Albert Einstein won the Nobel Prize in Physics for the photoelectric effect, not for relativity. The 1921 prize, handed to him in 1922, was given “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect”. That law is one short equation from 1905 about light knocking electrons out of metal. Relativity is not named. And the experiment that made the equation impossible to ignore was done by a physicist who thought the idea behind it was reckless, and who spent about ten years expecting it to fail.
This is Nobel week. On Tuesday 6 October 2026 the Royal Swedish Academy of Sciences names this year’s physics laureates. It is a good moment to look again at the most famous physics prize of all, and at what it was really for.
Light that kicks electrons
The case opens in 1887. Heinrich Hertz noticed that an electric spark jumped across a gap more easily when ultraviolet light shone on it. Wilhelm Hallwachs then showed that a negatively charged metal plate loses its charge under ultraviolet light. The light was pushing negative charge out of the metal. That charge, it turned out, is carried by electrons, thrown out of the metal by the light. This is the photoelectric effect.
Around 1900 light was understood as a wave, and a wave carries more energy when it is brighter. So the expectation was simple: brighter light should throw the electrons out faster.
In 1902 Philipp Lenard found the opposite. The energy of each electron did not depend on how bright the light was. It depended on the light’s colour, its frequency. Brighter light gave more electrons, not faster ones. And below a certain frequency no electrons came out at all, however bright the light.
There was a second problem, of timing. Robert Millikan later did the wave arithmetic for dim light: a standard candle three metres from a sensitive photoelectric cell. If light were a spread-out wave, he worked out, an atom would need “at least 12,000 seconds or 4 hours of illumination” before it had soaked up enough energy to release one electron. “Yet the corpuscle is observed to shoot out the instant the light is turned on.”
Einstein’s reckless idea
In 1905 Einstein was 26. He worked at the Swiss Patent Office in Bern and did physics in his spare time. On 18 March his paper on light reached the Annalen der Physik, under a careful title: “On a Heuristic Point of View Concerning the Production and Transformation of Light”.
His idea was that light itself comes in separate packets, quanta. Each quantum carries an energy of h × f, where f is the frequency of the light and h is Planck’s constant, from Max Planck’s work on radiation.
Now follow one quantum into the metal. It gives all its energy to one electron. Part of that energy pays a toll, the work needed to escape the metal, called the work function P (often written φ). Whatever is left is the electron’s energy of motion. That is the whole law:
One line, and every puzzle falls at once. A red quantum is too weak to pay the toll, so however many arrive, none escapes: that is the threshold. Brighter light means more quanta, so more electrons, not faster ones. And because each electron is paid in one go, there is no waiting.
The equation also made a prediction anyone could test. Plot the electrons’ maximum energy against the frequency of the light and you should get a straight line, with slope h, the same for every metal.
Almost nobody believed in the light quanta. Light makes interference patterns, and only waves do that. Millikan, writing in 1916, called Einstein’s proposal “the bold, not to say the reckless, hypothesis”, because it “flies in the face of the thoroughly established facts of interference”.
Millikan’s photoelectric experiment
Millikan worked at the Ryerson Physical Laboratory of the University of Chicago. Earlier measurements of the photoelectric effect scattered too much to decide anything. The trouble was the metal surface: a fresh surface quickly grows a film, and the film spoils every reading.
He chose the alkali metals sodium, potassium and lithium, which respond to light over a long range of frequencies, and which are also, as he noted, inflammable. He kept them in a vacuum, on a wheel, and worked them with electromagnets from outside the glass, including a knife that shaved a clean surface off the metal. He described the apparatus as “a machine shop in vacuo”. The tubes were built by the laboratory’s mechanician, Julius Pearson.
Each measurement went like this. Light of one colour falls on the freshly cut metal and electrons fly off. A voltage pushes back against them, and is raised until even the fastest electrons are stopped. That stopping voltage, times the electron’s charge, is their maximum energy. Change the colour, measure again, plot the point.
The points fell on a straight line, just as the equation said. The slope gave h divided by the electron’s charge, and Millikan had measured that charge himself, with his oil drops. His result, published in Physical Review in 1916, was h = 6.57 × 10⁻²⁷ erg seconds, that is 6.57 × 10⁻³⁴ joule seconds, “with a precision of about .5 per cent”. Today’s exact value is 6.62607015 × 10⁻³⁴ joule seconds. The video draws the graph point by point.
His summary was plain: Einstein’s equation “appears in every case to predict exactly the observed results”. In the same paper he wrote that the facts had been predicted “by a form of quantum theory which has now been pretty generally abandoned”. He accepted the law, and not the light quanta behind it.
Why Einstein’s Nobel Prize named the law, not relativity
Einstein had been nominated for the Nobel Prize in Physics since 1910. The Nobel archive lists 64 nominations from 1910 to 1922, rising to 14 in 1921 and 17 in 1922, and most of the attention was on relativity.
In 1921 the physics committee decided that none of that year’s nominations met the criteria of Alfred Nobel’s will, and the prize was reserved for a year. In 1922 a new member of the committee, Carl Wilhelm Oseen, nominated Einstein not for relativity but for the law of the photoelectric effect. On 9 November 1922 the Academy announced that Einstein would receive the reserved prize for 1921. The 1922 prize went to Niels Bohr.
The presentation speech, given by Svante Arrhenius on 10 December 1922, said that relativity “pertains essentially to epistemology”, and that Einstein’s law of the photoelectric effect “has been extremely rigorously tested by the American Millikan and his pupils and passed the test brilliantly”. The prize was for the tested law, not for the light quanta. Einstein was on a long tour of Asia and could not attend. When he gave his Nobel lecture, in Gothenburg on 11 July 1923, he spoke about relativity.
The light quanta won their own case soon after. In 1922 Arthur Compton bounced X-rays off electrons and found light behaving like particles; he received the Nobel Prize in 1927. Millikan came round as well. In 1949, writing for Einstein’s seventieth birthday, he looked back:
I spent ten years of my life testing that 1905 equation of Einstein’s, and, contrary to all my expectations I was compelled in 1915 to assert its unambiguous experimental verification in spite of its unreasonableness since it seemed to violate everything that we knew about the interference of light. — Robert A. Millikan, Reviews of Modern Physics, 1949
The man who doubted the idea is the one who proved the law. And that’s how we found out.
