5 4 D O C . 6 5 J U N E 1 9 1 9 65. From Gustav Mie Halle-on-S[aale], 47 Magdeburger St., 29 June 1919 Dear Colleague, We are in search of a successor here to our mathematician Wangerin, who wants to retire, and I would like to put Weyl in Zurich on the list, if possible in first place, as I have the impression that he is an excellent man. To find out whether my opin- ion is well founded, I would like to ask you what you think of him. First of all, you are obviously qualified and certainly can make a better assessment of Weyl’s pa- pers than I, particularly on the theory of general relativity. Besides that, you surely know Weyl personally, and it would also be very important to me to hear whether he is stimulating both as a teacher and as a person and anything else that can be said about his character. I am not acquainted with him in any way personally, and like- wise, scientifically only from the papers that happen to interest me. I am very willing to revise my opinion if you are of another view. I may have expressed my own view a little too bluntly at the beginning of this letter. Unfortunately, I have hardly managed to work in science lately, as my institute here gives me much worry and much work. Half a year ago I did complete a little investigation that will possibly interest you as well. I calculated the electric field of a charged massive sphere rotating around a center of gravity. As, according to your theory, this field can be derived from the case of a sphere at rest by some kind of transformation (I first calculated with uncurved space, but this surely comes to the same thing), it can consequently be predicted that the sphere does not radiate during this motion. And that is how it came out, too. I first thought that this result was particularly interesting. But unfortunately, upon closer reflection, the matter was very simply explained. What one derives in this way from the case of a normal sphere at rest is not the sphere rotating in an inherently radiation-free space. The resulting case is rather what one would get if, say, the outer space were enclosed in a very large, completely reflective hollow sphere that is concentric with the center of gravity. Then standing waves would form as a consequence of the reflection of the waves, and the revolving sphere would no longer give off any radiant energy. So we see that one must be very careful indeed with the general principle of relativity. A rotating charged sphere in completely free space does not result from 〈simple〉 “transformation” of a sphere at rest. (One must start out from a char[ged] sphere in a rotating electric field!) The motion must therefore be dem- onstrated “absolutely” in this case also, namely by the radiation and the energy loss associated with the radiation.