2 6 2 D O C U M E N T 2 4 8 A P R I L 1 9 2 6 248. From Paul Ehrenfest Leyden, 7 April 1926 Dear Einstein, I try as rarely as possible “to write at you” because I do know what a horrid amount of annoying correspondence you are saddled with.— But today you must put up with a fully laden letter. In responding to my questions you don’t need to repeat the questions it suffices if you simply number your answers according to the questions because then I’ll simply glance through a copy of my letter. 1st question, regarding conservation of angular momentum in emission and ab- sorption of Zeeman effect lines.— Formulated by means of the exemplifying ex- periment: Flame E emits, e.g., “the redshifted circular polarized” component of a Zeeman multiplet, and flame A absorbs this component. The angular momenta of the emis- sive and absorbtive atom change.[1] How does the conservation of angular momentum arise in experiment B? II.[2] Question regarding correct relativistic formulation of an angular- momentum hypothesis for corpuscles of light:[3] According to Compton-Uhlenbeck-Goudsmit, every electron in the world car- ries an angular momentum equal to h/2π.[4] — In order to formulate this relativisti- cally invariantly, one must, of course, add: “Observed from a system that cotranslates with the electron.” I would have a few questions to ask about the six- vector that one obtains with the transition to another system, but I don’t want to bore you with it. [5] — [From Frenkel’s letter, I know that he has done something very nice on this topic with Pauli. We had toiled over that in Leyden, but the nonpoint-like nature of the electron bothered us upon consideration.] However, I ask you please to tell me how the analogous hypothesis is correctly formulated relativistically FOR CORPUSCLES OF LIGHT.— Here one evidently cannot say: The corpuscle of light carries along an angular momentum h/2π, “mea- sured in the comoving system,” because the corpuscle obviously has the velocity of light.