Spacetime is as spacetime does

Abstract

Theories of quantum gravity generically presuppose or predict that the reality underlying relativistic spacetimes they are describing is significantly non-spatiotemporal. On pain of empirical incoherence, approaches to quantum gravity must establish how relativistic spacetime emerges from their non-spatiotemporal structures. We argue that in order to secure this emergence, it is sufficient to establish that only those features of relativistic spacetimes functionally relevant in producing empirical evidence must be recovered. In order to complete this task, an account must be given of how the more fundamental structures instantiate these functional roles. We illustrate the general idea in the context of causal set theory and loop quantum gravity, two prominent approaches to quantum gravity.

Spacetime functionalism from a realist perspective

“…In forthcoming work, David Yates challenges the idea that at least a very natural form of spacetime functionalism really helps in the case of the disappearance of spacetime in fundamental QG as we have argued in earlier work (Lam and Wüthrich 2018). He poses a dilemma for the realizer form of spacetime functionalism, at least insofar as it denies the existence of nonfundamental spacetime.…”

“…We proceed on the presupposition that spacetime disappears from the fundamental ontology, as is strongly suggested by current research in QG. In this case, we argued in Lam and Wüthrich (2018), spacetime functionalism becomes an indispensable element of understanding how spacetime emerges.…”

“…20 The issue in the QG context is about the emergence of spacetime from a possibly non-spatiotemporal level. In a previous article (Lam and Wüthrich 2018), we have articulated in some detail how a functionalist perspective can alleviate the conceptual difficulties specifically related to the emergence of spacetime from a QG level where it may be absent. In other words, given that spacetime may not be part of the ontological picture sketched by some of the main research programs in QG, functionalism provides conceptual tools to connect such an ontology without spacetime to the ordinary spacetime picture of the rest of classical (and, to some extent, quantum) physics.…”

“…Spacetime is as spacetime does was the slogan at which we arrived in a recent article (Lam and Wüthrich 2018) in which we argued that spacetime functionalism offers the most promising template to understanding the emergence of spacetime in the context of quantum gravity (QG), the domain of research aiming at a theory of gravity which incorporates both quantum and relativistic effects. The central tenet of spacetime functionalism is that spacetime, qua ontological posit of a theory, is dispensable if the structures postulated by that theory are sufficient to execute all the relevant functions which would otherwise be performed by spacetime itself.…”

The Emergence of Space and Time

“…Against these detractors, Lam and Wüthrich (2017) argue that there remains no task left undone once a research programme can show "how the fundamental degrees of freedom can collectively behave such that they appear spatiotemporal at macroscopic scales in all relevant and empirically testable ways" (10, emphasis in original). If this is right, then however counterintuitive a fundamentally non-spatiotemporal world may appear, it arises as a live possibility-one perhaps borne out in contemporary fundamental physics.…”