l i i I N T R O D U C T I O N T O V O L U M E 1 5 One of Reichenbach’s main observations was that while the metric tensor in general relativity has rods and clocks as its “physical indicators,” there is no such indicator for the separately defined affine connection in the attempts at a unified field theory by Weyl, Eddington, and Einstein. Reichenbach now aimed to make charged particles the physical indicator of his generalized connection and to have them move on the geodesics of that connection. Given that Einstein had pointed out that this would only work for one specific charge (such as that of the electron in particular), it is remarkable that he did not compare Reichenbach’s approach to Kaluza’s attempt at a unified field theory, which allows for all charged particles to move on the geodesics of a five-dimensional connection. Indeed, it seems it was Einstein who had introduced this idea and had suggested to Kaluza on 28 April 1919 that he incorporate it into his original paper (Vol. 9, Doc. 30). But Einstein ended up not communicating the paper to the Prussian Academy for publication, as he had originally offered to Kaluza to do only a week earlier (Vol. 9, Doc. 26). The reason was that Einstein worried about the status of the cylinder condition in the theory and connected to that, the nature of the fifth di- mension as compared to the other four spacetime dimensions. Kaluza’s original cylinder condition stated that no physical quantity could depend on the fifth coor- dinate, that is, that all derivatives with respect to the fifth coordinate vanish. Even after having reconsidered that it might have been a mistake not to communicate the paper and having offered again to send it to the Prussian Academy two years later (Einstein to Kaluza, 14 September 1921 [Vol. 12, Doc. 270]), Einstein ended up criticizing the cylinder condition in the first paper he cowrote with Jakob Grommer (Einstein and Grommer 1923a, 1923b [Vol. 13, Doc. 12]). This was also the first paper in which Einstein demanded that any satisfactory unified field theory needed to allow for electron solutions. He argued that Kaluza’s theory was not fit to accom- plish this end. In the present volume, we find Einstein revisiting Kaluza’s theory, especially the interpretation of the cylinder condition, and thus also the status of the fifth dimen- sion. He might have been prompted to do this by Ehrenfest and Lorentz, who alert- ed him to new work by Oskar Klein, who had likewise—and independently of Kaluza—attempted to use five-dimensional pseudo-Riemannian geometry to pro- duce a unified theory of gravity and electromagnetism (Doc. 302 and Abs. 506). But Klein was even more ambitious than Kaluza in that he attempted not only to unify gravity and electromagnetism, but also incorporate Schrödinger’s newly found wave function within this framework. Both Ehrenfest and Lorentz urged Einstein to come to Leyden and join their meetings with Klein. Einstein wrote that he had to finish some things before vaca-