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

• 0:08
  Speaker 1: Hey, Daniel, why do particle physicists obsessed so much about mass?
• 0:12
  Speaker 2: Well, mass is one of the basic properties of a particle.
• 0:15
  Speaker 2: It's like part of its identity.
• 0:17
  Speaker 1: Whoa is that? Healthy?
• 0:19
  Speaker 3: Though?
• 0:20
  Speaker 1: You think your mass should define who you are?
• 0:23
  Speaker 2: I don't think we have to worry too much about
• 0:25
  Speaker 2: like particle mental health.
• 0:26
  Speaker 1: Yeah, but shouldn't they be defined by their magnetism or
• 0:29
  Speaker 1: how colorful they are?
• 0:31
  Speaker 2: Well, we're all made of particles, so I guess we
• 0:32
  Speaker 2: can just decide for ourselves how to identify with them.
• 0:35
  Speaker 1: You are your particles, right, my particles are me? No,
• 0:39
  Speaker 1: I'm pretty sure it's the other way around.
• 0:41
  Speaker 2: It depends if you believe in strong or weak emergence.
• 0:43
  Speaker 1: That is a massive detail, right there. I am Poor Hamm,
• 1:02
  Speaker 1: a cartoonists and the author of Oliver's Great Think Universe.
• 1:05
  Speaker 2: Hi, I'm Daniel. I'm a particle physicist and a professor
• 1:08
  Speaker 2: at UC Irvine, and I really wish there was more
• 1:11
  Speaker 2: we could know about each particle.
• 1:13
  Speaker 1: What do you want to know?
• 1:14
  Speaker 2: I want to get to know them. You know, particles
• 1:16
  Speaker 2: are kind of like black holes. There's a few things
• 1:19
  Speaker 2: you can measure about it, the spin, the mass, the charge,
• 1:22
  Speaker 2: et cetera. But otherwise they're all totally identical. It's not
• 1:26
  Speaker 2: like this particle is Bob and that one is Sam
• 1:28
  Speaker 2: and this one is Juanita. You know, all electrons are
• 1:31
  Speaker 2: the same.
• 1:32
  Speaker 1: What if they don't want to be known? What if
• 1:35
  Speaker 1: they're private particles?
• 1:36
  Speaker 2: I see they're all spartacles.
• 1:38
  Speaker 4: Huh.
• 1:38
  Speaker 1: Yeah, they have secrets. They don't want the Walsh out
• 1:41
  Speaker 1: there on the internet.
• 1:43
  Speaker 2: Well, like I've said before, I don't think the universe
• 1:45
  Speaker 2: deserves any privacy. You know, we are curious creatures and
• 1:48
  Speaker 2: we're part of the universe, so knowing ourselves is sort
• 1:52
  Speaker 2: of like knowing the universe.
• 1:53
  Speaker 1: Are you saying. Physicists then are sort of like professional boxers.
• 1:58
  Speaker 2: I like to think of as more as the detectives
• 2:00
  Speaker 2: maybe private snoops. Yeah, we are snoops for sure, and
• 2:05
  Speaker 2: we're out to solve the biggest mystery in the universe,
• 2:08
  Speaker 2: which is like, how does this whole thing all work?
• 2:11
  Speaker 1: Did you change your jaw title then to a particle
• 2:13
  Speaker 1: snooper particle investigator? I'm a PI a PPI. I guess
• 2:18
  Speaker 1: I don't like having PP in my title. Yeah, PP's
• 2:20
  Speaker 1: not good on many things. But yeah, anyways, welcome to
• 2:24
  Speaker 1: our podcast Daniel and Jorge Explain the Universe, a production
• 2:28
  Speaker 1: of iHeartRadio in which we.
• 2:29
  Speaker 2: Try to lift the level of discourse as best we can,
• 2:33
  Speaker 2: elevating your mind to the deepest, biggest, most ethereal questions
• 2:37
  Speaker 2: in the universe. How does it all work, what's it
• 2:40
  Speaker 2: all made out of? What are the rules of the game,
• 2:43
  Speaker 2: and how is the game played in such a way
• 2:45
  Speaker 2: to give us this crazy, amazing, visceral conscious experience of
• 2:50
  Speaker 2: such a real world, which in the end is made
• 2:53
  Speaker 2: up of tiny, little, almost massless particles.
• 2:56
  Speaker 1: Yeah, because it is a pretty awesome experience to exist
• 2:59
  Speaker 1: in the universe and to look out there and appreciate
• 3:02
  Speaker 1: all the wonders and amazing things that are happening out
• 3:04
  Speaker 1: there in the universe that we can see and also
• 3:07
  Speaker 1: that we can't see.
• 3:08
  Speaker 2: And as we drill down into the nature of reality,
• 3:11
  Speaker 2: taking things apart into molecules and atoms and nuclei and
• 3:14
  Speaker 2: protons and neutrons, we like to give names to these things.
• 3:18
  Speaker 2: We say, oh, this kind of thing is an electron,
• 3:20
  Speaker 2: and that kind of thing is a neutrino, and this
• 3:23
  Speaker 2: kind of thing as a quark. It's just part of
• 3:25
  Speaker 2: who we are to want to attach labels to bits
• 3:28
  Speaker 2: and pieces of the universe.
• 3:30
  Speaker 1: Yeah, it's all part of humans quests to understand what's
• 3:33
  Speaker 1: going on out there, to get a handle on how
• 3:36
  Speaker 1: things work and how to predict what's going to happen
• 3:38
  Speaker 1: in the future.
• 3:39
  Speaker 2: And as we look at these tiny little particles, we
• 3:41
  Speaker 2: want to describe them in ways that make sense to us.
• 3:44
  Speaker 2: You know, how much spin does it have? What can
• 3:47
  Speaker 2: it do? And maybe at the most fundamental level, part
• 3:50
  Speaker 2: of the identity of a particle is how much mass
• 3:54
  Speaker 2: does it have?
• 3:55
  Speaker 1: Yeah, some particles have a little bit of mass, some
• 3:58
  Speaker 1: particles have a lot of and some particles have no mass. Right,
• 4:02
  Speaker 1: some particles adheres to a very impressive diet.
• 4:06
  Speaker 2: Photons have no mass, while top quarks, the heaviest known
• 4:09
  Speaker 2: fundamental particle, have the mass of like one hundred and
• 4:12
  Speaker 2: seventy five protons. So there really is an extraordinary range,
• 4:17
  Speaker 2: which is something that we don't understand at all. But
• 4:20
  Speaker 2: mass is also part of how we tell which particle
• 4:23
  Speaker 2: is which. I think about an electron and a muon.
• 4:27
  Speaker 2: What are the differences there between the two? They're almost
• 4:30
  Speaker 2: identical particles, except that muons have more mass than electrons do.
• 4:35
  Speaker 2: And when we produce particles in our experiments. That's how
• 4:38
  Speaker 2: we tell what's what. We measure the masses of these
• 4:41
  Speaker 2: particles and we say, oh, this one's got to be
• 4:43
  Speaker 2: an electron because look at its mass. So it's not
• 4:47
  Speaker 2: just that we take the particles, we assign mass labels
• 4:49
  Speaker 2: to them. We use the mass to tell us who
• 4:51
  Speaker 2: is who.
• 4:52
  Speaker 1: Yeah, and there are lots of particles out there. Some
• 4:54
  Speaker 1: of them are not shy at all about how much
• 4:57
  Speaker 1: mass they have. Some of them are a little bit
• 4:59
  Speaker 1: shy and don't necessarily want to reveal how much mass
• 5:02
  Speaker 1: he has.
• 5:03
  Speaker 2: Some of the weirdest particles out there are neutrinos, these
• 5:06
  Speaker 2: ghostly little particles that are everywhere but very hard to spot.
• 5:11
  Speaker 2: And in the case of neutrinos, their identity is something
• 5:14
  Speaker 2: of a more complex story. They have sort of two
• 5:17
  Speaker 2: different kinds of clothing they can wear, who they talk to,
• 5:20
  Speaker 2: and how they move through the universe. And because their
• 5:24
  Speaker 2: mass is so weird and so hard to nail down,
• 5:26
  Speaker 2: it's not something we actually know very well.
• 5:29
  Speaker 1: It's all a big mystery. And so today on the
• 5:31
  Speaker 1: podcast we'll be asking the question how massive is a neutrino?
• 5:40
  Speaker 2: Or maybe we should have said, how massive isn't a neutrino?
• 5:44
  Speaker 1: Wait? What why shouldn't we have not said.
• 5:46
  Speaker 2: That because neutrinos have some masks, but they definitely aren't very.
• 5:52
  Speaker 1: Massive, or how very little massive nutrina is? Is that
• 5:57
  Speaker 1: what I mean?
• 5:58
  Speaker 2: How dainty is a neutrino?
• 6:00
  Speaker 5: Yeah?
• 6:00
  Speaker 1: I thought you meant, like, how significant in nutrino is?
• 6:02
  Speaker 1: Like how massive it is it in a universal scale
• 6:06
  Speaker 1: of awesomeness?
• 6:08
  Speaker 2: Yeah. It actually turns out neutrinos are quite important and
• 6:11
  Speaker 2: play a big role in the physics of the universe
• 6:14
  Speaker 2: despite being almost invisible. So from a consequential point of view, right,
• 6:17
  Speaker 2: neutrinos are massive. Dude.
• 6:20
  Speaker 1: Well, I think what you're saying is that the mass
• 6:22
  Speaker 1: of the neutrino is not known. We don't know how
• 6:25
  Speaker 1: much mass it has.
• 6:25
  Speaker 2: We do not know how much mass the neutrino has.
• 6:28
  Speaker 2: We've only known that it has mass for a couple
• 6:30
  Speaker 2: of decades, which was a big shocker and sent quakes
• 6:33
  Speaker 2: through the theoretical community when we figured that out, and
• 6:37
  Speaker 2: it's still something that is very hard to pin down
• 6:40
  Speaker 2: and not something we know.
