D O C . 3 3 4 F E B R U A R Y 1 9 2 0 2 8 1 tude. Of most importance, however, is that no matter how small or large a system of this kind is, each represents its own Riemannian space-time with its own con- stant c. All these systems have their Maxwell equations and their energy conserva- tion law, that is, those physical foundations on which the theory of relativity is built. 4. These ideas could be made the basis of an as yet missing unified physics of stellar and atomic systems. The characteristic quantity c for each system indicates the typical properties of the atoms and spectrum lines of the elements. It determines crystal optics and correlates with the quantum hypothesis[10] and radioactivity. (Specifically the microworlds could provide Riemannian systems of such strong space-time curvature that experimental confirmation of the theory could be easier there than in the larger domains. In particular, perhaps one day something could be said about the relation between c quantities among the various atomic substances.) 5. In accordance with this, I pose the question: Does your theory of relativity ap- ply only to the universe as a whole, or is it valid for all closed systems in the above sense, whether molar or molecular in type?—by which the total universe would also be included. Doesn’t the theory of relativity agree with the assumption of a Euclidean (world-) space-time which, in comparison to the individual Riemannian systems, would represent something more absolute, and how are interior and exterior distin- guished here?– Highly esteemed Professor, you see that I am of the opinion that your theory has many more distinct ranges of validity than those emerging from known facts, that the theory is not just valid for the universe as a whole, for which I cannot imagine any absolute constant. The reply “those are simply other worlds” proves that the true meaning of the theory of relativity has not yet sunk into minds that have a purely mathematical command of it. I would be very thankful to hear from you whether and how far this extension of the theory would be possible and rational, and whether I am right that your existing theory already entails all that I have set forth. In utmost appreciation, Med. Prac[titioner] Dr. Albert Fleck. [4] Recipient’s marginal comment: “No.” [7] Recipient’s marginal comment: “Then we couldn’t see the other system, if it was in fact closed.” [8] Recipient’s marginal comment: “impossible for the s[ame] reason.”