‘Ghost particles’ detected from a galaxy far, far away
When the sun was young and faint and the Earth was barely formed, a gigantic black hole in a distant, brilliant galaxy spat out a powerful jet of radiation. That jet contained neutrinos — subatomic particles so tiny and difficult to detect they are nicknamed “ghost particles.”
Four billion years later, at Earth’s South Pole, 5,160 sensors buried more than a mile beneath the ice detected a single ghostly neutrino as it interacted with an atom. Scientists then traced the particle back to the galaxy that created it.
The cosmic achievement, reported Thursday by a team of more than 1,000 researchers in the journal Science, is the first time scientists have detected a high-energy neutrino and been able to pinpoint where it came from. It heralds the arrival of a new era of astronomy in which researchers can learn about the universe using neutrinos as well as ordinary light.
This is physics at its most mind-boggling and extreme. Researchers compared the breakthrough to the 2017 detection of ripples in space time caused by colliding dead stars, which added gravitational waves to scientists’ toolbox for observing the cosmos.
Scientists call the kinds of signals they can detect through space, like radio waves or gravitational waves or now neutrinos, “messengers.”
If you’re trying to understand complex and chaotic phenomena happening billions of light-years away, it’s helpful to have a messenger like a neutrino: one that doesn’t get lost.
“They’re very clean, they have simple interactions, and that means every single neutrino interaction tells you something,” said Heidi Schellman, a particle physicist at Oregon State University and computing coordinator for a different neutrino detection project, the Deep Underground Neutrino Experiment, who was not involved with the new research.
Neutrinos arrive on Earth at varying energy levels, which are signatures of the processes that created them. By pairing neutrino detections with light observations, Schellman said, scientists will be able to answer questions about distant cataclysms, test theories about the composition of the universe, and refine their understanding of the fundamental rules of physics.
The high-energy neutrino reported Thursday was created in the fast-moving swirl of matter around a supermassive black hole at the center of the galaxy. When this black hole generates a brilliant jet of radiation, and that jet is aimed directly at Earth, scientists call the galaxy a “blazar.” Subsequent analysis revealed this blazar had also produced a flare of more than a dozen neutrino events several years earlier.
The new discovery, from the South Pole neutrino detector called IceCube, has also solved a mystery that stumped scientists for generations: What is the source of mysterious cosmic rays? These extremely energetic particles have been detected raining down from space since 1912, but researchers could not figure out what phenomenon could produce particles moving at such high speeds.
Astroparticle physicist and IceCube spokesman Darren Grant said it’s as though scientists have spent 100 years listening to thunder with their eyes closed and never known what caused the booming sound. It wasn’t until they looked up and saw lightning that the spectacle finally made sense. Both sound and light — or in this case, cosmic rays and neutrinos — are coming from the same event.
“That’s why this is exciting,” Grant said of the neutrino detection. “It’s a brandnew vision on what’s happening in the universe.”
Our universe is suffused with neutrinos, so named because they are uncharged (or “neutral”) and infinitesimally puny (about a millionth of the mass of an electron).
They are created in nuclear reactions — at power plants, in the center of the sun, and amid even more extreme events — when protons accelerate, collide and then shatter in a shower of energetic particles.
Neutrinos are the second-most abundant type of particle in the universe, after photons (light particles). If you held your hand toward the sky, about a billion neutrinos from the sun would pass through it in a single second.
But you wouldn’t feel their presence, because these ethereal particles rarely interact with normal matter.
Unless a neutrino bumps right up against another particle, it passes through matter undisturbed and undetected.
And the reality is, most of what we call “matter” is just empty space. If a hydrogen atom were the size of Earth, the proton at its center would fit inside the Ohio State football stadium.
The electron orbiting it would be even smaller, and a neutrino could be compared to a lone ant.
Neutrinos are said to come in “flavors” — called electron, muon and tau — and on the rare occasions that they collide with other matter they generate corresponding charged particles. Many neutrino detectors work by looking for the flash of light emitted by these charged particles as they move through water or ice.
Flavored specks that are found everywhere yet felt by no one; matter that seems solid but is actually mostly empty — this is the bizarre science of particle physics. It’s difficult to wrap your mind around, and almost hard to believe.
Yet scientists assure us they are not just making things up. Since the 1950s, when neutrinos were detected for the first time, researchers have observed low-energy versions of these ghostly particles coming from the sun and a 1987 supernova in a nearby galaxy. Maps of neutrinos emanating from the surface of the Earth have even been used to identify the sites of nuclear reactors.
But high-energy neutrinos, generated only in extreme environments where protons are accelerated to astonishing speeds, have been challenging to pin down. To be detected, a neutrino had to form long ago in a far away cataclysm, travel across intergalactic space, fly through our galaxy, enter our solar system, sail on to Earth, and then happen to interact with a particle minding its own business in the ice below the South Pole.
And, in a process that seems just as improbable, in the time since the neutrino left its source 4 billion years ago, life on Earth had to arise, expand, and evolve to the point that a few enterprising Homo sapiens were willing to go to the extreme effort of detecting it.
“It’s crazy,” said Chad Finley, an astroparticle physicist at Stockholm University who spent 10 years coordinating the effort to pinpoint neutrinos’ origins for the IceCube team. “These are particles that seldom interact with anything. That has to be the unluckiest neutrino ever.”
On the other hand, he mused, he and his colleagues are some pretty lucky humans.
This was the detection scientists were dreaming of when the National Science Foundation began building the $279 million IceCube Neutrino Observatory in 2005.
Scientists and engineers melted dozens of mile-deep holes in the ice and dropped strings of spherical sensors into them. (Neutrino detectors are typically buried or submerged to filter out other cosmic signals that would obscure the tiny particles.)
The result was a grid array of sensors spread across a cubic kilometer of glacier and capable of catching a ghost. The sensors record the energy level and direction of the flash of light emitted by the charged particle created when a neutrino crashes into other matter.
From that information, scientists can extrapolate the energy level of the neutrino and where it came from.
Since the observatory was completed in 2010, IceCube scientists have detected dozens of high-energy neutrinos coming from outside the solar system. But they were never able to connect those particles with a source that could be observed by conventional telescopes.
Establishing such a connection was a “holy grail of the field,” Finley said, in large part because of the link between neutrinos and the enigma of cosmic rays.
“Neutrinos are the smoking gun,” Finley said.