• 6:42
  Speaker 1: It was a massive shock weighed heavily on the minds
• 6:45
  Speaker 1: of physicists for a long time.
• 6:47
  Speaker 2: They didn't take it lightly, that's for sure.
• 6:49
  Speaker 1: So yeah, this is an interesting question. How much maths
• 6:51
  Speaker 1: does and theatrino have. Apparently it's kind of tricky to
• 6:55
  Speaker 1: find out. So as usually, we were wondering how many
• 6:57
  Speaker 1: people out there had thought about this question or have
• 6:59
  Speaker 1: an idea about the mass of a nutrino.
• 7:01
  Speaker 2: So thanks very much to everybody who answers these questions
• 7:04
  Speaker 2: for this fun segment. If you'd like to hear your
• 7:06
  Speaker 2: voice speculating for everybody else's entertainment and education, please write
• 7:11
  Speaker 2: to us two questions at Danielandjorge dot com.
• 7:14
  Speaker 1: So think about it for a second. How massive do
• 7:17
  Speaker 1: you think a neutrino is? Here's what people have to say.
• 7:20
  Speaker 3: Not sure if the vibe was that there's more than
• 7:22
  Speaker 3: one type of neutrino. So maybe there's like some with
• 7:25
  Speaker 3: more mass. But I thought that neutrinos were like massless
• 7:29
  Speaker 3: or like had negligible mass and so like they travel
• 7:31
  Speaker 3: at the speed of light.
• 7:32
  Speaker 6: I think there's different types of neutrinos that are different sizes.
• 7:37
  Speaker 6: You talked about one of NASA finding another universe by
• 7:40
  Speaker 6: seeing neutrinos pass through Earth. So there's some massive ones
• 7:45
  Speaker 6: but not so massive. How big maybe like fifty protons
• 7:48
  Speaker 6: big or something, if that even makes sense, And maybe
• 7:52
  Speaker 6: neutrinos are also dark matters what you also said in
• 7:55
  Speaker 6: one of your earlier podcasts.
• 7:56
  Speaker 2: I would think that a neutrino is real light because't
• 8:00
  Speaker 2: interact with other particles, but it may interact with the
• 8:04
  Speaker 2: Higgs field. So I actually have no idea.
• 8:07
  Speaker 7: Well neutrino so eno means very small in Italian or smaller,
• 8:12
  Speaker 7: so I would assume that the mass of a neutrino
• 8:17
  Speaker 7: is much much much smaller than that of a neutron,
• 8:21
  Speaker 7: and I'm tempted to say that.
• 8:25
  Speaker 5: Neutrinos are massless.
• 8:27
  Speaker 4: Maybe mass is just I think the amount of energy
• 8:32
  Speaker 4: that's required to move something, so gravitational mass is just
• 8:38
  Speaker 4: a unique form of inertial mass, wherein it's the gravity
• 8:41
  Speaker 4: which is pulling you and that changes according to where
• 8:44
  Speaker 4: you are, whereas inertial mass is just independent of that.
• 8:49
  Speaker 4: I guess I.
• 8:50
  Speaker 5: Don't know how massive a neutrino is. I'm pretty sure
• 8:54
  Speaker 5: that I've heard that they have mass, and I think
• 8:56
  Speaker 5: it's extremely like neutrinos are very low mass, and it
• 9:02
  Speaker 5: would be great if they had the lowest amount of
• 9:05
  Speaker 5: mass allowed by quantum mechanics. That would be pretty.
• 9:09
  Speaker 1: Neat, right. I think a lot of people seem to
• 9:12
  Speaker 1: know it had very little mass.
• 9:14
  Speaker 2: I really like the linguistic analysis, reverse engineering the name
• 9:17
  Speaker 2: particle to infer what its mass has to be.
• 9:20
  Speaker 1: What do you mean it has neutral mass?
• 9:23
  Speaker 2: Well, you know, neutrino means little neutral particle. That was
• 9:27
  Speaker 2: the name given to it before we even really knew
• 9:29
  Speaker 2: what it was, because that's all we knew about it,
• 9:32
  Speaker 2: that it could be very massive and that it was
• 9:34
  Speaker 2: electrically neutral. So in that sense, you might even be
• 9:37
  Speaker 2: tempted to say that it's a well named particle.
• 9:39
  Speaker 1: They were going to say it has the mass of
• 9:41
  Speaker 1: a newt. But also you kind of have to know
• 9:44
  Speaker 1: Italian to know that the io ending, you know, know,
• 9:47
  Speaker 1: means small, don't you not everyone speaks Italian.
• 9:50
  Speaker 2: That's true. I guess if it had been named by
• 9:52
  Speaker 2: somebody who speaks Spanish, would be like nutrito.
• 9:54
  Speaker 1: Yeah exactly. Or in English, I guess, how would you
• 9:58
  Speaker 1: call it new trini neutral?
• 10:00
  Speaker 2: Do we have affectionate endings in English?
• 10:02
  Speaker 1: Tiny neutron? There you go, like tiny tim?
• 10:05
  Speaker 2: Or maybe we'd give it an ironic nickname you know,
• 10:08
  Speaker 2: like big.
• 10:08
  Speaker 1: Neutron, Yeah, neutronizer or something, or how about just neutron.
• 10:13
  Speaker 1: I mean that sounds pretty massive now in comparison to neutrino.
• 10:18
  Speaker 2: Neutron had already been discovered. Is the name of another particle?
• 10:21
  Speaker 1: Oh well, there you go, that one's misnaming them. All right, Well,
• 10:26
  Speaker 1: let's dig into this mystery. What is the massive a neutrino?
• 10:29
  Speaker 1: But I guess far as Daniel talk to us about
• 10:31
  Speaker 1: what a neutrino actually is.
• 10:33
  Speaker 2: Neutrino is a really fun particle because it's so weird
• 10:36
  Speaker 2: and yet so fundamental and so important and at the
• 10:40
  Speaker 2: same time not a part of the matter that's around us.
• 10:44
  Speaker 2: You know, if you take a part the stuff that
• 10:46
  Speaker 2: you're made out of, and that I'm made out of,
• 10:47
  Speaker 2: and that everything you've ever eaten is made out of,
• 10:50
  Speaker 2: you discover that it's made of atoms, and those atoms
• 10:52
  Speaker 2: are made of protons and neutrons and electrons. But the
• 10:56
  Speaker 2: protons and neutrons can be made out of quarks, up
• 10:58
  Speaker 2: quarks and down quarks. Specific that means that everything that
• 11:01
  Speaker 2: we know is made of two kinds of quarks, up
• 11:04
  Speaker 2: quarks and down corks, as well as electrons. So really
• 11:07
  Speaker 2: just three particles explain all of the matter that we know,
• 11:11
  Speaker 2: the stuff that the Earth is made out of, that
• 11:13
  Speaker 2: the Sun is made out of, that the visible matter
• 11:15
  Speaker 2: in the galaxy is made out of. Of course, put
• 11:17
  Speaker 2: dark matter aside because we don't know what that is
• 11:20
  Speaker 2: made out of. So those three particles sort of underlie
• 11:22
  Speaker 2: everything that exists. But there's another particle that's in the
• 11:26
  Speaker 2: same category as like one of the basic templates of
• 11:29
  Speaker 2: possible matter, and that's the neutrino. Because you notice that
• 11:33
  Speaker 2: the upcork and the down cork sort of have each other.
• 11:35
  Speaker 2: There's like a pair of quarks. You might wonder like, well,
• 11:38
  Speaker 2: who's the electrons partner, And the electron does have a partner,
• 11:42
  Speaker 2: it's the new trino. So it sort of like completes
• 11:44
  Speaker 2: the quartette of the fundamental bits of matter, even though
• 11:48
  Speaker 2: the neutrino doesn't appear in the atom and isn't used
• 11:51
  Speaker 2: to make up your lunch or your dinner or anything
• 11:54
  Speaker 2: you've ever eaten.
• 11:55
  Speaker 1: Hmmm, I guess maybe the first question I would have
• 11:57
  Speaker 1: is why not why they aren't neutrino's part of the
• 12:01
  Speaker 1: matter that we're made at it, or why don't we have,
• 12:03
  Speaker 1: you know, neutrino bits inside of us.
• 12:05
  Speaker 2: Yeah, it's a great question. You know, the universe has
• 12:08
  Speaker 2: these bits and pieces, and they have rules for how
• 12:10
  Speaker 2: they can come together, and then you get complex structures
• 12:13
  Speaker 2: emerging from that. You know, you have quarks bind together
• 12:16
  Speaker 2: to make protons and neutrons, which then bind with the
• 12:19
  Speaker 2: electron to make atoms, to make all sorts of other
• 12:21
  Speaker 2: complex stuff. I scream and stars and black holes and
• 12:24
  Speaker 2: all that stuff. And really it's the interaction. They're the
• 12:27
  Speaker 2: binding that's crucial. While quarks and electrons all have electric charges,
• 12:32
  Speaker 2: and quarks have strong charges, so they can use the
• 12:35
  Speaker 2: more powerful forces to build complex matter. Neutrinos are different
• 12:40
  Speaker 2: from the other three kinds of basic fundamental bits of
• 12:43
  Speaker 2: stuff in that they only feel the weak force, so
• 12:46
  Speaker 2: they have no electric charge, they're neutral, and they also
• 12:49
  Speaker 2: have no color, so they don't feel the strong nuclear force,
• 12:52
  Speaker 2: which means they're only left to interact via gravity, which
• 12:55
  Speaker 2: is basically negligible for a particle and the weak force.
• 12:59
  Speaker 2: So in order to bi old something out of neutrinos,
• 13:01
  Speaker 2: you'd have to have them bound together by the weak force,
• 13:04
  Speaker 2: but the weak force is just too weak to do that.
• 13:07
  Speaker 1: Interesting, What do you mean too weak? Like you can't
• 13:10
  Speaker 1: stick to nutriinas together with the weak force.
