Peconic,17 9 VIII 1939
Dear Schrödinger,
. . . . . . .
Now to physics. I am as convinced as ever that the wave representation of matter is an incomplete representation of the state of affairs, no matter how practically useful it has proved itself to be. The prettiest way to show this is by your example with the cat18 (radioactive decay with an explosion coupled to it.) At a fixed time parts of the ψ-function correspond to the cat being alive and other parts to the cat being pulverized.
If one attempts to interpret the ψ-function as a complete description of a state, independent of whether or not it is observed, then this means that at the time in question the cat is neither alive nor pulverized. But one or the other situation would be realized by making an observation.
If one rejects this interpretation then one must assume that the ψ-function does not express the real situation but rather that it expresses the contents of our knowledge of the situation. This is Born’s interpretation19 which most theorists today probably share. But then the laws of nature that one can formulate do not apply to the change with time of something that exists, but rather to the time variation of the content of our legitimate expectations.
Both points of view are logically unobjectionable; but I cannot believe that either of these viewpoints will finally be established.
There is also the mystic, who forbids, as being unscientific, an inquiry about something that exists independently of whether or not it is observed, i.e. the question as to whether or not the cat is alive at a particular instant before an observation is made (Bohr). Then both interpretations fuse into a gentle fog, in which I feel no better than I do in either of the previously mentioned interpretations, which do take a position with respect to the concept of reality.
I am as convinced as ever that this most remarkable situation has come about because we have not yet achieved a complete description of the actual state of affairs.
Of course I admit that such a complete description would not be observable in its entirety in the individual case, but from a rational point of view one also could not require this.
I write this to you, not with any illusions that I will convince you, but with the sole intention of letting you understand my point of view, which has driven me into deep solitude. I have also brought it to the point of a real mathematical theory, whose testing, however, is naturally very difficult.
Best regards from
Yours,
A. Einstein