Relativity theory—as it has convinced the Czechs of the fruitful functioning of the German University in terms of its outstanding cultural and economic significance—attempts, in the space of the city of Prague and in the difficult time in which we live, to squeeze these down to a quantité négligeable whereby the law of inertia is never taken into consideration. Alone this proves the greatness of Einstein and the grandeur of German science, which commands the attention of even a national opponent, that I must not fear that even the mayor, if he should learn of this greeting, would disavow me.4
This was not entirely unanticipated: German-identified residents of the city had shifted from being a tiny local minority within a polity that they dominated to being a tiny minority in a country run by the newly minted “Czechoslovaks.” Their expression of Teutonic pride was predictable, and Einstein made no comment about it. Instead, his letters to Elsa and friends focused on the debate he had with Oskar Kraus, a professor of philosophy at the German University. In recent years, Kraus had emerged as the most vocal Bohemian representative of a broader Germanophone attack on Einstein’s relativity theory. Now Kraus had an opportunity to confront Einstein in person.
“My lectures here, and also the discussion evening, are already done,” the physicist wrote to Elsa soon after the conclusion of the debate. “The latter was very amusing (also with Kraus).”5 He told his good friend Paul Ehrenfest almost two weeks later that the Kraus affair had been “a generally droll circus performance; he was however serious about it.”6 Kraus had followed Einstein’s lecture, which had focused on special relativity and the speed of light, with a witty and cutting broadside against the fundamental postulates of relative motion. Einstein had barely given it a response. Instead, claiming he did not want to exhaust the audience, he had picked up his violin and played a Mozart sonata. The Prager Tagblatt reporter noted that he hadn’t been half bad. Einstein had been a hit with the audience. He now beamed upon Prague. “The present stay was a great joy for me,” he told Elsa. “I have a strong desire to shorten Vienna and in the interim remain here.”7 But it was not to be. He went to the train station with the Winternitz family, the Franks, and the mathematician Georg Pick. He was off on his world travels, leaving Prague behind forever.
Prague did not let go of him so easily. The disciples who had been grouped together in the philosophy faculty of the German University in Einstein’s day had been divided a few months before his brief visit into a philosophy faculty, including the humanities and especially its titular discipline, and a natural sciences faculty.8 At the time of the performance at the Urania, Frank and Kraus, until recently close colleagues, were separated. For the next two decades, they would be sharp (though personally cordial) rivals in the pitched debates over relativity theory from within the walls of the German University. This chapter focuses on the two of them and their quest to define the universal philosophical significance of Einstein’s relativity within the local confines of Germanophone Prague. Their struggle was over nothing less than the status of Einstein’s theory for history.
Given the narrow nature of his social circle, it was common among Einstein’s former associates in Prague to think of Philipp Frank and Oskar Kraus together, though their personal affections for the former and slight distaste for the latter colored their comments. The mathematician Gerhard Kowaleski, for example, recalled the pair in his memoirs in starkly opposing terms. “Philipp Frank exercised a strong effect in Prague. His lectures stood out for calm and clarity. At the beginning of each hour he gave a brief overview of what was last presented. Frank was also active in the area of philosophy of science,” he wrote. “Besides that he was a linguistic genius. He knew fluent Arabic and Syriac. Also Frau Frank, née Gerson, was of superior cleverness. Both also appeared often at the lecture evenings of Frau Fanta”—since Bertha Fanta died in 1918, such joint visits would have taken place before their wedding. “I still remember an impressive lecture about relative motion that Philipp Frank gave there.”9 Indeed, Frank was already before the war making an international name for himself as an explainer of relativity theory and its potentially radical philosophical implications, a reputation that would grow in ensuing decades.
