At the center of many galaxies lies a black hole — predicted by Schwarzschild. Around it churns a scorching corona, similar to the Sun's but a billion degrees hot. Here, neutrinos are born — ghost particles (first guessed by Pauli) that can pass through a light-year of lead.
Protons, accelerated by the corona's magnetic fields, slam into X-ray light and transform into neutrinos. Previously, it was thought that such particles were spewed by powerful plasma jets, but data from the Antarctic IceCube Observatory showed otherwise. Quiet galaxies without jets, with ordinary coronas, turned out to be the main source — they explain almost the entire neutrino flux up to the peta-electronvolt range.
Only for record-breaking energies is this cauldron not enough — then galaxies with relativistic jets step in. By combining observations of these 'ghosts' with theory, we paint a complete picture of their origin.
To stop a neutrino, you'd need a lead shield one light-year thick — but even that's no guarantee.
In science fiction, neutrinos are the ideal courier through planets; in reality, they bring news from the blazing coronas of black holes.