Neutrinos may hold the key to quantum gravity, potentially leading scientists to a unified theory that combines quantum mechanics and general relativity, according to researchers at Spain’s Instituto de Física Corpuscular of the University of Valencia.
Such a theory would explain how the universe works at all levels, helping resolve today’s apparent scientific contradictions from cosmic to subatomic scales. With no direct evidence of quantum gravity yet uncovered, indirect effects on neutrinos may hold the only keys to the mysterious force.
Detecting Neutrinos
Neutrinos are electrically neutral particles so small that scientists once believed their tiny mass to be zero. The particles can pass through normal matter as they are immune to strong interaction or electromagnetic force. However, the short-range weak force exhibits some pull, and the neutrinos near zero mass leaves just enough for gravity’s grasp to hold on to, although it has little effect.
With such properties, neutrinos are highly elusive, but scientists have harnessed the power of Earth’s oceans in their pursuit. When neutrinos meet seawater, their interaction produces a blue glow called Cherenkov radiation. A large underwater observatory named the Kilometer Cube Neutrino Telescope (KM3NeT) detects neutrinos by imaging Cherenkov radiation on the ocean floor. Located 2,450 meters deep off the coast of Toulon, France, one of KM3NeT’s two detectors, Oscillation Research with Cosmics in the Abyss (ORCA), collected the data used in the new research.
Decoherence and Quantum Gravity
Studying quantum gravity sent the University of Valencia scientists looking for neutrinos evidencing a state of “decoherence.” Neutrinos do not have a definite mass but maintain a quantum superposition of three mass states, called “lepton flavors.” Coherence is a neutrino property whereby the superposition remains well defined as the neutrino oscillates between the lepton flavors of electron, muon, or tau as it moves through space, predictably changing its lepton number. Decoherence is when quantum gravity pulls on the neutrino, altering the oscillations.
“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 co-author Nadja Lessing, a physicist at the Instituto de Física Corpuscular of the University of Valencia.
“From the experimental point of view, we know the signal of this would be seeing neutrino oscillations suppressed,” Lessing added.
Null Results and Continued Search
While ORCA’s observations detected no decoherence, a negative result is still of value. Earlier neutrino experiments resulted in an expected upper limit to decoherence effects that the new research pushes down even further. With these results, scientists are closing in tighter open areas where decoherence can be located.
“This means that if quantum gravity alters neutrino oscillations, it does so with an intensity below the current sensitivity limits,” Lessing said.
With quantum gravity currently undetectable directly, scientists continue investigating neutrino decoherence as one of the few candidates to investigate the quantum effect. Discovering neutrino oscillations was one of the first steps past the Standard Model, and their decoherence potentially may lead to a complete, unified understanding of the universe on all levels.
“Finding neutrino decoherence would be a big thing,” concluded Lessing.“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.”
The paper “Search for Quantum Decoherence in Neutrino Oscillations with Six Detection Units of KM3NeT/ORCA” appeared in the Journal of Cosmology and Astroparticle Physics on March 20, 2025*.*
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted atryan@thedebrief.org, and follow him on Twitter@mdntwvlf.