From waves to particles: Schrödinger and Heitler in Zurich
2025 marks the centenary of quantum mechanics. Alongside the physicists Gregor Wentzel and Wolfgang Pauli, who we have discussed previously, two other heavyweights of quantum science left their mark on Zurich: Erwin Schrödinger and Walter Heitler.
Zurich: a fateful location for two quantum pioneers
While Wentzel and Pauli were based in Zurich for many years from 1928 onwards and left a lasting impression on this research hub, Schrödinger and Heitler spent a comparatively short stint in Zurich in the 1920s. Nevertheless, the period of time they spent in Zurich marked the most important turning point in both men’s lives.
It was here that Schrödinger achieved an epochal breakthrough with his theory of wave mechanics in 1925, making him one of the most important scientists of the 20th century. Shortly thereafter, Heitler, who held a scholarship under Schrödinger, successfully applied his theory to another field, laying the foundation for the emerging discipline of quantum chemistry.
This profile takes a look at these two very different individuals, who would later be reunited by fate and whose memories are now inextricably linked with the city of Zurich.
Early years
Erwin Schrödinger (1887−1961) grew up as the only child in a well-to-do Viennese family. Thanks to his British grandmother, he was fluent in English from an early age, which later made it easier for him to flee to England to escape the Nazis.
At school, he demonstrated a great aptitude for mathematics and physics. He also loved poetry, ancient and modern languages, and drama. Even as a boy, Schrödinger was hostile to the Church and critical of the Bible, but he maintained a lifelong interest in religious, philosophical and spiritual questions.
War-related hyperinflation impoverished Schrödinger’s parents – an experience of loss that shaped his way of dealing with money and sparked his constant search for more lucrative jobs later on.
Early years
Walter Heitler (1904−1981) was born in Karlsruhe. His Bohemian father was an engineer from a poor Jewish family.
To the annoyance of his older sister Annerose, as a child Walter set up a chemical laboratory in the bathroom – and failed to keep a close enough eye on the chemical reactions. At school, he read books on the theory of relativity under the table, arousing the displeasure of his conservatively minded physics teacher. Greek and Latin dominated the curriculum, bringing Heitler into contact with Plato’s philosophy from an early age. This would stay with him throughout his life.
Studies
Schrödinger studied physics and mathematics at the University of Vienna. His dissertation was titled ‘On the conduction of electricity on the surface of insulators in moist air’.
Schrödinger’s beloved teacher Fritz Hasenöhrl, a promising Austrian theoretical physicist, suffered a fatal head wound from a piece of shrapnel in 1915. Schrödinger also served in the war, which interrupted his academic career for four years and demanded a great deal from him both physically and mentally.
Studies
Heitler’s older brother Hans studied electrical engineering, while his sister became a doctor of economics. Walter Heitler himself found it difficult to decide on his career and remained torn between mathematics and chemistry for a long time. He only came across the existence of theoretical physics as a subject because this was mentioned to him personally.
He studied chemistry at the Technical University of Karlsruhe (now the Karlsruhe Institute of Technology) and physics in Berlin and Munich, where he received his doctorate in 1926 on a problem within the field of physical chemistry.
Heitler was a student of the physicist Arnold Sommerfeld, who attached great importance to independent thinking and creativity. At least six scientists under his tutelage won the Nobel Prize.
Research
Schrödinger is primarily known for his contributions to quantum mechanics, but he also conducted research into colour theory. Like Heitler, he also enjoyed teaching and publishing on cultural history, such as the history of the philosophy of ancient Greece.
Schrödinger’s ‘What is life?’ (1944) is a classic scientific work. In it, Schrödinger expresses a belief that genes control the development of cells with some kind of code. Although not always correct in terms of content, the text influenced the trajectory of an entire generation of researchers who later created the science of molecular genetics and discovered the structure of DNA.
Research
In addition to the Heitler-London theory presented in more detail below, Heitler also developed the Bethe-Heitler formula in collaboration with Hans Bethe. This describes how fast, charged particles lose energy as they fly through matter. Together with Homi J. Bhabha, Heitler created the cascade theory, which explains the development of electromagnetic showers in the atmosphere.
Heitler’s famous book The Quantum Theory of Radiation applies quantum mechanics to describe radiation processes. It immediately won great popularity amongst researchers around the world and is considered a classic reference work.
