What holds the world together at its core – Zurich and the birth of quantum mechanics

Around 1920, physics was in deep crisis. Seemingly eternal laws of physics suddenly failed as soon as they were applied in the atomic sphere. The behaviour of these spheres appeared to obey completely different, previously unexplored rules.

This new area of physics, which would soon become known as quantum mechanics, required contrarian and sometimes crazy-seeming thinkers. In constant dialogue (and frequent dispute), these physicists came up with the theory’s fundamental concepts, which are still used today, before the Nazi regime tore the group apart.

It comes as no surprise that Zurich played a leading role in this process, as the city has boasted 18 Nobel Prize winners in physics throughout its history. Erwin Schrödinger came up with his ‘Schrödinger equation’ here, which helped to establish modern quantum mechanics in 1925.

But it was two friends, Wolfgang Pauli and Gregor Wentzel, who turned Zurich into the international centre for quantum mechanics in the years that followed – with Pauli at ETH and Wentzel at the university.

Wolfgang Pauli

Background

Born in Austria in 1900, Wolfgang Pauli was one of the most prominent physicists. He came from a Jewish family from Prague, who settled in Vienna and baptised their son as a Catholic there. Pauli only learned of his Jewish heritage many years later.

Gregor Wentzel

Background

Gregor Wentzel was born in Düsseldorf in 1898 to a bourgeois, Catholic family. Gregor’s physics teacher inspired the young boy to build a telescope to observe the stars and planets. From that point on, Wentzel dreamed of becoming an astronomer – much to the disapproval of his family, who wanted their son to become a banker or lawyer.

Education

Pauli was identified early on as a child prodigy and obtained his doctorate at just 21 years old. Albert Einstein became aware of Pauli after he wrote an almost 250-page article on Einstein’s theory of relativity alongside his studies.

Education

Wentzel decided to study physics. He obtained his doctorate in Munich in 1921 and completed his habilitation in 1922. This is also where he met Pauli for the first time and befriended him. Their friendship lasted right up until the end of Pauli’s life.

Research

One of Pauli’s most famous achievements is his hypothesis on the existence of neutrinos, which could only be confirmed by experiments 26 years later. His ‘Pauli principle’ became one of the most important laws of quantum mechanics and had far-reaching implications for our understanding of the structure and stability of matter.

Research

Wentzel developed the WKB (Wentzel-Kramers-Brillouin) approximation, which can be used to approximate energy levels and wave functions of quantum-mechanical systems. In addition to other important works, he wrote the first detailed presentation of quantum field theory, the English translation of which soon became a standard text and had a significant impact on university teaching in the USA. Even decades later, many physics professorships in Switzerland and the USA were held by Wentzel’s students.

Honours

Pauli received invitations to conferences, guest lectures and talks all over the world. He acted as the ‘conscience of physics’. Many leading physicists, such as Werner Heisenberg, didn’t dare to publish work without first subjecting it to Pauli’s critical judgement. Throughout his life, Pauli corresponded with the leading physicists of his time, such as Niels Bohr, Albert Einstein, J. Robert Oppenheimer, Max Born, Paul Dirac, Otto Stern and Enrico Fermi.

Honours

Wentzel received, among other things, the Max Planck Medal, an honorary doctorate of ETH and membership of the National Academy of Sciences. He was President of the Swiss Physical Society from 1945 to 1947. The University of Chicago still awards a ‘Wentzel Research Prize’ each year to particularly outstanding physics students.

Other activities

Notably gregarious, Pauli spent evenings with writers such as James Joyce and Friedrich Dürrenmatt. The latter was fascinated by men like Pauli and incorporated their conversations into what is probably his most famous work, ‘The Physicists’ . Pauli also stayed in close contact with Carl Gustav Jung for almost 30 years. Both influenced each other’s way of thinking.

Other activities

Wentzel was involved in Zurich’s secondary schools and conducted exams there. Just a few months after his arrival, he was elected vice president of the Physics Society of Zurich, and then unanimously as its president in 1930. Under his leadership, the society organised several series of events, which provided the public with a simple introduction to subjects within physics. Around 200 people attended each lecture – four to five times the number of people at a regular society meeting. Decades later, Wentzel’s son organised similar courses for non-physicists, which drew up to 3,000 people each year.

