On 6 November, the last day of the PSA, the symposium on the emergence of spacetime in quantum theories of gravity which I co-organized with Nick Huggett will take place. Symposiast will also include, apart from Nick and myself, Sean Carroll and Tim Maudlin. I am very much looking forward to that!
Here are the abstract of our talks:
Christian Wüthrich: A priori vs. a posteriori non-fundamentality of spacetime
A surprisingly large number of competing approaches to quantum gravity either assume or conclude that the smooth spacetime structure of classical spacetime theories evaporates at the more fundamental quantum level. Typically, it is replaced by a discrete structure very unlike the spacetime we know and love. An example of an approach which posits, in an a priori fashion, the discreteness of the fundamental structure are the so-called causal sets. Loop quantum gravity is a prominent representative of the a posteriori camp which contains theories that arrive, by means of their own resources, at the conclusion that the underlying quantum structure is discrete. This talk shall explicate this basic dialectic and contrast the significance of the fundamental discreteness and the role played by the re-emergence of the classical spacetime structure in both the top-down and the bottom-up approaches.
Nick Huggett: Emergence and dualities in string theory
String theoretic claims of emergence are tied to `dualities’ between different theories. The simplest example `T-duality’, an expectation-value preserving isomorphism between the states and observables of string theories in spaces of radius R and 1/R (in suitable units). Such results have produced claims that there is no fact of the matter about the correct geometry, that it is merely something encoding observations of stringy matter. However, such arguments are structurally similar to traditional conventional arguments, potentially running into familiar objections and counter-moves; and, at least topologically, strings are still spatial, so T-duality does not realize full-blown emergence.
Therefore, this presentation will be devoted to an even more striking duality, between gauge theories in `noncommutative’ spaces (which do not correspond to classical spacetimes at all) and string theory. I will investigate the proposal that spacetime emerges, via string theory, from such an underlying–spacetime-less–theory. The main goals are to understand what `emergence’ means in this context and to illustrate how it might work. In the more critical part of the talk I will focus on two questions. First, in what ways does the idea agree with or differ from `emergence’ in the philosophical literature? Physicists typically have a weaker notion in mind than philosophers, along the lines of phenomenal reduction. Second, to understand why string theorists are interested in emergence I will emphasize that standard formulations of the theory problematically require a background spacetime of determinate metric. The existence of a duality with a theory without a background spacetime, is supposed to address that problem, by showing that the background is unnecessary–a key component of the notion of `emergence’ in string theory. To what extent does this notion fit or diverge with other philosophical and physical conceptions of emergence?
Sean Carroll: The future of time
Scientists and philosophers have long wondered whether time, space, or spacetime are necessary features of a fundamental description of nature. The unsolved nature of quantum gravity makes this problem especially pressing, while helping to obscure the eventual answer. In this talk I will argue that the most conservative option is to imagine that time truly is fundamental, while space is emergent. This view stems from the basic structure of quantum mechanics as well as some recent ideas in string theory. While this viewpoint is certainly speculative, it does lead to some interesting conclusions about the Hilbert space of the universe and the origin of the arrow of time.
Tim Maudlin: Can space-time emerge?
The notion that space-time itself could `emerge’ from non-spatio-temporal structure presents twinned conceptual and methodological problems. Methodologically, dynamical laws have always been couched in terms of a spatio-temporal setting. If the `underlying’ ontology has no similar structure, it is unclear how the laws are to be framed. And if it does, it is unclear in what sense it fails to be spatio-temporal. Special and General Relativity already provide examples of theories according to which space-time structure is unlike that postulated by classical physics, but we do not judge that they eliminate space-time. Furthermore, space-time descriptions have provided the locus of contact between the theoretical descriptions provided by a theory and empirical evidence. It nothing like space-time structure is postulated by a theory, one would need a completely novel account of how the theory makes contact with observational data.