When the rare neutrino interacts with the nuclei of hydrogen or oxygen, it creates a flash of light or radio waves that can be detected by nearby sensors. Instead, physicists look for the secondary effects of neutrinos interacting with a medium like water. Since neutrinos hardly ever interact with matter, it’s effectively impossible to detect the particle directly. In both cases, the methods for detecting the neutrinos are similar they mostly differ in their environments and scale. If you’re a neutrino hunter, the type of detector you want to use will depend on whether you’re looking for low-energy neutrinos or high-energy neutrinos. “They’re great for directly probing astrophysical sources because they travel cosmological distances without getting corrupted, but you need very large detectors to have a chance of seeing any.” “Neutrinos are a blessing and a curse,” says Cosmin Deaconu, a physicist at the University of Chicago and member of the Anita team. This means a cosmic neutrino detected on Earth is almost exactly the same as it was when it was spit out by the core of a galaxy on the other side of the universe. When a neutrino is produced, it keeps traveling in a straight line at the speed of light for eons. This makes neutrinos extremely difficult to detect, but it also makes them the best way to study exotic celestial objects like quasars and blazars that are billions of light years away. Unlike light and other forms of radiation produced by these galaxies, a neutrino doesn’t lose its energy as it travels through space, since it doesn’t interact with other matter. These have around a billion times more energy than solar neutrinos. At the other end are ultrahigh-energy cosmic neutrinos like the kind created around the supermassive blackholes in the turbulent hearts of so-called active galactic nuclei. At the low-energy end of the spectrum are neutrinos produced by our sun, which constitute the vast majority of natural neutrinos streaming through the Earth. ![]() But not all neutrinos are created equal-some have vastly more energy than others. This means that the Earth’s atmosphere, nuclear reactors, and even bananas are all neutrino factories. Neutrinos are produced whenever the nuclei of radioactive elements break down. The challenge for physicists is to figure out how to detect these rare interactions so the neutrinos can be studied. Consider this: Even though any given human encounters quadrillions of neutrinos per day, over the course of their lifetime only a single neutrino will interact with one of the billions of atoms in their body. On occasion, they do interact with other matter, but these instances are incredibly rare. They pass through solid material like sunlight streaming through a window. ![]() But neutrinos are the ghosts of the subatomic world. Given their abundance, you’d expect that detecting them would be about as challenging as catching fish in a barrel. At any given time, there are trillions of these nearly massless particles passing through your body at the speed of light. Gorham and his colleagues hoped Anita could lead them to the source of these particles like a hound tracking its prey-but first it had to find some. Some of the cosmic neutrinos that reach Earth have been traveling through space at the speed of light for billions of years, but their provenance remains a mystery. Anita was scouting for the faint burst of radio waves created when they pass through the Antarctic ice, one of only two proven ways to detect the elusive particles. Neutrinos are nearly massless and rarely interact with other matter, which makes them extraordinarily difficult to study. These rare subatomic particles could provide a window onto some of the most violent processes in the universe-but there’s a catch. It rustles in this majestic way as it rises up, and it’s just an unforgettable sound.”įor the past decade, Gorham and a small team of scientists had regularly traveled to Antarctica to send Anita on missions to detect signs of cosmic neutrinos. “The balloon looks like a giant jellyfish, and it just kind of hovers there for a moment when it’s released. ![]() When the balloon was full, it carried Anita 20 miles into the atmosphere, where it spent the next month riding the polar vortex in circles over Antarctica. The experiment was known as Anita-short for the Antarctic Impulsive Transient Antenna-and its hulking frame was a checkerboard of square white antennas and black solar panels. Attached to the balloon was a gondola the size of a semitruck cab that was designed to turn the entire frozen continent into the world’s largest radio dish. ![]() It was a crisp December morning in 2016 at the icy airfield near McMurdo Station in Antarctica, and Peter Gorham was watching a massive balloon fill with helium.
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