Daniel and Kelly’s Extraordinary Universe - How massive is a neutrino?

Episode Date: August 1, 2023

Daniel and Jorge talk about the mysterious mass of the neutrino and how we can measure it.See omnystudio.com/listener for privacy information.

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They give you the answers, and you still blew it. The Puzzler. Listen on the IHeart radio app, Apple Podcasts, or wherever you get your podcasts. Hey, Daniel. Why do a particle physicist obsessed so much about mass? Well, mass is one of the basic properties of a particle. It's like part of its identity. Whoa.

Is that healthy? though. You think your mass should define who you are? I don't think we have to worry too much about like particle mental health. Yeah, but shouldn't they be defined by their magnetism or how colorful they are? Well, we're all made a particle, so I guess we can just decide for ourselves how to identify with them. You are your particles, right? My particles are me. I'm pretty sure it's the other way around it. Depends if you believe in strong or weak emergence. That is a massive detail right there. Hi, I'm Foreham, a cartoonist, and the author of Oliver's Great Think Universe.

Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I really wish there was more we could know about each particle. Why do you want to know? I want to get to know them, you know? Particles are kind of like black holes. There's a few things. you can measure about it, the spin, the mass, the charge, et cetera. But otherwise, they're all totally identical. It's not like this particle is Bob and that one is Sam and this one is Juanita. You know, all electrons are the same. What if they don't want to be known? What if they're private particles?

I see. They're all Spartacus, huh? Yeah, they have secrets. They don't want the waltz out there on the Internet. Well, like I've said before, I don't think the universe deserves any privacy. You know, we are curious creatures and we're part. part of the universe. So knowing ourselves is sort of like knowing the universe. Are you saying physicists then are sort of like professional boxers?

I like to think of as more as detectives, maybe. Private snoops. Yeah, we are snoops for sure. And we're out to solve the biggest mystery in the universe, which is like, how does this whole thing all work? You should change your job title then to a particle snooper. Particle investigator. I'm a PI. A PPI, I guess. I don't like having Peepee in my title. Yeah, Pee is not good on many things. But yeah, anyways, welcome to our podcast, Daniel and Jorge, explain the universe, a production of IHeart Radio. In which we try to lift the level of discourse as best we can, elevating your

mind to the deepest, biggest, most ethereal questions in the universe. How does it all work? What's it all made out of what are the rules of the game? And how is the game played in such a way to give us this crazy, amazing, visceral, conscious experience of such a real world, which in the end is made up of tiny little almost massless particles. Yeah, because it is a pretty awesome experience to exist in the universe and to look out there and appreciate all the wonders and amazing things that are happening out there in the universe that we can see and also that we can't see. And as we drill down into the nature of reality, taking things apart into molecules and atoms and nuclei and protons,

and neutrons. We like to give names to these things. We say, oh, this kind of thing is an electron and that kind of thing is a neutrino and this kind of thing is a cork. It's just part of who we are to want to attach labels to bits and pieces of the universe.

Yeah, it's all part of humans' quest to understand what's going on out there to get a handle on how things work and how to predict what's going to happen in the future. And as we look at these tiny little particles, we want to describe them in ways that make sense to us.

You know, how much spin does it have? What can it do? And maybe at the most fundamental level, part of the identity of a particle is how much mass does it have? Yeah, some particles have a little bit of mass. Some particles have a lot of mass, and some particles have no mass, right? Some particles adhere to a very impressive diet. Photons have no mass, while top quarks, the heaviest known fundamental particle, have the mass of like 175 protons.

So there really is an extraordinary range, which is something that we don't understand at all. But mass is also part of how we tell which particle is which. When I think about an electron and a muon, what are the differences there between the two? They're almost identical particles except that muons have more mass than electrons do. And when we produce particles in our experiments, that's how we tell what's what. We measure the masses of these particles and we say, oh, this one's got to be an electron because, look at its mass. So it's not just that we take the particles, we assign mass labels to them.

We use the math to tell us who is who. Yeah, and there are lots of particles out there. Some of them are not shy at all about how much mass they have. Some of them are a little bit shy and don't necessarily want to reveal how much mass they have. Some of the weirdest particles out there are neutrinos, these ghostly little particles that are everywhere but very hard to spot. And in the case of neutrinos, their identity is something of a more. complex story. They have sort of two different kinds of clothing they can wear, who they talk to

and how they move through the universe. And because their mass is so weird and so hard to nail down, it's not something we actually know very well. It's all a big mystery. And so today on the podcast, we'll be asking the question, how massive is a neutrino. Or maybe we should have said how massive isn't a neutrino. Wait what? Why shouldn't we have not said that? Because neutrinos have some mass, but they definitely aren't very massive. Or how very little massive a neutrina is. Is that what you mean?

How dainty is a neutrino. I thought you meant like how significant a neutrino is. Like how massive is it in a universal scale of awesomeness. Yeah, it actually turns out neutrinos are quite important and play a big role in the physics of the universe, despite being almost invisible. So from a consequential point of view, right, neutrinos are massive, dude. Well, I think what you're saying is that the mass of the neutrino is not known.

We don't know how much mass it has. We do not know how much mass the neutrino has. We've only known that it has mass for a couple of decades, which was a big shocker and sent quakes through the theoretical community when we figure that out. And it's still something that is very hard to pin down and not something we know. It was a massive shock.

wait heavily on the minds of physicists for a long time. They didn't take it lightly, that's for sure. So yeah, this is an interesting question. How much math does a neutrino have? Apparently, it's kind of tricky to find out. So as usually, we were wondering how many people out there had thought about this question or have an idea about the mass of a neutrino. So thanks very much to everybody who answers these questions for this fun segment.

