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
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