Pauli Wolfgang Ernst
The Conscience of Twentieth-Century Physics


“Pauli was a physicist much respected by his colleagues for his deep insight into the newly emerging quantum theory…His name is mainly linked with two substantial achievements. The first, formulated in 1924, is known as the Pauli exclusion principle…With this principle the distribution of orbital electrons at last became clear, that is, they could be explained and predicted in purely quantum terms…Pauli’s second great insight was in resolving a problem in beta decay—a type of radioactivity in which electrons are emitted by the atomic nucleus.”
The Oxford Dictionary of Scientists

Pauli had a caustic wit; he was not a good lecturer and he was notoriously bad as an experimentalist; but he is one of the giants of 20th-century theoretical physics.
The Cambridge Dictionary of Scientists

“Pauli was one of the great theoretical physicists of his generation. As an originator of simple yet penetrating ideas he was perhaps the greatest. He was also a man of great charm, known personally to most of his colleagues, and held in affection as well as admiration by very many—young as well as old. He attained in his later years to a position approaching that of an oracle—one quick of opinion, yet happy as well as willing to change his mind when new facts had been discovered.”
Times obituary

Wolfgang Ernst Pauli was one of the most outstanding physicists of the twentieth century. In his work he was a perfectionist. He extended his rigorous criteria not only to his own work but also to the work of his friends as well. He was a great critique. This earned him the title of “conscience of Physics”. David C. Cassidy wrote: “Pauli was called “the conscience of physics” for his mastery of theoretical physics and his critical assessment of ongoing work. His review articles and books on relativity theory, statistical mechanics, and quantum physics are masterpieces of physical insight.” Pauli helped to lay the foundations of the quantum theory of fields. Pauli demonstrated that a fourth quantum number called ‘spin’ quantum number, taking on only half-integer values, was required to describe the state of an atomic electron. However, Pauli’s most significant contribution to physics was his exclusion principle, which states that no two electrons in an atom can exist in exactly same state, with the same quantum numbers. This enabled to give a clear quantum description of electron distribution within different atomic energy states. Pauli, and independently Arnold Sommerfeld, devised an atomic model that explained the electrical and thermal properties of metals. While explaining the process of radioactive beta-decay, Pauli was the first to recognize the existence of neutrino, an uncharged and almost massless particle which carries off energy in radioactive beta decay. Pauli also contributed in completing Dirac’s quantum electrodynamics. His other major contributions dealt with the electric properties of metals and with the relation between spin and statistics for elementary particles.

Pauli did not take much pain in publishing his ideas and results. Instead he announced them in lengthy correspondences with colleagues, with whom he had close friendships. He particularly undertook lengthy correspondences with Bohr and Heisenberg. It was not surprising that because of non-publishing, much Pauli’s work went un-credited. It was also true that the correspondences were often copied and circulated.

Wolfgang Ernst Pauli was born on April 25, 1900 to Wolfgang Joseph and Berta Camilla. In 1898, Wolfgang Joseph changed his name to Wolfgang Joseph Pauli. Wolfgang Joseph was originally a Jew but converted to a Roman Catholic. He was a qualified physician and he had earned a good name within a short span of time but later he gave up his medical practice for research in chemistry and physics and he went on to become a university professor in chemistry. Pauli’s father was influenced by his friend Ernst Mach, to study science. Pauli’s middle name “Ernst” was given by his father in honour of Ernst Mach. Pauli was also greatly influenced by Mach, who also happened to be his godfather. Pauli thought that his acquaintance with Mach was ‘the most important event’ that happened in his intellectual life. Pauli later wrote: “Among my books there is a somewhat dusty case; it contains a silver cup made in the art nouveau style and in there is a card…written in old-fashioned, adorned letters: ‘Dr. E. Mach, Professor at the University of Vienna’….[He] had kindly agreed to assume the role of my godfather …. (and) the result seems to be that I was baptized in this way to (become more) anti-metaphysical than Catholic…[That] cup…remained the symbol of aqua permanens (holy water), which drives away the evil metaphysical spirits.”

