Carlos Barcelo (University of Granada): Emergent gravity: pursuing an idea

Can gravity be an emergent phenomenon? For instance and as a proof of principle, is it possible to construct a theory of quantum gravity taking as starting point condensed-matter-like systems? I will motivate why the interest in pursuing this program, mentioning some of the lessons that we can learn in the process.

In particular, I will briefly describe a proposal to search for observable quantum gravity deviations in gravitational collapse. Besides, I will give an overview of the problems that one has to face in trying to fulfill the program, sketching where the solutions to these problems might reside.

Patrick Dürr (University of Oxford): The Geometric Approach to Spacetime — A Functionalist Rehabilitation

The talk aims at turning the wheel once more in the dialectic between proponents of the geometrical approach (GA) to spacetime and Brown’s dynamical view (DA). More clearly than is customary in the prevailing literature, I distinguish between the numerous different explananda with which participants in the debate are concerned. This allows us to identify what advocates of the DA and the GA really disagree about – and what is common ground. I review the responses which the GA is standardly taken to proffer on the controversial points, as well as Brown’s critique of it. The philosophical suppositions about explanation and causality that undergird his critique are explicated and re-assessed. The DA is then presented as an alternative approach to spacetime which Brown introduces in face of what he perceives as the GA’s shortcomings. I critically examine the DA’s supposed virtues. I argue that it’s unconvincingly motivated, resting on dubious preconceptions. Some of its difficulties, which I point out, considerably detract from its attraction. But our engagement with the DA suggests how to refine the GA in a manner that addresses Brown’s objections. The result is a construal of spacetime-geometrical explanations as an instance of what Dasgupta calls “constitutive explanations“: They ground special-relativistic phenomena in universal spatiotemporal part-whole relations of four-dimensional objects. This web of universal part-whole relation, I submit, is best understood as a kinematic constraint in Curiel’s sense. This is shown to capture key concerns in the accounts of other proponents of the GA.

Michael Esfeld (University of Lausanne): Functionalism: reduction instead of emergence

This talk draws a distinction between two versions of functionalism: a trivial one according to which function merely signifies playing the role of something, and a substantial one in the sense of causal role for something. In the trivial sense, more or less any position can count as spacetime functionalism, including the Leibnizian one of fundamental distance relations that play the role of world-making relations. In the substantial sense, spacetime functionalism turns out to be a reductionism about spacetime rather than an emergentism, because the functional role that spacetime plays is defined on the basis of some other primitive notions and can be deduced from these notions. I will illustrate this claim in terms of Lewis-style functionalist reductionism.

Rasmus Jaksland (Norwegian University of Science and Technology): The Versatility of Spacetime

In philosophy of mind, it is a central tenet of functionalism about consciousness that consciousness is multiply realizable. This paper explores and defends multiple realizability of spacetime in the context of quantum gravity. It is observed that despite differences between string theory, loop quantum gravity, causal set theory, and tensor networks, spacetime occurs — in a more or less emergent form — in all of them: various dissimilar microstructures can realize spacetime. The paper defends that this is indeed an instance of multiple realizability and not just an artefact of the type of theories that we find interesting to pursue.
The claim is founded on two pillars: (1) the occurrence in all the mentioned theories of similar relations between spacetime (connectivity) and entanglement and (2) an argument to the effect that entanglement in multiply realizable based on the observation that entanglement is oblivious to the nature of degrees of freedom so long as they are quantum degrees of freedom.
Finally, the paper considers two objections to (1): (a) that entanglement is not a sufficient condition for spacetime and (b) that the relations between spacetime and entanglement are known to hold only for semi-classical gravity.

Eleanor Knox (King’s College): Spacetime Functionalism and Inertial Frame Functionalism

This talk will have two purposes: first, to outline spacetime functionalism and examine its limits and applications, and second, to defend a type of spacetime functionalism called inertial frame functionalism. I’ll make some comments about the relation between spacetime functionalism and debates about emergence and substantivalism, before moving on to defend inertial frame functionalism.

Vincent Lam (University of Geneva): Ontological and epistemological aspects of spacetime functionalism in quantum gravity

Many research programs in quantum gravity seem to suggest a radical picture according to which crucial spatio-temporal features (and possibly even space and time themselves) are not part of the fundamental physical ontology. This talk discusses the main ontological and epistemological worries that may arise in this context and how a functionalist perspective on the emergence of spacetime can help to alleviate these worries.

