- Massless, uncharged nuclear particle whose main property is spin. Neutrinos are given off in immense numbers in nuclear reactions – a flood of uncounted billions of them pours off the Sun, e.g. – and they have immense importance in astrophysical and cosmological calculations. Because they are massless (or according to some cosmological theories possess a very tiny mass), they interact hardly at all with matter, and neutrinos generated in the Sun’s core pass out of the Sun with almost no probability of being affected by the rest of its matter. This means that solar neutrinos are the most direct form of information we can receive about the very centre of the Sun. The problem is that anything which will not interact with matter is hard to detect. Neutrino telescopes are the answer. They bear no resemblance to any other type of telescope. Instead they usually consist of a large cavern, perhaps a disused mine working, filled with a material like cleaning fluid. On the very rare occasions when a neutrino interacts with a chlorine atom in the fluid, the atom is transformed into an atom of argon which is then detectable. Assigning a very small mass to the neutrino is one possible solution to the missing mass problem.
- A neutral elementary particle with zero mass, or nearly zero mass. It is a type of lepton, and is produced, for instance during beta decay. Neutrinos travel at the speed of light, and pass through matter with little or no interaction.