What are sterile neutrinos?

00:00:08
Speaker 1: Ity, Daniel, you cook a lot. Are you the kind of person who keeps their kitchen clean? I don't think I'm excessive, you know, clean, but in a normal way. I think usually your definition of normal is different than most normal people. As a picture, they leave the dishes in the sink. What dishes in the sink? This isn't the frat house? I guess that's it. What about the counter? Do you have any of you know, stuff in it? I mean, we have cabinets and drawers. There's no reason to leave stuff on the counter. That's another no, I guess. So I'm getting the sense that you keep a pretty tighty kitchen. I mean we do our best to keep it need and organized right. And your office, how need do you keep that one? I think I focus my energies on the kitchen and not your work. You know, my priorities are the kitchen is sterile and the office is a disaster. I am for him and cartoonists and the co author of frequently asked Questions about the universe. Hi, I'm Daniel. I'm a particle physicist and a professor you see Irvine, And my office is a mess, but I promise you my mind is organized. What is that saying you're a mess in the office but a neat freak in the kitchen. Barack Obama said, physical discipline, mental discipline, and he's pretty good at what he does, so we should all follow his example. I mean, I've never read one of his physics papers, but he does seem like a smart dude. I think he probably funded a lot of your research indirectly, so maybe you should be a little bit more thankful. That's true. He used to be my boss's boss's boss's bosses boss's boss's boss's boss's boss. Yeah, there you go. He used to be a part of the administration practically. I mean you were right there. I mean you weren't in the white House, but you were in the white sun House. That's true, which, because I live in Irvine, is actually beige. But welcome to our podcast. Daniel and Jorge explained the Uni Verse, a production of I Heart Radio, in which we want to defeat all of the levels of the universe. We want to kill the lower minions and we want to work our way up to the boss levels of understanding of the universe. We want to break down all of those barriers. Talk about the deepest hardest, most confusing, trickiest questions about the nature of reality and show you what science does and does not understand about all of it. Yeah, because it is a pretty big universe, a pretty complicated with a kind of a complicated or chart. I guess a lot of middle management levels here in the universe that you have to go through, Daniel in in order to understand it. We are trying to make sense of it, and we're hoping that if we have a complete picture of everything that's out there and how they all talk to each other, that some patterns will emerge, that some sense will be a parent, that we will gain some understanding about how it all makes sense, or maybe the whole thing is just a big mess. What do you think is the universe's management style? Do you think it's pretty flat or is it kind of curved or is it multidimensional. It's definitely not Obama's management style. I mean, he was no drama Obama, but there's a lot of drama in the universe supernovas, that's pretty dramatic. Yeah, there's always those troublemakers in your organization, I guess who like to blow up exactly. But sometimes when things blow up they actually give us clues about how things work. When stars do go supernova, they give us a peek into the inside of those cosmic fusion reactors, so we can understand the crazy, intense forms of matter that are being created at their center. Because there are still a lot of incredible and tantalizing mysteries out there for us to discover. And that's just only on the things that we can see and detect. There's a lot about the universe that is actually hidden. You have these little windows into the universe through your senses. You can see some things, you can feel some things, you can taste a few things. But we know the universe is much richer and more complicated than the things we can detect directly with our senses. And over the years, we have built all sorts of technological eyeballs and ears and noses and tongues to experience the hidden parts of the universe. We can now see ultra violet and infrared. We can detect crazy particles that are invisible to our eyeballs. We know that there's a lot more going on out there than we can see directly. Wait. Wait, you're building like an electronic tongue. Is that what you're saying in a in a physics lab somewhere, or is this part of your kitchen endeavors. Well, the standard analogy is to talk about new technological eyeballs, like we are seeing the universe. I wonder sometimes why we always use the analogy of vision, because you could, in principle, translated to any of our senses. I don't know if people heard recently that the soonification of a black hole where they took like the oscillatory waves and the gas around the black hole and they turned it into sound. All these things are just an attempt to describe something that is unexperienceable into something that you know how to experience. Translate the unfamiliar into the familiar, so you can choose any of our senses, vision, hearing, taste, smell, right, what do electrons smell like? Are you trying to bring back to smell a vision? Are you trying to build the incredibly expensive smell atron. I think it's a whole new frontier for science communication. You know, people are making false color images of the James Webb pictures, people are making soonifications of black holes, but nobody's doing smellification of particles. Until we are today. I think if you made people smell physicists, they might be turned off by signs. So I'm not sure that's the best science communication strategy here. Yeah, I think we want to keep that in the office. But it is a pretty incredible universe because a lot of it is hidden. In fact, we've figured out that of the universe is totally invisible and we have no idea what it is. And stuff being invisible is not that unfamiliar. I mean, you are surrounded by air, but you can't see it, right, it's invisible to your eyes. Now, of course, you can touch it and you can tell that it's there, you can breathe it. But there are other things out there that are not just invisible. They're also intangible particles out there that fly right through your body and fly right through the Earth without even noticing. Does that mean they're also intasteable? Like? What does the neutrino taste like? I don't know, But we do have three flavors of neutrino, even though we have no idea which ones are delicious, So basically we can make up flavors for them, right, one one could be spicy, the other one could be sweet, the other one could be mommy, right. I was hoping you were gonna say, O, mommy neutrinos, because that's the most mysterious flavor. You know, it's the most delicious one, I guess if you're a savory kind of person. But there are a lot of mysterious things out there, some that are we are still trying to figure out, and so