Four billion years ago, an immense galaxy with a black hole at its heart spewed forth a jet of particles at nearly the speed of light. One of those particles, a neutrino that is just a fraction of the size of a regular atom, traversed across the universe on a collision course for Earth, finally striking the ice sheet of Antarctica last September. As it hit, a neutrino detector planted by scientists within the ice recorded the neutrino’s charged interaction, causing a blue flash of light that