Although Heitler mastered the most demanding mathematical methods and calculations – and demanded the same from his employees – he rarely used them when teaching. Rather, as a lecturer, it was important to him to convey physics-related relationships to his students in a comprehensible way.
Recognition
Glaciers, asteroids, lunar craters, public squares, roads, buildings and several science prizes have been named after Schrödinger. In addition to the Nobel Prize, he also received the Max Planck Medal. He was a member of the Pontifical Academy of Sciences, the British Royal Society, the Austrian Academy of Sciences and the Accademia Nazionale delle Scienze.
Recognition
Heitler received the Golden Medal from the Humboldt Society, honorary doctorates from the National University of Ireland as well as the Universities of Göttingen and Uppsala, the Max Planck Medal from the German Physical Society and the Swiss science prize Marcel Benoist. He was a member of the Royal Irish Academy, the German Academy of Sciences Leopoldina and the British Royal Society.
Heitler was also nominated for the Nobel Prize, but never won it – to the astonishment of certain contemporaries. However, he was permitted to attend the Lindau Nobel Laureate Conferences.
Personal life
Schrödinger had an open, childless marriage with his wife and had several daughters with other partners. His preference for much younger women is the subject of ongoing controversy.
He had an active lifestyle and enjoyed spending time with colleagues and friends. His hobbies included writing poetry and weaving rugs.
When it came to his work, Schrödinger was a lone wolf: he was significantly older than the other quantum mechanicists, dressed in eccentric clothing and showed no interest in training like-minded students. For his time, Schrödinger nominated an unusually large number of women for the Nobel Prize.
Personal life
Heitler’s colleagues described him as warm, helpful, understanding, humorous and kind, especially towards younger employees and students. He is said to have been reserved, sometimes almost dismissive when interacting with strangers.
Unlike Schrödinger, who essentially conducted his research alone, Heitler wrote all of his key works in collaboration with other physicists.
Away from his professional endeavours, he and his wife maintained close contact with Zurich’s artistic circles. Like Schrödinger, Heitler liked mountains and was a keen hiker and skier. This passion even influenced where he chose to live and work: his travels took him as far afield as Japan, India (as a guest professor), Russia and Columbia[A1] University in New York City.
Despite his many years spent working in Switzerland, he never acquired Swiss citizenship. Towards the end of his life, Heitler converted to Christianity.
Death
In 1961, Schrödinger died in Vienna of tuberculosis at the age of 73. He was probably already suffering from this disease when he moved to Zurich in 1921. The official records state that the cause of death was a hardening of the arteries. In line with his wishes, he was buried in his beloved mountains, specifically in Alpbach, Tyrol. His wife received condolences from Heitler, Karl Popper, Bruno Kreisky and many eminent physicists such as Paul Dirac, Werner Heisenberg, Max Born and Lise Meitner.
Death
Heitler’s faith gave him strength and comfort in dealing with the illness that marked his last years. He died at the age of 77 in November 1981; his wife survived him by almost 20 years. Heitler is buried in Witikon Cemetery.
Schrödinger at the University of Zurich
Schrödinger travelled to Zurich in 1921 to take up the Professorship of Theoretical Physics at the University, a post which had been vacant for seven years. Aged 34, he had not yet written any significant scientific works. Even over the next few years, there was no indication that Schrödinger would initiate a paradigm shift in physics in 1926 and bring the university global renown.
The city offered him stability in the midst of the turbulent post-war period, plus long-awaited financial security and the chance to follow in the footsteps of renowned predecessors such as Einstein. On sunny days, Schrödinger held his lectures on the shores of Lake Zurich, clad in swimming trunks and always carrying an improvised blackboard with him. Although he suffered from tuberculosis and often had to deal with illness and exhaustion, students liked his teaching style. While many lecturers were still reading their scripts verbatim, Schrödinger spoke – and wrote his calculations – off the cuff. He also organised social gatherings, which were known for their exuberant atmosphere.