Personality

Pauli’s sharp, sarcastic comments, his biting wit and sardonic smile were feared by students and physicist colleagues alike. He also showed no respect whatsoever for authority figures. According to Hans-Hennig von Grünberg, when he was a 19-year-old student he was said to have described Einstein’s statements as ‘not entirely stupid’ to a gathered audience (a rare compliment from him!).

Personality

His numerous students, co-workers and supervisors in Zurich and later the USA described Wentzel without exception as the ultimate gentleman and team player: pleasant to deal with, straightforward, serious, friendly but firm, modest and helpful. At the same time, he certainly knew how to get his wishes across to the university management (such as more teaching hours for quantum mechanics). 

Death and legacy

In 1958, Pauli had to cut short a lecture after suffering from severe stomach pains. He died of pancreatic cancer a few days later in the now closed Rotkreuzspital at Gloriastrasse 18. His grave is in Zollikon, where he lived for a long time.

Death and legacy

Following his retirement in 1969, Wentzel returned to Zurich from time to time to attend physics seminars and colloquiums. He is buried together with his wife and son in Ascona, where he loved to spend holidays and even lived in old age. His granddaughter Tania duBeau now runs a school in Maryland. 

Although Zurich certainly recognised the ground-breaking innovations in the field of quantum mechanics at the time, their practical applicability was still difficult to assess. Theoretical physics was seen as an unprofitable ancillary science to useful experimental physics.

Back then, the university’s physics institute consisted of two rooms for the two professors, two lecture halls for the few students, two practical rooms, a small library, a workshop and a few cellar-like offices for the private lecturers. Wentzel was solely responsible for theoretical physics. ETH gave Pauli an assistant, but this was an exception throughout Switzerland.

Pauli asked only the following of his assistants: Your job is to contradict me with detailed arguments whenever I say something. To the left of Pauli is his assistant Markus Fierz. (Image: CERN, Pauli archive)

The large physics lecture hall at ETH, 1928. (Image: ETH Library Zurich, image archive)

Physics students in front of ETH’s physics institute, around 1920. (Image: ETH Library Zurich, image archive)

Practical beginner’s course in physics, 1930. (Image: ETH Library Zurich, image archive)

ETH and the university ran their physics colloquium and theoretical seminar together.

Some students joked that Pauli’s lectures could only be understood by those who had previously studied the subject with Wentzel. The latter spoke clearly, elegantly and completely freely. He had an exceptional talent for expressing complex subject matter in a way that made everyone listening understand.

‘I have no idea what I was thinking about there. Oh well, it’ll be fine.’

Pauli describing his notes just a few minutes before the start of a lecture, as quoted by his assistant Markus Fierz.

Pauli’s course materials were notoriously unstructured, if available at all. He preferred to dedicate his time to his research and correspondence with other physicists around the world. During his time in Germany, holding a lecture at 11 am was an unreasonable demand, so much so that his boss regularly had to send round a housekeeper to get Pauli out of bed on time.

Both men were key to Zurich’s prominence as a centre for quantum mechanics: Wentzel, whose lectures and organisation laid the foundations for teaching and fostering young talent in the decades that followed, and Pauli, whose conviviality and fame drew the attention of researchers to Zurich.

Wentzel invested a lot of time in preparing and delivering his lessons. The picture shows the first page of his lecture on quantum theory, 1932. (Image: ETH Zurich University Archives)

A lecture by Wentzel providing an introduction to the quantum theory of wave fields, 1942. (Image: ZB Zürich, Gregor Wentzel estate)

Pauli wouldn’t tolerate any errors or inaccuracies – least of all his own. At the same time, however, he showed patience with supposedly stupid questions (‘because all questions were stupid to Pauli,’ said his assistant). (Image: CERN, Pauli archive)

At the time, physics in Zurich was already very international. For example, Pauli and Wentzel supervised Homi J. Bhabha, the name behind the famous Bhabha scattering process and father of the Indian nuclear programme. (Image: Credit: TIFR Archives)

In the beginning, Pauli and Wentzel felt at home in Zurich. Pauli enjoyed swimming in Lake Zurich and liked eating his lunch in his bathing suit together with his assistant. He would spend his afternoons at Sprüngli Gelateria on Paradeplatz, followed by evening concerts at the concert hall.

