4 July 1927
My dear Professor,
. . . . . . .
May I talk a little physics yet? I should like so much to know how the quantum situation is judged in Berlin and especially by you yourself. Is what the matrix-physicists and q-number-physicists say true—that the wave equation describes only the behavior of a statistical ensemble, just like the so-called Fokker partial differential equation perhaps? I would willingly believe it since the interpretation is really much more convenient, if I could only pacify my conscience and convince it that it is not frivolous to get off so easily in overcoming the difficulties. I believe I am right that you yourself wrestled with the first and most basic assumption of discontinuity (i.e. precisely “the quantum theory”) in its day, wrestled a hard intellectual struggle with your whole soul, as the “second version”7 which followed so long afterwards shows most clearly. I believe that one is obliged to take up this struggle anew with the same seriousness among today’s newly emerged points of view. I do not have the feeling that this is really happening on the part of those who today already announce categorically: the discontinuous exchange of energy must be adhered to.
What seems most questionable to me in Born’s probability interpretation is that when it is carried out in more detail (by its adherents) the most remarkable things come forth naturally: the probabilities of events that a naive interpretation would consider to be independent do not simply multiply when combined, but instead “the probability amplitudes interfere” in a completely mysterious way (namely, just like my wave amplitudes, of course). In a brand new article by Heisenberg even my much smiled at wave packets are said to have finally found their suitable interpretation as “probability packets”. The first is especially comical. It can also be expressed this way: the Born probability (more correctly its square root) is a two dimensional vector; its addition is to be carried out vectorially. The multiplication is still more complicated, I believe.
Well, as God wills; I keep quiet. That is, if one really must, I too will become accustomed to such things.
With kindest regards to your wife, Professor Planck, I remain