let's talk about one of them. So to be on the podcast, we'll be tackling the question what are sterile neutrinos, or for our UK fans, what are sterile neutrinos? They're apparently pronounced differently, although Daniel, you I think you have to pronounce it with the British accent sterile neutrinos. You want the full phrase with with that's kind of like a cockney accident maybe, or I don't know. And it's embarrassing that my UK accent is so bad because I actually am a citizen of the UK and my father grew up in London and my grandmother, who recently passed lived in London for fifty years, so I should be able to do it, but I can't. Yeah, I'm sure you're a big disappointment to Brits. Everywhere other. If you are British, doesn't that mean that any accent you have is a British accent? There you go, Yes, I define Britishness in just existing. Yeah, you're a pretty dear fellow. But this is an interesting question. Sterile neutrinos. Does that mean there are fertile neutrinas. Yeah, I think we're gonna have a lot of fun talking about the meaning of sterile and what physicists intended it to mean and how people interpreted. But no, there are not neutrinos out there that are becoming parents. Or is it more like sterile like clean, like bacteria free? Does that mean there are dirty neutrinos? Dirty fertile neutrinos sort of in the clean sense. These are more neutrinos that are totally isolated from everything else. They don't interact. They're more like standoffish neutrin learn or neutrinos. There you go. See, you should just put me in charge of naming things. Things would go a lot easier example number four seventy seven. But anyways, as usually, we were wondering how many people out there had heard of sterile neutrinos or have any idea about what they could be. So thanks very much to everybody who volunteers to answer these questions for the podcast. We really appreciate it, and everybody out there loves hearing what you think. So if you would like to hear your voice on the podcast for a future episode, please don't be shy. Right to us two questions at Daniel and Horre dot com. So think about it for a second. What do you think a sterile neutrino is? Here's what people have to say sterile neutrinos. I don't know what they are. I feel like a neutrino can turn into a different type of particle, though, so maybe a sterile neutrino cannot for some reason. And so I think that's as a sterile environment is an environment way there is no interaction between sales and bacterias or other um micro organism. I think the right neutrinos don't want to interact with um other particles like they should do. From what I've heard about sterile neutrinos, and like the word itself, um, I think they're just neutrinos that don't really interact with anything, so they're just existing. But I don't really serve much of a purpose sterile little trinos. They are very clear, probably they washed their heads, very awful. Keep social distance well. A neutrino, as far as I know, is um a particle that doesn't interact with anything, so it flies straight through and apparently it's flying through me right now, So that already seems pretty sterile. So I'm not sure how it could get more sterile because it doesn't really interact with matter, and I've not heard of sterile neutrinos. I feel like I have heard that term before. I know neutrinos come in three flavors, but I'm not sure what would make one st or how we would know. I mean, who's checking anyway. Ordinary neutrinos can only interact with other particles by means of the weak interaction. Sterile neutrinos don't interact with other particles at all. I assume that sterile neutrinos are neutrinos that are even more neutral than regular neutrinos, but I have no idea, all right, I am totally with the person here who says that this combination of words doesn't make any sense to me. I think that's a direct message to you, Daniel and to all physicists to maybe take words that make more sense. There are a lot of people who were totally misled by this phrase, but a couple of people actually got it right on the nose. So this pair of words did lead a few people to the right idea. Who got it right The people who said it doesn't make any sense, the people who doubted the ability of physicists to name things accurately. No, it's all about the interactions, right. What can particles do? They can fly through the universe and they can interact, So sterility refers to whether or not they interact with other particles. Well, let's dig into it, and let's start with the basics. First of all, what is an netrino? So neutrino is a really fun, very weird little particle. It's unusual because it's one of the sort of base particles we think of in the universe. There are four of them, the up cork, the down cork, the electron, and then the fourth one is the neutrino. The weird thing is that the neutrino, unlike the other three, does not appear in the atom. It's not technically a sub atomic particle because it's not part of the atom. Take the atom apart has electrons around it, and inside our protons and neutrons. You take those apart, you get up corks and down corks. So to make everything in the universe, you have upcorks, down corks, and electrons. But then there is this other particle out there, the neutrino. It definitely exists. It's created in the heart of the sun during fusion. There's a lot of them out there, but they're not part of the atom, and they're especially weird because they don't have any electric charge and so they don't give off any light, they don't absorb any light. They're essentially invisible. Yeah. Well, I mean, there are a lot of other particles besides the electron and the corks, but this one, is it a different type of particles out there? Is it like a whole different quantum field and it's hold own classification or is it kind of like a cousin of one of the other particles, So it's its own different type of particle at the same level as the upcork, the down cork, and the electron. There are other particles out there, you're right, like the charm cork and the top cork, but those are like reflections of the up cork. And there are muons and towels, but those are reflections of the electron. They're really like four base matter particles, the upcork, the down cork, the electron, and then the new trino, of which there are also three kinds. Now we do group them together, like the two corks we grouped together, and the electron we grouped together with the new trino, because then trino is related to the electron like the electron. It also doesn't feel the strong force, So the neutrino is super weird because it doesn't feel the strong force like corks do. It also doesn't feel electromagnetism like electrons do. The only force that it feels is the weak interaction. So like a neutrino can fly through a magnetic field and not get bent. If you've applied an electric field to a neutrino, it doesn't pull on it. A neutrino can fly through the heart of an atom without interacting with any of the corks and luons inside of it ignores almost all