Einstein’s light-hearted nonchalance about the meeting stemmed from the misalignment of his current research with the agenda of the conference, and this had everything to do with where he was then based geographically. If you were unaware of the details of Einstein’s biography and the mutable borders of European history, the list of attendees printed in the original French proceedings of the Solvay Conference might strike you as perplexing. The participants were separated by country, a fairly typical connection in this age of nation-based competitive internationalism (similar to the recently inaugurated Olympic Games), and down the list one comes across this line: “For Austria, professors A. Einstein of Prague; F. Hasenöhrl of Vienna.”10 This reminds us of two things: Einstein was coming to Brussels from Prague, and Prague was at this moment an Austrian city. Both facts transport us back to the physicist’s local context of 1911.
Although early in his time in Prague Einstein continued to pursue some questions related to quantum topics and occasionally fielded correspondence about them, he had sharply changed his research area when he arrived in Bohemia.11 Or, more precisely, he had changed it back. Einstein published almost a dozen papers while in Prague, and with very few exceptions they all concentrated on one topic: how to expand his special theory of relativity so it could encompass accelerated frames of reference, something that he had understood since 1907 would necessarily entail a new theory of gravity. Moving to Prague was a caesura in his personal life. It broke off relationships with friends and implanted him and his family in an alien city where they had no established ties. But it was also importantly an intellectual rupture, in this case with the quantum theory. Einstein took advantage of the lack of established local expectations, as well as what he interpreted as an impoverished intellectual community, to turn inward to explore the general theory of relativity.
Today, when we know that it is possible to formulate a self-consistent general relativity and that this theory provides a powerful interpretation of gravity—in fact, that it remains, over a century later, the centerpiece of gravitational physics and cosmology largely in the form Einstein endowed it—the choice seems a bold but reasonable one. It did not strike his peers that way. Around 1910, there were several unsolved anomalies in the dominant Newtonian theory of gravity that had been recognized for two centuries, but these were not seen as pressing problems, certainly not in the face of the fascinating vistas opened up by quantum physics. The physicists drawn to gravity were marginal players who often found it difficult to maintain a steady job while working in this backwater of physics. There was definitely nobody of the stature of an Einstein (even factoring in his relative youth) to measure himself against, yet he engaged in detailed and often blistering polemics during his three semesters in Prague as he struggled to develop what came to be known as the “static theory.”
He failed. His attempt to generalize special relativity, the most distinctive effort in physics of his time in Prague, foundered on a series of internal inconsistencies, and Einstein abandoned the project by the summer of 1912, when he left Prague and returned to Zurich for a new position at the Federal Institute of Technology. He then teamed up with his college classmate and ETH professor of mathematics Marcel Grossmann to develop what came to be called the “outline” (Entwurf) theory of general relativity, a problematic though promising approach that three years later would evolve into the gravitational field equations still in use today. Historians have reconstructed Einstein’s chaotic path from the Entwurf theory to general relativity in minute detail, scouring his famous “Zurich notebook” for clues to the course of the physicist’s negotiation of different puzzles posed by his new tensor formalism, which necessitated a curved spacetime.12 But this was not the theory he had crafted in Prague: that one worked with “flat” (i.e., Euclidean) spacetime, and the only lasting residue of it was a prediction of the bending of starlight around the sun’s gravitational field during a solar eclipse, an important conjecture that would play an outsized role in making Einstein’s reputation.
Had Einstein been killed by a runaway horse cart in July of 1912 as he was arranging his affairs to move back to Switzerland, we would not think of him today as the architect of general relativity. (The scenario is not fanciful; Marie Curie’s husband Pierre had died in precisely such a manner on 19 April 1906.) Einstein would be seen as a trailblazer for his insights in 1907 but ultimately as a failure in bringing the theory to fruition. That he would eventually solve the problem of general relativity was not obvious in 1912, yet implicitly historians and physicists have tended to assume this by concentrating on Einstein’s path once he arrived in Zurich—that is, when he was on a course that would eventually manifest as “the right track”—thus relegating the Prague theory to the shadows as an unsuccessful distraction. If what you are interested in is understanding the theory Einstein unveiled in Berlin in November 1915, this approach makes sense. If, on the other hand, we want to appreciate Einstein as a physicist like many others, wandering into mistaken cul-de-sacs and admitting his mistakes, and also to understand how the path toward one of the most successful physical theories of all time was deeply rooted in the opportunities and constraints imposed by Prague, we must treat the static theory as Einstein did: very seriously.
* * *
No idea has ever emerged full-blown from the mind of its creator, an Athena bursting forth from Zeus’s skull. All theories, in physics no less than in art, have contexts that shape and condition their birth. In the case of Einstein’s shift from quantum theory to gravity, that context was the German University in Prague, where he arrived in early April 1911 to take up a post as ordinary professor of mathematical physics. He was flush with a salary of 8,672 crowns and the resident of a brand-new apartment across the river from the new institute of theoretical physics that he would helm, located at Weinberggasse, or Viničná ulice 3 (later renumbered 7). The summer semester began on 20 April, and Einstein geared up to teach his courses and integrate into the community of the city on the Moldau.
