Wolfgang Ernst Pauli

Article by: J J O'Connor and E F Robertson (University of St. Andrews, Scotland)

Wolfgang Ernst Pauli

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Born: 25 April 1900 in Vienna, Austria
Died: 15 Dec 1958 in Zurich, Switzerland

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Wolfgang Pauli was the son of Wolfgang Joseph and Berta Camilla Schütz. Wolfgang Joseph had trained as a medical doctor in Prague. After qualifying, he practiced as a doctor in Vienna and quickly became popular. In 1898 he changed his name to Wolfgang Joseph Pauli and, in the following year, converted from Judaism to become a Roman Catholic. He married Berta Schütz in May 1899 but by this time he had given up his medical practice for research in chemistry and physics, becoming a university professor.

Wolfgang Joseph had been inspired to study science by Ernst Mach, and when his first child was born he named him Wolfgang Ernst Pauli, giving him the middle name of Ernst in honour of Mach. Not only did Pauli's middle name come from Mach, but Mach was also his godfather giving him a silver cup when he was christened on 31 May 1900.

Wolfgang attended school in Vienna where he began a deep study of mathematics and physics at the Döblingen Gymnasium. He was certainly not a typical pupil for he read Einstein's papers on relativity while he was still at the Gymnasium. School work was boring to the brilliant Pauli and he hid Einstein's papers under his school desk and studied them during the lessons. Not paying attention in class didn't hold Pauli back, for he graduated from the Gymnasium in July 1918 with distinction.

After leaving the Gymnasium he entered the Ludwig-Maximilian university of Munich. Within two months of leaving school he had submitted his first paper on the theory of relativity. While still an undergraduate at Munich he wrote two further articles on the theory of relativity. At Munich, Pauli was taught by Sommerfeld who quickly recognised his genius. Sommerfeld asked Pauli to write a review article on relativity for the Encyclopädie der mathematischen Wissenschaften when he had only been two years at university, a mark of the high regard in which he held Pauli. The respect was mutual, for Pauli showed more respect for Sommerfeld both as a person and a scientist than he did for any other.

Pauli, writing about his days as a student at Munich, wrote (see the extracts from Pauli's Nobel Prize lecture in 1945 given in [16]):-

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.

He wrote his first paper on quantum physics in June 1920, a work on the magnetic properties of matter. The year 1920 was when Heisenberg arrived in Munich, also to become a student of Sommerfeld. In [20] Pais quotes from Heisenberg's description of Pauli's way of life at this time:-

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.

Pauli received his doctorate, which had been supervised by Sommerfeld, in July 1921 for a thesis on the quantum theory of ionised molecular hydrogen. In his report on the thesis Sommerfeld wrote that it showed:-

... like his many already published smaller investigations and his larger encyclopedia article, the full command of the tools of mathematical physics.

Sommerfeld was certainly right to heap much praise on the thesis but it had been a disappointment to Pauli since the theoretical results he had proved did not agree with experimental evidence. Looking at it now one can see that it showed that quantum theory, as then formulated, was not in itself going to provide the necessary structure on which to build a logical theory of atomic structure which agreed with experimental evidence.

Two months after the award of his doctorate Pauli's survey of the theory of relativity appeared, by this time having grown into a work of 237 pages. His genius was immediately recognised by Einstein who, after reading Pauli's monograph on relativity, wrote a review [20]:-

Whoever studies this mature and grandly conceived work might not believe that its 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.

Pauli was then appointed to Göttingen as Born's assistant from October 1921. It was in Göttingen that he first met Niels Bohr in person and he said (see for example [16]):-

... 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 Göttingen 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 eagerly accepted the invitation and spent the year 1922-23 at Bohr's Institute [16]:-

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', ... a type of splitting of the spectral lines in a magnetic field which is different from the normal triplet.

In 1923, Pauli was appointed a privatdozent at Hamburg [16]:-

Very soon after my return to the university of Hamburg, 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 1924 Pauli proposed a quantum spin number for electrons. He is best known for the Pauli exclusion principle , proposed in 1925, which states that no two electrons in an atom can have the same four quantum numbers. Less than a year after this Heisenberg submitted his article on quantum mechanics which was to change the whole approach to the topic. Pauli, who before that had begun to feel that further advances could not be made with the theory as it then existed, quickly made progress using Heisenberg's new ideas and before the end of 1925 he had derived the hydrogen spectrum from the new theory.

