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EPISODE TRANSCRIPT (AUTO-GENERATED)

March 25th, 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 Corelli, director of the Naples Physics Institute. In the letter, he said, Dear Corelli, I have 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 Skewty. I will keep a fond memory of them all at least until 11 p.m. tonight, possibly later too. He was never seen again. Enrico Fermi, affectionately known as the Pope of Physics, had this to say about Ettore. 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 geniuses. One year before he disappeared, Majorana published his last paper, a strange, quiet little work 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 episode 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 here, 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 things are about to get weird. Particles aren't really particles. At least, not in the way we usually think about them. 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. But you know what's right? Contributing to Patreon. That's patreon.com slash P-M-S-U-T-T-E-R is how you can keep this show going, and I truly do appreciate it. Think about your hands. Or just look at them. Regard them. Your left hand and your right hand are mirror images of each other. Yeah, there may be a mole or a hangnail. Forget that. You get my point. 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... 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. You can't do anything short of radical body modification to change your left hand into a right hand and vice versa. 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? No one really knows. Different show. 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 or spinning on its axis in a quantum mechanical sense that doesn't map perfectly onto a spinning top, but is close enough for our purposes. You can imagine a particle for us, for what we need to talk about. You can imagine it 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. And if it's opposite the direction of motion, it's left-handed. Could have been the other way around, but we needed labels, and those are the labels we picked. I don't want to overcomplicate myself, but I do need to introduce a caveat here.

As usual, in particle physics, I'm being a little loose because the jargon just blows up in our faces all the time. The picture I just described is technically the spinning bullet moving direction. That's technically called something else. It's called helicity. which can change from your point of view. If you race past a speeding spinning bullet and you look behind you, the bullet now looks like it's moving away from you, not towards you, and so its helicity flips. So for particles, because of that, we use a different assignment of handedness called chirality that doesn't matter how you look at it. It's a real physical property, just like mass and charge, and it's related to the spinning particle. relative to its direction of motion, but it's not exactly it, the details are all in the math. And I'm just going to skip right over that. But it gives you a handy picture. Just know that there's some subtle mathematics underneath it. And for various reasons, we call it chirality instead of helicity.

Okay. Chirality is a property of particles, 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. They're like different orientations of existence. They're 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. You can't ever catch up to a photon and see it from a different point of view. We're always looking at photons from behind, as it were. taillights only. So once a photon is born with a particular handedness, either left or right handed, it stays that way forever. And the same is true for any other massless particle, like a gluon. So once you get a left-handed photon, you're stuck with a left-handed photon forever. If you get a right-handed photon, that's it. Until it dies, it's absorbed or whatever, it is always going to be a right-handed photon.

The same is not true for massive particles. Massive particles, as they travel, actually flip their chirality as they move left, right, left, right, as they move. Literally, if you took a 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. So say I shoot an electron at you and it comes out as a left handed electron, left chirality electron. But as it travels, it's actually going to flip back and forth between left and right handedness. And the thing that causes that flip from left and right handed, it's actually the Higgs field. As an electron moves, it's constantly interacting with the Higgs. It's constantly swimming through it, pushing through it, digging through it, bouncing through it, whatever you want to call it. A particle like an electron has to talk to the Higgs constantly. And every time it says hello, 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.

Same person, but different hand. And my friends, the Higgs mechanism is what gives particles mass. So it's the switching between left and right versions that is mass or causes mass, whichever you prefer. But the switching of chirality, the left and right versions, handedness of a particle, this constant flipping back and forth is due to the interaction with the Higgs, which is a particle's mass. When you watch an electron whiz by you, as they are wont to do, you're not watching a single whole unitary particle. You're watching two of them. We just established properties of particles included handedness, charge, mass, handedness. If I have two particles, one left-handed and one right-handed, they are different particles. They're just like mirror images of each other. Just my left and my right have the same mass, the same electric charge, but they're mirror images of each other. A left-handed electron is different than a right-handed electron. So how do they switch? They switch. This switching can happen.