If you'd like to hear your voice speculating for everybody else's entertainment and education, please write to us two questions at Danielanhorpe.com. So think about it for a second. How massive do you think a neutrino is? Here's what people had to say. Not sure if the vibe was that there's more than one type of neutrino. So maybe there's like some with more mass. But I thought that neutrinos were like massless or like had negligible mass.

And so like they travel at the speed of light. I think there's different types of neutrinos that are different sizes. You talked about one of NASA finding another universe by, I've seen neutrinos pass through Earth. So there's some massive ones, but not so massive. How big? Maybe like 50 protons big or something, if that even makes sense. And maybe neutrinos are also dark matter is what you also said in one of your earlier

podcast. I would think that a neutrino is real light because it doesn't interact with other particles. But it may interact with the Higgs field. So I actually have no idea. Well, neutrino, so Eno means very small in Italian. or smaller. So I would assume that the mass of a neutrino is much, much, much smaller than that of a neutron. And I'm tempted to say that neutrinos are massless, maybe. Mass is just, I think, the amount of energy that's required to move something. So

gravitational mass is just a unique form of inertial mass, wherein, It's the gravity which is pulling you, and that changes according to where you are. Whereas inertial mass is just independent of that, I guess. I don't know how massive a neutrino is. I'm pretty sure that I've heard that they have mass, and I think it's extremely light. Neutrinos are very low mass, and it would be great if they had the lowest amount of mass allowed by quantum mechanics. That would be pretty neat. Right. I think a lot of people seem to know it had very little mass.

I really like the linguistic analysis, reverse engineering the name particle to infer what its mass has to be. What do you mean? It has neutral mass? Well, you know, neutrino means little neutral particle. That was the name given to it before we even really knew what it was because that's all we knew about it, that it couldn't be very massive and that it was electrically neutral. So in that sense, you might even be tempted to say that it's a well-named particle. I thought you were going to say it has the mass of a newt. But also, you kind of have to know Italian to know that the I-N-O ending, E-Kno, means small, don't you? Not everyone speaks Italian. That's true.

I guess if he had been named by somebody who speaks Spanish, it would be like Nutrito. Yeah, exactly. Or in English, I guess, how would you call it? Nutrini? Little neutral. Do we have affectionate endings in English? Tiny neutron. There you go.

Like Tiny Tim. Or maybe we'd give it an ironic nickname, you know, like Big Neutron. Yeah, Neutronizer or something. Or how about just neutron? I mean, that sounds pretty massive now in comparison to neutrino. Neutron had already been discovered is the name of another particle. Oh, well, there you go. That one's misnamed them.

All right, well, let's dig into this mystery. What is the mass of a neutrino? But I guess as far as Daniel, talk to us about what a neutrino actually is. Neutrino is a really fun particle because it's so weird and yet so fundamental and so important. And at the same time, not a part of the matter that's around us. You know, if you take apart the stuff that you're made out of and that I'm made out of and that everything you've ever eaten is made out of, you discover that it's made of atoms and those atoms are made of protons and neutrons and electrons.

But the protons and neutrons can be made out of corks, up corks and down corks specifically. That means that everything that we know is made of two kinds of corks, up corks and down corks, as well as electrons. So really just three particles explain all of those. the matter that we know, the stuff that the earth is made out of, that the sun is made of that the visible matter in the galaxy is made out of, of course, put dark matter aside because we don't know what that is made out of. So those three particles sort of underlie everything that exists, but there's another particle that's in the same category as like one of the basic

templates of possible matter. And that's the neutrino. Because you notice that the up quark and the down quark sort of have each other. There's like a pair of corks. You might wonder like, well, Who's the electron's partner? And the electron does have a partner. It's the neutrino. So it sort of like completes the quartet of the fundamental bits of matter, even though the neutrino doesn't appear in the atom and isn't used to make up your lunch or your dinner

or anything you've ever eaten. I guess maybe the first question I would have is, why not? Why aren't neutrinos part of the matter that we're made at it? Or why don't we have neutrino bits inside of us? Yeah, it's a great question. And, you know, the universe has these bits and pieces and then they have rules for how they can come together. And then you get complex structures emerging from that.

You know, you have quarks bind together to make protons and neutrons, which then bind with the electron to make atoms to make all sorts of other complex stuff. I scream and stars and black holes and all that stuff. And really, it's the interaction there, the binding that's crucial. While quarks and electrons all have electric charges and quarks have strong charges so they can use the more powerful forces, To build complex matter, neutrinos are different from the other three kinds of basic fundamental bits of stuff in that they only feel the weak force. So they have no electric charge, they're neutral, and they also have no color so they don't feel the strong nuclear force, which means they're only left to interact via gravity, which is basically negligible for a particle and the weak force. So in order to build something out of neutrinos, you'd have to have them bound together by the weak force. but the weak force is just too weak to do that.

Interesting. What do you mean to weak? Like you can't stick to neutrinos together with the weak force? The weak force can be used to interact, but it's really very, very shockingly weak. That's why, for example, if you shoot a photon at the wall, it'll splat against the wall and interact with all the electrons inside of it. But if you shoot a neutrino against the same wall,

it will fly right through. It's not like it's finding holes in the wall. It's not like the wall is a screen or a mesh that it's slipping. through, it ignores all those particles because it doesn't interact with them. So it's really all about the strength of the interactions. If you wanted to like bind two neutrinos together into a more complex object, they'd have to be in a bound state. In order to be trapped together by an interaction that's so weak, they would have to be almost motionless. It wouldn't take very much energy to break it apart. So you'd have to have very cold bits fall together to make a bound state and then it'd be

very easy to break it apart. So it's basically not possible to build more complex structure. using the weak force. I think you're saying that you can, but maybe Matter would have to be super duper cold to put together things with the weak force. Yeah, matter would have to be super duper cold and there would have to not be other stronger forces disrupting it, right?

I don't know. How does the weak force work? Does it repel or attract or both? Does it have positive and negative charges to it? So the weak force is quite complicated. We talked once about whether the weak force can attract or repel. It actually can do both. There are two different charges for it.

