3 1 0 D O C U M E N T 2 9 3 M A Y 1 9 2 6 293. From George Y. Rainich Johns Hopkins University, Baltimore, Md., 23 May 1926 Dear Mr. Einstein, I cannot tell you how grateful I am for your letters, which give me the feeling that I am not working in a vacuum.— But I must say that your last letter[1] did not convince me that it is hopeless to solve the fundamental problems from field theo- retic point of view. You write: “… it seems to me certain that one would then” (i.e., based on the conception that electricity is “composed of singularities”) “have to dispense with … an explanation for the equality of the numerical values for electric charges… furthermore, it won’t be possible to arrive at a law of motion for electric charges… I am convinced that one could find a strict solution on the basis of the gravitational equations + Maxwell equations, which would represent the case of two electrons at rest. This would prove that your plan cannot be carried out.” To this I would like to reply that if it is possible to find a solution to a set of field equations that has two electrons at rest, then this could prove that the field equa- tions are unsatisfactory. You speak, in particular, of the basis consisting of “gravitational equations + Maxwell equations.” In my opinion, your original gravitational equations only state that electricity is absent or, better put, that we disregard electric phenomena.[2] Hence, only the Maxwell equations remain, or, I would say, the conditions M that must be demanded of the curvature field so that an electromagnetic field obeying the Maxw. equations can be embedded in the curvature field. (I set up these condi- tions before).[3] If in addition it were possible to find a static non-centrosymmetric field satisfying these conditions M, then that would only prove that we must impose further constraints on the curvature field. Either way, I do not see any reason for regarding these—¢very² quite minimal—conditions M as sufficient: The success of the theory in the centrosymmetric case evidently rests to a large extent on the con- straints that are being imposed precisely by the central symmetry.[4] That is why it seems to me that the cardinal question can be formulated in the following way:[5] Finding conditions or constraints for the curvature field which yield a theory that corresponds to reality specifically, it must yield: I equality of the charges II absence of magnetic charges (s[ee] f[urther] down) III laws of motion for electricity Naturally, this is not an answer to a question but rather itself the posing of a question. I am now trying to find suitable constraints, and I am only hoping to solve