A new study sets the most stringent limits yet on quantum gravity, which is considered the key to creating a “theory of everything” — a universal model of the universe that encompasses both quantum physics and classical mechanics. The research is based on studying the properties of neutrinos using the KM3NeT underwater detector in the Mediterranean Sea. The findings from the sensors point to further directions for the search.
Visualization of the experiment. Image source: KM3NeT
The KM3NeT Neutrino Telescope is a large underwater observatory designed to detect neutrinos by their interaction with water. It consists of two detectors. The ORCA detector, located at a depth of about 2,450 meters off the coast of Toulon (France), was used in the experiment to search for signs of quantum gravity.
Quantum gravity is the missing link between general relativity and quantum mechanics. For now, it remains a hypothesis, a potential key to a unified theory that can explain both the infinitely large and the infinitely small. The solution to this puzzle may lie in the humble neutrino, an elementary particle that has no electrical charge and is nearly invisible because it rarely interacts with matter, passing unimpeded through everything on our planet.
This is why neutrinos are so difficult to detect. However, in rare cases, the particle can interact, for example, with water molecules on the seabed. In this case, the so-called Cherenkov radiation occurs – a weak glow that can be recorded by photodetectors. The thickness of the water filters out most elementary particles of terrestrial and cosmic origin, while neutrinos freely penetrate to the depths.
It is known that during the journey, neutrinos oscillate — they change their mass. However, their total mass remains unchanged, being in a state of quantum superposition. This is a fundamental property of neutrinos, which can also be described by the concept of coherence. If quantum gravity exists (and some models predict it), then in some cases it can violate the coherence of neutrinos. It is this effect — decoherence — that the KM3NeT detectors tried to record.
However, the study found no abnormalities in the neutrino oscillations. They behaved as if quantum gravity did not exist. But even this result is significant because it imposes new, the most stringent constraints yet on models of quantum gravity.
«This, the scientists explain, means that if quantum gravity does influence neutrino oscillations, the strength of this influence is below the current sensitivity limits.” The study sets an upper limit on the strength of this effect, which is now tighter than that determined by previous experiments with atmospheric neutrinos. It also points to directions for future research.
«”Detecting neutrino decoherence would be a major breakthrough,” the researchers explain. “Until now, no direct evidence of quantum gravity has been found, so neutrino experiments are attracting more and more attention. Interest in this topic is growing. People studying quantum gravity are extremely interested in this, since decoherence probably cannot be explained by any other factors.”