Kowalewski held Kraus in less esteem. “In Prague Einstein found a fierce opponent in the philosopher Oskar Kraus.… Kraus remembered only sparse remnants of natural scientific and mathematical knowledge from his school days. Thus armed he charged into battle with relativity theory, which he only knew from Einstein’s short popular book,” he wrote. “As a jurist he clung to words and looked to dissect contradictions between a certain sentence on page x and one on page y. In Prague he had several public disputations with Einstein”—history registers but one of these—“from which naturally nothing emerged.” Kowaleski was pleased about the Urania clash solely because of the violin concert at the end. “That was the only pleasure that this evening brought the public,” he declared.10 In his canonical biography of Einstein, Frank echoed Kowaleski’s assessment of Kraus’s scientific acumen. The latter was “an acute thinker in the philosophy of law, whose conception of scientific discussions, however, was more like that of a counsel at a trial,” he concluded. “He made no attempt to explore the truth, but instead wanted only to refute his opponent by finding passages that were contradictory in the writings of Einstein’s supporters. In this he was successful.”11 He was also, although Frank did not mention it, a significant figure in the Habsburg and post-Habsburg intellectual tradition, cultivating the legacy (reputational and archival) of his mentor Franz Brentano into a dynamic and powerful philosophical school.
Kraus, no less than Frank, was a representative in Prague of a vibrant area of intellectual ferment: the multiple braided Austrian traditions in the philosophy of science. To a remarkable degree the major trends of this important domain of philosophy were crafted by people either originating from or working in the Habsburg Empire and its successor states. A partial recounting of some leading figures should suffice as an illustration: Bernard Bolzano, Ernst Mach, Franz Brentano, Carl Menger, Ludwig Boltzmann, Alois Höfler, Edmund Husserl, Hans Hahn, Otto Neurath, Rudolf Carnap, Moritz Schlick, Karl Popper, Ludwik Fleck, Friedrich von Hayek, Michael Polanyi, Imre Lakatos, and Paul Feyerabend. Almost every major philosopher working in this space contributed notably to the philosophy of science, something that cannot be said of their counterparts in Germany.12 To the extent that this fact has grabbed the attention of scholars, the emphasis has been on the undoubted significance of Vienna as a concentrating force. That was, to be sure, the main locus of the field’s energies, but it was not the only one.
Philipp Frank and (more rarely) Oskar Kraus are sometimes mentioned in passing as contributors to this dialogue, but their association with Prague has had the unfortunate effect of relegating them to the background.13 Even Frank, in an oral history interview from 1962 with the later famous (and non-Habsburg) philosopher and historian of science Thomas S. Kuhn, talked about the city that way at times. “The German University of Prague was really no different. It was just a branch of Vienna. It was different by the fact that Einstein was there, but that was casual circumstance,” he told Kuhn.14 But Prague was far from just that. Through the complex interaction of personalities, political transformations, and contingencies, Frank and Kraus both separately and in conflict with each other transformed Prague into one of the key nodes of European philosophy of science. The vehicle that enabled them to do this was the spirited debate over the epistemological significance, or lack thereof, of Albert Einstein’s theory of relativity that occurred following the First World War. By tracking these two figures and their engagement with relativity, we observe the centrality of Prague in shaping both the apotheosis and the demonization of Einstein in the interwar period.