• 13:13
  Speaker 2: The weak force can be used to interact, but it's
• 13:15
  Speaker 2: really very, very shockingly weak. That's why, for example, if
• 13:18
  Speaker 2: you shoot a photon at the wall, it'll splat against
• 13:22
  Speaker 2: the wall and interact with all the electrons inside of it.
• 13:25
  Speaker 2: But if you shoot a neutrino against the same wall,
• 13:27
  Speaker 2: it will fly right through. It's not like it's finding
• 13:30
  Speaker 2: holes in the wall. It's not like the wall is
• 13:32
  Speaker 2: a screen or a mesh that it's slipping through. It
• 13:35
  Speaker 2: ignores all those particles because it doesn't interact with them.
• 13:38
  Speaker 2: So it's really all about the strength of the interactions.
• 13:41
  Speaker 2: And if you wanted to like bind two neutrinos together
• 13:44
  Speaker 2: into a more complex object, that have to be in
• 13:47
  Speaker 2: a bound state in order to be trapped together by
• 13:50
  Speaker 2: an interaction that's so weak, they would have to be
• 13:52
  Speaker 2: almost motionless. It wouldn't take very much energy to break
• 13:54
  Speaker 2: it apart, So you'd have to have very cold bits
• 13:57
  Speaker 2: fall together to make a bound state and then be
• 14:00
  Speaker 2: very easy to break it apart. So it's basically not
• 14:02
  Speaker 2: possible to build more complex structure using the weak force.
• 14:07
  Speaker 1: I think you're saying that you can, but maybe matter
• 14:09
  Speaker 1: would have to be super duper cold to put together
• 14:12
  Speaker 1: things with the weak force.
• 14:13
  Speaker 2: Yeah, matter would have to be super duper cold, and
• 14:15
  Speaker 2: they would have to not be other stronger forces disrupting it.
• 14:18
  Speaker 1: Right, I don't know how does the weak force work.
• 14:22
  Speaker 1: Does it repel or attract or both? Does it have
• 14:24
  Speaker 1: positive negative charges to it?
• 14:26
  Speaker 2: So the weak force is quite complicated. We talked once
• 14:28
  Speaker 2: about whether the weak force can attract or repel. It
• 14:30
  Speaker 2: actually can do both. There are two different charges for it.
• 14:34
  Speaker 2: They're called isospin and weak hypercharge, and so it's a
• 14:37
  Speaker 2: complex combination of all these different numbers that tells you
• 14:40
  Speaker 2: what the weak force is going to do. But in short,
• 14:43
  Speaker 2: it can attract and it can repel. So it's very
• 14:45
  Speaker 2: similar to electromagnetism. Actually, electromagnetism and the weak force together
• 14:49
  Speaker 2: are part of a larger idea called electro week And
• 14:53
  Speaker 2: the reason that one of them is more powerful than
• 14:55
  Speaker 2: the other has to do with the Higgs boson, which
• 14:57
  Speaker 2: breaks the symmetry between the two forces, leaving one of
• 15:00
  Speaker 2: them very powerful and one of them very very weak.
• 15:04
  Speaker 1: So, like if I took two neutrinos, and I cooled
• 15:07
  Speaker 1: them down out there in space and I stuck them together.
• 15:10
  Speaker 1: Would they stick together due to the weak force?
• 15:12
  Speaker 2: You could put two neutrinos into a bound state if
• 15:14
  Speaker 2: they were very, very cold, so they didn't have enough
• 15:16
  Speaker 2: kinetic energy to escape these bonds and there was nothing
• 15:20
  Speaker 2: else bothering them. Yes, you could. And you could even
• 15:23
  Speaker 2: add more.
• 15:23
  Speaker 1: Yeah, you could add more. Maybe can you build a
• 15:25
  Speaker 1: whole planet out of neutrinos?
• 15:28
  Speaker 2: You could build larger, more complex structures, but it would
• 15:31
  Speaker 2: be very fragile, and it certainly wouldn't look like a planet,
• 15:34
  Speaker 2: and the whole thing could probably pass through the Earth
• 15:36
  Speaker 2: without even noticing, because neutrinos, again don't interact with normal matter.
• 15:40
  Speaker 2: So even if you build more complex structures out of neutrinos,
• 15:43
  Speaker 2: it exists sort of in parallel to us, the same
• 15:46
  Speaker 2: way that like dark matter does. Dark matter is here,
• 15:48
  Speaker 2: dark matters everywhere. Dark matter might make complex structures that
• 15:51
  Speaker 2: we can't see, but they passed right through us, and
• 15:54
  Speaker 2: we passed right through them because we don't have any
• 15:56
  Speaker 2: interactions with them, the same way a neutrino can pass
• 16:00
  Speaker 2: through like a light year thick wall of lead without
• 16:03
  Speaker 2: even interacting. And so a whole planet of neutrinos would
• 16:07
  Speaker 2: do the same thing. Like, right now, there's one hundred
• 16:09
  Speaker 2: billion neutrinos passing through every square centimeter of the surface
• 16:14
  Speaker 2: of the Earth every second, and yet we don't feel them.
• 16:17
  Speaker 2: So somebody could throw a planet of neutrinos at us
• 16:19
  Speaker 2: and we wouldn't even notice.
• 16:21
  Speaker 1: Would that neutrino planet break apart when it goes through
• 16:23
  Speaker 1: us or would it stay together?
• 16:25
  Speaker 2: A tiny fraction of those neutrinos would interact with us,
• 16:29
  Speaker 2: so those a little bonds would break up, but most
• 16:30
  Speaker 2: of it would totally ignore us. Neutrinos have a very
• 16:32
  Speaker 2: very tiny probability of interacting with electrons or with quarks.
• 16:38
  Speaker 1: So and then when you say weak, do you mean
• 16:40
  Speaker 1: like low probability or just that the force is weak?
• 16:44
  Speaker 2: We mean low probability, not small momentum exchange, but low probability.
• 16:48
  Speaker 2: Like you shoot a neutrino at another particle, it's very
• 16:51
  Speaker 2: unlikely to interact. If it does interact, it can impart
• 16:54
  Speaker 2: significant momentum, it's just a low probability of it happening.
• 16:58
  Speaker 1: Oh, that's interesting. So it's really called the weak force
• 17:01
  Speaker 1: because of its weak probability, not because like you wouldn't
• 17:04
  Speaker 1: feel it.
• 17:05
  Speaker 2: Yeah, exactly. The very strength of the forces are more
• 17:07
  Speaker 2: about the probability of that interaction, which, if you integrate
• 17:10
  Speaker 2: over all possibilities, does end up playing a role in
• 17:13
  Speaker 2: like its impact on the world, basically how massive is
• 17:16
  Speaker 2: its impact.
• 17:17
  Speaker 1: So then maybe like a better name for the weak
• 17:19
  Speaker 1: force would have been improbable for us, the unlikely.
• 17:23
  Speaker 2: Force, the unlikely force that makes it sound like it's
• 17:26
  Speaker 2: going to go on a hero's journey and in the
• 17:29
  Speaker 2: end it become the most powerful force in the universe.
• 17:31
  Speaker 1: That's right, the underdog for exactly What else do we
• 17:35
  Speaker 1: know about neutrinos.
• 17:36
  Speaker 2: We know that there are three kinds of neutrinos, the
• 17:39
  Speaker 2: way that there's like three different kinds of electron. There's
• 17:42
  Speaker 2: the more massive version that's the muon and the even
• 17:44
  Speaker 2: more massive version that's the tau, So there's three different
• 17:47
  Speaker 2: flavorers of electron. There's also three different flavors of neutrino.
• 17:53
  Speaker 2: So there's a neutrino associated with the electron, the electro
• 17:56
  Speaker 2: and neutrino, and one associated with the muon and one
• 17:58
  Speaker 2: associated with the taw.
• 18:00
  Speaker 1: What do you mean associated? What does that mean? They
• 18:02
  Speaker 1: signed a contract?
• 18:03
  Speaker 2: Well, these guys interact via the weak force, and so
• 18:05
  Speaker 2: for example, if you want to make an electron, you
• 18:07
  Speaker 2: can make it from a w boson. A W boson
• 18:10
  Speaker 2: can decate to an electron, but also decays to a neutrino.
• 18:13
  Speaker 2: And when you create an electron, you also create an
• 18:15
  Speaker 2: electron neutrino. You create a muon, then you also create
• 18:18
  Speaker 2: a muon neutrino. So when we say associated with, we
• 18:21
  Speaker 2: mean like grouped together with by the weak force. It
• 18:24
  Speaker 2: groups these guys together. Remember that we count the number
• 18:28
  Speaker 2: of eleptons in the universe and that's conserved. So for example,
• 18:31
  Speaker 2: you can't just like make more electrons. If you make
• 18:34
  Speaker 2: more electrons, you also have to make more anti electrons
• 18:37
  Speaker 2: to balance out the number of electrons in the universe.
• 18:40
  Speaker 2: But electron neutrinos fall into that category. So you can
• 18:44
  Speaker 2: make an electron and then you make an anti electron neutrino,
• 18:47
  Speaker 2: and the universe's books are all balanced.
• 18:49
  Speaker 1: Like an electron and a neutrino are sort of like twins,
• 18:52
  Speaker 1: like you can have you can make one without the other.
• 18:54
  Speaker 2: You can make an electron either with an anti electron
• 18:57
  Speaker 2: neutrino or with an anti electron, So like a boson
• 19:00
  Speaker 2: will de kate to an electron and an anti electron
• 19:03
  Speaker 2: neutrino together, or a z boson will dekate to an
• 19:07
  Speaker 2: electron and an anti electron. You can't just make an
• 19:10
  Speaker 2: electron by itself.
• 19:11
  Speaker 1: So there's it sounds like there are more electrons and
• 19:15
  Speaker 1: there are anti electrons and electron latrinos.
• 19:18
  Speaker 2: There's definitely more matter than antimatter. So yeah, they're more
• 19:21
  Speaker 2: electrons than anti electrons. But when it comes to the neutrinos,
• 19:25
  Speaker 2: like we have these pairings. So there's three different flavors
• 19:28
  Speaker 2: of neutrino, the muon, the electron, and the town neutrino.
