They are electrically neutral and have very small rest mass. They barely interact with anything, passing through planets, stars, and even you by the trillions every second without a trace.
Neutrinos belong to the lepton family, the same group as electrons and muons, but unlike their charged cousins, neutrinos have no electric charge. This means they don’t feel the electromagnetic force at all. They only interact via the weak nuclear force and gravity (though their mass is so tiny, gravity barely notices them). Because of this, they can travel cosmic distances without being stopped, making them messengers from the most extreme places in the universe: supernovae, black hole collisions, and even the Big Bang itself.
They come in three known flavors:
- Electron neutrinos (): produced in nuclear reactions, like those in the Sun.
- Muon neutrinos (): created in high-energy cosmic ray collisions.
- Tau neutrinos (): linked to the heavier tau particle, the rarest and hardest to detect.
And here’s where things get even weirder: neutrinos can actually change from one type to another as they travel, a phenomenon called neutrino oscillation. This was a groundbreaking discovery because it proved that neutrinos must have mass, something the original Standard Model of physics didn’t predict.
Do neutrinos explain why the universe has more matter than antimatter? Could there be a fourth “sterile” neutrino, one that doesn’t interact even via the weak force? We don’t know yet.