Chapter 8 Walther Gerlach (1889–1979): Precision Physicist, Educator and Research Organizer, Historian of Science Josef Georg Huber, Horst Schmidt-Böcking, and Bretislav Friedrich Abstract Walther Gerlach’s numerous contributions to physics include precision measurements related to the black-body radiation (1912–1916) as well as the first- ever quantitative measurement of the radiation pressure (1923), apart from his key role in the epochal Stern-Gerlach experiment (1921–1922). His wide-ranging research programs at the Universities of Tübingen, Frankfurt, and Munich entailed spectroscopy and spectral analysis, the study of the magnetic properties of matter, and radioactivity. An important player in the physics community already in his 20s and in the German academia in his later years, Gerlach was appointed, on Werner Heisenberg’s recommendation, Plenipotentiary for nuclear research for the last six- teen months of the existence of the Third Reich. He supported the effort of the German physicists to achieve a controlled chain reaction in a uranium reactor until the last moments before the effort was halted by the Allied Alsos Mission. The reader can find additional discussion of Gerlach’s role in the supplementary material provided with the online version of the chapter on SpringerLink. After returning from his detention at Farm Hall, he redirected his boundless elan and determination to the reconstruction of German academia. Among his high-ranking appointments in the Federal Republic were the presidency of the University of Munich (1948–1951) and of the Fraunhofer Society (1948–1951) as well as the vice-presidency of the German Science Foun- dation (1949–1961) and the German Physical Society (1956–1957). As a member of Göttinger Achtzehn, he signed the Göttingen Declaration (1957) against arming the Bundeswehr with nuclear weapons. Having made history in physics, Gerlach Electronic supplementary material The online version of this chapter (https://doi.org/10.1007/978-3-030-63963-1_8) contains supplementary material, which is available to authorized users. J. G. Huber Am Schnepfenweg 35a, 80995 München, Germany H. Schmidt-Böcking Institut für Kernphysik, Universität Frankfurt, 60348 Frankfurt, Germany B. Friedrich (B) Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany e-mail: [email protected] © The Author(s) 2021 119 B. Friedrich and H. Schmidt-Böcking (eds.), Molecular Beams in Physics and Chemistry, https://doi.org/10.1007/978-3-030-63963-1_8 120 J. G. Huber et al. became a prolific writer on the history of physics. Johannes Kepler was his favorite subject and personal hero—as both a scientist and humanist. 1 Introduction What Walther Gerlach said about his academic mentor, Friedrich Paschen (1865– 1947), could also be said about Gerlach himself (Gerlach 1935): The physicists saw him as a master of experimental physical research who carried on the great tradition of precision physics …With his unusual manual dexterity, he built the finest [scientific instruments], tirelessly trying to get the last out of them, in the conviction that every instrumental advance in physical research opens up new possibilities—and will enable new insights. And the fact that he succeeded in this …made him love his [scientific instruments] almost tenderly. By the time he earned his Ph.D. in Paschen’s Tübingen laboratory in 1912 at age 23, Gerlach was a major player in the research area of black-body radiation. He would pursue a related topic, that of light pressure, after an interruption due to World War One and his crucial involvement in the epochal Stern-Gerlach experiment during 1921–1922. In 1925, Gerlach would assume the chair of his mentor and in 1929 move on to Munich as the successor of Wilhelm (Willy) Wien (1864–1928), thereby receiving the accolade due to a leading experimental physicist. Gerlach’s tenure at Munich, which lasted until his retirement in 1957, would only be interrupted by his detention at Farm Hall (1945–1946) and a stint at the University of Bonn (1946– 1948), then in the British Zone of Occupation. In 1944, upon consulting Werner Heisenberg (1901–1976), Otto Hahn (1879– 1968), and Paul Rosbaud (1896–1963), Gerlach became the head of the Physics Section at the Reich Research Council and Reichsmarschall’s Plenipotentiary for nuclear physics responsible for the German Uranprojekt. Thereby, Gerlach entered higher echelons of Third Reich’s establishment (Walker 1995). As available testi- monials, including his own, suggest, in this capacity, Gerlach saved many young physicists from the service on the front—and, unbeknownst to him, likely kept the Allies abreast of the German nuclear research via Paul Rosbaud (1896–1963), a scientist and publisher who had become a British agent (Kramisch 1986). In his character testimonial about Gerlach, Rosbaud stated (Rosbaud 1945): Gerlach hated the Nazis, he had to suffer under their denunciations …he loved his country and wished the best to her and did not want her to perish …. During the last period of the war he only was interested in advancing pure research work and in saving the lives of scientists. He exceeded many times his competencies to save people …In contrast to many others, he was absolutely incorruptible and in consequence, despite [receiving] 2 or 3 Führerpakete,1 sometimes half starved. 