New Study: Gravity May Entangle Matter — Even If Gravity Isn't Quantum

Can a classical gravitational field produce quantum-like entanglement?

A recent theoretical paper by Joseph Aziz and Richard Howl (Royal Holloway, University of London) argues that a classical gravitational field, when coupled to quantum matter, could produce entanglement-like correlations without the gravitational field itself being fundamentally quantum. The result refines a long-standing thought experiment and forces a closer look at what experimental signatures would truly indicate a quantum theory of gravity.

Why this matters

The search for a consistent theory of quantum gravity aims to unite quantum mechanics (the physics of the very small) with general relativity (the physics of spacetime and gravity). For decades these frameworks have resisted unification: quantum theory describes forces in terms of discrete quanta (for example, photons for electromagnetism), while Einstein's general relativity describes gravity as the curvature of spacetime. Detecting unmistakable quantum behavior in gravity would be a major step toward resolving that tension.

Feynman's thought experiment

Aziz and Howl build on a famous idea first suggested by Richard Feynman in 1957. Feynman imagined placing a macroscopic mass—an apple, say—into a spatial quantum superposition so it exists simultaneously in two locations. If a nearby test mass were influenced differently by each branch of the superposition, and if the two masses became entangled through their gravitational interaction, one might infer that the gravitational field itself had quantum properties.

"When Feynman proposed this idea...he believed that it would mean that gravity is quantum," Howl told Space.com.

Aziz and Howl's proposal

Aziz and Howl show, via quantum-field-theory calculations, that entanglement-like correlations can arise even when the gravitational field remains classical. Their idea is that the classical spacetime curvature couples to quantum matter fields, and short-lived quantum fluctuations of the matter fields—what they describe as "virtual matter processes" or "virtual atoms"—can mediate correlations between the masses. In effect, the classical gravitational interaction can tap into quantum processes in matter and produce quasi-entanglement without invoking gravitons.

The authors emphasize that these classical-gravity-induced correlations are quantitatively much weaker than the correlations expected if gravity were fundamentally quantum and mediated by virtual gravitons. In practice, that means a strong, robust entanglement signal would still be strong evidence for quantum gravity, while a weak signal could in principle be explained by the mechanism Aziz and Howl describe.

Gravitons, virtual particles and measurable differences

In quantum descriptions of forces, interactions are carried by quanta—photons for electromagnetism, and hypothetical gravitons for gravity. No individual graviton has been detected, and virtual gravitons are invoked in quantum calculations as temporary, unobservable excitations. Aziz and Howl's work shows that virtual excitations of the matter fields can substitute for virtual gravitons in producing correlations when gravity itself is treated classically.

Crucially, the different origins of correlations lead to different statistical signatures. For example, in a pair of spins entangled by a quantum mediator one expects strong, repeatable correlations (if one spin is up, the other reliably reads down). The classical-mediated case predicts weaker correlations and altered probability distributions across repeated measurements, offering a possible, if challenging, way to discriminate the two scenarios experimentally.

Experimental challenges and outlook

Turning Feynman's thought experiment into a laboratory test remains extremely difficult. The experiment requires preparing macroscopic masses in long-lived quantum superpositions and eliminating all sources of decoherence. "It's still an open question as to whether you could do it," Howl admits. "There's nothing to say you can't do it in theory...but you have to eliminate all decoherence and it is an incredibly difficult challenge." Ongoing experiments in the U.K., Austria and elsewhere are attempting related feats, but practical realization will take time.

The Aziz–Howl result does not disprove quantum gravity. Instead, it adds nuance: some entanglement-like signals previously taken as definitive evidence for quantum gravity might, in principle, be mimicked by classical gravity coupled to quantum matter—albeit at a much weaker level. Other work has also explored hybrid models that mix classical gravity with quantum fields; for example, Jonathan Oppenheim (UCL) published a related model in 2023.

Howl expects debate and scrutiny: "I don't know if everyone is going to agree with us!" he says. Whether or not the community accepts the proposal, the paper sharpens the experimental and conceptual criteria for claiming a discovery of quantum gravity.

Publication: Aziz and Howl, Nature, Oct. 22.