When you buy through links in our articles, Future and its distribution partners may earn a commission. The view from inside a detector building block, with the yellow buoy of a nearby detection unit clearly visible. | Credit: Edward Berbee/Nikhef A particle-detecting telescope sunk more than a mile under the sea off the coast of France has tested a central principle of Einstein’s relativity and found no cracks. The detector, called Oscillation Research with Cosmic Abyss (ORCA), detects elusive particles called neutrinos. As a doctoral student, Alba Domi ventured into the Mediterranean aboard one of the two ships that helped build the telescope, lowering some of its first detection units to the seafloor about 2.4 kilometers (1.5 miles) below the waves off Toulon, France. Now, data from those early detection units has been used to test one of Einstein’s fundamental rules of relativity: the idea that the laws of nature are the same no matter which direction you look or how fast you move. The team found no sign that this rule, called Lorentz invariance, is broken. But even with a detector just a fraction of their planned size, they set limits that rival those of much larger and longer-running experiments. The results were published Wednesday (Nov. 4) in the Journal of Cosmology and Astroparticle Physics. A rift in space-time? Physics is based on two successful theories: general relativity, conceived by Albert Einstein more than 100 years ago, and the standard model of subatomic physics. and the Standard Model of particle physics actually describes the microscopic scale, the individual particles, how they behave,” said study co-author Lukas Hennig, a doctoral student in advanced physics at Erlangen-Nürnberg. Both theories assume that Lorentz invariance is true. But in extreme scenarios, such as the first moments after the Big Bang, the two theories don’t fit. This has fueled a decades-long search for a theory of quantum gravity that unites them. Some candidates, including versions of the theory of strings and loop quantum gravity, suggest that spacetime could have a hidden “grain” at the smallest scales, and that Lorentz invariance, “one of the most basic assumptions we make about our universe, could be violated,” Hennig said. To detect it, physicists would need a probe that could travel long distances and barely interact with anything along the way. form Neutrinos are almost massless particles that pass through the Earth every second and carry zero charge. They barely interact with anything and are of three types: electron, muon and tau neutrinos. As neutrinos travel, they transform from one type to another in a predictable pattern that physicists call neutrino oscillation. Lorentz, that oscillation would change, and the effect would become more pronounced the further the neutrinos traveled. So the team focused on neutrinos created when cosmic rays hit the atmosphere in other parts of the world before traveling across the planet to the detector. Some of the neutrinos tested crossed the entire 7,900-mile (12,700 km) width of the Earth. KM3NeT. Each of its detection units is anchored to the seafloor and contains 18 glass spheres with 31 light sensors each. When a neutrino collides with a particle in the water, it produces a faint flash of blue light that the sensors pick up. The numerous sensors in each sphere help the team determine which direction the light is coming from, Domi said. The researchers selected only particles that were traveling upwards through the planet. other particles in the atmosphere that would otherwise flood the signal. “Only neutrinos can get through,” Hennig said. That setup differs from IceCube, the Nobel Prize-winning detector buried in Antarctic ice. “Ice scatters light more than water, but on the other hand, water absorbs light more than ice,” Domi said, which changes how each team reconstructs the energy and trajectory of a neutrino. The researchers used 5,828 neutrinos collected over about 17 months between 2020 and 2021. They found that the oscillations were consistent with standard physics. That means that if a Lorentz violation exists, it must be weaker than the limits established by the team. Those limits are important to theorists if a model “predicts a violation of Lorentz invariance. [stronger than the new limits]The data rules it out. In that case, you can be pretty sure that this model is not correct,” Hennig said. The team also established the first direct limits on a type of violation that would treat muon and tau neutrinos differently. The detector limits on several key measurements are competitive with those of Japan’s Super-Kamiokande, which used 12 years of data, and IceCube, which used two years of data from its full array. Related Stories “I’m surprised that we have competitive results with only six detection units,” Hennig said. Today, 44 units are in the water, and the collaboration plans to activate 108 by 2030. The best tests may still be ahead. Because Lorentz violation effects grow with distance and energy, neutrinos from distant cosmic sources are “actually the best candidates to do this kind of research,” Domi said. The team plans to repeat the search using ORCA’s sister detector, ARCA (Astroparticle Research). with Cosmics in the Abyss), which is located off the coast of Sicily and is designed to catch those high-energy cosmic travelers If space-time has a grain, those particles may be among the first to sense it. See how much you know about Albert Einstein with our Albert Einstein quiz!
