Scientists are diving into the deep sea to study one of the universe’s biggest mysteries—quantum gravity. Using KM3NeT, a vast underwater neutrino telescope, researchers are watching ghost-like particles that may hold the key to uniting the very large and the very small physics. By analyzing how neutrinos oscillate—or don’t—during their journey through space, they’re searching for subtle signs of decoherence, a possible effect of quantum gravity.
Quantum gravity is the missing piece in physics: a theory that could unite general relativity, which describes the universe on large scales, with quantum mechanics, which governs the behavior of the tiniest particles. One key to this long-standing puzzle may be the neutrino, a tiny, electrically neutral particle nearly invisible because it almost never interacts with matter. Trillions pass through your body every second without leaving a trace.
Because neutrinos interact so rarely, they’re notoriously difficult to detect. But once in a while, a neutrino will collide with something, like a water molecule deep beneath the sea. When that happens, it can create a faint blue light known as Čerenkov radiation, which sensitive detectors like KM3NeT can pick up.
In the depths of the Mediterranean Sea, KM3NeT uses vast 3D arrays of optical modules and light sensors suspended in the water to detect neutrinos. KM3NeT (the Kilometer Cube Neutrino Telescope) is a vast underwater observatory designed to capture these rare neutrino interactions in the deep ocean. It includes two main detectors, one of which, ORCA (Oscillation Research with Cosmics in the Abyss), was used in this study.
ORCA sits about 2,450 meters below the surface, just off the coast of Toulon, France. But detecting neutrinos is just the first step.
Neutrino insights from KM3NeT study
To probe quantum gravity, researchers also look for subtle signs that “decoherence” affects these particles, a possible clue to physics beyond our current understanding. As they travel through space, neutrinos can “oscillate,” meaning they change identity—a phenomenon scientists call 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, known as “decoherence.”
“Several quantum gravity theories 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.
Image Credits: Photo by Muzammil Soorma on Unsplash
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