The writings in this volume date from the beginning of 1912 to the spring of 1914, the two years before Einstein left Zurich for Berlin. While his struggle with the problems of quanta, rather than those of relativity, had dominated his work from 1909 through 1911 (see Volume 3), he was now concentrating his efforts on attempting to construct a relativistic theory of gravitation. Einstein's efforts would finally achieve their goal in the autumn of 1915, when he completed his general theory of relativity.

Three scientific manuscripts, printed here for the first time, provide some insight into Einstein's efforts to generalize his original relativity theory into a relativistic theory of gravitation. The first is a review article on the special theory of relativity (Doc. 1); the second consists of notes documenting Einstein's research on gravitation as well as the support he received from his friend and former fellow student at the Swiss Federal Institute of Technology (ETH), the mathematician Marcel Grossmann (Doc. 10); and the third manuscript contains calculations by Einstein in collaboration with Michele Besso, another friend from his student days, on the problem of the motion of the perihelion of Mercury (Doc. 14). The three unpublished manuscripts document the kinds of trials and errors that cannot be reconstructed from Einstein's published papers, and in this way add to our understanding of the creation of general relativity.

During the period covered by this volume, Einstein's professional status rose rapidly. As 1912 began, he was professor at the German University of Prague, a relative backwater in scientific research. In the course of that year, however, Einstein declined offers of professorships at the universities of Utrecht and Leyden (where he would have succeeded H. A. Lorentz). He accepted instead a professorship in Zurich, but this time at the ETH rather than the university. A year after his return to Zurich in the summer of 1912, negotiations began about a research appointment in Berlin. In the spring of 1914 Einstein was about to depart from Zurich to take up a handsomely-paid position as a member of the Prussian Academy of Sciences, perhaps the crowning achievement for a German scientist of his generation.

During this same period major changes in Einstein's personal life had also begun. His marriage to Mileva Maric; was deteriorating; the couple would soon separate and eventually divorce. Einstein had also become reacquainted with his cousin Elsa when he visited Berlin in the spring of 1912, and they would marry in 1919, a few months after his divorce from Mileva. These developments, both professional and personal, are extensively documented in Einstein's correspondence during the Swiss Years, published in Volume 5.

For those who think of Einstein primarily as the creator of both the special and the general theories of relativity, it may come as a surprise to learn that his increasing prominence in the years from 1905 through 1914 was due in large part to his contributions to the quantum theory. Physicists generally were more interested in the unexpected experimental connections that even the early quantum theories predicted than they were in the abstractions of relativity, which were less capable of being tested experimentally.

When, for example, Max Planck, Walther Nernst, Heinrich Rubens, and Emil Warburg proposed Einstein for membership in the Prussian Academy of Sciences, they began by discussing his early work on relativity, but went on to describe his studies in the quantum theory, and particularly his work on the problem of specific heats. Their language leaves no doubt about what they emphasized: "His contributions to other questions on which contemporary interest is focused have proved to be far more significant."

Einstein contributed very little to quantum theory in the years 1912-1914. His major contribution was a derivation of the law of photochemical equivalence from purely thermodynamic considerations, together with the law of mass action, which led to the conclusion that photochemically active substances emit and absorb radiation energy in discrete quantities. This work added much to the understanding of the quantum hypothesis. His collaboration with Otto Stern on the application of the quantum theory to rotating molecules, on the other hand, at first seemed to provide an explanation of Arnold Eucken's measurements of the specific heat of hydrogen. But soon afterwards the explanation lost its foundation when Einstein lost confidence in the most important feature of the paper, namely the introduction of a term representing zero-point energy.

Einstein's first attempt at generalizing his relativity theory of 1905 to include accelerated frames of reference dates from 1907. His hypothesis, formulated in a review paper that year, concerning the equality of inertial and gravitational mass and the equivalence of a homogeneous gravitational field with a uniformly accelerated frame of reference became a corner stone for his subsequent attempts to generalize special relativity. Thus the separate problem of a generalization of the relativity theory of 1905 to accelerated frames of reference became linked to the development of a new theory of gravitation.

As heuristically fruitful as this "principle of equivalence" turned out to be for predicting such effects as the bending of light rays passing through a gravitational field and the gravitational redshift of spectral lines, Einstein initially hesitated to build a theory on it. He continued to stress his efforts to deal with the quantum riddle. It was only when his hope to solve this riddle waned that he began to concentrate on the gravitation problem. Einstein also decided to make his results public even when he had the feeling that they were not yet definitive. Cautiously proceeding step by step, he attacked first the case of a static gravitational field. At the end of March 1912 he was content with his preliminary results: "Lately, I have worked furiously on the problem of gravitation. I am now finally finished with the statics. I don't know anything yet about the dynamic field, that is to follow next. . . . Every step is devilishly difficult, and what I have derived so far is certainly only the easiest part."

