D O C . 2 1 9 A N E W U N I F I E D F I E L D T H E O R Y 2 1 7 219. “New Possibility for a Unified Field Theory of Gravitation and Electricity” [Einstein 1928o] Presented 14 June 1928[1] Published 10 July 1928 In: Preußische Akademie der Wissenschaften (Berlin). Physikalisch-mathematische Klasse. Sitzungsberichte (1928): 224–227. A few days ago, in a brief paper in these Proceedings,[2] I described how by us- ing an n-Bein orthogonal-frame field, a geometric theory can be formulated that is based on the fundamental principles of a Riemannian metric and distant parallel- ism. I initially left open the question of whether such a theory could serve to rep- resent physical phenomena. Since then, I have discovered that this theory–at least to first approximation–yields quite simply and naturally the field laws of gravita- tion and electromagnetism. It is therefore conceivable[3] that this theory will replace[4] the original formulation of the general theory of relativity. The introduction of distant parallelism entails that according to this theory, there is something like a straight line, i.e., a line whose elements are all parallel to one another such a line is naturally not at all identical to a geodesic. Furthermore, in contrast to previous versions of general relativity theory, it contains the concept of relative rest between two point masses (parallelism of two line elements that be- long to two different world-lines). To make the general theory directly applicable to the field theory in the form used, one must only make the following specifications: 1. The number of dimensions is 4 (n = 4). 2. The fourth local component A a (a = 4) of a vector is purely imaginary like- wise, as a result, so are the components of the fourth axis of the tetrad frame, i.e., the quantities hν 4 and h ν4. 1 The coefficients then naturally all become real. Therefore we will also choose the square of the magnitude of a timelike vector as negative. ____________________________________ 1. Instead of this, one could also define the square of the magnitude of the local vector A 1 2 +A 2 2 +A 3 2 A 4 2 and introduce Lorentz transformations in place of rotations of the local n-frame. Then the h would all become real, but the immediate connection to the formulation of the general the- ory would be lost. [p. 224] g  = h a h a  