My new paper on Einstein and the general theory of relativity:

# Tag: Marcel Grossmann

**מאמר שלי על דרכו של איינשטיין לתורת היחסות הכללית****: ****אודיסאת איינשטיין ליחסות הכללית **

“Einstein’s Odyssey to General Relativity”, *Scientific American Israel*

את המונח “אודיסאה” ליחסות הכללית טבע פרופ’ ג’ון סטצ’ל מאוניברסיטת בוסטון והוא מייצג את המסע המפרך של איינשטיין בדרכו ליחסות הכללית. ראו המאמר של סטצ’ל למטה

Odyssey to general relativity is John Stachel’s memorable phraseology. See:

Stachel, John (1979). “Einstein’s Odyssey**: **His Journey from Special to General Relativity”. In * Einstein from B to Z*, 2002

*.*

I am sorry but this piece is in Hebrew. You can read my book *General Relativity Conflict and Rivalries,* my papers on Einstein and general relativity and a short summary below.

מפייסבוק: מארחים את ד”ר גלי וינשטיין לדבר על איינשטיין

My drawing of Einstein: האיור שלי של איינשטיין

And the original (I tried as hard as I could to draw a young Einstein…): המקור

**The article discusses the following topics:**

1907. The Happiest thought of my life.

1907-1911. The equivalence principle and elevator experiments.

1911. Deflection of light and explaining deflection of light using an elevator thought experiment.

1911-1912 (1916). The disk thought experiment, gravitational time dilation and gravitational redshift.

1912. The disk thought experiment and non-Euclidean geometry.

1912. Einstein to Marcel Grossmann: “Grossmann, you must help me or else I’ll go crazy!”. Grossmann searched the literature, and brought the works of Bernhard Riemann, Gregorio Curbastro-Ricci, Tullio Levi-Civita and Elwin Bruno Christoffel to Einstein’s attention. With Grossmann’s help Einstein searched for gravitational field equations for the metric tensor in the Zurich Notebook.

1913-1914. The *Entwurf* theory. In 1913, Einstein and Michele Besso both tried to solve the new *Entwurf* field equations to find the perihelion advance of Mercury.

October 1915. Einstein realizes there are problems with his 1914 *Entwurf* theory. November 1915. Einstein’s competition with David Hilbert.

November 1915. Four ground-breaking papers: Einstein presents the field equations of general relativity, finds the advance of the perihelion of Mercury and predicts that a ray of light passing near the Sun would undergo a deflection of amount 1.7 arc seconds.

All the Berlin Century of General Relativity and MPIWG conference talks and discussions are on the web site of the Max Planck Institute for the History of Science in Berlin.

I shall begin by presenting the opening remarks of prof. Jürgen Renn’s talk, “The ‘Renaissance’ of General Relativity: Social and Epistemic Factors”, and thereafter I shall list my various comments on this talk. I shall present my views at the end regarding the term “Renaissance” describing the period from 1955 to the end of 1964.

Prof. Jürgen Renn:

“This talk is about explaining historical change; how we preliminary see the various stages of the history of general relativity: there was **the genesis of general relativity **[the period from 1907 to 1916] where Einstein worked not quite but almost alone with a few helpers. There were **the formative years **[the period from 1916 to 1925], in which the theory was discussed among a group of experts; there was then what Jean Eisenstaedt has epically termed **the low-water mark period **[the period from 1925 to 1955]; and then comes **the renaissance **[the period from 1955 to 1964]. Clifford Will gave the name to this period; and for us historians then came **the golden ages** [the period from 1964 to the mid 1970s] and today and the future, but I will not talk about that.

So the main challenge is to explain how development went from the low-water mark period to the renaissance; how what we today see as the basic theory of cosmology and astrophysics came back into the mainstream of physics after the low-water mark period. There were many factors of course and we have concentrated on several scenarios, examining them in preliminary ways to try to come up with what we think as a convincing explanation.

Let me start to outline the thesis. We think that the renaissance was mainly due to two factors:

One was the discovery of the untapped potential of general relativity as it has been created as a tool for theoretical physics. Hidden secrets that were discovered in this period, hidden potential of application, these would not have been explored and actively developed had it not been within a community that was just forming in this period.

[The second factor] We think that the renaissance is also very much history of a community, in which for the first time a real community of relativists and cosmologists emerged. We have kind of loosely been talking of relativists and cosmologists, even when referring to the twenties and the thirties [1920s and 1930s]. This is a somewhat anachronistic use of terms, because the real community only emerged as we see it during the period of the renaissance.

So you see already that the general approach is one that combines epistemological aspects with sociological aspects, and that is very much the spirit of our thinking. Robert Schulmann [a speaker in the conference] used the terminology of internalist and externalist. So we see this very much as a development that can only be understood if you combine the cognitive, the epistemological side, with the sociological developments parallel to it”.

__My comments on this talk__:

Prof. Jürgen Renn says: “There was the genesis of general relativity where Einstein worked not quite but almost alone with a few helpers. There were the formative years, in which the theory was discussed among a group of experts; there was then what Jean Eisenstaedt has epically termed the low-water mark period”.

I think that even during the genesis of general relativity, between 1912 and 1916, Einstein’s interaction with and response to eminent and non-eminent scientists and his ongoing discussions with other scientists contributed to the formation of general relativity. The efforts invested by physicists like Max Abraham, Gunnar Nordström, Gustav Mie, David Hilbert and others, which presented differing outlooks and discussions revolving around the theory of gravitation, were relegated to the background. Those works that did not embrace Einstein’s overall conceptual concerns – these primarily included the heuristic equivalence principle and Mach’s ideas (later called Mach’s principle) – were rejected, and authors focused on Einstein’s prodigious scientific achievements. However, one cannot discard Einstein’s response to the works of Abraham, Nordström, Mie, Tullio Levi-Civita, Hilbert and others. On the contrary, between 1912 and 1915 (the so-called genesis of general relativity) Einstein’s response to these works, and corrections made by these scientists to his work, constitutes a dynamic interaction that assisted him in his development (so-called genesis) of the general theory of relativity. In my book, *General Relativity Conflict and Rivalries* I show that general relativity was not developed as a single, coherent construction by an isolated individual, brooding alone. Instead, general relativity was developed through Einstein’s conflicts and interactions with other scientists, and was consolidated by his creative process during these exchanges.

Indeed, performing a historical research and also applying comparative research in a sociological context, we can emphasize the limits and associated problems tracing Einstein’s “odyssey”, i.e. intellectual road to the general theory of relativity. An intellectual approach emphasizes the simplistic hero worship narrative. In addition, some philosophers embrace externalist theories of justification and others embrace internalist theories of justification. There are many different versions of internalism and externalism, and philosophers offered different conceptions of internalism and externalism.

I would like to comment on the “formative years” and the “low-water mark” period. I show in my book *General Relativity Conflict and Rivalries* that between 1916 and 1955, Einstein was usually trusted as the authority on scientific matters. His authority in physics is revealed even on first-rate mathematicians and physicists. For example, in 1918 Felix Klein demonstrated to Einstein that the singularity in the de Sitter solution to the general relativity field equations was an artefact of the way in which the time coordinate was introduced. Einstein failed to appreciate that Klein’s analysis of the de Sitter solution showed that the singularity could be transformed away. In his response to Klein, Einstein simply reiterated the argument of his critical note on the de Sitter solution. In 1917-1918 the physicist-mathematician Hermann Weyl’s position corresponded exactly to Einstein’s when he criticized de Sitter’s solution; Weyl’s criticism revealed the influence of Einstein’s authority in physics even on first-rate mathematicians like Weyl.

Two other examples from the introduction of my book:

In March 1918, before publishing the book *Space-Time-Matter*, Weyl instructed his publisher to send Einstein the proofs of his book. In the same month, Weyl also instructed his publisher to send David Hilbert the proofs of his book. Hilbert looked carefully at the proofs of Weyl’s book but noticed that the latter did not even mention his first Göttingen paper from November 20, 1915, “Foundations of Physics”. Though Weyl mentioned profusely Einstein’s works on general relativity, no mention was made of Hilbert’s paper. Einstein received the proofs page-by-page from the publisher and read them with much delight and was very impressed. However, Einstein, an initial admirer of the beauty of Weyl’s theory, now raised serious objections against Weyl’s field theory. Einstein’s objection to Weyl’s field theory was Weyl’s attempt to unify gravitation and electromagnetism by giving up the invariance of the line element of general relativity. Weyl persistently held to his view for several years and only later finally dropped it.

Einstein also seemed to influence Sir Arthur Stanley Eddington when he objected to what later became known as “black holes”. In his controversy during the Royal Astronomical Society meeting of 1935 with Subrahmanyan Chandrasekhar, Eddington argued that various accidents may intervene to save a star from contracting into a diameter of a few kilometres. This possibility, according to Eddington, was a *reductio ad absurdum *of the relativistic degeneracy formula. Chandrasekhar later said that gravitational collapse leading to black holes is discernible even to the most casual observer. He, therefore, found it hard to understand why Eddington, who was one of the earliest and staunchest supporters of the general theory of relativity, should have found the conclusion that black holes may form during the natural course of the evolution of stars, so unacceptable. However, it is very reasonable that Eddington, who was one of the earliest and staunchest supporters of Einstein’s classical general relativity, found the conclusion that “black holes” were so unacceptable, because he was probably influenced by Einstein’s objection to the Schwarzschild singularity.

Prof. Jürgen Renn explained in his talk: “We think that the renaissance is also very much a history of a community, in which for the first time a real community of relativists and cosmologists emerged. We have kind of loosely been talked of relativists and cosmologists, even when referring to the twenties and the thirties. This is a somewhat anachronistic use of terms, because the real community only emerged as we see it during the period of the renaissance”.