the interactions that make the universe the way that it is. Right, it doesn't feel any charge because I guess the corks feel a positive charge, right, and the electrons feel a negative charge, but the nutrina doesn't feel any charge at all. Right, Yeah, the corks actually do. The corks have really weird charges. The upcork has a charge of plus two thirds it's a fraction, and the down cork has a charge of minus one third. So the upcork is a positive charge to down cork is a negative charge. Really interesting is that their charge difference is one, is exactly one, between two thirds and negative one third. It's the same for the electron and the neutrino. They also have a charge difference of one. In this case, the electron has charged negative one and the neutrino has charged zero. So then trino is the only fundamental matter particle that has no electric charge. And you're pretty sure about that. I guess could it be that it just feels it, but just super weakly or something. That's a super good question. How well do we know it? It's definitely part of the theory that it's exactly zero. How well do we know that experimentally? In the end, we never know anything perfectly well, Right, there's some limit on the charge of the neutrino. It's got to be something super duper duper tiny. Even a very small charge in the neutrino would upset all sorts of things because charge is conserved in the universe. That would mean, for example, if the neutrino had even a little bit of charge, would make it more complicated to keep charge conserved when you produce it together with an electron. For example, like we have a W boson that has charged positive one elects to decay into an electron and a neutrino. But if the neutrino has a little bit of charge, that has to come from somewhere, and that W only has charged one, so it can't create an electron and a neutrino if the neutrino has charged so we're pretty confident has zero charge, but you can never know absolutely for sure. But it does feel the weak force, which is part of the electromagnetic force, but it's kind of not it's like a sister force kind of yeah, it's not part of electromagnetism. It's part of a combined idea called electroweak, which puts electromagnetism and the weak force together into a single concept. It shows that mathematically, they obey some larger symmetries, some rules that tie them together, the same way that we think electricity and magnetism don't make as much sense separate as they do together, because we see that magnetic fields create electric fields and electric fields create magnetic fields. In that same way, it makes more sense to put the weak force together with electromagnetism into one holistic thing called electroweak. Right, and you said neutrinos are matter particles, which means they do actually have mass. Right, They do feel it, right, and they it does take them some time to accelerate. Right. That's a really interesting topic because it's actually one of the few hints we have that the standard model of particle physics is wrong. Neutrinos in our theory are perfectly massless. O. Our theory does not allow for them to have any mass, and yet in our experiments we measure them to have some mass. We know that they do have very tiny little masses, so you're right, they don't travel at the speed of light, but it's not something that we yet understand the masses of neutrinos. Wait, we've measured that they have mass, but in our theory they don't have mass. Don't you have to revise your theory. We do have to revise our theory. We know that the standard model is not correct and we need to somehow modify it to accommodate neutrino masses. But it's pretty tricky, and one way to do that actually is to add a new kind of neutrino called a sterile neutrino, which helps solve the problem, because that's a big question of the day. And just to clarifying, nutrients are not rare in the universe, right There's like a whole bunch of them going through us right now. Yeah, Our Sun is a huge neutrino factory. Fusion produces an enormous number of neutrinos, so many that they carry like one percent of the Sun's energy output in neutrinos. So the Sun isn't just a star that pumps out photons and light. It also pumps out neutrinos. Like you could use neutrinos to navigate around the Solar system because you can use it to see the Sun. There's so many that by the time they get to Earth, there's a hundred billion passing through a square centimeter per second. So if you like, hold out your hands and you count one Mississippi, a trillion neutrinos have passed through your fingernails. Wow. Unfortunately they don't interact with the electrons and the corks in my body. Otherwise would be a toast. Right, Yeah, you'd all get neutrino cancer or we'd be putting on neutrinos screen every time we go outside. All right, Well, those are neutrinos, and it seems like there's a different kind of neutrino called the sterile neutrino, which it sounds like solves a lot of problems in our theory of the universe. So let's get into that. But first let's take a quick break. All right, we're talking about neutrinos, which are plentiful in the universe. They're made in the Sun. The account for one percent of the Sun's output. So it's kind of a big deal in the universe, right, I mean, the universe seems to like making them. Yeah, the universe certainly pumps out a lot of them. They produced also in supernovas. We have a whole episode about how neutrinos let us see what's going on inside Supernova's one of My favorite applications of neutrinos, though, is that it lets us see that the sun is still there at nighttime because neutrinos hardly interact with anything, so they can pass all the way through the Earth. So if you're like up at three am and you're worried that the sun has disappeared, we can actually verify that the sun is still there on the other side of the Earth by looking for its neutrino signal, because that neutrino signal will pass all the way through the Earth to your neutrino detector. You could also just like call your friend in Japan, but that's less cool, or you could wait a few hours. I mean, the sun will rise tomorrow, isn't that what the song said, unless it's like disappeared or been blown up by aliens. And how would you even know if it's three am? Right? This keeps some people up at night, so this is a way for them to feel comfortable, or how how would it matter? You know, we'd all be toast if the sun disappeared, so you might as well sleep peacefully for the last couple of hours of life that you have. If I told you that aliens had blown up the sun, you'd go take a nap. Is that what you're saying? Yeah, basically, I mean what else would you do? Worry about it? Panic called Bruce Willis. So there's something called that's a sterile neutrino, which I guess it's a different kind of new trino. I mean you said there are different kinds of nutrinos already, right, there's three flavors of neutrinos, but this is something different. Yeah, it is something very different. We have three flavors of neutrinos. We