Mediterranean Neutrino Telescope Finds No Crack in Einstein's Relativity

Are the laws of physics the same everywhere in the universe? That principle, called Lorentz invariance, sits at the heart of Einstein's theory of relativity. A new analysis from the international KM3NeT collaboration, using a detector on the floor of the Mediterranean Sea, found no evidence that it fails, but it squeezed the room where any violation could hide. The results appear in the Journal of Cosmology and Astroparticle Physics.

The team studied 5,828 atmospheric neutrino candidate events recorded by KM3NeT/ORCA, a neutrino telescope deep in the Mediterranean. Atmospheric neutrinos are produced when cosmic rays hit Earth's upper atmosphere. The analysis used 1.4 years of data from the first six detection units of ORCA, comparing the observed neutrino energies and arrival directions with theoretical predictions.

Why test such a well-established principle? Some approaches to quantum gravity, the search for a theory joining general relativity with quantum physics, suggest spacetime may not be smooth but made of extremely small discrete units. That could produce tiny violations of Lorentz invariance. "We still don't know whether our spacetime is continuous or might be discrete," said Alba Domi, a researcher at the Erlangen Centre for Astroparticle Physics at Friedrich-Alexander-Universität Erlangen-Nürnberg and one of the study's two corresponding authors. "Looking for violations of Lorentz invariance is one way to investigate this possibility."

Neutrinos make good probes because they rarely interact with matter and can cross enormous distances, even the whole Earth. They also change from one type, or flavor, to another as they travel, a process called oscillation whose probabilities can be predicted with great precision. Any departure from those predictions could point to new physics.

The team saw none. That does not rule out violations, but it sets new limits on how large certain effects can be without contradicting the data. Despite being collected with an early, partial detector, the results are competitive with long-running experiments such as IceCube and Super-Kamiokande. They also provide the first direct limits on certain possible effects involving two neutrino flavors, muon and tau, and the way neutrinos oscillate between them.

"These results show the potential of KM3NeT/ORCA to test fundamental principles of physics, even with a relatively small detector," said Lukas Hennig, a Ph.D. student at ECAP and the other corresponding author. "With more data and an expanding detector, we will be able to search for even smaller effects."

ORCA is still being built out toward its full design. The researchers hope to extend the search to KM3NeT/ARCA, which targets much higher-energy neutrinos from distant astrophysical sources. Some possible violation effects grow with distance and energy, so ARCA could be especially well suited, Domi said, though there are not yet enough astrophysical neutrino events for the analysis. The study lands in a year when neutrino astronomy is in the spotlight: the 2026 Nobel Prize in Physics went to Francis Halzen for his work on the IceCube Neutrino Observatory and the discovery of high-energy neutrinos from space.