MCMP – Philosophy of Physics

Dennis Lehmkuhl (Wuppertal) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "Einstein's approach to Quantum Gravity". Abstract: It is common knowledge that despite being a pioneer of the early quantum theory, Einstein opposed the probabilistic interpretation of the new quantum mechanics of 1925 / 1926, and he would have opposed any approach of a quantization of gravity relying on this interpretation. What is less well-known is that Einstein had an alternative approach that bears some similarities to more recent ideas to general relativize quantum mechancis rather than quantizing general relativity. Einstein identified discreteness in nature with quantum theory: the existence of photons, the quantisation of electric charge, etc. Roughly speaking, his idea was that these quantum features of reality could be derived from a generally covariant field theory of gravitation and electromagnetism by finding overdetermined partial differential equations which allow for solutions capable of represtenting quantum particles. If this were to be achieved, the groundfloor ontology of the world would have turned out to be one of classical fields described by generally covariant partial differential field equations, while the quantum would only enter on the level of the solutions to these equations. The talk reviews the above research programme of Einsteins, and reflects on whether it can lend some inspiration to modern quantum gravity research.

Robert Helling (LMU) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "The "Shut up and Calculate" Approach to Quantum Gravity". Abstract: Quantum mechanics would have never been developed requiring to first understand all foundational issues. Rahter, it was approached in a pragmatic way summarized as shut up and calculate. For quantum gravity we already understand a number of its properties even if it is still unclear what form the final theory will have and how it solves its conceptual conundrums. The often repeated claim that quantum gravity lacks empirical data is - taken without qualification - not true. Rather, the requirement to reduce to known theories relativity in the appropiate limits togehter with our expericence of everyday physics rule out most creative proposals for a theory of quantum gravity. We want to pursue the effective-filed-theory line-of-thought neglecting conceptual expectations. The effective field theory approach still leaves room for a wide variety of ideas like string theory, loop quantum gravity or emergent gravity as we will show in a number of examples.

Claus Kiefer (Cologne) gives a talk at the MCMP workshop "Quantum Gravity in Perspective" (31 May-1 June, 2013) titled "Quantum Geometrodynamics: whence, whither?". Abstract: Quantum geometrodynamics is canonical quantum gravity with the three-metric as the configuration variable. Its central equation is the Wheeler-DeWitt equation. Here I give an overview of the status of this approach. The issues discussed include the problem of time, the relation to the convariant theory, the semiclassical approximation as well as applicatoins to black holes and cosmology. I conclude that quantum geometrodynamics is still a viable approach and provides insights into both the conceptual and technical aspects of quantum gravity.

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".

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.

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.

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.

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.

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.

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.