Quantum Gravity: Meaning and Measurement

TL;DR

This paper explores the conceptual foundations of quantum gravity, emphasizing the importance of measurement conditions, and advocates for a flexible, domain-specific approach to quantization, highlighting the significance of conformal and projective structures.

Contribution

It proposes an ecumenical approach to quantum gravity, emphasizing the role of measurement and advocating for a unimodular group-based formulation of general relativity.

Findings

Multiple valid approaches to quantization should be judged by their applicability.

The relation between different approaches can be a subject of study.

Gravitational radiation studies may benefit from retrodictive methods.

Abstract

A discussion of the meaning of a physical concept cannot be separated from discussion of the conditions for its ideal measurement. We assert that quantization is no more than the invocation of the quantum of action in the explanation of some process or phenomenon, and does not imply an assertion of the fundamental nature of such a process. This leads to an ecumenical approach to the problem of quantization of the gravitational field. There can be many valid approaches, each of which should be judged by the domain of its applicability to various phenomena. If two approaches have overlapping domains, the relation between them then itself becomes a subject of study. We advocate an approach to general relativity based on the unimodular group, which emphasizes the physical significance and measurability of the conformal and projective structures. A discussion of the method of matched asymptotic expansions, and of the weakness of terrestrial sources compared with astrophysical and cosmological sources, leads us to suggest theoretical studies of gravitational radiation based on retrodiction (observation) rather than prediction (experimentation).