MCMP – Philosophy of Physics

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.

Detlef Dürr (LMU) gives a talk at the MCMP Colloquium (23 January, 2013) titled "Bohmian Mechanics, speakable quantum physics". Abstract: I introduce Bohmian Mechanics, which is a theory of particles in motion. The law of motion is not classical, i.e. the particles do not move on Newtonian trajectories. As this is often not appreciated I shall discuss some features which will help to sharpen one's intuition about this theory of nature.

Karim Thébault (MCMP/LMU) gives a talk at the MCMP Colloquium (9 January, 2013) titled "Quantisation as a guide to ontic structure". Abstract: The ontic structural realist stance is motivated by a desire to do philosophical justice to the success of science, whilst withstanding the metaphysical undermining generated by the various species of ontological underdetermination. We are, however, as yet in want of general principles to provide a scaffold for the explicit construction of structural ontologies. Here we will attempt to bridge this gap by utilising the formal procedure of quantisation as a guide to ontic structure of modern physical theory. The example of non-relativistic particle mechanics will be considered and, for that case, it will be argued that, modulo certain mathematical ambiguities, a consistent candidate structural ontology can be established.

Igor Khavkine (Utrecht) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "Gravity. An exercise in quantization". Abstract: The quantization of General Relativity (GR) is an old and chellenging prob- lem that is in many ways still awaiting a satisfactory solution. GR is a partic- ularly complicated field theory in several respects: non-linearity, gauge invari- ance, dynamibal causal structure, renormalization, singularities, infared effects. Fortunately, much progress has been made on each of these fronts. Our under- standing of these problems has evolved greatly over the past century, together with our understandig of quantum field theory (QFT) in general. Today, the state of the art in QFT knows how to address each of these challenges, as they occur in isolation in ohter field theories. There is still an active research program aiming to combine the relevant methods and apply them to GR. But, at the very least, the problem of the quantization of GR can be formulated as a well defined mathematical question. On the other hand, quantum GR also faces a different set of obstacles: timelessness, non-renormalizability, naturality, unification, which reflect, not its technical difficulty, but rather the aesthetic and philosophical preferences of practing theoretical physicists. I will briefly discuss how the technical state of the art and a scientifically conservative philosophical position make these obstacles irrelevant. Time per- mitting, I will also briefly touch on some aspects of the state of technical state of the art that have turned the quantization of GR into a (still challenging) exercise: covariant Poisson structure, BV-BRST treatment of gauge theories, deformation quantization, Epstein-Glaser renormalization.

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).

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.

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.

Michael Stöltzner (South Carolina) gives a talk at the Irvine-Munich Workshop on the Foundations of Classical and Quantum Field Theories (14 December, 2014) titled "Best Possible Worlds and Random Walks: The Principle of Least Action as a Thought Experiment". Abstract: Over the centuries, no other principle of classical physics has to a larger extent nourished exalted hopes of a universal theory, has constantly been plagued by mathematical counterexamples, and has ignited metaphysical controversies than has the principle of least action (PLA). The aim of this paper is first to survey a series of modern approaches, among them the structural realist readings of Planck and Hilbert, a neo-Kantian relativized a priori principle, and more recent discussions about modality within the context of analytic metaphysics. But these considerations seem outrun by the broad applicability of the PLA beyond classical physics. In the case of Feynman’s path integral, the PLA does no longer amount to the distinction of the actual dynamics among the possible ones, but to the definition of a stochastic process to which all possibilities contribute with a certain probability. To reach a unified philosophical picture of all the various applications of the PLA and its kin, I suggest to consider them as a thought experiment about the applicability of mathematics to a physical problems.

