Talks

The Heisenberg microscope provides a powerful mental image of the measurement process of quantum mechanics (QM), attempting to explain the uncertainty relation through an uncontrollable back-action from the measurement device. However, Heisenberg's proposed back-action uses features that are not present in the QM description of the world, and according to Bohr not present in the world. Therefore, Bohr argues, the mental image proposed by Heisenberg should be avoided. Later developments by Bell and Kochen-Specker shows that a model that contains the features used for the Heisenberg microscope is in principle possible but must necessarily be nonlocal and contextual. In this paper we will re-examine the measurement process within a restriction of QM known as Stabilizer QM, that still exhibits for example Greenberger-Horne-Zeilinger nonlocality and Peres-Mermin contextuality. The re-examination will use a recent extension of stabilizer QM, the Contextual Ontological Model (COM), where the system state gives a complete description of future measurement outcomes reproducing the quantum predictions, including the mentioned phenomena. We will see that the resulting contextual Heisenberg microscope back-action can be completely described within COM, and that the associated randomness originates in the initial state of the pointer system, exactly as in the original description of the Heisenberg microscope. The presence of contextuality, usually seen as prohibiting ontological models, suggests that the contextual Heisenberg microscope picture could perhaps be enabled in general QM.

On March 27th, 1927, Heisenberg published the paper in which he introduced the uncertainty relations through his well-known $\gamma$-ray microscope thought experiment. Since then, the debates and commentaries over the origin of the uncertainty relations have continued for a century (see, for instance, Bacciagaluppi and Valentini 2009; Beller 1999; Brown and Redhead 1981; Busch 1990; Hilgevoord and Uffink 1985,2024; Jammer 1974; Uffink 1990). Going back to 1927 through a selection of letters -- including an unpublished letter from Jordan to Rosenfeld -- this talk aims to add a crucial piece to this century-old puzzle by shedding new light on the differences between Heisenberg and Bohr's conceptual basis of the uncertainty relations. According to the received view, Bohr derives the uncertainty relations from the \textit{wave-particle duality} of light. By contrast, it is commonly stated that Heisenberg based them on the \textit{discontinuity} in the interaction between the electron and the light quantum, i.e., the uncontrollable change resulting from the Compton effect (cf. Camilleri 2009). I will challenge the received view by arguing that: (i) Bohr’s derivation of the uncertainty relations fundamentally relies on the use of \textit{classical concepts}, consistent with their central role in his broader interpretation of quantum mechanics. (ii) The supposed contrast rooted in the wave-particle duality of light may stem from Heisenberg’s 1927 misrepresentation (or misinterpretation) of Bohr's ideas on the $\gamma$-ray microscope. This misrepresentation may have influenced the reception of Bohr’s interpretation, particularly regarding his ideas on the conceptual justification for the uncertainty relations, and contributed to obscuring the deeper significance of Bohr's complementarity principle - fully comprehensible only in light of (i).

We articulate a collection of desiderata for an account of the dynamical quantities of a physical theory, and we present a theory that meets these desiderata in the case of quantum mechanics. Our theory retains a distinction between the values of dynamical quantities and the truth values of sentences asserting that a system has a particular value of a particular quantity. This allows our theory to incorporate the phenomenon of quantum indeterminacy as a pattern in the properties instantiated by a system in a particular quantum state, while also retaining the semantics of classical logic. We contrast our theory with quantum logic, which flattens the distinction between quantity values and truth values. We discuss the historical origin of the assumptions leading to this feature of quantum logic, and we address a series of objections that have been raised to previous attempts to reconcile quantum indeterminacy with classical logic.

Since its inception, the quantum theory has been fraught with interpretive challenges. It has often been complained that the physical content of the theory is obscured by the mathematical resources used to articulate it. In recent decades, many have sought to reorganize the conceptual resources of quantum theory in terms of operational principles with clear physical meaning. These quantum reconstructions have been so successful that some people take them as evidence that the rightful subject matter of quantum theory—what the theory is about—is not the dynamical evolution of material systems at all, but rather the operational capacities of agents. Here, I argue against this agentive turn. Specifically, I show that such an account is not recommended by the success of quantum reconstruction and is, in fact, incompatible with it. I show how the operational language of quantum reconstruction principles can be given a clear, non-agentive interpretation. I further explain the practical and heuristic value of employing this operational language in facilitating the aims of the quantum reconstruction program.

