This is the first ever English translation of Heisenberg’s unpublished response to the EPR paper. In this chapter, Heisenberg uses his famous cut argument to argue against the possibility of hidden variables.

The famously controversial 1935 paper by Einstein, Podolsky, and Rosen (EPR) took aim at the heart of quantum mechanics. The paper provoked responses from leading theoretical physicists of the day, and brought entanglement and nonlocality to the forefront of discussion. This book looks back at when the EPR paper was published and explores those intense. conversations in print and in private correspondence. These offer significant insight into the minds of pioneering quantum physicists, including Bohr, Schrödinger and Einstein himself. Offering the most complete collection of sources to date – many published or translated here for the first time – this text brings a rich new context to this pivotal moment in physics history.

Schrödinger’s reaction to the EPR paper is less widely known than, say, Bohr’s, and yet our analysis shows that it fits rather nicely with contemporary concerns in foundations of quantum mechanics. Taking the lead both from the EPR paper and from Pauli’s remarks in their correspondence, Schrödinger shows that EPR’s locality considerations lead to the assignment of values to all quantum mechanical observables, but that under apparently mild assumptions this then leads to contradictions of the von Neumann type. This dilemma (as he explicitly calls it) is thus similar to more recent debates between nonlocality on the one hand and no-go results on the other (whether through violation of the Bell inequalities, the Kochen–Specker theorem, or what you will). We shall first look at Schrödinger’s fundamental worries in the years leading up to 1935. The chapter then discusses in detail the direct reaction by Schrödinger to EPR. It will, however, not exhaust our discussion of Schrödinger, who is a recurring character in the book, having poked and prodded his peers on EPR during the whole summer and autumn of 1935.

This is a reprinting of Bohr’s response to the EPR paper, wherein Bohr relies on his principle of complementarity to demonstrate an ambiguity in the criterion of reality as described by EPR and to argue that quantum mechanics is in fact a complete description of reality given the bounds of complementarity.

This is a reprinting of Margenau’s response to EPR (and to some extent, his evaluation of previous responses to EPR by Bohr, Kemble and Ruark). Margenau’s contribution to the EPR debate is certainly one of the most original, no doubt at least in part due to the meaty correspondence he had with Einstein while producing it. Margenau’s main strategy in this paper is to argue against the standard collapse postulate of quantum mechanics, suggesting that the EPR argument only applies to quantum mechanics with this postulate added. He also argues against the statistical interpretation of the collapse postulate suggested by Kemble and others.

This is a reprinting from Jammer (1974) of Podolsky’s unpublished response to Kemble’s criticisms of the EPR paper. Podolsky rightly criticises Kemble for missing the point of EPR’s argument and adds a few comments agreeing with Kemble that a statistical interpretation of quantum mechanics is best – yet Podolsky maintains such an interpretation is incomplete.

This is a reprinting of Flint’s response to EPR, originally signed only as ‘H.T.F.’ Flint begins with a fairly accurate outline of the argument in the EPR paper – with which he agrees – but then he expresses doubts as to the validity of the reality criterion. Without describing the nature of these doubts, he concludes by further agreeing with EPR in desiring a more direct description of reality than the one currently provided by quantum mechanics.

This chapter presents a collection of letters between the main protagonists in the EPR debate as analysed in the present volume. Among many other letters, it includes the first ever complete English translation of the correspondence Schrödinger held concerning the EPR paper with, e.g., Einstein, Bohr, Pauli, Born and Teller. He kept these letters in a special folder labelled ‘The Einstein Paradox’, only a small portion of which has previously been discussed in the foundations literature. These historical documents, many of which are published here for the first time, form the basis of our analysis in the beginning chapters of this book.

This chapter introduces in more comprehensive fashion than elsewhere in the literature the interesting role of Heisenberg in the EPR debate. Although we have already published an analysis of Heisenberg’s posthumously published draft response to EPR, only now are we able to situate this excellent primary source in its fullest context, by contributing a chapter describing, for example, Heisenberg’s thinking prior to EPR about interacting systems and hidden variables, the crucial role of Grete Hermann for Heisenberg’s thinking about separability, completeness and observational context, and describing the correspondence between Heisenberg and Bohr discussing Heisenberg’s manuscript.

This is a reprinting of Bohr’s note to Nature advertising his forthcoming response to the EPR paper. It is very brief but contains in essence the argumentative tack Bohr would in fact employ in his full response to EPR.

This is a reprinting of Furry’s response to EPR. Although his response misses the mark, his discussion of an example is intriguing for other aspects of the foundations of quantum mechanics.

This chapter provides a complete list and brief analyses of published and unpublished responses to EPR in 1935 (virtually all of which are reprinted as later chapters in this book). We invite a renewed consideration of certain contributors not much discussed elsewhere in the literature. These include going beyond Kemble’s short criticism of EPR to his ensuing disagreement with Margenau about the viability of an ensemble interpretation of the wavefunction, and also a response to Kemble’s note on EPR by Podolsky himself. We also examine the correspondence between Margenau and Einstein in the wake of EPR, discussing the role of the collapse postulate, and finally we discuss two papers by Furry, which although not entirely satisfactory qua a response to EPR’s arguments, are nevertheless of great potential interest for the foundations literature more generally.

This is a reprinting of Wolfe’s response to the EPR paper. Wolfe insists upon an epistemic reading of the wavefunction, arguing that, under such an interpretation, the EPR paradox dissolves.

This is a reprinting of the famous May 1935 paper in Physical Review by Einstein, Podolsky and Rosen. In this paper, the authors argued that the wavefunction fails to provide a complete description of reality unleashing the debate analysed in this volume.

This chapter details not only the prehistory of EPR but also examines the structure and logic of the EPR paper – including Einstein’s own preferred version of the argument for incompleteness. We here attempt a seamless interweaving of the excellent extant literature with additional details that have emerged from our work and the recent work of others. Some examples of new aspects in this prehistory of EPR include evidence of a ‘proto’ photon-box thought experiment Einstein had developed in connection with his ill-starred collaboration with Emil Rupp in 1926. We also describe the potential importance to this prehistory of Einstein’s paper with Tolman and Podolsky and of Einstein’s seminar and discussions with Schrödinger in Berlin in the early 1930s.

This is a reprinting of Edwin Kemble’s 1935 reply to the EPR paper. Kemble misses EPR’s point by taking their completeness criterion to be merely an epistemic concept; Kemble himself admits as much in a letter to Einstein later that year. His original response to EPR is nevertheless of interest, as Kemble there provides an argument for a statistical interpretation of the wavefunction – a view he attributes to Slater already in a 1929 paper, but for which Kemble provides greater clarity and motivation.

This is a reprinting of Furry’s response to Schrödinger’s cat paper and entanglement papers, as well as Furry’s response to other responses to the EPR paper, especially Bohr’s.

This is a reprinting of Schrödinger’s famous pair of papers delivered at the Cambridge Philosophical Society in late 1935 and 1936, wherein he first coins the term ‘entanglement’ to describe interacting quantum systems. The first paper (1935) is given here in full; section 4 of the second paper (1936) is reprinted as an appendix.

This is the first ever printing of a short unpublished note by Schrödinger discussing canonical conjugates, which he included among his correspondence in the folder he labelled ‘The Einstein Paradox’. The note references Flint’s response to EPR and contains ideas appearing also in a letter to Einstein in July 1935.

This is a transcription of a typescript Kemble had appended to a letter to Margenau in 1935. In this paragraph, Kemble admits that his initial published response to EPR missed the point of their argument.