Einstein and Freundlich remained in touch. The famous physicist—now in Berlin—helped find work for the perpetually underemployed astronomer, including having him design the solar telescope for the art nouveau “Einstein Tower” observatory that was under construction in nearby Potsdam. The ruptures following Adolf Hitler’s rise to power in Germany in 1933 dispersed both of them, Einstein first to Caltech (where the definitive confirmations of his gravity theory had been conducted) and then Princeton, and Freundlich to the winds. He ended up in Istanbul teaching until 1937, whereupon he moved to Prague, that city that had first brought the two of them together, as a professor of astronomy. He was not there for long, Central Europe being a dangerous neighborhood at the time, but did manage to have an important influence on Zdeněk Kopal, a Czech student who later occupied a distinguished professorship in astronomy at Manchester for three decades. In January 1939—after the amputation of the Sudetenland from Czechoslovakia but before the Nazi occupation of what would become the Protectorate of Bohemia and Moravia, Freundlich decamped for the University of St. Andrews in Scotland (on Eddington’s recommendation). He taught there until 1959.50 Upon retirement, he returned to Wiesbaden, his hometown, as a professor at the nearby University of Mainz. He died five years later.
* * *
Back in early 1912, in Prague, Einstein was hard at work pushing beyond his initial insights about the equivalence principle and the potential bending of starlight around massive bodies to produce a full-blown general theory of relativity. His starting point was already set: he would work with a modified Poisson equation—Δc = kcρ, where k is a constant, ρ is the mass density, the c on the right is the speed of light in a vacuum, and the Δ is an old-fashioned rendition of the Laplacian operator (which measures how rapidly a function varies across space)—to model the gravitational potential with spatially variable speed of light against a flat (Euclidean) spacetime. From here, he needed to work out a consistent theory. It was grueling labor. He wrote to his former assistant Hopf that he could be excused for his dilatoriness as a correspondent “because I am working like a horse, even if the cart does not always move very far from the spot.”51 Einstein liked the equine simile, which he repeated a week later to Heinrich Zangger: “But I have worked and been driven like a horse. The main subject was a relativity-theoretic work about gravitation. It was finished a few days ago.”52 Well, he had hoped it was. In March 1912 he was forced to confess again to Zangger that “I am up to my ears in the problem of gravitation, so that I cannot summon the energy to write a letter.”53
Nonetheless, despite the downs, there were definitely also ups, and Einstein repeatedly thought he had cracked the problem. “The research into the statics of gravitation (point mechanics[,] electromagnetics[,] gravitational statics) is finished and pleases me very much,” he wrote his friend and colleague Paul Ehrenfest in February 1912. “I truly believe that I have found a piece of truth. Now I am thinking about the dynamic case, once again moving from the more special to the more general.”54 He was confident enough to write enthusiastically to Lorentz, whose physical intuition he admired perhaps more than anyone else’s: “The second matter concerns the relationship of gravitational field—acceleration field—speed of light. Simple and beautiful things emerge entirely inevitably from this. The speed of light c is variable. It determines the force of gravity.”55
He wrote an article outlining what he believed was a complete static theory and sent it off to Wilhelm Wien, editor of the Annalen der Physik. “I am sending you a paper for the Annalen. A great deal of sweat went into it, but I now have total confidence in the matter,” he wrote in his cover letter. “Now I am looking for the dynamics of gravitation. That however won’t come quickly!”56 He was hoping that this theory would finally put to rest the criticisms of one of his rivals, a German physicist based in Turin named Max Abraham. His confidence was misplaced. Two weeks later, Einstein suddenly realized he had made a mistake and wrote Wien asking for him to refrain from publishing the paper. Then he had second thoughts about his second thoughts, sending a further letter on the same day: “This morning I asked you to send me back my manuscript and now I ask that you hold on to it. Certainly not everything that is in the work is tenable. But I believe the matter should be left as is so that those who are interested in the problem can see how I came to the formulas.”57 The problem was how to move from the static to the dynamic case, and it was a tough nut to crack.