Afterwards, Paul Scherrer, Pauli and his assistant would let loose in Zurich’s numerous nightclubs and bars. Popular meeting places included Mary’s Oldtimer Bar, the Odeon on Bellevue, the Schifflände Bar on Limmatquai and Cabaret Voltaire, the birthplace of the Dada movement. Sometimes Pauli would just drink schnapps with Wentzel until midnight.

However, Pauli’s early years in Zurich were not his happiest. His mother took her own life at the end of 1927. Although Pauli rejected marriage as an institution, he hastily married a dancer, who left him shortly after for a chemist named Goldfinger (‘If she had chosen a bullfighter... but such an ordinary chemist!’). He drank too much, got into fights and was thrown out of bars.

In a rare moment of emotional openness, Pauli confessed his loneliness and fear of dying alone to Wentzel. ‘The constant soliloquising is so tiring.’ The following year, Pauli underwent treatment with Carl Gustav Jung, who diagnosed him with a disturbed relationship with women and his feelings.

Little is known of Wentzel’s private life. According to his wife, most of his letters were destroyed. He also refused to record conversations. As a young man, he used every excuse he could find to travel from Germany to Paris to see his girlfriend (or rather, as Pauli suggested, to party at the Moulin Rouge). He got married at 31. Five years later, Donat Wenzel was born.

Pauli teaching with a broken shoulder, which he suffered when he fell down the stairs while drunk. The official explanation was a sporting accident. (Image: CERN, Pauli archive)

Therapy was successful. Pauli married for a second time in 1934 and had a childless, mostly happy marriage with his wife Franca up until his death. (Image: CERN, Pauli archive)

Pauli’s mother Bertha worked as a journalist right up until her death and was committed to women’s rights and pacifism. (Image: © A. Huber, Vienna / CERN, Pauli archive)

Donat inherited Gregor Wentzel’s love of astrophysics and his talent for teaching. (Image: AIP Emilio Segrè Visual Archives, John Irwin Slide Collection)

Wentzel with his wife Anna and Hideki Yukawa, the first Japanese winner of the Nobel Prize in Physics. (Image: AIP Emilio Segrè Visual Archives, gifted by Donat Wentzel)

Wentzel’s only vice was chain smoking Swiss Stumpen (later Cuban cigars). (Image: Photograph by J. D. Jackson, courtesy of AIP Emilio Segrè Visual Archives, Jackson Collection)

As several European physicists expressed great difficulty in dealing with J. Robert Oppenheimer, Paul Ehrenfest sent him to Pauli in 1928 to ‘kindly knock him into shape a bit’. Years later, Pauli came face to face with his former student under far more serious circumstances. Oppenheimer was now leading the Manhattan Project, while Pauli was one of many Jews who had fled to the USA.

Quantum mechanics was seen at the time as ‘Jewish physics’. The early onset of discrimination saved the lives of many researchers, as they were able to leave Germany in time. From Zurich, Pauli helped his colleagues to resettle in Britain or the USA, before he and his wife travelled there themselves in 1940.

ETH saw Pauli’s move to the USA as a kind of desertion. The universities wanted to transfer Pauli’s courses and ETH professorship to Wentzel and stop employing him. Wentzel resolutely refused to accept the professorship, believing that his friend would soon return. This assessment proved to be right.

All is well with the world here: Pauli (left) with Enrico Fermi (right), the ‘architect of the atomic bomb’, and Werner Heisenberg (middle), a principal scientist in the Nazi’s uranium project, 1927 at Lake Como. (Image: © Franco Rasetti / Cern, Pauli archive)

Before he fled the country, Pauli tried several times to become a citizen. To the Swiss, he was seen as an ‘unassimilable Eastern Jew’. Caricature by biophysicist Eugene Rabinowitch. (Image: Courtesy of Emilio Segrè Visual Archives, Weisskopf Collection)

