The results suggested that neutrinos can change their nature as they travel and hence that they have mass. These beams passed through several hundred kilometres of the Earth and were then detected in underground caverns. Further clues came earlier this year from experiments that measured neutrinos emitted by accelerators on Earth. However, neutrinos were widely assumed to be massless. This could only happen if neutrinos have a mass.

So was born the idea of the 'oscillating neutrinos'. The popular suggestion was that if the three varieties of neutrino oscillated from one form to another during their flight, then on average, only one- third would end up as electron-neutrinos by the time they reached Earth.Īs the earthly detectors were only sensitive to electron-neutrinos, this might explain the apparent shortfall. This became known as the solar neutrino problem. The shortfall was either evidence that the sun was emitting fewer than expected, as if its centre was unexpectedly cool - with potentially disastrous consequences - or something was happening to the neutrinos en route. This is a vast number, but if our understanding of the solar interior is correct, there should have been even more. Nearly a trillion electron-neutrinos are passing through you each second. The Earth is almost transparent to neutrinos from the sun, so they shine down on our heads by day and up through our beds by night undimmed. The sun emits 'electron-neutrinos', which are created in the thermonuclear reactions in the solar core. They exist in three types - the electron and its relatives the muon and the tau.

A neutrino contains very little