Nine days later, he sent another article to Wien, further elaborating the static theory: “I send you here the continuation of the research about the static gravitational field for the Annalen. I have tormented myself dreadfully over this matter, but believe now I have found the correct sense.”58 He sent the good news to Hopf as well: “I have now rigorously derived the theory of gravitation for the static field. The result is wonderfully beautiful and astonishingly simple. Abraham’s theory is entirely false. I will surely have a duel of pens with him.”59
On this, at least, Einstein was most definitely correct. Abraham was not one to take dismissal lying down. Born in 1875 in the then Prussian city of Danzig (now Gdańsk, Poland), Abraham was an astonishingly versatile physicist of Einstein’s own generation. Trained at the University of Berlin under Max Planck, he graduated and assumed a Privatdozent position at Göttingen, which he occupied for nine years. In this role he was situated better, academically speaking, than Einstein was at the Bern Patent Office. In every area of his research he came butting up against Einstein. In 1902 he published a theory of the electron that competed with Lorentz’s model, only to have Einstein’s own modification of Lorentz’s theory, special relativity, take over. (Abraham never relinquished his own theory.) He continued to look for professional advancement, taking a job at the University of Illinois in the United States in 1909 but abandoning it after a few months to assume a teaching post at the polytechnic in Milan. Still on Einstein’s heels, he sent an explication of his first theory of gravity to the Rendiconti della R. Accademia dei Lincei in December 1911.60 Einstein read it in early 1912 and thought it atrocious.
The feeling was mutual. Einstein, as we have seen, felt he had to give up the constancy of the speed of light in order to have a functional theory. “I have now worked out a theory of the statics of gravitation that steps out from the schema of relativity theory in that it violates the principle of the constancy of the speed of light,” he wrote to the Polish physicist Marian von Smoluchowski on 24 March. “I deem it rather certain that this principle only applies insofar as the gravitational potential can be considered as constant.”61 Abraham considered this a contradiction in terms. Given his steadfast opposition to Einstein’s theory of special relativity and his embrace of his own electron theory, he crowed that Einstein “has given up the postulate of the constancy of the speed of light, essential for his earlier theory; in a recently published work he allows the demand of invariance of the equations of motion under Lorentz transformations to lapse, and with that deals relativity theory its death blow.”62
This sparring back and forth formed a constant subtext in Einstein’s on-again, off-again submissions to Wien. In the February paper (which appeared only on 23 May), he noted that “Abraham’s system of equations cannot be reconciled with the equivalence hypothesis, and this conception of time and space cannot be considered correct already from a purely formal mathematical standpoint.”63 The equivalence principle lay at the root of their disagreement: for Einstein it was the entire justification for assuming that relativity theory would have any implications for gravity; for Abraham, it was nonsense. As Einstein himself would come to realize by late spring 1912, largely from the force of Abraham’s criticisms, it was not possible to maintain the equivalence principle as an absolute rule.64 Consider our elevator rider. She could imagine that there was a gravitational field emerging from the floor of the elevator car, but it would be a strange sort of gravitational field that emerged entirely perpendicular to the floor. On Earth, the planet’s spherical shape produces a ra
dial gravitational field that cannot be imitated by any elevator. An equivalence principle could only hold infinitesimally. Abraham preened at what he saw as Einstein’s retreat.65