• 19:31
  Speaker 2: Each one is connected to one of these leptons because
• 19:34
  Speaker 2: the weak force likes to make those together.
• 19:37
  Speaker 1: That's just something we've observed, right, Like we noticed that
• 19:40
  Speaker 1: the weak force, when it does things in the universe,
• 19:43
  Speaker 1: it creates these things in pairs. Like is there anything
• 19:46
  Speaker 1: else we know about them that associates them, Like do
• 19:48
  Speaker 1: they have the same quantum varible about it?
• 19:51
  Speaker 2: I like the way you say, that's just what we observed,
• 19:53
  Speaker 2: Like that's basically science, right. We observe the universe and
• 19:56
  Speaker 2: then we describe it, and then we try to boil
• 19:58
  Speaker 2: that description down to a couple a set of rules
• 20:00
  Speaker 2: as possible, and think about what that means. So, yeah,
• 20:04
  Speaker 2: that's just what we've observed. We've never seen this be violated.
• 20:07
  Speaker 2: So there's an asterisk there. We'll talk about neutrino oscillation
• 20:10
  Speaker 2: in a minute, but yeah, really that's the only difference
• 20:13
  Speaker 2: we know about it from these different kinds of neutrinos
• 20:15
  Speaker 2: that the weak force associates them with different leptons, with electron,
• 20:19
  Speaker 2: a muon, or a taw. The other question, of course,
• 20:21
  Speaker 2: is about their masses, like what are the masses of
• 20:24
  Speaker 2: these particles. We know that for normal matter, all the
• 20:27
  Speaker 2: quarks and the electron, the masses tend to increase as
• 20:30
  Speaker 2: you go to their copies, like the muon is heavier
• 20:33
  Speaker 2: than the taw. The upcork has heavier versions the charm
• 20:36
  Speaker 2: and the top. The down cork has heavier versions the
• 20:39
  Speaker 2: strange and the bottom. When it comes to the neutrinos,
• 20:41
  Speaker 2: we don't know so much about what their masses are
• 20:44
  Speaker 2: and how that's organized.
• 20:46
  Speaker 1: All right, it sounds like a good cue for us
• 20:48
  Speaker 1: to dig deeper into the mass of the neutrino and
• 20:50
  Speaker 1: talk about how we know it has mass and how
• 20:52
  Speaker 1: we measure that mass. So let's get into that, but
• 20:54
  Speaker 1: first let's take a quick break. All right, we're talking
• 21:09
  Speaker 1: about the mass of a neutrino specifically, what is its mass?
• 21:13
  Speaker 1: Is it a lot, is it a little? And why
• 21:15
  Speaker 1: is it the way it is? So we talked about
• 21:18
  Speaker 1: what a neutrino is. They are ghostly particles that fly
• 21:21
  Speaker 1: around the universe without really interacting with the rest of
• 21:25
  Speaker 1: the matter in the universe. Daniel a quick question. Do
• 21:27
  Speaker 1: they interact with dark matter?
• 21:29
  Speaker 2: Ooh, yeah, great question. We don't know. For a long
• 21:33
  Speaker 2: time we wondered if neutrinos were the dark matter, like
• 21:36
  Speaker 2: they kind of fit the bill because we can't really
• 21:38
  Speaker 2: see them and there's maybe a lot of them out there.
• 21:41
  Speaker 2: Turns out neutrinos can't be the dark matter because we
• 21:44
  Speaker 2: know the dark matter moves slowly, it's cold. We know
• 21:47
  Speaker 2: that from like how it's influenced the structure of the universe.
• 21:50
  Speaker 2: If dark matter moved faster, things would be less lumpy,
• 21:53
  Speaker 2: and neutrinos move really really fast, So neutrinos are too
• 21:56
  Speaker 2: hot to be the dark matter. Do neutrinos interact with
• 21:59
  Speaker 2: dark man? We don't think so, because we don't think
• 22:01
  Speaker 2: that dark matter feels the weak force or the improbable force,
• 22:05
  Speaker 2: as you'd like to call it, because if it did,
• 22:07
  Speaker 2: we would have seen it bump into some of our
• 22:08
  Speaker 2: big underground detectors. So because dark matter probably doesn't feel
• 22:12
  Speaker 2: the weak force, it probably doesn't interact with neutrinos.
• 22:15
  Speaker 1: Yeah, I feel the same way. I think I'm too
• 22:17
  Speaker 1: hot to be dark matter.
• 22:19
  Speaker 2: I'm always telling people that.
• 22:22
  Speaker 1: And ironically, I'm also pretty cool.
• 22:24
  Speaker 2: You're a paradox of physics.
• 22:26
  Speaker 1: Yes, I'm an enigma wrapped in a cartoonist. But talking
• 22:29
  Speaker 1: about the mass of a nutrino, I guess the first
• 22:31
  Speaker 1: question is like, first of all, how do you know
• 22:33
  Speaker 1: it has mass? Like, there are particles out there without mass, Right,
• 22:36
  Speaker 1: how do we know that neutrino has mass?
• 22:38
  Speaker 2: Yeah, you're right. There are particles out there that have
• 22:40
  Speaker 2: no mass, like the photon and the gluon. So it's
• 22:44
  Speaker 2: not impossible for the neutrino to have no mass, and
• 22:47
  Speaker 2: for a long time we assumed that it didn't. There's
• 22:50
  Speaker 2: even an argument about what we mean by the standard
• 22:53
  Speaker 2: model of particle physics, sort of our description of our
• 22:56
  Speaker 2: best understanding. Some people say that the standard model of
• 22:59
  Speaker 2: particle physics requires neutrinos to have no mass, though there
• 23:03
  Speaker 2: are extensions of it that allow them to have mass.
• 23:06
  Speaker 2: Some people say that's beyond the standard models. Some people
• 23:08
  Speaker 2: say that's the new standard model. As you might expect,
• 23:11
  Speaker 2: this big argument about how we name it. But for
• 23:13
  Speaker 2: a long time we assumed neutrinos had no mass. But
• 23:16
  Speaker 2: now we do know that they have mass, and we
• 23:18
  Speaker 2: know that in two different ways. We know that they
• 23:21
  Speaker 2: have mass even without knowing how much mass they have.
• 23:24
  Speaker 1: Interesting do you know because they I don't know, pass
• 23:27
  Speaker 1: around heavy objects, or because you've weight them.
• 23:30
  Speaker 2: So we know in a few different ways. Actually, one
• 23:31
  Speaker 2: of the first clues was looking at a supernova. There
• 23:35
  Speaker 2: was a supernova in nineteen eighty seven that was very,
• 23:38
  Speaker 2: very bright, and we saw a big flash of neutrinos
• 23:41
  Speaker 2: coming from that supernova. And the neutrinos actually arrived a
• 23:44
  Speaker 2: little bit before the photons because neutrinos come from the
• 23:48
  Speaker 2: center of the supernova and they aren't blocked by the
• 23:50
  Speaker 2: rest of the matter in the supernova, whereas the photons
• 23:53
  Speaker 2: come from the surface and it takes a while for
• 23:55
  Speaker 2: the energy to like propagate out and produce those photons.
• 23:58
  Speaker 2: But they looked at when the new trinos arrived and
• 24:01
  Speaker 2: realize that they don't all arrive at the same time.
• 24:04
  Speaker 2: We think they all leave the supernova basically the same moment,
• 24:08
  Speaker 2: but they don't all arrive at the same time. The
• 24:11
  Speaker 2: higher energy neutrinos arrive earlier than the lower energy once.
• 24:15
  Speaker 2: The higher the energy, the faster they go. That makes sense,
• 24:19
  Speaker 2: but it's actually a property you can only have if
• 24:22
  Speaker 2: you have mass. Massless particles like photons, all travel at
• 24:26
  Speaker 2: the same speed regardless of their energy. All photons travel
• 24:30
  Speaker 2: at the same speed because they're massless. Neutrinos have a
• 24:34
  Speaker 2: spread in their velocity, which means they have a mass.
• 24:38
  Speaker 1: But I guess it tells you that they're not as
• 24:40
  Speaker 1: fast as photons, which means they have mass. Right, because
• 24:44
  Speaker 1: anything that doesn't have mass would move at the speed
• 24:46
  Speaker 1: of light exactly.
• 24:47
  Speaker 2: Things that don't have mass always have to move at
• 24:49
  Speaker 2: the speed of light. There's no option there, right, Massless
• 24:52
  Speaker 2: objects always move at the speed of light.
• 24:55
  Speaker 1: Okay, so neutrinos don't move at this speed of light,
• 24:57
  Speaker 1: which means they have some mass. But then is that
• 24:59
  Speaker 1: the main way that we know they have mass?
• 25:01
  Speaker 2: So there's another really fascinating clue which comes from the
• 25:04
  Speaker 2: Big Bang. We think that a lot of neutrinos were
• 25:07
  Speaker 2: made in the Big Bang, Like all this energy was
• 25:09
  Speaker 2: hot and dense, and the quantum fields were frothing, and
• 25:12
  Speaker 2: as they cooled down, they sort of dribbled out into
• 25:14
  Speaker 2: all the different fields that are out there. So the
• 25:16
  Speaker 2: Big Bang made a lot of quarks and made a
• 25:18
  Speaker 2: lot of electrons, and made a lot of neutrinos as well.
• 25:21
  Speaker 2: And as those particles all mixed together, the amount of
• 25:24
  Speaker 2: photons and neutrinos and quarks determined like what kind of
• 25:28
  Speaker 2: stuff got made. Later as things cooled, like how much
• 25:31
  Speaker 2: hydrogen did you get and how much helium did you get?
• 25:33
  Speaker 2: And out of those things sort of sloshed together and
• 25:35
  Speaker 2: froth together in the Big Bang. So by studying the
• 25:38
  Speaker 2: relics of the Big Bang, the leftover bits of it,
• 25:41
  Speaker 2: we could actually get some clues as to like how
• 25:43
  Speaker 2: many neutrinos there were, and we can even figure out
• 25:46
  Speaker 2: something about the mass of those neutrinos.
