The gravity, explained in a simple way by Newton then a few centuries later in a more complete way by Einstein, may also have a quantum nature. Einstein's theory of relativity, presenting space-time as a “modelable fabric” drawing the force of gravity, may therefore not be sufficient to explain all the facets of this fascinating natural mechanics. Recently, a team of researchers has been studying gravitational waves - generated by the collision of black holes or neutron stars - in the hope of detecting and proving the quantum component of gravity.

The question of how gravity and quantum mechanics fit together has been one of the greatest lines of research in physics for decades. How quantum fluctuations affect gravitational waves (ripples in space-time caused by the movement and collision of massive objects) could provide physicists with a way to solve this mystery.

Gravity is the only area of physics that is not currently part of a quantum mechanical understanding of the universe. “Our fundamental physical theory is currently inconsistent: it is made up of two parts that do not go together,” explains Carlo Rovelli, from the University of Aix-Marseille, France, who did not participate in this work. "To have a coherent picture of the world, we have to combine the two halves."

### Gravitons: their effect could be detectable in gravitational waves

Much theoretical work has been devoted to this problem, but observations and experiments have not yet made it possible to solve it. This is mainly due to the fact that the energy levels at which the quantum effects on the behavior of gravity would be apparent are extraordinarily high. These high energy levels are found in particular in astronomical events that produce gravitational waves.

Waves produced by quantum fields, like light, are by nature both waves and particles. So if gravitational fields are indeed quantum fields, gravitational waves should also behave like particles. These hypothetical particles are called gravitons.

Maulik Parikh of Arizona State University and his colleagues calculated that the existence of gravitons could create disturbances in gravitational wave signals. They discovered that these could, in theory, be detected by current gravitational wave observatories such as LIGO and VIRGO.

“Maybe the quantum nature of gravity is not that out of reach, and maybe there is an experimental signature of it,” Parikh says. "Our prediction is that there is some kind of noise, interference, in gravity - and the characteristics of that noise depend on the quantum state of the gravitational field."

### Quantum gravity: unifying quantum mechanics and general relativity

It could be distinguished from other sources of noise by the fact that it is likely to manifest itself by exactly the same fluctuations of the signal in several detectors simultaneously. The observation of this noise would be the proof that gravity has a quantum component. “We have every reason to believe that is the case,” says Rovelli.

Parikh and his colleagues are currently modeling what quantum noise would look like in actual detections of gravitational waves from astronomical events, such as the fusion of black holes or neutron stars, so they know what to look for. . The discovery of this signal and the proof that gravity is a phenomenon at least in part quantum would constitute a major step towards the unification of general relativity and quantum mechanics, a research effort which one calls "quantum gravity".

Since gravity is a feature of all space-time, and quantum mechanics describes matter, this would bring us closer to a self-consistent theory of everything physics-related. “The whole story of gravity is actually the story of space and time,” Parikh says. "In a theory of everything, we would expect space, time and matter to be one in a sense, and observing this phenomenon would be a step towards proving that."

## Reference:

Quantum Mechanics of Gravitational Waves by Maulik Parikh, Frank Wilczek, and George Zahariade Phys. Rev. Lett. 127, 081602 – Published 19 August 2021 DOI: https://doi.org/10.1103/PhysRevLett.127.081602