MCMP – Philosophy of Physics

Meinard Kuhlmann (Mainz) gives a talk at the MCMP Colloquium (10 December, 2014) titled "A Trope Bundle Interpretation of Algebraic Quantum Field Theory". Abstract: Algebraic quantum field theory (AQFT) is a conceptually lucid reformulation of the conventional theory of quantum fields. I consider AQFT to be the appropriate starting point for ontological considerations about QFT because, like the philosophical discipline of ontology, AQFT strives for a clear, justified and parsimonious separation of basic and derived classes of entities. I argue that the one-category theory of particularized properties or 'tropes', which analyses all other entities in terms of the basic category of tropes, yields the most appropriate ontological reading of AQFT. Among other things I will show that trope ontology and AQFT have essential structural similarities. I argue in particular that the trope-ontological conception of objects as bundles of tropes is related to the pivotal net structure of observable algebras in AQFT. Eventually, I show that the identification of trope-like entities in AQFT is best achieved via the notion of representations in the algebraic theory of superselection sectors, where the notion of inequivalent irreducible representations allows for a neat distinction of essential and non-essential properties/tropes.

James Weatherall (UC Irvine) gives a talk at the Irvine-Munich Workshop on the Foundations of Classical and Quantum Field Theories (14 December) titled "Fiber Bundles, Yang-Mills Theory, and General Relativity". Abstract: I articulate and discuss a geometrical interpretation of Yang-Mills theory. Analogies and disanalogies between Yang-Mills theory and general relativity are also considered.

Brian Padden (MCMP/LMU) gives a talk at the Irvine-Munich Workshop on the Foundations of Classical and Quantum Field Theories (14 December, 2014) titled "Relativistic Quantum Particles the Feynman Way". Abstract: It is often believed, especially in light of theorems by Malament and others, that there is no relativistic theory of localizable quantum particles. However, an example of exactly such a theory seems to exist, and in fact occupies an important place in the storied history of quantum field theory: Feynman’s path integral approach to quantum electrodynamics. We introduce Feynman’s theory and show that, up to a few minor issues, it is satisfactory. Then, we turn to the theorems stating that such a theory is impossible and discuss which premises are violated by the Feynman theory.

Samuel Fletcher (MCMP/LMU) gives a talk at the Irvine-Munich Workshop on the Foundations of Classical and Quantum Field Theories (14 December, 2014) titled "Classical Field Theory and Intertheoretic Reduction". Abstract: In 1986, Ehlers set out a program on how to understanding the approximative relationships between different physical theories. However, he essentially only investigated the case of classical and relativistic spacetime theories, which have a number of special features that distinguish them from broader classes of physical theories. To what extent, then, can the Ehlers program be successful? I outline some of the challenges facing the program's generalization and argue that they can largely be overcome for classical field theories.

Sarita Rosenstock (UC Irvine) gives a talk at the Irvine-Munich Workshop on the Foundations of Classical and Quantum Field Theories (14 December, 2014) titled "On Fiber Bundle and Holonomy Interpretations of Yang-Mills Theories". Abstract: In the philosophy of Yang-Mills theories, there is an ongoing debate between rival interpretations that can be grouped into two rough categories: “holonomy interpretations” (supported by, e.g., Healey, Belot, and Lyre) and “fiber bundle interpretations” (supported by, e.g., Arntzenius, Maudlin, and Leeds). I present a theorem that I interpret as providing a precise sense in which these interpretations are equivalent.

Ben Feintzeig (UC Irvine) gives a talk at the Irvine-Munich Workshop on the Foundations of Classical and Quantum Field Theories (14 December, 2014) titled "Unitary Inequivalence in Classical Systems". Abstract: I provide an algebraic formulation of classical field theories and use this to probe our interpretation of algebraic theories more generally. I show that the problem of unitarily inequivalent representations, as discussed in Ruetsche (2011), arises in classical theories just as in quantum theories, and I argue that this gives reason to not be a Hilbert Space Conservative when interpreting algebraic theories.

