Neutrinos are elementary particles with practically zero mass which are produced by nuclear reactions. While low energy neutrinos are produced by the Sun and other similar phenomena, high energy neutrinos are produced in distant and extremely violent cosmic cataclysms such as black holes, supernovae and the Big Bang.
Once generated by cosmic cataclysms, neutrinos move at a speed close to that of light and do not stop. Their mass being virtually nil, they only very rarely interact with other particles, allowing them to move in a straight line to the frontiers of the universe, passing through stars, planets, vast magnetic fields and entire galaxies as if they did not exist. Trillions of neutrinos pass through the Earth every nanosecond and, for astrophysicists, each one of these infinitesimal particles is a potentially important messenger bringing information about its origin.
The problem for scientists is that the very properties that enable neutrinos to carry this information make them notoriously hard to detect. Fortunately, on rare occasions a high energy neutrino collides with an atom. In the collision the nucleus of the atom disintegrates and the neutrino is transformed into another particle called a muon. The muon continues along an identical trajectory to that of the neutrino and can be recognised by the cone of blue light that it generates. The cone of light is known as Cherenkov radiation and can be compared to the waves produced in the air by a bullet.