An artist's impression of a supermassive black hole shooting high-energy particles, including neutrinos, into spaceDESY, Science Communication Lab

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.

The observatory, known as IceCube, detects several hundred neutrinos every day, but these are produced near Earth. On September 22, 2017, the instrument caught something unusual: a single neutrino unlike all the rest. It had far more energy than the others, which suggested it came from somewhere beyond the solar system.

This time, astronomers have figured out exactly where it came from.

An international team of astronomers have traced this single, high-energy cosmic neutrino to a supermassive black hole at the center of a galaxy nearly 4 billion light-years away. The discovery, announced Thursday and published in Science, marks the first-ever detection of the origin of cosmic neutrinos.

The story of this discovery begins billions of years ago, when the Earth was just beginning to take shape. In another corner of the universe, a gargantuan black hole churned at the heart of a sparkling galaxy. Black holes love to gobble up everything that comes near them, but they’re also quite good at spitting matter back out, creating high-energy streams that light up the darkness. The black hole spewed a jet of particles, including neutrinos, and sent them flying through space.

Skip ahead a little bit—to last year—and one of these neutrinos finally made it to Earth. It passed through the planet as it would anything else, but it didn’t go unnoticed. Scientists were ready for it.

They had drilled more than 80 narrow holes into the thick Antarctic ice, each stretching about 8,200 feet into the depths. Into the holes went more than 5,000 light sensors. Neutrinos, the scientists knew, are not governed by electromagnetic forces, but they can, on rare occasions, interact with the nucleus of an atom. This interaction generates a new kind of particle, and that particle produces a tiny flash of light the underground sensors can detect.

This is what happened in Antarctica. When the neutrino struck the ice and continued on its merry way, IceCube recorded the glint of its passing.

The IceCube astronomers sounded the alarm within minutes, galvanizing their colleagues around the world to help. The flash of light they detected gave them a sense of the direction the neutrino had come from. They broadcast these coordinates to their peers, who aimed telescopes at the relevant slice of the sky, scanning it in virtually every wave on the electromagnetic spectrum. Meanwhile, the IceCube team pored over archival telescope observations of the region. They found more than 600 potential candidates for the source of the neutrino.

After countless analyses were run, only one candidate was left standing: a special kind of galaxy called a blazar. Blazars house supermassive black holes that spray jets of high-energy particles pointed directly toward Earth. This one, known as TXS 0506+056, is located in the constellation Orion, visible in the night sky throughout the world.

When the IceCube team sorted through their old data, they found that at least a dozen other, less energetic neutrinos had originated from this distant region from late 2014 to early 2015. Data from other telescopes further backed up their discovery. One of NASA’s space telescopes, Fermi, detected a flare of high-energy gamma rays that seemed to come from the blazar and followed a similar path to the neutrino. So did another observatory, known as MAGIC, based in the Canary Islands.

The discovery comes nearly a century after the the Austrian American physicist Victor Hess described cosmic radiation, and posited that high-energy particles can originate from beyond the solar system.

Astronomers have heralded this discovery as the beginning of a new chapter in astronomy, one that will allow them to better understand neutrinos, the tiny messengers from across the universe. If the concept of neutrinos—and of blazars, and other astrophysical creatures with funky names—feels difficult to grasp, consider this: Neutrinos are everywhere, and they pass through everything, all the time. This includes your little place on Earth: your home, your body, the membranes that enclose the cells that make you you. About 100 trillion neutrinos pass through your body every second. You can’t feel them, but perhaps you can feel some wonder in knowing they’re there, just passing through, a toll booth on an invisible journey without an end.

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