- Jonathan Bain
The Emergence of Spacetime in Condensed Matter Approaches to Quantum Gravity (slides)
Condensed matter approaches to quantum gravity suggest that spacetime emerges in the low-energy sector of a fundamental condensate. This talk considers what could be meant by this claim. In particular, I offer an account of low-energy emergence that is appropriate for effective field theories in general, and consider the extent to which it underwrites claims about the emergence of spacetime in effective field theories of condensed matter systems of the type that are relevant to quantum gravity.
- Aurelien Barrau
From Loop Quantum Cosmology to the Multiverse (slides)
In this talk I will first focus, as an example of a viable quantum theory of gravity, on Loop Quantum Gravity and its application to cosmology. I will review the basics of the model and explain how the Big Bang is replaced by a Big Bounce. Using recent results, I will underline that predictions can be made and compared with observations in the CMB. The appearance of “another” universe on the other side of the classical Big Bang singularity could be called a temporal multiverse. I will briefly review other possible multiverses and then turn to more philosophical issues, advocating for a relaxed definition of acceptable science.
- Gabriel Catren
On Cartan geometries and the formulation of a gravitational gauge principle (slides)
Whereas Ehresmann connections are the geometric counterpart of the gauge fields of Yang-Mills theory, Cartan connections allow us to reformulate (and generalize) general relativity as a theory in which the fundamental variable is not a metric but rather a connection. This reformulation opens the possibility of understanding general relativity as a gauge theory that results from the localization of the Poincaré group (or the affine group that acts transitively on the corresponding vacuum solution). In this talk we shall unpack the new insights on the geometric structure of space-time provided by the theory of Cartan connections and use this theory to revisit the problem of formulating a gauge principle for the gravitational interaction.
- Alexis de Saint Ours
Negative philosophical discoveries of Machian approaches to the problem of time (slides)
Attempts to construct a background independent quantum theory of gravity have renewed Machian understanding of time in which “It is utterly beyond our power to measure the changes of things by time. Quite the contrary, time is an abstraction at which we arrive through the changes of things (Mach, 1883).” Amongst the Machian positions that we will examine there will be the one of Carlo Rovelli. We will try to show that his relational solution to the problem of time – in which evolution is not measured with respect to an absolute external parameter – together with the thermal time hypothesis – proposed to identify a time variable within a background independent context – has notable philosophical consequences (“negative philosophical discoveries”, London&Bauer, 1939). In particular we will argue that this Machian perspective highlights – contrary to some physicist’s beliefs such as Lee Smolin – some of Bergson’s ideas about time and avoids his critic of spatialisation of time in physics.
- Fay Dowker
Materialism vs idealism in quantum gravity (slides)
In proposing this title I do not intend to stimulate a debate on what exactly materialism or idealism means in the philosophy of science –I am not qualified even to participate in such a debate, let alone lead one! I mean merely to draw attention to the fact that within certain (by no means all) current approaches to quantum gravity, two strands of idealism can be discerned. One is the contention that spacetime is nothing more than relationships between “matter” events (“after all, no-one directly experiences a gravitational field”). The second is the idealism that is forced not just on the quantum gravity researcher but on any physicist who takes the point of view that a physical quantum system should be understood in terms of the quantum state vector in Hilbert space (or wave function) which evolves by unitary evolution into a superposition of branches in each of which the state of the macroscopic pointer is correlated to the value of the measured variable (“all we need to predict are correlations”). In this presentation, I will point out that these (more or less) idealistic positions are free choices of quantum gravity workers because there are materialistic alternatives. I will describe the example I know best: the causal set approach to quantum gravity which is materialist on both the above counts. Within its framework, spacetime is material (and indeed atomic and discrete) and quantum theory, formulated fundamentally in terms of the path integral (and not the state vector), pertains to events in themselves and not merely to correlations.
- George Ellis
The Evolving Block Universe: A more realistic view of spacetime geometry (slides)
Usual spacetimes have no representation of the present time, or the difference between past, present, and future; thus they do not represent the flow of time as experienced in macrophysics, chemistry, biology, and the mind. I propose here a more realistic spacetime model: an evolving block universe, where the future boundary of spacetime represents the present time, and changes as time evolves along timelike worldlines. This necessarily involves existence of preferred surfaces of change that form the future boundary of spacetime; I argue these do indeed exist in any realistic spacetime model (the symmetry of the theory is broken by the geometry of the solution). I show how the evolution of these models may be expressed in usual ADM terms as long as the surfaces remain spacelike, and argue that this viewpoint automatically provides chronology protection. Issues remain as to what happens if these surfaces become timelike: I argue that this can only happen in extreme circumstances associated with back hole formation.
- Michael Esfeld
Against the emergence of space-time (slides)
The thesis of the paper is that, as things stand, there isno question of four-dimensional space-time (and what thereis located in space-time) emerging out of a fundamentalentity that is not spatio-temporal. The claims that suggestsuch an emergence stem from ignoring the problem of what theempirical content of a quantum theory is – in other words,from ignoring the measurement problem in quantum mechanicsthat carries on to quantum field theories and theories ofquantum gravity (instead of being solved or dispelled bythese theories). This problem leaves us with only twooptions: EITHER to recognize only the quantum state of theuniverse and to abandon the commitment to space-time and itscontent, OR to admit four-dimensional space-time includingobjects localized in it as primitive and regard the quantumstate as fixing the temporal development of such objects. Ishow how these options apply in quantum gravity and, inparticular, how a universal, stationary wave-function couldperform the latter task.