• 25:49
  Speaker 1: But wait, I thought, neutrinos don't interact with regular mass,
• 25:52
  Speaker 1: So how can like regular mass relics tell you about
• 25:55
  Speaker 1: how many the trinos there were in the Big Bank.
• 25:57
  Speaker 2: Yeah, you're right. The neutrinos almost never interact with matter,
• 26:01
  Speaker 2: but if matter is dense enough, they will, Like the
• 26:03
  Speaker 2: probability is not zero, it's greater than zero. And actually,
• 26:07
  Speaker 2: back in the earlier times, when the universe was hotter,
• 26:10
  Speaker 2: when things were denser, the weak force was not as
• 26:13
  Speaker 2: weak as it is today. We think back in the
• 26:16
  Speaker 2: very early universe, the weak force and the electromagnetic force,
• 26:19
  Speaker 2: before the Higgs boson broke the symmetry, the two were
• 26:22
  Speaker 2: actually equally as powerful. So neutrinos used to interact with
• 26:26
  Speaker 2: normal matter more than they do today.
• 26:28
  Speaker 1: I think what you're saying is that our models of
• 26:29
  Speaker 1: the Big Bang tell us that there were a lot
• 26:32
  Speaker 1: of neutrinos at the Big Band, and that they have passed.
• 26:34
  Speaker 2: The models of the Big Bang tell us something about
• 26:36
  Speaker 2: how many neutrinos there were, like the number of neutrinos,
• 26:41
  Speaker 2: because neutrinos back then were moving really really fast, they
• 26:44
  Speaker 2: were very very hot, and so they helped like spread
• 26:46
  Speaker 2: energy out. They sort of acted like photons because everything
• 26:49
  Speaker 2: was so hot. And when we study the early universe
• 26:52
  Speaker 2: we can see these acoustic oscillations, like there were these
• 26:56
  Speaker 2: density waves. In the early universe. Things were hot and dense,
• 27:00
  Speaker 2: and the created pressure waves in the matter photons and
• 27:03
  Speaker 2: neutrinos helped us sort of smooth that out a little bit.
• 27:06
  Speaker 2: So by looking at those oscillations they're called baryon acoustic oscillations,
• 27:10
  Speaker 2: which make these ringing patterns in the early universe, we
• 27:13
  Speaker 2: can measure how many neutrinos and how many photons there were,
• 27:16
  Speaker 2: So that tells us something about the number of neutrinos.
• 27:19
  Speaker 2: Then we can do a second thing to figure out
• 27:21
  Speaker 2: how massive the neutrinos had to be. Like we know
• 27:23
  Speaker 2: how many neutrinos there were, and then we can figure out, well,
• 27:27
  Speaker 2: how much mass could the neutrinos have without causing the
• 27:30
  Speaker 2: universe to collapse? Right, We know that the universe has
• 27:33
  Speaker 2: been expanding since it was very very young, and that
• 27:36
  Speaker 2: tells us something about like how much matter and radiation
• 27:38
  Speaker 2: and energy there is in the universe, because if there
• 27:41
  Speaker 2: was too much, then gravity would pull everything back together
• 27:44
  Speaker 2: very quickly into a big bang. So we know something
• 27:47
  Speaker 2: about how many neutrinos there were, we could put an
• 27:49
  Speaker 2: upper limit on how massive they could be without collapsing
• 27:52
  Speaker 2: the universe.
• 27:54
  Speaker 1: But I think the two are sort of tied together, right.
• 27:56
  Speaker 1: The number of neutrinos and how massive they are, right,
• 27:59
  Speaker 1: I mean you have to assume they have mass in
• 28:01
  Speaker 1: order for them to matter at the Big Bang, right.
• 28:03
  Speaker 2: Well, they don't have to have mass in order to matter.
• 28:06
  Speaker 2: It's funny that we use matter because remember, general relativity
• 28:09
  Speaker 2: is sensitive to energy density, whether it's in the form
• 28:12
  Speaker 2: of radiation or in the form of matter. It really
• 28:15
  Speaker 2: is just sensitive to energy density. So the Big Bang
• 28:18
  Speaker 2: analysis tells us the number of neutrinos totally independently of
• 28:21
  Speaker 2: their mass. And the second step is to say, well,
• 28:24
  Speaker 2: if neutrinos do exist, how much mass could you give
• 28:27
  Speaker 2: them without causing the universe to collapse? So that tells
• 28:30
  Speaker 2: us something about how massive they could be. Like an
• 28:34
  Speaker 2: upper limit, Yes, exactly, it's an upper limit. That number
• 28:37
  Speaker 2: is actually really really low. The number is less than
• 28:39
  Speaker 2: the tenth of an electron volt.
• 28:41
  Speaker 1: Which I guess to give us some context, how much
• 28:44
  Speaker 1: mass does an electron have?
• 28:45
  Speaker 2: So an electron has like five hundred thousand electron volts.
• 28:50
  Speaker 2: It's half of an MeV half of a mega electron bolt,
• 28:54
  Speaker 2: and so five hundred thousand electron vaults. That's not very much, right,
• 28:58
  Speaker 2: Electrons are very very low mass. Particles compared to like
• 29:01
  Speaker 2: a proton. A proton has like one giga electron bolts
• 29:04
  Speaker 2: one billion electron bolts. So we know from the Big
• 29:08
  Speaker 2: Bang that all neutrinos added together have to have less
• 29:12
  Speaker 2: than a tenth of an electron bolt, less than one
• 29:15
  Speaker 2: ten billionth of the mass of a proton.
• 29:17
  Speaker 1: You mean all the different kinds of neutrinos, not all
• 29:19
  Speaker 1: of the individual neutrinos in the universe.
• 29:22
  Speaker 2: Right, Yeah, that's exactly right. There are three neutrinos. When
• 29:24
  Speaker 2: you add up all their mass together, it has to
• 29:26
  Speaker 2: be less than a tenth of an ev where an
• 29:29
  Speaker 2: electron is five hundred thousand ev and a proton is
• 29:33
  Speaker 2: about a billion ev m.
• 29:35
  Speaker 1: Interesting, so then pretty light, very very light, Like how
• 29:39
  Speaker 1: much is a quark.
• 29:39
  Speaker 2: It depends a lot on which cork you're talking about.
• 29:41
  Speaker 2: The lowest mass quarks have like a few MeV a
• 29:44
  Speaker 2: few million electronvolts. The most massive ones, like the top quark,
• 29:48
  Speaker 2: is like one hundred and seventy five billion ev. So
• 29:52
  Speaker 2: these neutrinos have mass much much closer to zero than
• 29:55
  Speaker 2: anything we've ever seen before. They're like shockingly low mass.
• 30:00
  Speaker 1: Okay, so we have a sort of an upper limit.
• 30:01
  Speaker 1: You said for how much the three kinds of neutrinos
• 30:05
  Speaker 1: can add up together, But then how do we resolve
• 30:08
  Speaker 1: how much each one of them weighs?
• 30:09
  Speaker 2: So then we have another really fascinating clue which tells
• 30:12
  Speaker 2: us about the mass difference between the neutrinos. So so
• 30:15
  Speaker 2: far we know something about the sum of their masses.
• 30:17
  Speaker 2: We know it's less than point one ev. We also
• 30:20
  Speaker 2: know there are three neutrinos, when we're wondering, like, well,
• 30:22
  Speaker 2: they all have the same mass, is it like with
• 30:25
  Speaker 2: the other particles, where there's one low mass and then
• 30:27
  Speaker 2: another one and then another one. So we can do
• 30:29
  Speaker 2: another kind of experiment to measure the differences between the
• 30:32
  Speaker 2: masses of the neutrinos. And this comes from how they
• 30:35
  Speaker 2: actually change their identities. Neutrinos are weird compared to the
• 30:39
  Speaker 2: other particles. In even another way. They're different from the electron,
• 30:44
  Speaker 2: the muon, and the towel, and that they can change flavor.
• 30:46
  Speaker 2: Like if you create an electron neutrino and shoot it
• 30:49
  Speaker 2: through space and then wait like a light year two
• 30:52
  Speaker 2: light years and try to measure it, you might discover
• 30:54
  Speaker 2: it's no longer an electron neutrino. It's now a muon
• 30:58
  Speaker 2: neutrino or a neutrino. This is called neutrino oscillation.
• 31:03
  Speaker 1: M yeah, I think usually if you shoot anything the space,
• 31:06
  Speaker 1: no change flavor. But I guess how do we know this? Like,
• 31:10
  Speaker 1: how would we know if it changed flavors? And again,
• 31:12
  Speaker 1: flavor is kind of the charge of the weak force?
• 31:14
  Speaker 4: Right?
• 31:14
  Speaker 2: Flavor is actually which of these generations of particles? It
• 31:17
  Speaker 2: is like is it electron, is it muon? Is it tau? Right,
• 31:21
  Speaker 2: that's what we mean by flavor.
• 31:23
  Speaker 1: Oh, is there a charge the weak force or is
• 31:25
  Speaker 1: it just the weak charge?
• 31:26
  Speaker 2: The weak force does have a charge member, it's two
• 31:28
  Speaker 2: different charges. There's the isospin and the weak hypercharge, so
• 31:31
  Speaker 2: both of those count as weak charges. But the neutrinos
• 31:34
  Speaker 2: all have the same week charges where they have different
• 31:37
  Speaker 2: is this flavor? This different identity, But that identity actually
• 31:41
  Speaker 2: turns out to be different. When you create the neutrino
• 31:44
  Speaker 2: and when the neutrino flies through space, they have like
• 31:46
  Speaker 2: two different sets of identities. There's the identity we talked
• 31:49
  Speaker 2: about when a neutrino is made, like the weak force,
• 31:52
  Speaker 2: when it makes an electron, it makes an electron neutrino,
• 31:54
  Speaker 2: or if it makes a muan, it makes a muon neutrino.