Although difficulties with the principle that action equals reaction forced him to limit the validity of the principle of equivalence, Einstein did not give it up when he decided to move on to the dynamic case. Another guide Einstein found useful was Mach's critique of Newton's mechanics. It suggested to Einstein that a satisfactory generalization of the theory of relativity should allow him to consider rotational motion as being equivalent to a state of rest. A crucial insight, which occurred to Einstein probably in the summer of 1912, was the recognition of an analogy between the mathematical problems of a generalized relativity theory and Gauss's geometry of curved surfaces. From this analogy he concluded that the gravitational field had to be represented not by a scalar potential but by a ten-component metrical tensor, a crucial step towards general relativity. When Einstein returned from Prague to Zurich in August 1912, his friend Marcel Grossmann, professor of mathematics at the ETH, played an important role in extending his horizon to include the works of Riemann, Christoffel, Ricci, and Levi-Civita, and also in assisting him in his search for gravitational field equations based on tensor calculus.

In May 1913, more than a year after the completion of the static theory, Einstein and Grossmann published a preliminary but comprehensive version of a generalized theory of relativity and gravitation (Doc. 13). Although they had made important progress, their results remained unsatisfactory, not only to their critics but also to the authors themselves. The reason was that they had searched for a generally covariant theory but had not reached their goal. In fact, they were not even sure whether transformations to rotating coordinates were permitted by their equations so that it was not clear whether a rotating frame of reference could be considered as being equivalent to a rest frame. After the publication of the paper, Einstein wrote to H. A. Lorentz: "Thus, if not all systems of equations of the theory . . . admit transformations other than linear ones, then the theory violates its own point of departure; it is then left up in the air."

With hindsight it is easy to see how close Einstein and Grossmann came to the final general theory of relativity. In a retrospective account Einstein recalled how he had initially considered the Riemann curvature tensor as a possible basis upon which to construct generally covariant field equations, but had failed to understand that it was indeed applicable. Only after two years of hard work on conceptual as well as mathematical problems had he come back to it and based his final theory on it. What made Einstein reject the Riemann tensor is a question that has been widely discussed in the literature. Material presented in this volume provides important clues toward an answer. Einstein's research notes on gravitation, covering the period of his collaboration with Grossmann from the summer of 1912 to the publication of their joint paper and published here as Doc. 10, are an especially important source.

Attempts to understand the covariance properties of the new theory, attempts to relate it to empirical data, and efforts to convince his colleagues dominated Einstein's activities before his departure for Berlin in the spring of 1914. At the time, no empirical results on the gravitational bending of light or the gravitational redshift were available, and Einstein's attempt, undertaken with the help of Besso, to explain the perihelion advance of Mercury on the basis of the new theory of gravitation failed.

The lack of empirical support for Einstein and Grossmann's generalized theory of relativity became even more noticeable when, in 1913, Gunnar Nordstrom published a special relativistic scalar theory of gravitation. This theory avoided earlier objections by Einstein against such theories and thus was a serious competitor. Einstein discussed Nordstrom's theory as well as the Einstein-Grossmann theory in the masterful lecture he gave at the 1913 meeting of the Gesellschaft Deutscher Naturforscher und Arzte in Vienna and came to the conclusion that only experience could decide between the two (Doc. 17). Nordstrom's theory turned out to be more than a competitor for the Einstein-Grossmann theory. In a joint paper with Adriaan Fokker (Doc. 28), Einstein showed that it could be formulated within the same mathematical framework of tensor calculus and that Nordstrom's simpler theory served to illuminate the essential conceptual features of the application of this framework to a theory of gravitation.

The unfamiliar approach that Einstein took in his search for a theory of gravitation made his work controversial at first. Einstein expected little else: Even before the publication of his joint work with Grossmann he had written to Paul Ehrenfest that he expected "a murmur of indignation to go through the ranks of colleagues when the paper appears." Even his future colleagues in Berlin, Max Planck and Max von Laue, who had themselves made important contributions to the development of relativity, remained skeptical in their responses to Einstein's recent work. Einstein, on his part, used every occasion to expound the conceptual problems and epistemological considerations that had motivated him. As several papers in this volume clearly demonstrate, Einstein did not address his writings exclusively to specialists; he also hoped that his ideas would be understood by a wider public. One of his papers, for example, includes an imaginary dialogue between a specialist and a layman on the concept of absolute space: "Our fearless observer will interject: `You may be incomparably learned, but just as I could never be persuaded to believe in ghosts, can I believe in that giant object, of which you speak and that you call space. I can neither see such a thing, nor can I imagine it.'"

Einstein did not encounter skepticism from all sides, however. There were some who stimulated and encouraged him. Evidence is provided by lively correspondence with, among others, Michele Besso, Paul Ehrenfest, and H. A. Lorentz. He also used the opportunity of a lecture course on electricity and magnetism to present his ideas to students, although the amount of stimulation he received from them must have been limited. When Einstein moved to Berlin in April 1914, the early history of general relativity was not yet complete, but the foundations of the later development had been laid. Only a few weeks earlier Einstein had written: "Nature only shows us the tail of the lion. I am convinced, however, that the lion is attached to it, even though he cannot reveal himself directly because of his enormous size. We see him only as would a louse that sits on him."