- Domenico Giulini
Space-Time and Matter. Accidents and Essentials
So far physics did not succeed to overcome the old dichotomy of space(-time) and matter. The general feeling is that it should, though the reasons given vary as much as the approaches followed. Is matter an attribute of space(-time) geometry, or vice versa, or are both attributes of somethings else? It seems to me that some of the related conceptual issues concerning the distinction between “space” and “field” already afflict classical field physics, though they are often not taken very seriously. This I will try to elucidate in the first half of my talk. In its second half I will remark on the extent to which we can (and do) assign matter attributes to space in canonical (quantum) gravity. Here space’s topology will play an active role.
- Nick Huggett
Emergent Spacetime and Empirical (In)coherence (slides)
Numerous approaches to a quantum theory of gravity posit fundamental ontologies that exclude spacetime, either partially or wholly. This situation raises deep questions about how such theories could relate to the empirical realm, since arguably only entities localized in spacetime can ever be observed. Are such entities even possible in a theory without fundamental spacetime? How might they be derived, formally speaking? Moreover, since by assumption the fundamental entities can’t be smaller than the derived ones (since relative size is a spatiotemporal notion) and so can’t ‘compose’ them in any ordinary sense, would a formal derivation actually show the physical reality of localized entities? We address these questions via examples of theories of quantum gravity, and generally sketch how they may be answered positively. In so doing we aim to highlight the importance of conceptual analysis in the development of new physical theories, such as quantum gravity.
- Jerzy Lewandowski
Semiclassical geometry felt by quantum modes probing quantum spacetime
- Shahn Majid
Emergence of wave equations from quantum spacetime (slides)
We argue that classical geometry should be viewed as a special limit of noncommutative geometry in which aspects which are inter-constrained decouple and appear arbitrary in the classical limit. In particular, the wave equation is really a partial derivative in a unified extra-dimensional noncommutative geometry and arises out of the greater rigidity of the noncommutative world not visible in the classical limit. We provide an introduction to this ‘wave operator’ approach to noncommutative geometry as recently used to quantize any static spacetime metric admitting a spatial conformal Killing vector field, and in particular to construct the quantum Schwarzschild black hole. We also give an introduction to our related result that every classical Riemannian manifold is a shadow of a slightly noncommutative one wherein the meaning of the classical Ricci tensor becomes very natural as the square of a generalised braiding.
- Patrick Peter
The measurement problem for quantum cosmological perturbations (slides)
Inflation suggests that primordial perturbations, responsible for the observed large scale structure formation through gravitational collapse, originated from vacuum fluctuations of the metric. This means that cosmic microwave background anisotropies are a direct evidence for quantum gravity. The problem however is that there is only one universe, with no external observer, so the usual interpretation of quantum mechanics cannot hold in this context, and must thus be supplemented in order to render these fluctuations classical so that they can serve as sources for the Einstein equations: this is the measurement problem of quantum mechanics applied to the Universe as a whole. I will discuss two possible approaches, based either on an extension of the Schrödinger equation leading to dynamical wave packet reduction or on hidden variables; both these schemes allow for a derivation of the Born rules and can be tested in forthcoming experiments.
- Jurgen Renn & Don Salisbury
A semiclassical intrinsic canonical approach to quantum gravity (slides)
We propose a new semiclassical approach to quantum gravity that is based on historically motivated heuristics. We first discuss the role that semiclassical quantization played in the emergence of quantum physics, justifying a heuristics that focuses on the Hamilton-Jacobi equation and an invariant action. We then analyze the weaknesses of the standard semiclassical canonical approach, based on the Wheeler DeWitt equation. We argue that it does not adequately deal with the full diffeomorphism group of general relativity. We then turn to the role of intrinsic coordinates in constructing an action that is invariant under the full diffeomorphism group. We show, in particular, that they allow the formulation of a proper invariant Hamilton Jacobi equation, suitable as a starting point for semiclassical quantization. Finally, we treat the semiclassical quantization of a proper Hamilton-Jacobi equation for a midisuperspace model, showing that our approach provides a robust argument for the necessity to discretize both space and time in quantum gravity.
- Steve Weinstein
Nonlocal constraints for physical theories
The early universe is nearly homogeneous, and thus highly correlated from place to place. Microscopic quantum systems also exhibit striking nonlocal correlations when properly prepared. Together, these suggest the possibility of formulating our physical theories to include nonlocal constraints, constraints which encode correlations between spatially separated degrees of freedom. As with local constraints like the Gauss laws of electromagnetism, these constraints may be understood as lawlike if they are conserved under time evolution. In this talk I’d like to expand on this possibility, and raise the further question of how one should think about the lawlike (or not) nature of constraints in theories in which the notion of time-evolution is ill-defined at a fundamental level.