I would like to comment on the proposal to call the period from 1955 to 1964 the “renaissance” of general relativity. Clearly, in the atmosphere of cinquecento Florence or Milan, a scientist and artist needed a community. Actually for the first time in history, in renaissance Italy there were concentrations of scientists, writers, artists and patrons in close communities and courts. There were communication and interaction between those communities. Historians have reconstructed what went in these communities and how these communities functioned. Hence renaissance is a suitable historical term to describe the period from 1955 to 1964.

Updated July 2016, another talk by Jürgen Renn: the sixth biennial Francis Bacon Conference, “General Relativity at 100”, Bacon Award Public Lecture: (I did not attend this conference but the talk was uploaded to YouTube):

In 1907 in the patent office, Einstein was sitting down to write a review article in which he reviewed all the phenomena of physics in order to adapt them to the new framework of space and time established by special relativity. It was a routine task and dealing with gravitation in that context was also a routine task, so it seemed at the beginning.

[My comment: Einstein said in 1933, in the Glasgow lecture: “I came a step closer to the solution of the problem for the first time, when I attempted to treat the law of gravity within the framework of the special theory of relativity.” Apparently, sometime between September 1905 and September 1907 Einstein had already started to deal with the law of gravity within the framework of the special theory of relativity. When did he exactly start his work on the problem? Einstein did not mention any specific date, but in the Glasgow lecture he did describe the stages of his work presumably *prior to* 1907].

But then Einstein had to deal with the principle that was already established by Galileo hundreds of years ago, namely, the universality of free fall, the fact that all bodies fall with the same acceleration. How can these two be reconciled? When two people, one on the moving train and one on the platform each drop a stone, will the two stones hit the ground simultaneously? According to classical, Newtonian physics yes, but according to special relativity no. That is surprising. Was there any way to impose the universality of free fall and maintain Galileo’s principle?

The following is what inspired Einstein. Later he recalled in 1920: “Then came to me the happiest thought of my life in the following form. In an example worth considering the gravitational field only has a relative existence in a manner similar to the electric field generated by electromagnetic induction. Because for an observer in free-fall from the roof of a house, there is during the fall – at least in his immediate vicinity – no gravitational field”. Now the great thing is that this allowed Einstein to simulate gravity by acceleration and he could now treat accelerated frames of reference with the help of relativity, so he had a handler in the framework of special relativity, but in a new way that preserved universality of free fall. He could now predict that light bends in a gravitational field. The principle of equivalence (elevator experiments – linear acceleration, a heuristic guide).

Where did the idea of a generalization of the special relativity principle to accelerated motion actually come from? Einstein was particularly fascinated by Ernst Mach’s historical critical analysis of mechanics. What could Einstein learn from Mach? Mach had reconsidered Newton’s bucket experiment. Why does the water rise when it stars rotating? Newton’s answer was the following: because it moves with respect to absolute space. Mach’s answer was different: he claimed that it moves because it moves with respect to the fixed stars. That would make it a relative motion, an inertia, an interaction of bodies in relative motion with respect to each other: the water and the stars.

Rotation – heuristic guide

Besides Mach’s critique of mechanics, there is another element which can be identified in what Einstein called the happiest thought of his life. Einstein said (1920): “Then came to me the happiest thought of my life in the following form. In an example worth considering the *gravitational field* only has a relative existence in a manner *similar to the electric field generated by electromagnetic induction*. Because for an observer in free-fall from the roof of a house, there is during the fall – at least in his immediate vicinity – no gravitational field” (not italicized in the original). Use electromagnetic field theory as a mental model. We can see that it was the analogy of the gravitational field theory with the well-known theory of the electromagnetic field, on which Einstein of course was a specialist, that inspired him to the happiest thought as well, alongside the influence of Mach.

Now with the help of Mach, Einstein was in the position to complete the analogy between electromagnetic theory on the one hand and gravitational theory on the other hand by conceiving gravitation and inertia together as corresponding to the electromagnetic field. This analogy will guide him all these years from 1907 till 1915 to the completion of general relativity.

Let us look at the milestones of the genesis of general relativity. Usually this is portrayed as a drama in three acts:

[My comment: John Stachel wrote in his paper “The First Two Acts”, *Einstein from B to Z*, p. 261, that in 1920 Einstein himself wrote a short list of “my most important scientific ideas” in a letter to Robert Lawson (April 22, 1920):

1907 Basic idea for the general theory of relativity

1912 Recognition of the non-Euclidean nature and its physical determination by gravitation

1915 Field equations of gravitation. Explanation of the perihelion motion of Mercury.

Einstein’s words provide the warrant for comparing the development of general relativity to a three-act drama]:

According to Jürgen Renn: The problem here is that this portrait, this drama here leaves out the villain in this story, what is usually considered a villain, namely a theory on which Einstein worked between 1913 and 1915, in Zurich mostly but later also in Berlin, where he discarded it. It is called the preliminary or the draft and in German, the *Entwurf* theory. Now the point is what for other accounts is the villain of the story, a theory that was discarded, in my account, my book [? not yet published?] is the actual hero.

[My comment: I see what Jürgen Renn means, but I don’t think that for other historical accounts the *Entwurf* theory is a so-called villain of the story, a theory that is discarded in accounts of historians. Jürgen Renn even mentions these historians in his talk – Michel Janssen and John Stachel. What he jokingly calls the “villain”, the *Entwurf* theory, is a major part of my book, *General Relativity Conflict and Rivalries*, December 2015 and it is spread over many pages of it].

But let us proceed in order. At the begging of 1911 Einstein became the chair of physics at the German university of Prague, his first full professorship. Parague: What did Einstein achieve in 1912? He knew that the field had to be a combination of gravitation and inertia, but he did not know how to represent it mathematically. Fortunately the problem had two parts: equation of motion – the field tells matter how to move, and field equation – matter tells the field how to behave. He thus first tried to solve the problem of the equation of motion. And in particular the simple case how does a body move when no other forces, other than gravitation and inertia, act on it. Again the analogy with electromagnetism came to his rescue. It made sense that other special cases of dynamic gravitational fields such as the forces acting in a rotating frame of reference… [?] It should be possible to consider such a system at rest and the centrifugal forces acting there as dynamical gravitational forces. That’s what Einstein took from Mach. But the clue is a combination of gravity and inertia. So what could he learn?

Let us look at a rotating disk and try to measure its circumference with little roods. The disk is set into motion. Because of the length contraction predicted by special relativity, the rods would be shrunk, so that we need more of them to cover the circumference. In other words, the ratio between the circumference and the diameter would be larger than pi. This simple thought experiment gave Einstein the idea that, to describe general dynamical gravitational fields one needs to go beyond Euclidean geometry.

Non-Euclidean geometries were known. Einstein himself was not too familiar with non-Euclidean geometries. He had some courses at the ETH, he had skipped some courses at the ETH, but he did know that a straight line in such a geometry corresponds to a straightest line, or a geodesic. That solved for him the problem of the equation of motion, because when no other forces act in such a geometry, a particle would just follow the straightest possible line. Now the program of the new theory was clear. (John A, Wheeler: “matter tells space-time how to curve; curved space-time tells matter how to move”).

In the summer of 1912, Einstein returned to Zurich. He knew that he could describe a curved space-time by the metric tensor. He knew that he could define the deviation of space-time from Euclidean flat geometry in terms of the metric tensor. A metric tensor is a complicated object:

The metric tensor replaces the one Newtonian gravitational potential with ten gravitational potentials. He could even write down the equation of motion in terms of the metric tensor, he achieved that relatively quickly. But he had no clue as how to find a field equation for this complicated object, the metric tensor, these ten gravitational potentials.

One of the most important sources for our story is a notebook in which Einstein entered his calculations, in the winter of 1912-1913, the so-called *Zurich Notebook*.

The following fraternity of scholars provided historical-critical-mathematical-physical interpretation of the Zurich Notebook (it took them 10 years to interpret Einstein’s calculations):

Einstein’s first attempts to deal with the mathematics of the metric tensor look rather pedestrian. He tried to bring together the metric tensor with what he knew about the gravitational field equation, which was also relatively little.

When Einstein was desperate he called his old friend Marcel Grossmann: “Grossman you have got to help me or I will go crazy!” Grossman had helped Einstein to survive his exams and he got him his job at the patent office. Now he helped him master the problem of gravitation. Indeed, one immediately recognizes Grossman’s intervention in the notebook. His name appears next to the Riemann tensor, the crucial object for building a relativistic field equation. Einstein immediately used it to form what he considered a candidate to the left hand-side of the field equation.

Einstein and Grossmann found that these field equations do not match their physical expectations. It turned out to be difficult to reconcile Einstein’s physical expectations with the new formalism. Groping in the dark, Einstein and Grossmann essentially hit upon the correct field equations, in the winter of 1912-1913, three years before the final paper, in the weak field limit:

But Einstein and Grossmann found that these field equations do not match their physical expectations. Eventually they had to learn how to adapt these physical expectations to the implications of the new formalism: we are talking about a learning experience that took place between a mathematical formalism and new physical concepts that were being shaped during the process.

Jürgen Renn then gives the following metaphor [a machine] to explain the mechanism behind a field equation (the Einstein-Grossmann *Entwurf* gravitational field equation): Einstein started out with a hand made mathematical formalism, at least before Grossmann came into the game, but it did the job. Extracting from the source of the field (mass and energy) a gravitational field. That’s what a field equation is all about. Of course it was most crucial that the familiar special case of Newtonian gravity would also come out in the appropriate circumstances. And Einstein had to make sure that this machine was firmly grounded in basic physical principles and in particular in the conservation of energy and momentum. But he also wanted to generalize the principle of relativity to accelerated motions and he looked for a machine that worked in more general coordinate systems, but at that point he didn’t know quite at which. The later point was unclear to Einstein.

