A billiard ball clicks against another in complete darkness. The sound of the impact and the clatter of balls dropping into pockets: that's all you hear. This is roughly how physicists study the interactions of elementary particles at energies that no terrestrial accelerator can reproduce. In a new study, astrophysicists have turned entire galaxies into giant laboratories, with detectors made of cubic kilometers of Antarctic ice and Mediterranean seawater.

The key quantity here is the interaction cross section. It measures the probability that two particles passing by each other will actually interact. The larger the cross section, the more frequent the collisions. In terrestrial experiments, such as at the Large Hadron Collider, this quantity is measured directly by colliding particle beams. But when energies exceed the collider's capabilities, other methods are needed. The authors of the study proposed using neutrinos — nearly undetectable particles born in the most violent corners of the universe that travel unimpeded to Earth.

The idea behind the method is like a detective puzzle. Each neutrino source, whether it's the active nucleus of a distant galaxy or the center of our Milky Way, has a known power: how many cosmic rays it emits. One can also estimate the density of the target matter with which these rays collide. Knowing these inputs and measuring how many neutrinos reach the detector, one can calculate how efficiently the collisions occurred. The only unknown in the equation remains the interaction cross section itself.

The scientists applied their method to real data: the famous blazar TXS 0506+056, which was first linked to a neutrino signal in 2017, the active galaxy NGC 1068, emission from the plane of our Galaxy, and even a recent ultra-high-energy event recorded by the KM3NeT detector in the Mediterranean Sea. As a result, they were able to place constraints on cross sections over a vast range of energies — from a modest 1 GeV to a colossal 100,000 GeV. For comparison, the Large Hadron Collider reaches only 13,000 GeV. Thus, cosmic neutrinos become a tool for testing theories where terrestrial technology is currently powerless. And just as the sound of an impact in the dark reveals the collision of balls, these invisible visitors from space tell us about the forces governing matter at the edge of the possible.