9 4 D O C . 9 1 R E L A T I V I T Y & E Q U A T I O N S O F M O T I O N 91. “General Theory of Relativity and Equations of Motion” [Einstein 1928b] Dated 24 November 1927 Received 8 December 1927 Published 14 February 1928 In: Preußische Akademie der Wissenschaften (Berlin). Physikalisch-mathematische Klasse. Sitzungsberichte (1928): 235–245. In a recently published article,1 I investigated together with Dr. J. Grommer the question as to whether the law of motion of singularities is determined by the field equations of the general theory of relativity.2 It has been found that the previously generally accepted law of geodesics (possibly extended to include the electromag- netic forces) in the static or stationary case follows logically from the field equations.[4] However, it has also become clear that this conclusion cannot be ad- opted without additional hypotheses in the case that the field in which the singular- ity is embedded is not stationary. In that case, an additional hypothesis about the character of the singularity embedded in an external field must be proposed, whose justification could not be proven.[5] This result is of interest from the point of view of the general question as to whether or not the field theory stands in contradiction to the postulates of quantum theory.[6] The majority of physicists today are con- vinced that the empirical facts of quantum phenomena exclude a field theory in the usual sense of the word. But this belief is not founded on a sufficient knowledge of the consequences of field theory. Therefore, it appeared to me that a further inves- tigation of the consequences of the field theory in regard to the motions of singu- larities is at present desirable, even though other paths have led to a general mastery of the numerical relationships through quantum mechanics. 1 Sitzungsber. 1927. I.[1] 2 H. Weyl, in the later editions of his books Raum, Zeit, Materie,[2] has already expressed the opin- ion that the elementary particles can be considered to be singularities of the field. He also attempted to derive the equations of motion based on that point of view. See also K. Lanczos, Z. f. Phys. Vol. 44, p. 773.[3] [p. 235]