What is a Neutrino?

Fran takes this question...

Fran - Yeah, that's a good question. A neutrino is a very, very light particle. It's one of the particles of the standard model, which means we're very sure it exists. It's produced in radioactive decays, and around a hundred trillion neutrinos pass through your body every second. But because they're so weakly interacting, they just go straight through you and you don't notice them. To detect even a handful of neutrinos we have to build huge and very, very sensitive detectors; but they're actually all around us, they come from the sun, they come from nuclear reactors, they were produced in the early universe... so they're just everywhere.

Chris - So are they actually part and parcel of an atom or, are they only produced when there's a radioactive event - an atom decays, for example, and then the particle whizzes off - and if the latter, what's their ultimate fate then?

Fran - They're only produced in a radioactive decay, as you say. When they're produced by the atom they whizz off. And they don't really have an ultimate fate. They just stream through the universe forever.

Chris - Does that mean the number of them is therefore, theoretically, permanently increasing indefinitely?

Fran - Yes, but space is expanding. So more and more are being produced, but they're also being diluted by the expansion of space.

Chris - But the number of neutrinos out there is going up all the time, through radioactive decay? And they're not going to turn into anything else, there's nothing else they can do, once they're a neutrino?

Fran - They do occasionally turn into other things in a process that's like the inverse of radioactive decay. I don't quite know how those two effects balance out. My guess would be that whenever a radioactive decay happens, some decays always produce a neutrino, whereas once the neutrino is there it's very rare for it to ever interact with anything. So my guess would be the total number of neutrinos is always increasing. But I'd have to go and check that to tell you for sure.