Wolfgang Pauli: A Desperate Remedy That Turned Out to Be True
Physics rarely records the moment someone invents a particle out of embarrassment. Pauli's is documented, because he wrote it down in a letter he did not even deliver in person - he had a ball to attend. The letter proposed the neutrino to save a law he was not willing to give up, and he was so unconvinced by his own idea that he did not publish it for three years.
The Problem He Was Trying to Solve
By the late 1920s beta decay had become an embarrassment. When a nucleus emitted an electron, the energy of that electron should have been fixed: a specific initial state, a specific final state, a specific difference. Instead experiments found a continuous spectrum, with electrons emerging at every energy up to a maximum and most of them well below it.
The missing energy had to go somewhere or conservation of energy was wrong. Niels Bohr, seriously, proposed that it might be wrong - that energy conservation could hold only statistically at the nuclear scale. That was a respectable position held by the most respected physicist of the age.
Pauli refused it. His alternative was to suppose that a second particle left the nucleus alongside the electron, sharing the energy with it in varying proportions, and that this particle carried no charge and almost no mass, which is why nobody had seen it.
The proposal was economical: it saved conservation of energy, it explained the continuous spectrum exactly, and it required only one new object. It also required believing in something invisible, which is why he was so uncomfortable about it.
The Letter
On 4 December 1930 Pauli wrote to a meeting of physicists at Tübingen, addressing them as Dear Radioactive Ladies and Gentlemen. He set out the proposal, called the new particle a neutron - the actual neutron had not yet been discovered and would take the name two years later - and explained that he could not come in person because his presence was indispensable at a ball in Zurich.
The tone of the letter is the interesting part. He describes his own idea as a desperate remedy, says he does not feel secure enough to publish it, and asks the recipients to judge. This is not false modesty. He genuinely thought he might be proposing something unfalsifiable.
He is reported to have said to a colleague afterwards that he had done a terrible thing: he had postulated a particle that cannot be detected. Whether the wording is exact is uncertain, but the sentiment is consistent with everything else he wrote about it at the time.
He was right about the difficulty. A neutrino interacts so weakly that a light-year of lead would stop only about half of a beam. Detecting one requires either an enormous target or an enormous flux, and neither was available in 1930.
What Happened to the Idea
Pauli did not publish the proposal for three years. It was Enrico Fermi who took it seriously enough to build a full theory around it in 1934, and who gave the particle its modern name - the little neutral one, to distinguish it from the neutron discovered by James Chadwick in 1932.
Detection came in 1956, when Clyde Cowan and Frederick Reines found neutrinos from a nuclear reactor at Savannah River. They sent Pauli a telegram. He replied that everything comes to those who know how to wait, and reportedly drank a case of champagne with his colleagues.
He died in 1958, two years after the confirmation and thirty-seven years before Reines received the Nobel Prize for it. His own Nobel, awarded in 1945, was for the exclusion principle, which is a separate and arguably larger contribution.
The episode is worth remembering for a specific reason. Pauli was not proposing a particle to explain an inconvenience; he was proposing one to defend a conservation law against a serious alternative that would have abandoned it. The neutrino was the conservative option.
Frequently asked questions
Why did Pauli propose the neutrino?
To rescue conservation of energy. Beta decay produced electrons with a continuous range of energies rather than a fixed value, and the alternative on the table - proposed by Niels Bohr - was that energy conservation simply failed inside the nucleus.
Did he call it a neutrino?
No. He called it a neutron, before the actual neutron was discovered in 1932. Enrico Fermi later renamed it the neutrino, meaning the little neutral one.
Why did he apologise for it?
Because he believed he had proposed a particle that could never be detected, which he regarded as a poor thing for a physicist to do. He described the idea as a desperate remedy and did not publish it for three years.
Did he live to see it detected?
Yes. Cowan and Reines confirmed the neutrino in 1956 and sent him a telegram. Pauli died in 1958.
Was his Nobel Prize for the neutrino?
No. He received the 1945 Nobel Prize in Physics for the exclusion principle. Frederick Reines received a Nobel Prize for the neutrino's detection in 1995, thirty-seven years after Pauli's death.
