b1043_Chapter-2.4.qxd 1/27/2011 7:30 PM Page 319 b1043 Quantum Mechanics and Weimar Culture FA 319 Philosophical Rhetoric in Early Quantum Mechanics 1925–27: High Principles, Cultural Values and Professional Anxieties Alexei Kojevnikov* ‘I look on most general reasoning in science as [an] opportunistic (success- or unsuccessful) relationship between conceptions more or less defined by other conception[s] and helping us to overlook [danicism for “survey”] things.’ Niels Bohr (1919)1 This paper considers the role played by philosophical conceptions in the process of the development of quantum mechanics, 1925–1927, and analyses stances taken by key participants on four main issues of the controversy (Anschaulichkeit, quantum discontinuity, the wave-particle dilemma and causality). Social and cultural values and anxieties at the time of general crisis, as identified by Paul Forman, strongly affected the language of the debate. At the same time, individual philosophical positions presented as strongly-held principles were in fact flexible and sometimes reversible to almost their opposites. One can understand the dynamics of rhetorical shifts and changing strategies, if one considers interpretational debates as a way * Department of History, University of British Columbia, 1873 East Mall, Vancouver, British Columbia, Canada V6T 1Z1; [email protected]. The following abbreviations are used: AHQP, Archive for History of Quantum Physics, NBA, Copenhagen; AP, Annalen der Physik; HSPS, Historical Studies in the Physical Sciences; NBA, Niels Bohr Archive, Niels Bohr Institute, Copenhagen; NW, Die Naturwissenschaften; PWB, Wolfgang Pauli, Wissenschaftlicher Briefwechsel mit Bohr, Einstein, Heisenberg a.o., Band I: 1919–1929, ed. A. Hermann, K.V. Meyenn, and V.F. Weisskopf (New York: Springer, 1979); RKZ, Ralph Kronig Nachlass, Eidgenössische Technische Hochschule, Zurich; SHPS, Studies in the History and Philosophy of Science; WWM, Wilhelm Wien Nachlass, Deutsches Museum, Munich; ZP, Zeitschrift für Physik. 1 Niels Bohr to C.G. Darwin, draft of a presumably unsent letter, around July 1919, NBA. b1043_Chapter-2.4.qxd 1/27/2011 7:30 PM Page 320 FA b1043 Quantum Mechanics and Weimar Culture 320 Alexei Kojevnikov of justifying property claims over the emerging new revolutionary theory, at a time when its major concepts were in statu nascendi, while personal relations and insti- tutional hierarchies between its major contributors were still developing and negotiable. Ehrenfest, Darwin, and Pauli on the Forman Thesis In a June 1919 letter to his friend and colleague Niels Bohr, Paul Ehrenfest, pro- fessor of theoretical physics at the University of Leiden, commented insightfully on changes in the European intellectual climate after World War I: [I]t is remarkable that precisely here, in the circles of men having much to do with technology, production, industry, patents etc., opinions develop so uniformly about perspectives of culture. Overall there is building up an uncannily intensive reaction against rationalism ... If I am not entirely mistaken, in the next 5–10 years we will see the following happening at the institutes of higher learning (including technical!). Professors raised as relatively rational and disciplined individuals will despairingly and uncomprehendingly face the complaints and demands of a relatively “mystical” student body. At the same time, scientifically less clear but personally warmer teachers will gain the main influence over students.2 Ehrenfest’s observation remarkably and almost literally supports the core claim of Paul Forman’s article of 1971 that, in the immediate wake of World War I, a strong wave of reaction against rationalism and favouring more mystical lines of thought swept through not just the intellectual public in general, but also such professionals as engineers and exact scientists previously expected to strongly resist such trends.3 It is worth noting that the letter was from a physicist in the Netherlands to his colleague in Denmark, signifying the mood did not remain con- fined to Germany and Austria, the countries who had lost the war, but also affected at least the neighbouring neutral countries. Although not abandoning his personal rationalistic convictions, Ehrenfest appeared to defer to the opinions of the younger, as academics often do with the newest intellectual fashions. Ehrenfest’s letter also contains a strikingly self-conscious recognition and expectation that professors would adapt knowingly, rather than unreflectively, to 2 Paul Ehrenfest to Niels Bohr, 4 June 1919, NBA (emphasis in the original). 