MCMP – Philosophy of Physics

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.

Tim Maudlin (NYU) meets Sebastian Lutz (MCMP/LMU) in a joint session on "Reduction and the Ontology of Physical Theories" at the MCMP workshop "Bridges 2014" (2 and 3 Sept, 2014, German House, New York City). The 2-day trans-continental meeting in mathematical philosophy focused on inter-theoretical relations thereby connecting form and content of this philosophical exchange. Idea and motivation: We use theories to explain, to predict and to instruct, to talk about our world and order the objects therein. Different theories deliberately emphasize different aspects of an object, purposefully utilize different formal methods, and necessarily confine their attention to a distinct field of interest. The desire to enlarge knowledge by combining two theories presents a research community with the task of building bridges between the structures and theoretical entities on both sides. Especially if no background theory is available as yet, this becomes a question of principle and of philosophical groundwork: If there are any – what are the inter-theoretical relations to look like? Will a unified theory possibly adjudicate between monist and dualist positions? Under what circumstances will partial translations suffice? Can the ontological status of inter-theoretical relations inform us about inter-object relations in the world? Find more about the meeting at www.lmu.de/bridges2014.

Tim Maudlin (NYU) meets Sebastian Lutz (MCMP/LMU) in a joint session on "Reduction and the Ontology of Physical Theories" at the MCMP workshop "Bridges 2014" (2 and 3 Sept, 2014, German House, New York City). The 2-day trans-continental meeting in mathematical philosophy focused on inter-theoretical relations thereby connecting form and content of this philosophical exchange. Idea and motivation: We use theories to explain, to predict and to instruct, to talk about our world and order the objects therein. Different theories deliberately emphasize different aspects of an object, purposefully utilize different formal methods, and necessarily confine their attention to a distinct field of interest. The desire to enlarge knowledge by combining two theories presents a research community with the task of building bridges between the structures and theoretical entities on both sides. Especially if no background theory is available as yet, this becomes a question of principle and of philosophical groundwork: If there are any – what are the inter-theoretical relations to look like? Will a unified theory possibly adjudicate between monist and dualist positions? Under what circumstances will partial translations suffice? Can the ontological status of inter-theoretical relations inform us about inter-object relations in the world? Find more about the meeting at www.lmu.de/bridges2014.

Tim A. Koslowski (New Brunswick) gives a talk at the Mini-Workshop on the Foundations of Shape Dynamics (23 June, 2014) titled "Shape Dynamics". Abstract: Based on the introduction to shape dynamics by Sean Gryb, I will discuss the question: "Given that gravity (from the perspective of shape dynamics) is fundamentally the evolution of spatial conformal geometry and not spacetime: How is the arrow of time generated? How is the illusion of a spacetime generated? What are the limitations of the spacetime description? I will give explicit answers to several aspect of these questions and I will explain where the uncharted territory begins.

Sean Gryb (Nijmegen) gives a talk at the Mini-Workshop on the Foundations of Shape Dynamics (23 June, 2014) titled "An Introduction to Shape Dynamics: a New Perspective on Quantum Gravity". Abstract: Shape Dynamics is a theory of gravity where the fundamental ontology is that of evolving conformally invariant spatial geometry. This implements a notion of local spatial scale invariance such that what is seen to be physically meaningful is the information about the local "shapes" (as opposed to size) of a system. Perhaps surprisingly, this theory can be proven to reproduce a vast number of the solutions to the Einstein equations. However, black hole solutions are known to differ from those of GR past the horizon and do not lead to singularities. Shape Dynamics, thus, provides an intriguing new starting point for a theory of quantum gravity. In this introductory chalkboard talk, I will try to give some motivations for Shape Dynamics and will describe the basic structure of the theory, outlining how one can prove equivalence with GR. This will lay the ground work for Tim Koslowski's talk, which will discuss some recent developments of the theory.

Thomas Pashby (Pittsburgh) gives a talk at the MCMP Colloquium (28 May, 2014) titled "Against Dogma: Locality, Conditionalisation, and Collapse in Relativistic Quantum Mechanics". Abstract: I argue here against the widespread view (due to David Malament) that the non-commutativity of non-instantaneous localisation projections implies the existence of act-outcome correlations in relativistic QM. There are two facets to my argument: first, I claim that the interpretation of collapse as a process brought about by the experimenter is mistaken; second, I contend that a fully relativistic model should not condition on the occurrence of spacelike separated instantaneous events. This leaves the door open to define a relativistically invariant (but non-commuting) system of localization, which I interpret in terms of conditional probabilities for the occurrence of events. In accord with Tumulka (2009), I conclude that non-local correlations of events in a relativistic quantum theory need not imply the sort of action at a distance that worries Malament (1996).

