A new test for the quantum nature of gravity

Our physical models describe the world well - actually too well. Because one thing is certain: the current theories are not correct. For a world formula that describes phenomena at both the smallest and largest scales, the four known fundamental forces would have to be unified. So far, this has been achieved for three of them: the electromagnetic, weak and strong nuclear forces have been given a unified framework by quantum field theories, which describes the world of the subatomic excellently. However, gravity has so far eluded this formalism. One of the most pressing questions in fundamental physics is therefore whether gravity, like the other fundamental forces, has a quantum physical core at all - or whether it is fundamentally different from all other forces.

For several years, physicists have been working on experiments to answer this question. The most common idea for such an experiment is to connect two massive objects using a quantum mechanical effect: to create what is known as "entanglement" between the masses. However, this proves to be a mammoth task. Such correlated states are very sensitive; moreover, gravity is the weakest of the four fundamental forces, which is why a great deal of precision would be required for such an experiment. Until now, realisation seemed out of reach. Now a team led by physicist Ludovico Lami from the University of Amsterdam has presented an experimental idea that could test the quantum nature of gravity in a completely new way.

As the researchers explain in their paper published in May 2024 in the journal "Physical Review X", such an experiment would not require entanglement between two masses. Instead, they describe a setup with two dumbbell-shaped objects that weigh less than one gram. The two dumbbells are suspended parallel to each other so that they can each swing freely. The distance between them is chosen so that they feel their mutual gravitational attraction, but the Casimir force does not come into play. Shielding between the two objects is also intended to prevent electromagnetic or other forces from influencing the experiment.

A pendulum with two dumbbells

Lami and his team have investigated how the dumbbell-shaped objects swing if gravity really does have a quantum nature. As the researchers discovered, the movement of the dumbbells could be stopped by a minimal force. This means that one would only have to observe the oscillations of the tiny masses: If the objects stand still after the exerted force has been applied, gravity has a quantum physical core. However, such a measurement is extremely difficult to realise: The slightest vibrations in the environment would falsify the experiment.

For this reason, the researchers propose repeating the experiment extremely often and using lasers directed at the dumbbell-shaped objects to detect even the smallest displacements. If the frequency with which the objects are found to be stationary at the end of the procedure exceeds a certain threshold value, the experts write that quantised gravity can be assumed. To calculate this threshold, they had to "introduce and refine complicated mathematical techniques from quantum information theory", said Lami to the American Physical Society.

The experiment described is so far only a theoretical construct. Even if the necessary technologies are already available, the realisation of the experiment could pose unexpected practical problems. Which experimental setup will ultimately make it possible to fathom the true nature of gravity will be decided in a laboratory - and not on a piece of paper.

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