For the physicist in Prague, however, it was no such thing; it was simply a gradual revision of the theory in view of conceptual difficulties (such as another problem in accounting for momentum conservation). By the beginning of summer 1912, on the eve of moving back to Zurich, Einstein thought any further dialogue would be fruitless. He had begun his research on gravitation already with some reservations, tinged with respect, about this familiar sparring partner. “He must be a curious, immoderate person. Even his quite sharp judgment seems to be influenced by his passion,” he wrote in 1910. “I gladly concede to him that he calculates better than I do, that he knows more books than I do, and whatever else he wants.”66 These comments encapsulate the central element of his objection to Abraham, that he “operates formally without thinking physically!”67 The equivalence principle was a perfect battleground for precisely this conflict: Einstein considered it a wonderful heuristic guiding his physical intuition about how matter should behave in accelerated frames of reference, while all Abraham could focus on were its (undoubted) calculational flaws. For Einstein, this was simply not good physics, and his letters from this period are peppered with sarcastic and dismissive barbs that he tossed—behind Abraham’s back—to his friends about the man from Turin: “Abraham’s theory is created off the top of his head, i.e., created purely from considerations of mathematical beauty, and is completely untenable. I cannot at all comprehend how an intelligent man can let himself be enchanted by such superficiality.”68 In that same letter, he confessed that he had been hoodwinked by the theory for a whole fortnight before coming to his senses. Abraham’s theory was not invariant under Lorentz transformations yet relied on Minkowski’s spacetime geometry, which required such invariance. Fatal internal contradictions emerged in consequence.69
Einstein’s tendency to grumble about Abraham in his letters to friends presents another contrast between the two scientists—who were, it bears underscoring, two of the very few theorists working actively on developing a new theory of gravitation. Einstein may have had serious problems with the kind of physics Abraham was doing (and also his temperament), but he did not lower the tone in public. Although he considered Abraham’s “recently published theory of gravitation to be a severe blunder,” he made efforts in April 1912 to try to obtain for Abraham a better job in Zurich (of all places). “It is just incomprehensible that this truly significant man, because of some high-spirited sarcastic comments which he made several years ago, is to be shunned like a leper,” he wrote.70
Publicly, however, the exchange soon began to descend into polemic—pushed by Abraham’s tone and rhetorical style—until Einstein no longer felt dialogue was productive. On 4 July 1912 his final published response to Abraham concerning the static theory of gravitation was received at the Annalen der Physik. In it he stated that he was amazed that “Abraham writes that I have given the death blow to relativity theory through the denial of the postulate of the constancy of the speed of light and through it the corresponding renunciation of the invariance of systems of equations through Lorentz transformations,” even though he had always insisted that the two postulates of special relativity were “entirely independent of each other.”71 Relaxing the constraints on light in no way invalidated the more important first principle of relativity in all inertial frames of reference. “We do not have the slightest reason to doubt the general validity of the relativity principle,” he continued. “By contrast I am of the view that the principle of the constancy of the speed of light can be considered to be correct insofar that one constrains oneself to spacetime regions of constant gravitational potential. In my opinion here lies the limit of the applicability not of the relativity principle indeed but of the principle of the constancy of the speed of light and thus our current relativity theory.”72 He was ready to concede some of Abraham’s points about the validity of the equivalence principle only in infinitesimally local contexts. “But I see here no reason to forego the equivalence principle also for the infinitely small; nobody will be able to deny that this principle is a natural extrapolation of one of the most general experiential statements of physics.”73
That was his last explicit engagement with Abraham’s ideas. When the latter jumped in for yet another continuation of the fight, Einstein preferred to keep his own counsel. In late August 1912, he sent a final statement to the Annalen: “Since each of us has expressed his point of view with the necessary detail, I consider it unnecessary to answer Abraham’s preceding note. I would like here for the time being only to request that the reader not interpret my silence as agreement.”74 By this time he was already back in Zurich and ready to start a new approach to general relativity relying on tensor mathematics and curved spacetime. He discarded both Abraham and the static theory in a single move.
