1 5 0 D O C . 1 7 6 N O V E M B E R 1 9 1 9 I am immensely pleased that Planck has finally received the Nobel award. I am going to write to him. But do also let him know anyway that all of us here are gen- uinely delighted about it. I am also very glad that Stark got it.[20] Dear Einstein! —We all send you our greetings, starting with Aunt down to the little crawlikins.[21] —But perhaps not even I have been revived quite as much by your stay as my wife!!!—Only you could help her to grasp the covariant problem better again.[22] And this, and you as a whole, made her much more cheerful and self-assured again. Yours, Ehrenfest In the letter to your wife I am setting a trap for you! Beware. Is your mother with you all now?[23] 176. From Wilhelm Hort[1] Charlottenburg, 108 Tegeler Way, 25 November 1919 Esteemed Professor, A while ago, the chief planning officer, Privy Councillor Wittfeld, who may be familiar to you as director of electrification measures of the Prussian state,[2] men- tioned to me an idea concerning terrestrial proof of the field of centrifugal force fol- lowing from the general theory of relativity. In view of the fact that earlier experiments by the Friedländer brothers[3] fur- nished no first results, Mr. Wittfeld suggests accumulating the influence of the field of force by means of pendulum clocks such that, by extending the measurements over very long spans of time, the extremely small effects of the field of force are made detectable. The arrangement, as Wittfeld would conceive it, is indicated on the enclosed sketch. 2 pendulum clocks, I and II, are set up in proximity of a rotating mass so that the centrifugal force field has an accelerating influence on one clock and a de- celerating one on the other, so that theoretically one would be in a position to infer the centrifugal force field from a change in the running rates of the two clocks as a consequence of the rotation. I take the liberty of submitting this suggestion to you, since I am unable to judge whether even by this experimental method the expected influence by the centrifu- gal force field is too small for one to count on detection after taking into account a reasonable timespan (many years). Among the papers dealing with the calculation of rotational force fields I know the one by Thirring,[4] which, however, was not developed enough to report real
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