call them electron, muan in town neutrinos because they are produced with electrons, muans and towns. Like when a W boson decays into an electron and a neutrino, it doesn't just decay into any random neutrino decays into an electron neutrino, which is different from a muon neutrino. Muon neutrinos the kind produced when a W boson decays into a muon and a neutrino. In that case it produces a muon neutrino. So there's three different kinds electron, nuon, and town neutrinos. Those we know exist. We've definitely seen them. They interact with the weak force. We've measured each of their various kinds. We're sure that they exist. But the idea is maybe there's a fourth kind of neutrino, a sterile neutrino, which doesn't even feel the weak interaction. It doesn't have the strong force, it doesn't have electromagnetism, and it also doesn't have the weak force. It's totally without interactions, which is why they call it sterile. So it's kind of like a fourth kind of nutrio. Maybe step us through this first, though, like the three other kinds of neutrinos, how are they different? Because I think I've heard that we don't actually know their masses or we're not sure what their masses are, So how do we know they're different? So we know that they are different because we know that the universe keeps count of how many electrons there are, how many muons there are, and how many towels there are. Particle physics, we do this all we look for rules that the universe seems to follow, like conservation of momentum. We notice that in particle interactions, the amount of momentum afterwards is the same as the momentum beforehand and we go, oh, that's cool, what does that mean. We have a whole podcast episode about what that means about the universe. So we're always on the lookout for are the rules that are being followed, because then we can try to figure out what that means. And one thing we've noticed is that the universe keeps track of the number of electrons, so you can't just create one electron. If you do so, you have to also create an anti electron. But the electron neutrino is sort of in the same column. So when you produce an electron, if you produce an anti electron neutrino, then the universe is happy and all the accounts balance. That's why we think that there are different neutrinos. That's what it means. Means for these to be different neutrinos. There's the one that balances the electron book, another one that bounces the muon book, another one that balances the tao's books, which so you only think there are three different neutrinos. I mean, it sounds like what if just the one neutrino helps balance all the other electron, muon and tow particles, and it's not a universal currency. Well, if we produce just one kind of neutrino, for example, you start with muans. You can produce muon neutrinos from their decays, and then we smash it into a bunch of stuff. It tends to only interact with muans, and the same with electrons. You produce an electron neutrino beam and you smash it into a bunch of stuff. It tends to bounce off only electrons because those like to talk to each other. So we're pretty sure that those different flavors exist. Also, it would be very very weird in our theory if we had three kinds of charge lectons e. Mutao and only one kind of neutral lecton. That's not an argument for it not existing it being weird, but theoretically that to be very strange. But we do have experiments where we can isolate each different kind of neutrino and see that they really do have different properties. And we also know that there are three kinds of neutrinos because we have made measurements of their masses. We know that there are three different neutrino masses, all right, But I think you don't know exactly what those masses are right, we don't know what those masses are. In fact, we've only ever measured the mass differences. We know that there are differences between the masses of neutrinos because those mass differences help the neutrinos change flavor. So, for example, neutrinos produced in the Sun are all electron neutrinos because the Sun has electrons in it and almost no muans. By the time those electron neutrinos get to the Earth, a huge fraction of them have turned into something else like muon neutrinos, and that comes directly from the mass differences in the neutrinos. But it sounds like maybe it's a little bit hypothetical. Still, there's a lot we don't understand about neutrino mixing and oscillation and how this all works, and there's a bunch of things in the theory that don't quite fit together. Yeah, and there's also a bunch of experiments that all contradict each other and have had people arguing for about two decades. That's a physicist favorite activity, so they're pretty happy about that. But you said there's maybe a fourth kind of neutrino, or we know for sure there is a fourth kind of matrino called a sterile neutrino, or as they say in England, sterile neutrino. So we don't know for sure if there is a sterile neutrino. We have good reasons to suspect it, both theoretically and experimentally. We have a lot of hints from both directions. We definitely do not know for sure. Sterile neutrinos are still hypothetical, and there's a lot of very strong, very different opinions about the strength of the evidence for them. They're more hypothetically the other nutrina. Yeah, exactly, they're more hypothetical. And these things are really hard to observe because the only interaction they have is gravity, right, which, as we know, is the weakest force, And so that makes it very very difficult to like study sterile neutrinos directly or to discover them. All Right, well, it sounds like they're double they're hypothetical. Well, what makes us think they might exist or why did we give him a name and think that they are out there? So they started out in the theory, it's an idea of people were playing with. One of the central mysteries is why neutrino masses are so small, and actually how they get mass in the first place. Like most of the particles that are out there in the standard model, the electron, the corks, we think they get their masses by interacting with the Higgs boson. The electron flies through this field which fills the universe, the Higgs field, and it interacts with it a little bit, and that changes how it moves. The pure theoretical electron by itself has no mass, but the effective electron, the one that moved through the Higgs field, moves as if it had mass, and that's actually what we call mass. So that's how an electron gets mass, and that's how an upcork gets mass. But we don't think that happens for neutrinos. What do you mean you don't think neutrinos interact with the Higgs field. I thought we knew for sure, or that they have mass, or you're just talking about stole neutrinos. We're just talking about the standard model of neutrinos right now, we don't understand how they get mass. In our theory, they shouldn't have any mass, and that's because in order to interact with the Higgs field, and to get mass the way that