Carlo Rovelli (Aix-Marseille) gives a talk at the Workshop on the Problem of Time in Perspective (3-4 July, 2015) titled "Physics without Fundamental Time". Abstract: Rivers of ink have flown on the basic conceptual structure of quantum gravity -a theory where we expect the notions of classical spacetime, particles, fields, energy and momentum to require substantial revision-. I discuss a specific solution to these questions and apply concretely it to a physical calculation, the tunneling time of a Hajicek-Kiefer black-to-white hole transition. This is a quantum gravitational effect that might have some chance to be actually observable, or could have even been already observed in the "Fast Radio Bursts" observed by the Arecibo and Parkes radio-telescopes.

Donald Salisbury (Austin) gives a talk at the Workshop on the Problem of Time in Perspective (3-4 July, 2015) titled "The Intrinsic Hamilton-Jacobi Dynamics of General Relativity and its Implications for the Semi-Classical Emergence of Time". Abstract: The quantization of the general theory of relativity is notoriously difficult, in particular on account of the underlying general covariance and the consequent appearance of constraints in the classical Hamiltonian theory. The notion of time in the quantum theory is especially troubling since differing ideas of time suggest themselves depending on the quantum rules that are employed and the interpretations given to time in the classical theory itself. I will address the problem of time from a perspective in which constraints are implemented in a Hamilton-Jacobi framework through the use of intrinsic coordinates. The canonical approach is especially suited for this task. The decisive result is that the problem of time is even greater than one might have expected; there are arbitrarily many equally valid and possibly inequivalent time choices that one can introduce in this manner, all involving the use dynamical variables that are invariant under the action of the four-dimensional diffeomorphism-induced group as described in Pons, Salisbury, and Sundermeyer, Phys. Rev. (2009), 084015. I will review a Kuchař-inspired, but fully diffeomorphism covariant, classical Hamiltonian approach to general relativity in which spacetime scalar phase space variables are introduced that can serve as intrinsic coordinates. There corresponds to each choice a constraint which can be converted (as originally proposed by Asher Peres for conventional variables) into an Einstein-Hamilton-Jacobi (EHJ) equation. The choice of intrinsic coordinates is rendered simple in terms of these new variables, as are the choices in the new intrinsic EHJ equation. Indeed, the resulting intrinsic dynamics follows immediately from the EHJ equation. No Lagrangian is obtained, and this might be expected given that spacetime scalars must depend on time derivatives of the metric and their introduction into the Einstein action would result in the appearance higher derivative contributions. To each EHJ equation there corresponds a Wheeler-DeWitt quantum equation with its own emergent time. I will begin to examine possible quantum implications of the existence of distinct emergent intrinsic times.

Francesca Vidotto (Radboud) gives a talk at the Workshop on the Problem of Time in Perspective (3-4 July, 2015) titled "Discrete Time in Quantum Gravity". Abstract: We study the quantization of geometry in the presence of a cosmological constant, using a discretiza- tion with constant-curvature simplices. Phase space turns out to be compact and the Hilbert space finite dimensional for each link. Not only the intrinsic, but also the extrinsic geometry turns out to be discrete, pointing to discreetness of time, in addition to space. We work in 2+1 dimensions, but these results may be relevant also for the physical 3+1 case.

Henrique Gomes (Perimeter Institute) gives a talk at the Workshop on the Problem of Time in Perspective (3-4 July, 2015) titled "Timeless Quantum Mechanics in Configuration Space: an Outsider View". Abstract: In this talk, I will explore a timeless interpretation of quantum mechanics of closed systems, solely in terms of path integrals in non-relativistic timeless configuration space. What prompts a fresh look at the foundational problems in this context, is the advent of multiple gravitational models in which Lorentz symmetry is only emergent. In this setting, I propose a new understanding of records as certain relations between two configurations, the recorded one and the record-holding one. These relations are formalized through a factorization of the amplitude kernel, which forbids unwanted 'recoherence' of branches. On this basis, I show that in simple cases the Born rule is consistent with counting the relative density of observers with the same records. Furthermore, unlike what occurs in consistent histories, in this context there is indeed a preferred notion of coarse-grainings: those centered around piece-wise classical paths in configuration space (with a certain radius). Thus, this new understanding claims to resolve aspects of the measurement problem which are still deemed controversial in the standard approaches (but which probably leaves others open...).

