No, Albert Einstein did not win the Nobel Prize for relativity. He won the Nobel Prize in Physics 1921 “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect”. The word relativity does not appear in the citation. The law it names is a short equation from 1905 about light knocking electrons out of metal.
A prize held over for a year
The 1921 prize was not even awarded in 1921. That year the Nobel Committee for Physics decided that none of the nominations met the criteria in Alfred Nobel’s will. The Nobel Foundation’s statutes allow a prize to be reserved until the following year in such a case, and that is what happened: Einstein received the 1921 prize one year later, in 1922.
At the ceremony in Stockholm on 10 December 1922 the presentation speech made the committee’s position plain. Relativity, it said, “pertains essentially to epistemology”, the theory of knowledge, and was the subject of lively debate among philosophers. The photoelectric law was a different matter: it had been “extremely rigorously tested by the American Millikan and his pupils and passed the test brilliantly.”
The law of the photoelectric effect
In 1905, aged 26 and working at the Swiss Patent Office in Bern, Einstein proposed that light is not only a wave but comes in separate packets, quanta, each carrying an energy hf, where f is the frequency of the light and h is Planck’s constant.
One packet gives all its energy to one electron in a metal. Part of that energy pays the toll for leaving the metal, the work function, written P. The rest is the energy the electron flies off with:
That one line explains what the wave picture could not. The colour of the light, its frequency, sets the energy of each electron, not the brightness. Below a certain frequency no electrons come out at all, however bright the light: red light can be turned up as far as you like and the packets are still too weak to pay the toll. Violet light, with more energetic packets, kicks electrons straight out. Brighter light means more packets, so more electrons, not faster ones.
The test
Robert Millikan did not believe the idea behind the law. In 1916 he described it as “the bold, not to say the reckless, hypothesis” of a light corpuscle. He tested the equation anyway, and found that it “appears in every case to predict exactly the observed results.” That is the confirmation the presentation speech pointed to.
The long video and its story, What Did Einstein Win the Nobel Prize For?, follow Millikan’s experiment step by step. The Short does it in under a minute, starting from a search box that cannot find the word relativity.
The prize was for the law, not the theory. And that’s how we found out.
