Wait, what is… a neutrino?

Every second, your body is being bombarded with 100 trillion neutrinos — and no, that wasn't a typo. Put another way, that's 250 quintillion* neutrinos over a lifetime — passing through you, me, your neighbour Patrick, Carol down the road — everyone. After photons, neutrinos are the most abundant known particle in the universe — and yet, we know hardly anything about them. So... wait, what is a neutrino? In the rest of this article, we'll find out what they are, how we find them, and why we care in the first place. Let's dive in.

What is a neutrino?

Put simply, a neutrino is a fundamental particle (meaning it's not made of smaller stuff). There are 3 key things that identify a neutrino:

1. No charge

2. Almost no mass

3. Weakly interacting

And that's it! Simple; now, let's unpack these a bit.

No charge means they are electrically neutral — in the same way a neutron has no charge (and an electron has negative charge, a proton has positive), a neutrino has a charge of zero. This is actually where it got its name: neutrino is Italian for 'little neutral one'.

Point two — for a long time physicists thought neutrinos were massless, like photons. But it turns out they do have mass, and it's very, very small — over half a million times smaller than the next smallest particle we know, the electron. This tiny mass causes an interesting effect, which we'll talk about later.

Finally we have weakly interacting — which in this case, has a double meaning. Neutrinos only interact via the weak nuclear force**, one of the four fundamental forces. It's called the weak force for a reason: it's very very weak (shocking I know), and only acts over extremely short distances — as such neutrinos very rarely bump into anything. This last point is why billions and billions can pass through your body in a single second, and yet we don't feel a thing. They're the ghosts of the particle world.

Why neutrinos change flavour

Imagine you pick up a juicy red apple. You close your eyes, take a bite, and discover... it's now an orange. Weird, right? This is exactly what the small mass of neutrinos makes them do: have a minor identity crises. Every other fundamental particle we know, once created, stays exactly as it is unless it interacts with something else — sort of like Newton's first Law: an object in motion stays in motion.

Neutrinos, though, are a little different. They come in three types***, or flavours, called the electron, muon, and tau. As a neutrino travels it can switch between these three flavours in a process called oscillation. Just as it would be bizarre if an apple you bit into turned into an orange, it's just as strange to create one type of neutrino and detect it later as another.

Why do they do this? Well, most particles have a fixed, known mass. The electron, for example, has a mass of 511 keV: and that's part of what makes it an electron. Neutrinos, though, don't play by these rules. Each flavour of neutrino can be made up of 3 different masses. When a neutrino is born, you could imagine it's 'assigned' just one of those masses and goes about its merry way — and this is an okay way of thinking about it. But nature isn't quite that simple — the newborn neutrino is a blend of all three masses at once, something we call a quantum superposition. As the neutrino travels, these different masses drift slightly out of sync, and that is what makes the neutrino appear to change flavour over time.

So you can think about it this way: when a neutrino interacts, it shows its flavour. When it travels, it puts on its mass mask. Confused? Don't worry — so am I. This behaviour is key to understanding neutrinos, but it's also one of the strangest concepts in all of physics. Keep thinking about it — eventually, it'll click.

Why neutrinos matter

Considering they do... very little — why do physicists care about them?

For starters, neutrinos give us the first experimental hint of physics beyond the Standard Model of Particle Physics (SM). This SM is one of the most successful frameworks we have in physics, yet it has neutrinos as massless — and we know they're not. The fact that neutrinos have even this tiny mass indicates the SM isn't complete, and physicists are actively searching for more physics beyond it.

That was the more "boring physicist" answer, so let's zoom out. One of the biggest open questions in physics is why we exist in the first place. The SM tells us that matter and antimatter are always produced in equal amounts and when matter and antimatter meet, they annihilate with each other. But... we exist, and we're made of matter. So where did all the antimatter go?

Neutrinos might be the key! If they can break a set of fundamental rules, called symmetries, they could have tipped the balance, creating a tiny amount more matter than antimatter. That minute imbalance could explain why I can write this article, and you can read it.

Of course, there are plenty of other reasons why neutrinos are fascinating in their own right, but I'd argue that explaining why we're here is the most important.

Summary

There you have it — neutrinos: unsociable cosmic ghosts with an identity crisis that zip around the universe. We can't see them, we can't feel them, but they could be the reason me, you and Carol all exist in the first place.