Daniel and Jorge Explain the UniverseDaniel and Jorge talk about the mysterious mass of the neutrino and how we can measure it.
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Ayd Enniel. Why do particle physicists 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 of particles, So I guess we can just decide for ourselves how to identify with them.
You are your particles, right, My particles are me? No, 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. I am poor Ham, a cartoonist and the author of Oliver's Great Big 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.
What 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 Wanita. 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 spartacles.
Huh, Yeah, they have secrets. They don't want the walsh 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 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?
Did you change your job title end to a particle.
Snooper particle investigator?
I'm a PI a PPI.
I guess I don't like having pp in my title.
Yeah, PP's not good on many things. Yeah, anyways, Welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio 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 quark. 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 quests 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 it's 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 adheres to a very impressive diet.
Photons have no mass, while top quarks, the heaviest known fundamental particle, have the mass of one hundred and seventy five 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 mass 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 Massy have them. Some of them are a little bit shy and don't necessarily want to reveal how much Massy has.
Some of the weirdest particles out there are new trinos, these ghostly little particles that are everywhere but very hard to spot. And in the case of neutrino's, 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 masks but they definitely aren't very massive.
Or how very little massive neutrina is? Is that what I mean?
How dainty is a neutrino?
I thought you meant, like how significant a nutrina is? Like how massive it 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, neutrino's are massive.
Dude, Well, I think what you're saying is that the mass of the patrino 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 figured that out, and it's still something that is very hard to pin down and not something we know.
It was a massive shock weigh 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 maths does senatrino 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 natrino.
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 to questions at Danielanjorge dot 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 seeing neutrinos pass through Earth. So there's some massive ones, but not so massive. How big maybe like fifty 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 podcasts.
I would think that a neutrino is really 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. Well, 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.
Like 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 could 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 think you're going to say it has the mass of a newt. But also you kind of have to know Italian to know that the io ending you know, know, means small, don't you not everyone speaks Italian.
That's true. I guess if it had been named by somebody who speak Spanish, would be like nutrito.
Yeah, exactly. Or in English, I guess, how would you call it neutrini?
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 misnaming them. All right, Well, let's dig into this mystery. What is the massively neutrino? But I guess 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 a part 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 quarks, upquarks and down quarks. Specifically, that means that everything that we know is made of two kinds of quarks, upquarks and down quarks, as well as electrons. So really just three particles explain all of the matter that we know, the stuff that the Earth is made out of, that the Sun is made out 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 upcork and the down cork sort of have each other. There's like a pair of quarks. 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 quartette 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.
Hmmm, I guess maybe the first question I would have is why not Why aren't neutrino's part of the matter that we're made out of, or why don't we have, you know, neutrino bits inside of us?
Yeah, it's a great question. You know, the universe has these bits and pieces, and 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. They're 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 two weak like you can't stick to nutrias to get 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. And if you wanted to like bind two neutrinos together into a more complex object, that 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 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 for put together things with the weak force.
Yeah, matter would have to be super duper cold, and they 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 isospin 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.
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 build 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 passed right through us, and we passed right through them because we don't have any interactions with them, the same way a neutrino can pass through like a light year thick wall of lead without even interacting. And so a whole planet of neutrinos would do the same thing. Like right now, there's one hundred billion neutrinos passing through every three 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 probability 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 the low probability of it happening.
Oh, that's interesting. So it's really called the weak force because of its weak probability, not because like you wouldn't feel it.
Yeah, exactly. The very strength of the forces are more about the probability of that interaction, which, if you integrate over all possibilities, don't end up playing a role in like its impact on the world, basically how massive is its impact.
So then may be like a better name for the weak force would have been improbable for us, the unlikely.
Force, the unlikely force that makes it sound like it's going to go on a hero's journey and in the end it become the most powerful force in the universe.
That's right, the underdog for exactly 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 flavorers 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 tao.
What do you mean associated? What does that mean they signed 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 it cater to an electron, but it also decays to a neutrino. And when you create an electron, you also create an electron neutrino. 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 electons 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 have You can 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 dekate 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.
It sounds like there are more electrons and there are anti electrons and electron latrinos.
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 pairings. So there's three different flavors of neutrino, the muon, the electron, and the town 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 we noticed 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 viirable 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 simple a 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.
Yeah.