• 31:56
  Speaker 2: But when neutrinos fly through space, they have three different
• 31:59
  Speaker 2: ideas entities, and those are their masses. So there's three
• 32:03
  Speaker 2: different kinds of neutrinos for the weak force, and there's
• 32:06
  Speaker 2: three different kinds of neutrinos for the masses. But those
• 32:09
  Speaker 2: are not the same. They're like a mixture of each other.
• 32:12
  Speaker 2: So if you imagine this like M one, M two,
• 32:15
  Speaker 2: M three are the three neutrino masses. When you create
• 32:18
  Speaker 2: an electron neutrino, it's not like it's M one. It's
• 32:21
  Speaker 2: some weird mixture of all the masses of the three neutrinos.
• 32:25
  Speaker 1: You mean some kind of weird quantum mixture. Is that
• 32:27
  Speaker 1: what you mean?
• 32:28
  Speaker 2: Yeah, it's a superposition. So you create an electron neutrino,
• 32:31
  Speaker 2: it's a quantum superposition of the three different neutrino masses.
• 32:35
  Speaker 2: You create a mule on neutrino, it's a different superposition
• 32:38
  Speaker 2: of those masses. It's like having two different set of
• 32:41
  Speaker 2: axes that are not aligned. It's like a rotation between
• 32:45
  Speaker 2: your set of axes.
• 32:46
  Speaker 1: I guess maybe the question I have is so there's
• 32:48
  Speaker 1: three types of neutrinos, electronion, and town neutrinos, and the
• 32:52
  Speaker 1: only difference between them is the mass.
• 32:54
  Speaker 2: The only difference between the electron, muon, and town neutrino
• 32:57
  Speaker 2: is how they interact with the weak force. Three different
• 33:00
  Speaker 2: kinds of neutrinos. There's two different ways to break them down.
• 33:02
  Speaker 2: One is how do they interact with the weak force?
• 33:05
  Speaker 2: The other is what are their masses? So you get
• 33:08
  Speaker 2: two different ways to categorize the three neutrinos.
• 33:11
  Speaker 1: What do you mean how it interacts with the weak force?
• 33:13
  Speaker 1: Like like it's probability of interaction or its strength of interaction?
• 33:18
  Speaker 1: What do you mean by that?
• 33:19
  Speaker 2: Like what it's made in association with? Like, if you
• 33:21
  Speaker 2: make an electron, what kind of neutrino do you make? Well,
• 33:24
  Speaker 2: you make an electron neutrino. If you make a tow
• 33:27
  Speaker 2: what kind of neutrino do you make? You make a
• 33:29
  Speaker 2: town neutrino.
• 33:30
  Speaker 1: But if you already made it, does it matter or
• 33:32
  Speaker 1: does it matter in like what it can do later?
• 33:34
  Speaker 2: It matters in the accounting of the number of electrons
• 33:36
  Speaker 2: or muons or towels in the universe.
• 33:38
  Speaker 1: Yeah, But like if you just catch one in space.
• 33:40
  Speaker 1: How do you know what it is because you weren't
• 33:42
  Speaker 1: there when it was made.
• 33:44
  Speaker 2: Yeah, good question. Well, electron neutrino is more likely to
• 33:46
  Speaker 2: make electrons and a muon neutrino will make a muon,
• 33:49
  Speaker 2: and a town neutrino will interact and make a tow.
• 33:51
  Speaker 2: One of our neutrino experiments can see electrons, it can
• 33:54
  Speaker 2: also see muons, and it can also see tows And
• 33:56
  Speaker 2: so you can tell which kind of neutrino it was
• 33:59
  Speaker 2: by how how it interacts. Does it create an electron,
• 34:02
  Speaker 2: does it create a muon, Does it created.
• 34:03
  Speaker 1: A taw What it can do in the future kind.
• 34:06
  Speaker 2: Of yeah, what it can do in the future, because
• 34:08
  Speaker 2: the universe keeps track of this, accounting how many electrons
• 34:11
  Speaker 2: are there, how many muons are there, how many towels
• 34:13
  Speaker 2: are there. But again, that's just one way to see
• 34:15
  Speaker 2: these things. Another way to see these things is how
• 34:18
  Speaker 2: much mass do they have? And for most particles it's
• 34:21
  Speaker 2: the same thing. The weak force creates an electron, the
• 34:24
  Speaker 2: electron has a mass. All electrons have the same mass.
• 34:26
  Speaker 2: It's just a number. And if you ask, like what
• 34:28
  Speaker 2: are the masses of the eleptons, you get three different numbers,
• 34:31
  Speaker 2: those align with the flavors of the e leptons, but
• 34:33
  Speaker 2: when it comes to the neutrinos, they don't. So when
• 34:36
  Speaker 2: you create an electron neutrino, it's a weird mixture of
• 34:38
  Speaker 2: these different masses, and as it flies through space, those
• 34:42
  Speaker 2: that mixture can change because mass tells us how things
• 34:44
  Speaker 2: move through space. So these electron neutrinos and mew neutrinos
• 34:47
  Speaker 2: and town neutrinos, because they're made of three different masses,
• 34:50
  Speaker 2: and those masses are different, those masses like fly through
• 34:53
  Speaker 2: space slightly differently, and they can turn from one into another.
• 34:57
  Speaker 1: I think what you're saying is that, like, if you
• 35:00
  Speaker 1: make an electron neutrino like in the center of the
• 35:02
  Speaker 1: Sun and it's flying to us, and it has the
• 35:05
  Speaker 1: identity of an electron neutrino, it might have that identity,
• 35:08
  Speaker 1: but it might not necessarily have a particular mass, like
• 35:10
  Speaker 1: it might have one of three different masses exactly. Or
• 35:13
  Speaker 1: if you like find a neutrino during space with like
• 35:16
  Speaker 1: one of the masses, like the highest mass for neutrinos,
• 35:19
  Speaker 1: then that could still be either an electron neatrino or
• 35:22
  Speaker 1: town latrino or a unutrino.
• 35:25
  Speaker 2: Yes, that's exactly right. In mathematical terms, if you have
• 35:28
  Speaker 2: a weak eigen state, if you have an electron neutrino,
• 35:31
  Speaker 2: that's something produced by the weak force. In a pure
• 35:34
  Speaker 2: electron state, it's a mixture of the mass states. If
• 35:37
  Speaker 2: you have a pure mass state, it's a mixture of
• 35:39
  Speaker 2: the flavor states.
• 35:41
  Speaker 1: I think basically nutrino's kind of have an identity crisis
• 35:44
  Speaker 1: going on, both a mass crisis and an identity crisis,
• 35:48
  Speaker 1: Like it doesn't quite know what it is, or it
• 35:50
  Speaker 1: could be different things, but it could also weigh different things,
• 35:53
  Speaker 1: and it could also call itself different things. And it's
• 35:55
  Speaker 1: sort of like up in the air, like it can
• 35:57
  Speaker 1: change its fluid between these identities exactly.
• 36:00
  Speaker 2: Neutrinos have two different kinds of identities, and they do
• 36:03
  Speaker 2: not align. For most particles, these things align very well
• 36:06
  Speaker 2: for neutrinos.
• 36:07
  Speaker 1: They don't like an electron. For example, if it's born
• 36:10
  Speaker 1: an electron, it's going to have the mass of an electron.
• 36:12
  Speaker 1: It's not somethingly going to have the mass of a
• 36:14
  Speaker 1: Taue electron or a Newon electron.
• 36:16
  Speaker 2: Right, Yeah, And this calls in the question what I
• 36:18
  Speaker 2: was saying at the very beginning of the podcast about
• 36:20
  Speaker 2: mass being part of the identity of a particle, because
• 36:22
  Speaker 2: neutrinos can't really be defined by their mass, Like, well,
• 36:25
  Speaker 2: it depends are you talking about who I interact with
• 36:27
  Speaker 2: or how I fly through space? Because the same neutrino
• 36:30
  Speaker 2: can give you two different answers to that question.
• 36:33
  Speaker 1: Interesting, all right, Well, let's dig into how we actually
• 36:36
  Speaker 1: measure the mass of a neutrino and what those results
• 36:40
  Speaker 1: have found. But first, let's take another quick break. Right
• 36:55
  Speaker 1: we're talking about the mass of a neutrino. How massive
• 36:58
  Speaker 1: is this ghostly part of that flies through space, barely
• 37:01
  Speaker 1: interacting with everybody else in the universe, ignoring everyone. It's
• 37:05
  Speaker 1: kind of a snobby particle.
• 37:06
  Speaker 2: It's just got its own stuff to do, you know.
• 37:08
  Speaker 2: It just can't stop and chat with everybody. It's got
• 37:10
  Speaker 2: its list of barrens.
• 37:12
  Speaker 1: It's very aloof.
• 37:14
  Speaker 2: It's just busy, man, It's just busy.
• 37:16
  Speaker 1: Just more neutral, has less opinions. I guess it's not
• 37:19
  Speaker 1: as interesting, all right. And so we're talking about how
• 37:23
  Speaker 1: much mass it has, and we know from the Big
• 37:26
  Speaker 1: Bang models that we have that it nutrina has very
• 37:29
  Speaker 1: little mass, and the different kinds of nutrino thos can't
• 37:32
  Speaker 1: have a lot of masks combined. We talked about how
• 37:35
  Speaker 1: the nutrina kind of has an identity crisis, doesn't quite
• 37:39
  Speaker 1: knows for real what kind of neutrino it is and
• 37:42
  Speaker 1: how much it weighs. It's so sort of fluid and quantumy,
• 37:45
  Speaker 1: kind of complex and superposition. So then I guess the
• 37:48
  Speaker 1: big question is what can you do with that? How
• 37:50
  Speaker 1: do you measure these masses if the neutrino so wishy washing?
