Mediterranean neutrino observatory sets new limits on quantum gravity

Quantum gravity is the missing link between general relativity and quantum mechanics, the yet-to-be-discovered key to a unified theory capable of explaining both the infinitely large and the infinitely small. The solution to this puzzle might lie in the humble neutrino, an elementary particle with no electric charge and almost invisible, as it rarely interacts with matter, passing through everything on our planet without consequences.

For this very reason, neutrinos are difficult to detect. However, in rare cases, a neutrino can interact, for example, with water molecules at the bottom of the sea. The particles emitted in this interaction produce a “blue glow” known as Čerenkov radiation, detectable by instruments such as KM3NeT.

The KM3NeT (Kilometer Cube Neutrino Telescope) is a large underwater observatory designed to detect neutrinos through their interactions in water. It is divided into two detectors, one of which, ORCA (Oscillation Research with Cosmics in the Abyss), was used for this research. It is located off the coast of Toulon, France, at a depth of approximately 2,450 meters.

However, merely observing neutrinos is not enough to draw conclusions about the properties of quantum gravity—we must also look for signs of “decoherence.”

As they travel through space, neutrinos can “oscillate,” meaning they change identity—a phenomenon scientists refer to as flavor oscillations. Coherence is a fundamental property of these oscillations: a neutrino does not have a definite mass but exists as a quantum superposition of three different mass states. Coherence keeps this superposition well-defined, allowing the oscillations to occur regularly and predictably. However, quantum gravity effects could attenuate or even suppress these oscillations, a phenomenon known as “decoherence.”

“There are several theories of quantum gravity which somehow predict this effect because they say that the neutrino is not an isolated system. It can interact with the environment,” explains Nadja Lessing, a physicist at the Instituto de Física Corpuscular of the University of Valencia and corresponding author of this study, which includes contributions from hundreds of researchers worldwide.

“From the experimental point of view, we know the signal of this would be seeing neutrino oscillations suppressed.” This would happen because, during its journey to us—or more precisely, to the KM3NeT sensors at the bottom of the Mediterranean—the neutrino could interact with the environment in a way that alters or suppresses its oscillations.

However, in Lessing and colleagues’ study, the neutrinos analyzed by the KM3NeT/ORCA underwater detector showed no signs of decoherence, a result that provides valuable insights.

“This,” explains Lessing, “means that if quantum gravity alters neutrino oscillations, it does so with an intensity below the current sensitivity limits.” The study has established upper limits on the strength of this effect, which are now more stringent than those set by previous atmospheric neutrino experiments. It also provides indications for future research directions.

“Finding neutrino decoherence would be a big thing,” says Lessing. So far, no direct evidence of quantum gravity has ever been observed, which is why neutrino experiments are attracting increasing attention. “There has been a growing interest in this topic. People researching quantum gravity are just very interested in this because you probably couldn’t explain decoherence with something else.”