D O C S . 2 7 7 , 2 7 8 J A N U A R Y 1 9 2 0 2 2 9 277. To Paul Ehrenfest [Berlin, 23 January 1920] Dear Ehrenfest, You are quite right with your formula. It is expected that absolutely every law for bodies at rest be translatable without modification to bodies in motion. Another method for translating the equations to the moving case would be this: one seeks to set the energy tensor for the case of rest, substitutes the components for the case of motion (as the equation now) and finds the former from the energy-momentum equations. . This would even be the only really decent method, since otherwise one learns nothing about the energy-momentum outcome. I think one could set T11 . . . T33 equal to 0 for the case at rest, T14 = T41 = heat flow, T44 = energy density (+ mass density) [. . .] For you, however, the question probably is whether any law of motion can be extrapolated without alteration from the corre- sponding law for resting bodies. Later about money not pressing. The grand piano should be coming soon.[1] Let’s hope it arrives in good shape! It’s magnificent! Best regards, yours, Einstein. 278. To Friedrich Paschen Berlin, 23 January 1920 Highly esteemed Colleague, Your recent inquiry makes me somewhat anxious because I know very well that you can judge much better than I the suitability of the men under consideration.[1] But at least such information can free you a little from the oppressive feeling of re- sponsibility. Only in this sense do I think it excusable that I again express my opinion.[2] To relieve my conscience I must first say once more that I view the achievements of J. Franck superior, also against those of Edgar Meyer. Because the former is ruled out,[3] however, I shall just write about Edgar Meyer, who is, besides, a per- sonal friend of mine.[4] For this last reason I ask you please to treat this statement of opinion as discreetly as circumstances allow. I see as Edgar Meyer’s greatest scientific accomplishment his early recognition of the importance of statistical investigations for analysis of elementary processes of radiation and his develop- ment of useful experimental methods in this area. Naturally, I do believe that his ∂Tμν ∂xν ------------ - 0=
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