Quotations
(1) Neutrinos actually have some mass
It now seems to be the case that neutrinos actually have some mass. For a long time it was assumed that they had no mass at all, and therefore travelled at the speed of light. However it is now believed that they do have a tiny bit of mass and therefore do not travel at the speed of light. Any mass they have would be negligible, but because they are created in such huge quantities they may actually contribute the most mass of any particle to the universe.
(2) Neutrinos are incredibly numerous
The number of neutrinos out there is mind bogglingly huge. Every second there are literally billions of them passing straight through our bodies unaffected and unhindered as they interact so rarely with other matter. The neutrinos are mainly streaming through us from the sun, and at night they pass through us unhindered right through the planet from the other side of the earth, and also they are produced in natural radioactive processes in the earth too as well as some cosmic rays.
(3) Neutrinos can tell us about stars
Scientists have worked out the reactions that they think go on in our star, and specifically the fusion reactions and the various products, and how likely each reaction is to happen. These equations show that neutrinos should be formed, and predict the numbers that are formed as a result. Now it is impossible to really look into the heart of a star, but by measuring the number of neutrinos that come to earth (through working out how many are produced, then the probability of one interacting with matter, then measuring if the two numbers tally) we can check our understanding of the nuclear reactions within stars. And remarkably there is very good correlation between the two, implying that our understanding of the fusion reactions within the sun is very accurate. And even more, this gives us a very accurate idea of the temperature of the sun too, because the number of neutrinos output is very sensitive to the temperature, and would change significantly with a small temperature difference. Now neutrino astronomy could be used to probe our understanding of other stars around the universe and is a new field just opening up.
(4) Neutrinos come in different varieties
There is the electron neutrino, the muon neutrino and the tau neutrino. The muon and tau are other members of the electron family and are heavier in each case. They are unstable and rapidly decay. And it has been shown that each of them has a particular type of neutrino associated with it. And each of those neutrinos in turn has an anti-neutrino partner.
(5) Flavour oscillation
The different types of neutrino are called different flavours. Perhaps the most interesting property of neutrinos is the phenomenon that one form can apparently convert back and forth to another type. Thus an electron neutrino could for instance be detected as a muon neutrino! This is what implies that they have some mass, as if they were of zero mass under our current understanding of physics this oscillation could not occur.