Baptiste Le Bihan (University of Geneva): Spacetime Functionalism: No Hard Problem of Spacetime?

I will focus on spacetime functionalism as a strategy that may be used to fulfill two tasks: first, to answer the epistemological problem of empirical coherence in quantum gravity (QG) and second, more generally, to analyze the relation obtaining between general relativity and QG, namely to account for spacetime emergence in a robust and metaphysical sense. I will clarify the different metaphysical pictures behind spacetime functionalism and argue that functionalism is an interesting and viable epistemologicalposition but is more problematic as ametaphysical view. Indeed, although spacetime functionalism provides an answer to the problem of empirical coherence (as has been argued by Lam and Wüthrich, manuscript) it fails to provide a comprehensive account of spacetime emergence. By drawing a comparison to the philosophy of mind, I will show that when applied to spacetime emergence, the functionalist strategy comes in different flavors, each entailing a different metaphysical picture of the world. Therefore, it does not offer a solution to the hard problem of spacetime, which is to interpret the nature of the relation obtaining between spacetime roles and spacetime realizers (in the functionalist framework). What is more, it remains consistent with deflationist, eliminativist, dualist and reductive alternative approaches of this relation.

Joanna Luc (Jagiellonian University): What spacetime does?

What is the function of spacetime? Is there a single answer to this question? How to attack it? My proposal is to take Special Relativity and General Relativity as paradigmatic cases of spacetime theories and make them our reference point. From the functionalist’s perspective, the identity of an object fully relies on its (functional) relations to other objects. In the case of spacetime, these ‘other objects’ can be particles and/or matter fields. Both of them in some sense change with respect to spacetime. Therefore, the most general heuristics is that the role of spacetime in SR and GR is to give the framework for the description of distribution and change of matter. This is in fact a family of roles, which can be classified into three groups: 1) spacetime provides elements needed to pose the problem of evolution of matter, 2) spacetime provides elements needed to describe properties of solutions of this problem, 3) spacetime provides elements needed to analyse general properties of the problem itself. I will argue that the first group has more fundamental status than the other two and is indispensable for the proper understanding of them

Tushar Menon (University of Oxford): On the function of spacetime

Spacetime functionalism is an important research programme in the field of quantum gravity, given the resources it has to tell a story about the appearance of spacetime even in theories which do not obviously include fundamental spatiotemporal degrees of freedom. But what exactly is the role of spacetime that the functionalist relies on? I contend that there are two distinct conceptions of spacetime at work in physics, based on two separate roles.  `Theoretical spacetime’ is the structured set of independent variables of a matter field theory whose structure reflects the dynamical symmetries of the matter field theory. `Operational spacetime’, on the other hand, involves more than just symmetry structure, and codifies the behaviour of certain types of stable matter configurations. In appealing only to inertial structure, Eleanor Knox’s brand of functionalism can only be guaranteed to reproduce theoretical spacetime, although in many cases, further contingent facts about matter dynamics ensure that it also reproduces operational spacetimes. In this talk, I argue that the extra structure required to guarantee the recovery of operational spacetime in matter field theories can be obtained on a slightly liberalised conception of Brown and Pooley’s `dynamical approach’.

James Read (University of Oxford): Euclidean spacetime functionalism? And other puzzles.

I present three cases which indicate that some nuancing of Knox’s inertial frame functionalism about spacetime is in order.

First, I consider theories with Euclidean symmetries. In such theories, Knox’s approach picks out a Euclidean metric field as being ‘spatiotemporal’. But is this reasonable, since there appears to be nothing temporal in such a metric field? I suggest that a further criterion is needed in order to pick out spacetime: that the structure in question draw an intrinsic distinction between space and time.
Second, I consider T-dual string theories. Here, when considering background fields in such theories, Knox’s criterion picks out one such field—a metric field—as spatiotemporal. While this might appear the correct verdict, Huggett musters arguments that this metric field cannot correspond to “phenomenal space”, because it does not pick out “three large dimensions”. This, I suggest, constitutes another condition for spatiotemporality, beyond defining a structure of inertial frames.
Third, I consider recent work which demonstrates that, in some solutions to general relativity, matter fields behave such as to preclude the construction of rods and clocks which correctly read off intervals as given by the metric field. Knox’s functionalism identifies the metric field in such cases as spatiotemporal; however, one might again have concerns regarding whether such is the correct verdict.
I close with some clarifications regarding how Knox’s spacetime functionalism aligns with Brown’s ‘dynamical approach’ to spacetime theories.