electrons do and corks do, you have to have two different kinds of the particle. One time we talked about how the Higgs field can't give masses to particles that are their own antiparticle, because the Higgs field has to talk to two different sides of the particle, has to talk to the particle and the anti particle. But even more than that, particles have another quality to them. It's called holicity or chirality. Particles can be left handed or right handed, and to talk to the Higgs field, to get your mass from the Higgs field, you have to have both right and left handed versions of the particle. And that's true for the electron and for the up cork and for the down cork, but it's not true for the neutrino. Neutrinos only ever exist as left handed versions of the particle. Well, it's long, I'm not underhanded, it's I guess it's all right, But I think I'm getting a little loss in all this term anology. Here. You're saying that in neutrinos are do have mass, right, you confirmed that with experiments, it seems like and but it's very little mass. But that's weird because your theory says that they should have zero mass. Why does the theory say they should have zero mass, because neutrinos only interact via the weak force, and the weak force only talks to left handed particles. We have an episode about parody violation, how the weak force doesn't look the same in the mirror, and that's because the weak force has this really weird property where it ignores right handed particles and only talks to left handed particles. And this handedness refers to like the direction of the particle's motion relative to the direction of its quantum spin, and those two things translate differently in the mirror, so like a left handed particle in the mirror becomes a right handed particle. And so neutrinos are only left handed, and the weak force only talks too left handed particles, and so in our theory, only left handed neutrinos should exist, which means there should be no right handed neutrinos, which means they don't talk to the Higgs field, so they should have zero mass, and yet we see that they do have mass. So there's definitely some gap in the theory. Right the theory is not describing the universe that we see. Well, maybe you're just you know, wrong about that. Maybe you can interact with the Higgs field with the other kind of particle, you know, like what made you think that it can only the Higgs can only interact with one type of spinning particle and not the other if you have experimental evidence that it does. Well, we know that it has mass, but the Higgs field really mathematically just cannot give mass to particles that don't have a right handed component. But it does right well. We don't know that it gets its mass from the Higgs field. We know that it has mass. There are other ways for particles to get mass. We talked once about whether the neutrino is a myorona particle. Maybe it is its own antiparticle, so that's a different way to get mass. That's one possibility that maybe the neutrino gets mass but not from the Higgs field. Or maybe we're wrong about the neutrino not having a right handed component. Maybe there is a right handed neutrino out there. The weak force doesn't create it or talk to it, but maybe it still exists, and so that allows the neutrino to get mass from the Higgs field, all right, So that's one mystery how does the neutrino get its mass? And it's also kind of a mystery why that mass is so small, right, because it's a lot lighter than all the other particles. It would be really weird if the neutrino got its mass from the Higgs field and then it got such a tiny little serving of mass. Right, The neutrinos are like less than one millions the mass of the electron, which is already very very light compared to like the proton. So we have this really deep question in particle physics about why particles are interacting with the Higgs field at different strengths. And the neutrino would be like the outlier, be even crazier than all the other particles. So to make it sort of hang together more crisply mathematically, people like adding another neutrino. They say, what if there's another kind of neutrino and this one is only right handed, so it doesn't interact with the weak force at all. So this idea that maybe they're right handed neutrinos out there gets extended to like creating a pure right handed neutrino, a sterile neutrino. If you put that in and calculate all your equations, then it tends to pull on the masses of the other neutrinos and make them small. So the idea of a stero neutrino would explain how new trinos get mass and why they are so small. All right, So we have those two mysteries, why it neutrinos have mass and why they are so light. But you're saying the solution is to create a whole different kind of neutrinos. How would it hold different kind of natrino explain the mysteries and the other kinds of neutrinos. Well, it would demonstrate that right handed neutrinos exist, and that helps us understand how neutrinos get mass, because maybe we were wrong that neutrinos are only left handed. Maybe they all do have a little bit of a right handed component. It's possible for them to then talk to the Higgs field and to get mass. Then if you have a special fourth kind of neutrino through some mathematical mixing, it turns out that it gets most of the mass and it becomes very very heavy. And because the seesaw mechanism with like it sits on one side very very heavy, it makes the other particles very very light. Oh, I see, you've made up this new kind of neutrino because just to like balance out the weirdness of the other ones. It's like, all the other ones seem to be left handed. Maybe there's a huge ginormous right hand in the trin on the other side that's uh, you know, kind of taking up all of the neutrino mass in the universe. Is that kind of what you mean? Yeah, that's exactly it. And it's sort of a weird idea, but it also kind of works. Like this is part of the process of physics. It's just being creative and being like, well, what about this crazy idea? Now that doesn't work, And this is a crazy idea somebody had one day and they cranked it through the math and dot com. This actually kind of works. It's not that complicated, and it would explain what we're seeing. So it became sort of attractive theoretically for that reason. But I guess if there does exist humongous right handed netrino out there and tun when you have seen it already, wouldn't be like extra obvious. So that's the other fun clue. But Darren neutrinos is that maybe they are the dark matter. Maybe we have been seeing them right. We know that there is some sort of particle out there that only feels graft. It has no other kinds of interactions. We've been calling it dark matter on the podcast for years, and people have been talking about in physics for decades. We know that most of the mass in the universe is this new, weird kind of thing, and so sterile neutrino really fits that bill right. One reason we don't think that normal neutrinos e. Mute town neutrinos