30 May 1928
My dear Professor Einstein,
Enclosed is a letter from Niels Bohr14 who, at the end, expresses the wish that you and Planck might also be made aware of its contents. I also enclose the carbon copy of my letter just so that you can see what set off the discussion. The remark about the uncertainty relation in the ideal gas runs as follows, when worked out: if we quantize a molecule that is reflected back and forth on the segment l , then we have
Neighboring quantized values of the momentum therefore differ from each other by so little, (namely by only h\2l), that even with the largest possible uncertainty in the coordinate (Δx = l), I cannot buy enough accuracy in the momentum to allow me to distinguish between neighboring quantum states. What Bohr says about this case at the end of the third page, I do not understand at all.*
If it is agreeable to you, I would be glad to come over sometime to talk about the letter, but perhaps you do not have much time now, before your departure, and you need to be sparing with it.
With greetings and best regards to the whole family,
* Everything that I have just said is surely terribly trivial!