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The history of the Bohr model and the old quantum theory is as much a history of mechanical theorizing as it is a history of radiation theory. With respect to the latter, much scholarship has shown how Bohr, Kramers and others struggled with the “horrid assumption” of quantum transitions and radiation-free states, developed the correspondence principle and sought to establish the tools for describing spectral intensities etc. The situation seems to change with the advent of quantum mechanics. Matrix and wave mechanics are generally discussed as new frameworks for describing quantum mechanical systems, and radiation theoretical questions of the old quantum theory disappear from the narrative of the quantum revolution. This talk takes the opposite approach and explores how quantum mechanics, wave mechanics in particular, continued to struggle with radiation in formative periods of 1925 and 1926. I will discuss Schrödinger’s exploratory work on new “rules that take the place of the correspondence principle”, its surprising benefits and the ensuing discussions with the Göttingen-Copenhagen community on radiation in the new mechanics.

Philosophers and historians of philosophy have extensively discussed the key ideas of Neutral Monism as attempts to overcome the Mind/Body problem. This view, advocated by Ernst Mach and Bertrand Russell, resonates with the concepts developed by Niels Bohr and John C. Slater during the late 1920s in their development of quantum mechanics. The final layer of Bohr’s gradually developed and experimentally motivated complementarity approach to quantum phenomena aimed at offering a satisfactory comprehensive conceptualization of the approach. This provisionally ontological layer strikingly aligns with Russell’s and Mach’s influential (at the time) key ideas on Neutral Monism. It emerged from Bohr's exploration of a loosely related family of double-aspect philosophical accounts, with roots tracing back to Leibniz and Spinoza.

This talk is based on the book project “Reassessing Niels Bohr’s Philosophy: 100 years of Interpreting Quantum Mechanics”, which I am writing together with Rasmus Jaksland. The overall purpose is to show that Bohr was a naturalist, and that much of his thinking is best explained in terms of human evolution. For instance, it has been argued that Bohr’s insistence on the use of classical concepts in understanding quantum mechanics is the least convincing part of his philosophy. However, I intend to show that this is the strongest part of this philosophy if we allow ourselves to read Bohr as a naturalist. Moreover, I intend to present evidence that such a reading is the proper one, and that it supersedes earlier interpretations of Bohr’s thoughts. If we therefore take into consideration that cognitive science and evolutionary biology have developed tremendously up to now by assuming that the human cognitive apparatus is a result of natural selection and adaptation, these considerations indicate that Bohr’s understanding of quantum mechanics, by being supported by this development, is as modern as ever.

Naturalized metaphysics criticizes traditional metaphysics for relying on methods whose origin as cognitive capacities adapted to our predecessors’ environment render them unsuited for purposes of metaphysics. Naturalized metaphysics proposes instead that metaphysics can and should be based on science. The proponents of naturalized metaphysics, however, completely ignore the possibility that also such science-based metaphysics might face limitations due to our cognitive capacities. This talk argues that such limitations were exactly what Bohr grappled with, and Bohr’s work can therefore serve to exemplify what kind of limitations our cognitive capacities might have for even science-based metaphysics.

In this talk, I will report on an ongoing project of understanding the different roles of aesthetics, both in the historical development and in contemporary debates of quantum physics. I aim first to clarify the notion of aesthetics and to show that – far from being merely a question of subjective or personal taste – it is closely related to widely shared beliefs about scientific understanding. I then discuss how aesthetic considerations – in relation to visualization, determinism, and the scope of quantum theory – played an important role in the history of quantum interpretations. Particular attention will be given to the views of Niels Bohr, which were, contrary to some arguments found in the literature, also informed by aesthetics. I end by considering the merits and shortcomings of a recent critical account of the role of aesthetics in physics by Sabine Hossenfelder, and discuss how aesthetics is related to different views of unity, reductionism, and the limits of physics.

We argue that Bohr’s philosophy of complementarity has historically operated as a vehicle for epistemic infringement. Leydon-Hardy defines epistemic infringement as the systematic contravention of the interpersonal social and epistemic norms that an agent takes to constrain their relationship to the infringer in a manner that may encroach upon their epistemic agency. The infringer often appeals to the very norm they are violating to justify their actions, steering their victim into radical self-doubt.
Roughly put, complementarity requires rejecting any visualizable description of quantum objects. In a different history, this rejection might have aligned with a norm that Duhem took to govern the practice of physics: the introduction of novel, symbolic meanings for terms like “place” and “speed” with the ultimate aim of increasing the precision of our commonsense descriptions. In our history, however, Bohr’s complementarity served as a tool to undermine the commonsense understandings of laypersons and specialists alike. We can never know the full extent to which Bohr intended to use complementarity in this way. We argue, however, that regardless of Bohr’s intent, these mechanisms of infringement served to increase his social and political capital while suppressing the uptake of epistemic goods from contributors outside of Copenhagen.