Andreas Barrels (Bonn) gives a talk at the MCMP Colloquium (7 May, 2014) titled "How to Bite the Bullet of Quidditism - Why a Standard Argument against Categoricalism in Physics Fails". Abstract: Categoricalism is the statement that fundamental properties of physics are categorical, i.e., they have their dispositional characters not with metaphysical necessity. According to Black (2000), Bird (2005, 2007), and Esfeld (2009), categoricalism entails quidditism, the possible existence of properties which are not exclusively individuated by their dispositional characters. If quidditism is true, we cannot know, in principle, whether it is property F or its “Doppelgänger” G that shows up by exhibiting a certain set of dispositional characters. Since we cannot accept our metaphysics of properties to condemn us to necessary ignorance of fundamental properties, we must reject quidditism. Therefore, categoricalism fails. I argue that the possible epistemic situation revealed by quidditism is a case of empirical underdetermination of theoretical properties. This type of situation is not conceived, in general, as the occurrence of some necessary limit of knowledge. There are rational procedures to deal with empirical underdetermination in physics, and thus to decide about the properties the existence of which we are committed to accept. Thus, the unacceptability claim against quidditism is not well founded and categoricalism cannot be defeated that way.

Oliver Pooley (Oxford) gives a talk at the MCMP Colloquium (22 January, 2014) titled "New Work on the Problem of Time". Abstract: A central aspect of the "Problem of Time" in canonical general relativity is the result of applying to the theory Dirac's seemingly well-established method of identifying gauge transformations in constrained Hamiltonian theories. This "orthodox" move identifies transformations generated by the first-class constraints as mere gauge. Applied to GR the strategy yields the result that the genuine physical magnitudes of the theory (so identified) do not take on different values at different times. In the context of quantum gravity, this orthodoxy underwrites the derivation of the timeless Wheeler–DeWitt equation. It is thus intimately connected to one of the central interpretative puzzles of the canonical approach to quantum gravity, namely, how to make sense of a profoundly timeless quantum formalism. This talk reviews several disparate challenges to the technical underpinning of the orthodox view that are starting to gain prominence. Three issues, in particular, will be surveyed. One, explored in the work of Salisbury and collaborators and Pitts, concerns the true relationship between transformations identified as gauge symmetries in the context of a Lagrangian formalism and transformations generated by first-class constraints. A second, explored in the work of Barbour, Gryb and Thébault, concerns whether physical magnitudes are required to commute with all first-class constraints in order for a Hamiltonian theory to be manifestly deterministic. Taking on board the lessons from these two areas is not always sufficient to address all apparent indeterminism in the Hamiltonian formalism. The third topic concerns how this should be addressed.

Oliver Pooley (Oxford) gives a talk at the MCMP Colloquium (22 January, 2014) titled "New Work on the Problem of Time". Abstract: A central aspect of the "Problem of Time" in canonical general relativity is the result of applying to the theory Dirac's seemingly well-established method of identifying gauge transformations in constrained Hamiltonian theories. This "orthodox" move identifies transformations generated by the first-class constraints as mere gauge. Applied to GR the strategy yields the result that the genuine physical magnitudes of the theory (so identified) do not take on different values at different times. In the context of quantum gravity, this orthodoxy underwrites the derivation of the timeless Wheeler–DeWitt equation. It is thus intimately connected to one of the central interpretative puzzles of the canonical approach to quantum gravity, namely, how to make sense of a profoundly timeless quantum formalism. This talk reviews several disparate challenges to the technical underpinning of the orthodox view that are starting to gain prominence. Three issues, in particular, will be surveyed. One, explored in the work of Salisbury and collaborators and Pitts, concerns the true relationship between transformations identified as gauge symmetries in the context of a Lagrangian formalism and transformations generated by first-class constraints. A second, explored in the work of Barbour, Gryb and Thébault, concerns whether physical magnitudes are required to commute with all first-class constraints in order for a Hamiltonian theory to be manifestly deterministic. Taking on board the lessons from these two areas is not always sufficient to address all apparent indeterminism in the Hamiltonian formalism. The third topic concerns how this should be addressed.

Heinz-Jürgen Schmidt (Osnabrück) gives a talk at the MCMP Colloquium (27 November, 2013) titled "The Bohmian challenge". Abstract: The Bohmian extension of quantum theory claims to solve the measurement problem by re-establishing, in some sense, the classical ontology of particle trajectories on a microscopic level. In this talk I will not dwell upon the pros and cons to this claim but rather try to explain why this enterprise constitutes a challenge for philosophy of science, especially for those branches striving for rigorous methods. The Bohmian extension is empirically equivalent to standard quantum mechanics and mathematically only an extension by definitions. The main difference to other interpretations of quantum theory seems to be the belief into the reality of the Bohmian trajectories. Are the theory concepts used in philosophy of science rich enough to represent this crucial difference? In particular, I will scrutinize the theory concept of Günther Ludwig that was explicitly developed to foster his statistical interpretation of quantum theory and contains as a central notion the “reality domain” of a physical theory. A partial result will be that the Bohmian trajectories are not “objective properties” of particles in the sense defined by Ludwig, since the velocity of the Bohmian particles is not observable.