While many are aware of Bohr’s response to the Einstein-Podolsky-Rosen paper (EPR) arguing for the incompleteness of quantum mechanics, few know that Heisenberg also drafted a response to EPR; it was never published, save as an enclosure in a July 1935 letter to Pauli. This talk explores Heisenberg’s response (which is, unsurprisingly, quite different from Bohr’s), with special emphasis given to the treatment of hidden variables therein.
In Heisenberg’s response, he argues for the in-principle completeness of quantum mechanics using his idea of the “cut” between the classical and quantum regimes – a concept he employed frequently throughout his life, but most fully in the EPR response. It is within the larger context of Heisenberg’s cut argument that he discusses hidden variables theories; I argue that his particular approach to hidden variables not only enabled him to give what appears to be the first description of a contextual hidden variables theory (anticipating Bell by nearly thirty years), but also allowed Heisenberg to argue against hidden variables in a novel way.
After giving a synopsis of Heisenberg’s response to EPR, I provide a short history of the famed cut argument, followed by an analysis of his treatment of hidden variables as compared to the algebraic approach typified by von Neumann, Schrödinger and Pauli. I end by considering possible conceptual ties between Heisenberg’s cut and current discussions of decoherence and the quantum-to-classical transition.