They're called iso spin and weak hypercharge. And so it's a complex combination of all these different numbers that tells you what the weak force is going to do. But in short, it can attract and it can repel. So it's very similar to electromagnetism. Actually, electromagnetism and the weak force together are part of a larger idea called electro-week. And the reason that one of them is more powerful than the other has to do with the Higgs boson, which breaks the symmetry between the two forces, leaving one of them very powerful and one of them very, very weak. So like if I took two neutrinos and I cooled them down out there in space and I stuck them together, would they stick together due to the weak force?

You could put two neutrinos into a bound state if they were very, very cold so they didn't have enough kinetic energy to escape these bonds and there was nothing else bothering them. Yes, you could. And you could even add more. Yeah, you could add more. Maybe can you like build a whole planet out of neutrinos? You could build larger, more complex structures, but it would be very fragile and it certainly wouldn't look like a planet. And the whole thing could probably pass through the earth without even noticing because neutrinos again don't interact with normal matter.

So even if you built more complex structures out of neutrinos, it exists sort of in parallel to us. The same way that like dark matter does. Dark matter is here, dark matters everywhere. Dark matter might make complex structures that we can't see, but they pass right through us and we pass right through them because we don't have any interactions with them. The same way a neutrino can pass through like a light ear thick wall of lead without even interacting. And so a whole planet of neutrinos would do the same thing. Like right now, there's a hundred billion neutrinos passing through every square centimeter of the surface of the earth every second. And yet we don't feel them. So somebody could throw a planet of neutrinos

at us and we wouldn't even notice. Would that neutrino planet break apart when it goes through us? Or would it stay together? A tiny fraction of those neutrinos would interact with us. So those little bonds would break up, but most of it would totally ignore us. Neutrinos have a very, very tiny, of interacting with electrons or with quarks. So, and then when you say weak, do you mean like low probability or just that the force is weak? We mean low probability, not small momentum exchange, but low probability. Like you shoot a neutrino at another particle, it's very unlikely to interact. If it does interact, it can impart significant momentum.

It's just a low probability of it happening. Oh, that's interesting. So it's really called the weak force because of the weak probability, not because like, you wouldn't feel it. Yeah, exactly. The very strengths of the forces are more about the probability of that interaction, which if you integrate over all possibilities, does end up playing a role in like its impact on the world, basically how massive is its impact. So then maybe like a better name for the weak force would have been improbable force, the unlikely force. That makes it sound like it's going to go on a hero's journey and in the end become the most powerful force in the

universe. That's right. The underdog floors. What else do we know about neutrinos? We know that there are three kinds of neutrinos. The way that there's like three different kinds of electron, there's the more massive version that's the muon and the even more massive version that's the tau. So there's three different flavors of electron. There's also three different flavors of neutrino. So there's a neutrino associated with the electron, the electron neutrino and one associated with the muon and one associated with the tau. What do you mean associated. What does that mean? They sign a contract? Well, these guys interact via the weak force. And so, for example, if you want to make an electron, you can make it from a W boson.

A W boson can decay to an electron, but it also decays to a neutrino. And when you create an electron, you also create an electron neutrino. If you create a muon, then you also create a muon neutrino. So when we say associated with, we mean like grouped together with by the weak force. It groups these guys together. Remember that we count the number of leptons in the universe and that's conserved. So for example, you can't just like make more electrons. If you make more electrons, you also have to make more anti-electrons to balance out the number of electrons in the universe. But electron neutrinos fall into that category. So you can make an electron and then you make an anti-electron neutrino and the universe's books are all balanced. Like an electron and a

neutrino are sort of like twins. Like you can't have, you can't make one without the other? You can make an electron either with an anti-electron neutrino or with an anti-electron. So like a W boson will decay to an electron and an anti-electron neutrino together, or a Z boson will decay to an electron and an anti-electron. You can't just make an electron by itself. So there's, it sounds like there are more electrons than there are anti-electrons and electron neutrinos. There's definitely more matter than antimatter. So yeah, they're more electrons than anti-electrons.

But when it comes to the neutrinos, like we have these pairing. So there's three different flavors of neutrino. the muon, the electron, and the tau neutrino. Each one is connected to one of these leptons because the weak force likes to make those together. That's just something we've observed, right? Like, you notice that the weak force, when it does things in the universe, it creates these things in pairs. Like, is there anything else we know about them that associates them? Like, do they have the same quantum variable about it?

I like the way you say that's just what we observed. Like, that's basically science, right? We observe the universe and then we describe it and then we try to boil that description. down to a simplest set of rules as possible and think about what that means. So, yeah, that's just what we've observed. We've never seen this be violated. So there's an asterisk there. We'll talk about neutrino oscillation in a minute.

But, yeah, really, that's the only difference we know about from these different kinds of neutrinos that the weak force associates them with different leptons, with electron, a muon, or a tau. The other question, of course, is about their masses. Like, what are the masses of these particles? We know that for normal matter, all the corks and the electron, the masses tend to increase as you go to their copies. Like the muon is heavier than the tau. The upcork has heavier versions, the charm at the top.

The down cork has heavier versions, the strange in the bottom. When it comes to the neutrinos, we don't know so much about what their masses are and how that's organized. All right. Sounds like a good cue for us to dig deeper into the mass of the neutrino and talk about how we know it has mass and how we measure that mass. So let's get into that. But first, let's take a quick break. Don't let biased algorithms or degree screens or exclusive professional networks or stereotypes. Don't let anything keep you from discovering the half of the workforce who are stars.

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Wait, what? Oh, that's the run right. I'm looking at this thing. Listen to No Such Thing on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. All right, we're talking about the mass of a neutrino. Specifically, what is its mass? Is it a lot? Is it a little?

