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We may finally have a way to detect the quantum nature of gravity.

The question of how gravity and quantum mechanics fit together has been one of the biggest problems in physics for decades. The way that quantum fluctuations affect gravitational waves 鈥� ripples in space-time caused by the movements of massive objects 鈥� may give physicists a way to solve it.

Gravity is the one realm of physics that doesn鈥檛 currently fit into a quantum mechanical understanding for the universe. 鈥淥ur fundamental physical theory is currently incoherent: it is made up of two parts that do not fit,鈥� says at Aix-Marseille University in France, who wasn鈥檛 involved in this work. 鈥淭o have a coherent world picture we need to combine the two halves.鈥�

There has been a lot of theoretical work on this problem, but observations and experiments have yet to make a dent in it. This is mainly because the energy levels at which quantum effects on gravity鈥檚 behaviour would be apparent are extraordinarily high. One place we find those high energy levels are in astronomical events that produce gravitational waves.

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

Now, at Arizona State University and his colleagues have calculated that the existence of gravitons could create jitters in gravitational wave signals. They found that these could, theoretically, be detected with current gravitational wave observatories.

鈥淢aybe the quantum nature of gravity is not so out of reach, and maybe there is an experimental signature of it,鈥� says Parikh. 鈥淥ur prediction is that there鈥檚 a kind of noise, a graininess, to gravity 鈥� and the features of that noise depend on the quantum state of the gravitational field.鈥�

It could be distinguishable from other sources of noise, such as a truck driving past a detector, by the fact that it is likely to manifest as exactly the same fluctuations in the signal at multiple detectors simultaneously.

Observing this noise would be proof that gravity is a quantum force. 鈥淲e have all reasons to believe that this is the case,鈥� says Rovelli. 鈥淏ut in science we want hard empirical tests, not just 鈥榬easons to believe鈥�.鈥�

Parikh and his colleagues are now modelling what quantum noise would look like in real-life gravitational wave detections from astronomical events, such as merging black holes or neutron stars, so that we know what to look for. Finding this signal and proving that gravity is a quantum phenomenon would be a major step towards unifying gravity and quantum mechanics.

Because 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 in physics.

鈥淭he whole story of gravity is actually the story of space and time,鈥� says Parikh. 鈥淚n a theory of everything, we would expect space and time and matter to be one in a sense, and observing this would be a step towards proving that.鈥�

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