137 - Arthur I. Miller [35]
A week later, Bohr had clearly thought more seriously about Pauli’s new theory. He wrote to him, “I have the impression that we stand at a decisive turning point, now that the extent of the whole swindle has been so exhaustively characterized.” What struck him about Pauli’s proposal, he said, was its “complete insanity.” Bohr always condemned new proposals with the words “interesting but not crazy enough.” Saying that Pauli’s was completely insane meant he thought it was most probably right.
Pauli had still not solved the anomalous Zeeman effect, but he had accomplished something far more important. He suspected that the full significance of the exclusion principle would not become clear until there was a deeper understanding of quantum theory. “I will wait patiently; and be satisfied if I live to see the solution,” he wrote to Bohr. He hoped his “insane idea” would help toward understanding the structure of atoms made up of many electrons. If it did, “I would be the happiest man on earth.”
The fourth quantum number
The problem people had with understanding the exclusion principle was the lack of a visual model for the fourth quantum number. Pauli was well aware that it was essential for physicists to be able to visualize a theory—which was what made Bohr’s image of an atom as a miniscule solar system so pleasing. Nevertheless, he wrote to Bohr, although this need for visual images was “in part legitimate and healthy, it should never count as an argument for retaining systems of concepts. Once the systems of concepts are settled, then will visualizability be regained.” In effect, Pauli was suggesting strongly to Bohr that he should drop all visual images from his theory because they had proved to be incorrect and misleading. It was only once a new theory of atomic physics had emerged that it would be possible to develop a visual language to describe the atomic world.
Then Ralph Kronig, a twenty-year-old German American, noticed something Pauli had overlooked. The fourth quantum number of an electron has the mathematical properties of angular momentum, the momentum of an object moving in a circle. Every electron has an angular momentum from its orbit around the nucleus of the atom, like the earth revolving around the sun. Perhaps, thought Kronig, each electron also has an angular momentum of its own, like the earth spinning on its axis.
But whereas the earth’s spin is variable, the electron’s always remains the same. Kronig gave electron “spin” a value of a half, using the units of angular momentum. To explain spectral lines physicists had always assumed that the electron acted like a tiny magnet that could align itself in a magnetic field. Pauli’s and Kronig’s discovery had transformed it into a spinning magnet that could align itself along a magnetic field in one of two directions, depending on whether it had a spin of plus or minus a half. These were precisely the two values that Pauli had transferred from the inert closed core to the lone electron in the outermost shell of an alkali atom. Thus spin was recognized as a distinguishing feature of an electron. Every electron has its own spin, just as each of us has a nose, eyes, and lips that distinguish us from one another. Spin is an intrinsic property of an electron, and no matter where the electron is located it has a spin of a half.
Initially Pauli dismissed Kronig’s proposal, saying merely that it was “indeed a witty idea.” For imagining an electron as a spinning top, as Pauli and everyone else did, led to a serious conflict with relativity theory. It meant that a point on the surface of the electron might move with the velocity of light, which according to relativity theory is impossible. When Kronig visited Copenhagen, Bohr dismissed his proposal with the words “very interesting” Kronig dropped the idea.
Then, nine months later, two Dutch physicists, George Uhlenbeck and Samuel Goudsmit, rediscovered spin and staked their