Underwater Detector Tests Einstein’s Relativity, Finds No Flaws

Deep-Sea Detector Finds No Sign Einstein’s Relativity Falls Short

A partially built neutrino detector sitting 1.5 miles under the Mediterranean Sea off the coast of France has produced results that stand alongside experiments with a decade-plus head start. Using only six of the more than 100 sensor strings planned, the instrument, called KM3NeT/ORCA, collected about 1.4 years of data and set limits on a possible flaw in physics that are competitive with those from Japan’s Super-Kamiokande detector.

That flaw involves Lorentz invariance, the rule that the laws of nature stay the same no matter which way an experiment faces or how fast it moves at a steady speed. Einstein’s theory of relativity rests on it. Yet some theories that try to merge gravity with quantum physics suggest that spacetime, which looks smooth, could be built from extremely small, separate units, a picture hard to square with the rule. “We still don’t know whether our spacetime is continuous or might be discrete,” said Alba Domi in a press release, a researcher at Friedrich-Alexander-Universität Erlangen-Nürnberg and one of the study’s two corresponding authors.

Researchers with the KM3NeT collaboration, reporting in the Journal of Cosmology and Astroparticle Physics, found no sign of the rule being broken in 5,828 neutrino events.

Neutrino Detector Data Show No Departure From Standard Physics

Atmospheric neutrinos are ghostly particles that form when cosmic rays slam into the atmosphere. They almost never bump into anything, so they pass straight through the planet, crossing up to its full width before reaching the detector. Had the rules of physics bent slightly, that long trip would have given the effect time to add up.

ORCA catches the neutrinos that do collide with something. Its vertical strings of light sensors, anchored to the seafloor, record the faint flash each collision creates, revealing the particle’s energy and direction. This study used data gathered between January 2020 and November 2021, when six strings were installed, a setup nicknamed ORCA6. Quality checks left about 510 days, or 1.4 years, of data.

Noise from ocean radioactivity, glowing microorganisms, and stray atmospheric particles called muons had to be scrubbed out first. Because Earth blocks muons but not neutrinos, the team kept only particles traveling upward through the detector. The 5,828 surviving events were sorted into three groups by the shape of their light patterns, then compared with predictions by energy and arrival angle. Neutrinos change type as they travel, a process called oscillation, and a broken rule would have distorted that pattern.

Young Neutrino Detector Competes With Far Larger Rivals

Super-Kamiokande analyzed about 12 years of data, and IceCube, a huge detector buried in Antarctic ice, used two years. ORCA6 produced limits on one family of possible violations, ones that would make neutrino types mix in unexpected ways, that the paper calls competitive with theirs. A limit works like a ceiling: any violation larger than that value would likely have shown up in the data. ORCA’s design for low-energy neutrinos, where standard oscillation patterns are easy to distort, probably helped.

ORCA6 delivered something new. For the first time, researchers directly limited how a broken rule could affect muon neutrinos and tau neutrinos differently, across six versions of the theory. Earlier limits on related questions came from neutrinos arriving from deep space, which required assuming what mix of neutrino types left their sources. The ORCA6 limits skip that assumption.

One Statistical Blip Made One Limit Look Stronger Than Expected

Two limits came out tighter than simulations of a no-violation universe predicted. For one, most of that tightness traced back to a single slice of data, 79 high-energy, nearly vertical events, a mild excess over what standard physics expects. Rerunning the analysis without that slice loosened the limit by about 22 percent, back into the expected range.

Overall, the fit between data and standard predictions was poorer than expected. Only about 1 in 100 simulated experiments fit worse, and adding a possible violation to the math did not improve it. A later analysis of a larger, overlapping dataset fit much better, which supports the idea that ORCA6’s rough patch was a statistical fluke rather than a hint of new physics.

Full-Size ORCA Could Sharpen the Search

Researchers searched only a simplified version of Lorentz invariance violation, one that does not depend on which direction a detector points relative to the universe. Finding nothing is not proof that violations are impossible, only that any violation of this kind is small.

Next, researchers hope to turn to KM3NeT/ARCA, a sister detector off Sicily built for higher-energy neutrinos from deep space, where some effects could grow more pronounced over long distances. The catch, Domi said, is that “we don’t yet have enough astrophysical neutrino events to perform such an analysis.” For ORCA, growth is under way. “With more data and an expanding detector, we will be able to search for even smaller effects,” said Lukas Hennig, a doctoral student at the same university and the study’s other corresponding author.

Whether spacetime is smooth or grainy remains open, but a detector still under construction has narrowed where any violation could hide.

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