MCMP – Philosophy of Physics

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.

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.

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.

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.