Deep in the Mediterranean Sea, where water presses down with the force of hundreds of atmospheres, a network of light-sensitive spheres waited for a rare guest. And it came — a neutrino that carried through the bulk of the Earth an energy comparable to the impact of a flying brick. Except instead of a brick, it's a tiny particle, almost intangible, yet capable of telling us about the most violent corners of the cosmos.
This particle was named KM3-230213A. It is the first in history whose energy exceeded 100 PeV (petaelectronvolts, a unit of particle energy). For comparison: the energy of a proton in the Large Hadron Collider is 6.5 TeV, which is tens of thousands of times smaller. Neutron stars or active galactic nuclei could act as natural accelerators, boosting particles to such energies. But the specific source of this neutrino remains unclear.
There's also a puzzle: why haven't other major observatories, like IceCube in the Antarctic ice, seen similar events before? Perhaps we were lucky to catch an extremely rare burst, or our ideas about how often such neutrinos are born need revision. Scientists are already building hypotheses: maybe the source is a blazar (a supermassive black hole whose jet points directly at us), or the collision of cosmic rays with the relic radiation left over from the Big Bang.
Either way, a single event has already forced a rethink of the map of the ultra-high-energy Universe. Now that the window is open, we just have to wait for new visitors and learn to read their messages.
The energy of 100 PeV is roughly equal to the kinetic energy of a flying baseball, but compressed into a single subatomic particle.
In Peter Watts's novel Blindsight, neutrinos are used for communication with alien intelligence, so ubiquitous and elusive are they.