On the quantum nature of strong gravity

TL;DR

This paper argues that quantum fluctuations in gravitational waves emitted by strong gravity sources prevent superluminal signaling, implying that gravitational radiation must be quantized to be consistent with quantum mechanics.

Contribution

It reformulates a thought experiment to show that quantum fluctuations in gravitational waves prevent signaling, supporting the necessity of quantizing gravitational radiation.

Findings

Quantum fluctuations in gravitational waves prevent signaling.

Rotating black holes behave as extended quadrupolar objects in the Newtonian limit.

Quantization of gravitational radiation is required for consistency with quantum mechanics.

Abstract

Belenchia et al. [Phys. Rev. D 98, 126009 (2018)] have analyzed a gedankenexperiment where two observers, Alice and Bob, attempt to communicate via superluminal signals using a superposition of massive particles dressed by Newtonian fields and a test particle as field detector. Quantum fluctuations in the particle motion and in the field prevent signaling or violations of quantum mechanics in this setup. We reformulate this thought experiment by considering gravitational waves emitted by an extended quadrupolar object as a detector for Newtonian tidal fields. We find that quantum fluctuations in the gravitational waves prevent signaling. In the Newtonian limit, rotating black holes behave as extended quadrupolar objects, as consequence of the strong equivalence principle. It follows that consistency of the Newtonian limit of general relativity with quantum mechanics requires the quantization of gravitational radiation, even when the waves originate in strong gravity sources.