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HOME > PAST ISSUE > July-August 2005 > Article Detail

MARGINALIA

Storied Theory

Science and stories are not only compatible, they're inseparable, as shown by Einstein's classic 1905 paper on the photoelectric effect

Roald Hoffmann

The Puzzle of Dwarvish Work

Einstein's paper on the photoelectric effect, published that fecund year, was singled out by the 1921 Nobel Committee (late as usual, and perhaps still afraid of relativity) as the basis for their award. It is also the only one of the 1905 papers that Einstein himself deemed revolutionary. But when one reads the article, the photoelectric effect appears late, as a denouement; the paper begins elsewhere.

The unwritten prologue is the contemporary interest in black-body radiation—the tendency of any object, no matter what its composition, to radiate light when it is heated. We see it in iron nestled in the forge, glowing red, then yellow, then white.

Extremely hot objects emit light...Click to Enlarge Image

The intensity of this emitted light varies with the color (wavelength). At low temperatures, bodies radiate in the infrared. As the temperature rises, the maximum intensity of the radiated light moves into the red, then extends through the spectrum to the ultraviolet. At high temperatures, objects radiate intense light across the visible spectrum—that's white heat. The intensity of radiated light diminishes in the extreme ultraviolet and far infrared (see right). Astronomers estimate the temperatures of stars from just such curves.

The standard (and eminently successful) understanding of light in Einstein's day came from James Maxwell's electromagnetic theory. Coupled with thermodynamics and the kinetic theory of gases—a high expression of Newtonian mechanics—electromagnetic theory led to a "radiation law" that described how the intensity of light varied with wavelength at each temperature. The law fit the data—at long wavelengths. At short wavelengths, the equation derived from electromagnetic theory failed, in what became known as "the ultraviolet catastrophe."

In 1900, Max Planck found an expression that fit over the entire range of observations. Planck further perceived that his accurate radiation law could be obtained only if the energies of the little bits of oscillating charge that caused the light (he called them "resonators") assumed discontinuous values. So the quantum was born.

Planck had trouble believing that physics was, deep down, discontinuous. He spent many years searching for a way around what he discovered. But that is another story.





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