John Von Neumann on Quantum Correlations

Abstract

In an (unpublished) letter by von Neumann to Schrödinger (dated April 11, 1936) von Neumann replies to Schrödinger’s two famous 1935 papers, in which the notion of entanglement between spatially separated quantum systems is introduced and the probabilistic correlations arising from entanglement is discussed from the perspective of a possible clash between quantum mechanics and the principle of physical locality. By quoting extensively from von Neumann’s letter it will be seen that von Neumann position concerning such correlations is that they are unproblematic as long as (i) one can (at least in principle) assume that the correlations are explainable by common causes, or (ii) probabilities are interpreted subjectively. It will be argued that while a subjective interpretation of quantum probabilities is difficult to accept in a quantum context, a common cause type explanation of quantum correlations might be possible under a suitable specification of common cause.