(This is Part 4 of a series on neutrinos, Majorana fermions, and one of the strangest open questions in physics. Read Part 1, Part 2, and Part 3.)

It’s 1937. One year before Ettore Majorana vanishes. He is sitting with Dirac’s framework — the precise, picture-perfect vision of quantum mechanics — and doing what very few people in history have been capable of doing: going toe-to-toe with it.

He asks the kinds of questions nobody else is even thinking of asking. Does everything HAVE to work this way? Does a particle HAVE to have a distinct antiparticle?

He discovered that the answer is no. It’s not mandatory. It’s optional. It’s a choice. And it’s a choice that the universe, in all its infinite wisdom, made for electrons and quarks and every other charged particle we know. But neutrinos have no charge. Do they absolutely 100% HAVE to follow the same rules?

Majorana said “eh, maybe not.” And then disappeared.

These are what we call Majorana particles, as opposed to Dirac particles.

All Dirac particles have charge and have an antiparticle partner. All Dirac particles flip-flop between the two hands, but the universe doesn’t really care. Maybe neutrinos aren’t Dirac particles. Maybe they’re Majorana particles. Maybe their opposite partner doesn’t have opposite charge — it has opposite handedness. And the “charge” is the part that nobody cares about. Which is true, because neutrinos don’t have charge.

This means that neutrinos might be their own antiparticles.

Consider this: remember when 3D movies were briefly everywhere? Those work because light comes in two handednesses — left-circularly polarized and right-circularly polarized. One lens filters one out and passes the other, giving each eye a slightly different view. The photon is its own antiparticle. A left-handed photon and a right-handed photon aren’t particle and antiparticle of each other — they’re just the same particle with different handedness. The photon gets away with this because it carries no charge. Nothing forces the particle/antiparticle distinction to exist.

The Majorana idea is just: maybe the neutrino does the same thing. For the same reason.

In the Dirac picture we have four options. Left-handed neutrino — we see it. Right-handed antineutrino — check. Right-handed neutrino — invisible. Left-handed antineutrino — never seen. Two observable, two permanently hidden.

In the Majorana picture, we collapse that. The right-handed antineutrino and the right-handed neutrino? Same thing. The left-handed antineutrino and the left-handed neutrino? Same thing. Just two particles instead of four.

Most particles care about charge but not about handedness. Neutrinos might be the kind of particle that cares about handedness but not charge.

The Dirac picture asks us to believe in four kinds of particles when we only ever see two, and explains the missing two with “they exist but interact with literally nothing, deal with it.” The Majorana picture says: maybe there are only two particles. Maybe the universe isn’t hiding anything. Maybe we were just overcomplicating it.

But nature doesn’t care about elegance. You can have a beautiful, perfect, logical, completely wrong theory.

Watching Atoms Die

So how do we test it? How do you look at a neutrino and ask: hey buddy, are you your own antiparticle?

One option is to watch atoms die.

There’s a process called double beta decay. Sometimes two neutrons in a nucleus decay at the same time, producing two protons, two electrons, and two antineutrinos. We’ve seen this happen. It’s rare, but it’s real.

But if neutrinos are Majorana particles, then there’s really no such thing as “neutrino” versus “antineutrino” — they’re the same thing. And that changes what can happen inside the nucleus when the reactions go down. Instead of two antineutrinos coming out, you have one coming out of one neutron and going straight INTO the other. What comes out is two protons, two electrons…and nothing else.

We call it neutrinoless double beta decay. And right now, in deep underground laboratories that are absolutely not evil lairs, shielded from cosmic rays, surrounded by tons of carefully chosen isotopes, experiments are running and watching and waiting for exactly this signal.

We’ve got nothing.

That’s not a no. But it’s also not a yes. It’s just…not yet. The signal from neutrinoless double beta decay would be extraordinarily faint — neutrino masses are so vanishingly small that even if the process exists, it almost never happens. The non-observation just tells us it’s rare. It sets limits. But it’s not the final word.

Nobody knows what happened to Ettore Majorana. Some said it was suicide — that letter he sent wasn’t exactly the epitome of mental health. Some said he faked his death and fled to a monastery. There were reported sightings in South America, years later. Unverified, of course.

A lot like his namesake particle. A case that hasn’t been closed.