The famous EPR article of 1935 challenged the completeness of quantum mechanics and spurred decades of theoretical and experimental research into the foundations of quantum theory. A crowning achievement of this research is the demonstration that nature cannot in general consist in noncontextual pre-measurement properties that uniquely determine possible measurement outcomes, through experimental violations of Bell inequalities and Kochen-Specker theorems. In this article, I reconstruct an argument from Niels Bohr’s writings that the reality of the Einstein-Planck-de Broglie relations alone implies (...) that no such properties can exist for momentum and position measurements, show how this argument responds to the challenge of EPR on general physical grounds, and advance that this reconstruction shows that and how Bohr’s “complementarity” is a view of the objective content and logic of quantum theory. (shrink)

Niels Bohr famously argued that a consistent understanding of quantum mechanics requires a new epistemic framework, which he named complementarity . This position asserts that even in the context of quantum theory, classical concepts must be used to understand and communicate measurement results. The apparent conflict between certain classical descriptions is avoided by recognizing that their application now crucially depends on the measurement context. ;Recently it has been argued that a new form of complementarity can provide a solution to the (...) so-called information loss paradox. Stephen Hawking argues that the evolution of black holes cannot be described by standard unitary quantum evolution, because such evolution always preserves information, while the evaporation of a black hole will imply that any information that fell into it is irrevocably lost---hence a "paradox." Some researchers in quantum gravity have argued that this paradox can be resolved if one interprets certain seemingly incompatible descriptions of events around black holes as instead being complementary. ;In this dissertation I assess the extent to which this black hole complementarity can be undergirded by Bohr's account of the limitations of classical concepts. I begin by offering an interpretation of Bohr's complementarity and the role that it plays in his philosophy of quantum theory. After clarifying the nature of classical concepts, I offer an account of the limitations these concepts face, and argue that Bohr's appeal to disturbance is best understood as referring to these conceptual limits. ;Following preparatory chapters on issues in quantum field theory and black hole mechanics, I offer an analysis of the information loss paradox and various responses to it. I consider the three most prominent accounts of black hole complementarity and argue that they fail to offer sufficient justification for the proposed incompatibility between descriptions. ;The lesson that emerges from this dissertation is that we have as much to learn from the limitations facing our scientific descriptions as we do from the successes they enjoy. Because all of our scientific theories offer at best limited, effective accounts of the world, an important part of our interpretive efforts will be assessing the borders of these domains of description. (shrink)

This book presents a collection of novel contributions and reviews by renowned researchers in the foundations of quantum physics, quantum optics, and neutron physics. It is published in honor of Michael Horne, whose exceptionally clear and groundbreaking work in the foundations of quantum mechanics and interferometry, both of photons and of neutrons, has provided penetrating insight into the implications of modern physics for our understanding of the physical world. He is perhaps best known for the Clauser-Horne-Shimony-Holt (CHSH) inequality. This collection (...) includes an oral history of Michael Horne's contributions to the foundations of physics and his connections to other eminent figures in the history of the subject, among them Clifford Shull and Abner Shimony. Among the editors and contributors are John Clauser and Anton Zeilinger, recipients of the Nobel Prize in Physics 2022. (shrink)

In this paper I present and critically discuss the main strategies that Bohr used and could have used to fend off the charge that his interpretation does not provide a clear-cut distinction between the classical and the quantum domain. In particular, in the first part of the paper I reassess the main arguments used by Bohr to advocate the indispensability of a classical framework to refer to quantum phenomena. In this respect, by using a distinction coming from an apparently unrelated (...) philosophical corner, we could say that Bohr is not a revisionist philosopher of physics but rather a descriptivist one in the sense of Strawson. I will then go on discussing the nature of the holistic link between classical measurement apparatuses and observed system that he also advocated. The oft-repeated conclusion that Bohr’s interpretation of the quantum formalism is untenable can only be established by giving his arguments as much force as possible, which is what I will try to do in the following by remaining as faithful as possible to his published work. (shrink)

Einstein's philosophy of physics was predicated on his Trennungsprinzip, a combination of separability and locality, without which he believed objectification, and thereby "physical thought" and "physical laws", to be impossible. Bohr's philosophy, on the other hand, was grounded in a seemingly different doctrine about the possibility of objective knowledge, namely the necessity of classical concepts. In fact, it follows from Raggio's Theorem in algebraic quantum theory that - within an appropriate class of physical theories - suitable mathematical translations of the (...) doctrines of Bohr and Einstein are equivalent. Thus - upon our specific formalization - quantum mechanics accommodates Einstein's Trennungsprinzip if and only if it is interpreted a la Bohr through classical physics. Unfortunately, the protagonists themselves failed to discuss their differences in this constructive way, since their debate was dominated by Einstein's ingenious but ultimately flawed attempts to establish the "incompleteness" of quantum mechanics. This aspect of their debate may still be understood and appreciated, however, as reflecting a much deeper and insurmountable disagreement between Bohr and Einstein about the knowability of Nature. Using the theological controversy on the knowability of God as a analogy, we can say that Einstein was a Spinozist, whereas Bohr could be said to be on the side of Maimonides. Thus Einstein's off-the-cuff characterization of Bohr as a 'Talmudic philosopher' was spot-on. (shrink)

Recent years saw the rise of an interest in the roles and significance of thought experiments in different areas of human thinking. Heisenberg's gamma ray microscope is no doubt one of the most famous examples of a thought experiment in physics. Nevertheless, this particular thought experiment has not received much detailed attention in the philosophical literature on thought experiments up to date, maybe because of its often claimed inadequacies. In this paper, I try to do two things: to provide an (...) interesting interpretation of the roles played by Heisenberg's gamma ray microscope in interpreting quantum mechanics – partly based on Thomas Kuhn’s views on the function of thought experiments – and to contribute to the ongoing discussions on the roles and significance of thought experiments in physics. (shrink)