The problem was that it was not clear whether that machine would actually deliver the requested physical results. With Grossmann came the dream for a much more sophisticated machine, a machine that worked for all coordinate systems because it was generally covariant:

Here the starting point was a sophisticated mathematical formalism based on the Riemann tensor, and then of course the machine had to work in the same way: the source makes a field, but does the Newtonian limit come out right? And is the machine firmly grounded in the principles of energy and momentum conservation? It certainly doesn’t look that quite way. In any way it was generally covariant, working in all coordinate systems. The problem was that it was not entirely clear whether that machine would actually deliver the requested physical results and what kind of tweaking it would take to get them. In short, the mechanism was great but the output was uncertain. Given this situation, Einstein could now peruse two different strategies:

*physical requirements: the Newtonian limit and the energy momentum conservation.

In the winter of 1912-1913, Einstein and with him Grossmann constantly oscillated between these two strategies. At the end of the winter he decided for one of them: the physical strategy. Well not quite, but rather a physical strategy tweaked and adapted to match the requirements of energy-momentum conservation and a generalized principle of relativity. So it was a home made extension of the original machine.

[My comment: The “physical strategy” and the “mathematical strategy” and the “oscillation” between them are memorable phraseology of Jürgen Renn. These had already been invented by Jürgen Renn several years ago. You can find it in many papers by Jürgen Renn. For instance Renn, Jürgen and Sauer, Tilman, “Pathways out of Classical Physics”, *The Genesis of General Relativity 1*, 2007, 113-312].

The result of the collaboration between Einstein and Grossmann in the winter of 1912-1913 was a hybrid theory, the so-called *Entwurf* or draft theory, the villain or hero mentioned before. Why was the theory, “Draft of a Generalized Theory of Relativity and a Theory of Gravitation” a hybrid theory? It was a hybrid theory because the equation of motion (the field tells matter how to move) is generally covariant (retaining its form in all coordinate systems), but the field equation (matter tells the field how to behave) is not generally covariant. It was not even clear in which coordinate system the field equation would be covariant. Nevertheless, Einstein was quite proud. To his future wife Elsa he wrote: “I finally solved the problem a few weeks ago. It is a bold extension of the theory of relativity together with the theory of gravitation. Now I must give myself some rest, otherwise I will go kaput”. But was it worth the effort? Wasn’t the *Entwurf* theory just a blind alley and a waste of time (for more than two and a half years)? Most accounts say yes and speak of a comedy of errors.

[My comment: As I mentioned previously, I am afraid I don’t really agree with this conclusion. Other historians don’t say yes. In my account, for instance, in my book *General Relativity Conflict and Rivalries, *I demonstrate that the *Entwurf* theory plays a crucial role in Einstein’s development of the November 1915 theory. I refrain, however, from using the terms “bridges”, “scaffolding” and other metaphors Einstein did not use (see below) because I think that, these terms and metaphors embed constraints that impact the understanding of the historical narrative. I rather prefer Einstein’s own terms, for instance, “heuristic guide”. Hence, Einstein was guided by the 1913 calculation of the perihelion of Mercury. So was he guided by the 1914 variational principle, formalism].

But if the *Entwurf* theory was important, what was its role? What function did it have for the creation of the general theory of relativity, if it turned out to be a wrong theory at the end? To answer that question we shall use another metaphor: the *Entwurf* theory as a scaffolding for building an arch or a bridge between physics and mathematics (Michel Janssen’s metaphor). The building of a bridge between physics and mathematics. On its basis, Einstein first calculated the Mercury perihelion motion, and working out its mathematical structure, and by “its”, meaning the preliminary *Entwurf* draft theory. Einstein worked out its mathematical structure and set up a variational formalism for it.

The Mercury calculation eventually helped him to solve the problem of the Newtonian limit and the variational formalism helped him to solve the other problem he had encountered with the mathematical strategy, that is, the conservation of energy and momentum. All this prepared the situation of November 1915, when it eventually came to a situation when the scaffolding was torn down.

Let us look at the first issue, the issue of the Mercury perihelion motion (a problem studied in detail by Michel Janssen). In 1913 Einstein together with Michele Besso calculated the Mercury perihelion motion on the basis of the *Entwurf* theory and the value which they miscalculated came out too small. The theory predicated only 18″per century. This did not shatter, however, Einstein’s confidence in that theory. Einstein never mentioned this unsatisfactory result until 1915 but he reused this method developed under the auspices of the *Entwurf* theory in November 1915. The method of calculation, the scaffolding, could be used in the final November theory. The creation of general relativity was a team effort. Besso had a role in the perihelion calculation in building a scaffolding for the transition to the final theory. The Mercury calculation helped Einstein understand the problem of Newtonian limit and accept the field equation he had earlier discarded. The Mercury (perihelion) calculation of 1913 really did act as a scaffolding for what Einstein achieved in November 1915, when he redid this calculation now on the basis of the correct theory.

Einstein’s colleagues were amazed how quickly he could calculate. David Hilbert wrote in a postcard just a day after Einstein had submitted the paper: “Congratulations on conquering the perihelion motion. If I could calculate as fast as you can, the electron would be forced to surrender to my equations and the hydrogen atom would have to bring a note from home to be excused for not radiating”. However, all Einstein had to do is to redo the calculations for the perihelion motion in the *Entwurf* theory that he had done with Besso in 1913 but never published. Einstein did not bother to tell Hilbert about this earlier work. Apparently he wanted to give Hilbert a dose of his own medicine, seeing Hilbert as somebody who gave the impression of being superhuman by obfuscating his methods (Einstein to Ehrenfest, May 24, 1916).

In November 1915 the building of a scaffolding (Besso’s assistance) helped Einstein to overcome his earlier problems with extracting the Newtonian limit from the field equation found along the mathematical strategy.

But what about the second problem? The conservation laws of energy and momentum? Again the *Entwurf* theory served as a scaffolding. In 1914 Einstein and Grossmann set up a variational formalism for the *Entwurf* theory from which it was easy to derive the conservation laws. That formalism was general enough to allow the derivation of the conservation laws also for other theories, including the ones Einstein had discarded in the winter of 1912-1913. You can use the variational formalism to make a candidate field equation for the physical strategy or you can change the settings, and then you get a different machine, a candidate field equation for the mathematical strategy.

Having constructed such a formalism, the variational formalism, is what allowed Einstein to switch from the *Entwurf* theory to the theory he presented on November 4, 1915.

Einstein had resolved his two major problems that had prevented him in Zurich to accept candidate field equations along the mathematical strategy; namely the requirement to get out the Newtonian limit in the special case and the conservation laws. That confronts us with a puzzle: Why did Einstein not come back to the mathematical strategy right away once he had resolved these problems at the end of 1914? He firmly believed in the *Entwurf* theory and had concocted all kinds of arguments in its favor, for instance the hole argument.

[My comment: I am afraid I don’t agree with this conclusion: I don’t think that at the end of 1914 Einstein had already resolved his two major problems (Newtonian limit and conservation laws). He was only able to resolve them in November 1915].

But by October 1915, his perspective was gradually changing because problems with the *Entwurf* theory were gradually accumulated:

1) It did not explain the perihelion problem well, we have seen that. Einstein could live with it. He just put it under the rug and did not mention it in his publications.

2) It did not allow him to conceive rotation at rest. It was a major blow, considering his Machian vision.

3) And, it did not follow uniquely, as he had hoped, from the variational formalism that he had set up. But this failure was actually a blessing in disguise. It meant that the formalism was actually more general and not just tailor-made for the *Entwurf* theory. So he could use it.

So these problems were the prelude to the drama of November 1915 when Einstein published week after week his four conclusive publications on general relativity. On the 4th of November that is the transition from the *Entwurf* theory to the new mathematical objects; with an addendum on the 11th of November; the Mercury paper on the 18th of November, and the final field equations on the 25th. The first paper contains an interesting hint at what Einstein considered the “fatal prejudice” that had hindered him so far and also what was the key to the solution. The subsequent papers successively straighten out a logical structure of the theory, show that now the Mercury problem works and the final paper completes the logical structure of the theory.

In order to understand what the fatal prejudice was and what the key to the solution was, we have to once more time look at the mechanism at work here:

There is indeed not just the source and the field. There is also the gravitational potential represented by the metric tensor and its connection with the gravitational field. That connection is expressed by a differential operator. One way to express this differential operator turned out to be a fatal prejudice, the other a key to the solution. As it turned out, it was essentially sufficient to change one element in the variational formalism developed for the *Entwurf* field equations, in order to get the theory from November the 4^{th}, 1915. Namely, redefine the gravitational field.

Here are two ways in which Einstein expressed the connection between the field and the potential: One in the 1913 *Entwurf* theory and the other in the theory of November 4th. In the paper itself he speaks of the first way as a fatal prejudice and in a letter to Sommerfield he characterizes the second option as the key to the solution:

In the end the transition from the *Entwurf* theory, based on the physical strategy, to the November 1915 theory, based on the sophisticated math of the Riemann tensor, seems to have been a rather simple step. But what made this step possible was the scaffolding represented by the variational formalism Einstein had built for the *Entwurf* theory.

[My comment: In my book, *General Relativity Conflict and Rivalries*, I demonstrate an additional element. Einstein wrote to Sommerfeld the following:

“scalar derived from the energy tensor of matter, for which I write T in the following”.

In my book I show how one can derive the second term on the right-hand side of the above November 25, 1915 equation on the basis of the variational formalism and on the basis of Einstein’s 1914 *Entwurf* theory and November 4th, 1915 paper. I connect between this latter derivation and the derivation of the November 4, 1915 field equation from the 1914 variation principle].

A variational formalism is a machine for making machines: It made it simpler to pass from a machine based on the fatal prejudice:

to a machine that represented the key to the solution:

Einstein had to revise the architecture of his theory step by step and that is what happened in the final publication of November 1915. To Arnold Sommerfeld he wrote: “unfortunately I have immortalized my final errors in the academy papers” (November 28, 1915). And to his friend Paul Ehrenfest he wrote: “It is convenient with that fellow Einstein: every year he retracts what he wrote the year before” (December 26, 1915).