Wernher von Braun is supposed to have said, "Research is what I'm doing when I don't know what I'm doing." The path to a new idea is seldom straight, and travelers seldom know what their actual destination will be. I intend to illustrate all this by describing the origin of the word "qubit" and the concept of quantum data compression in the early days of quantum information theory.

In this talk, I trace one particular, personal thread in the history of quantum foundations and quantum information. It starts with a course taught by John Wheeler in the academic year 1977-1978 and ends with the 1992 discovery of the theoretical possibility of quantum teleportation. The story depends crucially on two insightful questions, posed by Asher Peres and Charles Bennett.

At the 1981 Physics of Computation Conference, held at MIT’s Endicott House, **John A. Wheeler** presented a paper entitled **“The Computer and the Universe”** where he put forward the idea of one day understanding ‘physics as information’ (Wheeler 1982). By the early 1980s, it was already well established that information is inseparably tied to physical degrees of freedom and therefore subject to physical law. Yet, several conference presentations advanced the counterpart idea: that physical laws themselves can be viewed as algorithms for information processing, and that their ultimate form must respect the limits of physically executable computation (see Landauer 1982; Fredkin 1982; Kantor 1982; Zuse 1982). After a distinguished career at Princeton, Wheeler joined the University of Texas at Austin in 1976, where he founded the Center for Theoretical Physics. His time there offers a perspective on how the informational turn, highlighted at the 1981 conference, influenced the trajectory of quantum mechanics research in the latter half of the twentieth century. During his time at Texas, Wheeler became intensely focused on the question, **“How come the quantum?”** He sought a deeper principle from which the quantum formalism could be derived. In this search, quantum theory became increasingly intertwined with computing and information theory. As director, he assembled a diverse group of graduate students, postdoctoral fellows, and visiting researchers, creating a dynamic research environment to help tackle this question. The group engaged deeply with emerging developments in computer science—reading widely on parallel computing, Turing machines, cellular automata, and cybernetics—and often held informal seminars where these topics converged with foundational issues in physics, particularly debates on the many-worlds interpretation (Everett, 1957). For Wheeler, the informational turn culminated in his proposal of **“it from bit,”** the idea that every physical entity—every “it”—derives its meaning from fundamental units of information, or “bits” (Wheeler, 1989). For many of the researchers who passed through Austin, this perspective translated into concrete efforts that laid the conceptual and technical groundwork for what would later be recognized as quantum information theory, quantum computation, and novel approaches to the foundations of quantum mechanics. This influence is particularly evident in the work of **David Deutsch** (quantum Turing machines), **William Wootters** (quantum distinguishability and teleportation), **Wojciech Zurek** (decoherence), and **Benjamin Schumacher** (quantum coding), with Wootters and Schumacher together introducing the concept of the qubit. The decade Wheeler spent at Austin provides a fresh perspective on the expanding role of computation and information theory in shaping modern physics in the late twentieth century. Computers were not merely pragmatic tools; they became conceptual models and heuristic devices that that helped steer the direction of research in quantum physics.

Once Heisenberg unlocked the Bohr frequency condition and the canonical commutation relation came out, Quantum Mechanics hit the ground running. In the rush of it all, it’s not clear to me who knew then (and who knows now) that the non-commutativity of phase space displacement is in fact an idea that has been around for at least as long as Jacobi, the father of canonical transformation theory. First conceived of in 1818 and then patented in 1854, the Amsler planimeter is a measuring instrument, known to even Maxwell, that in fact operates on exactly the same commutation relation, hiding in plain sight. Meanwhile, Lie's 1880 theory of transformation groups was also founded on exactly the same structure, what he called the contact element. Who knew? Why aren’t more physicists aware of this? Join me as we explore Poincare’s sudden death, the origins of the Gruppenpest, and Hilbert’s declining health when physicists like Wigner needed him more than ever.

