The simple reason why classical gravity can entangle

TL;DR

The paper examines whether gravity coupled to quantum matter can be unambiguously identified as non-classical via gravity-induced entanglement (GIE). It analyzes the locality assumptions behind LOCC-like no-go theorems and clarifies that mediation—not relativistic locality—underpins these results, arguing that mediation is not a natural or universal feature of gravity. Consequently, the no-go theorems do not exclude classical gravity, though GIE experiments remain valuable for discriminating concrete gravity–quantum-matter theories by their quantitative predictions. The author contends that, even if GIE is not a universal theory-independent certificate of non-classical gravity, a measurement consistent with perturbative quantum gravity would constitute strong evidence for quantum gravity, making GIE experiments urgent and informative for guiding quantum gravity research.

Abstract

Ever since gravity-induced entanglement (GIE) experiments have been proposed as a witness of the quantum nature of gravity, more and more theories of classical gravity coupled to quantum matter have been shown to predict GIE, despite the existence of several theory-independent no-go theorems purportedly claiming that it should not be possible. This note explains why this is possible, and why this makes the GIE experiments an even more urgent matter in quantum gravity research.

Figures (3)

• Figure 1: The assumptions of the LOCC-like no-go theorems are often presented as locality and classicality of the gravitational field. However, this presentation hides some important details about the locality assumption. A better presentation is as in figure \ref{['fig:no-go-real']}.
• Figure 2: Two different notions of locality used in physics. Left, a depiction of a spatiotemporal notion of locality, where information travels within lightcones. Right, a depiction of subsystem, or circuit, locality, where each line represents a system, and the boxes represent operations acting on at most two systems at a time. The first is the notion of locality most salient to relativity and field theory, while the latter is native to information theory and quantum foundations. It is the subsystem notion of locality that is involved in the LOCC-like no-go theorems.
• Figure 3: A better representation of the assumptions of the LOCC-like no-go theorems. Statespace means that gravity can be assigned an independent statespace like in equation \ref{['statespace']}, mediation means the evolution of the two masses and gravity factorises as \ref{['mediation']}, classicality is that gravity has a classical statespace.