The detection of a neutrino from a distant blazar, announced last week, is of paramount importance for current and future astrophysics. But detections of neutrinos can also have interesting consequences for other branches of physics. The high-energy particle was used to test one of the key features of Einstein’s theory of special relativity, known as Lorentz symmetry.
Lorentz symmetry simply states that the laws of physics stay the same between two moving observers. You would like to think that’s always the case, but maybe it's not. Maybe we haven’t looked at the most extreme cases in the universe where violations might be hiding. Researchers used the extremely high-energy neutrino because its tiny mass and high speed are a fantastic testbed for this principle of the theory. The findings, which once again confirm that special relativity is correct, are reported in Nature Physics.
"People love tests of Einstein's theory," stated Janet Conrad, professor of physics at MIT and a lead author on the paper. "I can't tell if people are cheering for him to be right or wrong, but he wins in this one, and that's kind of great. To be able to come up with as versatile a theory as he has done is an incredible thing."
Neutrinos, which hardly ever interact with ordinary matter, come in three flavors (don’t laugh, that’s actually the technical term): electron, muon, and tau. But this flavor property is not fixed. Neutrinos are flavor queer – they oscillate from one variety to the other – which means their abilities can be exploited.
Now let’s imagine that something somewhere violates Lorentz symmetry. We haven’t seen it in about 100 years of experiments, so if it exists it needs to be beyond where we looked. And that’s why high-energy neutrinos are perfect for this situation. If the violation exists, the highest energy neutrinos would be affected. If their hypothesis is correct, they would see a deficit of muon neutrinos.
The researchers used a high-energy neutrino dataset from IceCube, the neutrino observatory located in Antarctica. They combed through more than 35,000 observations in the search for potential deviations from the theory.
"We were looking to see if a Lorentz violation caused a deviation, and we didn't see it," Conrad says. "This closes the book on the possibility of Lorentz violation for a range of high-energy neutrinos, for a very long time."
Continued testing of well-established theories might seem trivial, but it is an important thing to do. Science is an approximation of reality, and even extremely good theories can be improved upon.