EINSTEIN, Albert. Typed letter signed ("A. Einstein"), to Professors Dr. R. Hosemann and Dr. Bagchi, with several autograph additions and a six-line autograph postscript, [Princeton, N.J., 9 August 1954. 1½ page, 4to, on Einstein's embossed Princeton stationery, two file holes in left-hand blank portion, margins with light pencilled notations (by the recipients?). In German.
CURRENT QUANTUM THEORY CANNOT BE OF UNIVERSAL APPLICATION "ANY MORE THAN ONE CAN MAKE NEWTON'S GRAVITATIONAL THEORY SATISFY GENERAL RELATIVITY FOR THE EQUIVALENCE OF ACCELERATION AND GRAVITY"
A thoughtful letter concerning quantum theory, written only seven months before Einstein's death, in reply to two physicists who had proposed a novel application of the theory. The elderly Einstein replies at considerable length: "You asked me to give my opinion about your efforts to complete the quantum theory so it can be considered to be a description of the individual systems (not just as an ensemble). But it was likely to have been primarily another theory that guided you: the De Broglie waves appear, like electrical radiation, as a field from whose field equation one would like to conceieve that it could be considered, in the first instance, to be a 'property of space' ("Raum-Eigenschaft"). However, this is contradicted by the fact that the De Broglie equation contains the mass of the m, which can in principle take on any value, or at least a large number of values. You have thus tried to replace the m by an expression from the field (\KS\k), and have done this (Eq.2) so that it matches a propagating wave. The fact that you did this without increasing the order of the differential equation is nice ("hübsch")."
"However, this required that you give up the linearity of the field equation (and thus the superposition principle), which is so essential for the interpretation of interferences, and even for the system of the current quantum theory. For a 'standing wave' this must be really ugly: Expression (2) looks as ugly as if it were cooked up in the devil's grandmothers kitchen ('Ueberhaupt sieht der Ausdruck (2) hässlich aus, wie wenn er in des Teufels Grossmutter Küche gekocht wäre'). The pilot principle that you have accepted for similar reasons is also questionable, since it also violates the superposition principle and can only produce reasonable motions for the simple propagating wave (as also for Bohm - De Broglie). In general it seems questionable to me to consider the \KS\k-field (analogous to the electromagnetic field) to be a carrier of energy and momentum, which just cannot be reconciled with the particle properties of the phenomena (also for electromagnetic radiation)."
"The current quantum theory is, in a certain sense, a marvelously self-contained theoretical system, but in my opinion, it cannot be made into an individual theory by supplementation, any more than one can, for example, make Newton's gravitational theory satisfy general relativity for the equivalence of acceleration and gravity. As difficult as it obviously is, one has to somehow build from the ground up." In an autograph postscript, Einstein adds: "It will generally become clear to everyone that the current quantum theory can only be considered as an ensemble description if one wishes to avoid obvious absurdities. Only the success [of the theory] can ultimately determine whether one needs to search for an individual description or not."