Heitler, Walter (1904–81), theoretical physicist and philosopher, was born 2 January 1904 in Karlsruhe, Baden, Germany, son of Adolf Heitler, a Jewish engineering professor, and Ottilie Heitler (née Rudolf ). His largely classical early education left him with a lifelong interest in philosophy. However, despite little encouragement at school, he also developed a strong interest in science, or rather, as he said, his ‘interest in science was always a desire for knowledge only, and it was never an interest in the practical side of science’ (quoted in Glass, 239). At about the age of 12 he made a telescope and later set up a chemical laboratory in the bathroom. He attended lectures outside school at the Karlsruhe Technische Hochschule, where he came across quantum theory. After secondary school he began studying chemistry at the same Technische Hochschule but soon decided to pursue his growing interest in physics. He moved to Berlin, where Einstein, Planck, and von Laue were among the eminent scientists. However, feeling his mathematical training and the available supervision were inadequate, he moved to Munich to carry out research for his Ph.D. (1925) on concentrated chemical solutions (physical chemistry) under Herzfield. Afterwards he went to Copenhagen (1926) on a Rockefeller fellowship.
Theoretical physics became his main interest and he wrote to Erwin Schrödinger (qv) to ask if he could come to Zurich to finish his fellowship. He moved to Switzerland (1927) and began working with Fritz London, which he described as a decisive turning point in his career. They both familiarised themselves with Schrödinger's work on wave mechanics and utilised this approach successfully to calculate the Van der Waals interaction between two atoms. Their paper, published (1927) when Heitler was just 23 years of age, is his best-known contribution to science. It became known as the Heitler–London theory of the covalent chemical bond, in which the two electrons forming the bond are indistinguishable from each other. This work laid the foundation for the modern theory of organic chemistry and has been described as ‘the greatest single contribution to the clarification of the chemist's conception of valence’ (quoted in Glass, 240). Max Born immediately offered him an assistantship in Göttingen (1928), where he remained till Hitler came to power (1933). He became a world expert on quantum electrodynamics but was dismissed from his position on account of his Jewish ancestry. He was offered a temporary position at Bristol University, where he continued his researches. In 1940, after the fall of France, he was interned for some months on the Isle of Man as one of a number of German refugees or ‘enemy aliens’ from the university, including his brother Hans. The university requested their release partly on the grounds that nuclear physics research was essential to the possible construction of an atomic bomb. Later however he declined to work on the atomic bomb project, a decision that caused a rift between himself and project workers.
In 1941 he was invited to a permanent position in the newly created Dublin Institute for Advanced Studies (DIAS), set up in 1940 on the initiative of Éamon de Valera (qv). Schrödinger was the first director. Heitler remained in Dublin for eight happy and successful years. He was promoted to professor in 1943 and succeeded Schrödinger as director (1945–9). The two men played an important role in establishing the DIAS's school of cosmic physics (1947). Continuing his work on quantum electrodynamics and cosmic rays, he devoted a significant proportion of his research in Dublin to the theory of the cosmic ray meson (now called the muon). He also collaborated with experimental physicists in Dublin including Ernest Walton (qv) (TCD), Thomas Nevin (qv) (UCD), Lanos Janossy (DIAS), and Cormac Ó Ceallaigh (qv) (DIAS). He established a vibrant research group of young theoretical physicists, including Jim Hamilton, Sheila Power, P. Walsh, and several overseas postdoctoral students (Walter Thirring, H. W. Peng, Cecile de Witt (née Morette), Suraj Gupta), many later becoming international names in physics. He was always courteous and helpful to his students, and his Dublin seminars and lectures were remembered for their clarity and interest. He was also involved in developing theoretical physics courses and gave lectures on wave mechanics for chemists, published as Elementary wave mechanics (1945). This followed his earlier publication The quantum theory of radiation (1936), which is still relevant and widely used today. His most important work in Dublin was on radiation damping.
In 1949 he was offered and accepted the professorship of theoretical physics at the University of Zurich. He was particularly happy to return to the German-speaking world, and he remained in Switzerland for the rest of his life. On his appointment he also became director of the institute, retaining this position till his retirement at the age of 70. Here he continued his work in electrodynamics for a period, but only wrote two scientific papers after 1960, having previously written over eighty. He returned to his earlier interests in philosophy and religion and spent the last twenty years of his life preoccupied with these subjects, some say due to his dismay at the scientific world in the aftermath of the atomic bomb (N. Mott, 1982). Of his four published philosophical books, Man and science (1961) was the most popular and was translated into many languages. In spite of the success of reductionist science, he felt that it could never explain all aspects of nature, and that many of the processes of biology and evolution had other non-quantitative forces at work. At one stage he stated his regret at studying physics rather than biology or philosophy. He had an interest in the teachings of Rudolf Steiner, which were based on the belief that creative activities such as myth-making are beneficial from an educational and therapeutic viewpoint. Heitler became a member of the Swiss Reformed Church towards the end of his life.
During his life he received many honours: MRIA (1943), FRS (1948), honorary member of the Kungl. Vetenskaps Societetens of Uppsala (1967), the Leopoldina in Halle (1968), and the Akademie der Wissenschaften und der Literatur of Mainz (1970), and member of the Norwegian Royal Society (1974). He also received the Max Planck medal (1968), the Swiss Marcel Benoist prize (1970), and the gold medal of the Humboldt Society (1979), as well as honorary doctorates from the NUI, Dublin, Göttingen, and Uppsala.
Soon after his arrival in Dublin he married (1942) Kathleen Winifred Nicholson, a researcher in biological science whom he had met in Bristol. They lived at 21 Seapark Road, Clontarf, and had one son, Eric, in 1946, the same year Heitler took out Irish citizenship. After their departure from Ireland (1949), they lived in Zurich at Drusbergstrasse 59 (1949–58) and at Am Guggenberg 5 (1958–81). In Switzerland he could indulge his love of the Black Forest and participate in mountain climbing and skiing. His sister and mother continued to live in Ireland till their deaths. Heitler died in Zurich 15 November 1981. Many contemporaries were surprised that he was not awarded the Nobel prize.