From a historical and biographical perspective, I want to tell the story of why and when Bohr introduced an analogy to psychology into his explanation of the new epistemological situation in quantum mechanics. Bohr’s psychological ideas did not arise in a vacuum; I will therefore briefly sketch the psychological tradition in Denmark with focus on the philosopher Harald Høffding’s contributions to psychology as an important context for the development of Bohr’s views. Høffding had been Bohr’s father’s close friend, and he was Bohr’s philosophy teacher at the university. Despite the age difference, Bohr and Høffding developed a close friendship from Bohr’s student years onwards. Already as a student, Bohr showed a keen interest in psychological questions. Besides psychology, the two discussed the epistemology of modern physics on and off during the 1920s culminating in the period 1927-1929 when Bohr developed the concept of complementarity, as has been shown by Jan Faye. This was also the period where Bohr introduced an analogy to psychology in his writings about the interpretation of quantum mechanics. However, the conversations with Høffding were not the only inspiration for Bohr at the time. He also discussed issues of free will and the psycho-physical parallelism with Pascual Jordan, and in 1932 Bohr (re)discovered William James’ psychology.

Bohr’s call for a clear distinction between subject and object has not been taken seriously since John Bell made it into the butt of a joke (“the shifty split”). And even before Bell tried to laugh Bohr’s views into irrelevance, they had already been completely detached from their roots in nineteenth century psychology and epistemology. The goal of this talk is to re-contextualize Bohr’s philosophical psychology so that it can be evaluated on its merits. I will show that the original context for Bohr’s philosophical psychology was the rejection by Scandinavian philosophers of G.W.F. Hegel’s claim that the subject-object distinction is “aufgehoben”. I trace the development of a dissenting view through Sibbern, Møller, Nielsen, and Høffding, who together form the philosophical backdrop for Bohr’s views about the task of science.

This talk takes up the discussion of explanation across disciplines that Niels Bohr developed in his 1932 address to the International Congress on Light Therapy held in Copenhagen. I aim to illuminate his careful analogies, account of the relations between physics, physiology and biology and discussions of psycho-physical parallelism by considering similarities and distinctions between Bohr’s account and Ernst Mach’s treatment of similar themes seventy years earlier.

The theory of quantum mechanics has often been thought to show an affinity with logical empiricism: in both, observation plays a central role, and questions about what is unobservable are dismissed. However, there were also tensions between logical empiricism and the views of quantum physicists. In the 1920s and 1930s, many physicists thought that quantum mechanics revealed a limit to what could be known scientifically, and this opened the door to a wide range of speculations, in which quantum mechanics was connected with free will, organic life, psychology, and religion. The quantum physicist Pascual Jordan presented his philosophical views as in line with those of logical empiricism, while at the same time engaging in speculative thought which members of the Vienna Circle, such as Philipp Frank and Moritz Schlick, found unacceptable. Frank and Schlick attempted to develop a common position with Niels Bohr about the philosophical implications of quantum mechanics, but this too turned out to be not without difficulties.

I flesh out the sense in which the informational approach to interpreting quantum mechanics, as defended by Pitowsky and Bub and lately by a number of other authors, is (neo-)Bohrian. I argue that on this approach, quantum mechanics represents what Bohr called a “natural generalisation of the ordinary causal description” in the sense that the idea (which philosophers of science like Stein have argued for on the grounds of practical and epistemic necessity) that understanding a theory as a theory of physics requires that one be able to “schematise the observer” within it, is elevated in quantum mechanics to the level of a postulate in the sense that interpreting the outcome of a measurement interaction as providing us with information about the world, requires as a matter of principle the specification of a schematic representation of an observer in the form of a “Boolean frame” – the Boolean algebra representing the yes-or-no questions associated with a given observable representative of a given experimental context. I argue that the approach’s central concern is with the methodological question of how to assign physical properties to what one takes to be a system in a given experimental context, rather than the metaphysical question of what a given state vector represents independently of any context, and I show how the quantum generalisation of the concept of an open system may be used to assuage Einstein’s complaint that the orthodox approach to quantum mechanics runs afoul of the supposedly fundamental methodological requirement to the effect that one must always be able, according to Einstein, to treat spatially separated systems as isolated from one another.