This identity changing can happen between left and right, left and right, left and right, left and right as it moves. because you're not watching a single particle. A single particle moving past you is not a single particle. It's actually two of them. There's a left-hander and a right-hander, and they constantly trade back and forth. If you were to freeze frame, and you can, so don't try, you wouldn't see an electron as you think of it. At one moment, you would actually see a massless left-handed particle moving And then the next instant, you would see a massless right-handed particle. And then back to the left-handed, then back to the right-handed. How frequently these particles swap hands is controlled by how easily they talk to the Higgs. Every time they talk, they switch, and that frequency of switching is the mass. I know, this is wild to think about, that a particle like an electron, as it's moving, is actually made of two different kinds of particles simultaneously. that are mirror images of each other and are each individually massless.

There's a left-handed one and a right-handed one, and they constantly swap back and forth. The left-handed one goes for a little bit, meets the Higgs, then becomes a right-handed one, meets the Higgs, becomes a left-handed one, meets the Higgs, becomes a right-handed one. Wait, did I do right-handed twice? I don't know. It's just, it's going to go back and forth. And that frequency of switching is the mass. The more frequently it switches, the more massive a particle is. In other words, what we call the property of mass is really a measure of how often these twin left and right-hand particles swap places as they travel. All massive particles in the universe do this. Every single massive particle, could be a top quark, could be an electron, does this swapping of two massless particles swapping hands, a left-handed one and a right-hand one constantly, And that's what we interpret as a single particle with mass. Everyone does it except the neutrinos. What I'm about to say may be of some small comfort for those of you who are left-handed and feel like the world isn't constructed with you in mind.

When it comes to fundamental particles, the universe doesn't care if you're left or right-handed. It doesn't make a difference. Think of all the forces of nature and the ways they interact. Gravity and mass, electromagnetism and charge, strong force and... Well, let's not talk about that right now. Left-handed, nobody cares or notices. Everything is the same. Right-handed, same deal. An electron isn't really a single, solitary, massive particle. It's actually two massless particles, one left-handed, one right-handed, that are constantly swapping back and forth, constantly flitting, changing identities. One mirror image to the other. The massless twins, when they combine, they give us the electron. But when it hits you, you don't care if it's in left-handed mode or right-handed mode. You feel its mass and charge. That's it. Except the weak nuclear force. The weak nuclear force, the quirky cousin in the family of the forces, the eccentric one, the one that's living in its own little world, living its life to the beat of a different drum or accordion or vuvuzela.

The weak force really cares about handedness. In fact, it's the only force that cares about handedness. Gravity doesn't care if you're left or right-handed. Electromagnetism doesn't care if you're left or right-handed. Strong force doesn't care if you're left or right-handed. So when a particle like an electron is constantly flipping back and forth, all the other forces of nature, strong, electromagnetic, and gravity, don't care. The weak force does. The weak force cares so much that it only, and I mean only, talks to left-handed particles. It is blind to right-handed particles. It's like some sort of germaphobe that will only shake hands with left-handed people. Famed physicist and all-around curmudgeon Wolfgang Pauli once quipped, quote, I cannot believe God is a weak left-hander. He said this because even though the weak force is the odd one out, it's absolutely critical for, well, most of the universe as we know it. One of its superpowers is beta decay, which allows it to reach inside a neutron, grab one of its quarks, and change it, transforming the neutron into a proton.

That transformation makes nuclear fission and fusion possible, which is responsible for, among other things, I don't know, making the stars shine. He also spent years disparaging, in typical Pauli fashion, the experiments conducted by Chinese-American physicist Qiancheng Wu, aka Madam Wu, the first lady of physics, who conclusively demonstrated that the weak force only works with left-handed particles. Her experiments showed that the radioactive decay of Cobalt-60 tended to prefer one direction over another. And when she first made the announcement, nobody liked it because it broke up the nice and tidy picture of the universe we had been so meticulously cultivating for decades. But evidence is evidence, and Madame Wu was exceptionally good at getting it, so even the old curmudgeons like Polly relented. Okay, that's weird. Fine, universe, whatever. In most cases, this doesn't matter at all. That's because every particle like an electron is constantly flitting between left and right handed mode.