Really, that's the only difference we know about it from these different kinds of neutrinos that the weak force associates them with different leptons, with electron, a muon, or a towel. 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 quarks and the electron, the masses tend to increase as you go to their copies, like the muon is heavier than the taw. The upcork has heavier versions, the charm and the top. The down cork has heavier versions the strange and 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, it 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.
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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. They're 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?
Ooh yeah, great question. We don't know. For a long time we wondered if neutrinos were the dark matter, like to 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 that 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 nutrino, 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 that 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 models. Some people say that's the new standard model. As you might expect, this 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 a heavy objects, or because you've weight 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 nineteen eighty seven that was very, very bright, and we saw a big flash of neutrinos coming from that super nica, 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 like propagate out and produce those photons. But they looked at when the neutrinos arrived and realize that they don't all arrive at the same time. We think they all leave the supernova 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 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 this 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 and 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 froth together in the Big Bang. So by studying the relics of the Big Bang, the leftover bits of it, we could actually get some clue as to like how many neutrinos there were, and we can even figure out something about the mass of those neutrinos.
Wait, I thought, neutrinos don't interact with regular mass, So how can like regular mass relics tell you about how many patrinos there were in the Big Bank.
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 Bank, and that they have mass.
The models of the Big Bank 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 the 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 Buryon 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 can 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 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 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. The number is less than the tenth of an electron volt.
Which I guess to give some context, how much mass does an electron have?
So an electron has like five hundred thousand electron bolts. It's half of an MeV, half of a mega electron bolt, and so five hundred thousand 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 bolts one billion electron bolts. So we know from the Big Bang that all neutrinos added together have to have less than a tenth of an electron bolt, less than one ten 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 five hundred thousand ev and a proton is about a billion ev.
Hmm.
Interesting, so then pretty light, very very light. Like how much is a quark?
It depends a lot on which quirk you're talking about. The lowest mass quarks have like a few MeV a few million electron bolts. The most massive ones, like the top quark, is like one hundred and seventy five billion ev. So these neutrinos have mass much much closer to zero than anything we've ever seen before. They're like shockingly low mass.
Okay, so we have a 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 point one ev. We also know there are three neutrinos, when 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, different from the electron, the muon, and the towel, and that they can change flavor like if you create an electron neutrino and shoot it through space and then wait like a light year, two light years and try to measure it. You might discover it's no longer an electron neutrino. It's now a Muon neutrino or a Towe neutrino. This is called neutrino oscillation.
M yeah, I think usually if you shoot anything, the space will change flavor. But I guess how do we know this? Like, 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 taw Right? That's what we mean by flavor.
Oh, is there a charge the weak force or is it just a weak charge?
The Week force does have a charge, member, it's two different charges. There's the isospin and the weak hypercharge, so both of those count as weak charges. But the neutrinos all have the same week charges. Where they have different is this flavor? It's 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 mule on, 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 a like M one, M two, M three are the three neutrino masses. When you create an electron neutrino, it's not like it's M one. 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 mule on neutrino, it's a different superposition of those masses. It's like having two different set of axes that are not alive. Is like a rotation between your set of axes.
I guess maybe the question I have is, so there's three types of neutrinos, electronion and town 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. There's two different ways to break them down. One is how do they interact with the 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 tow what kind of neutrino do you make? You make a town 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 towels 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 or 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 town neutrino will interact and make a tao. One of our neutrino experiments can see electrons, it can also see muons, and it can also see tows 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 created a taw?
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 towels are there. But again, that's just one way to see these things. Another way to see these things is how much mass do they have? And for most particles it's the 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 eleptons, 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 mew 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, like, 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 during space with like one of the masses, like the highest mass for neutrinos, then that could still be either an electron neutrino or town latrino or a mew neutrino.
Yes, that's exactly right. In mathematical terms, if you have a weak eigen state, 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, what say, you 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. 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 its 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 somethingly going to have the mass of a Tauel electron or a Newon electron.
Right, Yeah, and this calls in the question what I was saying at 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.
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Right, we're talking about the mass of a neutrino. How so 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, just more neutral, has 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 it nutrina has very little mass, and the different kinds of nutrinos can't have a lot of mass combined. We talked about how the neutrino kind of has an identity crisis, doesn't quite knows for real what kind of neutrino it is and how much it weighs. It's so 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 neutrinos so wishy washing.
