Quantum Gravito-Optics: A Light Route from Semiclassical Gravity to Quantum Gravity

TL;DR

This paper proposes a novel approach called quantum gravito-optics, linking semiclassical gravity and quantum gravity through the quantum nature of gravitational waves, and suggests observable quantum effects in gravitational wave detection.

Contribution

It introduces the concept of quantum gravito-optics, establishing an equivalence between quantum gravity and semiclassical theory in the radiation sector for observable gravitational waves.

Findings

Gravitational waves in detectors are fully quantum states of radiation.

The equivalence of quantum gravity and semiclassical theory holds in the radiation sector.

Detection of waves from binary systems aligns with quantum optical analogies.

Abstract

Quantum gravity remains an elusive theory, in spite of our thorough understanding of the quantum theory and the general theory of relativity separately, presumably due to the lack of any observational clues. We argue that the theory of quantum gravity has a strong constraining anchor in the sector of gravitational radiation ensuring reliable physical clues, albeit in a limited observable form. In particular, all types of gravitational waves expected to be observable in LIGO-like advanced detectors are fully quantum mechanical states of radiation. Exact equivalence of the full quantum gravity theory with the familiar semiclassical theory is ensured in the radiation sector, in most real situations where the relevant quantum operator functions are normal ordered, by the analogue of the optical equivalence theorem in quantum optics. We show that this is indeed the case for detection of the waves from a massive binary system, a single gravitational atom, that emits coherent radiation. The idea of quantum-gravitational optics can assist in guiding along the fuzzy roads to quantum gravity.