D O C U M E N T 1 7 9 J A N U A R Y 1 9 2 6 1 9 5 179. To Arthur S. Eddington [Berlin, after 22 January 1926][1] Dear, esteemed Mr. Eddington, I would certainly have used this opportunity to travel to England, despite all aversion of mine to official affairs, if I could have conversed with you orally. But since it is our fate now not to be able to communicate with our ears, I shall just have to deny myself again. My wish to speak with you is so strong that it would be worth it for me to learn to speak English for that alone. As regards Heisenberg-Born, I am entirely of your opinion.[2] It is mathemati- cally unnatural to form the concept of arbitrary functions of matrices. Since God did have a choice on the day of creation, he certainly did not make this one. But as an achievement of thought, this theory is admirable. Like you, I also have the im- pression that there should be some truth to it. Under no condition do I believe that one would have to dispense with a spatiotemporal description of reality. But it really does seem questionable whether we can manage with differential equations.[3] This seems to me the big question, for which we know too little ex- perimentally to answer it. The general theory of relativity seems to me to point par- ticularly naturally to the equations[4] of which I believe that they uniquely determine development over time.[5] However, these equations do not determine the mass and charge of electrons and protons hence they still need amendment in order to express the entire laws of nature.[6] I am not able to see with certainty whether these equations are in contra- diction to the quantum facts. One never knows with such nonlinear equations what constraints of the possibilities arise from the prohibition of singularities.[7] One does not know whether properties of integrals exist that would permit one to turn quantum conditions into initial conditions. The only thing that is immediately clear is that the scalar R is constant along any world line of electrically charged particles, which indicates a temporal invariance of electrons and protons.[8] One does not know whether every solution that exists to the 1st approximation of the field equa- tions corresponds to an exact (singularity-free) solution.[9] As long as these ques- tions cannot be answered, one cannot know whether the general theory of relativity in its current simplest form fails in the face of quantum phenomena. I am at quite a loss when faced with these issues. Rik 1 4 -- - gikR – κTik electromagnetic –= Tik 1 4 -- - gikϕαβϕαβ ϕiαϕkα – = ϕαβ ∂ϕα xβ --------- ∂ϕ xα ---------β –=