• 37:55
  Speaker 2: Yeah, So the fact that neutrinos can change flavor was
• 37:58
  Speaker 2: a big mystery in particles physics for many decades. Like
• 38:01
  Speaker 2: we count the number of neutrinos we see from the
• 38:03
  Speaker 2: Sun electron neutrinos, and we don't see as many as
• 38:06
  Speaker 2: we thought we should, which is a big puzzle. For
• 38:08
  Speaker 2: a long time, we predicted a certain number of electron
• 38:10
  Speaker 2: neutrinos being created in the Sun, and we just didn't
• 38:13
  Speaker 2: see as many. We saw like a third as many
• 38:15
  Speaker 2: as we expected. Now we understand that's because they're oscillating.
• 38:18
  Speaker 2: They're changing from electron neutrino to something else, and so
• 38:21
  Speaker 2: we're not seeing them because they're not interacting with our electrons.
• 38:24
  Speaker 2: But we can also use that to measure the differences
• 38:28
  Speaker 2: in the masses of the neutrinos. It's because there's a
• 38:31
  Speaker 2: mass difference between the neutrinos because they fly differently through
• 38:34
  Speaker 2: space that they're changing their identity as they go. So
• 38:38
  Speaker 2: what we can extract from this are two numbers, the
• 38:41
  Speaker 2: mass differences. Like you imagine this M one, M two,
• 38:43
  Speaker 2: M three. We can measure the separation between those three.
• 38:46
  Speaker 2: We can't tell the overall mass, but we can tell
• 38:49
  Speaker 2: how different they are. But the gaps are between the
• 38:52
  Speaker 2: neutrino masses.
• 38:54
  Speaker 1: I guess the question is can we why can we
• 38:57
  Speaker 1: measure the absolute value of these masses.
• 38:59
  Speaker 2: Because this oscillatetiontion doesn't depend on the absolute value. It
• 39:01
  Speaker 2: only depends on the difference. Like if all the neutrinos
• 39:04
  Speaker 2: had the same mass, then there wouldn't be any oscillation.
• 39:07
  Speaker 2: If the mass differences were really really large, they would
• 39:09
  Speaker 2: oscillate more. So by measuring how much they oscillate, we
• 39:12
  Speaker 2: can measure this mass difference, but the oscillation doesn't depend
• 39:16
  Speaker 2: on the total mass. This's a separate experiment we'll talk
• 39:18
  Speaker 2: about in a minute, called the Caturing experiment, which is
• 39:21
  Speaker 2: going to try to measure the overall mass of the neutrino.
• 39:24
  Speaker 2: But this oscillation, something which is quite well established, gives
• 39:27
  Speaker 2: us a precise measurement only of the differences between the masses.
• 39:31
  Speaker 1: I guess maybe I didn't quite understand why we can
• 39:33
  Speaker 1: only measure the differences.
• 39:34
  Speaker 2: Because the oscillation comes from the differences, Like if there
• 39:36
  Speaker 2: weren't any differences, you would see no oscillation. And the
• 39:39
  Speaker 2: larger the difference, the more the oscillation. It's kind of
• 39:42
  Speaker 2: like measuring interference between two laser beams. If they're in sync,
• 39:47
  Speaker 2: you see no interference. If one of them is delayed,
• 39:49
  Speaker 2: then they're out of phase and they interfere with each
• 39:52
  Speaker 2: other and giving an effect you can measure. But all
• 39:55
  Speaker 2: you can measure from the interference is the difference between
• 39:58
  Speaker 2: the beams, because that's what causes the interference. And neutrino
• 40:02
  Speaker 2: is a mixture of different masses, and each of those
• 40:05
  Speaker 2: masses flies through space differently, and it's that difference that
• 40:09
  Speaker 2: causes them to change flavor to oscillate.
• 40:11
  Speaker 1: But then how do we measure the oscillations, Like we
• 40:13
  Speaker 1: can only measure one neutrino at a time, we don't
• 40:16
  Speaker 1: know what it was before. How do we know what
• 40:18
  Speaker 1: it was after.
• 40:20
  Speaker 2: Well, we don't measure oscillations for an individual neutrino, You're right.
• 40:22
  Speaker 2: What we do is measure them statistically. So we have
• 40:24
  Speaker 2: like a bunch of neutrinos made in the Sun, and
• 40:26
  Speaker 2: we know those are all electron neutrinos because the Sun
• 40:28
  Speaker 2: has electrons in it and not muons and towels, So
• 40:31
  Speaker 2: we can measure how many of those have disappeared by
• 40:33
  Speaker 2: the time they get to Earth. We can also make
• 40:35
  Speaker 2: a bunch of muon neutrinos and a particle beam on
• 40:38
  Speaker 2: Earth and then see how often they disappear. So we
• 40:41
  Speaker 2: can make a bunch of these measurements of neutrino oscillation,
• 40:44
  Speaker 2: not by looking at an individual neutrino and seeing it oscillate,
• 40:46
  Speaker 2: but by making a huge number of neutrinos and seeing
• 40:49
  Speaker 2: how many of them disappear from their original identity.
• 40:53
  Speaker 1: Because you're saying, like the way you measure them, when
• 40:55
  Speaker 1: you catch a neutrino, you sort of know what it was,
• 40:58
  Speaker 1: or at least the detectors can only as you're one
• 41:00
  Speaker 1: kind of neutrino at a time.
• 41:02
  Speaker 2: Exactly, and all you can do is measure is flavor.
• 41:03
  Speaker 2: That's the way we detect them is we interact with them.
• 41:06
  Speaker 2: The only way to interact with them is through the
• 41:07
  Speaker 2: weak force, and that means using electrons, viuans, and towels.
• 41:11
  Speaker 2: That's how we interact with them.
• 41:12
  Speaker 1: And then how does that tell us their mass differences?
• 41:15
  Speaker 1: Like if I catch in the trino, can I just
• 41:18
  Speaker 1: infer its mass from like how much energy it has
• 41:20
  Speaker 1: and how fast it was going.
• 41:22
  Speaker 2: So there are experiments that are going to try to
• 41:23
  Speaker 2: do exactly that, which we can talk about in a minute.
• 41:25
  Speaker 2: The oscillation experiments are just counting how many neutrinos have disappeared.
• 41:29
  Speaker 2: Neutrinos have such low mass that's very, very difficult to
• 41:33
  Speaker 2: measure them individually on a per neutrino basis. But there
• 41:36
  Speaker 2: is an experiment in Germany which is trying to do
• 41:38
  Speaker 2: exactly that.
• 41:40
  Speaker 1: Okay, so then you're saying that we have measured kind
• 41:42
  Speaker 1: of the differences between the masses. So what are those numbers?
• 41:45
  Speaker 2: Those numbers are really small. There's two numbers there. One
• 41:48
  Speaker 2: of them is ten mili electron volts. A milli electric
• 41:50
  Speaker 2: bolt is one thousands of an electron volts. The other
• 41:53
  Speaker 2: one is fifty milli electron volts. So some of them
• 41:56
  Speaker 2: has to be less than one hundred and twenty milli
• 41:58
  Speaker 2: electron volts. We know that the gaps between them are
• 42:01
  Speaker 2: ten and fifty.
• 42:03
  Speaker 1: This feels like a fourth grade logic problem, like Sally, Paul,
• 42:08
  Speaker 1: and John money in their pockets and it adds up
• 42:12
  Speaker 1: to a dollar twenty. But the difference between Sally and
• 42:15
  Speaker 1: Paul is fifty cents, and between Sally and John is
• 42:18
  Speaker 1: Paul sense how much does Sally have exactly?
• 42:22
  Speaker 2: And so we know that there's two possible solutions. We
• 42:25
  Speaker 2: know that two of the neutrinos are close to each other.
• 42:27
  Speaker 2: This is a small gap ten MeTV. We also know
• 42:29
  Speaker 2: that the third one is further away, it's fifty MeTV away.
• 42:32
  Speaker 2: We don't know if the two ones that are closer
• 42:35
  Speaker 2: are heavier or lighter, So like are the two ones
• 42:38
  Speaker 2: that are near each other at the top of the
• 42:39
  Speaker 2: spectrum or the bottom of the spectrum. We don't know.
• 42:42
  Speaker 2: There's two possible answers there. We also don't know quite
• 42:44
  Speaker 2: how it adds up, like the number we have from
• 42:47
  Speaker 2: the early universe is an upper limit, and they could
• 42:49
  Speaker 2: all still be very very low values. So there's a
• 42:52
  Speaker 2: lot of open questions there. We'd love to know the
• 42:54
  Speaker 2: sum of the masses of all the neutrinos.
• 42:57
  Speaker 1: Well, you sort of just need to know one of
• 42:59
  Speaker 1: the masses right, and then that would click the other
• 43:01
  Speaker 1: ones in place.
• 43:02
  Speaker 2: Well, there's still two possible solutions if you just know
• 43:04
  Speaker 2: one of them. You don't know if you have like
• 43:05
  Speaker 2: the inverted hierarchy where the two close ones are the top,
• 43:08
  Speaker 2: or if you have the other hierarchy where the two
• 43:10
  Speaker 2: close ones are at the bottom.
• 43:11
  Speaker 1: Oh, I see, but you're saying, we know this very precisely.
• 43:14
  Speaker 1: Like our models of the neutrino. When you shoot a
• 43:17
  Speaker 1: bunch of them out and you see how many transform
• 43:19
  Speaker 1: into different kinds, dat somehow tells you the difference in
• 43:22
  Speaker 1: their masses because it, I guess it affects the probability
• 43:24
  Speaker 1: of these transformations.
• 43:26
  Speaker 2: Yeah. And we've been doing these neutrino oscillation experiments for decades,
• 43:29
  Speaker 2: and we've done them in all sorts of ways, with
• 43:31
  Speaker 2: all sorts of different combinations. Make this kind of neutrino
• 43:34
  Speaker 2: disappear that kind of trino, make this kind of measure
• 43:36
  Speaker 2: the appearance of the other one. We've triangulated that whole matrix,
• 43:39
  Speaker 2: and we know exactly how these numbers work out. What
• 43:42
  Speaker 2: we don't know is the overall mass, only the differences.
