Astronomers have become incredibly skilled at detecting cosmic radiation in all its forms—not just ordinary, visible light, but also light we can’t see, everything from infrared to ultraviolet to gamma rays. They’ve seen bursts of energy from black holes halfway across the universe, blips of radio noise from neutron stars spinning at hundreds of revolutions per second, and even the faint glow of microwaves emitted more than 13 billion years ago, in the immediate aftermath of the Big Bang itself.
Trying to detect neutrinos, on the other hand, has proven hellishly difficult, and no wonder: these elementary particles are so elusive that the average neutrino could zip through a chunk of lead five trillion miles (8 trillion km) thick without the slightest problem. So while they stream across the universe in vast numbers—literally trillions of them pass through your body every second—building a telescope to catch them is no mean trick.
But that’s what a team of scientists from 41 institutions in 12 countries has pulled off, and the first results have just come out in the Science. Using a neutrino telescope known as IceCube, located appropriately enough at the South Pole, astronomers have detected 28 high-energy neutrinos that almost certainly came from the depths of the universe.
“This is quite a big deal,” says Ray Jayawardhana a University of Toronto astrophysicist and author of a new book called Neutrino Hunters, who wasn’t part of the research team. “This is a new window on the universe, and especially on the most ferocious, violent cosmic events.” When Jayawardhana says violent, he means it: events like jets of matter spewing from giant black holes at the cores of massive galaxies or gamma-ray bursts from the most powerful stellar explosions, both of which create neutrinos in droves.
IceCube is not the first neutrino detector ever created. That one, which confirmed the particle’s existence, was set up in the 1950’s just outside a nuclear reactor, where physicists suspected neutrinos were being churned out by the quadrillions. Other detectors have managed to pick up neutrinos created in the Sun’s core, and even from an exploding star, or supernova, just outside the Milky Way—the only neutrinos ever confirmed from beyond the Solar System.
But IceCube puts those earlier efforts to shame. It’s made up of more than 5,000 individual detectors, strung on 86 cables and sunk up to 1.5 mi (2.4 km) into the East Antarctic Ice Sheet. The detector-studded cables were lowered into holes drilled with high-pressure hot-water hoses; then the holes, filled with water, were allowed to re-freeze, sealing the cables permanently in place. “It’s like launching a satellite,” says Francis Halzen, a University of Wisconsin physicist and IceCube’s lead scientist. “You know you’ll never get your hands on it again.”
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What the scientists see with all those detectors isn’t the neutrinos themselves; it’s tiny flashes of blue light, called Cherenkov radiation, triggered on those occasions when a neutrino slams into a hydrogen or oxygen atom in the frozen H2O that surrounds the detectors. By measuring the intensity and direction of the flash, the scientists can calculate the energy level and flight path of the neutrino that caused it. And those two details make all the difference.
The vast majority of flashes come from neutrinos created right in Earth’s atmosphere, as cosmic-ray particles smash into air molecules in tiny, violent collisions. The new report in Science is based on IceCube’s first two years of operation; in that time, the instrument picked up several hundred thousand such neutrinos. Only 28 turned out to be extragalactic, judging by their trajectory and the punch they packed. But that’s just about the number theorists predicted.
“It sounds a bit like looking for a needle in a haystack,” says Halzen, “but it turns out to be amazingly easy. When you see one of these events on the display, you know you’ve never seen anything like it before.”
With little more than two dozen neutrinos in hand so far, scientists can’t say much of anything about the violent events that created them. But IceCube’s electronics were designed to survive for 20 years in the South Polar ice, and Halzen is convinced that the project’s scientists will find ways of upping the rate of needles they can pull from their haystack. “I don’t think it will take twenty years, before we learn some very interesting things about the universe,” he says. In a cosmos that’s been around for nearly 14 billion years, that’s not much of a wait.