3 9 0 D O C U M E N T 2 4 5 J U L Y 1 9 2 8 Wenn ich wieder zurück komme,[11] wollen wir ausführlich über die Sache re- den. Man kann natürlich die ganze Bemühung als utopisch erklären. Aber eine ma- thematisch so natürliche Theorie ist ernsthafter Beachtung wert, zumal bei der gegenwärtigen disperaten Lage der theoretischen Physik.[12] Beste Grüsse von Ihrem A. Einstein. ALS. [18 311]. [1] Dated on the assumption that the letter is a reply to Doc. 244. [2] On the latest efforts to purchase a summer house, see Doc. 242, note 6. [3] In Doc. 244, Müntz showed how one can integrate the linearized field equations following from the second invariant introduced at the end of Einstein 1928n (Doc. 216), and again in a note added at proof stage in Einstein 1928o (Doc. 219). [4] Jakob Grommer for some of his concerns, see Doc. 232. [5] This interpretation was introduced in Doc. 219, §1. Immediately after identifying the compo- nents of the torsion tensor with the components of the electromagnetic potential , Einstein writes in a footnote that this interpretation is not the only possibility. [6] These are equations 8 and 9 of Doc. 219. [7] In the previous year, sparked by correspondence with George Yuri Rainich, Einstein had like- wise worried whether, and if so, under which conditions, solutions to the linearized purely gravita- tional field equations correspond to solutions of the fully nonlinear Einstein equations that the linearized equations approximate. The discussion with Rainich had been concerned with two-particle solutions to the vacuum Einstein equations and launched Einstein’s work on the problem of motion in general relativity see Vol. 15, Introduction, sec. II Einstein and Grommer 1927 (Vol. 15, Doc. 443), especially note 14 and Einstein 1928b (Doc. 91). [8] Einstein had long used the question of whether such spherically symmetric solutions exist as a litmus test for the viability of a given unified field theory approach. He took them to be a necessary condition for reducing electrons to field theory, and thus to overcoming the dualism between field and matter. See Einstein and Grommer 1923a, 1923b (Vol. 13, Doc. 12) for the first appearance of this criterion. [9] Niels Bohr. James Clerk Maxwell. [10] “Approximate equations” may include the Einstein-Maxwell equations, as Einstein’s work on the problem of motion shows (Einstein and Grommer 1927 [Vol. 15, Doc. 443] and Einstein 1928b [Doc. 91]). See Lehmkuhl 2019 for further analysis on how Einstein saw the role of singularities in general relativity vis à vis unified field theory. [11] To Berlin. [12] Einstein likely refers to his unhappiness with the developments surrounding the new quantum mechanics. For details on Einstein’s rising trepidation in this respect, see Vol. 15, Introduction, sec. IX.