Yeah, So the fact that neutrinos can change flavor was 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 M one, M two, M three. We can measure the separation between those three. We can't tell the overall mass, but we can tell how different they are, but 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 Catrin 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. And 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, Like we can only measure one neutrino at a time, Like 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 towels, 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 and 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, when you catch in neutrino, you sort of know what it was, or at least the detectors can only measure one kind of neutrino at a time.
Exactly, and 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 towels. That's how we interact with them.
And then how does that tell us their mass differences? Like if I catch in the trino, can I just infer its mass from 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 ten mili electron volts. A milli electric volt is one thousands of an electron volt. The other one is fifty milli electron volts. So some of them has to be less than one hundred and twenty milla electron vaults. And we know that the gaps between them are ten and fifty.
This feels like a fourth grade logic problem, like Sally, Paul and John moneying their pockets and it adds up to a dollar twenty. But the difference between Sally and Paul is fifty cents, and between Sally and John is paul sense how much does Sally have exactly?
And so we know that there's two possible solutions. We know that two of the neutrinos are close to each other, there's a small gap ten MeTV. We also know that the third one is further away, it's fifty MeTV 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, and 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 the top, or if you have the other hierarchy where the two close ones are 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, dat somehow tells you the difference in their masses because it, 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 trino, make this kind of measure the appearance of the other one. We've triangulated that whole matrix, and we know exactly 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 one hundred and twenty mili electron bolts. But now, this is really cool experiment in Germany called the Katrine 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, and now what is it? How does it work?
So this experiment is called the Carlsrua tritium neutrino experiment, which is a tortured way to make catrin as an acronym to say the least. But it starts from tritium and tretium decays two helium, which is like two protons a neutron, and then it also produces an electron and a neutrino.
And tretium 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 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, the comp 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. 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 the electron energy very very precisely. So that's why this experiment does. It measures those electrons really really precisely, and if it 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 it slurps out from this reaction.
Okay, so let me see if I got this straight. You start with an isotope of hydrogen called tritium, which is two neutrons in the nucleus around it by an electron, and then you just let it hang out and eventually it's going to decay into a hydrogen atom like right, Like one of those neutrons is just going to diss You're transformed to something else. And you're saying that this reaction shoots out an electron and an antineutrino, and the electron we can measure its mass and speed because it's an electron, and so whatever's left because 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 So it's sort of like a budget. You know, you have a budget for the whole thing. The electron mask 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. If 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 is taken from the budget.
M I think I get it. So, like you start with let's say one hundred 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 ninety nine out of a whole bunch of times, did you do this? Ninety nine 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 reveals 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 some of the distinct neutrino mass values weighted by how much of each one is in that electron neutrino. So remember, electronutrinos don't have a definite mass, so you're measuring this like weird average mass of a neutrino.
If you're going to sort of for the like the minimum amount of mass that the neutrino has, then must be giving you the minimum mass for one of them.
Right, ys 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, but we know, oh 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 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 massive 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 higher can we have? Like are the two close ones to the top? Were 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 eight hundred mili electron bolts. Now that's not much information because we already know from the Big Bang that it's less than one hundred and twenty. This is just sort of like their first result. They're going to keep running the 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 one hundred and twenty, but the first measurements say it's less than eight hundred.
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 mess in the tree that was eight hundred million electron bolts, because that's way too much.
Yeah, exactly, this sets an upper bound of less than eight hundred. We already know they're less than one twenty, so it'd be pretty weird to measure it at like six hundred or five hundred, 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 but this one is we keep running and so we can keep getting more and more precise measurements. And so they're hoping by twenty twenty four, twenty twenty five, they can get their sensitivity down to like two hundred MEB, 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 this 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 three hundred kilometers to the experimental site. They actually had to put it on a boat and float it down river through the Mediterranean, out through the Atlantic over to the Netherlands, and then up another river to the experiment. So it's only like three hundred and fifty kilometers away, but I have to take like a nine thousand 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, and 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. 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 deniers 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 US specifically has decided to double down on neutrinos. We didn't build the next greatest best particle collider to compete with CERN. Instead, the US has 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 nutrino 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 anti neutrino, that the neutrino is its own anti particle. This is a fun story about a physicist called Mayorana who thought about these Mayorana particles that might be their own anti particles and give themselves mass in this weird way. So it might even teach us about what mass is for a particle.
Hmmm. Cool, And that's 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, hysterile 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 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. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases. Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last sustainability to learn more.
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