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This chapter provides a complete list and brief analyses of published and unpublished responses to EPR in 1935 (virtually all of which are reprinted as later chapters in this book).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 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.

In this chapter, we dive deeply into Bohr’s views on (in)completeness and (non)locality. Perhaps the most outspoken and famous respondent to EPR, Bohr is generally thought to be obscure in his reply. We analyse it afresh (at least to our satisfaction), in particular in regard to its argumentative structure, the role of Bohr's examples and that of his 'non-mechanical disturbance'. We also assess its limitations as a reply to Einstein's wider concerns.

This chapter gives a quantitative introduction to decoherence theory, including density matrix formalism in the context of quantum field theory, and a survey of the quantum trajectories method. Finally, the mathematical structure for a new proposal for spontaneous collapse, introduced nonmathematically in Chapter 6, is given.

This chapter surveys modern progress in physics on the topic of “decoherence,” the physical process by which irreversible behavior can occur in wave systems. A substantial part of the chapter discusses a proposal by the author of this book for a spontaneous collapse theory that is connected to decoherence.

This chapter begins a short, two-chapter section on calculations that specifically impact the philosophy of quantum mechanics. A quantitative discussion of the famous Einstein–Podalsky–Rosen (EPR) experiment is given, as well as a mathematical discussion of problems with the many-worlds hypothesis, the Bohmian pilot-wave hypothesis, and the “transactional” hypothesis for interpreting quantum mechanics.

Given the philosophical problems of the Copenhagen interpretation, several other approaches to interpreting quantum mechanics have been proposed over the years. This chapter surveys four of these approaches, namely the many-worlds hypothesis, Bohmian “pilot waves,” positivist approaches, and spontaneous collapse of the quantum wave function. Problems with each of these approaches are discussed.

After having shown in previous chapters that wave-particle duality is not a fundamental problem for quantum mechanics, this chapter introduces the really strange effect of quantum mechanics, namely “nonlocal correlations” that appear to act over long distances faster than the speed of light. The “Copenhagen” interpretation of quantum mechanics is introduced, which puts human knowledge in a special role, and some of the philosophical objections to it.

Entanglement is one of the most fundamental, and intriguing, properties of quantum mechanics. It is also at the heart of quantum cryptography! In this chapter we start by giving a clear mathematical definition of entanglement. We give two classic applications, to superdense coding and to secret sharing. We then investigate two complementary properties of entanglement that we will use deeply in cryptographic applications. The first is nonlocality, which we investigate through the famous CHSH game. The second is the monogamy of entanglement, which we demonstrate using a three-player version of the CHSH game.

This chapter serves as an elementary introduction to the basic quantum puzzles such as the measurement problem, indeterminacy, nonlocality, entanglement, and ambiguity surrounding the projection postulate. It briefly reviews the standard extant interpretations such as the Copenhagen interpretation, many-worlds interpretation, and hidden variables interpretation, and introduces the idea that quantum theory is about a new metaphysical category: physical possibility or res potentia.