To begin unravelling a century-long mystery in cosmology, astronomers first had to catch a ghost.
And recently, they did, detecting a single neutrino, a particle so small that it was historically treated as a dimensionless point without width or volume. The detection pointed researchers to a supermassive black hole in a distant galaxy, suggesting an intergalactic source for a variety of exotic and enigmatic cosmic rays that have puzzled the scientific community since the early 20th century.
Elusive neutrinos are nicknamed “ghost particles”—their tiny size and lack of electrical charge mean they rarely interact with physical matter. But they’re everywhere. If you are tanning at noon, 65 billion neutrinos from the sun will pass through every square centimeter of you each second without any measurable interactions with the molecules of your body.
Detecting them requires capturing evidence of rare moments when they crash into the cores of atoms.
Higher energy neutrinos coming from other galaxies are easier to spot than those from our sun, said particle physicist Michelle Dolinski of Drexel University, because faster neutrinos are more likely to smash into things. They present their own challenges, though. “These very high energy neutrinos are more likely to interact” with matter, she said, “but they deposit so much energy in a detector that in order to contain and detect all that energy, the detector has to be very large.”
Such a detector is buried deep beneath the South Pole, encompassing 35 billion cubic feet of ice. Fittingly, it’s called IceCube. Its 3D grid of sensors embedded in the ice detected the neutrino when it smashed into the nucleus of a water molecule. The impact exploded the nucleus, blasting out a cone of rippling radiation, mostly in the form of light waves. Computers monitoring the sensors used these ripples to reconstruct the path of the neutrino. Within a minute, they had relayed a message to telescopes around and orbiting the globe to train their sights on a small patch of sky within the Orion constellation.
There, 19 telescopes saw a distant galaxy occupied by a supermassive black hole called a blazar. It was actively flaring gamma rays, a signature of this type of black hole and a telling coincidence that it was the source of the neutrino.
The blazar was almost certainly shooting out cosmic rays that pack an even bigger punch, too. Astronomers have long puzzled over the origin of rare but incredible high energy events where atomic nuclei travelling close to the speed of light come crashing into the Earth’s atmosphere. Some of these are called “Oh-My-God particles,” because a single particle can smash into the Earth with the force of a 50-plus mph baseball—an object that’s more than a trillion trillion times more massive.
Before now, tracing the origins of this type of cosmic ray has been impossible, Dolinski said, because most are charged particles. “They experience deflections from electric and magnetic fields, so they don’t necessarily point back to the astronomical objects they came from.”
But these ultra-high-energy cosmic rays are also the only plausible source of the high-energy neutrinos, which fly true, even across intergalactic distances. This is why the teams rely on neutrinos to sniff out the origin of the other particles.
The discovery also marks a turning point in astronomy. “It’s basically a new window on the universe,” Dolinski said. A decade ago we essentially could only use telescopes to see different waves of light. Then, with the detection of gravitational waves, we gained the ability to hear the universe too. Now, with IceCube recording the impacts of intergalactic neutrinos, we have a way to feel the cosmos.
“It’s the beginning of a new channel in astronomy,” Dolinski said.