Princeton, 8 October 1952

On the flaws in the current state of quantum theory, and on the direction of time's arrow.

Besso has posed two questions. The first is on the relationship between quantum theory and physical reality: 'In what relationship stands the "state" ("quantum state") described by a ψ-function to a defined real situation (let's call it a "real state"). Does the quantum state characterise a real state (1) completely or (2) incompletely?'. Einstein reminds Besso that the question cannot immediately be answered, because 'every measurement involves an uncontrollable real intervention in the system (Heisenberg)'. The 'real state' is not therefore empirically accessible. Nevertheless, Einstein presents the argument for rejecting option (1), 'since this conception obliges the presumption that a rigid coupling exists between parts of the system spatially separated to any extent (direct remote action, which does not diminish with increasing distance)'. He goes into further details for favouring this conclusion: 'If one considers the method of current quantum theory as in principle definitive, this implies the renunciation of a complete description of real states'. Einstein however, cannot accept this: 'I think therefore that the statistical character of the present theory [of quanta] is simply conditioned by the choice of an incomplete description'.

Besso's second question is whether there exists a direction for the movement of time, from the point of view of the fundamental laws. There is nothing to support this in the experiential propositions on the elementary processes, nor in classical mechanics. 'We are still far from possessing a rational theory of light and matter which is in accord with the facts!'. Einstein underlines his long-held belief that it is speculation, not an accumulation of experimental data, which will bring us closer to the truth.