26 April 1926
Many thanks for your letter. I am convinced that you have made a decisive advance with your formulation of the quantum condition, just as I am equally convinced that the Heisenberg-Born route is off the track. The same condition of system additivity is not satisfied in their method.
I have now discovered considerations that nearly rule out the existence of elementary spherical waves, so that I am pretty well convinced that the experiment I proposed will turn out negatively. Its simplest realization is, in principle:
A direction of emission R corresponds to a point in the focal plane of the telescope. Rays emitted in the direction R by a particle reach, or do not reach, the telescope (alternately); for a suitable relationship between the particle velocity and the path difference the interference would have to be destroyed, which, however, I do not believe. Diffraction at the grating acts as a disturbance, but not so strongly as to destroy the demonstrative power of the experiment.