Walter Heinrich Heitler (German: [ˈvaltɐ ˈhaɪnʁɪç ˈhaɪtlɐ]; 2 January 1904 – 15 November 1981) was a German–Irish theoretical physicist who made contributions to quantum electrodynamics and quantum field theory. He brought chemistry under quantum mechanics through his theory of valence bonding.
Walter Heinrich Heitler was born on 2 January 1904 in Karlsruhe, Germany, the son of Adolf Heitler, a Jewish engineering professor, and Ottilie Rudolf.
Heitler studied physics at the University of Karlsruhe (TH) (1922), at the Friedrich Wilhelm University of Berlin (1923), and at the Ludwig-Maximilians-Universität München (1924), where he studied under both Arnold Sommerfeld and Karl Herzfeld. Heitler received his Ph.D. in 1926, with Herzfeld as his thesis advisor. Herzfeld taught courses in theoretical physics and one in physical chemistry, and often substituted for Sommerfeld.
From 1926 to 1927, Heitler was a Rockefeller Foundation Fellow for postgraduate research with Niels Bohr at the Institute for Theoretical Physics at the University of Copenhagen and with Erwin Schrödinger at the University of Zurich. He then became an assistant to Max Born in the Institute for Theoretical Physics at the University of Göttingen. In 1929, Heitler completed his habilitation (Dr. habil.) under Born, and remained as a Privatdozent until 1933, when he was dismissed by the University because he was Jewish.
At the time Heitler obtained his doctorate, three Institutes for Theoretical Physics formed a consortium which worked on the key problems of the day, such as atomic and molecular structure, and exchanged both scientific information and personnel in their scientific quests. These institutes were located at the Ludwig-Maximilians-Universität München, under Arnold Sommerfeld; the University of Göttingen, under Max Born; and the University of Copenhagen, under Niels Bohr. Furthermore, Werner Heisenberg and Born had just recently published their trilogy of papers which launched the matrix mechanics formulation of quantum mechanics. Also, in early 1926, Erwin Schrödinger, at the University of Zurich, began to publish his quintet of papers which launched the wave mechanics formulation of quantum mechanics and showed that the wave mechanics and matrix mechanics formulations were equivalent. These papers immediately put the personnel at the leading theoretical physics institutes onto applying these new tools to understanding atomic and molecular structure. It was in this environment that Heitler used his Rockefeller Foundation Fellowship, leaving the Ludwig-Maximilians-Universität München and within a period of two years going to do research and study with the leading figures of the day in theoretical physics—Bohr at the University of Copenhagen, Schrödinger at the University of Zurich, and Born at the University of Göttingen.
At the University of Zurich, with Fritz London, Heitler applied the new quantum mechanics to deal with the saturable, non-dynamic forces of attraction and repulsion, i.e., exchange forces, of the hydrogen molecule. Their valence bond treatment of this problem, was a landmark in that it brought chemistry under quantum mechanics. Furthermore, their work greatly influenced chemistry through Linus Pauling, who had just received his doctorate and on a Guggenheim Fellowship visited Heitler and London in Zurich. Pauling spent much of his career studying the nature of the chemical bond. The application of quantum mechanics to chemistry would be a prominent theme in Heitler's career.
While Heitler was at Göttingen, Adolf Hitler came to power in 1933. With the rising prominence of anti-Semitism under Hitler, Born took it upon himself to take the younger Jewish generation under his wing. In doing so, Born arranged for Heitler to get a position that year as a research fellow at the University of Bristol, with Nevill Francis Mott.
Heitler was a research fellow of the Academic Assistance Council in the H. H. Wills Physics Laboratory. Among other things, he worked on quantum field theory and quantum electrodynamics on his own, as well as in collaboration with other scientific refugees from Hitler, such as Hans Bethe and Herbert Fröhlich, who also left Germany in 1933.
With Bethe, he published a paper on pair production of gamma rays in the Coulomb field of an atomic nucleus, in which they developed the Bethe–Heitler formula for Bremsstrahlung (braking radiation).
In 1936, Heitler published his major work on quantum electrodynamics, The Quantum Theory of Radiation, which marked the direction for future developments in quantum theory. The book appeared in many editions and printings and has been translated into Russian.
Heitler also contributed to the understanding of cosmic rays,
as well as predicting the existence of the electrically neutral pi meson.
While developing the theory of cosmic ray showers in 1937, he became aware of the latest experimental work in the field; the observation of cosmic ray interactions in nuclear emulsion by Austrian physicists Marietta Blau and Hertha Wambacher. He mentioned this to Cecil Powell, saying that the method appeared so straightforward that "even a theoretician might be able also to do it". This intrigued Powell, and he convinced theoretician Heitler to travel to Switzerland with a batch of llford emulsions and expose them on the Jungfraujoch at 3500m. In a letter to Nature in August 1939, Heitler and Powell were able to confirm the observations of Blau and Wambacher—thus Heitler had some influence in setting Powell on the first step of his path to the 1950 Nobel Prize in Physics "for his development of the photographic method of studying nuclear processes and his discoveries regarding mesons made with this method".
After the fall of France in 1940, Heitler was briefly interned on the Isle of Man for several months.
In 1941, Heitler became a professor at the Dublin Institute for Advanced Studies (DIAS), which was arranged there by Erwin Schrödinger, director of the School for Theoretical Physics. He has been described as the "unsung hero of DIAS in the 1940s".
Heitler's work with Peng Huanwu on radiation damping theory and the meson scattering process resulted in the Heitler–Peng integral equation.
In 1942, Heitler married Kathleen Winifred Nicholson, a biologist whom he met in Bristol. They lived at 21 Seapark Road, Clontarf (down the road from Schrödinger), and had a son, Eric, in 1946. Heitler became a naturalised Irish citizen the same year.
During the 1942–1943 academic year, Heitler gave a course on elementary wave mechanics, during which W. S. E. Hickson took notes and prepared a finished copy. These notes were the basis for Heitler's book Elementary Wave Mechanics: Introductory Course of Lectures, first published in 1943. A new edition was published as Elementary Wave Mechanics in 1945. This version was revised and republished many times, as well as being translated into French and Italian and published in 1949 and in German in 1961. A further revised version appeared as Elementary Wave Mechanics With Applications to Quantum Chemistry in 1956, as well as in German in 1961.