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 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 These historical documents, many of which are published here for the first time, form the basis of our analysis in the beginning chapters of this book.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 is a revision of John Trimmer’s English translation of Schrödinger’s famous ‘cat paper’, originally published in three parts in Naturwissenschaften in 1935.

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 translation of notes by Schrödinger wherein he draws the implications of Einstein’s photon-box thought experiment.

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.

The topic of this chapter is the wave function – what it is, how it is to be interpreted and how information can be extracted from it. To this end, the notion of operators in quantum physics is introduced. And the statistical interpretation called the Born interpretation is discussed. This discussion also involves terms such as expectation values and standard deviations. The first part, however, is dedicated to a brief outline of how quantum theory came about – who were the key people involved, and how the theory grew out of a need for understanding certain natural phenomena. Parallels are drawn to the historical development of our understanding of light. At a time when it was generally understood that light is to be explained in terms of travelling waves, an additional understanding of light consisting of small quanta turned out to be required. It was in this context that Louis de Broglie introduced the idea that matter, which finally was known to consist of particles – atoms – must be perceived as waves as well. Finally, formal aspects such as Dirac notation and inner products are briefly addressed. And units are introduced which allow for convenient implementations in the following chapters.

The purpose of this chapter is to clearly define the mathematical objects that describe particles of various kinds: bosons (spin-0 and spin-1) or spin-1/2 fermions. Starting from the Schrödinger equation, the Klein–Gordon equation, the Dirac equation and the Maxwell equations are detailed, leading to the description of the associated quantised field – a well-adapted framework to treat states composed of many particles that can be created or annihilated when they interact. The notion of 4-current is introduced, and the quantisation of the various fields is presented. With the Dirac equation, the spinor’s properties are described extensively. The interpretation of the solutions of the Dirac equation in terms of antiparticles and spin or helicity degrees of freedom is then detailed. Helicity and chirality are also treated carefully. Finally, the Maxwell field and the Proca field are described, highlighting their specificities in terms of polarisation degrees of freedom.