And why is it the way it is? So we talked about what a neutrino is. There are ghostly particles that fly around. the universe without really interacting with the rest of the matter in the universe. Daniel, a quick question. Do they interact with dark matter? Oh, yeah, great question. We don't know.

For a long time, we wondered if neutrinos were the dark matter. Like, they kind of fit the bill because we can't really see them and there's maybe a lot of them out there. Turns out neutrinos can't be the dark matter because we know the dark matter moves slowly. It's cold. We know that from like how it's influenced the structure of the universe. If dark matter moved faster, things would be less lumpy. And neutrinos move really, really fast.

So neutrinos are too hot to be the dark matter. Do neutrinos interact with dark matter? We don't think so because we don't think that dark matter feels the weak force or the improbable force, as you'd like to call it. Because if it did, we would have seen it bump into some of our big underground detectors. So because dark matter probably doesn't feel the weak force, it probably doesn't interact with neutrinos. Yeah, I feel the same way. I think I'm too hot to be dark matter. I'm always telling people that.

And ironically, I'm also pretty cool. You're a paradox of physics. Yes, I'm an enigma wrapped in a cartoonist. But talking about the mass of a neutrino, I guess the first question is, like, first of all, how do you know it has mass? Like, there are particles out there without mass, right? How do we know the neutrino has mass? Yeah, you're right.

There are particles out there that have no mass, like the photon and the glue on. So it's not impossible for the neutrino to have no mass. And for a long time, we assumed that it didn't. There's even an argument about what we mean by. the standard model of particle physics, sort of our description of our best understanding. Some people say that the standard model of particle physics requires neutrinos to have no mass. Though there are extensions of it that allow them to have mass. Some people say that's beyond the standard model.

Some people say that's the new standard model. As you might expect, there's a big argument about how we name it. But for a long time, we assumed neutrinos had no mass. But now we do know that they have mass. And we know that in two different ways. We know that they have mass even without knowing how much mass they have. Interesting. Do you know because they, I don't know, pass around heavy objects or because you've weighed them?

So we know in a few different ways, actually. One of the first clues was looking at a supernova. There was a supernova in 1987 that was very, very bright. And we saw a big flash of neutrinos coming from that supernova. And the neutrinos actually arrived a little bit before the photons because neutrinos come from the center of the supernova. and they aren't blocked by the rest of the matter in the supernova, whereas the photons come from the surface

and it takes a while for the energy to propagate out and produce those photons. But they looked at when the neutrinos arrived and realized that they don't all arrive at the same time. We think they all leave the supernova at basically the same moment, but they don't all arrive at the same time. The higher energy neutrinos arrive earlier than the lower energy ones. The higher the energy, the faster they go. That makes sense, but it's actually a property you can only have if you have mass.

Massless particles like photons all travel at the same speed regardless of their energy. All photons travel at the same speed because they're massless. Neutrinos have a spread in their velocity, which means they have a mass. But I guess it tells you that they're not as fast as photons, which means they have mass, right? Because anything that doesn't have mass would move at the speed of light. Exactly. Things that don't have mass always have to move at the speed of light. There's no option there, right? Massless objects always move at the speed of light. Okay, so neutrinos don't move at the speed of light, which means they have some mass, but then is that the main way that we know they have mass? So there's another really fascinating clue, which comes from the Big Bang. We think that a lot of neutrinos were made in the Big Bang.

Like all this energy was hot and dense and the quantum fields were frothing. And as they cooled down, they sort of dribbled out into all the different fields that are out there. So the Big Bang made a lot of quarks, made a lot of electrons, and made a lot of neutrinos as well. And as those particles all mixed together, the amount of photons and neutrinos and quarks determined, like, what kind of stuff got made later as things cooled. Like, how much hydrogen did you get? And how much helium did you get? And out of those things sort of sloshed together and frothed together in the Big Bang. So by studying the relics of the Big Bang, the leftover bits of it, we can actually get

some clues as to, like, how many neutrinos there were. And we can even figure out something about the mass of those neutrinos. But wait, I thought neutrinos don't interact with regular mass. So how can it like regular mass relics tell you about how many neutrinos there were in the Big Bang? Yeah, you're right. The neutrinos almost never interact with matter. But if matter is dense enough, they will. Like the probability is not zero.

It's greater than zero. And actually back in the earlier times when the universe was hotter, when things were denser, the weak force was not as weak as it is today. we think back in the very early universe, the weak force and the electromagnetic force, before the Higgs boson broke the symmetry, the two were actually equally as powerful. So neutrinos used to interact with normal matter more than they do today. I think what you're saying is that our models of the Big Bang tell us that there were a lot of neutrinos at the Big Bang and that they had mass.

The models of the Big Bang tell us something about how many neutrinos there were, like the number of neutrinos, because neutrinos back then were moving really, really fast. They were very, very hot. And so they helped, like, spread energy out. They sort of acted like photons because everything was so hot. And when we study the early universe, we can see these acoustic oscillations. Like there were these density waves in the early universe. Things were hot and dense, and they created pressure waves in the matter.

Photons and neutrinos helped us sort of smooth that out a little bit. So by looking at those oscillations, they're called baryon acoustic oscillations, which make these ringing patterns in the early universe. We can measure how many neutrinos and how many photons there were. So that tells us something about the number of neutrinos. Then we can do a second thing to figure out how massive the neutrinos had to be. Like we know how many neutrinos there were. And then we can figure out, well, how much mass could the neutrinos have without causing the universe to collapse?

