Astronomers have spent a century searching for the source of cosmic rays, streams of high-energy particles that are key to understanding the nature of the universe. Now, thanks to a single ghostly neutrino, they have finally succeeded.
The neutrino in question is so tiny that it would take more than 10 billion of them to equal the mass of a proton. But it made history by striking a network of sensors buried deep within Antarctic ice.
It was an auspicious end for a journey that began 4 billion light-years away. By analyzing the collateral damage of the impact, scientists traced the neutrino's origin to a powerful quasar known as a blazar far beyond our galaxy — and launched a new era of neutrino astronomy.
"We are not going to solve high-energy astrophysics in the old-fashioned way anymore," said Francis Halzen, a particle astrophysicist at the University of Wisconsin-Madison and principal investigator for IceCube, the frozen observatory that made the discovery described Thursday in the journal Science.
Astronomers' earliest observations of the universe were made by observing visible light, which they could see directly. Then they expanded their search to other portions of the electromagnetic spectrum, such as radio waves, microwaves, X-rays and gamma rays. This allowed them to find black holes, neutron stars and the cosmic microwave background radiation that's a remnant of the big bang.
More recently, scientists have looked beyond the light spectrum altogether. The detection of gravitational waves made it possible to eavesdrop on some of the most cataclysmic events in the universe, including mergers of black holes. Gravitational waves could help scientists understand dark matter, the mysterious stuff that dominates the universe but can't be seen or touched.
Now astronomers will add neutrino astronomy to their toolbox.
"In my opinion, this is as significant as the first steps in X-ray astronomy, which were awarded the Nobel Prize," said Alexander Kusenko, a particle astrophysicist at UCLA who was not involved in the work.
Neutrinos are valuable to astronomers because they hardly interact with matter. That means they can pass right through planets, stars and even entire galaxies.
Although they are exceedingly tiny, they are also plentiful. Many billions of these subatomic particles pass through your fingertip every second.
Despite their inherently elusive nature, astronomers hunt for neutrinos in the hope that they'll help solve the mystery of the origins of cosmic rays that bombard Earth from space.
Cosmic rays are highly energetic charged particles, mostly protons, that have been revved up to enormous energies and hurled across the universe. These particles can reach Earth with energies of hundreds of trillions of electron volts — dwarfing the mere 6.5 trillion electron volts of the protons circulating through the Large Hadron Collider in Europe. It would take a powerful cosmic engine — say, a supermassive black hole at a galaxy's heart, or an enormous supernova — to accelerate these atomic fragments to such high energies.
But until now, scientists haven't known for sure where these cosmic rays came from. That's because as these particles travel intergalactic distances, their paths are warped by the magnetic fields that permeate space — which means that by the time they get to Earth, they're no longer pointing back at their source.
Neutrinos offer a solution to this problem because these neutral particles are unaffected by magnetic fields. By the time they reach Earth, they're still pointing the way home. On top of that, the kinds of powerful cosmic forces that would generate high-energy cosmic rays also would produce a torrent of high-energy neutrinos.
But the very quality that makes these ghostly particles so useful — the fact that they don't interact with matter — also makes neutrinos exceedingly difficult for scientists to catch in action. For every individual high-energy neutrino hit, Halzen said, roughly 10,000 or 100,000 more pass through unscathed.
The IceCube collaboration set out to detect that rare, singular neutrino strike. Composed of more than 5,000 sensors embedded in a cubic kilometer of ice sitting deep beneath the Antarctic surface, IceCube picks up the flashes of blue light caused by secondary particles after a neutrino makes contact. The scientists can analyze that resulting light track to tell what direction the particle came from and how energetic it was when it hit.
In 2013, the collaboration announced it had found 28 high-energy neutrinos that had originated from deep space, but the group was not able to tell where exactly any of them came from.
Then, on Sept. 22, 2017, the scientists picked up an energetic neutrino that had clearly originated far outside our interstellar neighborhood.
Within 60 seconds, the news had reached a host of other observatories, including gamma ray, infrared, radio and X-ray telescopes. They turned toward the apparent source and picked up a light signal at wavelengths across the electromagnetic spectrum.
The light was coming from a blazar named TXS 0506+056, which sits just beneath the arm of the constellation Orion. This blazar is a giant elliptical galaxy with a spinning black hole at the center that's gobbling up material and shooting out twin beams of light on either side of its disk. In this case, one of those beams is pointed directly at Earth, like a flashlight.
Still, there was a small chance — about 1 in 1,000 — that the neutrino's apparent origin and the blazar signal were mere coincidence. So the researchers went back in the archives, looking for previous neutrino measurements that also could have come from the blazar's direction.
Sure enough, the researchers found more than a dozen neutrinos from September 2014 to March 2015 that appeared to be coming from the direction of the blazar. Those results were published in a second paper in Science.
Such neutrino discoveries could help astronomers to better understand the inner workings of these cosmic events, Kusenko said.
It also may allow scientists to see old events in a new light, Halzen said.
"We are not seeing the vanilla blazar that astronomers are seeing," Halzen said. "I think in the end there will be many surprises about this source. ... It won't be business as usual."
For one thing, it would have taken an extremely powerful source to push these particles to such high energies and then send them across nearly 4 billion light-years, he pointed out.
"So there's something special about this source," Halzen said — something special that was not obvious from the blazar's light profile and which will require further study to understand.
Already, he added, neutrino astronomy is revealing extraordinary events right in front of scientists' eyes.
9:50 a.m.: This article was updated with additional information from Francis Halzen.