Wolfgang Pauli

Wolfgang Pauli was an Austrian theoretical physicist noted for his work on spin theory and quantum theory, and for the remarkable discovery of the Pauli exclusion principle, which underpins the structure of matter and the whole of chemistry.

Education and Influences

Wolfgang Ernst Pauli was born on 25 April 1900 in Vienna, Austria (then Austria-Hungary). He was raised as a Roman Catholic, although both his parents came from prominent Jewish families, and his godfather was the noted Austrian physicist and philosopher Ernst Mach. He was something of a youth prodigy, graduating with distinction from the Döblinger-Gymnasium in Vienna in 1918, despite paying scant attention to his classes and publishing his first paper (on Albert Einstein’s General Theory of Relativity) just two months after graduation. He attended the Ludwig Maximilian University in Munich, working under Arnold Sommerfeld, and he received his Ph.D. there in 1921 for his thesis on the quantum theory of ionized molecular hydrogen. His 1921 monograph on the Theory of Relativity remains a standard reference on the subject to this day.

Pauli spent 1922 at the University of Göttingen as the assistant to Max Born, and 1923 at Niels Bohr’s Institute for Theoretical Physics in Copenhagen. From 1923 to 1928, he lectured at the University of Hamburg, during which time he was instrumental in the development of the modern theory of quantum mechanics, particularly his formulation of the exclusion principle and the theory of non-relativistic spin.

Contributions and Impact

In 1924, he had proposed a quantum number for the “spin” of electrons, with two possible values, “up” and “down.” He then extended this to formulate the Pauli exclusion principle in 1925, perhaps his best known and most important work, which states that no two electrons (or, technically, fermions) can exist in the same quantum state (i.e., no two electrons in an atom can have the same four quantum numbers).

By the end of 1925, he had used Heisenberg’s newly developed matrix theory of quantum mechanics (which Pauli saw as the only way forward for quantum physics) to derive the observed spectrum of the hydrogen atom, which was an essential step in securing credibility for Heisenberg’s theory. He went on to use a 2 × 2 matrix as a basis of spin operators, thus solving the non-relativistic theory of spin.

Pauli seldom published papers, preferring lengthy correspondences with colleagues such as Niels Bohr and Werner Heisenberg, with whom he had close friendships. Many of his ideas and results were therefore never actually published and appeared only in his letters, which were often copied and circulated by their recipients, with Pauli apparently unconcerned that much of his work thus went uncredited. He earned a reputation as a perfectionist, both concerning his own work and that of others, but he could also appear arrogant and was particularly scathing about theories which were untestable or unevaluatable, famously dismissing one such idea with: “Not only is it not right, it’s not even wrong.”

In 1928, Pauli was appointed Professor of Theoretical Physics at the Swiss Federal Institute of Technology in Zürich, where Einstein had earlier studied and taught. In 1929, (two years after his mother, to whom he had been very close, committed suicide), he officially left the Roman Catholic Church, and later that year, he married Käthe Margarethe Deppner. The marriage was an unhappy one, however, ending in divorce after less than a year.

In 1930, while considering the problem of beta decay, he proposed the existence of a hitherto unobserved neutral particle with a small mass (less than 1% the mass of a proton), which he initially proposed to call the “neutron”, but which became better known under the name given to it later by Enrico Fermi, the “neutrino”. Neutrinos were finally confirmed experimentally in 1956 (the neutron which makes up part of the nucleus of an atom had been discovered in the meantime by James Chadwick in 1932).

Pauli was awarded the Lorentz Medal for outstanding contributions to theoretical physics in 1931, but his failed marriage and the strain of his work resulted in a nervous breakdown around this time. He began drinking heavily and had consultations with the psychologist Carl Jung, who also lived near Zürich. He re-married in 1934 to Franciska Bertram, and this marriage was much more successful. Franciska proved a great support to him over the years, although they were to have no children.

In addition to his post at the Swiss Federal Institute of Technology, Pauli also held visiting professorships in the United States, at the University of Michigan in 1931, and at the Institute for Advanced Study at Princeton, New Jersey in 1935. The German annexation of Austria in 1938 made him a de facto German national, which became a difficulty with the outbreak of World War II in 1939, and made the decision to move to the United States in 1940.

From 1940, he held the position of Professor of Theoretical Physics at Princeton (during which time he showed that particles with half-integer spin are fermions, while particles with integer spin are bosons). This was followed by more visiting professorships at the University of Michigan in 1941 and at Purdue University in 1942. In 1945, he received the Nobel Prize in Physics (nominated by Albert Einstein) for his “decisive contribution through his discovery in 1925 of a new law of Nature, the Exclusion Principle or Pauli Principle”.

After the end of the War, in 1946, he became a naturalized citizen of the United States, but then returned to Zürich, where he mostly remained for the rest of his life. During the last ten to fifteen years of his life, Pauli spent much time studying the history and philosophy of science. He was elected a Fellow of the Royal Society of London in 1953, and he was also elected a member of the Swiss Physical Society, the American Physical Society and the American Association for the Advancement of Science.

In 1958, Pauli was awarded the Max Planck Medal for extraordinary achievements in theoretical physics.

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