Pauli's letter

An invisible particle to explain the missing beta energy puzzle

Austrian-born Swiss physicist, Wolfgang Pauli (1900-1958) was one of the founders of quantum field theory. He formulated the exclusion principle and later the neutrino hypothesis. The Pauli exclusion principle, enunciated by Pauli in 1925, states that two electrons, or more generally two fermions can not be in the same quantum state. It explains the shell structure of atoms and why they are quasi incompressible.

Pauli put forward in 1930 the hypothesis of a new and invisible particle, the neutrino. It took 25 years to confirm what appears in retrospect an extraordinary step forward in the understanding of the infinitely small. Pauli was born in Vienna. He taught theoretical physics at several universities among them Princeton. He received the Nobel Prize in Physics in 1945.

Wolfgang Pauli
Wolfgang Pauli is remembered by his famous assumption in 1930 of a particle whose existence was experimentally proved only 26 years later, the neutrino !
CERN - Archives Pauli

Translation of the open letter sent by Wolfgang Pauli to Lise Meitner and Hans Geiger and a group of radioactive people at the Gauverein meeting in Tübingen.

Zürich, Dec. 4, 1930

Physics Institute of the ETH


Dear Radioactive Ladies and Gentlemen,

As the bearer of these lines, to whom I graciously ask you to listen, will explain to you in more detail, because of the "wrong" statistics of the N- and Li-6 nuclei and the continuous beta spectrum, I have hit upon a desperate remedy to save the "exchange theorem" (1) of statistics and the law of conservation of energy. Namely, the possibility that in the nuclei there could exist electrically neutral particles, which I will call neutrons, that have spin 1/2 and obey the exclusion principle and that further differ from light quanta in that they do not travel with the velocity of light. 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 mass. - 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.

Now it is also a question of which forces act upon neutrons. For me, the most likely model for the neutron seems to be, for wave-mechanical reasons (the bearer of these lines knows more), that the neutron at rest is a magnetic dipole with a certain moment μ. The experiments seem to require that the ionizing effect of such a neutron can not be bigger than the one of a gamma-ray, and then μ is probably not allowed to be larger than e • (10 -13cm).

But so far I do not dare to publish anything about this idea, and trustfully turn first to you, dear radioactive people, with the question of how likely it is to find experimental evidence for such a neutron if it would have the same or perhaps a 10 times larger ability to get through [material] than a gamma-ray.

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, and the seriousness of the situation, due to the continuous structure of the beta spectrum, is illuminated by a remark of my honored predecessor, Mr Debye, who told me recently in Bruxelles: "Oh, It's better not to think about this at all, like new taxes." Therefore one should seriously discuss every way of rescue. Thus, dear radioactive people, scrutinize and judge. - Unfortunately, I cannot personally appear in Tübingen since I amindispensable here in Zürich because of a ball on the night from December 6 to 7. With my best regards to you, and also to Mr. Back, your humble servant

W. Pauli

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