The question of the quantum nature of gravity remains one of the sharpest in fundamental physics. The effective quantum field theory of gravity works well in weak fields, but its applicability to strong fields, for instance near black holes, remains an open question. This work belongs to a line of research that uses thought experiments to expose the logical requirements for quantizing gravity. Related studies examine quantum fluctuations of gravitational waves, gravitational decoherence, and quantum limits on measurement, but this paper is the first to carry those arguments over to the case of strong fields.
The authors modify the thought experiment of Belenchia and co-authors. Alice prepares a binary system of masses in a superposition of two orientations, creating a tidal field. Instead of a test particle, Bob uses a spinning black hole, which in the Newtonian limit behaves as an extended quadrupole object. The motion of the black hole under the influence of Alice's tidal field generates gravitational waves. Bob measures these waves, trying to obtain information about Alice's state. The key step is accounting for quantum fluctuations of the gravitational radiation within the effective field theory.
It is established that quantum fluctuations of the gravitational waves emitted by the moving black hole prevent Bob from distinguishing Alice's states over a spacelike separation. The decoherence function, which characterizes the overlap of coherent states of the gravitational waves, is bounded from above by a quantity that tends to zero when the conditions of spacelike separation and slow preparation of the superposition are met. This means that no information travels faster than light and the complementarity principle is not violated. Consequently, quantizing gravitational radiation is necessary for the consistency of quantum mechanics and general relativity even in strong fields.
The work extends arguments about the inevitability of quantizing gravity to the regime of strong fields. Unlike earlier studies, which used test particles or weak gravitational fields, here the detector is a spinning black hole, an object with strong gravity. This is a first step toward proving that quantum effects of gravitational radiation must also show up in astrophysical sources, such as merging black holes.
The topic could develop toward the study of quantum states of gravitational waves from realistic astrophysical sources. In particular, nonlinearities in the oscillations of black holes (quasinormal modes) may lead to nonclassical states of radiation, such as squeezed states or entanglement. This opens a path to searching for quantum signatures in gravitational-wave data.
The results will affect fundamental physics, gravitational-wave astronomy, and quantum information theory.
The next step could be a quantitative estimate of quantum corrections to gravitational waves from black holes and the development of methods to detect them.
The work is directly connected to the problem of quantizing gravity and to the question of whether quantum effects show up in strong gravitational fields. It also touches on the problem of decoherence in the presence of event horizons.
The thought experiment features observers named Alice and Bob, the traditional characters of quantum cryptography and information theory.
The idea of using black holes as detectors of quantum states echoes Liu Cixin's novel The Three-Body Problem, where gravitational waves are used for faster-than-light communication.