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The decades-long quest to understand how quantum physics and gravity mix may have just become even more complicated. It has long been thought that gravity may need to be quantum, a force carried by quantum particles called gravitons. But a new calculation suggests that quantum phenomena could arise from gravitational fields that aren’t fully quantum themselves.

Many physicists think that at the level of its smallest building blocks, our world is quantum. This means objects are made from quantum particles, and the forces that act on those objects – such as the strong and weak forces, the electromagnetic force and gravity – are carried by quantum particles. We know which particles carry the first three of those forces, but gravity – a fundamental force that stems from the very shape of our space-time – has so far resisted such quantisation.

To remedy this, researchers have been searching for gravitons, and experiments are being built to look for signs that gravity creates the inextricable and fundamentally quantum link between particles called entanglement.

and at Royal Holloway, University of London, have now found that the issue of gravity’s quantumness may not be so simple.

They began their mathematical analysis with a gravitational field – a map of gravitational force that any object that has mass would feel at any point in space. Crucially, the field they analysed was not quantum in nature. This field couldn’t assume a quantum superposition state in which it could simultaneously have more than one quantum property, and we couldn’t tell which is unambiguously its truth.

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Then, they plugged this non-quantum field into the full mathematical machinery of quantum field theory. When they used this framework to calculate how two objects with mass would interact, they found that sometimes, the gravitational field could give rise to an effect that made them quantum entangled.

Howl says this is because the two objects can exchange “virtual matter” that emerges from the gravitational field. This isn’t an oddity in the physics of fields – it is similar to an effect seen with electromagnetic fields, where objects exchange virtual particles of light, or virtual photons. These have the same effect that a real photon would, but they cannot be measured. But there is one big difference: the electromagnetic field is known to be quantum. Because Aziz and Howl formulated their gravitational field as non-quantum, and there are no gravitons in the picture, what does their finding then say about the quantumness of gravity?

“If we were to say whether gravity is quantum or classical, then I guess we would be saying that question doesn’t have a binary answer,” says Howl. In other words, even without being explicitly quantised, the gravitational field may not exclude all quantum phenomena.

The team’s calculations add nuance to how we ought to interpret experiments aiming to diagnose gravity’s quantum nature, says at the University of Vienna in Austria. “It says, look, if you do your experiment and you see entanglement when you let two particles interact gravitationally, there is actually a parameter regime within which there can be another explanation that is actually not based on any quantum gravity assumptions. That is a very, very nice insight,” he says.

This may be interpreted as a new way to think about gravity, but it could also be seen as a separate effect from gravity altogether. In this scenario, the gravitational field would merely influence the strength of the interaction but not directly cause it, says at University College London.

Similarly, at University College London says the exchange of virtual matter is qualitatively different from the exchange of virtual gravitons, which is the conventional view of how quantum gravity would work. In his view, the research has revealed a new way for massive bodies to interact, distinct from gravity.

The new effect would be most prominent for objects with masses much larger than those in experiments currently being constructed to probe fully quantum gravity. Bose, who has initiated some of those experiments, says there is no chance that this new effect will show up as a spurious signal there.

Additionally, Aspelmeyer says that for extremely small masses – for instance, microdiamonds that weigh only a quadrillionth of a kilogram – an explanation for entanglement through classical gravity is still not possible. Because of this, the new work doesn’t negate the need for a quantum theory of gravity, he says: “This door is not closed.”

Luckily, determining what lies behind it is on the horizon. At one point, it may have seemed that directly testing ideas about quantum gravity was impossible, but now concrete experiments stand a chance at offering real insights within a decade, says Aziz. “Interest in these experiments has motivated people to look more closely at the fundamental reasoning behind them,” he says.