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Einstein may have publicly abandoned Abraham, but Abraham had not abandoned him, predicting the eventual capsizing of relativity theory against the shoals of his continued reliance on the equivalence principle.75 In Einstein’s letters to friends discussing, among other things, the development with Marcel Grossmann of the Entwurf theory of general relativity, he still could vent his spleen in acerbic asides: “Abraham’s new theory is, so far as I can see, logically correct, yet it is still an embarrassing stillbirth,” he wrote to Arnold Sommerfeld in October 1912, a year after the Solvay Conference.76
Einstein could ignore Abraham as he continued to work on gravitation, but Abraham could not symmetrically ignore him. Einstein was a major player in theoretical physics, despite gravity not being a major playing field of that discipline. Any mainstream engagement with gravitation had to pay attention to the publications coming out of the ETH and, after 1914, from Berlin. Abraham continued to pursue his agenda, but he was getting less and less attention, even as he drew novel implications from his theories, such as gravitational waves.77 In 1914, Abraham announced the end of relativity:
Any relativity theory, both the special one of 1905 and the general one of 1913, will thus founder on the shoals of gravity. The relativistic ideas are obviously not broad enough to serve as the framework for a complete world picture.
Yet relativity theory remains a historic achievement in the criticism of the concepts of space and time. It has taught us that these concepts depend upon their representations, that we form them from the behavior of measurement of length and time intervals using yardsticks and clocks, and they are subject to the vicissitudes of those. This secures for relativity theory an honorable burial.78
From above the fray, Einstein continued to grudgingly admire Abraham’s persistence: “He complains powerfully in Scienza against everything relativity, but with understanding.”79 Einstein even wrote yet another letter in support of Abraham’s hiring, this time in Aachen in 1921.80 The irascible antagonist died just one year later.
Once the two ceased jousting, the field of interlocutors in gravitational physics narrowed considerably. The two main alternatives to the line Einstein was pursuing after his abandonment of the static theory were those of Gustav Mie, then at the small northern German university in Greifswald, and Gunnar Nordström, a talented theorist from Helsinki, at the time a city within the Russian Empire. Einstein did not have much patience for Mie, but he followed Nordström’s work carefully, regarding it as the most significant alternative to his own line of research, and one that had the potential to be correct.81 Educated at the universities in both Helsinki and Göttingen (he received his doctorate from the former in 1910), Nordström was only two years younger than Einstein and in precisely the same generation.82 He was also, as his geographic origins indicate, marginal to the mainstream of Germanophone physics, as was every researcher on gravitation in this period with the exception of Einstein.
Nordström’s work was published in late 1912—thus after Einstein had already left Prague—but his imp
etus for it stemmed from the static theory Einstein had developed in Bohemia. He was impressed with Einstein’s results that showed the connection between the generalization of relativity to accelerated frames of reference and a new theory of gravitation, but he wanted to develop it without the problematic equivalence principle. He also insisted on using flat, Euclidean spacetime. In the appendix to one of his important early articles, Nordström acknowledged his fruitful constructive correspondence with Einstein, a compliment that Einstein returned in print in 1913 when he maintained that Nordström’s static theory was the only alternative to his own that he considered viable.83 In many respects, the theories were equivalent; where they differed most substantially was in the prediction of whether light would bend in strong gravitational fields: Einstein’s theory said it would; Nordström’s said it would not. That question would be spectacularly resolved in 1919. In this moment before the Great War, however, Einstein continued to encourage Nordström, and during the war he helped him cross heavily militarized borders by issuing him bona fides.84 The relationship remained cordial until the Finn’s premature death in 1923. In 1915, in fact, Einstein was involved in placing Nordström’s name in second position on the hiring committee list for a post in Berlin.85
Speaking of those in secondo loco who were associated with Einstein, there was another marginal player in the pursuit of gravity in 1912 who was, however, never acknowledged by him: Gustav Jaumann, the man who had initially been offered the Prague position—after some ministerial legerdemain—instead of Einstein. Among theoretical physicists in this period, Jaumann had a reputation for maintaining a rigid and unwavering commitment to continuum physics. Following his mentor Ernst Mach’s epistemological skepticism about the ontological reality of atoms, Jaumann had built an entire physics out of continuous fields and substances, a technical feat as mathematically challenging as it was out of step with the mainstream. In his treatment, gravity was just like temperature, a potential built out of the Poisson equation, and he claimed that his great achievement was to explain the long-term stability of gravitational systems—as well as the anomalous perihelion advance of Mercury, which later became one of the three classic tests of Einstein’s theory of general relativity.86
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