If the weak nuclear force wants to talk to the electron, it just in a sense here, I'm playing a little loose with the math here for the sake of a better story. It like waits for the left handed version to show up, which isn't very long at all. And it grabs it. It doesn't need the right handed version. It doesn't talk to the right handed version. The weak force doesn't talk to right handed particles at But every massive particle that we care about constantly swaps back and forth between left and right, so the weak force, if it wants to talk to an electron, just waits for the left-handed version to show up, and then it grabs that. To torture my analogy even further, if our germaphobe only shakes hands with left-handed people, that would normally be incredibly restrictive and largely unfriendly, except this germaphobe happens to live in a world filled with ambidextrous people. So it doesn't matter. Except for the neutrinos. Neutrinos are exceptionally selective when it comes to the forces.

They have no electric charge, so they don't see light. They absolutely refuse to communicate via the electromagnetic force. They have no color charge, which is the term I wanted to avoid earlier. Dang it. So the strong force is out for them. They feel gravity because if you exist in this universe, you don't really get a choice about it. But the weak force... Oh, they love the weak force. They're the one kind of person our left hand only germophobic force will actually seek out at an event. They speak the same language. They buy clothes from the same store. They eat the same kinds of foods with their left hands, of course. And this is where the story, which seems to be building to some grand and inevitable conclusion that wraps this all up in a nice tidy bow, comes screeching to a halt. And that's because there are only left Seriously, every single neutrino we have ever observed, ever, in every physical reaction is only left-handed. They don't flip back and forth. They don't switch identities.

They just are left-handed. Oh, and the anti-neutrinos? Oh yeah, those exist, but those are only right-handed. This is completely unlike any other particle. Any other particle, matter or antimatter, has both left- and right-handed identities that constantly swap back and forth. But the neutrino is weird. Only left-handers for normal neutrinos, and only right-handers for anti-neutrinos. That's it. Trillions of neutrinos passing through your thumbnail every single second. Not a single one is right-handed. For a long time, this was classified under the heading of weird, but fine. We thought neutrinos were massless, and massless particles, like our friend the photon, are locked in either left or right-handed modes forever. Okay, there was the weirdness that while we see both left and right-handed photons in equal measure, for the ultra-curious, these are the different polarizations of light. We only ever see left-handed neutrinos and right-handed anti-neutrinos, but that was tagged as a problem for another day.

And then we discovered that neutrinos have mass. Massless particles are locked into one hand. Massive particles swap back and forth. The neutrino is both massive and locked. I don't know about you, but this sounds like a problem. Neutrinos have mass. We know this. And massive particles, all massive particles, as we just established, flicker between left and right-handed modes. That flickering is the mass. The constant Higgs handshake. The endless switching. That's the deal. That's what makes it all hang together. But neutrinos don't flicker. Left-handed neutrinos stay left-handed. Right-handed anti-neutrinos stay right-handed. No switching. No flickering. Nothing. And yet they have mass. So either everything we said about mass is wrong, and I'm pretty sure it isn't, or something very strange is going on with the neutrino. The most straightforward solution is this. The right-handed neutrinos are there. They exist. We just can't see them. Here's why that works. Think about the electron. The electron has two completely independent ways to describe it.

One is handedness, left or right. But we found that for a massive particle, that this is just the flickering. It's transient. It's constantly changing. It's not a permanent label. Handedness for an electron is almost incidental. It doesn't define what the electron fundamentally is. The universe doesn't respond to the handedness of an electron. But there's another way to describe an electron. Particle versus antiparticle. Electron versus positron. This one is permanent. It's important. pinned open by electric charge. An electron has charge. Positron has the opposite. If they meet, they find each other and they annihilate in a flash of pure energy. The universe treats this distinction as sacred because charge is conserved and the universe does not mess around with conserved quantities. So for the electron, handedness flickers and it doesn't really matter. Particle versus antiparticle is locked in fundamental and really, really matters. Once an electron with negative charge is born, it stays that way forever.

