PAULI, WOLFGANG

born April 25, 1900, Vienna, Austria died Dec. 15, 1958, Zrich, Switz. Wolfgang Pauli, 1945. Austrian-born physicist and recipient of the 1945 Nobel Prize for Physics for his discovery in 1925 of the Pauli exclusion principle, which states that in an atom no two electrons can occupy the same quantum state simultaneously. This principle clearly relates the quantum theory to the observed properties of atoms. When he was 20, Pauli wrote a 200-page encyclopaedia article on the theory of relativity. He was appointed a lecturer at the University of Hamburg in 1923, and the following year he proposed that a fourth quantum number, which may take on the numerical values +1/2 or -1/2, was necessary to specify electron energy states. It was later found that the two values represent the two possible directions of spin for fermions. In 1925 he introduced his exclusion principle, which immediately made clear the reason for the structure of the periodic table of the elements. In 1928 Pauli became professor of theoretical physics at the Federal Institute of Technology, Zrich. Under his direction the institution became a great centre of research in theoretical physics during the years preceding World War II. In the late 1920s it was observed that when a beta particle (electron) is emitted from an atomic nucleus, there is generally some energy and momentum missing, a grave violation of the laws of conservation. Rather than allow these laws to be discarded, Pauli proposed in 1931 that the missing energy and momentum is carried away from the nucleus by some particle (later named the neutrino by Enrico Fermi) that is uncharged and has little or no mass and had gone unnoticed because it interacts with matter so seldom that it is nearly impossible to detect. This particle was not observed until 1956. In 1940 Pauli was appointed to the chair of theoretical physics at the Institute for Advanced Study, Princeton, New Jersey, and in 1946 he became a naturalized citizen of the United States. Following World War II he returned to Zrich.