Zürich, December 4, 1930
Dear radioactive ladies1 and gentlemen,

I beg you to most favorably listen to the carrier of this letter.  He will tell you that, in view of the wrong statistics of the N and the Li6 nuclei and of 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.  This is the possibility that electrically neutral particles exist which I will call neutrons,2 which exist in nuclei, which have a spin 1/2 and obey the exclusion principle, and which differ from the photons also in that they do not move with the velocity of light.  The mass of the neutrons should be of the same order as those of the electrons and should in no case exceed 0.01 proton masses.  The continuous beta spectrum would then be understandable if one assumes that during beta decay with each electron a neutron is emitted in such a way that the sum of the energies of neutron and electron is constant. . . .

I admit that my remedy may seem incredible, because one should have seen these neutrons long ago if they really exist.  But only he who dares can win and the difficult situation caused by the continuous beta spectrum is illuminated by a remark of my honored predecessor, Mr. Debye, who told me recently in Brussels: Oh, it is best not to think at all, just as with the new taxes.  Henceforth every possible solution must be discussed.  So, dear radioactive people, examine and judge.  Unfortunately I will not be able to appear in Tübingen personally, because I am indispensable here due to a ball which will take place in Zürich during the night from December 6 to 7.  With my best regards to you and also Mr. Back.

Your humble servant,
W. Pauli


  1. The radioactive lady was Lise Meitner.
  2. What we presently call neutron had not yet been discovered so the name was available for this use.  Enrico Fermi later proposed the now accepted name of neutrino.  They are no longer thought to exist inside nuclei or to have a mass equal that of an electron.

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transcribed 11/23/2004
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