Chasing the ghost particle
A century-old mystery is solved at the South Pole
Reported by Ben Gilliland
For nearly 4 billion years a ghostly traveller hurtled towards our planet at nearly the speed of light – passing through galaxies and skirting alien worlds, but never once deviating from its course.
When its journey began, the distant Earth had been a lifeless lump of rock orbiting a young star. The ghost’s journey was so vast that as it travelled, the first single-celled life evolved on Earth, singlecelled life became multicellular, complex life blossomed in the seas and conquered the land, the dinosaurs rose and fell and humans took their first tentative steps out of Africa.
Through the entirety of the history of life on Earth, the ghost had journeyed unhindered through the cosmos. Then, on 22 September 2017, it was detected on Earth. And, deep beneath the South Pole it perished. In doing so, it imparted a secret.
This is the story of the hunt for that ghost and the century-old mystery its demise is helping to solve.
At the turn of the 20th century physicists were busy experimenting with the radioactive element radium (radiation itself had only recently been discovered) when they started to notice something strange – even when the radium was removed their instruments were still registering the presence of energetic particles.
Most scientists of the time believed that the radiation must have been coming from minerals in the ground. But in 1910, German physicist Theodor Wulf, took an electroscope to the Eiffel Tower and tested radiation levels at ground level and at the top of the tower. He found that the effect was actually stronger at altitude – the radiation wasn’t coming from the ground.
The mystery was picked up by an Austrian physicist, Victor Hess, who, in a series of daring balloon flights in 1911 and 1912, took radiation readings from ground level to an altitude of 5,300 metres (17,390 feet). He found that although radiation levels initially dropped off, as he gained
altitude the readings increased until, at the highest altitudes, the radiation levels were many times greater than they were at ground level. Hess concluded that “a radiation of very high penetrating power enters our atmosphere from above”. Or, to put it another way, the particles were coming from space. But just how far out in space were they coming from?
One obvious candidate was the Sun, but Hess ruled that out by performing one of his balloon experiments during a total eclipse in 1912 – had particles been coming from the Sun the readings would have dropped off as the Moon covered it up, but the levels remained the same. The only possible explanation was that they came from further out in space. Hess had discovered cosmic rays.
Space is flooded with cosmic radiation in the form of high-energy, charged particles, such as protons and atomic nuclei. These particles can be produced in all sorts of astronomical processes, such as the nuclear reactions in stars.
But then there are the highest energy cosmic rays. These are particles that are imbued with hundreds of millions, or perhaps even billions, of times more energy than those created in humanmade particle accelerators. In fact, physicists would
need to build a particle accelerator the same size as the orbit of the planet Mercury – about 360 million kilometres (224 million miles) – to reach the energy of the particles in cosmic rays.
What mechanism could accelerate these particles to the sort of energies that make the Large Hadron Collider look like a particle pea shooter? They must surely come from some of the most violent and least understood objects and events in the universe, such as a massive star dying in a supernova explosion, or from an active black hole in the centre of some distant galaxy. It is a mystery that has endured for more than a century.
Cosmic rays are the fragments of atoms, such as protons, electrons and atomic nuclei, that have been torn apart and accelerated in violent cosmic events. The trouble with these particles is they all carry an electric charge, which means as they fly through space they interact with the magnetic fields of stars and other objects. These magnetic fields attract or deflect the charged particles, which alters their trajectory and makes them swerve around space like a cosmic drunk driver. This means that by the time they reach Earth they can tell us very little about where they came from.
“Space is flooded with cosmic radiation in the form of high-energy, charged particles, such as protons and atomic nuclei”
Ghost particles are elementary According to the Standard Model of particle physics, a neutrino is an elementary particle. The elementary particles are building blocks of the universe.
A rendering of the track detected by IceCube’s sensors on 22 September 2017. It tracks the course of a muon created when the neutrino collided with an atom in the ice. The colours show its direction of travel – from red (first detection) to green and blueThey are born from violence Neutrinos are created as a by-product of nuclear fusion in stars and in violent astrophysical events like supernovae.