Right. We know that the universe has been expanding since it was very, very young. And that tells us something about like how much matter and radiation and energy there is in the universe. because if there was too much, then gravity would pull everything back together very quickly into a Big Bang. So we know something about how many neutrinos there were. We could put an upper limit on how massive they could be

without collapsing the universe. But I think the two are sort of tied together, right? The number of neutrinos and how massive they are, right? I mean, you sort of have to assume they have mass in order for them to matter at the Big Bang, right? Well, they don't have to have mass in order to matter. It's funny that we use matter. Because remember, general relativity is sensitive to any.

energy density, whether it's in the form of radiation or in the form of matter, it really is just sensitive to energy density. So the Big Bang analysis tells us the number of neutrinos totally independently of their mass. And then the second step is to say, well, if neutrinos do exist, how much mass could you give them without causing the universe to collapse? So that tells us something about how massive they could be. Like an upper limit. Yes, exactly.

It's an upper limit. That number is actually really, really low. that number is less than a tenth of an electron volt. Which, I guess, to give us some context, how much mass does an electron have? So an electron has like 500,000 electron volts. It's half of an MEV, half of a mega electron bolt. And so 500,000 electron volts, that's not very much, right? Electrons are very, very low mass particles compared to like a proton.

A proton has like one giga electron volts, one billion electron volts. So we know from the Big Bang that all the neutrinos added together have to have less than a tenth of an electron bolt, less than one 10 billionth of the mass of a proton. You mean all the different kinds of neutrinos, not all of the individual neutrinos in the universe, right? Yeah, that's exactly right. There are three neutrinos. When you add up all their mass together, it has to be less than a tenth of an EV, where an electron is 500,000 EV and a proton is about a billion EV. Interesting. So then pretty light. Very, very light.

Like how much is a cork? It depends a lot on which cork you're talking about. The lowest mass corks have like a few M.EV, a few million electron volts. The most massive ones, like the top cork, is like 175 billion EV. So these machinos have mass much, much closer to zero than anything we've ever seen before. They're like shockingly low mass. Okay, so we have sort of an upper limit, you said, for how much the three, kinds of neutrinos can add up together, but then how do we resolve how much each one of them

weighs? So then we have another really fascinating clue, which tells us about the mass difference between the neutrinos. So so far we know something about the sum of their masses. We know it's less than 0.1 EV. We also know there are three neutrinos. We're wondering like, well, they all have the same mass. Is it like with the other particles where there's one low mass and then another one and then another one? So we can do another kind of experiment to measure the differences between the masses of the neutrinos. And this comes from how they actually change their identities. Neutrinos are weird compared to the other particles in even another way.

They're different from like the electron, the muon, and the tau, and that they can change flavor. Like if you create an electron neutrino and shoot it through space and then wait like a light ear, two light ears and try to measure it, you might discover it's no longer an electron neutrino. It's now a muon neutrino or a tau neutrino. This is called neutrino oscillation. Yeah. I think usually if you shoot anything, do space will change the flavors.

But I guess how do we know this? How would we know if it changed flavors? And again, flavor is kind of the charge of the weak force, right? Flavor is actually which of these generations of particles it is. Like, is it electron, is it muon, is it tau? Right? That's what we mean by flavor. Oh, is there a charge of the weak force?

Or is it just a weak charge? The weak force does have a charge. Remember, it's two different charges. There's the isos spin and the weak. hypercharge. So both of those count as weak charges. But the neutrinos all have the same weak charges. What they have different is this flavor, this different identity. But that identity actually turns out to be different when you create the neutrino and when the neutrino flies through space. They have like two different sets of identities. There's the identity we talked about

when a neutrino is made, like the weak force when it makes an electron, it makes an electron neutrino or if it makes a muon, it makes a muon neutrino. But when neutrinos fly through space, they have three different identities, and those are their masses. So there's three different kinds of neutrinos for the weak force, and there's three different kinds of neutrinos for the masses, but those are not the same. They're like a mixture of each other. So if you imagine this like M1, M2, M3 are the three neutrino masses. When you create an electron neutrino, it's not like it's M1.

It's some weird mixture of all the masses of the three neutrinos. You mean some kind of weird quantum mixture. Is that what you mean? Yeah, it's a superposition. So you create an electron neutrino. It's a quantum superposition of the three different neutrino masses. When you create a muon neutrino, it's a different superposition of those masses. It's like having two different set of axes that are not aligned. It's like a rotation between your set of axes. I guess maybe the question I have is, so there's three types of neutrinos, electron neon and tau neutrinos. And the only difference between them is the mass? The only difference between the electron, muon, and town neutrino is how they interact with the weak force. Three different kinds of neutrinos is two different ways to break them down. One is how do they interact with a weak force?

The other is what are their masses? So you get two different ways to categorize the three neutrinos. What do you mean how it interacts with the weak force? Like it's probability of interaction or its strength of interaction? What do you mean by that? Like what it's made in association with? Like if you make an electron, what kind of neutrino do you make? Well, you make an electron neutrino.

If you make a tau, what kind of neutrino do you make? You make a tau neutrino. But if you already made it, does it matter? Or does it matter in like what it can do later? It matters in the accounting of the number of electrons or muons or taos in the universe, yeah. But like if you just catch one in space, how do you know what it is? Because you weren't there when it was made. Yeah, good question.

Well, electron neutrino is more likely to make electrons. And a muon neutrino will make a muon and a tau neutrino will interact and make a tau. One of our neutrino experiments can see electrons. It can also see muons and it can also see taos. And so you can tell which kind of neutrino it was by how it interacts. Does it create an electron? Does it create a muon? Does it create a tau?

What it can do in the future, kind of. Yeah, what it can do in the future. Because the universe keeps track of this accounting. How many electrons are there? How many muons are there? How many taus are there? But again, that's just one way to see these things. Another way to see these things is how much math do they have?

