Testing Einstein's laws from the bottom of the Mediterranean Sea

Could spacetime be different from what we think?

Why question such a fundamental principle? One reason lies in the search for a theory of quantum gravity, capable of reconciling general relativity with quantum physics. Some theoretical approaches suggest that spacetime, which we normally consider continuous, might actually be made up of extremely small, discrete units. This could have profound implications for the laws of physics, potentially including violations of Lorentz invariance.

“We still don't know whether our spacetime is continuous or might be discrete,” explains Alba Domi, researcher at Erlangen Centre for Astroparticle Physics (ECAP), 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 are particularly promising probes for signs of such new physics, as Domi explains. These tiny particles rarely interact with matter, allowing them to travel enormous distances, even through the Earth. They also have a remarkable property: they can change from one type, or “flavour” (this is the term used by scientists), to another as they travel, a phenomenon known as neutrino oscillation. The probability of these transformations can be predicted with great precision. If the observed behaviour differs from these predictions, it could be a sign of new physics, potentially including a violation of Lorentz invariance.

To look for such deviations, the researchers analysed 1.4 years of data collected with the first six detection units of KM3NeT/ORCA, comparing the observed neutrino energies and arrival directions with theoretical predictions.

New limits, new possibilities

The researchers found no evidence of Lorentz invariance violation. However, this does not mean that such violations are impossible: their results establish new limits on how large certain effects could be without contradicting the observations. Despite being obtained with an early, partial configuration of the detector, the results are competitive with those of long-running experiments such as IceCube and Super-Kamiokande. For the first time, they also established direct limits on certain possible effects of Lorentz invariance violation involving two neutrino flavours, muon and tau, and the way neutrinos oscillate between them.

These results are particularly promising considering that, at the time the data were collected, ORCA was still in an early stage of construction, operating with just six detection units. The detector is still being expanded towards its full planned configuration [see box].
“These results show the potential of KM3NeT/ORCA to test fundamental principles of physics, even with a relatively small detector,” says Lukas Hennig, a PhD student at ECAP and the study's other corresponding author. “With more data and an expanding detector, we will be able to search for even smaller effects.”

In the future, the researchers hope to extend their investigations to KM3NeT/ARCA, which focuses on much higher-energy neutrinos arriving from distant astrophysical sources. Such neutrinos could be especially promising for these searches, as some possible effects of Lorentz invariance violation become more pronounced over very long distances and at higher energies. “ARCA could be particularly well suited to this kind of research, but we don't yet have enough astrophysical neutrino events to perform such an analysis,” explains Domi.

The growing importance of high-energy neutrino research was recently recognised with the 2026 Nobel Prize in Physics, awarded to Francis Halzen for his pioneering contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos from space.

The analysis was carried out as part of Domi’s Marie Skłodowska-Curie Postdoctoral Fellowship (QGRANT), funded by the European Union’s Horizon Europe programme.