Sean Gryb (Radboud) gives a talk at the Workshop on the Problem of Time in Perspective (3-4 July, 2015) titled "In Favour of a Schrödinger Evolution of the Universe". Abstract: In the canonical formulation of reparametrization invariant systems, time evolution on phase space is generated by a fully constrained Hamiltonian. On the orthodoxy view, the quantum formalism for such systems is constructed through Dirac quantization, which leads to a real, time-independent constraint on the quantum state. The question then remains how to extract a notion of time evolution from this frozen formalism. On one predominant view, the system is to be split in terms of ''partial observables'' -- which may be used as internal clocks -- and a set of ''complete observables'' -- which are understood to evolve in terms of the former. This has led to a controversy around the interpretation of the partial observables within the formalism. In this talk, we will provide a negative argument against the orthodoxy view that clarifies the role that should be played by the partial observables. We then present a proposal for the canonical quantization of reparametrization invariant systems that naturally encodes a genuine notion of time evolution and illustrate how this proposal can be applied to gravity.

Bianca Dittrich (Perimeter Institute) gives a talk at the Workshop on the Problem of Time in Perspective (3-4 July, 2015) titled "The Consistent Boundary Formulation". Abstract: I will introduce the consistent boundary formulation which allows to express renormalization flow in a background independent context. I will discuss consequences of this formulation for the Hamiltonian framework and explore in which sense Hamiltonian constraints do actually exists in this context and how this influences the notion of time.

Philipp Hoehn (Perimeter Institute) gives a talk at the Workshop on the Problem of Time in Perspective (3-4 July, 2015) titled "Constraints, Dirac Observables and Chaos". Abstract: I will discuss fundamental challenges to the standard relational paradigm arising from chaotic dynamics.

J. Brian Pitts (Cambridge) gives a talk at the Workshop on the Problem of Time in Perspective (3-4 July, 2015) titled "Changing Observables in Canonical General Relativity from Hamiltonian-Lagrangian Equivalence". Abstract: Is change missing in classical canonical General Relativity? If one insists on Hamiltonian-Lagrangian equivalence, then there is Hamiltonian change just when there is no time-like Killing vector field. Change has seemed missing partly due to Dirac’s belief that a first-class constraint, especially a primary, generates a gauge transformation. Pons showed that Dirac’s argument stops too soon: working to second order in time brings in first-class secondaries and hence the gauge generator G, a tuned sum of first-class constraints used by Anderson and Bergmann (1951) and recovered by Mukunda, Castellani et al. from the 1980s. I observe that trouble happens immediately: a first-class primary constraint generates an illegal change of initial data in GR, Maxwell and Yang-Mills. Dirac’s subtractive derivation misses it by cancellation; confusion between the electric field E(dA) and canonical momenta p (auxiliary fields in the canonical action int dt (p dot{q}-H) also obscures the problem. Dirac’s conjecture that a first-class secondary constraint generates a gauge transformation rests on a false assumption. Looking for gauge symmetries of the canonical action, one finds that the gauge generator G changes the action by at most a boundary term, but an isolated first-class constraint does not. The gauge generator G generates spatio-temporal coordinate transformations (not just spatial ones) for the space-time metric (not just the spatial metric). But are there locally varying _observables_ in canonical General Relativity? Hamiltonian-Lagrangian equivalence guarantees that Hamiltonian observables are equivalent on-shell to Lagrangian observables. (Historically, Lagrangian-inequivalent observables may have arisen within Bergmann’s school due to novel postulation in Bergmann-Schiller 1953.) With first-class constraints exposed as not generating gauge transformations, observables’ Poisson brackets should be taken with the gauge generator G, as noted by Pons, Salisbury and Sundermeyer. Heeding Einstein’s point-coincidence argument excludes primitive point individuation and thus active diffeomorphisms in favor of (4-d) tensor calculus. Kuchař’s unsystematic waiver of the vanishing Poisson brackets condition to permit change has a more principled extension: observables should be internally gauge _invariant_ (0 Poisson bracket with G for Maxwell, Yang-Mills, etc.) but externally gauge _covariant_. Hence the Poisson bracket with the coordinate-changing G should be the Lie derivative, indeed the Lie derivative of a geometric object (on-shell). For GR with no matter gauge group, observables are (on-shell) space-time geometric objects (components in coordinates with a transformation law). Hence the space-time metric and its concomitants (connection, curvature, etc.) are locally varying observables. Questions regarding Legendre projectability when an internal gauge group is also present and regarding the mixed supergravity transformations are noted. Velocity-dependent gauge transformations call for phase space extended by time---“phase space-time”; GR’s Lie derivative is an example. Vacuum GR’s phase space-time has 20 infinity^3 + 1 dimensions and 8 infinity^3 first-class constraints; one should not have expected a reduced phase _space_ description of a theory with many-fingered time. Classical clarity might be of some use in quantization.

