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Logbook of March 2007
Pauli Letter Collection, CERN

Neutrino invention

Wolfgang Pauli postulated the existence of a neutral, light-weight particle that could save the fundamental law of the conservation of energy.

Wolfgang Pauli, at age 30, had a bold idea on how to solve a perplexing problem in nuclear physics. To explain the apparent disappearance of energy in the decay of certain atomic nuclei, he postulated the existence of a neutral, light-weight particle, saving the fundamental law of the conservation of energy.

Pauli proposed that “neutrons” could emerge from decay processes, carrying away energy while escaping direct experimental detection.

Worried that nobody would ever be able to observe this particle, Pauli did not dare to publish his invention without consulting some experimental physicists. On December 4, 1930, Pauli wrote an open letter to a group of nuclear physicists, the “dear radioactive ladies and gentlemen,” who were going to meet a few days later in Tübingen, Germany. The document shown here is a machine-typed copy that Pauli obtained in 1956 from Lise Meitner, a well-regarded scientist who had attended the Tübingen meeting.

In the early 1930s, scientists elaborated on Pauli’s idea and concluded that the new particle must be extremely light and very weakly interacting. When James Chadwick discovered a neutral particle in 1932, it received the name neutron. But the particle turned out to be too heavy to fit Pauli’s prediction. Enrico Fermi, developing a theory of weakly interacting particles, introduced a new name for Pauli’s particle: neutrino, which means “little neutral one.” A quarter-century later, scientists observed for the first time collisions of neutrinos with matter, the long-sought-after evidence for Pauli’s ghost-like invention.

Logbook of March 2007
Pauli Letter Collection, CERN
  1. Dear Radioactive Ladies and Gentlemen!
  2. I have hit upon a desperate remedy to save…the law of conservation of energy.
  3. …there could exist electrically neutral particles, which I will call neutrons, in the nuclei…
  4. The continuous beta spectrum would then make sense with the assumption that in beta decay, in addition to the electron, a neutron is emitted such that the sum of the energies of neutron and electron is constant.
  5. But so far I do not dare to publish anything about this idea, and trustfully turn first to you, dear radioactive ones, with the question of how likely it is to find experimental evidence for such a neutron…
  6. I admit that my remedy may seem almost improbable because one probably would have seen those neutrons, if they exist, for a long time. But nothing ventured, nothing gained…
  7. Thus, dear radioactive ones, scrutinize and judge.

Full Translation of the letter