Spacetime and the Philosophical Challenge of Quantum Gravity

TL;DR

This paper explores the philosophical challenges of quantum gravity, focusing on how it questions the classical treatment of spacetime as a 4D manifold, and discusses various research approaches and their limitations.

Contribution

It provides a philosophical survey of quantum gravity research, highlighting how it challenges traditional notions of spacetime and proposing directions beyond metric quantization.

Findings

Quantum gravity questions the classical spacetime concept.

Main research programs accept modified metrics but face fundamental limitations.

Potential future directions include quantizing topology or treating structures phenomenologically.

Abstract

We survey some philosophical aspects of the search for a quantum theory of gravity, emphasising how quantum gravity throws into doubt the treatment of spacetime common to the two `ingredient theories' (quantum theory and general relativity), as a 4-dimensional manifold equipped with a Lorentzian metric. After an introduction, we briefly review the conceptual problems of the ingredient theories and introduce the enterprise of quantum gravity We then describe how three main research programmes in quantum gravity treat four topics of particular importance: the scope of standard quantum theory; the nature of spacetime; spacetime diffeomorphisms, and the so-called problem of time. By and large, these programmes accept most of the ingredient theories' treatment of spacetime, albeit with a metric with some type of quantum nature; but they also suggest that the treatment has fundamental limitations. This prompts the idea of going further: either by quantizing structures other than the metric, such as the topology; or by regarding such structures as phenomenological. We discuss this in Section \ref{Sec:TowardsQuST}.