Leopold Ružička, winner of the Nobel Prize in Chemistry, recommended Pauli for citizenship, but most of his ETH colleagues were of a different opinion. (Image: ETH Library Zurich, image archive)

ETH geoscientist Paul Niggli was also prepared to vouch for Pauli’s successful integration. (Image: Franz Schmelhaus / ZB Zürich)

Wentzel decided to apply for Swiss citizenship too. Despite his broken Swiss German, his application was approved. (Image: Zurich State Archives, StAZH Z 1139.1112)

Pauli and Chien-Shiung Wu, the ‘Chinese Marie Curie’. She played a key role in the Manhattan Project and liked to call Oppenheimer ‘Oppie’. (Image: AIP Emilio Segrè Visual Archives, Segrè Collection)

During the Second World War, Pauli was one of the few physicists in the USA who wasn’t involved in any way in constructing the atomic bomb (shown here with students at the University of Michigan, 1941). (Image: Ivory Photo, Ann Arbor, courtesy of AIP Emilio Segrè Visual Archives, Goudsmit Collection)

Pauli’s former ETH assistants Victor Weisskopf, Rudolf Peierls and Felix Bloch (1952 Nobel Prize) worked on the Manhattan Project. The picture shows Victor Weisskopf (right) in 1936. (Image: Photograph by Paul Ehrenfest Jr., AIP Emilio Segrè Visual Archives, Weisskopf Collection)

Hans Bethe (seated on the right), who turned down a job as Pauli’s assistant, became head of the theoretical division of the Manhattan Project (shown here with Victor Weisskopf and Enrico Fermi in Los Alamos, standing on the left and right). (Image: Photo by Emilio Segrè, courtesy of AIP Emilio Segrè Visual Archives, Segrè Collection)

When Pauli won the Nobel Prize in 1945, and Princeton offered him the position as Einstein’s successor, Zurich’s objections to him vanished into thin air. He finally received his long-awaited Swiss citizenship and remained at ETH Zurich.

To his disappointment, in 1948 Wentzel left Zurich for the USA. He taught at Stanford, Berkeley and ‘Mecca of physics’ at that time, the University of Chicago, where he worked together with Enrico Fermi and other leading physicists.

Quantum mechanics brought about revolutionary technical developments in the decades that followed – and still does today. The University of Zurich is currently researching quantum materials through scattering experiments, for which Wentzel laid the first theoretical foundations. Other focus areas include quantum computers and the potential of quantum effects for data storage. ETH is working together with the Paul Scherrer Institute on the topics of quantum information processing, quantum simulation and quantum sensors. The impact of quantum mechanics on our everyday lives has never been greater.

Wentzel, along with other physicists, put together a Festschrift to mark Pauli’s 60th birthday. As Pauli died unexpectedly, the birthday present became a commemorative book. The image shows the ’Salle Pauli’ room at CERN with the commemorative book. (Image: © CERN)

The first quantum computer system for commercial use. It should enable the use of quantum computers beyond lab facilities. (Image: © IBM Research / ITWM)

Some maglev trains use superconductors for their magnetic systems. Superconductivity is a phenomenon of quantum mechanics and a research area of the university and ETH Zurich. (Image: Chainwit / Wikimedia Commons)

Quantum communication via glass fibres could lead to significant improvements in internet security in future. (Image: Infestor / Wikimedia Commons)

Irina Morell, historian and employee at the University Library Zurich, natural sciences

January 2024

From 27 June 2025, the Physics Institute of the University of Zurich is showing the exhibition Quantum Century - Zurich and the birth of quantum mechanics.

Header image: Pauli at a lecture in Copenhagen, 1929. (© Goudsmit / CERN, Pauli archive)

References 

Unless otherwise noted, all quotations are taken from the source edition of Wolfgang Pauli: Scientific Correspondence With Bohr, Einstein, Heisenberg and Others and from Markus Fierz ‘Rückblick vom Hönggerberg (April 1975)’ in Naturwissenschaft und Geschichte.

The author also drew on the following books: Wolfgang Pauli und sein Wirken an der ETH Zürich, No Time to Be Brief: A Scientific Biography of Wolfgang Pauli and Einstein und Co.: Nobelpreisträger in Zürich.