Quantum Machian Time in Toy Models of Gravity

TL;DR

This paper explores relational particle models inspired by General Relativity to understand the Problem of Time in quantum gravity, demonstrating how constraints like the Mach constraint lead to a temporally relational theory.

Contribution

It introduces a novel approach to best-matching time invariance in toy models, linking relational constraints to the emergence of time in quantum gravity.

Findings

The Mach constraint removes background dependence.

Relational models can interpolate between absolute and relational time.

Quantization reveals how time may emerge in quantum gravity.

Abstract

General Relativity on closed spatial topologies can be derived, using a technique called "best-matching", as an evolving 3-geometry subject to constraints. These constraints can be thought of as a way of imposing temporal and spatial relationalism. The same type of constraints can be used in non-relativistic particle models to produce relational theories that suffer from the same Problem of Time as that encountered in General Relativity. As a result, these simple toy models are well suited for studying the Problem of Time in quantum gravity. In this paper, a version of these particle models is studied where we "best-match" the time translational invariance of the theory. Using insights gained from this procedure, we can move back and forth between absolute and relational time by changing the way in which the relational fields are varied. We then proceed to quantize this theory using Dirac and path integral quantizations. We discover that one of the constraints of the theory, which we call the Mach constraint, is responsible for removing the dependence of the theory on a background structure. It is this Mach constraint that is responsible for making the theory temporally relational. Because of the deep relationship between these models and General Relativity, this work may shed new light on the Problem of Time in quantum gravity and how one might expect time to emerge on quantum subsystems of the universe.