5 1 0 D O C U M E N T 5 1 4 A P R I L 1 9 2 7 I don’t want to torture you any longer. One can speak endlessly about this entire wonderful development. How nice it would be if I could speak about all these things with you personally once again. As I understand, it is Heisenberg’s intention to try to meet you in Berlin on his trip back. I have long had the intention to try and clarify my thoughts about the general questions in a short paper, but the develop- ments storm by so quickly that everything becomes trivial. Yet I do hope soon to be able to finish such a paper. With kindest regards, your N. Bohr 514. To Hermann Weyl [Berlin,] 26 April 1927 Dear Mr. Weyl, In response to your kind letter,[1] I have of course carefully studied your proof about the law of motion of the electron[2] and was at first very enthused by its beauty and clarity. But I finally encountered difficulties that do not seem to me eas- ily solvable. 1) It cannot be seen readily that terms with in the electric field are to be ne- glected. For such terms, which are proportional to , were the ones that Maxwel- lian theory had used in an attempt to explain inertia electromagnetically. This I consider as less important though. But the following does seem important to me: 2) To be able to choose the fictitious field in the interior of the bounding sphere, or channel,[3] using the normal coordinate system, as a static one, as you do in your proof, the field must be suitably constituted at the boundary. But we know nothing about this, especially if the electron is accelerated (is under the influence of an elec- tric field). I attach so much importance to the entire matter because it would be very im- portant to know whether the field equations as such should be considered as refuted by the fact of quanta or not.[4] One is after all inclined to believe this and most do. But to this day, nothing about this appears to me to have been proven. f 2 c2