In 1912, scientists discovered that the Earth is bombarded by a constant stream of high-energy particles from beyond the solar system. They called the mysterious particles ‘cosmic rays’ and set out to find out where they were coming from – 100 year later, they are still looking.
The charged particles that make up cosmic radiation can be produced in all sorts of astronomical processes – such as the nuclear reactions in stars. But the origin of the highest energy of these particles, which can strike the Earth with energies up to a hundred million times higher than those created in man-made particle accelerators, remains a mystery.
Over the years there have been several contenders, but the most promising was thought to be a phenomenon known as a ‘gamma ray burst’ (GRB) – a massive burst of energy released when a massive star explodes in a supernova.
To test the theory, scientists built an array of particle detectors 2.4km high and 800m wide and inserted it beneath the ice at the South Pole.
But the aptly-named IceCube observatory wasn’t built to look for the high-energy particles themselves – they come from all over the sky and can tell us nothing about where they came from. It was built to detect neutrinos – a sort particle version of a Jedi Ninja, that is thought to be produced alongside the charged particles in cosmic rays.
Neutrinos posses almost no mass, have no electric charge and can pass through matter without even having to say "excuse me". These properties allow the neutrino to travel undisturbed through the cosmos in an “as the cosmic crow flies” fashion.
So, if scientists find neutrinos coming from the direction of a gamma ray burst, its a fair bet that that is where they were produced. Find the neutrinos and you have found the source of the cosmic rays.
The IceCube neutrino telescope has been watching 300 gamma ray bursts for signs of one of these tell-tale neutrinos and, in two years of observations, they have spotted precisely... none – not one neutrino has been detected coming from the most likely source of cosmic rays.
So it’s back to cosmic drawing board and time for IceCube to move on to the next most likely source – black holes.
Graphic below: How neutrinos can guide scientists to the source of cosmic rays. Click to zoom
Find out how scientists discovered cosmic rays 100 years ago this month. Click here