A particle-detecting telescope sunk more than a mile beneath the Mediterranean Sea has tested a core principle of Einstein's relativity and found no cracks, Live Science reported. The detector, called Oscillation Research with Cosmics in the Abyss, or ORCA, sits about 1.5 miles (2.4 kilometers) down off Toulon, France. Even though it is only a fraction of its planned size, the team used it to test Lorentz invariance, the idea that the laws of nature look the same no matter which way you face or how fast you move. The results, published Nov. 4 in the Journal of Cosmology and Astroparticle Physics, set limits that compete with those of far larger and longer-running experiments.
Physics rests on two successful theories: general relativity and the Standard Model of particle physics. Both assume Lorentz invariance is true. In extreme settings, such as the first moments after the Big Bang, the two theories do not mesh, which has fueled a hunt for a theory of quantum gravity that unites them. Some candidates, including versions of string theory and loop quantum gravity, suggest that space-time might have a hidden "grain" at the tiniest scales, and that Lorentz invariance might be violated, said co-author Lukas Hennig, a doctoral student at Erlangen-Nürnberg. Any such violation would be extremely small, so physicists need a probe that can travel long distances and barely interact along the way. Neutrinos fit that description. They are nearly massless, uncharged particles that pass through Earth every second and morph from one type into another, a process called oscillation. If Lorentz invariance were broken, that oscillation would change, and the effect would grow the farther the neutrinos traveled.
ORCA is part of the KM3NeT neutrino telescope. Each detection unit is anchored to the seabed and holds 18 glass spheres packed with 31 light sensors. When a neutrino collides with a particle in the water, it produces a faint flash of blue light. The team focused on neutrinos created when cosmic rays slam into the atmosphere elsewhere on the globe, then travel up through the planet to the detector. Some crossed the entire 7,900-mile (12,700-kilometer) width of Earth. Earth itself served as a filter: only neutrinos get through, Hennig said. The setup differs from IceCube, the detector buried in Antarctic ice, because ice scatters light more than water, while water absorbs light more than ice, said Alba Domi, who helped lower some of the first units as a doctoral student.
The researchers used 5,828 neutrinos collected over about 17 months in 2020 and 2021. The oscillations matched standard physics. If Lorentz violation exists, it must be weaker than the limits they set. Those limits already rule out some theoretical models that predicted a stronger effect. The verified result is a null finding with competitive precision from a still-unfinished underwater telescope, not a discovery of new physics.