The saga of Einstein's search for his general theory of relativity could have successfully ended as early as in 1913, with the completion of the Outline of a Generalized Theory of Relativity and of a Theory of Gravitation^{[1]}, known also as the "*Entwurf*".

Indeed, in this paper Einstein opens the Section 5 in a very promising way by writing the field equations in the following general form:

where k is a constant, Θ_{μν} is the contravariant stress-energy tensor and Γ_{μν} is the still to be found gravitational tensor "which has to be derived from the fundamental tensor g_{μν} by differential operations. In line with **the Newton-Poisson law** one would be inclined to require that these equations be** second-order**."

But then Einstein drops his bombshell: "It must be stressed that, it proves impossible to find a differential expression Γ_{μν} that is a generalization of ΔΦ and that proves to be a **tensor** with respect to **arbitrary transformations**". This assertion being justified by a reference to a particular subsection of Grossman's "Mathematical part" of the paper.

In this passage - see below extract, Grossman notes that the second-rank contraction of Riemann tensor, known as the "**Ricci tensor**" would be a good candidate to fit the right side of the above field equations, but then rejects this hypothesis as it** does not reduce properly into the Newtonian limit**.

By this decision, Einstein and Grossman did not know it yet but** they had just lost two more years** : Einstein would eventually go back exactly to the same point on his 4 November, 1915 paper, by proposing as the gravitationnal tensor, guess what....the Ricci tensor!

[1] *Entwurf einer verallgemeinerten Relativitätstheorie und eine Theorie der Gravitation*. I. Physikalischer Teil von A. Einstein II. Mathematischer Teil von M. Grossmann's rejection of Ricci tensor as gravitationnal tensor candidate

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