3 2 0 D O C U M E N T 1 8 0 J A N U A R Y 1 9 2 6 (singularitäten-freie) Lösung entspricht.[9] Solange diese Fragen sich nicht beant- worten lassen, kann man nicht wissen ob die allgemeine Relativitätstheorie in ihrer heutigen einfachsten Gestalt den Quanten-Erscheinungen gegenüber versagt. Ich stehe recht ratlos vor diesen Fragen. Von der Unmöglichkeit, die Gesetze der Elektrizität durch Verallgemeinerung des Riemannschen Begriffs-Schemas abzuleiten, bin ich nun fest überzreugt, leider.[10] Es war alles vergebliche Mühe. Herzlich grüsst Sie Ihr A. Einstein. ALSX (UkCF). [73 946]. [1]Dated from Doc. 178, to which this is a reply. [2]For Eddington’s opinion, see Doc. 178. [3]See Einstein 1924d (Vol. 14, Doc. 170) for similar statements, and Stachel 1993 for an analysis of Einstein’s recurring doubts on whether differential equations would allow to give an account of quantum particles. [4]These are eq. (3) and (2a) of Einstein 1927a (Doc. 158). Einstein also discusses them in Doc. 138. [5]Einstein refers to an initial value formulation of the field equations. See Einstein 1924d (Vol. 14, Doc. 170), note 3, for details. [6]See Doc. 216 for Einstein elaborating in detail on the difficulty of deriving the masses and charges of electrons and protons in the context of a field theory. [7]The role of singularities and whether they should be forbidden in field theory is the principal topic of the part of Einstein’s correspondence with George Y. Rainich that starts with Doc. 245. By the time of writing Einstein and Grommer 1927 (Doc. 443), Einstein had come to the opinion that in general relativity singularities are permissible at the location of particles (that are not included in a purely gravitational theory), but are to be forbidden outside of the particles (the domain of application of general relativity). See Doc. 443, notes 6 and 22, for the context of making this distinction. [8]Einstein had previously shown that the field equations discussed in this letter imply that the Ricci scalar R is zero throughout spacetime (see Doc. 138, note 7, for details). [9]In Einstein and Grommer 1927 (Doc. 443), pp. 6–7, the authors discuss conditions that they believe have to be fulfilled in order for solutions to the linearized field equations to become approx- imate solutions to the nonlinear field equations. [10]In the addendum added in the proofs of Einstein 1925w (Doc. 92), he had already claimed that the goal of unifying electrodynamics and the law of gravity no longer seemed justified to him. To Michele Besso he voiced the conviction that the approach via Weyl’s and Eddington’s generalizations of Riemannian geometry would not be successful (Doc. 138). 180. From Arnold Sommerfeld München, den 23. I. 26 Lieber Einstein! Als ich Sie im Herbst besuchte, sprachen wir u. a. über Aberration und über Thirrings Noten dazu.[1] Der Vortrag von Emden, der heute an die „Naturwiss.“ ab- geschickt ist,[2] giebt soviel ich sehe, Ihren Standpunkt wider und scheint mir die