Pauli completed his schooling at the Doblingen Gymnasium. He was not an ordinary student. He read Einstein’s paper on relativity while still he was in the Gymnasium. He graduated from the Gymnasium in July 1918 with distinction and then joined the Ludwig-Maximilian University at Munich. He submitted his first paper on the theory of relativity within two months of leaving school. While pursuing his undergraduate studies he wrote another two papers on the theory of relativity. Among his teachers at Munich was Sommerfeld, who had great regard for extraordinary talent of Pauli. Sommerfeld asked Pauli to write a review article on relativity for the prestigious Encyclpaedia der mathematischen Wissenschaften (German Encyclopadia of Mathematics). Pauli completed his review article two months after receiving his PhD. The article, which ran into 233 pages, was published as a monograph. The work remains a standard reference to this day. Einstein himself appreciated the report very much. He said: “Whoever studies this nature, grandly conceived work would not believe that the author is a twenty-one year old man. One wonders what to admire most, the psychological understanding for the development of ideas, the sureness of mathematical deduction, the profound physical insight, the capacity for lucid, systematical presentation, the knowledge of the literature, the complete treatment of the subject matter, or the sureness of critical appraisal.”

Heisenberg commenting on Pauli’s way of life in his student days at Munich wrote: “Wolfgang was a typical night bird. He preferred the town, liked to spend evenings in some café, and would thereafter work on his physics with great intensity and great success. To Sommerfeld’s dismay he would therefore rarely attend morning lectures and would not turn up until about noon.”

At Munich, Pauli came in contact with Heisenberg and both became friends. They developed a life-long professional collaboration. They encouraged each other to make most significant contributions in quantum physics. They undertook an extensive correspondence and today this is an important source of history of twentieth century physics. Cassidy wrote: “Like most quantum physicists of the 1920s, Heisenberg and Pauli were products of the European upper-middle-class cultural elite. Both of their fathers were professors: Pauli’s, a professor of colloid chemistry at the University of Vienna; Heisenberg’s, a professor of Byzantine philology at the University of Munich. Both sons attended outstanding humanistic gymnasia (high schools), which emphasized classical languages and literature. Both were attracted to physics by the excitement attending Albert Einstein’s theory of relativity, and both earned doctorates in theoretical physics with Arnold Sommerfeld in Munich. Pauli arrived at the University of Munich in 1918, two years ahead of Heisenberg. Heisenberg’s early encounter with Pauli, then in his last semester, helped to turn Heisenberg toward atomic theory.”

Further he continues: “Heisenberg and Pauli entered their profession at a time of great ferment. New data and analyses indicated the inadequacy of the planetary quantum model of the atom developed earlier by Bohr and Sommerfeld. At the same time, turmoil surrounding the German defeat in World War I, hyperinflation, and a boycott of German science made professional advancement difficult for young physicists.” Commenting on his student days at Munich, Pauli in his Nobel Lecture, said: “I was not spared the shock which every physicist accustomed to the classical way of thinking experienced when he came to know Bohr’s basic postulate of quantum theory for the first time.”

Pauli was awarded PhD in 1921. In his report on the thesis Sommerfeld wrote: “…like his many already published smaller investigations and his larger Encyclopaedia article, the full command of the tools of mathematical physics.”

In October 1921 Pauli was appointed as Max Born’s assistant at the University of Gottingen. He spent about a year at Gottingen. It was at Gottingen, Pauli first met Niels Bohr. Recalling his first meeting with Bohr, Pauli wrote: “…a new phase of my scientific life began when I met Niels Bohr personally for the first time. This was in 1922, when he gave a series of guest lectures at Gottingen when he reported on his theoretical investigations on the periodic system of elements. During these meetings, Bohr asked me whether I could come to Copenhagen for a year.” Pauli accepted Bohr’s invitation and he spent one year (1922-23) at Copenhagen. “Following Bohr’s invitation, I went to Cpenhagen in autumn of 1922, where I made a serious effort to explain the so-called ‘anomalous Zeeman effect’, …a type of splitting of the spectral lines in a magnetic field which is different from the normal triplet.”