* * *
Einstein chaired the committee that sought his successor, and he placed Philipp Frank in the first position, even over his close friend Paul Ehrenfest, who was desperately searching for a job at that moment; they were trailed b
y Emil Kohl, the Vienna Privatdozent who had also been in third position in the search that had led to Einstein’s own call to the German University. Anton Lampa, Pick, and Einstein had clearly learned from the frustrations of that previous round, when the Education Ministry had flipped the order of the first and second positions, and so they “came to the view that among the theoretical physicists within the country some are found of such ability that it is natural to propose only Austrians.” This conclusion implicitly recognizes that the difficulty with Einstein’s own candidacy had been his citizenship status, not his religious affiliation: all three candidates for his successor were Jews by confession, but they were also—as the report explicitly noted—“of German nationality,” a nod to the increasingly tense relations that were developing between the Czech and German institutions.15 Frank came out on top in no small part because he had already begun to publish important articles on relativity.16
The committee also thought it significant to note “that Frank has written several original essays with epistemological content (The Law of Causality and Experience. Mechanism or Vitalism?), that give evidence of the versatility of the author as well as of his ambition to engage with the general problems of knowledge.”17 The causality article, published in 1907, had been a landmark for Frank, marking the beginning of a career interrogating the implications of causality long before the paradoxes of quantum mechanics had raised this issue to primacy. Einstein’s first contact with Frank, in fact, had occurred when the then Zurich-based physicist had written the other man to praise him for his analysis.18 In his 1932 book The Law of Causality and Its Limits, published in Prague, Frank would thank Einstein for many years of exchanges on this question, as well as cite local friends like Hugo Bergmann in ripostes to Einstein nemeses Hugo Dingler and (from a very different point of view) Henri Bergson.19 And so, in 1912, Frank arrived at the German University, sporting a full beard and, as one of his former students recalled, “by far the youngest professor at the University.”20
Frank was widely praised for his astute lectures in theoretical physics, and he performed his pedagogical duties in the field with aplomb, but it soon became clear that his interest in the philosophy of science was no minor matter, but rather a core aspect of his research. Especially at this moment, in the early days of relativity and quantum theory, the boundary between physics and the philosophy of physics proved particularly blurry. While a student in Vienna, Frank had studied with Ludwig Boltzmann (until his suicide in 1906) and other philosophically inclined professors, but found his main interlocutors, the mathematician Hans Hahn and the philosopher Otto Neurath, in a tight discussion circle that in later years came to be called “the first Vienna Circle,” a precursor to the famous movement in logical empiricism. The close-knit group met regularly from 1907 until 1912, when Frank departed for Prague. Together, they read cutting-edge works by philosophical scientists like Pierre Duhem, Henri Poincaré, and Abel Rey, but their main fascination was with the retired Viennese professor Ernst Mach.21
We have already encountered Mach as the rector of what was then the Charles-Ferdinand University, reluctantly negotiating the settlement that would lead to the split of that institution into the Czech and the German Universities in 1882. It was Frank’s devotion to Mach’s ideas and legacy that rendered his epistemological emphasis not a defect in the eyes of his colleagues but a great asset. For them Frank’s investigations represented—much as Einstein’s might have done had he stayed at the German University—a line of continuity that reached back to the great experimentalist and philosopher of science. It is impossible to understand Frank’s impact on the philosophy of relativity theory, or the intellectual culture surrounding epistemology in Germanophone Prague, without placing Mach at their foundation.
Ernst Mach was born on 18 February 1838 in the town of Chirlitz (today Chrlice), just outside of the city of Brno in Moravia. His family stemmed from Bohemia and Moravia, which meant he had ancestors on either side of the German–Czech language divide and grew up speaking both languages at home and during his education at the gymnasium in Kremsier (Kroměříž).22 After obtaining his doctorate in physics in 1860 from the University of Vienna, Mach took a position at the University of Graz before moving to Prague, where he would spend the longest stretch of his career. Given that Mach eventually left Prague in frustration—with both his assistant Gustav Jaumann and his counterparts at both universities—one might suspect that he experienced his time in Bohemia as a kind of exile from Vienna, to which he returned in 1895. In fact, he turned down other job offers (from Graz in 1876, from Jena in 1882, from Munich in 1885, and from Graz again in 1890) to stay and pursue his experimental program. The bulk of his publications across his career stemmed from his time in Prague, especially his authoritative experiments concerning the Doppler effect and his acoustic research that led to the dubbing of speeds above the sound barrier as “Mach 1,” “Mach 2,” and so on. (Christian Doppler, as it happened, had come up with his eponymous theory of the shift of frequencies—at first light, and later, more famously, sound—as a function of the velocity of its source while serving as a professor at the Polytechnic in Prague a generation earlier.)23 Prague was also where Mach became interested in the history and philosophy of science (due to reading during a bout of illness that kept him from the laboratory), which he would incorporate into his teaching repertoire.24
