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Pauli, Wolfgang

quantum principle electron energy

[pow lee] (1900–58) Austrian–Swiss–US physicist: discovered the Pauli exclusion principle in quantum mechanics.

The son of a professor of physical chemistry at the University of Vienna, Pauli obtained his PhD at Munich in 1921. Encouraged by , Pauli had written (when he was only 19) an article, subsequently published as a small book, on relativity that was admired by for its ‘deep physical insight’. Pauli studied further with in Copenhagen and in Göttingen. He then taught at Hamburg and gained a professorship in 1928 at the Federal Institute of Technology, Zürich, remaining there until his death, except for 5 war years spent at Princeton (when he became a US citizen).

Working on quantum mechanics, he contributed the Pauli exclusion principle (1924), which explained much about atomic structure. The principle requires that no two electrons in an atom can be in the same quantum state. The original Bohr–Sommerfeld model of the atom (1915) specified for each electron in an atom three quantum numbers (n,l,m) and Pauli additionally required the electron to have another, called the spin quantum number s = ±½. Pauli’s principle that no more than one electron is able to occupy a state described by n, l, m and s then gave the correct formation of electronic shells in atoms, gave a theoretical basis for the periodic classification, and explained the effect of atomic spectra. This concept of spin, able to have one of two values, was verified experimentally by in 1926. For his idea of the exclusion principle Pauli was awarded the Nobel Prize for physics in 1945. It was much overdue.

Pauli also studied the relation between the spin of a particle and the statistics of energy level occupancy (quantum statistics); the paramagnetic properties of gases and metals (including electrons in metals); the extension of quantum mechanics from one to a large number of particles; the explanation of the meson and the nuclear binding force.

Furthermore, Pauli solved a major problem concerning beta decay, in which atomic nuclei eject electrons and apparently contravened the conservation of energy principle. The energies of emitted electrons cover a continuous range up to a maximum value, and it was unclear what happened to the ‘missing’ energy if an electron had less than the maximum. Pauli realized that this energy could be carried off by an undetected, very light neutral particle (named the neutrino, Italian for ‘little neutral one’, by ) emitted at the same time as the electron. This was correct and the neutrino was first observed more directly by F Reines (1918–98) in 1956.

Pauli had a caustic wit; he was not a good lecturer and he was notoriously bad as an experimentalist; but he is one of the giants of 20th-c theoretical physics.

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