On the quantum nature of strong gravity

TL;DR

The paper extends the argument for the quantum nature of gravity from weak-field, linearized regimes to strong-field contexts by analyzing gravitational waves emitted by the motion of extended quadrupolar objects (notably rotating black holes) as detectors of Newtonian tidal fields. Using an effective field theory description of Einstein gravity, it shows that GW quantum fluctuations induce decoherence or indistinguishability constraints that prevent faster-than-light signaling and preserve both quantum mechanics and general relativity in spacelike separated experiments. The results imply that quantization of gravitational radiation is required across regimes, and they point toward observable quantum signatures in strong-field gravitational waves via BH dynamics and quasinormal mode excitations. Taken together, the work strengthens the case that gravity must be quantized and that strong-gravity systems may harbor detectable quantum gravitational phenomena with potential implications for gravitational-wave astronomy.

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.

Figures (3)

• Figure 1: Schematic representation of the Belenchia et al. setup, adapted from belenchia2018quantum.
• Figure 2: Modified version of the gedankenexperiment, where we consider tidal fields produced by Alice's binary mass system placed in the $xy$ plane. The binary mass system is prepared in a superposition of distinct orientations $\vert \pm \rangle$, corresponding to angles $\pm \psi$ with respect to the $x$ axis. To detect the superposition, Bob measures gravitational waves emitted by the motion of a rotating black hole at position $\mathbf{x} = (b,\theta,\phi)$, with quadrupole moment $Q_{ij}^{B}(t)$. Depending on Alice's tidal field, the waves are emitted in coherent states $\vert \alpha^{\pm} \rangle$; Bob can obtain which-path information on Alice's state when $\vert\langle \alpha^{+}\vert\alpha^{-}\rangle\vert \approx 0$.
• Figure 3: Plot of $\gamma = \gamma(\theta,\phi)$ as a function of Bob's polar and azimuthal angles $\theta,\phi$, for fixed orientation of Alice's superposition state $\psi = \pi/4$; left: lateral view, right: top view.