31 May 1928
Dear Schrödinger,
I think that you have hit the nail on the head. It is true that the evasion using the arbitrarily large domain of cyclic variables to limit the value of Δ p is very ingenious.15 But an uncertainty relation interpreted that way does not appear to be very illuminating. The thing was invented for free particles, and it fits only that case in a natural way. Your claim that the concepts p,q will have to be given up, if they can only claim such a “shaky” meaning, seems to me to be fully justified. The Heisenberg-Bohr16 tranquilizing philosophy—or religion?—is so delicately contrived that, for the time being, it provides a gentle pillow for the true believer from which he cannot very easily be aroused. So let him lie there.
But this religion has so damned little effect on me that, in spite of everything, I say
not: E and ν
but rather: E or ν;
and indeed: not ν, but rather E (it is ultimately real). But I cannot make head or tail of it mathematically. My brain is also too worn out by this time. If you would give me the pleasure of a visit from you again sometime it would be good of you and very fine for me.
Best regards from
A. Einstein