Schrödinger (back, middle) at Mythenquai lido. (Image: University of Vienna, Austrian Central Library for Physics, Erwin Schrödinger Archive, F33-RB-51c)
Instead of the formal outfit you might expect, Schrödinger would wear a jumper with a bow tie or an open shirt, as here with his subsequent assistant Fritz London. (Image: © Edith London)
One of Schrödinger’s famous parties, here in Berlin in 1933. Schrödinger is in the bottom right. (Image: provided by Brenner Forum together with Brenner Archive)
Schrödinger enjoyed swimming in Lake Zurich, especially on the island of Ufenau, and even dedicated a poem to this place. (Image: provided by Brenner Forum together with Brenner Archive)
Schrödinger wrote poems in German and English and published some of them in a volume of poetry. (Image: provided by Brenner Forum together with Brenner Archive)
Schrödinger cloaked his ideas of physics in vivid, almost lyrical language. For example, he described particles as ‘a head of foam atop the waves of radiation that form the foundations of the earth.’ (Image: provided by Brenner Forum together with Brenner Archive)
In 1926, Schrödinger took part in the ETH Physics Weeks, which brought important physicists such as Wolfgang Pauli, Otto Stern, Arnold Sommerfeld and Lise Meitner to Zurich every year. (Image: provided by Brenner Forum together with Brenner Archive)
The programme also included an excursion on Lake Zurich, complete with boat trip. (Image: provided by Brenner Forum together with Brenner Archive)
Other participants in the ETH Physics Weeks. (Image: provided by Brenner Forum together with Brenner Archive)
Exhausted by the war, the strenuous application process and his illness, in the early days Schrödinger cancelled a comparatively large number of his lectures. Sick note, 1922. (Image: Zurich State Archives, U 110.3.60.18.2)
Schrödinger’s sick note, addressed to his employer, 1927. (Image: Zurich State Archives, Z 1139.918.2)
Schrödinger’s tuberculosis did not stop him from smoking. Here, he can be seen around 1926 with Arnold Sommerfeld and Wolfgang Pauli (right). (Image: provided by Brenner Forum together with Brenner Archive)
Schrödinger in an unknown lecture hall. (Image: provided by Brenner Forum together with Brenner Archive)
‘Zurich superstition’: Schrödinger’s wave mechanics
‘Where did we get that from? Nowhere. It's not possible to derive it from anything you know. It came out of the mind of Schrödinger.’
Nobel Laureate Richard Feynman
In 1924, French nobleman Louis de Broglie presented a bold but unproven thesis to the somewhat perplexed Parisian Faculty of Natural Sciences: all matter has wave properties. Schrödinger, immediately fascinated by this, set out to develop a comprehensive theory. His breakthrough came in 1925 during his Christmas skiing holiday in Arosa. Schrödinger sent his paper ‘Quantisation as an eigenvalue problem’ to the editors of the journal Annalen der Physik in January. The text contained his famous wave equation, later also called the Schrödinger equation.
The equation represents the mathematical heart of wave mechanics, which Schrödinger developed over the course of just a few months. Wave mechanics enables the energy states of electrons in atoms to be precisely calculated and can thus predict their behaviour.
Schrödinger’s answer to Wolfgang Pauli’s pointed remark that his wave mechanics is mere superstition has been lost to history. However, the comment does not criticise Schrödinger’s calculations, but only Schrödinger’s interpretation of his own theory.
Schrödinger’s grave with his equation. (Image: © Wolfgang Morscher, www.SAGEN.at)
Einstein immediately recognised the importance of de Broglie’s notion that all matter has both particle and wave properties. The picture shows him in Innsbruck, where he probably discussed it in person with Schrödinger in 1924. (Image: Ciwl Photo Library)
Schrödinger’s colleague from ETH and friend Peter Debye urged him to prove De Broglie’s hypothesis with an equation instead of just talking about it in seminars. (Image: Franz Schmelhaus/ETH Library, image archive)
Schrödinger was assisted in the calculations by his friend Hermann Weyl. Weyl was an ETH professor and one of the most important mathematicians of the last century. (Image: Institute for Advanced Study, Princeton)
Schrödinger had to share the Nobel Prize in 1933 with Paul Dirac, a British citizen with Swiss nationality. Dirac later said: ‘Of all the physicists that I met, I think Schrödinger was the one that I felt to be most closely similar to myself.’ (Image: AIP Emilio Segrè Visual Archives, gift of Mrs Zemansky)
Werner Heisenberg’s older matrix mechanics was equivalent to wave mechanics, but it was too abstract for many people. Here are Heisenberg and Schrödinger at Schrödinger’s Nobel Prize ceremony. (Image: Max Planck Institute, courtesy of AIP Emilio Segrè Visual Archives)
A letter from Wolfgang Pauli to Schrödinger. Schrödinger and Einstein could not accept that the quantum world was determined by chance and did not follow fixed rules, bringing them into conflict with other quantum physicists. (Image: University of Vienna, Austrian Central Library for Physics, Erwin Schrödinger Archive)
The Heitler-London theory
23-year-old Heitler was inspired by Schrödinger’s publication (‘everyone was,’ said Heitler himself) and, thanks to a scholarship, travelled to his home in Zurich in early 1927. He hoped that Schrödinger would provide him with ideas for his research and professional support, but only received invitations for convivial weekend trips to the countryside and joint skiing holidays. His collaboration in Zurich with fellow scholarship-holder Fritz London, with whom he immediately struck up a rapport, became much more important.