Heisenberg’s 1925 Matrizenmechanik (matrix mechanics) can be seen as a radical paradigm shift. While established scientists such as Bohr and Sommerfeld worked within the paradigm that electrons must be in a definite position at a definite time, the young Heisenberg abandoned these notions for electrons inside the atom. He wrote: > *“… in this situation it seems more advisable to give up completely any > hope of an observation of hitherto unobservable quantities (such as > position, orbital time of the electron) …”* > *“… it was not possible to assign a point in space (as a function of > time) to an electron by means of observable quantities …”* (W. Heisenberg, Über quantentheoretische Umdeutungen kinematischer und mechanischer Beziehungen, Zeitschrift für Physik, 33, pp. 880–881; received 29 July 1925.) Even radical ideas rarely occur in isolation. It is argued that Heisenberg’s early exposure to Plato’s philosophy and his discussions with friends on philosophical themes shaped by the Judeo-Christian tradition created fertile ground for his bold 1925 insight. The value of classical education was deeply rooted in his family: his father, a professor of Greek Philology, and his maternal grandfather, Nikolaus Wecklein—a noted scholar of Greek Sophists—ensured that classical education was a central pillar of his upbringing. Growing up in Würzburg and Munich, he read Latin and Greek, studied Kant’s Critique of Pure Reason at 16, and read Plato’s *Phaidon* and *Apology* in the original language. At 17, Plato’s *Timaios* introduced him to Greek atomist theory from a primary source. Lesser known is his exposure to French philosopher Malebranche, acknowledged when he later wrote: > “Robert’s remark about Malebranche had made it clear to me that > experience about atoms can only be of a rather indirect kind and that > atoms are likely not things.” (W. Heisenberg, Der Teil und das Ganze: Gespräche im Umkreis der Atomphysik, p. 21.) These two streams of thought may have aided Heisenberg's 1925 discovery because 1. they avoid a framework presupposing the independent existence of objects with definite positions at definite times, and 2. they offer a philosophy in which mathematics and abstract ideas can provide genuine insights about the world.

That there’s a sense in which classical physics reduces to and emerges from quantum physics is relatively uncontroversial but this talk will introduce much needed clarity on the role of the $\hbar$ limit for such inter-theoretic relations. This talk divides into two parts: I'll consider how classical physics emerges from quantum physics via decoherence, and I'll articulate the role for the $\hbar\rightarrow0$ limit. First I'll argue that decoherence provides the unique and interpretation-neutral way to understand the emergence of classical from quantum physics, I'll do this by reference to an account of emergence, and I'll argue that this account leaves interpretations with the more metaphysical task of articulating how ontology supervenes upon the decohered quantum description. Decoherence entails a form of dynamical independence between the branches of a superposition in the relevant basis to a very good degree of approximation. This means that the evolution in any such branch is screened off from the other branches and from the peculiarly quantum effects. The upshot of this is that in specific contexts classical dynamics are instantiated in fundamentally quantum systems. I'll claim that decoherence is thus responsible for the emergence of classical behaviour. Second I'll consider the role of the $\hbar$ limit. I’ll mention the history going back to Bohr and Dirac (see Bokulich (2008)) but focus on more recent analyses, especially Feintzeig’s work (though see also Landsman (2017)). Feintzeig’s rigorous treatise establishes a relation between the algebras of classical and quantum mechanics: he demonstrates that as $\hbar$ goes to zero, non-commuting observables commute. He argues that this is a reduction of classical by quantum mechanics, in the sense that it is “an explanation of the success of classical mechanics on the basis of quantum mechanics”. However, I’ll demonstrate that this limit is neither necessary nor sufficient for such explanations of classicality. First there are circumstances where classicality is instantiated but $\hbar$ is not small relative to the action; second there are circumstances where $\hbar$ is small relative to the action but classical behaviour is not instantiated. I discuss alternative accounts of what the $\hbar$ limit achieves and suggest that it may provide an empirical grounding for quantum mechanics in the sense that it tells us how to understand the theory, rather than informing us of the relation between the theory and its predecessors. I’ll conclude with some more general reasons why the distinction between the roles of decoherence and the $\hbar$ limit should be expected: limits are rather blunt tools in that they don’t provide the context-specificity that one finds in decoherence theory. Given that classical mechanics is only instantiated in certain contexts rather than brutely at large energy or timescales, the explanation of its success ought to depend on something more subtle than a limiting procedure. Bokulich, Alisa (2008). Reexamining the Quantum-Classical Relation: Beyond Reductionism and Pluralism. Cambridge University Press. Feintzeig, Benjamin H (2022). The classical–quantum correspondence. Cambridge University Press. Landsman, Klaas (2017). Foundations of Quantum Theory: From Classical Concepts to Operator Algebras. Springer Open.