And for most particles, it's the same. same thing. The weak force creates an electron. The electron has a mass. All electrons have the same mass. It's just a number. And if you ask like, what are the masses of the leptons? You get three different numbers. Those align with the flavors of the leptons. But when it comes to the neutrinos, they don't. So when you create an electron neutrino, it's a weird mixture of these different masses. And as it flies through space, those, that mixture can change because mass tells us how things move through space. So these electron neutrinos and muon neutrinos and town neutrinos, because they're made of three different masses

and those masses are different, those masses like fly through space slightly differently and they can turn from one into another. I think what you're saying is that if you make an electron neutrino like in the center of the sun and it's flying to us and it has the identity of an electron neutrino, it might have that identity, but it might not necessarily have a particular mass.

Like it might have one of three different masses. Exactly. Or if you like find a neutrino out there in space with like one of the masses, like the highest mass for neutrinos, then that could still be either an electron neutrino or tau neutrino or a new neutrino.

Yes, that's exactly right. In mathematical terms, if you have a weak eigenstate, if you have an electron neutrino that's something produced by the weak force in a pure electron state, it's a mixture of the mass states. If you have a pure mass state,

it's a mixture of the flavor states. I think basically the neutrinos can have an identity crisis going on. Both the mass crisis and an identity crisis. Like it doesn't quite know what it is or it could be different things but it could also weigh different things

and it could also call itself different things and it's sort of like up in the air like it can change, it's fluid between these identities. Exactly. Neutrinos have two different kinds of identities and they do not align. For most particles, these things align very well. For neutrinos, they don't.

Like an electron, for example, if it's born an electron, it's going to have the mass of an electron. It's not suddenly going to have the mass of a tau electron or a muon electron, right? Yeah, and this calls in the question what I was saying the very beginning of the podcast about mass being part of the identity of a particle because neutrinos can't really be defined by their mass. Like, well, it

depends. Are you talking about who I interact with or how I fly through space? Because the same neutrino can give you two different answers to that question. Interesting. All right, well, let's dig into how we actually measure the mass of a neutrino and what those results have found. But first, let's take another quick break. I always had to be so good, no one could ignore me. Carve my path with data and drive. But some people only see who I am on paper. The paper ceiling.

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See? Listen to no such thing on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. All right, we're talking about the mass of a neutrino. How massive is this ghostly particle that flies through space, barely interacting with everybody else in the universe, ignoring everyone. It's kind of a snobby particle. It's just got its own stuff to do, you know.

It just can't stop and chat with everybody. It's got its list of errands. It's very aloof. It's just busy, man. It's just busy. It's just more neutral. That's less opinions, I guess. It's not as interesting.

All right, and so we're talking about how much mass it has, and we know from the Big Bang models that we have, that Nutrune has very little mass, and the different kinds of neutrinos can't have a lot of mass combined. We talked about how the neutrino kind of has an identity crisis. It doesn't quite knows for real what kind of neutrino it is and how much it weighs. It's all sort of fluid and quantumy kind of complex and superposition.

So then I guess the big question is, What can you do with that? How do you measure these masses if the neutrino is so wishy-washing? Yeah. So the fact that neutrinos can change flavor was a big mystery in particle physics for many decades. Like we count the number of neutrinos we see from the sun electron neutrinos. And we don't see as many as we thought we should, which is a big puzzle for a long time. We predicted a certain number of electron neutrinos being created in the sun.

And we just didn't see as many. We saw like a third as many as we expected. Now we understand that's because they're oscillating. they're changing from electron neutrino to something else. And so we're not seeing them because they're not interacting with our electrons. But we can also use that to measure the differences in the masses of the neutrinos. It's because there's a mass difference between the neutrinos because they fly differently through space, that they're changing their identity as they go.

So what we can extract from this are two numbers, the mass differences. Like you imagine this M1, M2, M3. We can measure the separation between those three. We can't tell the overall mass, but we can tell how different they are, what the gaps are between the neutrino masses. I guess the question is, why can we measure the absolute value of these masses? Because this oscillation doesn't depend on the absolute value. It only depends on the difference. Like if all the neutrinos had the same mass, then there wouldn't be any oscillation.

And if the mass differences were really, really large, they would oscillate more. So by measuring how much they oscillate, we can measure this mass difference. But the oscillation doesn't depend on the total mass. There's a separate experiment we'll talk about in a minute called the Katrin experiment, which is going to try to measure the overall mass of the neutrino. But this oscillation, something which is quite well established, gives us a precise measurement only of the differences between the masses. I guess maybe I didn't quite understand why we can only measure the differences.

Because the oscillation comes from the differences. Like if there weren't any differences, you would see no oscillation. And the larger the difference, the more the oscillation. It's kind of like measuring interference. between two laser beams. If they're in sync, you see no interference. If one of them is delayed, then they're out of phase and they interfere with each other and give you an effect you can measure.

But all you can measure from the interference is the difference between the beams because that's what causes the interference. A neutrino is a mixture of different masses, and each of those masses flies through space differently. And it's that difference that causes them to change flavor to oscillate. But then how do we measure the oscillations? It's like we can only measure one neutrino at a time. We don't know what it was before.

How do we know what it was after? Well, we don't measure oscillations for an individual neutrino. You're right. What we do is measure them statistically. So we have like a bunch of neutrinos made in the sun. And we know those are all electron neutrinos because the sun has electrons in it and not muons and tau's. So we can measure how many of those have disappeared by the time they get to Earth. We can also make a bunch of muon neutrinos in a particle beam on Earth and then see how often they disappear.

So we can make a bunch of these measurements of. neutrino oscillation, not by looking at an individual neutrino and seeing it oscillate, but by making a huge number of neutrinos and seeing how many of them disappear from their original identity. Because you're saying like the way you measure them, like when you catch a neutrino, you sort of know what it was. Or at least the detectors can only measure one kind of neutrino at a time. Exactly.

All you can do is measure its flavor. That's the way we detect them is we interact with them. The only way to interact with them is through the weak force. And that means using electrons, muons, and tau's. That's how we interact with them. And then how does that tell us there are mass differences? Like if I catch a neutrino, can I just infer its mass from like how much energy it has and how fast it was going?