In Einstein’s defense one had to remember that what contributed to the drama was that the mathematical David Hilbert was or at least seemed to have been hot on Einstein’s trail. Hilbert presented his field equations in Göttingen on November 20, 1915, five days before Einstein. He used the Riemann curvature scalar in his variational formalism. He did not explicitly write down the field equation but he could have easily calculated it of course. In the late 1990s, page proofs of Hilbert paper, which itself was not published until March 1916, turned up. These page proofs carry a date of December 6, 1915, after Einstein’s publication.

Did Hilbert beat Einstein to the punch? His page proofs show that the original version of Hilbert’s theory was conceptually closer to the *Entwurf* theory than to Einstein’s final version. Hilbert’s theory of December 6, 1915 was just as the *Entwurf* theory, a hybrid theory with extra conditions on the coordinate systems. This restriction on the coordinate systems was later dropped in the published version appearing in March 1916. Hence, the moral is: Einstein could have taken his time in November 1915 and need not have worried about Hilbert stealing his thunder. Hilbert did not build a bridge between mathematics and physics as Einstein had done. In fact he didn’t worry about these problems of Newtonian limit and energy momentum conservation in the way that Einstein had done. So on November 25, 1915, the edifice of general relativity seemed complete.

This is the first part of Jürgen Renn’s lecture, it deals with the genesis of general relativity (until approximately 49 minutes after the YouTube video start time). The second part of the lecture, not given here, is quite disappointing compared to the first part. In the first part of the lecture, Jürgen Renn is inspired by great scholars, notably John Stachel. Unfortunately, the second part of the lecture lacks the inspiration and sensation of the greatness of the fraternity of Einstein scholars (see photo further above).

Here is the dust jacket of my new scholarly book on the history of general relativity, to be released on… **my Birthday**:

*General Relativity Conflict and Rivalries: Einstein’s polemics with Physicists*.

The book is illustrated by me and discusses the history of **general relativity**, **gravitational waves**, **relativistic cosmology** and **unified field theory** between 1905 and 1955:

The development of general relativity (1905-1916), “low water mark” period and several results during the “renaissance of general relativity” (1960-1980).

Conversations I have had more than a decade ago with my PhD supervisor, the late Prof. Mara Beller (from the Hebrew University in Jerusalem), comprise major parts of the preface and the general setting of the book. However, the book presents the current state of research and many new findings in history of general relativity.

My first book:

*Einstein’s Pathway to the Special Theory of Relativity *(April, 2015)

includes a wide variety of topics including also the early history of general relativity.

“Arch and scaffold: How Einstein found his field equations” by Michel Janssen and Jürgen Renn. *Physics Today* 68(11), 30 (2015). The article is published in November 2015, which marks the centenary of the Einstein field equations. (Renn co-authored with Gutfreund *The Road to Relativity, *Princeton Press)

This is a very good article. However, I would like to comment on several historical interpretations. . Michel Janssen and Jürgen Renn ask: Why did Einstein reject the field equations of the first November paper (scholars call them the “November tensor”) when he and Marcel Grossmann first considered them in 1912–13 in the Zurich notebook?

They offer the following explanation: In 1912 Albert Einstein gave up the November tensor (derived from the Ricci tensor) because the rotation metric (metric of Minkowski spacetime in rotating coordinates) did not satisfy the Hertz restriction (the vanishing of the four-divergence of the metric). Einstein wanted the rotation metric to be a solution of the field equations in the absence of matter (vacuum field equations) so that he could interpret the inertial forces in a rotating frame of reference as gravitational forces (i.e. so that the equivalence principle would be fulfilled in his theory).

However, the above question – why did Einstein reject the November tensor in 1912-1913, only to come back to it in November 1915 – apparently has several answers. It also seems that the answer is Einstein’s inability to properly take the Newtonian limit.

Einstein’s 1912 earlier work on static gravitational fields (in Prague) led him to conclude that in the weak-field approximation, the spatial metric of a static gravitational field must be flat. This statement appears to have led him to reject the Ricci tensor, and fall into the trap of *Entwurf* limited generally covariant field equations. Or as Einstein later put it, he abandoned the generally covariant field equations with heavy heart and began to search for non-generally covariant field equations. Einstein thought that the Ricci tensor should reduce in the limit to his static gravitational field theory from 1912 *and then* to the Newtonian limit, if the static spatial metric is *flat*. This prevented the Ricci tensor from representing the gravitational potential of any distribution of matter, static or otherwise. Later in the 1920s, it was demonstrated that the spatial metric can go to a flat Newtonian limit, while the Newtonian connection remains non-flat without violating the compatibility conditions between metric and (affine) connection (See John Stachel).

Phys. Today 68, 11, 30 (2015).

As to the “archs and scaffolds” metaphor. Michel Janssen and Jürgen Renn demonstrate that the Lagrangian for the *Entwurf* field equations has the same structure as the Lagrangian for the source-free Maxwell equations: It is essentially the square of the gravitational field, defined as minus the gradient of the metric. Since the metric plays the role of the gravitational potential in the theory, it was only natural to define the gravitational field as minus its gradient. This is part of the *Entwurf* scaffold. The authors emphasize the analogy between gravity and electromagnetism, on which Einstein relied so heavily in his work on the *Entwurf* theory.

However, I am not sure whether in 1912-1913 Einstein was absolutely aware of this formal analogy when developing the *Entwurf* field equations. He first found the *Entwurf* equations, starting from energy-momentum considerations, and then this analogy (regarding the Lagrangian) lent support to his *Entwurf* field equations. Anyway, I don’t think that this metaphor (analogy between gravity and electromagnetism) persisted beyond 1914. Of course Einstein came back to electrodynamics-gravity, but I think that he discovered his 1915 field equations in a way which is unrelated to Maxwell’s equations (apart from the 1911 generally covariant field equations, influenced by Hilbert’s electromagnetic-gravitational unified theory, but this is out of the scope of this post and of course unrelated to the above metaphor).

As to the November 4, 1915 field equations of Einstein’s general theory of relativity: When all was done after November 25, 1915, Albert Einstein said that the redefinition of the components of the gravitational field in terms of Christoffel symbols had been the “key to the solution”. Michel Janssen and Jürgen Renn demonstrate that if the components of the gravitational field – the Christoffel symbols – are inserted into the 1914 *Entwurf* Lagrangian, then the resulting field equations (using variational principle) are the November tensor. In their account, then, Einstein found his way back to the equations of the first November paper (November 4, 1915) through considerations of physics. Hence this is the interpretation to Einstein’s above “key to the solution”.

I agree that Einstein found his way back to the equations of the first November paper through considerations of physics and not through considerations of mathematics. Mathematics would later serve as heuristic guide in searching for the equations of his unified field theory. However, it seems to me that Michel Janssen and Jürgen Renn actually iterate Einstein’s November 4, 1915 variational method. In November 4, 1915, Einstein inserted the Christoffel symbols into his 1914 *Entwurf* Lagrangian and obtained the November 4, 1915 field equations (the November tensor). See explanation in my book, *General Relativity Conflict and Rivalries*, pp. 139-140.

Indeed Janssen and Renn write: There is no conclusive evidence to determine which came first, the redefinition of the gravitational field (in terms of the Christoffel symbols) or the return to the Riemann tensor.

Hence, in October 1915 Einstein could have first returned to the November tensor in his Zurich Notebook (restricted to unimodular transformations) __and only afterwards__ in November 1915, could he redefine the gravitational field components in terms of the Christoffel symbols. Subsequently, this led him to a redefinition of the *Entwurf* Lagrangian and, by variational method, to a re-derivation of the 1912 November tensor.

Van Gogh had nostrified Hilbert (Hilbert visited Van Gogh, closed time-like loops…. ….)

Finally, Michel Janssen and Jürgen Renn write: Despite Einstein’s efforts to hide the *Entwurf *scaffold, the arch unveiled in the first November paper (November 4, 1915) still shows clear traces of it.

I don’t think that Einstein tried to hide the *Entwurf* scaffold. Although later he wrote Arnold Sommerfeld: “Unfortunately, I have immortalized the last error in this struggle in the Academy-papers, which I can send to you soon”, in his first November paper Einstein had explicitly demonstrated equations exchange between 1914 *Entwurf* and new covariant November ones, restricted to unimodular transformations.

Stay tuned for my book release, forthcoming soon (out by the end of 2015) on the history of general relativity, relativistic cosmology and unified field theory between 1907 and 1955.

**My new paper on General Relativity: Einstein and Schwarzschild.**

Sometime in October 1915 Einstein dropped the Einstein-Grossman theory. Starting on November 4, 1915, Einstein gradually expanded the range of the covariance of his field equations. On November 11, 1915 Einstein was able to write the field equations of gravitation in a general covariant form, but there was a coordinate condition (there are no equations here so I cannot write it down here).

On November 18, 1915, Einstein presented to the Prussian Academy his paper, “Explanation of the Perihelion Motion of Mercury from the General Theory of Relativity”. Einstein reported in this talk that the perihelion motion of Mercury is explained by his theory. In this paper, Einstein tried to find approximate solutions to his November 11, 1915 field equations. He intended to obtain a solution, without considering the question whether or not the solution was the only possible unique solution.

Einstein’s field equations are non-linear partial differential equations of the second rank. This complicated system of equations cannot be solved in the general case, but can be solved in particular simple situations. The first to offer an exact solution to Einstein’s November 18, 1915 field equations was Karl Schwarzschild, the director of the Astrophysical Observatory in Potsdam. On December 22, 1915 Schwarzschild wrote Einstein from the Russian front. Schwarzschild set out to rework Einstein’s calculation in his November 18 1915 paper of the Mercury perihelion problem. He first responded to Einstein’s solution for the first order approximation from his November 18, 1915 paper, and found another first-order approximate solution. Schwarzschild told Einstein that the problem would be then physically undetermined if there were a few approximate solutions. Subsequently, Schwarzschild presented a complete solution. He said he realized that there was only one line element, which satisfied the conditions imposed by Einstein on the gravitational field of the sun, as well as Einstein’s field equations from the November 18 1915 paper.

