Neutrinos Have Mass! (And Not Just on Sunday)

Image courtesy of Argonne National Laboratory

We used to think neutrinos were massless, because from what little we knew about them, they appear to travel at the speed of light. Massive particles cannot do that, because it would take infinite energy to accelerate them that much.

Neutrinos don't interact with matter very much at all. It has been estimated that a ball of lead the size of our solar system would only affect about half of the enormous stream of neutrinos emitted by the sun each second. Those babies fly through everything at nearly the speed of light, as if it were nothing. Consequently, they're very hard to measure.

Somehow, we figured out that neutrinos come in three flavors: electron, muon, and tau. Yeah, whatever. They just do. And yes, particle physicists call them 'flavors'. But recently, scientists discovered that neutrinos can change flavors on the fly. Apparently, this can only happen if neutrinos have mass.

This is the part that mystifies me. All the articles I've read simply say that the ability to change flavors requires (or implies) that neutrinos have mass, but they don't say why. That's understandable, I guess, when the explanation looks like this. I'm going to have to study mass eigenstates for a while, I guess. Yowsa!

Recent experiments have established the maximum range for the mass of a neutrino at about 1eV. This is still a miniscule mass-slash-energy, and neutrinos of such a small mass can still move so fast that for all intents and purposes, they appear to be moving at the speed of light. The difference is negligible, and neutrinos are hard to measure anyway.

It's a strange, strange world, quantum physics is. A great deal of it has been experimentally verified to very high precision, with reproducible regularity. But if you're into weird, you can't make this stuff up.