Einstein's philosophy of physics (as clarified by Fine, Howard, and Held) was predicated on his Trennungsprinzip, a combination of separability and locality, without which he believed objectification, and thereby "physical thought" and "physical laws", to be impossible. Bohr's philosophy (as elucidated by Hooker, Scheibe, Folse, Howard, Held, and others), on the other hand, was grounded in a seemingly different doctrine about the possibility of objective knowledge, namely the necessity of classical concepts. In fact, it follows from Raggio's Theorem in algebraic (...) quantum theory that - within an appropriate class of physical theories - suitable mathematical translations of the doctrines of Bohr and Einstein are equivalent. Thus - upon our specific formalization - quantum mechanics accommodates Einstein's Trennungsprinzip if and only if it is interpreted a la Bohr through classical physics. Unfortunately, the protagonists themselves failed to discuss their differences in this constructive way, since their debate was dominated by Einstein's ingenious but ultimately flawed attempts to establish the "incompleteness" of quantum mechanics. This aspect of their debate may still be understood and appreciated, however, as reflecting a much deeper and insurmountable disagreement between Bohr and Einstein about the knowability of Nature. Using the theological controversy on the knowability of God as a analogy, we can say that Einstein was a Spinozist, whereas Bohr could be said to be on the side of Maimonides. Thus Einstein's off-the-cuff characterization of Bohr as a 'Talmudic philosopher' was spot-on. (shrink)

It is well known that Niels Bohr insisted on the necessity of classical concepts in the account of quantum phenomena. But there is little consensus concerning his reasons, and what he exactly meant by this. In this paper, I re-examine Bohr’s interpretation of quantum mechanics, and argue that the necessity of the classical can be seen as part of his response to the measurement problem. More generally, I attempt to clarify Bohr’s view on the classical/quantum divide, arguing that the relation (...) between the two theories is that of mutual dependence. An important element in this clarification consists in distinguishing Bohr’s idea of the wave function as symbolic from both a purely epistemic and an ontological interpretation. Together with new evidence concerning Bohr’s conception of the wave function collapse, this sets his interpretation apart from both standard versions of the Copenhagen interpretation, and from some of the reconstructions of his view found in the literature. I conclude with a few remarks on how Bohr’s ideas make much sense also when modern developments in quantum gravity and early universe cosmology are taken into account. (shrink)

Although Bohr's reply to the EPR argument is supposed to be a watershed moment in the development of his philosophy of quantum theory, it is difficult to find a clear statement of the reply's philosophical point. Moreover, some have claimed that the point is simply that Bohr is a radical positivist. In this paper, we show that such claims are unfounded. In particular, we give a mathematically rigorous reconstruction of Bohr's reply to the _original_ EPR argument that clarifies its logical (...) structure, and which shows that it does not rest on questionable philosophical assumptions. Rather, Bohr's reply is dictated by his commitment to provide "classical" and "objective" descriptions of experimental phenomena. (shrink)

Quantum mechanics is an area of Physics that deals with subatomic phenomena. It can be extracted from a vision of the physical world which contradicts many aspects of our everyday perception, prompting many philosophical debates and admitting different interpretations. Among the wide range of problems within the interpretation of quantum theory, there is the measurement problem. Some philosophical aspects of the problems concerning the notion of “measurement” in quantum mechanics are analyzed in order to identify how the problem arises in (...) discussions about the foundations of the interpretation of quantum theory and how it holds up even in the most recent interpretations, conflicting in various ways (including ontological) with a worldview that can be drawn from classical physics – assuming that it provides a worldview. Although Bohr has, on several occasions, addressed the measurement problem, he did not get to properly formulate a measurement theory. This was done by von Neumann, who presented an axiomatic formulation of a measurement theory along with a critique of the Bohrian model and, at the same time, an alternative grounded in the introduction of a dualistic notion of consciousness (nonphysical) with causal power in quantum measurement. It is precisely the introduction of the dualistic concept of consciousness within the scope of the concept of measurement that inserts into the philosophical discussion as an ontological problem insofar as it comes to the introduction of a new entity in the universe. The first subjetivistic interpretations, proposed by London and Bauer, are analysed, passing through the solipsistic difficulty to this subjectivist interpretations placed through Wigner’s work, up to the interpretations inspired by the late work of the physicist Erwin Schrödinger, which proposes a monistic interpretation of the notion of “consciousness”. Some distinct attitudes towards measuring the philosophical problems are outlined very briefly in the form of sampling, to illustrate the plural character of the proposals for interpretation of the term “measurement” in quantum mechanics. Although strictly speaking there is no “best” interpretation of quantum mechanics, it is suggested that the formulation of an ontology that takes into account quantum mechanics could help in understanding certain concepts, such as “measurement” or “consciousness” without that philosophical difficulties – as dualism – or paradoxical situations – such as solipsism – necessarily accompany them. (shrink)

If a specific question has meaning, it must be possible to find operations by which an answer may be given to it. It will be found in many cases that the operations cannot exist, and the question therefore has no meaning. —Bridgman, The Logic of Modern Physics..

It is argued that Niels Bohr ultimately arrived at positivistic and antirealist-flavored statements because of weaknesses in his initial objective of accounting for measurement in physical terms. Bohr’s investigative approach faced a dilemma, the choices being conceptual inconsistency or taking the classical realm as primitive. In either case, Bohr’s ‘Complementarity’ does not adequately explain or account for the emergence of a macroscopic, classical domain from a microscopic domain described by quantum mechanics. A diagnosis of the basic problem is offered, and (...) an alternative way forward is indicated. (shrink)