Science has been studying Einstein’s theory of relativity for years and analyzing whether the pillars on which it rests are strong enough. From time to time, some falter, while others gain more strength with each new discovery. One of those that is still in doubt is Lorentz invariance, a principle that broadly states that the laws of physics are the same anywhere in the universe. There are scientists who believe that this principle can be broken to extremes. If so, Einstein would be wrong. Some researchers want nothing more than to rob a bit of talent, which is why they want to prove it.
Many techniques are hypothesized to be useful for achieving this, but undoubtedly the most studied is based on neutrino detection. So a team of scientists is experimenting with them deep in the Mediterranean Sea.
KM3NeT/ORCA. It is the name of the neutrino detector located at a depth of 2,450 meters off the coast of Toulon, France. In a recently published study, project scientists describe the detection of 5,828 atmospheric neutrinos, whose properties do not appear to break Lorentz invariance. At this point, Einstein is still right, even though this is the first part of his experiment.
Why neutrinos? Neutrinos are very light particles that have no charge and interact mainly through the weak force, so they can travel through the universe without virtually ever interacting with matter. This makes them very useful tools for mapping the universe, since it is enough to trace their path back to see where they came from. It would be strange if something removed them.
There are three types – or flavors – of neutrinos depending on the particle they are associated with: electronic, muon and taun. It is known that they can change flavors during their journey through the cosmos in a phenomenon known as oscillations. The laws of physics are very precisely established when these changes occur. This is why they are useful for studying Lorentz invariance. If the result is always expected, regardless of which direction the measurement is taken, then the law is correct. If not, some modifications may be required under certain conditions.
Why under the sea? Every second, billions of neutrinos pass through our bodies, without us realizing it. These are very difficult particles to detect. However, it has long been known that water can be a good neutrino detector. If enough volume is used, there may come a time when they interact with a particle. When this happens, they release charged particles in virtually the same direction as the neutrino was traveling. These particles, in turn, emit a blue light that, since water is transparent, is easily detected. A light that differs from one neutrino flavor to another.
Atmospheric neutrinos form in mixtures of flavors that theory predicts. As they travel, muon neutrinos also change flavor at a known rate. Therefore, by identifying which type of remains reach the ocean after traveling a certain distance, you can see if all the predictions come true.
Depth is very important. The detectors are placed very deep to avoid low particles that could interfere with the results, such as muons from cosmic rays. At depth, it is easy to capture neutrinos without interference. Ice also contains detectors, such as the famous icecube that recently won a Nobel Prize for one of its creators. However, in the study in question, the Mediterranean Sea was used, also with good results.
even better. The study used only 1.4 years of data from the first six detection units, and even they match the accuracy of experienced experiments such as the aforementioned Icecube at the South Pole or Super-Kamiokande in Japan.
And now what? In addition to ORCA, the KM3NeT project includes another detector called ARCA, located at a depth of 3,500 meters off the coast of Sicily. While ORCA specializes in low-energy atmospheric neutrinos, ARCA focuses on neutrinos with much higher energies from deep space. The authors of the study also want to analyze the oscillations of neutrinos in this second detector, since in this case, if there is more energy, possible variations that would invalidate the principle of Lorentz invariance would be pronounced.
For now, Einstein is still right about this principle, but there are still tests. We can’t take anything for sure.
Photo by Edward Barbee Nikhef
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