The year 1927 saw personal tragedy for Pauli when his mother, to whom he had been very close, committed suicide. In the following year his father remarried making an even more unhappy situation for Pauli who referred to his father's new wife as "the evil step-mother". On 6 May 1929 Pauli left the Roman Catholic Church, but his reasons for this are not entirely clear. Further unhappiness was to follow when he married Käthe Margarethe Deppner in Berlin on 23 December 1929. The marriage was never a success even in the first few months and they were divorced in Vienna on 29 November 1930.

Despite the personal problem, Pauli's career progressed well. In 1928 he was appointed Professor of Theoretical Physics at the Federal Institute of Technology in Zurich and soon made some remarkable progress. He predicted mathematically, in 1931, that conservation laws required the existence of a new particle which he proposed to call the "neutron". He first mentioned his theoretical evidence for this particle in a letter written on 4 December 1930 and his public announcement came at a conference in Pasadena on 16 June 1931. The New York Times of 17 June reported:-

A new inhabitant 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 or entities which he christened "neutrons".

The existence and properties of the particle were still not clear to Pauli, however, and it was not until 1933 that he published his prediction in print. At that time he made the claim, for the first time, that the particle had zero mass. The particle which we now know as the neutron had been discovered by Chadwick in 1932. Pauli's particle was named the neutrino by Fermi in 1934 and at that time he correctly stated that it was not a constituent of the nucleus of an atom. It was later found experimentally.

This period of scientific discovery by Pauli coincided with a period of increasing personal difficulties for him. Perhaps as a consequence of his disastrous marriage, he began drinking and as a result consulted the psychologist Carl Gustave Jung. He was not treated by Jung, rather one of his assistants helped Pauli. However, Pauli detailed over 1000 dreams which he sent to Jung over many years and Jung published work based on some of the dreams. Pauli clearly believed in psychology as much as he did physics. He wrote later in his life in a letter to Pais (see for example [20]):-

It is my personal opinion that in the science of the future reality will neither be "psychic" nor "physical" but somehow both and somehow neither.

Things went better for Pauli after he married Franciska Bertram on 4 April 1934. In contrast with his first disastrous marriage his second marriage proved a great support to him. After his death Franciska Pauli said this of her late husband:-

He was very easily hurt and therefore would let down a curtain. He tried to live without admitting reality. And his unworldliness stemmed precisely from his belief that this was possible.

In 1931 Pauli was visiting Professor at the University of Michigan, then in 1935-1936 he was visiting Professor at the Institute for Advanced Study, Princeton. He returned to Zurich but after the Second World War broke out in 1939 he found himself in an awkward situation since Germany having annexed Austria in 1938 had made him a German citizen. In 1940 he was greatly relieved to receive an offer from Princeton and he was appointed to the chair of theoretical physics there, spending 1941 as visiting Professor at the University of Michigan, and 1942 as visiting Professor at Purdue University. Pauli worried that fascism might bring about the end of scientific life in Europe. For this reason he actively encouraged scientific developments in the United States and also in the Soviet Union. He was keen to participate in conferences in the Soviet Union, attending the All-Union physics conference in Odessa in 1939 and the All-Union physics conference in Moscow in 1937. Pauli also tried to encourage those scientists who could remain in Italy and Germany to do so, for he believed this might ensure that scientific culture survived after the War. Pauli did not remain in the United States but he returned to Zurich after World War II. It was not an easy decision for him but basically he always felt European and never quite felt that he fitted in the United States.

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.

He had been nominated for the prize by Einstein. He did not go to Stockholm for the prize ceremony in 1945 but there was special ceremony at Princeton for him on 10 December. In Stockholm Professor I Waller delivered a presentation speech in Pauli's absence. He explained the importance of the exclusion principle:-

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 a "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.

The spin proposal, which gave meaning to Pauli's fourth quantum number, which first suggested by Uhlenbeck in 1925. Pauli delived his Nobel Lecture in Stockholm on 13 December in the following year.

In [19] Laurikainen writes about other directions which Pauli's work took him in the years following World War II:-

During the last 10-15 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 to natural science.

Pauli received many honours for his work in addition to the Nobel Prize. 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. He was awarded the Lorentz Medal in Amsterdam in October 1931.