Physicist Wolfgang Pauli (Photo: https://www.nobelprize.org/prizes/physics/1945/pauli/facts/)
Electrons are precisely alike and any two detest being in the same state or configuration at the same time. When two electrons meet, they look at each other and ask, “If you are exactly me, one of us is not needed here.” That is the essence of the Pauli Exclusion Principle as I understand it. Today happens to be the 119th birth anniversary of the man behind that principle, Wolfgang Pauli.
My comprehension of Pauli’s work pretty much ends with that opening. I do experience many moments of lucid understanding of the ideas of quantum mechanics in whose creation Pauli was a key player in 1924. His introduction of a new quantum number, which later came to be known as spin, was described by him as “a two-valuedness not describable classically.” By “not describable classically” Pauli meant the way classical physics looked at matter. Hence the beginnings of quantum mechanics, which Albert Einstein had already signaled quite some time prior to that.
In order to get a little more detailed view of his work I read Pauli’s 1945 Nobel Prize lecture. I am happy to report that I could read every word of it without comprehending any meaning of it. This is a lifelong battle for me with quantum mechanics. I seem to understand some of it in the moment but it disappears almost as soon as I disengage from it.
Notwithstanding, I was able to get at the core of the paper rather quickly. It is this: “The complicated numbers of electrons in closed subgroups are reduced to the simple number one if the division of the groups by giving the values of the four quantum numbers of an electron is carried so far that every degeneracy is removed. An entirely non-degenerate energy level is already « closed », if it is occupied by a single electron; states in contradiction with this postulate have to be excluded. The exposition of this general formulation of the exclusion principle was made in Hamburg in the spring of 1925 , after I was able to verify some additional conclusions concerning the anomalous Zeeman effect of more complicated atoms during a visit to Tübingen with the help of the spectroscopic material assembled there.”
When a single electron occupies a state, it is only that electron that occupies that state. Like I said, an electron can’t stand another electron perhaps because it reminds itself too much of itself.
One of the main lures of my repeated return to quantum mechanics for more 40 years now is that I fail consistently but within those failure there are moments of great awakening when I understand it very well. That understanding is evanescent and that is the thrill for me. In the case of the Pauli Principle case too, I do feel excluded.
Here is to Wolfgang Pauli.