Neutrinos are the universe’s ghosts. Trillions of these particles move through us all the time at close to the speed of light. We cannot see, feel or sense them.
For the first time, scientists have captured a neutrino and traced its origin - a supermassive black hole 4 billion light-years away.
The discovery could reveal more about some of the universe's great mysteries - black holes and dark matter.
“We are truly in the era of astro-particle physics," says Swinburne University astronomer Associate Professor Alan Duffy. "This may lead to us finally learning how black holes grow and feed, as well as shed a light on dark matter."
What is a neutrino?
A neutrino is one of the universe's fundamental particles, says Adelaide University Associate Professor Gary Hill, who is a member of the international team behind the discovery.
But unlike nearly every other particle, it has next-to-no mass, no electric charge, and does not interact with anything. It just exists, alone, in the universe.
They are produced when very small particles called protons smack into each other at high speed.
To find one, you dig
To search for a ghost, you go under the ice.
Scientists dug 2.5 kilometres beneath the South Pole, where they built the IceCube Neutrino Observatory.
Professor Hill spent several seasons living on the ice cap to help build it. “It was like being on a big construction site, except it was minus 20 or 30 degrees all the time. It was a massive engineering feat.”
Very, very rarely, neutrinos bump into other particles, releasing a tiny burst of radiation and a pale blue light. This is the only way of knowing they are there. IceCube’s sensors monitor one square kilometre of ice for those telltale bursts of light.
So far, they’ve been operating for almost eight years. In that time, they’ve found maybe 1000 high-speed neutrinos - including this one.
Scientists determined that the neutrino found on the South Pole was shot out of the jetstream of an extremely powerful black hole in a blazar galaxy 4 billion light-years away.
Blazar galaxies have huge black holes at their centre. These black holes are so powerful they produce a huge jet of red-hot energy - like the exhaust of a jet engine.
The jet is so powerful it accelerates protons to such a speed that when they collide a neutrino is produced.
“This guy [the black hole] is 4 billion light-years away from Earth. That means this neutrino started its journey probably around the time the Earth was born,” says Professor Hill.
Scientists are already thinking up ways of using this discovery to find out more about the universe, especially about dark matter.
Dark matter makes up 25 per cent of the universe, but is even more ghostly than a neutrino – we are yet to detect it at all.
“We know there is dark matter all around us,” says Professor Hill. “Some of it will get trapped in the core of the Sun and the core of the Earth, and eventually release neutrinos – now we’re hunting for those.”