MCMP – Philosophy of Physics

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".

James Owen Weatherall (California) gives a talk at the MCMP Colloquium (19 July, 2013) titled "Inertial Motion, Explanation, and the Foundations of Classical Spacetime Theories". Abstract: There is an influential view in physics and philosophy of physics, originating with Einstein and Eddington, that holds that general relativity is distinctive in the history of physics because it can be used to "explain" inertial, or unforced, motion. In this talk, I will describe how a reformulation of Newtonian gravitation may be used to provide insight into claims concerning the (allegedly) distinctive explanatory resources of relativity theory. I will then argue that Newtonian gravitation can be understood to explain inertial motion in much the same way as general relativity. However, a careful comparative study of the status of inertial motion in the two theories reveals that neither explanation is as clean or straightforward as adherents to the view noted above believe. I will conclude by presenting a view about the interdependencies of the central principles of physical theories that I will argue provides some insight into a sense in which inertial motion is explained in both of these theories.

Michael Stöltzner (South Carolina) gives a talk at the MCMP Colloquium (17 July, 2013) titled "Feynman Diagrams as Models". Abstract: While Feynman considered his diagrams as pictorial representation of (real and virtual) physical processes, Dyson took then as a mere bookkeeping tool for mathematical expressions in a perturbation series. This split perspective has persisted since, especially when Feynman diagrams gradually extended their sway into modern particle physics. I argue that the modern debates on models in science can build a bridge between both perspectives by granting Feynman diagrams some explanatory autonomy and representative features. While a single Feynman diagram remains isomorph to a mathematical expression, it also represents a family of diagrams that model a measurable physical effect.

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.