So there are experiments that are going to try to do exactly that, which we can talk about in a minute. The oscillation experiments are just counting how many neutrinos have disappeared. Neutrinos have such low mass that's very, very difficult to measure them individually on a per neutrino basis. But there is an experiment in Germany, which is trying to do exactly that. Okay. So then you're saying that we have measured kind of.

the differences between the masses. So what are those numbers? Those numbers are really small. There's two numbers there. One of them is 10 mill electron volts. A mill electron volt is 1,000th of an electron volt. The other one is 50 mill electron volts. So some of them has to be less than 120 milo electron volts.

And we know that the gaps between them are 10 and 50. This feels like a fourth grade logic problem. Like Sally, Paul, and John have money in their pockets. and it adds up to $1.20, but the difference between Sally and Paul is 50 cents and the difference between Sally and John is 12 cents. How much does Sally have? Exactly.

And so we know that there's two possible solutions. We know that two in the neutrinos are close to each other. There's a small gap 10 M.EV. We also know that the third one is further away. It's 50 MEV away. We don't know if the two ones that are closer are heavier or lighter. So like are the two ones that are near each other on the top of the spectrum or the bottom of the spectrum.

We don't know. There's two possible answers there. We also don't know quite how it adds up. Like the number we have from the early universe is an upper limit. They could all still be very, very low values. So there's a lot of open questions there. We'd love to know the sum of the masses of all the neutrinos. Well, you sort of just need to know one of the masses, right? And then that would click the other ones in place.

Well, there's still two possible solutions. If you just know one of them, you don't know if you have like the inverted hierarchy where the two close ones are at the top or if you have the other hierarchy where the two close ones at the bottom. Oh, I see. But you're saying we know this very precisely, like our models of the neutrino when you shoot a bunch of them out and you see how many transform into different kinds. That somehow tells you the difference in their masses because I guess it affects the probability of these transformations. Yeah. And we've been doing these neutrino oscillation experiments for decades and we've done them in all sorts of ways with all sorts of different

combinations, make this kind of neutrino, disappear that kind of neutrino, make this kind of measure the appearance of the other one. We've triangulated that whole matrix and we know exactly how how these numbers work out. What we don't know is the overall mass, only the differences. So the differences are very precisely known. The overall mass is limited by this Big Bang cosmology stuff to less than 120 mill electron volts. But now this is this really cool experiment in Germany called the Katrin experiment, which is going to try to measure the mass of the electron neutrino as precisely as possible. All right. Let's talk about this experiment. Now, what is it, how does it work? So this experiment is called the Karlsrua Tridium Neutrino experiment, which is a tortured way to make Katrin as an acronym.

To say the least. But it starts from Tridium and Tridium decays to helium, which is like two protons and a neutron. And then it also produces an electron and a neutrino. And tritium is just an element, right? Yeah, Tritium is two neutrons and a proton. So it's like an isotope of hydrogen. Basically what happens is one of those neutrons turns into a proton and then emits a element. an electron and a neutrino.

And this is a nice way to measure the neutrino mass because the electron neutrino don't have a lot of energy. They come out moving really, really slow. And so basically you can see the effect of the mass of these particles on how fast they're moving. There's like not a whole lot of energy made in this reaction, so not a lot of despair. So if the electron and the neutrino have a lot of mass, they'll come out moving slower. They have less mass, they'll come out moving faster.

And so we can't see the neutrino directly, but we can measure this. the electron energy very, very precisely. So that's what this experiment does. It measures those electrons really, really precisely. And if he sees electrons moving with more energy, it means that the neutrino mass hasn't taken up some of that energy budget. And if it doesn't see electrons moving

with sort of near the maximum possible energy that this decay can make, it means that the neutrino has used up some of the energy budget that otherwise could have made the electron go faster. And that means the neutrino has some mass. So it's sort of like a way to measure the neutrino mass by seeing how much energy slurps out from this reaction. Okay, so let me see if I got this straight.

You started with an isotope of hydrogen called Tridium, which is two neutrons in the nucleus surrounded by an electron, and then you just let it hang out, and eventually it's going to decay into a hydrogen atom, right? Like one of those neutrons is just going to disappear, transform into something else. And you're saying this reaction shoots out an electron and an antineutrina. And the electron we can measure, it's mass and speed,

because it's an electron and so whatever is left because we so I guess you assume a certain amount of energy at the beginning we know very well how tritium decays and how it turns into helium and how much energy is available yeah and we know that energy has to go to the electron and the neutrino and so the difference between what you started with and how much you measure the electron is the energy that goes into the neutrino exactly but then how does that tell you the mass it could just be like something light moving fast or something heavy moving slow yeah so there's a spectrum of possibilities and what we're looking for is the maximum scenario. Like, are there any cases where the electron takes all of the energy available? There's like a certain energy budget for producing

this, subtract out the electron mass, and then we wonder like, are there scenarios where the electron takes all of the energy? If we see cases where the electron takes all of the energy, that means the neutrino hasn't taken any. It's sort of like a budget. You know, you have a budget for the whole thing. The electron mass gets taken out. Then we wonder, like, does the neutrino take a cut? If the neutrino takes a cut, that leaves a smaller budget for the electron, and you'll never see an electron having energy higher than that limit. The neutrino doesn't take a cut. It leaves more energy for the electron, and you'll see faster moving electrons. So you look at the tail of the distribution, like, what's the fastest electron you ever see?

