This book, like the earlier compilation of translations and critical assessments of the proceedings of the 1927 Solvay conference by Bacciagaluppi and Valentini ([2009]), is at once an invaluable resource as well as a scientifically and philosophically acute scrutiny of a defining ‘moment’ in quantum foundations. References to the Einstein–Podolsky–Rosen (EPR) ‘paradox’ have largely vanished from the literature in recent decades, but the book’s title appears to have been taken from a folder of correspondence, found in Schrödinger’s Nachlaß in Alpbach, from the summer and autumn of 1935 and entitled ‘Korrespondenz betreffend das Einstein-Paradoxon’ (p. xvi). As it turns out, Schrödinger in fact considered EPR a genuine paradox.

The book is partitioned under five major headings. Part 1 is the longest (145 pp.) and the philosophically richest part of the book. It contains Bacciagaluppi and Crull’s detailed consideration of anticipations of the central themes of the EPR paradox (incompleteness, hidden variables, non-separability, entanglement) in the writings of Einstein, Schrödinger, Heisenberg, and Bohr, as well as extensive discussion of the substantive responses of these thinkers to EPR.

Part 2 comprises both familiar and unfamiliar papers, including a translation of the short report of Einstein’s Berlin University colloquium in late 1931, published in 1932 in the journal Zeitschrift für Angewandte Chemie, and another unpublished paper by Schrödinger, both concerning Einstein’s ‘photon-box’ Gedankenexperiment. These two notes are ‘introduced’ in part 1 by a thorough discussion of the thought experiment, from its origin and subsequent development in the evolution of Einstein’s thinking. Since Jammer’s ([1974], pp. 170–72) discussion, it is known that, contra to Bohr’s retrospective 1949 account, by 1931 Einstein no longer viewed the photon-box experiment as a frontal attack on the Heisenberg uncertainty relations but as prototype of an argument for quantum-mechanical incompleteness.

Part 3 features the EPR paper itself, Bohr’s reply in Physical Review, as well as Schrödinger’s key paper, ‘Discussion of Probability Relations between Separated Systems’, and an amended translation of the latter’s well-known three-part essay ‘Die gegenwärtige Situation in der Quantenmechanik’. Lesser-known papers are also included: two by Wendell Furry, a translation of Heisenberg’s unpublished response to EPR (‘Is a Deterministic Completion of Quantum Mechanics Possible?’), and a translation of Grete Hermann’s essay, ‘The Natural-Philosophical Foundations of Quantum Mechanics’.

Part 4 consists of period papers and letters previously discussed in Jammer’s ([1974]) comprehensive treatment of early reactions to the EPR argument, from Kemble’s ([1935]) letter to Physical Review to that of Hugh C. Wolfe ([1936]).

Part 5 consists of letters regarding EPR sent between Schrödinger, Einstein, Bohr, Teller, Pauli, Heisenberg, von Laue, and Born. An additional treat are letters between Schrödinger and Arnold Berliner, editor of Die Naturwissenschaften, the latter humorously suggesting reference to Einstein as a Manzoni character (L’) Innominato (The Unnamed), ‘one who must not be named’ within the Third Reich. Despite sharing Schrödinger’s loathing of the Nazi regime, Berliner was able to persuade Schrödinger to publish ‘Die gegenwärtige Situation in der Quantenmechanik’ in his journal.

Gathering this comprehensive collection of original material into a single volume is a tremendously valuable stimulus for future scholarship. To cite but one example, reading Furry’s astute appraisals of the EPR argument occasions surprise at how ably he articulated and thus made more intelligible the physical details behinds Bohr’s overarching philosophical claim made both at the time and in many of his later writings (that is, ‘the difficulty, often remarked upon by Bohr, which is inherent in the distinction between subject and object’).

Within the space of this review I will comment only on three issues, all treated in part 1. There are many treasures in this book and I shall point out two of them. Chapter 3 of part 1 is the best discussion I know of the development of Schrödinger’s thinking about EPR-type scenarios ‘about which’, he wrote to Einstein on 7 June 1935, ‘we had talked so much already in Berlin’ (p. 281). Bacciagaluppi and Crull document why Schrödinger considered EPR truly paradoxical: he viewed the argument as forcing a choice between violation of a separability principle and the impossibility of defining values for observables in terms of functional relations, which rendered inexplicable the fact that matching pairs of observables have matching values when measured.