The semester lasted until the end of July, and he spent many hours a week teaching three different courses: “The Mechanics of Discrete Mass Points,” which he taught to 13 students for three hours a week; “Thermodynamics,” to 12 students for two hours a week; and a weekly seminar to 6 additional students. His winter semester resembled the summer one. From October 1911 until Palm Sunday 1912, he taught the same courses, with doubled enrollments in “Thermodynamics,” at the Clementinum, right at the base of the Charles Bridge in the historic halls of the sixteenth-century Jesuit college, while the seminar was conducted back at the institute in the New Town. His final semester—by which point his departure had become public knowledge—comprised courses on “Mechanics of Continua,” “The Molecular Theory of Heat,” and another seminar, held on Friday evenings, in which seven students forsook the pleasures of the town to discuss theoretical physics in detail.13 There was nothing extraordinary about this teaching load: these were basic, foundational courses, disconnected from his current research, and he garnered the usual enrollments one would expect in the increasingly strapped German University. The only significant change across these three semesters was that the number of auditors in Einstein’s courses started high with eight and then fell to two for the following semesters, as his novelty wore off.
Three of those auditors came from the Czech University, which was not especially unusual in technical subjects like theoretical physics. Perhaps more remarkable is that Einstein did not seem to recognize that there was a nationalist valence to these students’ choice to audit his classes. Here, again, Einstein’s ignorance of the local context did not mean that his auditors did not experience some tension, or relief that Einstein did not make an issue of it. Since student reminiscences of Einstein’s teaching from this period are rare, and these particular ones are largely unknown outside Czech scholarship, it is worth quoting one of them at length. This comes from Miroslav Hrabák, himself a resident of Einstein’s neighborhood of Smíchov and the son of a
metal lathe operator:
Einstein was calm, of small stature, with flowing hair. We met him always with a Virginia cigar, even during lectures, when he constantly walked around with the index finger of his right hand on his nose and his thumb on his chin. His presentation of the material was very demanding, he presupposed a high level of knowledge of the students. He wrote the formulas and examples messily on the board; even a colleague, confused by a disorganized calculation, once complained about something. Usually placid, he made a fuss—he retorted that he is not a sergeant (feldvébl), but then he wrote it in order. He in general looked down on the Austro-Hungarian order which reigned in the school. I took two examinations with him: I remember that for one of them he examined me from eight in the morning until noon, when the famous midday firing of the cannon alerted him that time was up. He interrogated me about all of physics. Then he put on his coat and we went down together, across the Palacký bridge to Smíchov, where I lived.14
In the wake of the tumults of the twentieth century, the interviewer had found Hrabák working at the Slavonic Library in Prague. This report, and the few others like it, engage in the posthumous romanticization of Einstein that seems unavoidable among those who had encountered him decades earlier. Nonetheless, we obtain a picture of students exhibiting the respect for an instructor typical of that time and place.
The respect was not necessarily mutual. Einstein walked into his first class with relatively low expectations. “My colleague Lampa tells me that the scientific interest of the students here is very feeble,” he wrote to Zangger three days after his arrival in the city, and two weeks before his first lecture. “But I believe one’s voice will also ring through this forest according to the manner in which one calls. I will not let my illusions be devastated so easily.”15 His lecture notes that survive show that his courses derived from the similar ones he had taught in Zurich, but now he evidently gave less attention to his pedagogy.16 Presumably, this had something to do with his focus on his own research. According to another later reminiscence from an auditor who had graduated in philosophy a few years earlier, Hugo Bergmann, Einstein tried to put a positive spin on what must have seemed like mindless drudgery. “Actually I am glad to be allowed to lecture on these elementary matters, for in my own research it often happens that I pursue and explore some thought,” Bergmann remembered him saying, “only to realize in the end that I have been wandering in a maze; all those weeks of hunting a phantom would have been lost, had I not been giving my lectures and thus done something useful during all that time.”17 It seems, as we saw earlier, that the students did not notice his abstraction, and a later professor at the German University who had arrived for graduate study just after Einstein had left recalled that he had a reputation for being accessible to students.18
If stimulation were not going to come from his students, Einstein might have hoped he could engage his colleagues on the details of his scientific research, but this was also largely a mirage. His research assistant during his final semester in Prague, Otto Stern, related in an interview preserved on a magnetic tape stored in the archives of the ETH that “Einstein was completely isolated in Prague (1912), even though there were four colleges there: a German university, a Czech university, a German polytechnic, and a Czech polytechnic. At none of them was there a single person with whom Einstein could talk over the matters which truly interested him. He did so nolens volens with me.… The only really intelligent man there was a mathematician named Pick.”19
Georg Pick was the only one Einstein claimed he would miss upon his return to Zurich.20 Pick had been born in August 1859 in Vienna and had studied physics at the university there from 1875 to 1879, the year of Einstein’s birth. The differences in their ages and career experiences marked them as different sorts of scholars. Pick earned his doctorate at Königsberg with legendary mathematician Karl Weierstrass in 1880, and then moved to Prague to serve as assistant to Ernst Mach, then professor of experimental physics. He filed his habilitation there in 1882 (the year of the split); it was entitled “On the Integration of Hyperelliptical Differentials Through Logarithms.” He stayed at the German University for the rest of his career, during which he published 67 works mostly in complex analysis and differential geometry, but with individual studies in a wide array of other specializations. When Anton Puchta, a professor of mathematics, resigned his post in 1888, Pick stepped in and was appointed ordinary professor four years later. He taught for 46 years in the faculty of philosophy, even serving as dean during the 1900–1901 academic year, and upon retirement moved to Vienna, only returning to Prague with the Anschluss between Austria and Nazi Germany, 79 years old and in poor health. He would eventually die in 1942 shortly after his deportation to the concentration camp at Theresienstadt, a victim of Hitler’s Protectorate of Bohemia and Moravia.21 That, of course, lay in the future when Einstein met him. The physicist admired this senior mathematician, and the two discussed his ongoing research into gravitation and relativity theory. This was his only significant interchange with local faculty about these topics.
