Swimming is rhythmic and repetitive. It relaxes stress, relieves anxiety and takes you back to the womb. During the summer, at a local lake in New Hampshire, I do it every day. But the flip side of swimming’s sedative virtues is that swimming is boring. Since it’s best not to fall asleep in the water, I often plan something to think about. What could be more natural than to think about the physics around swimming?

Viewed at a microscopic scale, water no longer seems smooth and placid. Its true nature, as a collection of jiggling atoms, gets revealed. Dust from pollen grains, for example, gets buffeted in random directions, driven to “swim” aimlessly and inefficiently. The botanist Thomas Brown first noted such agitated micro-motion in 1827.

Albert Einstein,

in the “miracle year” of 1905 when he published on relativity, E=mc² and photons, interpreted Brown’s observations on the basis of atomic theory. By connecting the so-called Brownian motion to other observations about diffusion and viscosity, he was able to make a convincing case for the existence of atoms and to derive a good estimate of atomic sizes.

Organisms slightly larger than dust, like bacteria, can resist the molecular weather, but its influence remains. To appreciate the problem, imagine trying to walk through rapidly shifting gusty winds, or to swim through ever-shifting currents. To a bacterium, water feels extremely viscous.

The rules that govern bacterial swimming are peculiar. Progress requires continuous effort. Inertia is quickly dissipated. Then we have the dynamics proposed by


dominated by friction, where no force means no progress and velocity is proportional to force. To-and-fro strokes, whatever their timing, don’t work either. If scallops were bacteria-sized, their usual strategy, to progress by closing their shells fast and opening them slowly, would get them nowhere. Real bacteria, to move forward, often resort to using screwlike flagella which they turn in only one direction.

Other swimming situations feature far less resistance—or none at all. That’s not an unmixed blessing. It makes swimming challenging in different ways.

Even with nothing to push against, there is one important thing you can still do. By contorting your body, you can change your orientation. Divers “swim” through air and must arrange to hit the water just so. They generally do their reorientations in a practiced way, aiming for a chosen angle. But if divers slip off their diving board unexpectedly, like cats falling from a tree, they have to improvise.

In contrast to reorienting yourself, lacking something to push against you can’t change your forward motion, or momentum. (Though if you happen to have a jetpack, you can eject mass and recoil.) This brings to mind the peculiar horror of astronauts losing connection with their spacecraft, and then drifting forever helplessly through outer space, doomed by the laws of physics. In an iconic scene from “2001,” the highly advanced computer HAL ejects astronaut Frank Poole into space. That seals Frank’s fate. It also seals HAL’s, as the surviving astronaut, Dave, resolves then and there to lobotomize him.

Electrons swim, too—through materials—and a lot of technology revolves around helping them do so efficiently. Even when electrons are not bound in atoms, and are therefore nominally free to move, they have to weave their way through a lattice of nuclei and get buffeted both by vibrations (phonons) and by each other. That’s what underlies electrical resistance in metals.

But in superconductivity—a state of matter that, at least for now, requires very low temperatures—electrons learn to cooperate. Resistance vanishes, and currents will flow without generating heat, in principle forever. It’s as if the ocean were chock-a-block with fish, all moving together. To swim, you’d simply go with the flow.

Athletes speak of being “in the zone,” when you do wonderful things automatically, without conscious effort. Superconductivity is electrons swimming in the zone. And sometimes while swimming, when my thoughts take me out of body, I get there too.


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