fluid flow, he knew he was returning to a childlike, elemental fascination with the world as it is. The pleasure of watching water in bathtubs or mud puddles on the sidewalk, of trying to dam a curbside rivulet after a rainstorm, of contemplating the movement in waterfalls and whirlpools—that was what made every child a physicist, he felt. In trying to understand superfluidity, he began once again with first principles. What was a fluid? A substance, liquid or gas, that cannot withstand a shear stress, but moves under the force. The tendency of a fluid to resist the shear is its viscosity, its internal friction—honey being more viscous than water, water more than air. Nineteenth-century physicists creating the first effective equations for fluid flow found viscosity especially troublesome, so uncomputable were its consequences. For the sake of simplicity, they often created models that ignored viscosity—and for that John von Neumann later mocked them. Modelers always tried to omit unnecessary complication—that was one thing. But classical fluid dynamicists had omitted what seemed an essential, defining quality. Sarcastically von Neumann called them theorists of “dry water.” Superfluid helium, Feynman said, resembled that impossible idealization, fluid without viscosity. It was dry water.
Superfluidity had an equally bizarre twin, superconductivity, the flow of electricity with no dissipation or resistance. Both were phenomena of low-temperature experimentation. Superconductivity had been discovered in 1911; superfluidity not until 1938, because of the difficulties of watching the behavior of a liquid inside a pinhead-size container in a supercooled cryostat. Esoteric though they were, by the fifties this pair of phenomena had become crown jewels of the side of theoretical physics not devoted to elementary particles. Little progress had been made in understanding the perpetual-motion machinery that seemed to be at work. It seemed to Feynman that they were like “two cities under siege … completely surrounded by knowledge although they themselves remained isolated and unassailable.” Besides Landau, the chief contributor to the theorizing on superfluidity was Lars Onsager, the distinguished Yale chemist whose notoriously difficult courses in statistical mechanics were sometimes called (in allusion to Onsager’s accent) Norwegian I and Norwegian II.
Nature had exhibited another kind of perpetual motion, familiar to quantum physicists: motion at the level of electrons in the atom. No friction