From Copenhagen, Pauli went to the University of Hamburg. Pauli later wrote: “Very soon after my return to the University of Hamburgh, in 1923, I gave there my inaugural lecture as privatdozent on the periodic system of elements. The contents of the lecture appeared very unsatisfactory to me, since the problem of the closing of the electronic shells had been clarified no further.” In 1928, Pauli became Professor of Theoretical Physics at the Federal Institute of Technology at Zurich. Under his guidance the institute became a great centre of theoretical physics.

In 1924, Pauli proposed that a fourth quantum number is needed in quantum theory. Niels Bohr’s atomic model proposed in 1913 was extended by Sommerfeld in 1915. According to the Bohr-Sommerfeld model, each electron orbiting the nucleus of an atom had three quantum numbers—principle quantum number (n), azimuthal quantum number (l), magnetic quantum number (m). Pauli introduced a fourth quantum number, which may take numerical values +1/2 or -1/2. This was necessary to specify electron energy states. It was later found that the two values correspond to possible values of the ‘spin’ of the electron. The concept of spin was experimentally verified in 1926. A set of four quantum numbers could give a complete picture of the quantum states of atoms.

In 1925, Pauli introduced an additional principle, called exclusion principle, for understanding the structure of all atoms with more than one electron. According to Pauli’s principle: “There cannot exist two or more equivalent electrons in the atom for which…[the values of all four] quantum numbers coincide. If the atom contains an electron for which these quantum numbers have certain values then this state is “occupied””. Thus according to this principle no two quantum states of an atom can have the same fingerprint. This principle explained why not all the electrons in an atom occupy the orbit nearest to the nucleus, where the least amount of energy is required to complete an orbit. Pauli discovered the exclusion principle as he was trying to understand the anomalous Zeeman effect, named after the Dutch Physicist, Pieter Zeeman. Pauli in his Nobel lecture, while describing how he discovered the exclusion principle, said: “The history of the discovery of the ‘exclusion principle’…goes back to my student days…(It) was at the University of Munich that I was introduced by Sommerfeld to the structure of the atom—somewhat strange from the point of view of classical physics…A new phase of my scientific life began when I met Niels Bohr personally for the first time. This was in 1922, when he gave a series of guest lectures at Gottingen, in which he reported on his theoretical investigations on the Periodic System of the Elements … Following Bohr’s invitation, I went to Copenhagen in the autumn of 1922, where I made a serious effort to explain the so-called ‘anomalous Zeeman effect’…”

In 1926, Pauli used Heisenberg’s matrix theory of modern quantum mechanics to derive the observed spectrum of the hydrogen atom. The result provided credence to Heisenberg’s theory.

In 1927, he introduced Pauli matrices as a basis of spin operators. This solved the non-relativistic theory of spin, which in turn influenced Dirac in his discovery of the Dirac equation for the relativistic electron.

In the late 1920s it was found that when a beta particle is emitted from an atomic nucleus, invariably there was some missing energy and momentum. This was viewed as grave violation of laws of conservation. To save the situation Pauli proposed that the missing energy and momentum is carried away from the nucleus by some particle. In December 1930 Pauli proposed, in an “open letter” to Meitner and Geiger, that the beta puzzle…might be solved by introducing into the nucleus a new, neutral particle: “[There is] the possibility that there could exist in the nuclei electrically neutral particles that I wish to call neutrons, which have spin ½ and obey the exclusion principle, and additionally differ from light quanta in that they do not travel with the velocity of light: The mass of the neutron must be of the same order of magnitude as the electron mass and, in any case, not larger than 0.01 proton mass. The continuous beta-spectrum would then become understandable by the assumption that in beta-decay a neutron is emitted together with the electron, in such a way that the sum of the energies of neutron and electron is constant.” Pauli did not publish his idea immediately. He first publicly defended his hypothesis at a conference in Pasadena on June 16, 1931. His announcement was reported by the New York Times on June 17, 1931. It wrote: “A new habitant of the heart of the atom was introduced to the world of physics today when Dr W Pauli of the Institute of Technology in Zurich, Switzerland, postulated the existence of particles which he christened “neutrons.” Pauli’s prediction was published in print 1933. By that time “heavy neutron” had already been discovered by Chadwick. Pauli had no clear idea about the properties of the particle. In 1934, Enrico Fermi proposed that Pauli’s particle be called a “neutrino”, which is Italian for “little neutral one.” Fermi also correctly stated that the particle was not a constituent of the nucleus as Pauli originally believed. Neutrinos are so small that they were almost impossible to detect with the technologies available at the time of their prediction. And years passed but scientists failed to prove their existence. The existence of the particle predicted by Pauli was finally experimentally verified in 1956.

