Wolfgang Pauli

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Wolfgang Pauli


Austrian-American Physicist

Wolfgang Pauli developed one of the most important ideas in modern physics, which came to be known as the Pauli exclusion principle. It holds that no two electrons in an atom can exist in exactly the same quantum state. Pauli put forth the principle in an effort to explain a feature in atomic spectra called the anomalous Zeeman effect. He was awarded the 1945 Nobel Prize in physics for this work.

Pauli was born on April 25, 1900, in Vienna, Austria. His father was a professor of physical chemistry. Pauli received his Ph.D. in 1921 from the University of Munich, where he studied under Arnold Sommerfeld (1868-1951) and wrote a 200-page encyclopedia article on the theory of relativity.

In 1916 Sommerfeld had extended the atomic model of Niels Bohr (1885-1962), in which electrons orbited the atomic nucleus in specified circular paths, to include elliptical orbits. The Bohr-Sommerfeld model, with three "quantum numbers" specifying the state of the electron, was able to account for most of the features seen in atomic spectra. One of the few exceptions was the anomalous Zeeman effect. In the normal Zeeman effect, a weak magnetic field caused a single spectral line to split into a triplet. Sometimes, however, it split into many lines.

In 1924 Pauli, then a lecturer at the University of Hamburg, was investigating this phenomenon. He concluded that an additional quantum number was required to completely specify the state of an electron. If a state was so specified, only one electron could occupy it. This "exclusion principle" solved the remaining problems in the atomic spectra. In 1925 George Uhlenbeck (1900-1988 ) and Samuel Goudsmit (1902-1978) provided a physical model for the fourth quantum number, with a value of either +½ or -½, in their hypothesis of electron spin.

In 1928 Pauli moved to Zurich, Switzerland, in order to accept an appointment as professor of theoretical physics at the Federal Institute of Technology. He soon began attempting to uncover the reason for the wide variation in the energies of particles emitted during beta decay. "Missing" energy was a major problem, because this would contradict the conservation laws of physics. In 1931 Pauli proposed that a previously unknown particle, neutral in charge and with negligible mass, carried away the energy that was not otherwise accounted for. Enrico Fermi (1901-1954) coined the term neutrinos for these particles. Evidence of their existence was first observed by Frederick Reines (1918-1998) and C. L. Cowan (1919-1974) in 1953. The neutrino is difficult to detect because it rarely interacts with other matter.

Pauli was known for the clarity of his scientific publications and the influence of his extensive correspondence. During his time in Zurich, he helped build the Federal Institute of Technology into a major research center in theoretical physics. In 1940 he was appointed to the Institute for Advanced Study in Princeton, New Jersey, and began the process of obtaining United States citizenship, which was finalized in 1946. However, with World War II over, he returned to Zurich, where he died on December 15, 1958.