Wolfgang Ernst Pauli

The Austrian theoretical physicist Wolfgang Ernst Pauli (1900-1958) was awarded the Nobel Prize in Physics for his discovery of the exclusion principle, known as the Pauli principle.

Wolfgang Pauli the son of Wolfgang Joseph Pauli, a professor in the University of Vienna, was born in that city on April 25, 1900. Brilliant at school, he studied theoretical physics in the University of Munich under Arnold Sommerfeld (1918-1921) and graduated as a Doctor of Philosophy. Sommerfeld asked him to write the article on relativity for the Encyclopedia of Mathematical Sciences. The article, over 200 pages long, was published in 1921; it was translated into English and Italian in 1958 and is still definitive.

Pauli was an assistant to Max Born at Göttingen (1921-1922) and to Niels Bohr at Copenhagen (1922-1923). He then spent 5 years as a lecturer in the University of Hamburg, and in 1928 he became professor of physics in the Federal Institute of Technology at Zurich.

In 1921 the generally accepted theory of the atom was that advanced by Bohr in 1913. In the case of the hydrogen atom with its single electron, the state of the atom was defined by a single quantum number representing the energy in the possible circular orbits of the electron. By postulating an additional set of quantum numbers Sommerfeld later extended Bohr's theory to cover the elliptical orbits in complex atoms, and a third set was later postulated to explain the atom in a magnetic field. The Bohr-Sommerfeld theory explained the hydrogen atom satisfactorily; but in the case of complex atoms it did not explain the doublet nature of the series of the alkali spectra, nor did it explain the anomalous Zeeman effect which Pauli had tried to elucidate while he was at Copenhagen.

In 1924-1925 Pauli published his theoretical solution of the anomalous Zeeman effect. To explain it, others had suggested that the third, or magnetic, quantum number should be regarded as having a half-integer value. But Pauli postulated a fourth quantum number, a fourth degree of freedom. This he regarded as having one of two values only—a property he later defined as "two-valuedness not describable classically." He then defined his "principle," which is now usually stated as follows: no two electrons in the same atom can have all four quantum numbers equal. Recognized from the time of its publication as important, it was not at once called the exclusion, or Pauli, principle. In 1925 G. E. Uhlenbeck and S. A. Goudsmit introduced the hypothesis of electron spin, with possible quantum numbers of either + ½ or -½. About this time the new mechanics, as exemplified by Werner Heisenberg's matrix mechanics and Erwin Schrödinger's wave equation, was making headway, but these methods did not easily explain the problem of the hydrogen atom because it involved the inverse-square law in the attractive force. In 1926 Pauli solved this problem brilliantly by identifying his hypothetical fourth degree of freedom with Uhlenbeck and Goudsmit's "spin," and since then this degree has been called the spin quantum.

Between 1928 and 1930 Pauli first attempted—partly in collaboration with Heisenberg—to apply the quantum principle to the interaction of radiation and matter. These three papers constituted the first steps in quantum field theory. In the early 1930s, to explain the phenomenon of beta decay of nuclei, by which an unpredictable amount of energy appeared to be lost, Pauli postulated the existence of a neutral particle of low mass but with spin ½ For this particle Enrico Fermi later coined the name "neutrino."

Pauli was visiting professor at the University of Michigan (1931, 1941) and at the Institute for Advanced Study, Princeton (1935-1936, 1940-1945). He received many honors, including the Nobel Prize for Physics in 1945. In 1953 he was elected a Foreign Member of the Royal Society. He died in Zurich on Dec. 15, 1958.

Further Reading on Wolfgang Ernst Pauli

There is a biography of Pauli in Nobel Lectures, Physics, 1942-1962 (1964), which also includes his Nobel Lecture. For his work see N. H. de V. Heathcote, Nobel Prize Winners, Physics, 1901-1950 (1953); B. Hoffmann, The Strange Story of the Quantum (2d ed. 1959); and A. d'Abro, The Rise of the New Physics, vol. 2 (1951).