Hints towards the Emergent Nature of Gravity

TL;DR

This paper explores the idea that gravity and spacetime might emerge from microscopic structures, reviewing evidence and arguments supporting this hypothesis within quantum gravity research.

Contribution

It provides a systematic assessment of the main arguments and evidence supporting the emergent gravity hypothesis, clarifying its theoretical foundations.

Findings

The metric's universal coupling to matter supports emergent gravity.

Black hole thermodynamics suggests underlying microstructure.

Holographic principle indicates a microscopic origin of spacetime.

Abstract

A possible way out of the conundrum of quantum gravity is the proposal that general relativity (GR) is not a fundamental theory but emerges from an underlying microscopic description. Despite recent interest in the emergent gravity program within the physics as well as the philosophy community, an assessment of the theoretical evidence for this idea is lacking at the moment. We intend to fill this gap in the literature by discussing the main arguments in favour of the hypothesis that the metric field and its dynamics are emergent. First, we distinguish between microstructure inspired from GR, such as through quantization or discretization, and microstructure that is not directly motivated from GR, such as strings, quantum bits or condensed matter fields. The emergent gravity approach can then be defined as the view that the metric field and its dynamics are derivable from the latter type of microstructure. Subsequently, we assess in how far the following properties of (semi-classical) GR are suggestive of underlying microstructure: (1) the metric's universal coupling to matter fields, (2) perturbative non-renormalizability, (3) black hole thermodynamics, and (4) the holographic principle. In the conclusion we formalize the general structure of the plausibility arguments put forward.