D O C . 7 7 J U L Y 1 9 1 9 6 5 In Brussels I saw Mr. Solvay,[7] who is doing quite well, although the difficult years he has had to endure have visibly left their mark. With the Commission ad- ministrative I discussed the reactivation of the Institut international de physique. After the old Comité scientifique was dissolved, a new Comité was formed in which all the former members were absorbed, of course, with the exception of Warburg, Nernst, and Goldschmidt, whose places will be taken by W. H. Bragg, Righi, and van Aubel.[8] The replacement of Goldschmidt by van Aubel is due to the fact that we want to limit ourselves to physics more strictly than before. Thus the institute will take up its work again. It is clear that for the time being no Germans will be invited (it would be difficult for them to come to Brussels), but there is no question of formal exclusion the door will be held ajar for them so that general cooperation can return in the future. Unfortunately this must take many more years.[9] I very much hope that Germany will succeed in finding the necessary calm and a chance for undisturbed work then, thanks to the great energy of its people, it will recover from the deep fall and once again take up its appropriate role in the peaceful development of the world. I wish you personally well. I hope you found your chil- dren in good health. With cordial regards, devotedly yours, H. A. Lorentz. Since I do not know whether you are still in Switzerland,[10] I am writing this letter in duplicate and also sending it to Berlin. Recipient’s note: “File away.” [1]Sender’s note: “We are staying here until the end of August, as are de Haas and his family.” 77. From Joseph Petzoldt[1] Spandau, 6 Wröhmänner St., 26 July 1919 Highly esteemed and dear Professor, May I bother you with a problem that is of no great significance but that I have repeatedly come up against? In your “Foundation of the Gen. Theory of Relativity,” Leipzig, 1916, you say, p. 12,[2] that for the observer K “at rest,” the relation between circumference and diameter of the rotating disk would be larger than π. The same then also ap- pears in your popular account and likewise with Weyl, Bloch, and Schlick.[3] How- ever, since according to the spec. th. of rel. a “moving” length is shortened for an observer “at rest,” surely the observ. in K must think that the “moving” disk’s cir- cumference is smaller than 2rπ, hence also the relation between circumference and K′
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