Heitler struggled with the foehn wind in Zurich. During a particularly pronounced foehn storm, he was unable to concentrate for hours, slept through the afternoon and was suddenly startled awake by an image of hydrogen molecules, wave functions and electron exchange in front of his eyes. He called London, who stopped by immediately. Driven by a frenzy of creativity, they worked late into the night. The next day, their Heitler-London theory – the first successful quantum mechanical explanation of a chemical bond – was largely complete. Their article would soon be published in the journal Zeitschrift für Physik.
Heitler and London in Bristol, 1934 or 1935. Their mutual research interests led to the development of a long-standing friendship. (Image: © Edith London)
Heitler and London with Ava Helen Pauling, an American human rights activist and the wife of quantum chemist Linus Pauling. Heitler and London wrote their article on the terrace of a now-closed restaurant, Globus. (Image: Oregon State University Libraries)
London wrote the introduction and the discussion of the results; Heitler wrote the calculations and the sections on molecular forces in the article. Heitler can be seen here shortly before his arrival in Zurich. (Image: ETH Zurich University Archives)
As a penniless scholarship-holder, Heitler saw a veritable gulf between him and Schrödinger in terms of reputation and wealth. When talking to London, he once referred to him as the ‘bigwig’. (Image: provided by Brenner Forum together with Brenner Archive)
The bonds between neutral atoms, such as two hydrogen atoms in a hydrogen molecule, were first explained via quantum mechanics by London (centre) and Heitler (right). (Images: Anna C. Véron; Francis Simon/AIP Emilio Segrè Visual Archives; ETH Zurich University Archives)
Together with a theory by physicists Friedrich Hund (centre) and Robert Mulliken (right) – molecular orbital theory – the London-Heitler theory still forms the basis of calculating molecular structures and properties. (Image: Anna C. Véron; GFHund/Wikimedia Commons)
Goodbye, Zurich – and a reunion
Schrödinger’s wave mechanics and the Heitler-London theory immediately sparked enthusiasm in the world of physics due to their mathematical elegance, explanatory power and broad scope of application.
To the likely disappointment of many students, Schrödinger soon left for the US, where he would give 50 lectures over the course of several months. He then departed Zurich as a celebrated star and headed for Berlin, together with London, who had been promoted to the role of his assistant. Heitler became Max Born’s assistant and later professor at Göttingen, the leading centre of quantum physics.
As a Jew, Heitler was dismissed from his position in 1933 and fled to England. Schrödinger was classified by the Nazis as ‘politically unreliable’ and ‘friendly to Jews’, which is why he emigrated to Dublin in 1939. Two years later – now Director of the Dublin Institute for Advanced Studies – he brought Heitler to his institution. During the war, Heitler fell in love for the first time, married a biologist and became Schrödinger’s successor, but he missed the vast forests of his homeland, the Alps and German-speaking culture.
Schrödinger’s appreciation of Heitler is made clear in a letter to Max Born:
‘Since Heitler has been here for some time now, I feel obliged to thank you, in particular, for recommending him so strongly. He is an extremely valuable asset in every respect. Scientifically, he is at least equal to his “milk brother” London (I mean this brotherhood in terms of its first great achievement), but he is incomparably superior to him as a person and as a teacher. In fact, he has a wonderful gift for understanding other people’s difficulties and objections.’
