D O C U M E N T 2 6 0 A U G U S T 1 9 2 8 4 1 7 setzen, d.h. die Glieder von der Ordnung berücksichtigen. Dann liefert unsere Gleichung für räumliche  oder bei Weglassung der in den  quadratischen Glieder . Dies sind die Bewegungsgleichungen, wobei natürlich die auszurechnen wären.— Mathematisch ist dies alles noch recht hässlich. Aber wenn es erst in Ordnung gebracht ist, dann können wir analog auch berechnen, wie sich in der neuen Theorie Singularitäten verhalten. Denn die Methode ist offenbar übertragbar. Einstweilen bitte ich Sie, dies zu lesen und zu versuchen, die Sache mathematisch zu säubern, zu glätten und auszuführen, bevor wir das neue Problem weiter bear- beiten. Wir schaffen damit ein Instrument, welches für die Interpretation jeder relativistischen Feldtheorie brauchbar sein wird. Senden Sie mir bitte, wenn Sie mir wieder schreiben, diesen Brief zurück, damit ich alles beisammen habe. Bestens grüsst Sie Ihr A. E. ALSX. [18 332]. [1] In Einstein and Grommer 1927 (Vol. 15, Doc. 443), Einstein and Grommer derived the equa- tions of motion of uncharged mass points from , where is the Ricci tensor in Einstein 1928b (Doc. 91), Einstein derived the equations of motion of a charged mass point from the Einstein- Maxwell equations. In both cases, the derivation depended on an approximation scheme of the field equations that was carried through up to second order terms. In Doc. 257, Einstein had first told Müntz that he was revisiting this earlier work on the problem of motion in the context of general rel- ativity in order to transfer it to the teleparallel approach. A day later (in Doc. 258), Einstein voiced hope that achieving this would allow him to determine whether the contracted torsion tensor should really be identified with the electromagnetic potential, a question that had plagued Einstein since his first paper on using teleparallel geometry to unify gravity and electromagnetism see Einstein 1928o (Doc. 219), footnote 1 on p. 225. In Doc. 259, Einstein then reported to Paul Ehrenfest that he had mastered the problem of motion to the extent that he could now transfer it to any field theory. The present letter is the most detailed elaboration of the fruits of all these efforts. [2] See. Docs. 255 and 256. Although in Doc. 257 Einstein claimed that this was not a solution due to an alleged miscalculation by Müntz, he reported in Doc. 258 that Müntz’s electron was possible for a specific Lagrangian. [3] See the corresponding equation (10) of Einstein 1928b (Doc. 91), p. 238, where Einstein had introduced an expansion in the potentials. [4] The factor of 2 should not be there. While in the first term  is a constant, in the second term  denotes an index, and summation over  is implied. [5] In Einstein 1928b (Doc. 91), p. 243, Einstein had also introduced retarded potentials when cal- culating the metric field components at second order, pointing out that, in singling out a direction of time, the restriction to retarded potentials involves a moment of arbitrariness see Doc. 91, p. 243, note 1. [6] This equation corresponds to approximations expressed in equations (7) and (8) of Einstein 1928b (Doc. 91). 3 2 d dt ---- - --  r A  r ----- - + 0 =  ·· A  + 0 = A  R ik 0 = R ik