A cornerstone of the modern understanding of matter, the exclusion principle garnered Pauli the Nobel Prize in Physics in 1945. According to the principle, no two electrons in an atom can share all four quantum numbers at the same time. Prior to Pauli’s work, only three quantum numbers describing the energy state of an electron were recognized. The fourth, which, as he described it, could only have one of two values, arose from his work involving the Zeeman effect and was later identified with the physical phenomenon of electron spin discovered by Samuel Goudsmit and George Uhlenbeck.
Pauli was born in Vienna on April 25, 1900. His parents were well educated and created an environment conducive to learning for their son. When Pauli’s father, Wolfgang Joseph Pauli, married Berta Camilla Schütz, he ran a medical practice, but by the time their first child was born he had begun carrying out research in biochemistry and teaching at the University of Vienna. Pauli’s mother was a writer. Their professions put them in contact with many prominent individuals, which helped instill high standards in Pauli in his youth. Ernst Mach, a well-known Austrian physicist and philosopher, was the boy’s godfather.
Pauli’s early education, including his introduction to mathematics and physics, took place in Vienna. Shortly after he graduated from Döblingen Gymnasium, his first paper, which discussed the theory of general relativity, appeared in print. He moved to Munich to study at Ludwig-Maximilian University. There, along with many other up-and-coming young scientists, he had the opportunity of learning from Arnold Sommerfeld, an exceptional German physicist. Pauli’s brilliance and skilled work on relativity inspired Sommerfeld to invite him to write the chapter on the theory of special and general relativity for the Encyclopedia of the Mathematical Sciences. The chapter, which exceeded 200 pages and has since been published as a monograph, is still considered one of the preeminent treatises of the subject. It first appeared in print in 1921, only a couple months after Pauli received his Ph.D in physics. Albert Einstein, the originator of relativity theory, commended Pauli’s article:
"No one studying this mature, grandly conceived work would believe that the author is a man of twenty-one. One wonders what to admire most, the psychological understanding for the development of the ideas, the sureness of mathematical deduction, the profound physical insight, the capacity for lucid, systematic presentation, the knowledge of the literature, the complete treatment of the subject matter, or the sureness of critical appraisal."
After receiving his degree, Pauli accepted an assistantship position from Max Born at the University of Göttingen. The following year he traveled to Copenhagen, Denmark, per the request of Niels Bohr, who invited him to work at the research institution that later came to be called the Niels Bohr Institute for Theoretical Physics. There Pauli tried to develop an explanation for the Zeeman effect, which is the splitting of a spectral line associated with a strong magnetic field first observed by Dutch physicist Pieter Zeeman. Pauli left Copenhagen after a year to teach at the University Hamburg, but he persisted in his work on the Zeeman effect.
During his early days at Hamburg, Pauli’s consideration of the Zeeman effect led to his positing the existence of a fourth quantum number that exhibits two-valuedness. Not long after, a paper by Edmund Stoner inspired him to extend his work in order to explain why all of the electrons in an atom are not able to occupy the lowest energy level. The result was the Pauli exclusion principle. In addition to theoretically accounting for the capacities of electron orbitals, the exclusion principle elucidated the periodic table of elements and a number of characteristics of matter in general. Particles that obey the Pauli exclusion principle are known as fermions; those that do not are called bosons.
Following his development of the exclusion principle, Pauli helped give credence to the matrix theory of quantum mechanics formulated by Werner Heisenberg in 1926. He did so by using matrix theory to derive the spectrum of hydrogen, which can be observed via experiment. Also in 1926, Pauli penned another important scientific treatise, a chapter on quantum theory for the Handbook of Physics. The following year he extended his work with matrix theory to spin, leading to his development of what are now termed the Pauli matrices.
Pauli left Hamburg for Zürich in 1928 to accept the chair of theoretical physics vacated by Peter Debye at the Swiss Federal Institute of Technology (ETH). In 1929, Pauli married Käthe Margarethe Deppner, but the couple divorced less than a year later. Around the same time, Pauli’s mother passed away. These events had a deep effect on Pauli, who suffered from depression severe enough to seek the help of Carl Jung, the renowned Swiss psychiatrist. Pauli married again in 1934, and ended his therapy sessions with Jung, although the two regularly corresponded for much of their lives. Pauli’s second union, with Franciska Bertram, lasted until his death. He had no children from either marriage.
During his tenure as department chair, Pauli helped make ETH one of the best research institutions in the field of theoretical physics in the world. In 1931, Pauli made another important contribution to physics: the theoretical discovery of the neutrino. The discovery arose from beta-decay research carried out in the late 1920s that indicated that a small amount of energy and momentum dissipates when an atomic nucleus emits a beta particle. This loss clearly violated the laws of conservation. In order to explain the anomaly while keeping the conservation laws intact, Pauli suggested the existence of an uncharged particle of little or no mass that conducted energy and momentum away from the atomic nucleus, thus explaining the apparent loss. Pauli’s theoretical particle later came to be called the neutrino (a term coined by Enrico Fermi) and was first discovered through experimental means in the late 1950s.
In 1933, Pauli wrote another chapter for the Handbook of Physics, this time covering wave mechanics theory. The treatise was so well received that the physics community came to refer to it as the "New Testament," and to his earlier chapter on relativity as the "Old Testament." In 1938, Pauli left ETH because Nazi-controlled Germany had annexed Austria, rendering him a German citizen. Although raised Roman Catholic, Pauli had Jewish roots, which made him a possible target of persecution by the Germans. After two failed attempts to acquire Swiss citizenship, in 1940 he immigrated to the United States and joined the faculty at Princeton University.
Pauli won the 1945 Nobel Prize in Physics for his discovery of the exclusion principle. The war, however, made it impossible for him to travel to Sweden to attend the award ceremony. To mark the occasion, Pauli’s colleagues held a ceremony for him in the United States. Albert Einstein gave the final oration, in which he declared that Pauli was his scholarly successor. The following year Pauli became a U.S. citizen. But instead of remaining in America, he returned to Zürich to once again chair the theoretical physics department at ETH, where he worked for the rest of his career.
Pauli was giving a lecture in 1958 when he suddenly experienced a bout of violent pain. He died a few weeks later of pancreatic cancer on December 15. The last hospital room that he stayed in was numbered 137, a fact that he pointed out to his visitors. The number had special significance to Pauli, who had always been intrigued and mystified by the fine structure constant, which is approximately equal to 1/137.