For all his intellectual isolation from both students and colleagues, Einstein seemed to work happily at Prague. He was most pleased with his professorial accommodations. “I have a magnificent institute here in which I work very contentedly,” he bubbled to Marcel Grossmann a few weeks after his arrival in Bohemia. However, he could not resist immediately following this statement with a few complaints about the city surrounding the Institute for Theoretical Physics: “Otherwise it is less homey (the Czech language, bedbugs, miserable water, etc.). The Czechs are, by the way, much more harmless than one thinks. I scarcely know my colleagues yet. The administration is very bureaucratic. Infinite quantities of paper-pushing for the most insignificant nonsense.”22 Einstein mostly appreciated the rich library on Weinberggasse, where Ernst Mach had established the new physics institute in 1879, moving it from the center of the Old Town on Obstmarkt (Ovocný trh), just before the division of the university. As was the rule, the institute followed its director to the German University.23 Einstein had an office on an upper floor, and he essentially cabined himself in there with notebooks and references. “This summer I will not leave Prague because I urgently need the vacation for work,” he told Michele Besso. “My position and my institute here make me very happy. Only the people are so alien to me.”24 Intellectually alone, he settled into what he came to Prague for: thinking about gravitation. He had been waiting a long time for this opportunity.
* * *
The year 1905 has long been known as Einstein’s annus mirabilis, or “miracle year,” and for good reason. In a rapid burst of productivity in the winter and spring, he sent off articles detailing his new theory of Brownian motion, the photoelectric effect, and of course the special theory of relativity. Each of these could (and eventually did) transform separate domains of theoretical physics: atomic theory, statistical mechanics, and electromagnetism. The attention of theoretical physicists homed in on the patent clerk from Bern, whose theories were rapidly assimilated into the mainstream of the science. As noted earlier, the implications of Einstein’s light quanta for almost every area of physics became a dominant, though disputed, topic for more than a decade to come among his peers, but that was only because the tremendous consequences of relativity theory had in large part been widely accepted. It would take some time yet for Einstein’s bold fiat excommunicating from existence the luminiferous ether—the hypothesized and, until Einstein, universally accepted medium upon which light waves and other forms of electromagnetic radiation were presumed to travel—to become commonplace, ushered in by the development of a four-dimensional mathematical interpretation of spacetime created by Einstein’s former mathematics teacher at the ETH, Hermann Minkowski (now at Göttingen), in 1907.25
That same year, Johannes Stark invited Einstein to write a review essay on relativity theory for the Jahrbuch der Radioaktivität und Elektronik, an opportunity that allowed him to summarize and tie together the various refine
ments that had been made to special relativity during the prior two years—including the derivation of the mass-energy relation encoded in his most famous equation, E = mc2—and to lay out some directions for future research. In the final section, he considered what would be required to generalize the special theory of relativity. The latter was “special” because it described the relationship between space, time, and the speed of light solely for inertial frames of reference, that is, those moving at a constant speed with respect to each other. What would happen, Einstein wondered, if one tried to apply the theory to all frames of reference, including those that were accelerating?
To understand the challenge of the problem, it helps to review the basic features of special relativity. Imagine—to take one of Einstein’s classic examples—a train hurtling along at an exceedingly high, but constant, velocity (think science-fictional speeds, such as a significant fraction of the speed of light), passing by a stationhouse on the embankment (see figure 2). There is a gentleman sitting on the roof of the train at its exact midpoint, with two mirrors angled to each other so that he can view the front and the back of the train at the same time. In the middle of the station platform, another person is standing; she also has a similar contraption that lets her observe both ends of the train. Just as the two individuals are opposite each other, the woman notices a freak of nature: lightning bolts have streaked out of the heavens and struck the tracks just at the front and back of the train! Holy smokes, she thinks, I’ll have to speak to the rooftop rider later about this event. When the two get together, however, they find they have different accounts of the event. Where the woman saw both lightning bolts strike simultaneously, the man observed a lightning bolt strike the front of the train, and then a few moments later a second bolt hit the back of the train. Who is correct? As special relativity shows, they both are.
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