“Raffiniert ist der Herrgott, aber boshaft ist er nicht” (Einstein might have already said….), because the problem with Schwarzschild’s line element was that a mathematical singularity was seen to occur at the origin! Oh my, Einstein abhorred singularities.

Actually, Schwarzschild “committed another crime”: he did not satisfy the coordinate condition from Einstein’s November 11 or November 18, 1915 paper. Schwarzschild admitted that his coordinates were not “allowed” coordinates, with which the field equations could be formed, because these spherical coordinates did not have determinant 1. Schwarzschild chose then the non-“allowed” coordinates, and in addition, a mathematical singularity was seen to occur in his solution. But Schwarzschild told Einstein: Don’t worry, “The equation of [Mercury’s] orbit remains exactly as you obtained in the first approximation”! See my paper from 2012.

Einstein replied to Schwarzschild on December 29, 1915 and told him that his calculation proving uniqueness proof for the problem is very interesting. “I hope you publish the idea soon! I would not have thought that the strict treatment of the point- problem was so simple”. Subsequently Schwarzschild sent Einstein a manuscript, in which he derived his solution of Einstein’s November 18, 1915 field equations for the field of a single mass. Einstein received the manuscript by the beginning of January 1916, and he examined it “with great interest”. He told Schwarzschild that he “did not expect that one could formulate so easily the rigorous solution to the problem”. On January 13, 1916, Einstein delivered Schwarzschild’s paper before the Prussian Academy with a few words of explanation. Schwarzschild’s paper, “On the Gravitational Field of a Point-Mass according to Einstein’s Theory” was published a month later.

Karl Schwarzschild

In March 1916 Einstein submitted to the *Annalen der Physik* a review article on the general theory of relativity, “The Foundation of the General Theory of Relativity”. The paper was published two months later, in May 1916. The 1916 review article was written after Schwarzschild had found the complete exact solution to Einstein’s November 18, 1915 field equations. Even so, in his 1916 paper, Einstein preferred NOT to base himself on Schwarzschild’s exact solution, and he returned to his first order approximate solution from his November 18, 1915 paper.

A comment regarding Einstein’s calculations in his November 18, 1915 paper of the Mercury perihelion problem and Einstein’s 1916 paper. In his early works on GTR, in order to obtain the Newtonian results, Einstein used the special relativistic limit and the weak field approximation, and assumed that space was flat (see my paper). Already in 1914 Einstein had reasoned that in the general case, the gravitational field was characterized by ten space-time functions of the metric tensor. g were functions of the coordinates. In the case of special relativity this reduces to g_{44 }= c^{2}, where c denotes a constant. Einstein took for granted that the same degeneration occurs in the static gravitational field, except that in the latter case, this reduces to a single potential, where g_{44 }= c^{2} is a function of spatial coordinates, x_{1}, x_{2}, x_{3. }

Later that year David Hilbert (with a vengeance from 1915?…) arrived at a line-element similar to Schwarzschild’s one, and he concluded that the singularity disappears only if we accept a world without electricity. Such an empty space was inacceptable by Einstein who was apparently much attracted by Mach’s ideas! (later termed by Einstein “Mach’s Principle”). Okay, Einstein, said Hilbert: If there is matter then another singularity exists, or as Hilbert puts it: “there are places where the metric proves to be irregular”…. (See my paper from 2012).

Strange Days at Blake Holsey High: a student is sucked into the black hole…

Einstein’s first big project on Gravitation in Berlin was to complete by October 1914 a summarizing long review article of his Einstein-Grossmann theory. The paper was published in November 1914. This version of the theory was an organized and extended version of his works with Marcel Grossmann, the most fully and comprehensive theory of gravitation; a masterpiece of what would finally be discovered as faulty field equations.

On November 4, 1915 Einstein wrote his elder son Hans Albert Einstein, “In the last days I completed one of the finest papers of my life; when you are older I’ll tell you about it”. The day this letter was written Einstein presented this paper to the Prussian Academy of Sciences. The paper was the first out of four papers that corrected his November 1914 review paper. Einstein’s work on this paper was so intense during October 1915 that he told Hans Albert in the same letter, “I am often so in my work, that I forget lunch”.

In the first November 4 1915 paper, Einstein gradually expanded the range of the covariance of his field equations. Every week he expanded the covariance a little further until he arrived on November 25 1915 to fully generally covariant field equations. Einstein’s explained to Moritz Schlick that, through the general covariance of the field equations, “time and space lose the last remnant of physical reality. All that remains is that the world is to be conceived as a four-dimensional (hyperbolic) continuum of four dimensions” (Einstein to Schlick, December 14, 1915, *CPAE* 8, Doc 165) John Stachel explains the meaning of this revolution in space and time, in his book: Stachel, John, *Einstein from ‘B’ to ‘Z’*, 2002; see p. 323).

These are a few of my papers on Einstein’s pathway to General Relativity:

http://xxx.tau.ac.il/abs/1201.5352

http://xxx.tau.ac.il/abs/1201.5353

http://xxx.tau.ac.il/abs/1201.5358

http://xxx.tau.ac.il/abs/1202.2791

http://xxx.tau.ac.il/abs/1202.4305

http://xxx.tau.ac.il/abs/1204.3386

http://xxx.tau.ac.il/abs/1309.6590

http://xxx.tau.ac.il/abs/1310.1033

http://xxx.tau.ac.il/abs/1205.5966

http://xxx.tau.ac.il/abs/1310.2890

http://xxx.tau.ac.il/abs/1310.6541

Stay tuned for my next centenary of GTR post!

Albert Einstein? or Albert Einstein, Michele Besso, Marcel Grossmann?… Read my latest paper

**Besso, Special Relativity: **Einstein ends his 1905 relativity paper by saying that he is indebted to Besso for several valuable suggestions. What could Besso’s valuable suggestions have been? Einstein’s biographer, Carl Seelig, wrote: “Later Besso […] used the following analogy: Einstein the eagle has taken Besso the sparrow under his wing. Then the sparrow fluttered a little higher: ‘I could not have found a better sounding-board in the whole of Europe’, Einstein remarked when the conversation turned one day to Besso. This way Einstein and Besso became inseparable”. x

In 1952 Besso recounted, “Another little fairy tale of mine concerning my view that I had participated in [the formulation of] the special theory of relativity. It seemed to me, as an electrical engineer, I must have brought up, in conversations with you, the question, within the context of Maxwell’s theory, of what is induced in the inductor of an alternator […]”: the Magnet and Conductor thought experiment that opens Einstein’s 1905 Relativity Paper. Maxwell’s theory was not yet on the official program of the Polytechnic School ETH (Einstein’s and Besso’s collage). It was probably Einstein’s self-reading about Maxwell’s theory, who explained to Besso about this theory. Only after such explanation could Besso within the context of Maxwell’s theory refer to his technical work and speak with Einstein or remind him about induction of which Einstein had already read about in books

Einstein and his closest friend, Michele Besso

**Grossmann, General Relativity: **When Einstein came back to Zurich in 1912 Marcel Grossmann looked through the literature, and discovered that the mathematical problem was already solved by Riemann, Ricci and Levi-Civita. Einstein collaborated with Grossmann and this led to the Einstein-Grossmann theory published in two joint papers. Just before writing the first paper with Grossmann, Einstein had struggled with these new tools in the Zurich Notebook. Einstein wrote Grossmann’s name and considered candidate field equations he would come back to in the first 1915 paper on General Relativity

In this paper Einstein wrote in the introduction, “I completely lost trust in my established field equations [of the Einstein-Grossmann theory], […]. Thus I arrived back at the demand of a broader general covariance for the field equations, from which I parted, though with a heavy heart, three years ago when I worked together with my friend Grossmann. As a matter of fact, we then have already come quite close to the solution of the problem given in the following”. x

**Besso, General Relativity: **During a visit by Besso to Einstein in Zurich in June 1913 they both tried to solve the Einstein-Grossmann theory field equations to find the perihelion advance of Mercury in the “Einstein-Besso manuscript”. Besso was inducted by Einstein into the necessary calculations. Besso collaborated with Einstein on the wrong gravitational Einstein-Grossmann theory, and their calculation based on this theory gave a wrong result. In October 1915 Einstein abandoned the Einstein-Grossmann theory; he transferred the basic framework of the calculation from the Einstein-Besso manuscript, and corrected it according to his new 1915 General Relativity Theory with which he got the correct precession so quickly, because he was able to apply the methods he had already worked out two years earlier with Besso. Einstein though did not acknowledge his earlier work with Besso, and did not mention his name in his 1915 paper that explains the anomalous precession of Mercury

Einstein considered his best friend Michele Besso as a sounding board and his class-mate from the Polytechnic Marcel Grossman – as his active partner. Yet, Einstein wrote that Grossman will never claim to be considered a co-discoverer of the Einstein-Grossmann theory – a theory very close to Einstein’s general theory of relativity that he published in November 1915. He only helped in guiding Einstein through the mathematical literature, but contributed nothing of substance to the results of the theory. Hence, *Einstein neither considered Besso or Grossmann as co-discoverers* or *co-inventors *of the relativity theory which he himself invented

Read also this paper, “How many scientists does it take to make a discovery? The era of the lone genius , as epitomised by Albert Einstein, has long gone”. Prof. Athene Donald, the author of the paper writes, “Ask people to conjure up an image of a scientist and Albert Einstein is most likely to pop into their head. The iconic image is of a lone genius beavering away in some secluded room until that familiar equation – E=mc2 – crystallised in his brain sufficiently to be written down. I very much doubt doing science was ever quite like that, but it is even more unlikely to apply now”. What do you think? x

**Genesis of General Relativity – History of General Relativity and Gravitation **

** Talk **

**Gali Weinstein**

** **

In 1955 Einstein wrote his short *Autobiographical Sketch* dedicated mainly to his relations with his close friend Marcel Grossmann. In this *Skizze* Einstein told the story of his collaboration with Grossman which led to the Einstein-Grossmann theory. I will analyze Einstein’s recollections, and while doing so outline Einstein’s efforts at searching for a gravitational theory between 1912 and 1914.

