On December 4, 1930, Wolfgang Pauli — at 30 already one of the most influential theoretical physicists in Europe — wrote what is now one of the most famous letters in the history of science. He had been invited to a small conference on radioactivity in Tübingen, Germany. He did not want to go. By his own admission, he was planning to spend the relevant weekend at a ball in Zürich.
So he sent a letter instead. It began: “Liebe Radioaktive Damen und Herren” — “Dear radioactive ladies and gentlemen.” It contained an apology for not attending, a few lines of social pleasantry, and then a proposal that would eventually rewrite particle physics.
The problem Pauli was trying to fix
By 1930, nuclear physics had a serious puzzle on its hands. In beta decay — a kind of radioactive decay in which a nucleus emits an electron — the energy of the outgoing electron was supposed to be fixed. A two-body decay, by simple conservation of energy and momentum, must produce particles with specific energies.
But that was not what experiments showed. The electrons came out with a continuous spectrum of energies, from almost zero up to a maximum. Most electrons carried away less than they should have. Energy seemed to be going missing.
This was not a minor anomaly. The conservation of energy is one of the deepest principles in physics. The great Niels Bohr, at one point, was prepared to consider abandoning it for these small-scale interactions. “Energy conservation,” he reasoned, “may simply not hold at the nuclear level.”
Pauli disagreed. He thought that abandoning a sacred law was too high a price. He preferred a more modest sin: inventing a particle.
The letter
The letter, addressed to Lise Meitner and Hans Geiger, ran in part:
“Dear radioactive ladies and gentlemen, I have hit upon a desperate remedy to save the law of conservation of energy. Namely the possibility that there could exist in the nuclei electrically neutral particles, that I wish to call neutrons, which have spin 1/2 and obey the exclusion principle… The mass of the neutrons should be of the same order of magnitude as the electron mass and in any event not larger than 0.01 proton masses. The continuous beta spectrum would then become understandable by the assumption that in beta decay a neutron is emitted in addition to the electron such that the sum of the energies of the neutron and electron is constant.”
He went on to acknowledge the problem with his own proposal:
“For the time being I dare not publish anything about this idea, and I address myself confidentially first to you, dear radioactive ones, with the question how the situation would be with the experimental proof of such a neutron, if it had about ten times the penetrating power of a gamma ray.”
Pauli was suggesting a particle so weakly interacting that he himself believed it might never be detected. He famously remarked to a colleague: “I have done a terrible thing. I have postulated a particle that cannot be detected.”
What Pauli got right and what he got wrong
The particle Pauli called a “neutron” in his 1930 letter is what we now call the neutrino. Two years later, in 1932, James Chadwick discovered a different neutral particle inside the nucleus — much heavier — and that one took the name “neutron.” Enrico Fermi, in 1934, gave Pauli’s particle the diminutive neutrino, “little neutral one,” and built the first quantitative theory of beta decay around it.
Pauli got several things right:
- The particle should be electrically neutral.
- It should have spin 1/2.
- It should be emitted alongside the electron, sharing the available energy.
- It should be very light — much lighter than the proton.
He got one thing approximately wrong:
- He guessed the mass would be “of the same order as the electron mass.” It is, in fact, at least a million times lighter.
The order of magnitude of the interaction strength turned out to be smaller than even his worst-case estimate. A neutrino can travel a light-year through solid lead with only about a 50% chance of interacting. That’s why Pauli thought his particle might be undetectable. It took 26 years to prove him wrong.
The proof
In 1956, Clyde Cowan and Frederick Reines — physicists with access to the high-flux nuclear reactor at the Savannah River Site in South Carolina — caught the neutrino in a 400-liter tank of water laced with cadmium chloride. They sent Pauli a telegram. He replied: “Everything comes to him who knows how to wait.”
A full account of how Cowan and Reines pulled it off is in our piece Cowan and Reines: how a reactor experiment caught the neutrino.
Pauli died in 1958, two years after the confirmation, before the broader implications of his “desperate remedy” became clear. He never lived to see his particle detected from the Sun (1968), oscillating between flavors (1998), arriving from a supernova (1987), or pointing back to a cosmic blazar (2017). All of that physics rested on his weekend off in 1930.
