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

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