1921 · Einstein and the Photoelectric Effect

In May 1905, a 26-year-old patent clerk in Bern wrote to his friend Conrad Habicht promising four papers. "The first," he wrote, "deals with radiation and the energy properties of light and is very revolutionary." He did not mean relativity. He meant the paper that would win him the Nobel Prize.

The puzzle was the photoelectric effect: shine light on metal and electrons pop out. Philipp Lenard — Hertz's own student — had nailed down the strange facts by 1902 with a carbon arc and a retarding voltage. Double the light's intensity and you double the number of electrons — but their energy doesn't change at all. What sets the electrons' energy is the light's colour: above a threshold frequency they fly out fast; below it, nothing comes out, however blinding the light. Classical wave theory was helpless: bigger waves should mean more energetic electrons, and dim light should need time to "pump up" an electron before it escapes. Instead the emission was instantaneous.

Einstein's paper — "On a heuristic point of view concerning the production and transformation of light," received 18 March 1905 — appealed to no new experiments. There were none demanding it. His argument was thermodynamic: in the Wien regime, radiation's entropy changes exactly the way an ideal gas's entropy changes under compression. Radiation, he concluded, "behaves thermodynamically as if it consisted of mutually independent energy quanta" — each of energy \\(E = h\\nu\\), localized in space, moving without dividing, absorbed or emitted only as a whole. One photon strikes one electron and hands over everything. A dim blue beam works; a blazing red beam never can.

He applied the idea to three phenomena in the same paper — Stokes' rule of photoluminescence, the photoelectric effect, and the ionization of gases — and wrote down the law: \\(K_{\\max} = h\\nu - \\phi\\), where \\(\\phi\\) is the work function, the escape cost. Robert Millikan spent years trying to disprove it, building what he called "a machine shop in vacuo" to scrape metal surfaces atomically clean. By 1916 he had confirmed the equation exactly — measuring \\(h\\) to within half a percent of Planck's value — and still called the photon "bold, not to say reckless," flying "in the face of thoroughly established facts of interference."

Then came the politics. Einstein was nominated for the prize nearly every year from 1910 — first by Wilhelm Ostwald, the same Ostwald who had rejected his 1901 job application. But the committee's relativity referee, the ophthalmologist Allvar Gullstrand, judged relativity's effects "below the limits of experimental error," and chairman Arrhenius blocked a second quantum prize so soon after Planck's 1918 award. The 1921 prize was simply not awarded — reserved under the statutes. In 1922 the deadlock broke: Planck proposed giving Einstein the overdue 1921 prize (and Bohr the 1922 one), and Uppsala's Carl Oseen supplied the compromise citation — the photoelectric law.

So Einstein received the 1921 Nobel Prize in Physics — announced 9 November 1922 — "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect." He wasn't there: he was en route to Japan. In Stockholm the German ambassador accepted the prize on behalf of "a German" — Einstein travelled on a Swiss passport — while the Swiss ambassador merely watched; months later the medal was formally handed to Einstein in Berlin, at his request, via the Swiss embassy. The prize money never touched his hands either: his 1919 divorce settlement had promised it all to his first wife Mileva Marič, who bought a house in Zurich with it.

Acceptance of the photon took decades more. Planck himself apologized for Einstein's light quanta when nominating him to the Prussian Academy in 1913; Bohr's own 1922 Nobel address rejected them; only Compton's 1923 X-ray scattering experiments — light bouncing off electrons like billiard balls — convinced the holdouts, with Bohr surrendering around mid-1925. The particle got its name, photon, from Gilbert Lewis in 1926. And in a final irony, Einstein's official Nobel lecture — delivered at Gothenburg on 11 July 1923 before two thousand people including the King of Sweden — was about relativity, the work the prize had pointedly not honoured. The Nobel Foundation's own footnote disowns the mismatch.

Common myth: "Einstein won the Nobel for relativity." He didn't — not special, not general, not \\(E = mc^2\\). The Academy "did not accept the quantization of light and would not recognize the theory of relativity" — so it honoured the one quantum paper nobody could argue with. His Nobel was a quantum prize.