Mach’s seminal contributions to epistemology developed from a series of articles on perceptual psychology and empiricism and his landmark historical monographs, The Principles of the Theory of Heat (1896) and especially The Science of Mechanics (1883).25 Even in his more abstract pieces about the analysis of sensations, which primarily aimed to break down the notion that there was a unitary perceiving subject, one can find traces of his local context, such as a vivid invocation of a walk with his three-year-old son “upon the walls about Prague.”26 The guiding principle of Mach’s research, the insight that grounded the tremendous influence his work would have—especially upon Philipp Frank—was an implacable hostility to “metaphysical” thought in the sciences. For Mach, good science was about observations made by the senses, and only about that. Contrary to caricatures by later opponents, Mach did not reject all conceptual tools in scientific reasoning; rather, he considered such notions essential to give “economy” to scientific analysis. If, however, scientists were unable to point to concrete perceptions and observations that undergirded a concept, then that concept should be discarded from their toolkit. By reforming how they talked about ideas, Mach insisted, physicists could build a metaphysics-free science in which every theory was bound by ironclad links to the empiricism that validated all epistemology. The American philosopher and psychologist William James, upon visiting Mach in Prague in 1882, wrote to his wife in admiration: “I don’t think anyone ever gave me so strong an impression of pure intellectual genius.”27
Someone else who thought so, at least at first, was the young Albert Einstein. While living in Bern and working in the patent office, Einstein and a set of close friends formed a group they called the “Olympia Academy,” which was dedicated to reading and discussing philosophy, physics, and also Don Quixote. Ernst Mach’s Mechanics was a central text; its sharp epistemological analysis of concepts and their relation to experience clearly left an imprint on Einstein’s 1905 declaration that since the luminiferous ether could not be detected by experiment, it was “superfluous” to electrodynamic theory.28 As his career became more established, Einstein reached out to Mach in a flattering letter of August 1909: “Besides I naturally know your major works very well; among them I especially admire the one about mechanics. You have had such an impact on the epistemological views of the younger generation of physicists, that even your present opponents, such as e.g. Mr. Planck, would without doubt be called ‘Machian’ by one of those physicists as they generally were a few decades ago.”29 However complimentary Einstein was of Mach’s lucid criticisms of physical theory at this early stage, he did not hesitate to point
out where he thought the venerable professor had slipped. One of the key points of this letter was to open a dialogue to persuade Mach that his well-known opposition to atomism as “metaphysical” was misguided.
It was a friendly exchange, and Einstein sent another letter 12 days later stating that he was “very delighted that you are pleased with the relativity theory.”30 This understanding of special relativity as essentially “Machian” was widespread among certain circles, especially in Prague. Georg Pick, Einstein’s colleague at the German University who had studied with Mach decades earlier, assured his former teacher that there was nothing amiss about the strange ideas of relativity: “That the physicists who have formed the new theory were essentially influenced by your thoughts I learned, were it not objectively clear to me, from one of them himself: Einstein, through personal communication.”31 (Mach’s pacifism and vocal opposition to anti-Semitism were also very congenial to the younger scientist.32) Einstein’s actions at this period suited his words: not only did he continue to correspond with Mach about his current research on general relativity—a frustratingly undated letter mentions his work on a “new theory,” but it is unclear whether this was the Prague static theory or the Zurich Entwurf—but he also signed onto the program of a recently founded “Society for Positivist Philosophy” dedicated to the antimetaphysical epistemological program. Ernst Mach was another signatory.33 Upon Mach’s death in February 1916, Einstein wrote a generous obituary, though he was more impressed by the encomium to the sage of positivism penned by Philipp Frank.34
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