are the dark matter is because they have very very low mass because they fly through the universe very very fast, and so they would have given a different shape to the universe. It wouldn't get like the clumpiness that we see in the universe today. They're too hot, is what we say. But this could be cold dark matter. They could be very massive and just sort of like hanging out there. So the universe could be filled with these sterile neutrinos. You ask, why wouldn't we have seen them, Well, maybe we have been Maybe they are the dark matter. What it's like a huge plot twist if it turns out the dark matter was just a big neutrino in the universe. So let's get into this idea a little bit more and whether or not we've seen these in experiments. But first let's take another quick break. We're in the third act of the movie here, which just revealed the big plot twist in our story here. It turns out the dark matter of the universe that we have no idea what it is. Maybe we do know what it is, and it could be maybe a fourth kind of neutrino, which kind of totally fits the bill right. It's like an almost invisible type of matter. It's heavy, and that white did means you think of this before. It's been a candidate for dark matter for a while, but it's become more popular in the last couple of decades because we have a bunch of really confusing experimental results in neutrino physics that have been hinting towards a sterile neutrino, but then also kind of contradicting other experiments. So it's a bit of a confusing landscape right now about whether the experiments are consistent with a sterile neutrino or not? Wait, which experiments? The dark matter experiments tell us there is dark matter out there. We know that it's out there, we know how much it is, but we don't know what particle it's made out of, or if it is even made out of a particle. The dark matter experiments have never seen a particle of dark matter. But there are people studying neutrinos in great detail, watching them change from one kind to another, electrons to muans, two towers to back, and they've been seeing some weird things that suggest maybe there's another kind of neutrino out there that would help explain these weird neutrino experiments. And that could also maybe be the dark matter that we're seeing. Right, that's the idea exactly. Maybe it all come together into a beautiful resolution at the end of the movie, right, Yeah, bring together the A B storylines. That's that's always the coal. But um, I guess one question is why are they called sterile matrinos or sterile if you're in England, are there neutrinos who are not sterile or reproductive or fertile or dirty? Well, there are neutrinos that interact electron muan in town neutrinos do feel the weak force, so we are capable of detecting them. If you send a bunch of electron neutrinos into a huge vat of liquid, one out of a billion of them will bang into an electron and will give it some recoil. So you can see that if you send muon neutrinos into a huge vat of liquid, one of them will bang into a muan if it happens to be created as a virtual particle temporarily, so we can see electron muan and town neutrinos sort of indirectly. But sterile neutrinos don't have any interaction at all other than gravity, so we can't do particle physics experiments with them. But what we can do is try to see if there's a missing part of the story. We watch electron Muan and town and Trina sort of turn into each other back and forth, and we try to notice if somebody accounting doesn't seem to be adding up, is like a missing person. It sounds sort of like the definition of a sterile metrino is the same definition is dark matter. I mean you're saying asteria lutrino is massive, it has mass, it feels gravity, but it doesn't feel the weak force, which is a sort of kind of what devolves. It's turn about about dark matter, and so you can't see this potential type of neutrino, and so you're saying there might be other ways to see it. Yeah, there might be other ways to see it. There are various candidates for dark matter. You know, some generic particle out there that only feels gravity. This is a special type of dark matter candidate because it's not just a generic particle. It's a neutrino, and it actually does mix with the other kinds of neutrinos. See neutrinos do this really weird thing, which is that they mix with each other. We've been talking about neutrinos in two different ways. There's different flavors of neutrinos Electron, Muan, and tao. And then this is the different masses of neutrinos, like call those one, two, and three. The super duper confusing thing about neutrinos is that those are not the same things. Like we know there are three different mass neutrinos, and we know there are three different flavor neutrinos. But it's not the case that electron neutrino has one specific mass and the muon neutrino has another specific mass and the town neutrino has a third specific mass. Instead, they're all mixtures of each other. So the electron neutrino is a mixture of the three different neutrino masses. Wait, what do you mean? What? How can something have different masses? What even the words different masses have mean? Make some one thing having three different masses? What does that mean? Yeah, So, as you fly through the universe, your mass determines how you move, right, So that's the property of a particle. It doesn't determine how you move. It determs how you speed up or slow down. Right, If you're just moving through the universe, your mass is kind of irrelevant. Yeah, that's what I mean. It determines how you're creative, determines how fast you go based on your energy. This kind of stuff. The mass is part of the property determines how you move through the universe and includes acceleration and momentum and all that kind of stuff. So neutrinos have different kinds of masses. Yeah, normally particles are created with a definite mass. You create an electron, it has a certain mass. It flies through the universe. Happy you create an electron neutrino, it's a combination of three different masses. This is super duper weird, and so as it flies through the universe, those masses do move differently. And because the muon neutrino is a different combination of those masses, that's how electron neutrinos can turn into muon neutrinos as they fly through space. It's sort of like two different ways to see the neutrinos. Imagine you have like you know, an X and Y axis that's like the masses, and now you have like a rotated version like X prime and y prime. You create something along the y prime axis. It's like a mixture of X and y and so in the same way, electron neutrinos are a mixture of the three different masses. Muon neutrinos are a different mixture of those masses. He lost me a little bit there with the rotating xs that, but I guess I'm trying to understand what you mean by like having a combination of different masses. What does it mean like I'm heavy and I'm light at the same time. Or are these like types of masses. No, that's exactly what