Oliver Pooley (Oxford) gives a talk at the Workshop on the Problem of Time in Perspective (3-4 July, 2015) titled "First-Class Constraints, Gauge, and the Wheeler-DeWitt Equation". Abstract: Recently, Pitts (2014) has argued that the claim that first-class constraints generate gauge transformations (hereafter “orthodoxy”) fails even in electromagnetism, which is standardly taken to illustrate its correctness. Independently, Barbour and Foster (2008) have argued that a key presupposition of the primary argument for orthodoxy (due to Dirac, 1964) is not satisfied in the important case of reparameterization-invariant theories. In assessing these claims, one needs to distinguish between, (i) transformations that relate points of phase space that represent the same instantaneous state and (ii) transformations that map curves on phase space to curves that represent the same history. Pitts shows that arbitrary first-class constraints fail to generate transformations of type (ii), but leaves untouched the orthodox position concerning (i). Barbour and Foster show that we have no reason to regard transformations generated by Hamiltonian constraints as type (i), but that leaves open that they may be regarded as transformations of type (ii). I will discuss whether the latter possibility allows one to reconcile applying Dirac’s constrained quantization procedure to the Hamiltonian constraint and interpreting the Wheeler–DeWitt wavefunction as representing genuine change.

Tim A. Koslowski (Brunswick) gives a talk at the Workshop on the Problem of Time in Perspective (3-4 July, 2015) titled "The Gravitational Arrow of Time". Abstract: The arrow of time appears to always to point in one direction (i.e. we can clearly tell whether a movie is played forward or backward) although the underlying physics is time-reversal symmetric. The most widely accepted explanation for this is that the experienced arrow is the thermodynamic arrow of time (i.e. the direction of entropy growth). This scenario requires the past hypothesis, i.e. an atypical initial condition. We propose an alternative mechanism: The arrow of time is the direction in which the complexity of the universe grows. Gravity generates this arrow of time and creates subsystems with low entropy initial conditions spontaneously. I show this in detail in the Newtonian limit and discuss the extension to cosmological models in GR.

Kurt Sundermeyer (FU Berlin/MPIWG) gives a talk at the Workshop on the Problem of Time in Perspective (3-4 July, 2015) titled "Facets of Time in Physics". Abstract: In an attempt to understand slogans such as "The End Of Time", "Forget About Time", "Time Reborn", "Time Remains", I started to write an essay about the various notions of time in physics (with a glimpse to philosophy) from classical mechanics to quantum gravity. By this curiosity-driven motivation I realized that there are not only various notions of time, but that there are diverse problems with time and problems of time. In my talk I will present the structure and inputs of this still unfinished treatise, and I expect to receive valuable feedback from the competent participants of this workshop.