And that'll tell you how much the neutrino has taken from the budget. I think I get it. So like you start with, let's say, 100 units of energy, and you measure how much energy electron that comes out has. and you look for like what's the maximum energy that the electron can take away from this and let's say it's like 99 out of a whole bunch of times that you do this 99 is the maximum

which means like the minimum amount of energy the neutrino can take is one which since it's the maximum for the electron it must mean that it's like it created a neutrino that wasn't moving at all maybe and so they're looking for those scenarios like when you make a motionless neutrino and the electron takes all of its energy

that reaction revealed the mass of the neutrino in the energy of the electron. It reveals, I guess, the mass of an electron neutrino. Yes, it's revealed the mass of a neutrino created with an electron. What does that really mean? Remember, the electron neutrino doesn't have a definite mass. So actually, what it's measuring is a combination of all the masses of the neutrinos. It's just like incoherent sum of the distinct neutrino mass values

weighted by how much of each one is in that electron neutrino. So remember, electron neutrinos don't have a definite mass. So you're measuring this weird average mass of a neutrino. If you're going sort of for the minimum amount of mass that the neutrino has, then it must be giving you the minimum mass for one of them, right? Yeah, it's a bit of a subtle point of quantum mechanics. The mass of that neutrino is not actually determined, right? It's not like it has a certain number and we don't know it.

What we know is it's an electron neutrino, which means we don't know what its mass is. And so overall, on average, what you'll be sensitive to is the average mass of those neutrinos. But you're right. What we're doing is looking for the most energetic electron, which means we'd be sensitive to the lower end of the neutrino masses of that electron neutrino. Which would maybe give you like the lightest of the three neutrino masses. Yeah.

And what we're looking to do is combine this with our measurements from neutrino oscillation, which tells us very precisely the separation between the neutrinos. And now we want to anchor the overall scale and slide it up or slide it down. But I guess even if you do, like you said, there's two possibilities for the other two, right? So like you might know the mass of one of the masses, but you wouldn't necessarily know the mass of the other two. But I guess you would narrow it down to two possibilities. Yeah, we'd narrow down to two possibilities. You're right.

This would still leave ambiguity for which hierarchy we have, like are the two close ones at the top or the two close ones at the bottom. So this experiment's been running for a couple of years and they have some. preliminary results, their measurement says that this mass they're measuring is less than 800 mill electron volts. And that's not much information because we already know from the Big Bang that it's less than 120. This is just sort of like their first result. They're going to keep running, collecting more data, and they hope they'll be able to measure

this thing more precisely. Wait, so we know that they can't be more than 120, but the first measurements say it's less than 800. Yeah. So this is not as sensitive as the Big Bang measurement so far. But it would be really weird if they found that the mass of the neutrino was 800 mill electron bolts because that's way too much. Yeah, exactly.

This sets an upper bound of less than 800. We already know they're less than 120. So it'd be pretty weird to measure it at like 600 or 500, you know. But these are very, very different measurements, right? The Big Bang versus like experiments we're doing here on Earth. So it's not always the case that they're going to agree. There's a lot of theoretical assumptions that go into both of them. But the good thing about this one is we can keep running and so we can keep getting more

and more precise measurements. And so they're hoping by 2024, 2025, they can get their sensitivity down to like 200 MEP, and then they can push even further. Because I guess it's all statistical, right? And so just the longer you run it, the more accurate you can say what the minimum is. And this experiment is also super fun because it involves this huge metal container. They shoot these electrons into this mammoth vacuum chamber to measure their energy super duper precisely the spectrometer.

It required a really specialized shop to build this thing. You should go online and Google a picture of this thing. It's like a big steel blimp, basically. And it was so big that it was really hard to transport from the factory where they built it like 300 kilometers to the experimental site. They actually had to put it on a boat and float it down river through the Mediterranean, out of the Atlantic, over to the Netherlands, and then up another river to the experiment. So it was only like 350 kilometers away, but it had to take like a 9,000 kilometer long detour because it was too big to like put on a flatbed truck and drive around.

Wow. Sounds like they should have thought about it before they built it. I mean they have built it on site. Yeah, exactly. But, you know, you take specialized techniques just to build this thing and then specialized techniques just to move this thing. There's some awesome videos of it making its last seven kilometer journey across land from the docks to the laboratory. how they like squeezed it through these old villages, you know, with like a centimeter to spare on each side. It's pretty awesome.

All right. Well, again, a neutrino is part of our standard model of the universe. And so, and it's also kind of like one of the last frontiers in terms of what we know about the standard model, right? Like once we found the Higgs boson and we know about all the matter particles, the neutrino is sort of one of the last big questions we have about it, right? And which means it sort of helps complete our understanding of matter particles in the universe. Yeah, you're absolutely right. It's the frontier particle physics. And the U.S. specifically has decided to double down on neutrinos. We didn't build the next greatest, best particle collider to compete with CERN. Instead, the U.S. is decided to build big neutrino experiments to measure these masses, to measure the neutrino interactions, to understand this weird sector of the universe in more detail. We think there's probably a lot more interesting hints there. And so learning more about the neutrino, what would that tell us about the universe? Well, understanding the neutrino mass will help us understand the Big Bang and like what was going on and the neutrino contributions there.

We also don't really know how the neutrino gets mass. Like, does it get mass from the Higgs boson the way other particles do? Or does a neutrino give itself mass? Like, it might be that there is no antineutrino, that the neutrino is its own antiparticle. This is a fun story about a physicist called myerana who thought about these myerana particles that might be their own antiparticles and give themselves mass in this weird way. So it might even teach us about what mass is for a particle. Cool. And that's very important because it would tell us why we have mass, right?

Yeah, absolutely. It would tell us more about what the meaning of mass is. They might also give us some clues about the nature of dark matter. We know that these three neutrinos are not the dark matter, but there might be a fourth kind of neutrino, a sterile neutrino that could be out there. And understanding the neutrino masses and how they mix and interact with each other might clear up some nagging questions about whether they, there are other flavors of neutrinos out there.

That would be massive. All right. Well, we hope you enjoyed that. Thanks for joining us. See you next time. Thanks for listening. And remember that Daniel and Jorge Explain the Universe is a production of IHeart Radio.

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