On the quantum nature of strong gravity

Abstract Belenchia et al. [A. Belenchia et al, PRD 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 signalling. In the Newtonian limit, rotating black holes behave as extended quadrupolar objects, as a 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.