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.