The line element

Einstein had started to study Minkowski’s four-dimensional reformulation of the special theory of relativity in earnest around 1910. However he did not use this formalism in his theory of static gravitational fields of 1912. Einstein wrote in his *Skizze*,

“From the experience of the kind of scientific research which those happy Berne years have brought, I mention only one, the idea turned to be the most fruitful of my life. The special theory of relativity was several years old […].

1909-1912, while I was teaching at the Zurich [1909-1910] and at the Prague universities of theoretical physics, I mused incessantly over the problem [gravitation]. 1912, when I was appointed to the Zurich Polytechnic, I was considerably closer to the solution of the problem. Of importance here proved to be Hermann Minkowski’s analysis of the formal basis of the special theory of relativity.”

Max Abraham told Einstein that in his theory of static gravitational fields of 1912 he failed to implement Minkowski’s reformulation of special relativity (the “usual” theory of relativity as they put it) in terms of a four-dimensional space-time manifold. After the controversy with Abraham, Einstein realized that Minkowski’s formalism was as crucial instrument for the further development of his theory of gravitation. However, a successful application of Minkowski’s formalism to the problem of gravitation called for a mathematical generalization of this formalism.

In late spring 1912 Einstein found the appropriate starting point for such a generalization of Minkowski’s formalism: the “line element” used in Minkowski’s formalism, which is invariant under the Lorentz group.

###### The metric tensor

Einstein went on to say in the *Skizze*, “[…] the Gravitational field is described by a metric, a symmetric tensor-field of metric *gik*“.

The crucial breakthrough had been that Einstein had recognized that the gravitational field should not be described by a variable speed of light as he had attempted to do in 1912 in Prague, but by the metric tensor field; a mathematical object of ten independent components, that characterizes the geometry of space and time.

Einstein wrote about his switch of attitude towards mathematics in the oft-quoted letter to Sommerfeld on October 29, 1912,

“I am now occupied exclusively with the gravitational problem, and believe that I can overcome all difficulties with the help of a local mathematician friend. But one thing is certain, never before in my life have I troubled myself over anything so much, and that I have gained great respect for mathematics, whose more subtle parts I considered until now, in my ignorance, as pure luxury! Compared with this problem, the original theory of relativity is childish”.

Einstein indeed was occupied with the more subtle parts of mathematics after adopting the metric tensor. He wrote in the *Skizze*,

The problem of gravity was thus reduced to a purely mathematical one. Are there differential equations for *gik*, which are invariant under non-linear coordinate transformations? Such differential equations and *only* those were taken into consideration as the field equations of the gravitational field. The law of motion of material points was given by the equation of the geodesic line”.

###### The Einstein-Grossman collaboration

After arriving back to Zurich in summer 1912 Einstein was searching his “local mathematician friend” (as he told Sommerfeld above) from collage, Marcel Grossmann,

“With this task in mind, in 1912, I was looking for my old student friend Marcel Grossmann, who had meanwhile become a professor of mathematics in the Swiss Federal Polytechnic institute. He was immediately caught in the fire, even though he had as a real mathematician a somewhat skeptical attitude towards physics. When we were both students, and we used to exchange thoughts in the Café, he once said such a beautiful and characteristic remark that I cannot help but quoting it here: ‘I admit that from studying physics I have benefitted nothing essential. When I sat on the chair earlier and I felt a little of the heat that came from my “pre-seated” it grazed me a little. This has completely passed, because physics has taught me to consider writing that heat is something very impersonal’.

So he arrived and he was indeed happy to collaborate on the problem, but with the restriction that he would not be responsible for any statements and won’t assume any interpretations of physical nature”.

Einstein’s collaboration with Marcel Grossmann led to two joint papers: the first of these was published before the end of June 1913, and the second, almost a year later, two months after Einstein’s move to Berlin.

Einstein and Grossmann’s first joint paper entitled, “Entwurf einer verallgemeinerten Relativitätstheorie und einer Theorie der Gravitation” (“Outline of a Generalized Theory of Relativity and of a Theory of Gravitation”) is called by scholars the “Entwurf” paper. Actually Einstein himself also called this paper “Entwurf”, and he and Grossmann referred to the theory presented in this paper as the “Entwurf” theory: in their second joint paper they wrote, “Gravitationsgleichung (21) bzw. (18) des ‘Entwurfes'”.

Grossmann wrote the mathematical part of this paper and Einstein wrote the physical part. The paper was first published in 1913 by B. G. Teubner (Leipzig and Berlin). And then it was reprinted with added “Bemarkungen” (remark) in the *Zeitschrift für Mathematik und Physik* in 1914. The “Bemarkungen” was written by Einstein and contained the well-known “Hole Argument”.

The “Entwurf” theory was already very close to Einstein’s general theory of relativity that he published in November 1915. The gravitational field is represented by a metric tensor, the mathematical apparatus of the theory is based on the work of Riemann, Christoffel, Ricci and Levi-Civita on differential covariants, and the action of gravity on other physical processes is represented by generally covariant equations (that is, in a form which remained unchanged under all coordinate transformations). However, there was a difference between the two theories, the “Entwurf” and general relativity. The “Entwurf” theory contained *different field equations* that represented the gravitational field, and these were *not* generally covariant. At first though – when Einstein first collaborated with Grossmann – he considered (in what scholars call the “Zurich Notebook) field equations that were very close to the ones he would eventually choose in November 1915. But he gave them up in favor of the “Entwurf” field equations.

Starting in 1912, Einstein went through a long odyssey in the search after the correct form of the field equations of his new theory. The first trail of Einstein’s efforts appear to be documented in a blue bound notebook – known as the “Zurich Notebook” – comprised of 96 pages, all written in Einstein’s hand. The back cover of the Notebook bears the title “Relativität” in Einstein’s hand (probably an indication that he began his notes at the end). Two pieces of paper were probably taped later to the front of the notebook by Einstein’s secretary, Helen Dukas. The subject matter of the calculations in the notebook includes statistical physics, thermodynamics, the basic principles of the four-dimensional representation of electrodynamics, and the major part of the notebook is gravitation.

The notebook was found within Einstein’s papers after his death. The calculations that Einstein had done in the final pages of the Notebook indicate continues path towards the “Entwurf” paper of 1913 with Marcel Grossman.

###### Grossman brings to Einstein’s attention the absolute Differential Calculus

At this very early stage during summer 1912 of calculations with the metric tensor, Einstein explained in the *Skizze* that Grossmann,

“He looked through the literature, and soon discovered that the particular implied mathematical problem was already solved by Riemann, Ricci and Levi-Civita. The entire development followed the Gaussian theory of curvature-surfaces, which was the first systematical use of generalized coordinates. Reimann’s achievement was the biggest. He showed how a field of *gik* tensors of the second differentiation rank can be formed”.

About thirty years earlier, in 1922, Einstein was reported to have said, “I had the key idea of the analogy between the new mathematical problems connected with the new theory and the Gaussian theory of surfaces, however only after my return to Zurich in 1912, without at first studying Riemann and Ricci, as well as Levi-Civita. This was first brought to my attention by my friend Grossmann in Zurich, when I presented to him the problem of looking for generally covariant tensors whose components depend only on derivatives of the coefficients of the quadratic fundamental invariant [g_{mn}]”.

Einstein seemed to have had a key idea of the Gaussian theory of curvature-surfaces, as discussed below. Grossmann brought to his attention the works of Riemann, Ricci and Levi-Civita; and in addition he showed him the work of Christoffel. Louis Kollros, a professor of geometry and mathematics at the ETH, who was a fellow student of Einstein, wrote in memory of Einstein in 1955 that, sometime upon his arrival, Einstein spoke about his concern with Grossmann and told him one day, “Grossmann, you have to help me, or I shall go crazy! And Marcel Grossman had managed to show him that the mathematical tools he needed, were created just in Zurich in 1869 by Christoffel in the treatise ‘On the Transformation of the Homogeneous differential Forms of the Second Degree’, published in Volume 70. of the ‘Journal de Crelle’ for pure and applied mathematics”.

###### Geiser’s lectures at the Polytechnic on Gauss’ theory of curved surfaces

Already before arriving at Zurich Einstein knew he needed a theory of invariants and covariants associated with the differential line element. Kollros wrote, “Even in Prague he had foreseen that his generalized theory of relativity demanded much more than the mathematics of the elegant special relativity. He now found that elegance should rather remain a matter for tailors and shoemakers”.

Einstein was already aware of the need of the implementation of Minkowski’s formalism and of Gauss’ theory of curved surfaces. Marcel Grossmann’s notebooks from the Polytechnic demonstrate that the lectures which Einstein heard as a student at school from Prof. Carl Friedrich Geiser had familiarized him with the Gaussian theory of two-dimensional surfaces.

Einstein writes in his *Skizze*, “also I was fascinated by professor Geiser’s lectures on infinitesimal Geometry, the true master pieces of art were pedagogical and helped very much afterwards in the struggle with the general theory of relativity”.

In the Kyoto lecture Einstein is reported to have said, “This problem was unsolved until 1912, when I hit upon the idea that the surface theory of Karl Friedrich Gauss might be the key to this mystery. I found that Gauss’ surface coordinates were very meaningful for understanding this problem. Until then I did not know that Bernhard Riemann had discussed the foundation of geometry deeply. I happened to remember the lecture on geometry in my student years [in Zurich] by Carl Friedrich Geiser who discussed the Gauss theory. I found that the foundations of geometry had deep physical meaning in this problem.

When I came back to Zurich from Prague, my friend the mathematician Marcel Grossmann was waiting for me. […] First he taught me the work of Curbastro Gregorio Ricci and later the work of Riemann”.