Letter dated 5 October 1941
Schrödinger’s fame attracted further researchers to Zurich in 1926, such as the two-time Nobel Prize winner and co-founder of quantum chemistry Linus Pauling (in the middle with his wife Ava, left: Heitler, right: London). (Image: Oregon State University Libraries)
Oppenheimer described Schrödinger’s theory as ‘probably one of the most perfect, precise and beautiful ever formulated.’ The picture shows Oppenheimer’s official ID photo while working for the Manhattan project. (Image: Los Alamos National Laboratory/Wikimedia Commons)
Schrödinger’s resignation in 1927 hit Zurich unexpectedly hard, as Peter Debye, a second Nobel Prize winner, emigrated to Germany at the same time. Telegram to the Director of the Institute of Physics. (Image: Zurich State Archives, Z 1139.918.2)
Students organised a torchlight procession to stop Schrödinger from leaving for Germany – albeit without success. He was touched, and the NZZ reported on the procession on 15 July 1927. (Image: University of Vienna, Austrian Central Library for Physics, Erwin Schrödinger Archive, C94–77)
Schrödinger liked Zurich and Switzerland, even though the people sometimes seemed to him ‘too cold’. Copy of his farewell letter to the university. (Image: Zurich State Archives, Z 1139.918.1.1)
Max Born, Heitler and Schrödinger reunited in Dublin. Heitler’s mentor Born remained in contact with the pair for the rest of his life. (Image: © Dublin Institute for Advanced Studies)
Heitler’s brother Hans was living in Stuttgart when he saw his photo – along with his name and address – identifying him as Jewish on the front page of the newspaper Flammenzeichen in December 1938.
Since Heitler was German, he was temporarily detained in an internment camp upon arrival in England as a ‘enemy alien’. He recorded this experience in his handwritten memoirs. (Image: ETH Zurich University Archives)
Walter Heitler was able to bring his two siblings and his mother to England just in time; his father had already died by that point. Here you can see the family in Baden-Baden in 1929. (Image: ETH Zurich University Archives)
The later Nobel laureate and co-founder of quantum mechanics Max Born used his influence to secure jobs abroad for his assistant Heitler and other Jews.(Bild: Churchill Archives Centre, The Papers of Professor Max Born, BORN 6/1/22)
Born was also Oppenheimer’s and Wolfgang Pauli’s former mentor, although their behaviour sometimes brought him to the brink of despair. Pictured: Pauli (left) ca. 1926 with Max Born in Hamburg. (Image: © CERN).
Although high-energy physics was one of Heitler’s main focuses, he was not involved in the Manhattan project. On the right, Hans Bethe, the head of the theoretical department, with whom Heitler published one of his most important works in 1934. (Image: Acc. 90–105 – Science Service, Smithsonian Institution Archives)
Heitler’s ‘turn to the humanities’: philosophy, religion and quantum physics
‘After Hitler’s war, the United States held very great prestige, including morally. In a matter of minutes, it was destroyed. […]
What became of physics – my science – what became of the physicists!’
Quote from Heitler’s memoirs
Although he was now an Irish citizen, Heitler returned to the city of Zurich after the end of the war and remained there until his death. For a quarter of a century, he headed up the Institute of Theoretical Physics at the University, published and lectured, but physics was increasingly becoming a science of death for him. He met Japanese radiation victims and was disturbed by the political, military and economic appropriation of physics. The idea of a purely materialistic, soulless world, in which the divine has no place, felt incomplete and alien to him.
He vehemently defended his convictions publicly, be it at panel discussions in the town hall or in newspaper interviews. His publications, such as Naturwissenschaft ist Geisteswissenschaft (‘Natural science is spiritual science’), Naturphilosophische Streifzüge (‘A foray through natural philosophy’), Die Natur und das Göttliche (‘Nature and the divine’) and Wahrheit und Richtigkeit in den exakten Naturwissenschaften (‘Truth and correctness in the exact natural sciences’), also demonstrate his commitment. His most successful book on this subject was Der Mensch und die naturwissenschaftliche Erkenntnis (‘Man and Science’).
