This week’s guest blogger, Frank Close, is a particle physicist, author and speaker. He is Professor of Physics at the University of Oxford and a Fellow of Exeter College, Oxford. He is the author of several books, including the best-selling Antimatter, and the winner of the Kelvin Medal of the Institute of Physics for his “outstanding contributions to the public understanding of physics.”
Of all the things that make the universe, the commonest and weirdest are neutrinos*. Able to travel through the earth like a bullet through a bank of fog, they are so shy that half a century after their discovery we still know less about them than all the other varieties of matter that have ever been seen.
These will o’ the wisps are coming up from the ground beneath our feet, emitted by natural radioactivity in rocks, but most of those hereabouts were born in the heart of the Sun less than 10 minutes ago. In just a few seconds the Sun has emitted more neutrinos than there are grains of sand in the deserts and beaches of the world, greater even than the number of atoms in all the humans that have ever lived. As you read this, billions of them are hurtling, unseen, through your eyeballs at almost the speed of light. They pass through the earth as easily as a bullet through a bank of fog.
If we could see with neutrino eyes, night would be as bright as day: solar neutrinos shine down on our heads by day and up through our beds by night, undimmed. To capture even a few of them requires thousands of tonnes of material. When Ray Davis began chasing solar neutrinos in 1960, many thought he was attempting the impossible. It nearly turned out to be: 40 years were to pass before he was proved right, winning his Nobel Prize in 2002, aged 87.
Patience is an asset in the neutrino business. Not only was Davis the first human to look inside a star, his legacy is a new science: neutrino astronomy. Not just the Sun; each of the stars visible to the naked eye, and the countless ones seen by the most powerful telescopes, are all filling the void with neutrinos. The neutrinos born in the Sun and stars, numerous though they are, are relative newcomers. Most are fossil relics of the Big Bang, and have been travelling through space unseen for over 13 billion years.
Scientists are now decamping to Antarctica, in the hope of achieving things even more remarkable than even Davis – capturing neutrinos from distant stars, and even some that are remnants of the big bang.
Neutrinos from afar
Apart from the sun, the only star ever seen to shine in neutrinos has been a supernova.
On 23 February 1987, utterly without warning, a supernova was seen to have erupted in the Large Magellanic Cloud, a satellite galaxy of the Milky Way in the southern skies. A blast of neutrinos from this explosion, having travelled across space for 170,000 years, passed through the Earth during about 15 seconds that day. Underground experiments detected a handful of neutrinos from the supernova.
Astrophysicists had long believed that the gravitational collapse of a supernova is a copious source of neutrinos; that the brilliant flash of light, the traditional manifestation of a supernova that can briefly outshine an entire galaxy, is only a minor part of the drama. Powerful though this intense electromagnetic radiation is, the visible light, radio waves, X rays and gamma rays all add up to less than 1 per cent of the whole. The bulk of the energy radiated by the supernova is carried away by neutrinos.
For the first time, we had detected neutrinos emanating from outside our galaxy, and proved that the theory of a supernova is right: when stars collapse they throw off their energy as neutrinos, up to 1059 – that’s one followed by 59 zeroes – a hundred billion trillion trillion trillion trillion of them.
Most had spread around the cosmos; only a few passing through the detectors on Earth. Even so, by detecting this momentary blast of neutrinos, we had our first look into the workings of a supernova. This confirmed everything that had previously been just theory: a supernova is the result of a star collapsing to form a neutron star.
Neutrinos on Ice
With the singular exception of supernova, neutrinos from stars in our galaxy and beyond are probably as faint compared to solar neutrinos as is starlight to daylight. To have any chance of capturing them requires detectors containing over a cubic kilometre of matter.
The ingenious solution is Ice Cube, an experiment just beginning at the South Pole, which uses the ice in the Antarctic as a natural detector of the vast numbers of neutrinos that fill the void
Ice in the Antarctic is not like ice that we are used to on a cold winter’s day at home. In the Antarctic, snow has fallen on ice for much longer than recorded-history. Deep down, the pressure is so great that all the air bubbles have been squeezed out, leaving ice so pure that light flashes, produced by neutrinos, can travel hundreds of metres.
Photomultiplier tubes – devices for recording the tell-tale flashes of light – have been lowered into the ice, down shafts that are made by a special drill that sprays out hot water and melts a hole. The detector is attached to a long cable, lowered into the ice, which then freezes it into place. From then on it records data continuously. The set-up is so sensitive that it regularly records neutrinos produced by cosmic rays hitting the atmosphere from all around the globe; some come from directly above the Antarctic, while others have travelled all the way through the Earth, from the North Pole.
Ice Cube will look further into space and into areas – such as the galactic core of the Milky Way – that we’ve never been able to see before. It is possible that neutrinos will interact with the background radiation from the big bang. There may be surprises, even more sensational than anything that has happened so far.
To learn more on this thrilling subject why not come to one of Frank Close’s talks, or read his book Neutrino. Details can be found on his website
*Definition: The neutrino is a sub atomic particle which holds no electrical charge, travels at nearly the speed of light, and passes through ordinary matter nearly unharmed. Neutrinos are emitted in huge numbers by stars like the sun.