Elusive ‘ghost particle’ is trapped in giant ice cube
For seven years a giant astrophysical trap had been set around a cubic kilometre of ice beneath the South Pole.
On September 22 last year, it finally caught its prey: a microscopic shower of blue light from an elusive space particle crashing into a molecule of water.
The detection has allowed scientists to trace the particle back to a supermassive black hole that is emitting light and cosmic rays at the centre of a galaxy four billion light years away.
This extraordinary feat, described in two papers published in the journal Science, has been hailed as the opening move in a new era of astronomy.
The particle was a bizarre specimen known as a high-energy neutrino. First hypothesised by Wolfgang Pauli, a Swiss-American Nobel laureate, in 1930, neutrinos are strange even by the baffling standards of sub-atomic physics. They are nicknamed ‘‘ghost particles’’ with good reason. They zip around the universe at the speed of light, changing their physical properties and almost never troubling themselves with the world of matter. They have no charge and virtually no mass. About 65 billion pass through every square centimetre of the human body each second.
Neutrinos are born when heftier particles are dismembered or smashed together in cosmic furnaces such as supernovas and the sun. To catch one, you need a very large box of liquid and a good deal of patience.
Working out where it came from is even harder. The instant scientists spot the trace of a neutrino in one of their traps, they must swivel their telescopes to track its path back to a source in space. That is what hundreds of researchers around the world, including the University of Leicester, appear to have achieved.
The breakthrough originated in the IceCube neutrino observatory, where 5106 light detectors were drilled nearly 3km down into the ice below the Amundsen-Scott South Pole station.
Each time a neutrino interacts with an atomic nucleus in the water ice, it gives off a tiny flash of blue light known as Cherenkov radiation. IceCube had spotted 81 flashes without being able to find the neutrinos that made them.
Then came a stroke of luck. The Fermi gamma-ray space telescope, orbiting the Earth, and another observatory in the Canary Islands picked up a burst of radiation coming from the same direction as the neutrino.
The source was a blazar, a vortex of spinning energy around a black hole in the middle of a distant galaxy.
Excitement about the discovery is caused partly by its technical difficulty but largely by the possibilities it opens up.
Doug Cowen, professor of astrophysics at Penn State University, said that it would lead to ‘‘exciting breakthroughs in our understanding of the universe and fundamental physics’’. – Telegraph Group