1 8 4 D O C S . 2 1 5 , 2 1 6 D E C E M B E R 1 9 1 9 215. From Heinrich Zangger [before 15 December 1919] [Not translated for this volume.] 216. To Arthur S. Eddington Berlin, 15 December 1919 Dear Mr. Eddington, First of all, I congratulate you on the happy success of your difficult expedition. From the great interest you showed already in the past for relativity theory, I believe I may assume that it was due primarily to your initiative that the expeditions were carried out.[1] I am astonished at the great interest the theory has generated among our English colleagues despite its complexity. With regard to science, I would like to make the following comment. I am con- vinced that the redshift of spectrum lines is an absolutely compelling consequence of relativity theory. If it were proved that this effect did not exist in nature, then the whole theory would have to be abandoned. This is most convincingly evident in the following example: Let a coordinate system K rotate uniformly about its z-axis rel- ative to an inertial system. Of two identically made clocks [U], let one be posi- tioned at the origin, the other at a point on the x-axis. Since is moving relative to the inertial system, whereas is not, according to the special theory of relativity[2] consequently runs more slowly relative to the inertial system than . The same obviously applies if one chooses K as the coordinate system, and the time relative to K is chosen such that the one in the grav- itational field becomes static relative to K. In this case, the difference in operating speeds between the two clocks must evidently be traced back to the gravitational field. (Potential differences of the centrifugal field).[3] A translation to gravitational fields of other sorts is probably inescapable. I do not consider Weyl’s theory (metric interpretation of the electromagnetic po- tential) correct. It seems to me incompatible with the fact that measuring rods and clocks exhibit a behavior independent of their prehistories.[4] It pleased me exceedingly that you wrote me personally, and no less that you found a few words of acknowledgment for Mr. Freundlich.[5] He is very enthusias- tic but, as a consequence of material and personal obstacles, still has not been able to contribute very much toward testing the theory. U2 U1 U2 U1