1A food allocation provided during WWII once a year to the military and other choice personnel on behalf of Adolf Hitler. 8 Walther Gerlach (1889–1979): Precision Physicist, Educator … 121 In the aftermath of World War Two and beyond, Gerlach directed his boundless elan and determination to the reconstruction of German academia. He built up anew the Institute of Physics at Munich’s Ludwig-Maximilans-Universität and served as the university’s Rector (1948–1951); during the same period he served as the founding President of the Fraunhofer-Gesellschaft for applied research; was Vice-President of the Deutsche Forschungsgemeinschaft (1949–1961) and of the Deutsche Physikalis- che Gesellschaft (1956–1957). “Making friends and cultivating friendships was one of his greatest talents” (Gentner 1980), which Gerlach amply deployed throughout these years. Gerlach was also engaged in attempts to limit the spread of nuclear weapons and signed as a member of Göttinger Achtzehn the Göttingen Declaration opposing the move by the West-German government to arm the Bundeswehr with tactical nuclear weapons (12 April 1957). Since the late 1940s, Walther Gerlach’s interest turned increasingly to the history of science. He would write about 500 didactic, biographical, and memorial articles— apart from about 320 research papers and monographs (Nida-Rümelin 1982). His essay on Max Planck (Gerlach 1948) or book on Johannes Kepler (Gerlach 1980) belong to his most acclaimed history works. Gerlach was co-nominated, with Otto Stern, thirty-one times for the Nobel Prize in Physics for the Stern-Gerlach experiment, Fig. 1. Gerlach’s contributions to the fields of black body radiation, light pressure, magnetism, and spectroscopy were no less demanding but remain much less known. In this chapter, we revisit Gerlach’s seminal works in an attempt to do justice to his scientific legacy. We conclude by showcasing his work in the history of science. 2 Walther Gerlach’s Social Background, Upbringing, and Education Walther Gerlach was born on 1 August 1889 in Wiesbaden-Biebrich (Huber 2015). His father, Valentin Gerlach (1858–1957), came from a family of craftsmen based in Frankfurt and became a doctor. However, he only practiced medicine for a short time and soon turned to experimental chemistry. His mother, Maria, neé Niederhaeuser (1868–1941), also came from a family of craftsmen, from the nearby Wiesbaden area. Figure 2 shows Walther Gerlach in the first year of his life. When he turned two, his twin brothers Werner and Wolfgang were born. Formal upbringing in the family was primarily set by the father and took place within the framework of the conservative value system of the time. Figure 3 shows Gerlach as a school child. However, more strongly yet, it was shaped by the Enlight- enment ideas of the Freemasons, of whose order the father was a member. Freedom, Equality, Brotherhood, Tolerance, and Humanity were at the foundation of their creed. The father, Figs. 4 and 5, was also an admirer and connoisseur of Johann Wolfgang Goethe, whose understanding of education played an important role in the 122 J. G. Huber et al. Fig. 1 Walther Gerlach’s nominations for a Nobel prize in Physics. The compilation is based on the information available at the nomination archive https://www.nobelprize.org/nomination/archive/. The 1924 nomination by Albert Einstein was not a valid one, as Einstein nominated additional candidates apart from Stern and Gerlach that year Gerlach family as well. Not to forget Valentin Gerlach’s membership in a student association Corps Alemannia to whose events he would often take his children along. The upbringing in the Gerlach family was both highly demanding and encour- aging, characterized by rigor and devotion. The father himself had learned that one can only achieve something in life through determined work and self-discipline and wanted to pass on this realization to his children. The parents set at first narrow bound- aries but gradually expanded them as the children grew older and could increasingly take responsibility for their own actions. Walther Gerlach’s first diary tells of exten- sive hikes, preoccupation with flora, fauna and minerals, visits to the theater, literary, artistic and musical activities as well as photography and much more. He played the piano and organ and tried his hand at drawing and poetry. 8 Walther Gerlach (1889–1979): Precision Physicist, Educator … 123 Fig. 2 Walther Gerlach in 1889 (Heinrich and Bachmann 1989) Fig. 3 Walther Gerlach as a pupil (Heinrich and Bachmann 1989) 124 J. G. Huber et al. Fig.