Refrigerators, Magnets, and Blue Skies

Light Is Both Wave and Particle (1909)

In a lecture at Salzburg in 1909 Einstein analyzed the tiny random fluctuations in the energy of radiation inside a hot cavity. He found two contributions added together: one exactly what you would expect from waves, the other exactly what you would expect from particles. He predicted that the next stage of physics would bring a theory of light fusing the two pictures. It was the first clear statement of wave–particle duality, fifteen years before quantum mechanics delivered it.

Why the Sky Is Blue (1910)

Near the point at which a liquid and its vapor become indistinguishable, a clear fluid turns milky. Einstein explained this "critical opalescence" as light scattering from random fluctuations in density, which become enormous near that point. The same reasoning, applied to the far smaller fluctuations in ordinary air, gives a molecular account of why the sky scatters blue light most strongly.

The Experimenter (1915)

Einstein is remembered as a pure theorist, but in 1915 he published an experiment, carried out in Berlin with the Dutch physicist Wander de Haas. They suspended an iron cylinder from a fine thread and reversed its magnetization. The cylinder twisted.

The Einstein–de Haas effect shows that magnetism in iron is tied to angular momentum carried by its electrons. The two reported a number matching the theory of the day. More careful measurements by others found a value about twice as large, a discrepancy that was only explained a decade later by the discovery of electron spin.

The Einstein Refrigerator (1926 – 1930)

In the 1920s household refrigerators used toxic gases, and Einstein is said to have read a newspaper report of a Berlin family killed in their sleep by fumes leaking from their refrigerator's pump. With the young physicist Leo Szilard he set out to design a refrigerator with no moving parts and therefore no seals to fail.

Their absorption refrigerator runs on heat alone, circulating butane, ammonia, and water. The pair were granted a United States patent on 11 November 1930, and sold designs to manufacturers including Electrolux and AEG. The arrival of the safer refrigerant Freon, and the Depression, made the invention commercially unnecessary, and it was never mass-produced. Their related idea of a pump that moves liquid metal with a magnetic field later found use in cooling nuclear reactors.

Szilard, a decade later, was the man who brought Einstein the letter to President Roosevelt. See Nuclear Energy.

A Patent Clerk's Habits

Einstein spent seven years examining patent applications in Bern, and he never lost his liking for practical devices. He held patents, acted as an expert witness in patent disputes, and, with a physician friend, designed a self-adjusting camera. The image of a man lost in abstraction is only part of the picture.