Universe Today
On March 25, 1938, a 31-year-old physicist named Ettore Majorana bought a ticket for a ferry from Palermo to Naples. That night, before boarding, he sent a letter to Antonio Carrelli, director of the Naples Physics Institute:
Dear Carrelli,
I made a decision that has become unavoidable. There isn't a bit of selfishness in it, but I realize what trouble my sudden disappearance will cause you and the students. For this as well, I beg your forgiveness, but especially for betraying the trust, the sincere friendship, and the sympathy you gave me over the past months.
I ask you to remember me to all those I learned to know and appreciate in your Institute, especially Sciuti: I will keep a fond memory of them all at least until 11 pm tonight, possibly later too.
*— E. Majorana*
He was never seen again.
Enrico Fermi — affectionately known as the Pope of Physics — had this to say about him: "There are several categories of scientists in the world. Those of second or third rank do their best but never get very far. Then there is the first rank, those who make important discoveries, fundamental to scientific progress. But then there are the geniuses, like Galileo and Newton. Majorana was one of these."
One year before he disappeared, Majorana published his last paper. A strange, quiet little paper that most physicists ignored at the time. It described a theoretical possibility — a particle that is its own antiparticle. Something that shouldn't be able to exist. Something that, if it does exist, just might unlock the future of physics.
We still don't know what happened to Ettore Majorana.
And we still don't know if he was right.
Yes this series is going to be about neutrinos, because of course it is. Neutrinos, if you'll recall from previous rage-filled diatribes on the subject, are awful. They spoil everything. We basically had all of physics figured out (I'm exaggerating but vibe with me) until the neutrino came along. It doesn't behave. It doesn't follow the rules. It doesn't care about your standard model and your Nobel prizes and your fancy equations. The neutrino is just going to exist and it's going to do its own thing and it's going to be PERFECTLY HAPPY WITH THAT.
Listen folks, if you PROPOSED the existence of neutrinos, even TODAY, RIGHT NOW, with no evidence you would be LAUGHED OUT OF THE ROOM, which would be humiliating.
Neutrinos don't care about your feelings.
And I'm going to tell you EXACTLY how neutrinos break every single rule that we happen to care about. But to do that, I need to explain what a particle is, probably in a way that you've never heard before, so please secure any loose items and keep your arms and legs inside the ride at all times, because we're about to get weird.
Particles aren't really particles.
At least, not in the way we usually think about it.
We're used to a simple conception of a particle. Like an electron. It has properties that we can measure. Mass, charge, spin. We can point to it. We can throw them across the room. We can trade them. Even in the quantum field picture, we can still treat an electron as an OBJECT, a thing, a concrete fixture that exists as a unique and independent entity in the universe.
So...that's wrong.
Think about your hands. Or just look at them. Regard them. Your left hand and your right hand are mirror images of each other. They have the same fingers, the same basic structure, the same everything — except they're fundamentally, irreducibly different. You can't rotate your left hand into your right hand. You can flip it, you can twist it, you can wave it around like you just don't care, and it will always be a left hand. There is no sequence of moves that transforms one into the other.
That property — that built-in handedness that can't be changed — is called chirality. And it shows up everywhere in nature. Certain molecules are chiral. DNA is chiral. Life itself has a preference: almost every amino acid in your body is left-handed. Why? Nobody really knows. But the universe apparently has opinions about handedness.
So do particles.
A particle moving through space has a direction — it's going somewhere. And a particle also has spin — it's rotating, in a quantum mechanical sense that doesn't map perfectly onto a spinning top but is close enough for our purposes. Like a spinning bullet flying through the air. The relationship between those two things — which way the particle is moving, and which way it's spinning — gives the particle a handedness. If the spin aligns with the direction of motion, we call it right-handed. If it's opposite, left-handed.
Now, the picture I just described is called HELICITY, which can change from your point of view. If you race past a speeding, spinning bullet and look behind you, it looks like it's moving AWAY from you, not TOWARDS you, so its handedness flips.
So for particles we use a different assignment of handedness called CHIRALITY that doesn't depend on how you look at it. It's a real physical property, just like mass and charge. A left-handed particle and a right-handed particle are, in a deep sense, different objects, just like your hands. Almost perfectly the same, but different orientations of existence. Mirror images of each other.
For massless particles, the chirality can't change. Neither can the helicity. In fact for massless particles they're the exact same thing. Why? Because you can't race past a photon and look behind you. So once a photon is born with a particular handedness, either left- or right-handed, it stays that way forever.
The same is NOT true for massive particles. Massive particles, as they travel, actually flip their chirality as they move! Left, right, left, right. It's like if you took a right-handed glove and threw it, and as it traveled it magically switched between right and left hands.
And the thing that causes, say, an electron to flip back and forth? Why, it's the Higgs field. As an electron moves, it's constantly interacting with the Higgs. And every time it does, it switches between right- and left-handed. Like someone walking through a crowded room, and every time they reach out for a handshake, it's a different hand.
The Higgs mechanism is what gives particles mass. The switching BETWEEN left- and right-handed versions IS mass.
When you watch an electron whizz by you, you're not watching a single, whole, unitary particle. You're watching TWO of them. A left-hander and a right-hander, constantly flipping back and forth. At one moment, you would see a MASSLESS left-handed particle. Then in the next instant you would see a MASSLESS right-handed particle.
How frequently these particles swap hands is controlled by how easily they interact with the Higgs. Every time they interact, they switch, and that frequency of switching IS THE MASS.
So in other words, what we call the property of "mass" is really a measure of how often twin left- and right-hand particles swap places as they travel.
All massive particles in the universe do this.
Except neutrinos.
In Part 2, we meet the one force in nature that actually cares which hand you use — and why the neutrino's relationship with it is so deeply strange.