###### The Riemann Tensor and the Ricci Tensor

Pais recalls a discussion with Einstein in which he asked Einstein how the collaboration with Grossmann began.”I have a vivid though not verbatim memory of Einstein’s reply: he told Grossman of his problems and asked him to please go to the library and see if there existed an appropriate geometry to handle such questioning. The next day Grossman returned (Einstein told me) and said that there indeed was such a geometry, Riemannian geometry”.

Grossmann helped Einstein in his search for a gravitational tensor, and like Grossmann Einstein “was immediately caught in the fire”. Just before writing the “Entwurf” paper with Grossmann, Einstein struggled with these new tools in a small blue Notebook –named by scholars the “Zurich Notebook”. Einstein filled 43 pages of this notebook with calculations, while he was fascinated with Riemann’s calculus.

While filling the notebook he received from time to time the new mathematical tools from Grossmann, and he wrote Grossmann’s name in the notebook every time he got something new to indicate the tensors that he received from him. At the top of the 14L page Einstein wrote on the left: “Grossmann’s tensor four-manifold” and next to it on the right he wrote the fully covariant form of the Riemann tensor. On top of the 22R page he wrote Grossmann’s name.

On page 14L of the Zurich Notebook Einstein systematically started to explore the Riemann tensor. It appears that Grossman suggested the four-rank Riemann tensor as a starting point. In the course of this exploration Einstein considered on page 22R candidate field equations with a gravitational tensor that is constructed from the Ricci tensor; an equation Einstein would come back to in November 1915.

In the Zurich Notebook Einstein searched for a gravitational theory, and a gravitational field equation, that would satisfy some heuristic requirements. Einstein had to cope with new mathematical tools, and at the same time he was guided by few heuristic principles: the principle of relativity, the equivalence principle, the correspondence principle, and the principles of conservation of energy and momentum.

In the Zurich Notebook Einstein first tackled relativity and equivalence and arrived at general covariance, and then moved on to correspondence and conservation. But afterwards it was just the other way round. Einstein first tackled correspondence and conservation and then relativity and equivalence, and lost general covariance. The interplay of the four heuristic principles with the new tools of absolute differential calculus of 1912 that Einstein was exploring governed the form of the field equations Einstein was finally left with at the end of the Zurich Notebook. Hence, at the end of the day Einstein’s conditions over determined Einstein’s research between 1912 and 1913.

Renn and Sauer formulated Einstein’s heuristic principles that played a role in Einstein’s search and rejection of generally covariant field equations between 1912 and 1913. They explain that “Each of Einstein’s heuristic principles against which constructions would have to be checked could be used either as a construction principle or as a criterion for their validity”:

1) *The Equivalence principle*: After 1912 Einstein implemented the equivalence principle by letting the metric field g_{mn} represent both the gravitational field and the inertial structure of space-time. The metric field is a solution of the same field equations in the coordinate systems of both observers. This is automatically true if the field equations are generally covariant. But if the field equations are of restricted covariance, then Einstein tried to implement the equivalence principle in other ways. The equivalence principle became the principle core of Einstein’s search for a generalization of the relativity principle for non-uniform motion.

2) *The generalized principle of relativity*: From 1912 onward, Einstein attempted to generalize the principle of relativity by requiring that the covariance group of his new theory of gravitation be larger than the group of Lorentz transformations of special relativity. In his understanding this requirement was optimally satisfied if the field equation of the new theory could be shown to possess the mathematical property of general covariance.

Einstein was thus under the impression that the principle of relativity for uniform motion of special relativity could be generalized to arbitrary motion if the field equations possessed the mathematical property of general covariance. And if the principle of relativity is generalized then the equivalence principle is satisfied, because according to this principle, an arbitrary accelerated reference frame in Minkowski space-time can precisely be considered as being physically equivalent to an inertial reference frame if a gravitational field can be introduced which accounts for the inertial effects in the accelerated frame. Einstein then tried to construct field equations of the broadest possible covariance. In the Zurich Notebook Einstein succeeded in formulating a generally-covariant equation of motion for a test particle in an arbitrary field. In this equation, the gravitational potential is represented by a four-dimensional metric tensor, which became the key object for Einstein’s further research in the following years.

3*) The conservation principle*: The energy-momentum conservation principle played a crucial role in Einstein’s static gravitational theory of 1912. Einstein started with the relation between mass and energy from special relativity. He extended it within his theory of static gravitational fields. However, scientists were already extending special relativity to relativistic dynamics, and conservation of energy and momentum centered at that time upon a four-dimensional stress-energy tensor. Starting in 1912 Einstein embodied the mass and energy relation in the energy-momentum tensor as the source of the gravitational field. Einstein also required that the gravitational field equation should be compatible with the generalized requirement of energy and momentum conservation. In 1912 this caused Einstein to think that energy-momentum conservation required that the covariance of the field equations be restricted.

4) *The correspondence principle*: Einstein was searching in two directions: he was looking for an equation of motion for bodies in a gravitational field and for a field equation determining the gravitational field itself, the generalization of the Poisson equation. The Poisson equation of classical gravitation theory describes how gravitating matter generates a gravitational potential. This potential can then be related to the gravitational field and to the force acting on the material particles exposed to it. Einstein’s theory of static gravitational fields provided a starting point, since it represents a step beyond the Poisson equation towards a generalized relativistic theory. One of Einstein’s earliest attempts in his Zurich notebook was to construct equations that would mimic the way in which the classical Laplace operator was formed.

However, Einstein was examining various candidates for generally covariant field equations in his Zurich Notebook; and he wanted to check whether these unknown gravitational field equations for the metric tensor fulfilled the requirement of recovering the familiar Newtonian gravitation theory (the Poisson equation of Newtonian theory for the scalar Newtonian potential) in the special case of the low velocity limit and weak static gravitational field. Einstein expected that under appropriate limiting conditions, the theory he was developing would first reproduce the results of his 1912 theory of static gravitational fields; that is, he expected his new theory would reduce to his own earlier theory of static fields in which gravitational potential is represented by a variable speed of light. And then, under further constraints, he would be able to recover the classical Poisson equation.

While searching for the gravitational field equation, Einstein had an additional problem, to ensure the compatibility of the different heuristic requirements by integrating them into a coherent gravitation theory represented by a consistent tensorial framework. This complexity led to the following situation: The correspondence principle became the weightiest of Einstein’s heuristic principles.

At this stage Einstein explored a different candidate for the metric tensor, and implemented differently the correspondence principle. And in fact he eventually perused all options. Of course he could not give up so easily the conservation principle and the settings of the stress-energy tensor. He had to solve the conflict between them in order to solve the incompatibility problem between the correspondence and conservation principles. Einstein changed the left hand side of the field equations and solved the conflict between the two above principles, but had to check whether the conservation principle was fully satisfied. Once this issue was settled the correspondence principle caused problems with the theory of static gravitational fields that was obtained using the equivalence principle.

At the end the match between the correspondence principle and the conservation principle was achieved at the expense of the generalized principle of relativity. At some stage, thus, Einstein appeared to somewhat forgot a little from the generalized principle of relativity; the starting point of his research project. Meanwhile he had developed many tools that would finally lead him to the goal of generally covariant equations. But at this stage he was not quite sure about the most important principles his novel theory should fulfill. He found it difficult to establish a match between the equations and the principles. But during 1912-1913 he gradually created the tensorial framework for his future general relativity.

It appears that Einstein’s long vacillating between general covariance and the correspondence principle is a symptom of his fixation on the older 1912 paradigm of the static gravitational fields. He needed an extra two years to gradually and mentally switch into the new paradigm of differential calculus; yet he would always envision Riemann’s calculus in terms of his heuristic principles.

The Zurich Notebook shows that Einstein already considered the field equations of general relativity about three years before he published them in November 1915. Einstein had come close to his November 4, 1915 field equation on page 22R when he considered the Ricci tensor given to him by Grossmann as a possible candidate for the left hand-side of such a field equation. As we have seen, the analysis of the notebook by the scholars even revealed that Einstein got so close to his November 4, 1915 breakthrough at the end of 1912, that he even considered on page 20L another candidate – albeit in a linearized form – which resembles the final version of the November 25, 1915 field equation of general relativity.

Thus Einstein first wrote down a mathematical expression close to the correct field equation and then discarded it, only to return to it more than three years later. Scholars asked: Why did Einstein discard in winter 1912-1913 what appears in hindsight to be essentially the correct gravitational field equation, and what made his field equation acceptable in late 1915? Why did he reject equations of much broader covariance in 1912-1913? The answer the scholars gave was that accepting the correct mathematical expression in 1912 required abandoning a few heuristic conditions that Einstein could not yet reconcile with his field equations of 1912. This path, says Norton, was reasonable at the time, 1912-1913. His rejection of the Ricci tensor need not be explained in terms of simple error. He was rather not prepared to accept generally covariant equations as a result of a number of misconceptions.

Einstein’s work in the Zurich Notebook was so laborious and painful but he did think that this work was rewarded when he published the “Entwurf” paper. The scholars jointly summarized the situation in the spring of 1913 in the following way: Through trial and error Einstein found a body of results, strategies, and techniques that he drew on for the more systematic search for field equations. He checked a series of candidate field equations against a list of criteria they would have to satisfy. All these candidates were extracted from the Riemann tensor. In the Zurich Notebook Einstein eventually gave up trying to extract field equations from the Riemann tensor. Drawing on results and techniques found during the earlier stage instead, he developed a way of generating field equations guaranteed to meet what he deemed to be the most important of the requirements to be satisfied of such equations. In this way he found the field equations of severely limited covariance published in the spring of 1913 in the paper co-authored with Marcel Grossmann, the “*Entwurf*” paper.