Why the letter still matters
The 1930 letter is studied today partly as a piece of physics history, partly as an example of how good guesses are sometimes worth more than careful caution. Niels Bohr, whose response had been to consider giving up energy conservation, was almost certainly the deeper thinker. But Pauli was the better gambler.
Pauli won. Energy conservation was saved. A new particle was added to the physical world. And a century later, dozens of experiments, hundreds of papers per week, and an entire branch of astronomy trace their origin back to one letter, written by a Viennese theoretician who chose a ball over a conference.
For the historical arc that followed Pauli’s letter, see our full neutrino timeline. For what neutrinos actually are, see What is a neutrino, anyway?.
Further reading
Primary sources
- Pauli’s original 1930 letter (German transcript, CERN Document Server) — the so-called “Tübingen letter” of December 4, 1930
- Wolfgang Pauli, “On the Earlier and More Recent History of the Neutrino” (1957 lecture text) — Pauli’s own retrospective on his proposal
Background and context
- Wikipedia: Wolfgang Pauli
- Wikipedia: History of the neutrino
- CERN Courier — “The pauli neutrino” — historical feature on the 1930 proposal and its 26-year journey to detection
Frequently asked
What did Pauli propose in 1930?
A new, electrically neutral, almost massless particle that he suggested must be emitted along with the electron in beta decay. Without such a particle, the apparent energy and angular momentum discrepancies in beta decay could not be reconciled with conservation laws. Pauli called the proposal 'a desperate remedy' in his original letter.
Why did he call it desperate?
Because postulating a brand-new particle to solve a problem felt theoretically extravagant by 1930 standards. Pauli also worried that the proposed particle would be too weakly interacting to ever be detected — a concern that turned out to be partly justified but ultimately surmountable. The Cowan-Reines team caught the particle 26 years later, in 1956.
What did Pauli originally call the particle?
He called it the 'neutron' in 1930. That name became inconvenient in 1932 when James Chadwick discovered the actual neutron — the heavier neutral particle inside the nucleus. The community needed a new name, and Enrico Fermi suggested 'neutrino' — Italian for 'little neutral one.'
Why did Pauli skip the Tübingen conference?
By his own admission, he wanted to attend a ball in Zürich that same weekend. Instead of presenting his proposal in person, he wrote his now-famous open letter to the conference attendees: 'Dear radioactive ladies and gentlemen, I have hit upon a desperate remedy to save the exchange theorem of statistics and the law of conservation of energy.'
Did Pauli live to see the neutrino confirmed?
Yes — Cowan and Reines telegraphed Pauli in June 1956 to inform him that they had directly detected the antineutrino through inverse beta decay. Pauli responded: 'Thanks for message. Everything comes to him who knows how to wait.' He died two years later in 1958, before the Nobel Prize for the discovery was awarded (Reines received it in 1995).
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APA
Neutrino Times Editorial Team. (2025, June 14). Dear radioactive ladies and gentlemen: how Pauli invented the neutrino in 1930. Neutrino Times. https://neutrino-times.com/articles/pauli-1930-letter-neutrino-proposal/
Chicago
Neutrino Times Editorial Team. "Dear radioactive ladies and gentlemen: how Pauli invented the neutrino in 1930." Neutrino Times, June 14, 2025. https://neutrino-times.com/articles/pauli-1930-letter-neutrino-proposal/.
MLA
Neutrino Times Editorial Team. "Dear radioactive ladies and gentlemen: how Pauli invented the neutrino in 1930." Neutrino Times, 14 Jun. 2025, https://neutrino-times.com/articles/pauli-1930-letter-neutrino-proposal/.
BibTeX
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author = {Neutrino Times Editorial Team},
title = {Dear radioactive ladies and gentlemen: how Pauli invented the neutrino in 1930},
howpublished = {Neutrino Times},
year = {2025},
month = {jun},
url = {https://neutrino-times.com/articles/pauli-1930-letter-neutrino-proposal/},
note = {Accessed: 2025-06-14}
} RIS
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