it means. Electron neutrinos. These are quantum particles, right, so they are a superposition of three different masses. I'm heavy and light at the same time. What does that mean? Doesn't energy need to be conserved? How can I switch back and forth? Willy Nili, Well, the different states have different energies, right, They have different masses and different energies in the same way the particle can be like, Oh, I'm a combination of spin up and spin down. Electron neutrinos are a combination of three different masses, and muon neutrinos are a different combination of those masses. Oh, you mean quantum mechanically combined, right, like probabilistically combined. Like it could be when I measure it could be heavy, light, or medium. But since it's quantum and I haven't measured it, you say that it's a combination of the three. Yeah, But there's another layer of nuance, which is about how you measure it. If you measure it, if you try to measure its mass, then you get either mass one, two, or three. If you're trying to measure using the weak force, then you get electron, muan or tow. Right, So the weak force looks at these neutrinos differently than mass looks at these neutrinos. There's like a rotation between them. There's like offset. The weak forces like twisted in this strange way, so the neutrinos it likes to create are not lined up with the masses of the neutrinos. There this weird twist you mean, like they're not correlated. It's not like the tow is always one type of mass and the men is another type of mass. It's like each of the three kinds have different you know, probabilities of being three different masses exactly. So we do these experiments where we see one count of neutrino turning into another one. What I mean, it turns into another one, like it flies through the air and it turns into a muon or a tower or something like that. That's exactly right. An electron neutrino produced in the Sun, by the time it gets to the Earth, a big fraction them are now muon neutrinos. Why, because they're created with these superposition of different masses and those different masses moved through the universe differently, right, because they have different masses, and so by the time it gets to the by the time it gets to the Earth, it's a different combination of those masses which has a good chance to line up with the Muon kind of neutrino. So that's how they can change. They can change from one flavor into another because the masses are not all the same, so they fly through the universe differently. Wait do they actually change or is it again one of these quantum mechanical tricks where it's like it has the probability of being one of the three, but it's one depending on how you measure it. Well, it's created, it has chance to being an electron neutrino as it flies through the universe. That probability changes when you measure it after one a U it has like a sixty chance of being a Muan neutrino or in a five chance of being a town neutrino. And so yes, it's a quantum mechanical trick, but also that's the trick that describes our universe. But then you say, these things have to be conserved, so like if one of them changes from a meal into a tower or something. Doesn't that, you know, throw the balance sheet of the universe off, like if a dollar turned into a British pound exactly. And that's why it was such a huge discovery because it breaks the standard model. In the standard model, these things are totally conserved, but in the universe they are not. This breaks that rule. You're exactly right, very sharp point. And so we know that the standard model is violated, right, because neutrino oscillations, that's what this is called, breaks the conservation of lefton number. And that's why people won several Nobel Prizes for these discoveries because it's a hint that the standard model is wrong. I feel like Daniel, this whole episode has been us saying there's a rule, but you know, it turns out that rule doesn't work. So why even have rules? Maybe you shouldn't call them rules such a questions. Yeah, there you go. We were looking for the suggestions of the universe, not the laws of the And to me, the really fascinating part experimentally is that we look at all these experiments that see one kind of neutrino changing into another, and they don't all make sense. Like there was one experiment in Los Almos in the nine nineties they trying to see muon neutrinos turning into electron neutrinos, and they saw way too many. We couldn't explain them even with a modified version of the standard model that had three neutrino types in it. But you can explain it if you add a fourth type of neutrino, a sterile neutrino, would explain that experiment. It like helps participate in this weird mixing that neutrinos do with each other and would change those rates of mixing and would explain that experiment. Wait, so I guess this helps us get back on the topic of sterile neutrinos because we think it's maybe an interesting possibility that could explain maybe dark matter. But I guess then the question is if they're so hard to see, just like dark matter is hard to see, how are we trying to see this kind of neutrino. And you're saying that maybe you can see it by looking at experiments with regular neutrinos, right, I think that's what you're saying, because this fourth potential kind of neutrino can also turn into the other three kinds of neutrinos you can't directly, but it's also a weird mixture of the mass states. If it turns out there's four different masses of neutrinos, and stero neutrinos are a weird mixture of those four mass states. So the short version is it changes how the other two neutrinos turned into each other. Like stero neutrinos can turn sometimes into electron neutrinos, muon neutrinos could be turning into stero neutrinos because of this weird mixing that neutrinos can do with each other. So it's sort of like discovering that there's a fourth person in your conversation but you haven't been hearing them, but it explains what everybody else has been doing, you know, why they've been acting sort of strangely. So the muon, electron and town atrinos have been acting kind of weird. We can't explain how they're turning into each other and back and forth without invoking this other fourth neutrino I see. So in the experiments that you've have been doing with neutrinos, there's something maybe missing some unexplainable things, and you think that me me, this new neutrino would fix it. But I mean, if it is kind of part of dark matter, wouldn't that mean there are there's a lot of this fourth kind of a netrino, Like, wouldn't that just totally throw the balances off because there is of the universe that is supposed to be dark matter. Wouldn't that be a huge amount of sterile neutrinos. Yes, absolutely, it would be a huge amount of steril neutrinos, and it would throw a lot of other things off. Like people who study the early universe, they can actually measure how many different kinds of neutrinos there are because it would affect like how the universe expanded