Removing Classical Inputs from Quantum Gravity Tests
Is gravity quantum or classical? This fundamental question is tackled in proposals for tabletop experiments that seek gravity-induced quantum effects. But these effects, if observed, could be explained without invoking a quantum description of the gravitational field. Now Lin-Qing Chen from IQOQI Vienna and Flaminia Giacomini from the Swiss Federal Institute of Technology (ETH) Zurich have used a low-energy quantum gravity model to devise an experimental protocol that could probe the quantumness of the gravitational field [1].
One way to test the quantum nature of gravity is to place a small, massive object in a quantum superposition of two positions, say, “left” and “right,” as would result from passing the object through a kind of double slit. Bringing this two-position object close to another two-position object causes them to interact gravitationally, potentially revealing quantum correlations, so-called gravitationally induced entanglement.
However, the predicted correlations could appear even if the gravitational field is classical. To devise a genuine test, Chen and Giacomini propose starting with a linearized quantum gravity model, which is a low-energy approximation that is expected to be consistent with other theories (such as string theory and loop quantum gravity). Within this quantum gravity framework, the researchers imagine two test objects that are “delocalized”—that is, their possible positions are spread out over a range rather than being limited to two locations. In this case, the researchers find two effects—additional features in the correlation signal—that would appear only if the gravitational field is quantized rather than classical. The researchers admit that these types of experiments remain extremely challenging, but several groups are working in this direction (see Synopsis: Unleashing the Quantumness of a Nanoparticle).
Correction (5 September 2025): A previous version of this story misrepresented previous proposals for experiments, suggesting that they could be interpreted classically. In fact, only the underlying field framework tested in such experiments would be open to a nonquantum interpretation.
–Michael Schirber
Michael Schirber is a Corresponding Editor for Physics Magazine based in Lyon, France.
References
- L.-Q. Chen and F. Giacomini, “Quantum effects in gravity beyond the Newton potential from a delocalized quantum source,” Phys. Rev. X 15, 031063 (2025).