Coherent State Description of Gravitational Waves from Binary Black Holes

TL;DR

The paper tackles whether gravitational waves from binary black holes can be encoded as quantum states of gravitons, deriving a coherent-state description for the inspiral in the transverse-traceless gauge. The leading-order interaction reproduces the classical waveform as $\langle h_{ij} \rangle$, while quadratic terms generate squeezed graviton states with a small squeezing parameter of order $\zeta \sim 10^{-4}$ for GW150914. The authors argue that decoherence during propagation is negligible, so the detector signal reflects the source state, and they discuss experimental probes such as Hanbury Brown–Twiss interferometry to detect nonclassicality. They also note that inflationary primordial GWs could further enhance squeezing, linking astrophysical observations to early-universe physics.

Abstract

Quantum mechanics is the fundamental framework of nature, and gravitational waves from binary black holes during the inspiral phase should likewise be analyzed quantum mechanically. It is commonly assumed that their classical description corresponds to a coherent state, so any deviation would signal genuinely quantum nature of gravity. We show that the coherent-state description reproduces classical gravitational waves at leading order, while next-order effects generate squeezed states of gravitons. For GW150914, we estimate the squeezing parameter to be $\sim 10^{-4}$. We find that gravitational waves from binary black holes are well described by a coherent state.