Miklós Rédei (LSE) gives a talk at the MCMP Colloquium (13 May, 2015) titled "A Categorial Approach to Relativistic Locality". Abstract: In the talk relativistic locality of a probabilistic physical theory is interpreted as an interconnected web of properties which express compatibility of the theory with the underlying causal structure of spacetime. Four components of this web are distinguished: spatiotemporal locality, causal locality-Independence, causal locality-Dependence, and causal locality-Dynamic. These four conditions will be specified in terms of concepts from the categorical approach to quantum field theory and results are recalled indicating the extent to which an algebraic quantum field theory satisfying the Haag–Kastler axioms is causally local.

Richard Dawid (MCMP) gives a talk at the MCMP Colloquium (29 April, 2015) titled "Bayesian Perspectives on the Higgs Search". Abstract: The history of the Higgs discovery is characterized by a specific constellation: trust in the existence of a Higgs particle was very strong already before the particle's discovery. This raises the issue of a Bayesian perspective on data analysis in high energy physics in an interesting way that differs from other contexts in the field where the deployment of Bayesian strategies was proposed.

Elena Castellani (Florence) gives a talk at the MCMP Colloquium (28 January, 2015) titled "Dualities". Abstract: Dualities are a key, intriguing ingredient in mathematics, logic and physics. This talk is concerned with physical dualities, in particular those dualities that have played "a central role in mapping out the structure of theoretical physics" in the past two decades (quoting Polchinski, 2015). Despite the importance of duality in field and string theory, philosophers are just starting to pay the due attention to the subject. This talk is meant as an introduction to discussing the significance of physical dualities, by pointing out the different relevant functions they play and the kinds of philosophical issues they connect or give rise to (in particular: theoretical equivalence and ontological indeterminacy).

Rüdiger Schack (Royal Holloway London) gives a talk at the Workshop on Quantum Computation, Quantum Information, and the Exact Sciences (30-31 January, 2015) titled "QBism and the Born rule". Abstract: By adopting a strictly personalist approach to probability, QBism takes the view that quantum states, and therefore also quantum information, reflect an agent’s personal degrees of belief about the consequences of his actions on the world. The quantum formalism enables the agent to make better decisions in the light of his previous experiences. This talk focuses on the central role played in quantum mechanics by the Born rule, which in QBism has a normative character similar to the rules of probability theory.

Chris Timpson (Brasenose College, Oxford) gives a talk at the Workshop on Quantum Computation, Quantum Information, and the Exact Sciences (30-31 January, 2015) titled "Quantum Information: Conceptual and Ontological Aspects". Abstract: In this talk I will explore some ways of thinking about what quantum information is. This topic has a certain intrinsic interest, but it is also important when trying to assess in a careful way what role the concept of information might have to play in fundamental physics. I shall argue for a view which is fairly ontologically deflationary about quantum information (this has significant impact on how we should understand the slogan ‘information is physical’ and for informational immaterialist views) and which sees quantum and classical (Shannon) information both as species of a single genus.

Gia Dvali (LMU-MPI & NYU) gives a talk at the MCMP Colloquium (14 January, 2015) titled "Corpuscular structure of geometry". Abstract: We review some recent ideas on quantum-corpuscular structure of gravitational metric backgrounds, such as black holes and cosmological spaces. We show how this picture sheds light on seemingly-mysterious properties, such as, black hole information processing and evaporation, as well as how it excludes eternal de Sitter space. This picture sheds a very different light on notion of "holography" and cosmological constant problem.

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.

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.

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.

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.

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.

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.

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.

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.

