Pauli's letter of the 4th of December 1930

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, how because of the "wrong" statistics of the N and Li6 nuclei and the continuous beta spectrum, I have hit upon a desperate remedy to save the "exchange theorem" of statistics and 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 and which 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 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 the electron is constant...

I agree that my remedy could seem incredible because one should have seen these neutrons much earlier if they really exist. But only the one who dare can win and the difficult situation, due to the continuous structure of the beta spectrum, is lighted by a remark of my honoured predecessor, Mr Debye, who told me recently in Bruxelles: "Oh, It's well better not to think about this at all, like new taxes". From now on, every solution to the issue must be discussed. Thus, dear radioactive people, look and judge.

Unfortunately, I cannot appear in Tubingen personally since I am indispensable here in Zurich because of a ball on the night of 6/7 December. With my best regards to you, and also to Mr Back.

Your humble servant,

W. Pauli


1) The text above is an abridged version of the original. (See here for a copy of the original, kept in the Pauli Archive at CERN.)

2) In his letter (1930), Pauli refers to his new proposed particle, the "neutron". The neutron (as we know it today) was discovered, by J Chadwick, two years after Pauli's proposal. In 1934, at a seminar on his recent theory of beta-decay, Fermi was asked whether the neutral particle emitted in the nuclear beta-decay was the same as Chadwick's neutron. Apparently, Fermi clarified that he was talking about a different particle which he referred to as neutrino ("little neutral one").

3) Pauli thought his proposal of the "neutron" was too speculative, and did not publish it in a scientific journal until 1934, by which time Fermi had already developped a theory of beta decay incorporating the neutrino.