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In 1930, Wolfgang Pauli wrote a letter to Lise Meitner for a convention in Tübingen, considering the problem of beta decay.

Does anybody know, where to find the original letter online ?

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Here is a pdf file with the letter:… – Qmechanic Mar 3 '12 at 22:30
@Qmechanic that would be an answer ;-) – David Z Mar 4 '12 at 0:37
up vote 10 down vote accepted

Here is a pdf file with the letter in both German and English. Screenshot:

enter image description here

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“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.” — Lovely! – F'x Mar 13 '12 at 11:15
Note that this letter predates the discovery of neutrons as we know them today, and Pauli is actually talking about neutrinos (and presumably their relativistic mass) – MSalters Mar 13 '12 at 12:39
@nibot: Thanks for the screenshot. – Qmechanic Mar 20 '12 at 0:24
@MSalters: true about the neutrinos (Fermi coined the Italian-style term after the "neutron" was named), but the mass is the invariant scalar mass, the relativistic mass can be huge, you can dump nearly all the energy in beta decay into the outgoing neutrino. The cleanest case is neutron beta decay, which obviously was not yet available, neutrons are harder to detect than protons or electrons--- they're neutral. – Ron Maimon Mar 20 '12 at 4:15

Pasted text of the letter in English - the link also contains the original typed German letter.

Open letter to the 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

signed W. Pauli

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Is the 'radioactive people' merely a translation artifact; or is he jokingly using the term? – Manishearth Mar 4 '12 at 1:13
@Manishearth It's not a translation artifact ;-) – mate64 Mar 4 '12 at 1:27

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