Erik Curiel (MCMP/LMU) gives a talk at the MCMP Colloquium (18 December, 2013) titled "Are Classical Black Holes Hot or Cold?". Abstract: In the early 1970s it is was realized that there is a striking formal analogy between the so-called laws of black-hole mechanics and the laws of classical thermodynamics. Before the discovery of Hawking radiation, however, it was generally thought that the analogy was only formal, and did not reflect a deep connection between gravitational and thermodynamical phenomena. In particular, it is still commonly held that the surface gravity of a stationary black hole can be construed as a true physical temperature only when quantum effects are taken into account; in the context of classical general relativity alone, one cannot cogently construe it so. Does the use of quantum field theory in curved spacetime offer the only hope for taking the analogy seriously? I think the answer is 'no'. To attempt to justify that answer, I shall begin by arguing that the standard argument to the contrary is not physically well founded, and in any event begs the question. Looking at the various ways that the idea of "temperature" enters classical thermodynamics then will suggest arguments that, I claim, show the analogy between classical black-hole mechanics and classical thermodynamics should be taken more seriously, at least so far as temperature goes, without the need to rely on or invoke quantum mechanics. If this is correct, then there may be a deep connection between classical general relativity and classical thermodynamics on their own, independent of quantum mechanics.

Karen Crowther (Sydney) gives a talk at the MCMP Colloquium (13 November, 2013) titled "Emergent spacetime in condensed matter analogues of general relativity". Abstract: It has been claimed, based on a few different lines of reasoning, that the notion of spacetime will not appear in a quantum theory of gravity. If this is the case, then spacetime is an emergent concept. Analogue models of general relativity based in condensed matter systems present us with concrete examples of emergent spacetime, and could potentially help us understand the nature of emergent spacetime in the context of quantum gravity. These models present a curved spacetime metric that is described using an effective field theory. In this talk, I explore the conception of emergence that is relevant to analogue spacetime in the condensed matter models. This is a conception of emergence that also applies more generally to other effective field theories, and, as I argue, one that is best understood without appeal to the idea of reduction. I finish by briefly mentioning some potential implications for quantum gravity.

Fay Dowker (Imperial College London) gives a talk at the 17th UK and European Meeting on the Foundations of Physics (29-31 July, 2013) titled "Things happen, they just happen in a partial order". Abstract: In causal set quantum gravity spacetime is hypothesized to be atomic and causal order is the most basic organising principle. Fundamental discreteness brings with it novel possibilities for "dynamical laws" in which spacetime grows by the accumulation of new atoms, potentially realising within physics C.D. Broad's concept of a growing block universe in which the past is real and the future is not. That a growing block can be compatible with general covariance and the lack of a global time, is demonstrated by the Rideout-Sorkin Classical Sequential Growth models in which the “present” is identified with the growth process itself.

Jean Bricmont (Université catholique de Louvain) gives a talk at the 17th UK and European Meeting on the Foundations of Physics (29-31 July, 2013) titled "From the microscopic to the macroscopic world". Abstract: The derivation of the laws describing the macroscopic world from those governing the microscopic one is a very difficult problem. The root of the difficulty is sometimes seen as arising from the fact that the first set of laws are often time-irreversible, while the second ones are time-reversible. The goal of the talk will be to explain precisely these notions (macroscopic, microscopic, (ir)reversibility) and why this difference does not constitute an insuperable difficulty. We will also discuss the role of probability in the derivation of the macroscopic laws from the microscopic ones and criticize several misleading attempts at justifying this derivation.

Julian Barbour (Oxford) gives an evening lecture at the 17th UK and European Meeting on the Foundations of Physics (29-31 July, 2013) titled "A Dynamical Origin of the Arrow of Time".

Robert Spekkens (Perimeter Institute) gives a talk at the 17th UK and European Meeting on the Foundations of Physics (29-31 July, 2013) titled "On Causal Explanations of Quantum Correlations". Abstract: If correlation does not imply causation, then what does? Causal discovery algorithms take as their input facts about correlations among a set of observed variables, and they return as their output causal structures that can account for the correlations. We show that any causal explanation of Bell-inequality-violating correlations must contradict a core principle of these algorithms, namely, that an observed statistical independence between variables should not be explained by fine-tuning of the causal parameters. The fine-tuning criticism applies to all of the standard attempts at causal explanations of Bell correlations, such as superluminal causal influences, superdeterminism, and retrocausal influences that do not introduce causal cycles. This suggests a novel perspective on the assumptions underlying Bell's theorem: the nebulous assumption of realism can be replaced with the principle that all correlations ought to be explained causally and Bell's notion of locality can be replaced with the assumption of no fine-tuning. Finally, we discuss the possibility of salvaging a causal explanation of quantum correlations by casting quantum theory as an innovation to the theory of Bayesian inference.