Chapter 5 of part 1 is also remarkable in arguing, through a detour into Pauli’s ([1933]) Handbuch treatment of general quantum measurements, that the single- and double-slit examples in Bohr’s reply to EPR are ‘strictly analogous’, as Bohr indeed claimed. In addition, an extensive discussion probes Bohr’s inchoate idea of ‘non-mechanical disturbance’ via different reconstructions, the first, admittedly anachronistically, in terms of Everett’s ‘relative states’ and then, more illuminatingly, following Howard’s ([1994]) analysis of Bohr’s doctrine of classical concepts.

My last remark, of a more critical nature, concerns claims of ‘the crucial role of Grete Hermann for Heisenberg’s thinking about separability, completeness, and observational context’ (p. xvii), and even of her possible influence on Bohr’s response to EPR.^[1] Hermann’s influences are purportedly readily present in Heisenberg’s unpublished reply to EPR, mentioned above, and this influence is discussed in various subsections 4 and 5 of part 1. While the evidential details are too intricate to be fully considered here, the following facts are not in dispute.

Hermann studied mathematics and philosophy in Göttingen, obtaining her doctorate on polynomial ideals under Emmy Noether and Edmund Landau in 1926. Influenced by the Friesian neo-Kantian Leonard Nelson while in Göttiingen, she sought to excise the indeterministic aspect from quantum mechanics in accord with her goal of ‘reconciling a neo-Kantian law of causality with quantum mechanics’ (Bacciagaluppi and Crull, p. 110). In 1933 she wrote an essay arguing for a deterministic completion of quantum mechanics (Hermann [1935a]), sending the paper to Heisenberg and von Weizsäcker. Both responded ‘with nearly identical criticisms’ (Cassidy [2025], p. 40) and so she joined Heisenberg’s Leipzigerkreis for the 1934–35 winter semester, to learn more of quantum mechanics and, presumably, to confront Heisenberg’s assertions against causality, for example, ‘that the classical formulation of the law of causality has proved itself not to be incorrect, but only to be empty; it no longer has validity or domain of application (Anwendungsbereich), and for this reason, is of no interest to the physicist’ (Heisenberg [1931], p. 175). As Bacciagaluppi and Crull note (p. 111), after discussions in Leipzig, Hermann gave up her earlier hope, expressed in the 1933 essay. She subsequently argued in two 1935 papers that retroductive causation sufficed to forge agreement between causation and quantum mechanics, preserving causal completeness but severing the link between causality and prediction.^[2] Any difficulties in reconciling the causality principle with quantum mechanics, she claimed, are due not to the causality principle but ‘to the tacitly added assumption [from classical physics] that physical knowledge accounts for natural events adequately and independently of observational context’ (Bacciagaluppi and Crull, p. 242).

This was the principal message of two 1935 papers, perhaps written during and then after her visit to Leipzig. The first, a lengthy article entitled ‘Die naturphilosophischen Grundlagen der Quantenmechanik’ (Hermann [1935a]) originally published in an obscure journal (Abhandlungen der Fries’schen Schule) and later appearing as a monograph, contains important critiques of von Neumann’s impossibility proof for hidden variables.^[3] There is also an extended exposition of Carl Friedrich von Weizsäcker’s ([1931]) ‘Ortbestimmung eines Elektron durch ein Mikroskop’. Von Weizsäcker, who became Heisenberg’s assistant in Leipzig in the summer of 1934 after spending a half-year at Bohr’s institute in Copenhagen (p. 38, note 6), had published this paper as a nineteen-year old. As a thought experiment, von Weizsäcker argued that the same apparatus (Heisenberg’s 𝛾-ray microscope) might be used to determine either the position or the momentum of an electron (in Compton scattering), depending on whether a photographic plate (the relevant observational context) was placed in the image plane of the electron or in the focal plane of the lens. Characterized by Bacciagaluppi and Crull merely as ‘illustrating the uncertainty relations’ (p. 112), the paper certainly contains the idea of an experimenter’s free choice regarding placement of the photographic plate, and so is a precursor of Wheeler’s delayed choice experiments. A much shorter article under the same title (Hermann [1935b]), without the critique of von Neumann but with a brief discussion of the von Weizsäcker paper, to illustrate ‘the relative character of the quantum mechanical mode of description’, appeared in the 18 October 1935 issue of Die Naturwissenschaften. It is this shorter version that appears in translation in this volume. There are reasons to think that the von Weizsäcker paper played a somewhat larger role in Hermann’s thinking about quantum mechanics than the shorter paper alone would indicate.^[4] In any case, it would have been beneficial to have included the earlier, longer 1935 paper in the volume, since claims of Hermann’s influence on Heisenberg do not seem to me to be substantiated in the shorter text.