At that time, Heitler was no different from Schrödinger and other physicists such as Heisenberg and Oppenheimer with his close attachment to nature and his pronounced philosophical, religious and spiritual interests. (Image: ETH Zurich University Archives)
Heitler warned against applying the strict rationality of the natural sciences to socio-political questions, as in this article in Die Tat on 26 November 1976. He was particularly alarmed by a study that calculated the value of a human being based on its chemical components. (Image: ZB Zürich)
Gregor Wentzel and Max Born appreciated Heitler’s appeals, and he also received a good deal of support from the public. Letter from Born to Heitler, 16 February 1968. (Image: ETH Zurich University Archives)
Nuclear physicist Lise Meitner (right) also praised Heitler’s writings. As a co-discoverer of nuclear fission, she was nominated for the Nobel Prize in Chemistry and Physics almost 50 times. (Image: Francis Simon/AIP Emilio Segrè Visual Archives)
Heitler was concerned about alienation from nature, which he perceived in the canalisation of Zurich’s Limmat. The city was also giving more and more space to traffic, for example via the reconstruction of the station bridge around 1950. (Image: Comet Photo AG/ETH Library, image archive)
The Rudolf Steiner School on Plattenstrasse in Zurich. Although Heitler had little time for anthroposophy and homeopathy, he permitted his disabled son to attend this school. (Image: Juliet Haller/Architectural History Archive of the City of Zurich)
Schrödinger visited Switzerland only occasionally after the war, as here for a meeting with Carl Gustav Jung at a conference. Both gave lectures there on topics from the humanities. (Image: provided by Brenner Forum together with Brenner Archive)
Legacy: Schrödinger, Heitler and Zurich today
Ironically, it was not the Schrödinger equation that found its way into pop culture, but his cat. Its presence in literature, film, music and internet memes illustrates how closely science and culture can overlap. Today, it is seen as a symbol of ambiguity and uncertainty in everyday situations.
With his thought experiment about the cat, Schrödinger was criticising a certain interpretation of quantum mechanics, which he was hostile to throughout his life. This interpretation asserts, among other things, that quantum mechanical particles are in several possible states at the same time as long as they are not observed. Schrödinger wanted to show how absurd this principle looks when applied directly to everyday things.
Nowadays, Zurich’s relationship with Schrödinger is straightforward: guided city tours visit the places where he lived and worked, and his house at Huttenstrasse 9 is a minor tourist attraction. Nothing comparable exists for Heitler.
The property ‘Zu vier Wachten’, Schrödinger’s former home at Huttenstrasse 9 in Zurich. In addition to quail figurines, there is also a life-size Schrödinger’s cat in the garden. (Image: Irina Morell)
Depending on the time of the visit, the cat is alive or, as in this case, dead. (Image: Irina Morell)
Heitler’s property at Guggenberg 5, occupied until 2024 by his successor and good friend, UZH physics professor Günther Rasche. (Image: Irina Morell)
Heitler’s papers, which are currently largely unresearched, are located in the ETH University Archives. The image shows him with his wife Kathleen and his mentor Max Born. (Image: ETH Zurich University Archives)
The annual Schrödinger Colloquium at the University of Zurich honours his scientific legacy and is aimed at a broad audience. (Image: Physics Institute of the University of Zurich)
Globi visited ETH Zurich in 2017 and went on numerous adventures with Professor Pauli Schrödinger. (Image: © 2025, Orell Füssli Ltd, Globi Verlag Imprint Orell Füssli Verlag, Zurich, illustration from Globi und die verrückte Maschine (Volume 87))
Pauli Schrödinger’s cat Erwin is fiddling with a machine that makes Pauli and Globi shrink to the size of a pea. (Image: © 2025, Orell Füssli Ltd, Globi Verlag Imprint Orell Füssli Verlag, Zurich, illustration from Globi und die verrückte Maschine (Volume 87))
As a lifelong fan of dogs, Schrödinger probably never owned a cat in real life. Here he is with his dog Barney and his wife Annemarie. (Image: University of Vienna, Austrian Central Library for Physics, Erwin Schrödinger Archive/shelf mark F94–148)
Irina Morell, historian and employee of Universitätsbibliothek Zürich, Natural Sciences
June 2025
From the end of June 2025 onwards, the Physics Institute at the University of Zurich will be hosting the exhibition ‘Quantum Century – Zurich and the birth of quantum mechanics’ at the Universitätsbibliothek, Natural Sciences.
Header image: Schrödinger with Pauli, Heisenberg, Einstein, Max Born and Louis de Broglie at the Solvay Conference of 1927. (Colourised by Sanna Dullaway, photograph by Benjamin Couprie, Institut International de Physique Solvay, courtesy of AIP Emilio Segrè Visual Archives)
Acknowledgements & sources
The author would like to thank Mr Axel Rasche for the generous provision of information and kind support in connection with the estate of Walter Heitler.
Unless otherwise noted, all quotations are from the source edition Eine Entdeckung von ganz ausserordentlicher Tragweite: Schrödingers Briefwechsel zur Wellenmechanik und zum Katzenparadoxon (‘A discovery of exceptional significance: Schrödinger’s correspondence on wave mechanics and the cat paradox’) and Walter Heitler’s unpublished memoirs.