J. Brian Pitts (Cambridge) gives a talk in the colloquium "On the Split Between Gravity and Inertia in Different Spacetime Theories" at the 17th UK and European Meeting on the Foundations of Physics (29-31 July, 2013) titled "Inertia and the Conformal-Projective Decomposition for Nordström-Einstein-Fokker, Massive Scalar, Einstein, and Massive Spin 2 Gravities". Abstract: The Ehlers-Pirani-Schild (EPS) construction, which derives a metric tensor from a projective connection and a conformal metric density, has sometimes been thought to undermine the conventionality of geometry. It might be of renewed interest due to the appearance of the dynamical or constructivist ap- proach to space-time geometry of Brown and Pooley. Constructivism shares with conventionalism the modally cosmopolitan awareness of a multiplicity of options, not all so tidy as to fit a unique geometry, leaving the ‘true’ geometry ambiguous. An EPS-inspired decomposition is applied to Nordström-Einstein-Fokker (massless spin 0) scalar gravity and its belatedly studied cousin, massive spin 0, which agree on the geometry seen by matter (conformally flat). For mas- sive scalar gravity, the symmetry group of the whole theory is the Poincaré group of Minkowski geometry, not the 15-parameter conformal group as in Nordström-Einstein-Fokker. By focusing only on the matter action, the EPS construction fails to notice the key geometrical diferences between massless and massive spin 0 theories and hence fails to address key issues motivating conventionalist and constructivist positions. For both massless and massive scalar gravities, inertia has an absolute core but is modifiable invariantly by gravity. The decomposition is then applied to Einstein’s General Relativity (mass-less spin 2) and its recently revived cousin(s), massive spin 2 gravity(s). Similar issues to the spin 0 comparison arise prima facie, but complicated by gauge freedom (in both cases but for different reasons) as well as the greater number of fields.

Eleanor Knox (London) gives a talk in the colloquium "On the Split Between Gravity and Inertia in Different Spacetime Theories" at the 17th UK and European Meeting on the Foundations of Physics (29-31 July, 2013) titled "The Gravity-Inertia split in Newtonian and Relativistic Contexts". Abstract: Relative to Newton-Cartan theory, Newtonian gravitation involves the split- ting of a single curved connection into gravitational and inertial parts. I examine the prospects for imposing an analogous division of the connection in general relativity. It’s well-known one cannot split the Levi-Civita connection in quite the same way as one does the Newton-Cartan connection, into a symmetric connection and gravitational field. However, it is possible to divide the Levi-Civita connection into a non-symmetric connection and a part that has sometimes been held (in Teleparallel theories) to represent the gravitational field. I’ll argue that non-symmetric connections are not candi- dates for representing full inertial structure, and hence that general relativity unites the gravitational and inertial field in a particularly profound sense.