Karim Thebault (MCMP/LMU) gives a talk at the Irvine-Munich Workshop on the Foundations of Classical and Quantum Field Theories (14 December, 2014) titled "A New Prescription for the Quantization of Refoliation Invariant Field Theories". Abstract: Imagine a loaf of bread that we can irregularly cut up into a sequence of slices. The loaf is spacetime and the slices are instantaneous spatial surfaces. A foliation is a parameterization of a spacetime by a time ordered sequence of spatial slices. In a field theory such a parametrization can be local in the sense that it is defined for every point on every spatial slice. Diffeomorphism invariance implies that spacetimes described by general relativity that are related by refoliations are physically equivalent. Classically the symmetry is therefore directly connectable to the idea that only the coordinate-free information contained in a spacetime geometry has a physical basis. The implications of this symmetry for quantization are notoriously problematic. Here we offer a new prescription for the canonical quantization of gravity that side-steps the issues with refoliations via the adoption of the 'shape dynamics’ reformulation. We then offer our thoughts as to whether this is a satisfactory resolution for the problem for understanding refoliation symmetry in the context of a quantum field theory of gravity.

Michael Esfeld (Lausanne) gives a talk at the MCMP Colloquium (26 November, 2014) titled "The Primitive Ontology of Quantum Physics". Abstract: In this talk, I will first recall the arguments why we need what is known as a primitive ontology of quantum physics and then argue that this ontology consists in primitive stuff that is structurally individuated through metrical relations (but without a commitment to absolute space). Against this background, the only reason to admit physical properties then is that they play a causal role for the temporal development of the primitive stuff. I will sketch out two metaphysical views of these properties, namely Humeanism and dispositionalism.

Christopher Fuchs (MPQ Garching) gives a talk at the MCMP Colloquium (20 November, 2014) titled "QBism: A Subjective Way to Take Ontic Indeterminism Seriously". Abstract: The term QBism, invented in 2009, initially stood for Quantum Bayesianism, a view of quantum theory a few of us had been developing since 1993. Eventually, however, I. J. Good's warning that there are 46,656 varieties of Bayesianism came to bite us, with some Bayesians feeling their good name had been hijacked. David Mermin suggested that the B in QBism should more accurately stand for "Bruno", as in Bruno de Finetti, so that we would at least get the variety of (subjective) Bayesianism right. The trouble is QBism incorporates a kind of metaphysics that even Bruno de Finetti might have rejected! So, trying to be as true to our story as possible, we momentarily toyed with the idea of associating the B with what Chief Justice Oliver Wendell Holmes Jr. called bettabilitarianism. It is the idea that the world is loose at the joints, that indeterminism plays a real role in the world. In the face of such a world, what is an active agent to do but participate in the uncertainty that is all around him? As Louis Menand put it, "We cannot know what consequences the universe will attach to our choices, but we can bet on them, and we do it every day." This is what QBism says quantum theory is about: How to best place bets on the consequences of our actions in this quantum world. But what an ugly, ugly word, "bettabilitarianism"! Therefore, maybe one should just think of the B as standing for no word in particular, but a deep idea instead: That the world is so wired that our actions as active agents actually matter. Our actions and their consequences are not eliminable epiphenomena. In this talk, I will describe QBism as it presently stands and give some indication of the many things that remain to be developed.

Michael Esfeld (Lausanne) gives a talk at the MCMP Colloquium (26 November, 2014) titled "The Primitive Ontology of Quantum Physics". Abstract: In this talk, I will first recall the arguments why we need what is known as a primitive ontology of quantum physics and then argue that this ontology consists in primitive stuff that is structurally individuated through metrical relations (but without a commitment to absolute space). Against this background, the only reason to admit physical properties then is that they play a causal role for the temporal development of the primitive stuff. I will sketch out two metaphysical views of these properties, namely Humeanism and dispositionalism.