One of the fundamental particles that makes up the universe is also one of the most mysterious.
Neutrinos, Italian for “little neutral one,” are everywhere. They emerged soon after the Big Bang, and, later on, from black holes, exploding stars, the nuclear reaction that fuels our sun, even from the interaction between cosmic radiation and Earth’s atmosphere. The tiny particles have very, very little mass—how much exactly, no one knows—and don’t abide by the same rules as other particles with which we’re more familiar. Unlike electrons, for example, neutrinos lack an electric charge, so the usual electromagnetic forces in space that jostle other particles have no effect on them. Neutrinos roam freely in space, zipping across great distances at nearly the speed of light without slowing down or changing direction. They pass through planets, stars, and whole galaxies, imperceptible to all.
Frederick Reines, the physicist who co-discovered neutrinos in 1956, described them as “the most tiny quantity of reality ever imagined by a human being.”
Scientists have spent decades designing and testing experiments to detect the elusive particles, particularly the high-energy kind that originates from mysterious sources in the depths of the cosmos. Five years ago, an observatory near the South Pole in Antarctica managed to detect, for the first time, neutrinos from beyond our solar system. They couldn’t figure out where they were coming from, though; the particles appeared to be bombarding Earth from random directions across the sky. Which was perfectly fine, for the most part. After all, just detecting these things was a tremendous scientific feat. Scientists buckled down and waited to find more.