Remarks on the history of the exclusion principle

Abstract

The history of the discovery of the exclusion principle, for which I have received the honor of the Nobel Prize award this year, goes back to my student days in Munich. While, in school in Vienna, I had already obtained some knowledge of classical physics and the then-new Einstein relativity theory, it was at the University of Munich that I was introduced by Sommerfeld to the structure of the atom — somewhat strange from the point of view of classical physics. I was not spared the shock which every physicist accustomed to the classical way of thinking experienced when he came to know of Bohr’s”Basic postulate of quantum theory“ for the first time. At that time there were two approaches to the difficult problems connected with the quantum of action. One was an effort to bring abstract order to the new ideas by looking for a key to translate classical mechanics and electrodynamics into quantum language which would form a logical generalization of these. This was the direction which was taken by Bohr’s Correspondence Principle. Sommerfeld, however, tried to overcome the difficulties which blocked the use of the concepts of kinematical models by a direct interpretation of the laws of spectra in terms of integral numbers, following, as Kepler once did in his investigation of the planetary system, an inner feeling for harmony. Both methods, which did not appear to me irreconcilable, influenced me.

„One can call particles obeying the exclusion principle the ‘antisocial’ particles, while photons are ‘social’.”

Pauli (1946a), S. 214

„It was all before your time. I met Bohr the first time at the ‚Bohr-Festspiele’ in Göttingen (1922). This is all published in Zeitschrift für Physik 9, 1 (1922). [...] Some main points were:

1. The ground state of the He-atom consists of two orbits in crossed planes with a resulting angular momentum 1 ħ. Hence this top was as K-shell in the ‘stomach’ of every atom.

2. The ortho-He consists of orbits in the same plane. By some auto suggestive arguments Bohr believed to be able to explain on the basis of the classical mechanics, that the states with crossed orbits could not combine with the states with coplanar orbits of the two electrons in the He-atom.

3. According to his correspondence principle Bohr correctly concluded that the doublet-structure of the spectra of alkali metals must correspond to the type of motion of a precession of the orbital plane of the valence electron around an other axis. But his mechanical interpretation of this precession was the idea of a resultant angular momentum of the atomic core of the alkali metals (therefore also of the noble gases). And this was identified with the ‘top in the stomach’ mentioned sub 1.

4. A particular complication was introduced by the asumption that the number of electrons in a closed shell can depend on the presence (or absence) of electrons in other shells. This gave rise to wrong numbers of electrons in closed shells.

You will get a certain insight in the struggle against this errors by reading the preface of Sommerfelds Atombau und Spektrallinien, 4^th edition 1924. I warmly supported Sommerfelds opposition against the ‘top in the stomach’ and was sure that the resultant angular momentum of the ground state of the He-atom (hence of the K-shell) has to be zero....

Bohr was full of self criticism and doubts in these decisive years 1922–24. He came down from the upper floor of Blegdamsvej 15, where he lived at that time, made drawings of the terms of the mercury spectrum on the blackboard (oh, these singlet and triplet-terms, and these ‘relativistic’ and ‘screening’ doublets in the X-ray spectra!) plucked his hairs — and was running back in the upper floor again!

Sommerfeld was great when be did not care of mechanical models. But he still clung to them, on the other hand. He wanted to replace Bobrs models by better ones. But gradually Bohr ceased to believe in mechanical pictures at all and had some advantage relativ to Sommerfeld. And all this together with this anomalous Zeeman-effect, a little bit a complicated story. First we did not know, what the right Hamiltonian was and second we did not know, once a Hamiltonian was given, bow to treat it ‘unmechanically’. So I was running restlessly (1923) through the streets of Copenhagen with a grim face thinking on the anomalous Zeeman-effect. And suddenly I heard a voice behind me saying: ‘Think on Jesus Christ!’ — Very much startled I turned around — it was an old women of the salvation army. (That is a true story; when I remember it correctly it was in the Kobnragergade.)

This seemed to be an entirely hopeless situation in the early autum days of 1923! But even without the salvation army the top was out of the stomach of the atoms only one year later — like Schneewittchens apple in the fairy tale!

But the ‘top ‘ started to wander, he had to be somewhere and that is another story until to Dvrac’s wave equation (which Dirac founded on entirely wrong arguments).”

Pauli an Rosenfeld, 31. Mai 1954