Dennis Lehmkuhl (Wuppertal) and Oliver Pooley (Oxford) give a talk in the colloquium "On the Split Between Gravity and Inertia in Different Spacetime Theories" at the 17th UK and European Meeting on the Foundations of Physics (29-31 July, 2013) titled "Against the Gravity/Inertia split?". Abstract: To make sense of talk of a frame-dependent inertia–gravity split in General relativity, one needs to relate the theory to Newtonian gravity, and to recognise that two routes to privileged frames of reference need not yield the same sets of frames. On the first route, which paths in spacetime correspond to unaccelerated (“inertial”) motions is an absolute, coordinate-independent matter. The privileged frames are those whose standard of rest corresponds to inertial motion. On the second route, privileged frames are identified via classes of co-moving coordinate systems with respect to which dynamical equations take a simple, canonical form. In Newtonian gravity, the second route yields globally-defined frames with respect to which freely-falling bodies are (in general) accelerating. In practice, however, the theory cannot distinguish between frames that are relatively translationally accelerated. At best, therefore, an empirically undetectable proper subset of these frames encode inertial motion. The idea of a frame-dependent inertia–gravity split arises when one combines the idea that these frames encode inertia (and thus that free-fall motions involve gravitational deflection from inertial motion) with the idea that they are fundamentally physically equivalent. This combination, however, is not coherent. A preferable viewpoint reconciles an absolute notion of inertia with the physical equivalence of the frames identified via the second route by denying that they encode inertial motion. They are, instead, frames with respect to which the components of the connection take a particularly simple form, even though they do not all vanish. We will argue that Einstein’s central claims concerning the equivalence principle, and the frame-dependence of the gravitational field, are compatible with this second viewpoint.

Sean Gryb (Radboud) gives a talk in the colloquium "Journeys in Platonia: Celebrating 50 Years Since The End of Time" at the 17th UK and European Meeting on the Foundations of Physics (29-31 July, 2013) titled "Barbour's Shape Space as an Ontology for Gravity". Abstract: I will give a personal account of the development of the conformally invariant version of ‘Shape Dynamics’. The story will be told from three perspectives: i) a historical one, highlighting the role of College Farm and the unique interactions with Julian Barbour, ii) a philosophical one, describing a simple observation about the meaning of local scale in physics, and iii) a formal one, showing how an early observation of Poincare ́ combined with York’s method for solving the initial value problem in General Relativity led a concrete implementation of Barbour’s ontology.

Edward Anderson (Paris Diderot and Cambridge) gives a talk in the colloquium "Journeys in Platonia: Celebrating 50 Years Since The End of Time" at the 17th UK and European Meeting on the Foundations of Physics (29-31 July, 2013) titled "Kendall’s Shape Statistics as a Classical Realization of Barbour-type Timeless Records Theory Approach to Quantum Gravity". Abstract: I already showed that Kendall’s shape geometry work was the geometrical description of Barbour’s relational mechanics’ reduced configuration spaces (alias shape spaces). I now describe the extent to which Kendall’s subsequent statistical application to such as the ‘standing stones problem’ realizes further ideas along the lines of Barbour-type timeless records theories, albeit just at the classical level.

Harvey Brown (Oxford) gives a talk in the colloquium "Journeys in Platonia: Celebrating 50 Years Since The End of Time" at the 17th UK and European Meeting on the Foundations of Physics (29-31 July, 2013) titled "Leibniz, Mach and Barbour". Abstract: My comments will be concerned with the way that Leibniz's and Mach's thinking on the nature of space have influenced Julian Barbour's approach to the formulation of dynamical theories.

John D. Norton (Pittsburgh) gives a talk at the 17th UK and European Meeting on the Foundations of Physics (29-31 July, 2013) titled "The Neglect of Fluctuations in the Thermodynamics of Computation". Abstract: The thermodynamics of computation assumes that thermodynamically reversible processes can be realized arbitrarily closely at molecular scales. They cannot. Overcoming fluctuations so that a molecular scale process can be completed creates more thermodynamic entropy than the small quantities tracked by Landauer's Principle. This no go result is the latest instance of a rich history of problems posed by fluctuations for thermodynamics.

Michael Krämer (RWTH Aachen) gives a talk at the MCMP Colloquium (23 October, 2013) titled "The (philosophy of the) Higgs". Abstract: I will review the physics of the Large hadron Collider LHC, including the recent the discovery of a Higgs particle, and the search for new physics beyond the Standard Model of particle physics. While the talk focusses on the physics, I will also mention various philosophical questions which are being addressed in the context of an interdisciplinary research group on the "Epistemology of the LHC".