D O C U M E N T 1 1 3 N O V E M B E R 1 9 2 5 1 3 5 able toward such an assignment. One question unrelated to this is whether the times of the transitional processes also ought to enter into the theory as observable quan- tities. However, the whole theory is still in such an unfinished state that nothing can be said about that yet. Your third objection, however, doesn’t seem to me too weighty. That Coulomb forces seem to be more complicated in the new theory than in the existing seems to me attributable rather more to our habit. In fact Pauli has now also succeeded in deriving the Balmer formula on the basis of the new mechanics and the calcula- tions for it are scarcely longer than in the existing theory. Even if the attempted new theory does cause greater complication in the computations, one could still think that Nature simply “doesn’t do it more cheaply.” I don’t know whether the basic assumptions of the theory are unappealing to you from the outset but an escape from this accumulation of difficulties in the past few years of quantum theory did not seem at all feasible to me, if one did not reflect precisely on quantities observable in principle. It seemed very bad to me, too, that intuitive comprehensibility is so entirely lost by it, and I did not dare to publish this stuff for quite some time. But I managed to assuage my conscience with the thought that atoms certainly would not exist if our space-time concepts were even only ap- proximately correct for very small spaces. Obviously, I am also still very unhappy about the present state of the theory because everything is so unfinished and a pre- cise determination of the space-time conditions really governing atoms refuses to succeed. The best thing about the theory up to now is probably the mathematics of it done by Born and Jordan. But please excuse me for having detained you with such unfinished matters. With kind regards from Prof. Franck, yours sincerely, W. Heisenberg 113. To Paul Ehrenfest [Berlin, 18 November 1925] Dear Ehrenfest, So, I’m now coming [on] 1 December and am very much looking forward to it. Frost finally asked me for the Julius biography. So, please prepare the material so I can write the article in Leyden. Regards to all of you, particularly to your well-traveled, learned daughter×  from your Einstein Albert has a cross to bear, analogous to that of the latter×. Perhaps the two could be cured by each other.