Some 9.1 billion years ago, a giant galaxy burped a blast of neutrinos – the most energetic particles in the Universe. And today, an international team of scientists reports it caught one of the elusive particles in a massive cube of ice buried near the South Pole.
The study, led by Matthias Kadler from the University of Würzburg in Germany and published in Nature Physics, describes the high-energy particle, whose arrival coincided with an outburst in a distant galaxy.
Neutrinos are one of the fundamental building blocks of the Universe. They're tiny – about the size of an electron – and travel almost at the speed of light, but carry no charge. This means they're able to whiz through solid matter and electromagnetic fields with ease. Trillions of neutrinos flow through your body every second.
But it's their size and neutrality that makes them so hard to capture, even though they're the most abundant particles.
Enter the South Pole Neutrino Observatory, better known as the IceCube: a cubic kilometre of ice filled with sensors hanging from "strings" but encased in Antarctic ice up to 2.5 kilometres deep. The sensors are designed to capture flashes of light, created on the odd occasion a neutrino interacts with water molecules as it barrels through Earth.
Almost 40 flashes have been picked up by IceCube since 2010, but all bar three have been less than a petavolt in energy.
One of these high-energy neutrinos was picked up in 2012, when IceCube detected a flash of two petavolts – the most powerful yet. But while scientists had some idea of the neutrino's path, it couldn't pinpoint where the neutrino came from. To trace its origin, the team turned to radio telescopes.
A number of telescopes are trained on interesting objects, such as big old galaxies. One galaxy – dubbed PKS B1424-418 – has been under surveillance for decades. It's 9.1 billion light-years from Earth, so formed when the Universe with relatively young.
From 2011 to 2014, it changed dramatically, morphing shape and brightening to four times its previous luminosity. When the researchers examined the galaxy's energy output, it was high enough to explain the neutrino.
It was, they calculate with a 5% margin of error, the neutrino's source.
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