In the longer paper, Hermann’s ([1935a]) argument, based on detailed analysis of von Weizsäcker’s ([1931]) paper, is that quantum theory in its present form is already causally complete and therefore not in need of supplementation by hidden variables. This is because after the observer knows the outcome of the measurement with the photon, it is possible ‘with sufficient reason’ to reconstruct the events leading to the measured outcome. So while it is impossible to predict at which point the light quantum will darken the photographic plate, it is possible to infer precisely from the darkened spot on the photographic plate (say, in the focal plane of the lens) ‘in this exchange of momentum the cause for the light quantum being found exactly at this point on the plate’ (Hermann [2016], p. 259).

It is this argument for quantum-mechanical completeness that is alleged to have influenced Heisenberg. Bacciagaluppi and Crull point in particular to the passage in Heisenberg’s draft: ‘The causal flow can be followed only within a given observational context; in the discontinuous transition from one observational context to another (in fact, a ‘complementary’ one in the Bohrian sense)’ (p. 109; citing Heisenberg [1931], p. 233). In fact, the authors themselves do not insist on this point but tell us that this sentence is ‘perhaps a nod in the direction of the strategy preferred by his mentor, Bohr, in defending quantum mechanics’ (p. 109). In any case, influences cannot be traced from the shorter paper whose treatment of von Weizsäcker is greatly compressed, and the lesson drawn there regarding causal completeness is rather muted:

Even for unpredictable events, quantum mechanics gives a causal explanation and verifies this through predictions. But this verification comes about indirectly. One infers backwards from the unpredictable events to their cause, and from the assumption that this cause was present, to predictions of forthcoming events whose occurrence can be verified empirically […] That this new possibility of only indirect verification of causal claims was not appreciated by classical physics, is grounded in the fact that the relative trait of the quantum mechanical description of nature is foreign to classical physics. For it [classical physics], the characterization of any system is unique and independent of the manner in which the observer procures knowledge of it. (p. 242)

In sum, the shorter paper does not, in this reviewer’s opinion, support a definitive assertion of influence.

Claims of influence of X on Y are often hard to establish. Some years ago, Beller ([1999], p. 7), in her historical examination of the ‘dialogical’ origins of the Copenhagen interpretation, argued that attempts to establish such claims often overlook what in retrospect might appear obvious, that scientific ideas emerge in ‘dialogical’ contexts where ‘a creative scientist […] is linked fundamentally to the efforts and concerns of others’. Jammer ([1974], p. 208) nonetheless credited Hermann with being the first to recognize ‘the relational, or […] “relative” character of the quantum mechanical description, which she regarded as the “decisive achievement of this remarkable theory”’. Yet neither Jammer nor Bacciagaluppi and Crull mention Heisenberg’s retrospective account of discussions with Hermann and von Weizsäcker in Leipzig. Loosely set as a dialogue between the three, an entire chapter entitled ‘Quantenmechanik und Kantsche Philosophie’ in (Heisenberg [1969]) makes it abundantly clear that it was von Weizsäcker who convinced Hermann that measurement outcomes depend on the context of experiment, contrary to the Kantian orthodoxy to which Hermann at the time seemed wedded: ‘Every perception refers to an observational situation that must be specified if experience is to result from perception’.^[5] While more loosely expressed in von Weizsäcker’s own Kantian jargon, this would seem to be Bohr’s idea that laboratory practice is where ‘physical reality’ is to be found, in the carefully controlled contact of microphysical entities and macroscopic apparatus, the necessary context for definition of physical quantities attributable to microentities.

Whether or not one can unravel the murky trail of who influenced whom in 1935, the authors have given the foundations community a work of extraordinary value.

Thomas Ryckman
Stanford University
tryckman@stanford.edu

Notes

^[1] ‘One may legitimately ask whether Hermann’s analysis of the 𝛾-ray microscope and her argument for the completeness of quantum mechanics had an influence also on Bohr’s reply to EPR’ (p. 113, note 19).