###### The “Entwurf” field equations

On May 1913 Einstein was more confident that he was probably not deceived with the “Entwurf” theory of him and Grossman. Einstein is reported to have said in the Kyoto lecture “I discussed with him [Grossmann] whether the problem could be solved using Riemann theory, in other words, by using the concept of invariance of line elements. We wrote a paper on this subject in 1913, although we could not obtain the correct equations for gravity. I studied Riemann’s equations further only to find many reasons why the desired results could not be attained in this way”.

At first, Einstein was not quite satisfied with the “Entwurf” equations. He wrote his friend Paul Ehrenfest in May 1913,

“I am now deeply convinced of having taken the thing right, and also, of course, that a murmur of indignation will spread through the ranks of our colleagues when the work [“Entwurf”] appears, which will take place in a few weeks. Naturally, I will send you right away a copy. The conviction which I have slowly struggled my way through is that *privileged coordinate systems do not exist at all*. However, I succeeded only partially, to formally penetrate to this position as well”.

By this time Einstein knew he only succeeded partially from the formal point of view – he did not possess generally covariant field equations. However, to use Einstein’s own chair story that he told of Grossmann before in the *Skizze*, now when Einstein was going to sit on his new honored chair, and feel a little of the heat that came from his “pre-seated” giant, Newton, and this heat grazed him a little; it did not completely pass. Because physics has taught him to consider writing that the Newtonian limit was something very important.

At the beginning Einstein was confused from the abrupt transition from scalar theory to tensors: “I thought again about the scalar theory [from Prague] when I was at first a bit overawed by the complexity of the equations which Grossmann and I wrote down a little later. Yes there was confusion at that time, too. But it was not like the Prague days. In Zurich I was sure that I had found the right starting point”.

Conservation of momentum and energy is also a basic pillar of physics. This brought problems when creating a theory in which there were generally covariant equations that completely determined the gravitational field from the matter tensor. Einstein was at first dissatisfied with the lack of general covariance of his field equations. He did not have much faith in the new theory. He continued to search for the most general transformation under which the gravitational field equations of the “Entwurf” theory are invariant, but by mid August he was still unable to find a single non-linear transformation admitted by the “Entwurf” field equations.

###### The Hole Argument

Einstein thought for a while – or persuaded himself – that generally covariant field equations were not permissible; one must restrict the covariance of the equations. He gave two arguments for this:

1) Einstein supposed that the stress-energy tensors for matter and for gravitation are generally covariant. He then considered the conservation law for matter and gravitational field together. However, Einstein realized that this expression cannot be covariant with respect to arbitrary transformations. Therefore, by formulating this expression Einstein created an equation which was restricted. He restricted himself to privileged systems in which the law of conservation of momentum and energy holds in the form he had written it. If one privileges a group of systems, it means that the conservation law for matter and gravitational field is only covariant with respect to arbitrary linear transformations.

2) He introduced an ingenious argument – the Hole Argument – to demonstrate that generally covariant field equations were not permissible. The Hole Argument seemed to cause Einstein great satisfaction, or else he persuaded himself that he was satisfied. Having found the Hole argument, Einstein spent two years after 1913 looking for a non-generally covariant formulation of gravitational field equations.

Around this time, Einstein used to write to his friends, I am completely satisfied now from my theory of gravitation, and I no longer have doubts of the correctness of the theory. Although he thought he had invoked an ingenious demonstration, according to which it was unavoidable that the gravitational equations were not generally covariant with respect to arbitrary transformations, the issue seemed to still bother him very much. He wrote Hopf on November 1913 that this topic bothered him so much for so long. But now it was settled. Einstein needed the support of his friends when struggling with the development of the new theory of gravitation. He was rather isolated because physicists (and the highest in ranks, Plank) behaved passively towards his new gravitation theory. He felt he was on the right track with the equivalence principle, and the equality of inertial and gravitational mass, and this caused him great satisfaction. But he did have this inkling that something was missing. He wrote Zangger on March 10, 1914,

“Now the harmony of the mutual relationship in the theory is such that I no longer have the slightest doubt about its correctness. Nature shows us only the tail of the lion. But I have no doubt that the lion belongs with it even if he cannot reveal himself to the eye directly, because of his huge dimension”. It appears that Einstein was aware that at this point he could only see a little of the tail of the lion; but he did not have the slightest doubt of the correctness of the main scaffold of his physical theory and of the direction that he chose: the absolute differential geometry.

###### Mercury, Rotation, and Light Deflection

Einstein’s collaboration with his close friends included Michele Besso as well. During a visit by Besso to Einstein in Zurich in June 1913 they both tried to solve the “Entwurf” field equations to find the perihelion advance of Mercury in the field of a static sun in what is known by the name, the “Einstein-Besso manuscript”. Besso was inducted by Einstein into the necessary calculations. The “Entwurf” theory predicted a perihelion advance of about 18” per century instead of 43” per century.

As shown later, Einstein did not mention Besso’s name in his 1915 paper that explains the anomalous precession of Mercury. Besso collaborated with Einstein on the wrong gravitational (the “Enrwurf”) theory, and their calculation based on this theory gave a wrong result. Towards the end of 1915 Einstein abandoned the “Entwurf” theory, and with his new theory got the correct precession so quickly because he was able to apply the methods he had already worked out two years earlier with Besso. Einstein though did not acknowledge his earlier work with Besso.

It appears that Einstein did not mention Besso because he considered him as a sounding board even though Besso was calculating with Einstein in Zurich; this as opposed to his other friend, Marcel Grossmann, who was his active partner since 1912 in creating the “Entwurf” theory. Einstein wrote Besso a series of letters between 1913 and 1916, and described to him step by step his discoveries of General Relativity, and thus Besso functioned again as the good old sounding board as before 1905.

Alberto Martínez objected to characterizing Besso as a “sounding board” for Einstein’s ideas. Martínez wrote, that this description was “first used by Einstein but repudiated by Besso as downplaying his role in their discussions and collaborations”. However, it seems that from 1912 Einstein considered Grossmann, and not Besso, as his partner; and Besso remained Einstein’s closest friend and sounding board.

On pages 41-42 of the Einstein-Besso manuscript, Einstein checked whether the metric field describing space and time for a rotating system was a solution of the field equations of the “Entwurf” theory. Einstein’s answer was yes, but he later discovered that he made a mistake in the calculations.

Like its predecessor for the static gravitational field from 1911-1912, the “Entwurf ” theory predicted the same value for the deflection of light in a gravitational field of the sun, 0.83 seconds of arc. Einstein, however, could not yet send an expedition to check this prediction of his theory. He made many efforts to obtain empirical data on light deflection, first from already existing photographs and later by involving himself in the organization of an expedition for the 1914 total solar eclipse; but the empirical verification of the light-bending effect remained elusive until 1919.

Meanwhile Gunnar Nordström accepted the equivalence of inertial and gravitational masses. His theory of gravitation was more natural than the “Entwurf” theory, simpler, and more related to the original theory of relativity, and to its light postulate. According to Nordström there existed a red shift of spectral lines, as in Einstein’s “Entwurf” theory, but there was no bending of light rays in a gravitational field. It could also not explain the anomalous motion of Mercury. But Nordström’s theory became a true option for a gravitational theory.

At that time the “Entwurf” theory remained without empirical support. Thus a decision in favor of one or the other theory – the “Entwurf” or Nordström’s – was impossible on empirical grounds. Einstein began to study Nordström’s theory from the theoretical point of view. He realized that it did not satisfy Mach’s ideas: according to this theory, the inertia of bodies seems to not be caused by other bodies. In a joint 1914 paper with Lorentz’s student Adrian Fokker, Einstein showed that a generally covariant formalism is presented from which Nordström’s theory follows if a single assumption is made that it is possible to choose preferred systems of reference in such a way that the velocity of light is constant; and this was done after Einstein had failed to develop a generally covariant formulation for the “Entwurf” theory.

###### Einstein and Grossmann’s second paper

Meanwhile Einstein and Grossmann wrote their second paper, published almost a year after the “Entwurf” paper. They elaborated the gravitational “Entwurf” field equations. At this stage Einstein possessed the Hole Argument, and Einstein convinced himself by calculations that the “Entwurf” equations hold good in a uniformly rotating system. Einstein’s desire was that acceleration-transformations – nonlinear transformations – would become permissible transformations in his theory. In this way transformations to accelerated frames of reference would be allowed and the theory could generalize the principle of relativity for uniform motions. Einstein wrote Besso before publishing the paper,

“This shows that there exists acceleration transformations of varied kinds, which transform the equations to themselves (e.g. also rotation), so that the equivalence hypothesis is preserved in its original form, even to an unexpectedly large extent”. Einstein was so happy that he concluded, “Now I am perfectly satisfied and no longer doubt the correctness of the whole system, regardless of whether the observation of the solar eclipse will be successful or not. The logic of the thing is too evident”. But Einstein continued on the one hand to struggle with the problem of generally covariant field equations, and on the other, with arguments (like the Hole Argument) intended to explain why he could not arrive at such equations.

###### End of collaboration with Grossmann

Einstein left Zurich in March-April 1914, and by this ended his collaboration with Marcel Grossmann. After real hard work in Berlin Einstein would finally find a brilliant way to create a theory, in which the generally covariant field equations he had already obtained in 1912 (in the Zurich Notebook) are interwoven with his physics of principles.

Einstein wrote towards the end of the *Skizze*,

“While I was busy at work together with my old friend, none of us thought of that tricky suffering, now this noble man is deceased. The courage to write this little colorful Autobiographical Sketch, gave me the desire to express at least once in life my gratitude to Marcel Grossmann”.

The intermediate stage of the development of general relativity is indeed inseparable of Grossmann’s mathematical assistance. Einstein acknowledges Grossmann’s help to the development of general relativity during 1912-1913. In fact, as with special relativity so was it with General relativity; at this intermediate stage Einstein received assistance only from his old friends, Marcel Grossmann and Michele Besso.

התגלית הגדולה: טנסור נובמבר לתפארת מדינת ישראל :)…