Once a positron with positive electric charge is born, it stays that way forever. Once a particle or a positron or an electron, once they're born right-handed, it flips back and forth constantly. It doesn't matter. So charge seems to be important while handedness does not seem to be important. This gives us what we can reasonably call the Dirac picture, named after our good friend Paul Adrian Maurice Dirac, who first worked out the mathematics of relativistic quantum mechanics. In this picture, neutrinos work exactly the same way as electrons. There are two options for handedness, two options for charge. That makes four total combinations. You can have an electron, left or right-handed. You can have a positron, left or right-handed. You can have a neutrino, left or right-handed, and an antineutrino, left or right-handed. Now, as usual with anything regarding particle physics, I have to add a little bit of clarifier. Feel free to skip this if you want. Neutrinos do not have the usual sort of electric charge.

They are, after all, little neutral ones. So to make this picture work, we assign them their own quantum number to tell particles from antiparticles apart. Because of course we do. Listen, if you want a deep dive into all the different kinds of quantum properties and charges and labels in the universe, let me know and I'll fire up an episode about it. Or maybe I'll charge up an episode. Anyway, I just wanted to bookmark that so it's complete. But let's continue with the story. Left-handed neutrino. We see them. The weak force loves them. Right-handed anti-neutrino. We see them too. The weak force produces them in beta decay. These are both observable. But then there are the other two, the right-handed neutrino and the left-handed antineutrino. These exist in theory in the direct picture. They just don't interact with anything. Our germophobic weak force won't touch them. The wrong hands, remember? There's nothing for them to grab onto. And they have no electric charge, so electromagnetism ignores them.

They have no color charge, so the strong force ignores them. The only force they ever feel is gravity. They are, in the most complete and total sense imaginable, invisible. Not hard to detect. Not rare. Not shy. Invisible. Completely, permanently, in principle, undetectable by any instrument we could ever possibly build or conceive. They could be in this room right now, and we would have no way of detecting them. The other forces won't touch it. The weak nuclear force will talk to a normal neutrino, but it will not talk to a right-handed neutrino. Look, it works. The math is consistent. It explains why we only see left-handed neutrinos. It preserves everything we think we know about particles and antiparticles being fundamentally distinct things. Now, we still need to explain why neutrinos don't constantly flip back and forth in handedness like all the other particles, but that seems like a smaller problem to solve... than the complete lack of existence of right-handed neutrinos. They are there, they're just invisible.

When it comes to the electron, its two descriptions, handedness and particle versus antiparticle, are kept independent by electrocharge. Charge is what forces them apart. Charge is what insists that electron and positron are categorically different things that cannot be confused or collapse into each other. Handedness is there, But it doesn't matter. But the neutrino has no electric charge. We have bookkeeping devices that keep neutrinos distinct from anti-neutrinos in our equations. But unlike electric charge, that quantity, that number, that bookkeeping device isn't sacred. It's not protected by any deep principle. It's accidental. The universe didn't mandate those rules like it did for electric charge. It just fell out of the math because we designed the math that way and we wrote it down. Here's the thing, nothing is forcing the distinction between neutrino and anti-neutrino to be fundamental. So here's how we get to the mystery of Ettore Majorana, not his disappearance, he was never seen again, end of that story.

I'm talking about his last work, where he asks precisely this question. For all other particles, charge matters, but handedness doesn't. What if for the neutrinos, it's the reverse? So let's talk about Ettore Majorana's last paper. It's 1937, one year before he vanishes. I should also mention that he was a diehard fascist and while visiting Germany was excited to see Hitler, so that's a fact. Anyway, Majorana was sitting with Dirac's ultimate work, where he gave the precise, picture-perfect version of quantum relativity. Now, it's a rare person that can go toe-to-toe with the likes of Dirac, but Majorana was that kind of guy. He was also the kind of guy to ask the kinds of questions that 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. It's a choice that the universe and 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 he disappeared. These are what we call Majorana particles, as opposed to Dirac particles, which should be obvious by now. All Dirac particles have a charge and have an antiparticle partner. And all Dirac particles flip-flop between the two hands, left and right, but the universe doesn't really care about that part. Maybe neutrinos aren't Dirac particles. Maybe they're Majorana particles. Maybe their opposite partner doesn't have opposite charge. Maybe it has opposite handedness. And the charge is the part that nobody cares about, which is true because neutrinos don't have any charge. This means neutrinos might be their own antiparticle. This is pretty intense, I know. But there's something, there's an analogy here hiding in plain sight. Remember when 3D movies were all the rage for like 18 months? You ever wondered how they work? No? Well, you're about to find out whether you like it or not.

