Living Reviews in Relativity: "The causal set approach to quantum gravity"
Surya, S., "The causal set approach to quantum gravity", Living Rev Relativ (2019) 22: 5. https://doi.org/10.1007/s41114-019-0023-1
Open Access | Review Article
First Online: 27 September 2019
"This is a very nice review of a subject that has badly needed a comprehensive review article. The causal set approach to quantum gravity may not be as developed as some of the others, because as the author writes it is a minimalistic approach and many of the technical tools used in other approaches are not available, and because of the smaller number of researchers actively involved in it. It is however viewed as an elegant and promising theory, and this review is a welcome addition to the literature on the subject."
The causal set theory (CST) approach to quantum gravity postulates that at the most fundamental level, spacetime is discrete, with the spacetime continuum replaced by locally finite posets or “causal sets”. The partial order on a causal set represents a proto-causality relation while local finiteness encodes an intrinsic discreteness. In the continuum approximation the former corresponds to the spacetime causality relation and the latter to a fundamental spacetime atomicity, so that finite volume regions in the continuum contain only a finite number of causal set elements. CST is deeply rooted in the Lorentzian character of spacetime, where a primary role is played by the causal structure poset. Importantly, the assumption of a fundamental discreteness in CST does not violate local Lorentz invariance in the continuum approximation. On the other hand, the combination of discreteness and Lorentz invariance gives rise to a characteristic non-locality which distinguishes CST from most other approaches to quantum gravity. In this review we give a broad, semi-pedagogical introduction to CST, highlighting key results as well as some of the key open questions. This review is intended both for the beginner student in quantum gravity as well as more seasoned researchers in the field.
Sumati Surya is a Professor of Theoretical Physics at the Raman Research Institute. She works in the area of classical and quantum gravity and has an associated interest in quantum foundations. Her primary focus for several years has been on the causal set approach to quantum gravity where the spacetime continuum is replaced by a locally finite partially ordered set. She is also interested in aspects of classical gravity related to Lorentzian geometry and causal structure.