In 194, Pauli proved the spin-statistics theorem. This theorem, an important result of quantum mechanics, states that particles with half-integer spin are fermions, while particles with integer spin are bosons.

In 1931, Pauli was awarded the Lorentz Medal in Amsterdam. Pauli was awarded the Nobel Prize in 1945 for his “…decisive contribution through his discovery in 1925 of a new law of Nature, the exclusion principle or Pauli principle.” Pauli was first nominated for Nobel Prize in 1933 and continued to receive nomination for eight years but not the Prize. Finally he got the Prize in 1945 after Einstein nominated him. Einstein in a telegram sent to the Nobel Committee wrote: “Nominate Wolfgang Pauli for physics prize… [His] contribution to modern quantum theory consisting in the so-called Pauli or exclusion principle became fundamental part of modern quantum physics….”

Pauli could not be present at the Prize giving ceremony at Stockholm. However, a special ceremony was organized at Princeton on December 10, 1945. In this function I. Waller delivered a presentation speech in Pauli’s absence. He said: “Pauli based his investigation on a profound analysis of the experimental and theoretical knowledge in atomic physics at the time. He found that four quantum numbers are in general needed in order to define the energy state of an electron. He then pronounced his principle, which can be expressed by saying that there cannot be more than one electron in each energy state when this state is completely defined. Three quantum numbers only can be related to the revolution of the electron round the nucleus. The necessity of a fourth quantum number proved the existence of interesting properties of the electron.

Other physicists found that these properties may be interpreted by stating that the electron has ‘spin’, i.e., that it behaves to some extent as if it were rapidly rotating round an axis through its centre of gravity.

Pauli showed himself that the electronic configuration is made fully intelligible by the exclusion principle, which is therefore essential for the elucidation of the characteristic physical and chemical properties of different elements. Among those important phenomena for the explanation of which the Pauli principle is indispensable, we mention the electric conductivity of metals and the magnetic properties of matter.

In 1925 and 1926 essential progress of another kind was made in the quantum theory, which is the foundation of atomic physics. New and revolutionary methods were developed for the description of the motion of particles.”

At personal level Pauli suffered much. Pauli’s mother committed suicide in 1927. He was much attached to his mother. This made him lonely. The situation worsened when his father remarried in the following year. He referred to his father’s new wife as “the evil step-mother.” His first marriage with Kathe Margarethe Deppner lasted less than year. After the divorce Pauli had a severe breakdown and resorted to drinking. Pauli consulted the psychiatrist and psycho therapist Carl Jung.

Pauli took keen interest in history and philosophy of science. Laurikainen wrote: “During the last 1-15 years of his life, Pauli spent much time studying the history and philosophy of science. His starting point was the philosophy of quantum mechanics, but this led him to psychology, the history of ideas and many other fields, not least the relation of religion of natural science.”

In 1944, Pauli was appointed to the chair of theoretical physics at the Institute for Advanced Study, Princeton, New Jersey and then in 1946 he became a naturalized citizen of the United States. After the Second World War was over, Pauli returned to Zurich. Pieter Zeeman. In 1953, Pauli was elected a Fellow of the Royal Society of London. He was elected Fellow of the Swiss Physical Society, the American Physical Society, and the American Association for the Advancement of Science. In 1958, Pauli was awarded the Planck Medal.

Pauli died on December 15, 1958 in Zurich.


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