3D movies work because light comes in two hands. Left circular polarized and right circular polarized. Light has no charge. by itself. It's not a charged particle, but it does have two hands. And the lens filters in your glasses filter out one and passes through the other. Left-handed photons for one eye and right-handed photons for the other, and then your brain assembles a fake three-dimensional world out of that. 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, but with just different hands. The photon gets away with this because it carries no charge. Nothing forces open the particle-antiparticle distinction for photons. So it gets to be its own antiparticle, and it gets to express itself in either left-handed or right-handed modes that are locked permanently. The Majorana idea is just this. Maybe the neutrino does the same thing, even though it has mass. Because mass with particles, totally easy.

This is no problem at all. Majorana is just like, well, okay. Maybe this can also happen for massive particles for the exact same reason. That's it. That's the whole idea. 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, nope. Two observable and two are permanently hidden. In the Majorana picture, we just collapse that. The right-handed antineutrino and the right-handed neutrino, are the same thing. The left-handed antineutrino and the left-handed neutrino, those are the same thing. They're just the two particles, not four, just two. Just like with an electron, there are only two particles. There's the electron and the positron. Handedness doesn't matter. Now with the neutrino, the charge doesn't matter, but the handedness does. There are still only two particles. Most particles care about charge, but not about handedness. Neutrinos are just the kind of particle that care about handedness, but don't care about charge.

Now, I'm no particle physicist, but that doesn't stop me from having opinions about aesthetics and beauty and symmetry. And to me, and trust me, this is just me, my own personal opinions that are absolutely not endorsed by the international confederation of people who actually have to work through the math and observational consequences, I like the Majorana picture better. Not because it's proven, it's not. But because the Dirac picture asks us to believe in four kinds of particles when we only ever see two. And it explains the missing two with, well, they exist, but they interact with literally nothing. Deal with it. The Bajorana picture says maybe there are only two particles. There are two neutrinos, a left-hander and a right-hander. And yeah, we call the left-hander one a neutrino and we call the right-handed one an antineutrino. But that's us mixing up the labels because we're so used to particles working the other way. Maybe we're just overcomplicating stuff. But at the end of the day, nature doesn't care about my opinions.

You can have a beautiful, perfect, logical, completely wrong theory. So if we want to say that neutrinos are their own antiparticles, we have to see it. How do we test it? How do you look at a neutrino and ask, hey buddy, are you your own antiparticle? Well, one option is to watch atoms die. There's this process called double beta decay. Sometimes two neutrons in a nucleus decay at the same time, and they produce two protons, two electrons, and two antineutrinos. We've seen this happen. It's rare, but it's real enough for our purposes. But if the neutrinos are Majorana, then there's really no such thing as neutrino versus antineutrino. They're the same dang thing. And that changes what can happen inside the neutrons when the reactions are going down. Instead of two anti-neutrinos coming out, you instead have one coming out of one neutron and going inside the other. And what comes out is two protons, two electrons. That's it. We call this neutrino-less double beta decay, which sounds about right.

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 got nothing. That's not a no. It's also not a yes. It's just not yet. The signal from neutrino-less 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 that it's rare. Sets limits, but it's not the final word. Nobody knows what happened to Ettore Majorana. Some say it was suicide. I mean, the letter he sent wasn't exactly the epitome of mental health. Some say 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. Thanks to at Mobius tester and Todd C for the questions that led to today's episode.

Thank you for all of your questions. That's ask a spaceman at gmail.com or the website, ask a spaceman.com. And of course, please keep telling everyone in leaving reviews on your favorite podcasting platform. It really helps the show visibility. And if you can, I would appreciate you contributing to Patreon. That's patreon.com slash PM center. I'd like to thank you. my top contributors this month. They are Keep those questions coming. And I will see you next time. for more complete knowledge of time and space.

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