[This is a transl ation of a m achine-typed copy of a letter that W olfgang Pauli sent to a g roup of physicis ts

meeting in Tübingen in Decem ber 1930. Pauli ask ed a colleague t o take the le tter to the m eeting, and the

bearer was to prov ide more info rmation as n eeded.]

Copy/Dec. 15, 195 6 PM

Open letter to the group of radi oactive peopl e at the

Gauverein meet ing in Tübi ngen.

Copy

Physics Institute Z ürich, Dec. 4, 1930

of the ETH Glorias trasse

Zürich

Dear Radioact ive Ladies and G entlemen,

As the bearer of these lines, to whom I graciously ask y ou to listen, w ill explain to you in m ore

detail, because of the "wrong " statistics of the N- and Li-6 nuclei and the continuous be ta spectrum , I

have hit upon a desp erate rem edy to sav e the "exchang e theorem " (1) of statistics and th e law of

conservation of energy . Namely, the possibi lity that in the nu clei there could exist elect rically neutral

particles, which I will call n eutrons, that hav e spin 1/2 and obey the exc lusion prin ciple and th at further

differ from light quanta in that they do not trav el with the v elocity of lig ht. The m ass of the neutron s

should be of the sam e order of m agnitude as the e lectron m ass and in any ev ent not larger tha n 0.01

proton mass. - The continuous be ta spectrum would th en mak e sense with the ass umption that in be ta

decay, in addition t o the ele ctron, a neutron is emitted s uch that the sum of the ene rg ies of neutron and

electron is const ant.

Now it is also a que stion of which forces act upon neutrons. Fo r me, the m ost likely model for the

neutron seem s to be, for wa ve-mechanical reason s (the bearer o f these lines know s m ore), that the neutron

at rest is a m agnetic dipole with a certain m om ent μ. The experim ents seem to require th at the ion izing

effect of such a neutron can not be bigger than th e one of a gamm a - ray , and then μ is probably not

allowed to be la rger than e • (10

-13

cm).

But so far I do not dare to p ublish anything about this i dea, and trustfu lly turn fi rst to you, dear

radioactive peopl e, with the question of how likely it is to find expe rimental evide nce for such a neu tron

if it would hav e the sam e or perhaps a 10 t imes larger ability to g et throug h [materi al] than a gamm a-ray.

I adm it that my rem edy may seem almost improbable because one probab ly woul d have seen

those neutrons, if they exist, for a long tim e. But nothing ventured, nothing gained, and the s eriousness of

the situation, due to the con tinuous structure of the beta spectrum, is illuminated by a remark of my

honored predecess or, Mr Deby e, who told m e recently in Brux elles: "O h, It's better not to think about this

at all, like new taxes." Ther efore one shou ld seriously discuss ev ery way of rescue. Thus, de ar radioactiv e

people, scrutiniz e and judge. - Unfortunately, I cannot personally appear in Tübing en since I am

indispensable h ere in Züric h because of a b all on the n ight from Decem ber 6 to 7. With my best regards to

you, and also to M r. Back, y our humble servant

signed W. Pauli

[Translation: Kurt Ries selman n]