Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [85]
Aside from helping Bohr, Pauli made a serious effort in Copenhagen to explain the 'anomalous' Zeeman effect – a feature of atomic spectra that the Bohr-Sommerfeld model could not explain. If atoms were exposed to a strong magnetic field, then the resulting atomic spectra contained lines that were split. It was quickly shown by Lorentz that classical physics predicted a splitting of a line into a doublet or a triplet: a phenomenon known as the 'normal' Zeeman effect which Bohr's atom could not accommodate.21 Fortunately, Sommerfeld came to the rescue with two new quantum numbers and the modified quantum atom resolved the problem. It involved a series of new rules governing electrons jumping from one orbit (or energy level) to another based on three 'quantum numbers', n, k, and m, that described the size of the orbit, the shape of the orbit, and the direction in which the orbit was pointing. But the celebrations were short-lived when it was discovered that the splitting of the red alpha line in the spectrum of hydrogen was smaller than expected. The situation grew worse with the confirmation that some spectral lines actually split up into a quartet or more instead of just two or three lines.
Although called the 'anomalous' Zeeman effect because the extra lines could not be explained using either existing quantum physics or classical theory, it was in fact far more common than the 'normal' effect. For Pauli it signalled nothing less than the 'deep seated failure of the theoretical principles known till now'.22 Having set himself the task of rectifying this miserable state of affairs, Pauli could not come up with an explanation. 'Up till now I have thoroughly gone wrong', he wrote to Sommerfeld in June 1923.23 Consumed by the problem, Pauli later admitted that he was in complete despair for some time.
One day another physicist from the institute met him while strolling around the streets of Copenhagen. 'You look very unhappy', said his colleague. Pauli turned on him: 'How can one look happy when he is thinking about the anomalous Zeeman effect?'24 The use of ad hoc rules to describe the complex structure of atomic spectra was just too much for Pauli. He wanted a deeper, more fundamental explanation of the phenomena. Part of the problem, he believed, was the guesswork involved in Bohr's theory of the periodic table. Did it really describe the correct arrangement of electrons inside atoms?
By 1922 the electrons in the Bohr-Sommerfeld model were believed to move in three-dimensional 'shells'. These were not physical shells, but energy levels within atoms around which electrons seemed to cluster. A vital clue in helping Bohr construct this new electron shell model was the stability of the so-called noble gases: helium, neon, argon, krypton, xenon and radon.25 With atomic numbers of 2, 10, 18, 36, 54 and 86, the relatively high energies required to ionise any noble gas atom – to rip away an electron and turn it into a positive ion – together with their reluctance to chemically bond with other atoms to form compounds, suggested that the electron configurations in these atoms were extremely stable and consisted of 'closed shells'.
The chemical properties of the noble gases were in stark contrast to the elements that preceded them in the periodic table – hydrogen and the halogens: fluorine, chlorine, bromine, iodine, and astatine. With atomic numbers 1, 9, 17, 35, 53 and 85, all of these elements easily formed compounds. Unlike the chemically