3 Paul Forman, ‘Weimar Culture, Causality, and Quantum Theory, 1918–1927: Adaptation by German Physicists and Mathematicians to a Hostile Intellectual Environment’, HSPS 3 (1971): 1–115. b1043_Chapter-2.4.qxd 1/27/2011 7:30 PM Page 321 b1043 Quantum Mechanics and Weimar Culture FA Philosophical Rhetoric in Early Quantum Mechanics 321 the direction of the prevailing intellectual wind. He did not state it explicitly in this letter, but his other correspondence of the time reveals quite clearly that, in his own field of theoretical physics, he admired and regarded Bohr as precisely the kind of professor who would resonate with and inspire the younger generation of students.4 The quoted description, indeed, sits well with characteristic hagio- graphic references in existing recollections regarding the way Bohr’s charisma influenced younger students. Bohr appears there as a kind of philosophical guru, whose thoughts were too profound to be understood or even expressed clearly, but this only helped them to be tremendously inspiring. Whether or not Ehrenfest’s letter thus contained implicit advice to Bohr, and whether or not Bohr accepted the hint or arrived at similar ideas on his own, around the same time he was already inclined ‘to take the most radical or rather mystical views imaginable’ with regard to the daunting problem of the quantum interaction of matter and radiation. That much Bohr admitted himself while composing a reply to his British friend and col- league Charles Galton Darwin.5 Darwin’s letter and his enclosed manuscript suggested several possible ideas of this kind, including acausality. Darwin ‘felt that the fundamental basis of physics is in a desperate state. The great positive successes of the quantum theory have accentuated … also the essential contradiction on which it rests.’ As a rem- edy, he proposed ‘to knock away the props of classical physics one by one and find, after a particular one has been removed, that our difficulties have become reconciled. It may be that it will prove necessary to make fundamental changes in our ideas of time and space or to abandon the conservation of matter and electric- ity or even as a last forlorn hope to endow electrons with free will.’ Personally, Darwin preferred to think that ‘contradictions in physics all rest on the exact con- servation of energy’ and his favourite solution thus involved ‘denying that conservation is anything more than statistical.’6 Four years later, this acausal hypothesis would become Bohr’s preferred choice, too, but in 1919 he was not yet ready to decide which, if any, of the mentioned ‘mystical’ hypotheses could be endorsed. Ehrenfest’s 1919 formulation of the Forman thesis avant la lettre makes an important addition to it by specifying the effective milieu whose demands made 4 For example, Ehrenfest to A.F. Joffe, 6 September 1920, in Erenfest–Ioffe: Nauchnaia Perepiska, 1907–1933 (Leningrad: Nauka, 1973), 146–150. 5 Bohr to Darwin, July 1919, NBA (ref. 1): ‘or rather mystical’ is inserted into the sentence above the line. Cf. also Bohr to Ehrenfest, 22 October 1919, NBA. 6 Darwin to Bohr, 20 July 1919, with an enclosed manuscript ‘A Critique of the Foundations of Physics’, NBA. b1043_Chapter-2.4.qxd 1/27/2011 7:30 PM Page 322 FA b1043 Quantum Mechanics and Weimar Culture 322 Alexei Kojevnikov professors adapt — students in auditoriums. Indeed, students may well have been the immediate audience many professors cared about the most. In this respect, however, Bohr’s situation was rather peculiar and differed from Ehrenfest’s. From early on in his professorship at the University of Copenhagen, Bohr delegated the task of giving general lectures to his assistants and did not teach undergraduate students. With only a couple of exceptions, he did not train doctoral students either. Almost all of the young physicists who worked with him received their degrees elsewhere and usually came to his institute as postdoctoral fellows. Arguably the most outspoken among them, Wolfgang Pauli, has also left a well- documented manuscript record that reflects the development of his views on quantum acausality. Pauli’s probably earliest extant comment on the unpredictability of quantum behaviour can be found in his letter written in Copenhagen in June 1923 to Arnold Sommerfeld in Munich. The remark, however brief, is quite provocative, since it compared Pauli’s personal feeling of insecurity and professional anxiety with the uncertainty of a microscopic object, thus suggesting a possible additional motiva- tion for the development of acausal thought. The letter primarily dealt with Sommerfeld’s proposal that Pauli obtain his Habilitation and become a Privatdozent at the University of Munich. Pauli declined the tempting, if belated, offer from his former teacher and doctoral adviser, hoping instead that Bohr, upon returning from an American trip, would invite him again for another stay in Copenhagen. He did understand,