Abraham Pais’s biography of Niels Bohr reports that after the publication of the Einstein, Podolsky and Rosen (EPR) paper ‘no one … paid any attention’ (1991, pp. 430–1). Pais argues that ‘there are a few who opine that Einstein concluded with a bang, while most consider it a whimper’. Guido Bacciagaluppi and Elise Crull have performed a valuable service to the field by firmly putting this idea to bed. What emerges from the pages of this wonderful text is not only a clear recognition of the profound significance of the ‘Einstein paradox(s)’ for the central figures of early twentieth-century physics, but also a far deeper appreciation of the decade-long debate out of which they developed.
This debate unfolded primarily in seminars and extended correspondence, which has been held in diverse archival collections and remained largely inaccessible to many historians and philosophers of science. It is collected and translated here for the first time. These sources make an invaluable contribution to the extant literature, but the true significance of this monograph lies in its four expository chapters, which are devoted to the development of Einstein’s thought leading up to the EPR paper (Chapter 1), and the considered responses of Erwin Schrödinger (Chapter 3), Werner Heisenberg (Chapter 4) and Bohr (Chapter 5).
Chapter 1 masterfully elucidates the key stages of Einstein’s exploration of the problem of the completeness of quantum mechanics. One particularly important element of this account is the explicit challenge to Bohr’s long-canonical characterization of the photon box thought experiment as critique of the validity of the uncertainty relations. Following Don Howard (2007), Bacciagaluppi and Crull show that Einstein aimed not to challenge the uncertainty relations (the validity of which he long accepted), but their ontological import. He favoured an epistemological reading motivated by the apparent non-local or non-separable nature of the delayed choice variants of the photon box thought experiment. It was enlightening to observe how this reasoning was developed and eventually generalized in the EPR paper in support of their challenge to the apparent completeness of quantum mechanics.
Chapter 3 stands as a real highlight. The discussion of Schrödinger’s criticism of Bohr’s use of classical concepts (provocatively framed as a crutch) is highly illuminating of the middle period in Schrödinger’s philosophy of science – in particular, relating to his search for new concepts in quantum theory. The account of the ‘cat paper’ is exemplary. There is a clear clarification of Schrödinger’s reproach of the epistemic reading of the uncertainty relations developed in his correspondence with Pauli. This nicely complements the classic criticism of the ontological reading in the case of macroscopic systems (that is, live cats). Yet, with these two options barred, and given Schrödinger’s searching criticism of the usual response offered by the Copenhagen orthodoxy (measurement = meaning), we are left with a true sense of the bewilderment that characterized Schrödinger’s thought at the time.
The Heisenberg chapter retraces well-trodden territory for the renowned authors, although with a few new twists to reward the renewed effort. The discussion is centred on Heisenberg’s unpublished response to EPR and its historical and philosophical context. This work aimed to demonstrate the incompletability of quantum mechanics, and it was focused on Heisenberg’s famous account of the arbitrariness of the ‘cut’ or divide between the measurement apparatus and a physical system in quantum theory. Through a discussion of the influence of the Kantian thought of Grete Hermann, Bacciagaluppi and Crull provide a clear and insightful discussion of Heisenberg’s understanding of the ‘context of observation’ as the key element responsible for the statistical nature of quantum theory.
Chapter 5’s reconsideration of Bohr’s canonical response to EPR might be the most cogent to date. The summary of Bohr’s view concerning the nature of non-mechanical disturbances – the sense in which the choice of experimental procedure limits the reconstruction of past interactions – is particularly helpful. This limitation constrains the definition of phenomena in the ‘measurement = meaning’ context of the orthodox interpretation. Here, the authors suggest once more that Grete Hermann’s account of observational contexts might hold the key insight, as it is only relative to the measurement context of a given EPR particle that claims concerning its entangled pair make sense in the first place. This seems to be a natural way to read Bohr’s response to EPR, keeping in mind the ‘no-independent-reality’ principle concerning quantum phenomena and measurement arrangements in the Copenhagen interpretation.
In this text, the historical narrative concerning one of the central debates in the history of quantum theory has been greatly clarified. The authors present a remarkable retrospective account of the thought leading up to one of the main turning points in the history of the interpretation of quantum theory and its import for contemporary debate. Yet lingering questions remain about the prospective thought of the central figures at the time.
The leading theorists all agreed that quantum theory was successful but disagreed about the lessons to be drawn from its success. The debate centred on the interpretation of quantum theory, but it really concerned the appropriate methodological framework for the future development of quantum theory. In this new light, we are compelled to reassess the debate’s philosophical import. Why did Einstein feel that the delayed-choice photon box thought experiments required that the uncertainty relations be interpreted epistemologically? Why did he abhor an ontological account of non-separability? Was this belief based on a methodological point concerning the nature of field theory and the future of physical enquiry, or was there a more unconventional philosophical context at play? What led Schrödinger to interrogate the ‘intuitive’ nature of quantum theory? Was this based on a realist sentiment or a commitment to Ludwig Boltzmann’s methodological appeal to models as reference points for the progress of science? What motivated Schrödinger’s stringent commitment to a unified wave theory? How are we to understand the philosophical context of Heisenberg’s unpublished response to EPR? And, more generally, how broadly did philosophical considerations influence methodological views on the future of quantum theory in this period?