very very early on, and those measurements are very conclusive. There are exactly three kinds of neutrinos and no more and no room for any other kind, thank you very much. So the very early universe picture from like studying the cosmic microwave background says there's absolutely only three kinds of neutrinos. But then we have this weird experiment from Los Almos that can only be explained using a sterile neutrino or maybe they messed it up. That's the other possibility. I guess maybe they just didn't care at the zero or something. Well, you know, these experiments are very very hard to do, and they were doing something nobody had ever done before, and it was a big puzzle. And so they actually did a follow up experiment like ten years later. It was called Mini Boon, where they used like a lot of the same devices, but they made it a little bit more clever to see if they could like nail this down. They changed it so they weren't just looking at muon neutrinos, they were looking also at anti muon neutrinos. And I remember being in the room at Fermilab when they unveiled the results of this experiment. It's like fifteen years ago, and everybody was wondering, like, are we going to see conclusive evidence us there on neutrinos, And unfortunately what they saw doesn't agree with the original Los Almos experiment, but it also doesn't agree with the standard model. So it's like they saw something else weird that they can't explain. So then we had like two mysteries that were inconsistent with each other. But from the same group of people. So yeah, it was a bit of a puzzle what was going on there, and they didn't carry the two this time. Yeah, And you know, sociologically it's fascinating because there's some very prominent neutrino physicists in the community, very fancy universities, so they have a lot of credibility. But you know, if you ask neutrino physicists, what do you think is the most likely explanation. Do you think it's star on neutrinos or do you think they've like misunderstood some of the background or mess something up. They start off talking about stering neutrinos, but in the end, I think a lot of people are convinced that these experiments there's something missing about our understanding of them. Well, I mean, the trinos are kind of a big deal right now in physics, and so there are a lot of ma trino experiments going on. Haven't have any of them found any indication that sterling neutrinos exist? Or have they all just been telling us the same picture. All the other experiments are consistent with each other and inconsistent with these two experiments that point to sting netrinos. So Without these two experiments, everything fits together very very nicely. Wait, without these two experiments, we wouldn't even be talking about sterl netrinos. Yeah, that's right. I mean, theoretically they're very attractive and they could explain the dark matter. But the early universe says no to star on the trinos. All the other neutrino experiments out there, not led by the group of people, are all consistent with each other and say no to stare on neutrinos, even very specifically cross checking the exact kind of stare on the trino predicted by this Los Almos experiment and the miniboon experiment. But these two other experiments, you know, we don't understand their results. Oh man, I feel a little bit let down. I feel like they shouldn't be called ster on lutrinos. Maybe they should just be called futile because it sounds like I'm not really believe that these exists, Right, You're not painting a very convincing picture from me here. Well, I'll say, you know, personally, I'm not convinced by these things at all. When I was in the room and they unveiled the results of this second experiment, the follow up one, I thought the results look pretty wonky. Not just the actual results sometimes you see a surprise from the universe, but they had a bunch of control regions where they were supposed to be able to understand the results if they knew what they were doing, and those also looked wonky. So immediately I was like, I'm just not sure that these folks understand the backgrounds. It's very personal, my personal opinion about these experimental results. And you know, these things are really really hard to do. I'm not saying I know how to do these things better than those experts. I'm just not sure that we really understand everything that's going on in those experiments. Well, I would hear about what happened, and I think that pretty much explains it. Yeah, I think their kitchens were just dirty and not though, and that probably uh you know, ended up correcting their data. Right, Yeah, maybe that's it. Actually, these experiments are amazing technological feats involved like a hundred and sixty tons of pure mineral oil in these vats underground. They're very difficult. You have to be very careful and exacting about your cleanliness to even do these experiments. So I'm pretty sure their kitchen, metaphorically speaking, is pretty clean. Maybe that you said the oven wrong, All right, Well, it sounds like it's another one of these interesting mysteries in physics, as as they seem to be everywhere and where you have some experiments that point to something interesting and some that don't, and if one of them is correct, and Mike tell us a lot about a huge portion of the universe, right Like, if something like this could explain dark matter, it would be a humongous deal. It would be twenty seven per end of the universe. It would be a humongous deal. And even without these experiments, even without sterio neutrinos, we know there's something weird going on with neutrinos, right because we can't explain why they have mass, why they seem to be right handed neutrinos, why they have such a little mass. Neutrinos themselves are very very strange objects. And in the physics community, a lot of people are persuaded that this is the next frontier, this is where we're going to make big discoveries. And that's actually why the United States has decided to make a big bet on neutrino physics. We're not going after like Energy Frontier super high energy colliders were letting the Europeans and the Chinese do that, but the U S community is betting big on neutrino experiments. Alright, well, stay tuned as we learn more about neutrinos, or maybe not learn much about neutrinos. Either way, it would point to some big mysteries in the universe. And we definitely know that the universe seems to be sort of a hot, delicious mess. Doesn't seem to make any sense to us, but maybe one day if we rotate our internal matrice is the right way, it'll all click into place and hopefully be delicious. That's the taste I'm going for in life. Let's open an ice cream shop called Neutrino Flavor. Well, we hope you enjoyed that. Thanks for joining us, See you next time. Thanks for listening, and remember that Daniel and Jorge explained. The Universe is a production of I Heart Radio. For more podcast from my Heart Radio, visit the I Heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows. H