^[2] Hermann’s argument is paraphrased by the authors: ‘although the ability to predict the outcomes of measurements on quantum systems is certainly lost, this does not entail the nonexistence of the requisite chain of cause–effect relations. Such a chain, stretching continuously from initial conditions to pointer reading, can indeed be identified—but only after measurement—by inferring backwards the evolution of the system from the measurement outcome to the initial state’ (p. 111).

^[3] Hermann argued that von Neumann’s proof was circular, pointing out that von Neumann’s linearity assumption, while valid for quantum mechanical expectation values, could not be assumed to hold for dispersion-free states in which expectation values and eigenvalues are identical. However, von Neumann was well aware that addition of non-commuting observables is not physically definable, it was not ‘im Rahmen unserer Bedingungen’ (within the domain of our assumptions). See (Landsmann [2017], p. 342).

^[4] As Bacciagaluppi and Crull then note, the shorter paper omits discussion of an interesting third possibility discussed in the longer paper (no photographic plate at all). Hence after scattering, a joint wave-function describes a superposed state independent of observational context, a linear combination jointly of electron and photon only in relation to one another; that is, entangled. Hence the electron does not have a definite state until a basis is chosen for the description of the photon. The authors recognize this to be an anticipation of ‘the very notion of nonseparability undergirding Einstein’s troubles’ (p. 113). See also (Filk [2016]).

^[5] ‘Jede Wahrnemung bezieht sich auf eine Beobactungssituation, die angegeben werden muß, wenn aus Wahrnehmung auch Erfahrung folgen soll.’

References

Bacciagaluppi, G. and Valentini, A. [2009]: Quantum Theory at the Crossroads: Reconsidering the 1927 Solvay Conference, Cambridge: Cambridge University Press.

Beller, M. [1999]: Quantum Dialogue: The Making of a Revolution, Chicago, IL: University of Chicago.

Cassidy, D. [2025]: ‘Werner Heisenberg and Carl Friedrich Freiherr von Weizsäcker: A Fifty-Year Friendship’, Physics in Perspective, 17, pp. 33–54.

Filk, T. [2016]: ‘Carl Friedrich von Weizsäcker’s “Ortsbestinnung eines Elektrons” and Its Influence on Grete Hermann’, in E. Crull and G. Bacciagaluppi (eds), Grete Hermann: Between Physics and Philosophy, Dordrecht: Springer, pp. 71–83.

Heisenberg, W. [1931]: ‘Kausalgesetz und Quantenmechanik’, Erkenntnis, 2, pp. 172–82.

Heisenberg, W. [1969]: Der Teil und das Ganze, Munich: Piper.

Hermann, G. [1935a]: ‘Die naturphilosophischen Grundlagen der Quantenmechanik’, Abhandlungen der Fries’schen Schule, 6., pp. 75–152.

Hermann, G. [1935b]: ‘Die naturphilosophischen Grundlagen der Quantenmechanik’, Naturwissenschaften, 23, pp. 718–21.

Hermann, G. [2016]: ‘Natural-Philosophical Foundations of Quantum Mechanics’ in E. Crull and G. Bacciagaluppi (eds), Grete Hermann: Between Physics and Philosophy, Dordrecht: Springer, pp. 239–78.

Howard, D. [1994]: ‘What Makes a Classical Concept Classical? Towards a Reconstruction of Niels Bohr’s Philosophy of Physics’, in J. Faye and H. J. Folse (eds), Niels Bohr and Contemporary Philosophy, Dordrecht: Kluwer, pp. 201–30.

Jammer, M. [1974]: The Philosophy of Quantum Mechanics: The Interpretation of Quantum Mechanics in Historical Perspective, New York: John Wiley, pp. 170–72.

Kemble, E. C. [1935]: ‘The Correlation of Wave Functions with the States of Physical Systems’, Physical Review, 47, p. 973.

Landsmann, K. [2017]: ‘Bohrification: From Classical Concepts to Commutative Operator Algebras’, in J. Faye and H. J. Folse (eds), Neils Bohr and the Philosophy of Physics: Twenty-First-Century Perspectives, London: Bloomsbury, pp. 335–66.

Pauli, W. [1933]: ‘Die Allgemeinen Prinzipien der Wellenmechanik’, Handbuch der Physik I, Berlin: Springer.

Wolfe, H. C. [1936]: ‘Quantum Mechanics and Physical Reality’, Physical Review, 49, p. 274.

von Weizsäcker, C. F.  [1931]: ‘Ortbestimmung eines Elektron durch ein Mikroskop’, Zeitschrift für Physik, 70, pp. 114–30.