• 43:45
  Speaker 2: So the differences are very precisely known. The overall mass
• 43:48
  Speaker 2: is limited by this Big Bang cosmology stuff to less
• 43:51
  Speaker 2: than one hundred and twenty mili electron bolts. But now,
• 43:54
  Speaker 2: this is really cool experiment in Germany called the Katrine experiment,
• 43:58
  Speaker 2: which is going to try to measure the mass of
• 44:01
  Speaker 2: the electron neutrino as precisely as possible.
• 44:03
  Speaker 1: All right, let's talk about this experiment, and now what
• 44:06
  Speaker 1: is it? How does it work?
• 44:07
  Speaker 2: So this experiment is called the Carlsrua tritium neutrino experiment,
• 44:11
  Speaker 2: which is a tortured way to make catrin as an
• 44:14
  Speaker 2: acronym to.
• 44:15
  Speaker 1: Say the least.
• 44:17
  Speaker 2: But it starts from tritium and tretium decays two helium,
• 44:21
  Speaker 2: which is like two protons a neutron, and then it
• 44:24
  Speaker 2: also produces an electron and a neutrino.
• 44:26
  Speaker 1: And tretium is just an element, right.
• 44:29
  Speaker 2: Yeah, Tritium is two neutrons and a proton, so it's
• 44:32
  Speaker 2: like an isotope of hydrogen. Basically, what happens is one
• 44:34
  Speaker 2: of those neutrons turns into a proton and then emits
• 44:37
  Speaker 2: an electron. And a neutrino, And this is a nice
• 44:40
  Speaker 2: way to measure the neutrino mass because the electron neutrino
• 44:43
  Speaker 2: don't have a lot of energy. The comp moving really
• 44:46
  Speaker 2: really slow, and so basically you can see the effect
• 44:49
  Speaker 2: of the mass of these particles on how fast they're moving.
• 44:52
  Speaker 2: It's like, not a whole lot of energy made in
• 44:54
  Speaker 2: this reaction, so not a lot of despair. So if
• 44:58
  Speaker 2: the electron and the neutrino have a lot of mass,
• 45:00
  Speaker 2: they'll come out moving slower. They have less mass will
• 45:02
  Speaker 2: come out moving faster, And so we can't see the
• 45:06
  Speaker 2: neutrino directly, but we can measure the electron energy very
• 45:09
  Speaker 2: very precisely. So that's why this experiment does. It measures
• 45:12
  Speaker 2: those electrons really really precisely, and if it sees electrons
• 45:15
  Speaker 2: moving with more energy, it means that the neutrino mass
• 45:18
  Speaker 2: hasn't taken up some of that energy budget. And if
• 45:21
  Speaker 2: it doesn't see electrons moving with sort of near the
• 45:24
  Speaker 2: maximum possible energy that this decay can make, it means
• 45:27
  Speaker 2: that the neutrino has used up some of the energy
• 45:30
  Speaker 2: budget that otherwise could have made the electron go faster,
• 45:33
  Speaker 2: and that means the neutrino has some mass, so it's
• 45:36
  Speaker 2: sort of like a way to measure the neutrino mass
• 45:38
  Speaker 2: by seeing how much energy it slurps out from this reaction.
• 45:43
  Speaker 1: Okay, so let me see if I got this straight.
• 45:45
  Speaker 1: You start with an isotope of hydrogen called tritium, which
• 45:48
  Speaker 1: is two neutrons in the nucleus surrounded by an electron,
• 45:51
  Speaker 1: and then you just let it hang out and eventually
• 45:53
  Speaker 1: it's going to decay into a hydrogen atom like right,
• 45:56
  Speaker 1: Like one of those neutrons is just going to disappear,
• 45:58
  Speaker 1: transformed to something else. And you're saying that this reaction
• 46:01
  Speaker 1: shoots out an electron and an antineutrino, and the electron
• 46:05
  Speaker 1: we can measure its mass and speed because it's an electron,
• 46:09
  Speaker 1: and so whatever's left because we I guess you assume
• 46:13
  Speaker 1: a certain amount of energy at the beginning.
• 46:14
  Speaker 2: We know very well how tritium decays and how it
• 46:17
  Speaker 2: turns into helium and how much energy is available. Yeah,
• 46:20
  Speaker 2: and we know that energy has to go to the
• 46:22
  Speaker 2: electron and the neutrino, and.
• 46:23
  Speaker 1: So the difference between what you started with and how
• 46:26
  Speaker 1: much you measure the electron is the energy that goes
• 46:28
  Speaker 1: into the neutrino exactly, But then how does that tell
• 46:31
  Speaker 1: you the mass? It could just be like something light
• 46:33
  Speaker 1: moving fast or something heavy moving slow.
• 46:35
  Speaker 2: Yeah, So there's a spectrum of possibilities, and what we're
• 46:37
  Speaker 2: looking for is the maximum scenario, Like are there any
• 46:41
  Speaker 2: cases where the electron takes all of the energy available?
• 46:44
  Speaker 2: There's like a certain energy budget for producing this subtract
• 46:48
  Speaker 2: out the electron mass, and then we wonder like, are
• 46:50
  Speaker 2: there scenarios where the electron takes all of the energy.
• 46:53
  Speaker 2: If we see cases where the electron takes all of
• 46:56
  Speaker 2: the energy, that means the neutrino hasn't taken any So
• 46:58
  Speaker 2: it's sort of like a budget. You have a budget
• 47:00
  Speaker 2: for the whole thing. The electron mask gets taken out,
• 47:03
  Speaker 2: then we wonder like does the neutrino take a cut?
• 47:06
  Speaker 2: If the neutrino takes a cut, that leaves a smaller
• 47:08
  Speaker 2: budget for the electron, and you'll never see an electron
• 47:11
  Speaker 2: having energy higher than that limit. The neutrino doesn't take
• 47:15
  Speaker 2: a cut, it leaves more energy for the electron and
• 47:17
  Speaker 2: you'll see faster moving electrons. So you look at the
• 47:20
  Speaker 2: tail of the distribution, like what's the fastest electron you
• 47:23
  Speaker 2: ever see, and that'll tell you how much the neutrino
• 47:25
  Speaker 2: is taken from the budget.
• 47:27
  Speaker 1: M I think I get it. So, like you start
• 47:29
  Speaker 1: with let's say one hundred units of energy, and you
• 47:32
  Speaker 1: measure how much energy electron that comes out has, and
• 47:35
  Speaker 1: you look for, like, what's the maximum energy that the
• 47:38
  Speaker 1: electron can take away from this? And let's say it's
• 47:40
  Speaker 1: like ninety nine out of a whole bunch of times,
• 47:42
  Speaker 1: did you do this? Ninety nine is the maximum, which
• 47:44
  Speaker 1: means like the minimum amount of energy the neutrino can
• 47:47
  Speaker 1: take is one, which, since it's the maximum for the electron,
• 47:53
  Speaker 1: it must mean that it's like it created a neutrino
• 47:55
  Speaker 1: that wasn't moving at all.
• 47:56
  Speaker 2: Maybe, And so they're looking for those scenarios like when
• 47:59
  Speaker 2: you make a motionless neutrino and the electron takes all
• 48:02
  Speaker 2: of its energy, that reaction reveals the mass of the
• 48:06
  Speaker 2: neutrino in the energy of the electron.
• 48:09
  Speaker 1: It reveals I guess, the mass of an electron neutrino.
• 48:12
  Speaker 2: Yes, it's revealed the mass of a neutrino created with
• 48:15
  Speaker 2: an electron. What does that really mean? Remember, the electron
• 48:17
  Speaker 2: neutrino doesn't have a definite mass, So actually what it's
• 48:21
  Speaker 2: measuring is a combination of all the masses of the neutrinos.
• 48:25
  Speaker 2: It's just like incoherent some of the distinct neutrino mass
• 48:30
  Speaker 2: values weighted by how much of each one is in
• 48:33
  Speaker 2: that electron neutrino. So remember electro neutrinos don't have a
• 48:37
  Speaker 2: definite mass, So you're measuring this like weird average mass
• 48:40
  Speaker 2: of a neutrino.
• 48:42
  Speaker 1: If you're going to sort of for like the minimum
• 48:44
  Speaker 1: amount of mass that the neutrino has, then must be
• 48:46
  Speaker 1: giving you the minimum mass for one of them. Right.
• 48:48
  Speaker 2: Yes, it's a bit of a subtle point of quantum mechanics.
• 48:51
  Speaker 2: The mass of that neutrino is not actually determined, right,
• 48:54
  Speaker 2: It's not like it has a certain number and we
• 48:56
  Speaker 2: don't know it. What we know is it's an electron neutrino,
• 49:00
  Speaker 2: which means we don't know what its mass is. And
• 49:03
  Speaker 2: so overall, on average, what you'll be sensitive to is
• 49:07
  Speaker 2: the average mass of those neutrinos. But you're right, what
• 49:11
  Speaker 2: we're doing is looking for the most energetic electron, which
• 49:15
  Speaker 2: means we'd be sensitive to the lower end of the
• 49:18
  Speaker 2: neutrino masses of that electron neutrino.
• 49:21
  Speaker 1: Which would maybe give you like the lightest of the
• 49:23
  Speaker 1: three neutrino masses.
• 49:24
  Speaker 2: Yeah, And what we're looking to do is combine this
• 49:26
  Speaker 2: with our measurements from neutrino oscillation, which tells very precisely
• 49:30
  Speaker 2: the separation between the neutrinos, and now we want to
• 49:33
  Speaker 2: anchor the overall scale and slide it up or slide
• 49:36
  Speaker 2: it down.
• 49:38
  Speaker 1: But I guess even if you do, like you said,
• 49:39
  Speaker 1: there's two possibilities for the other two, right, so like
• 49:43
  Speaker 1: you might know the massive one one of the masses,
• 49:45
  Speaker 1: but you wouldn't necessarily know the mass of the other two.
• 49:49
  Speaker 1: But I guess you would narrow it down to two possibilities.
• 49:51
  Speaker 2: Yeah, we'd narrow down to two possibilities. You're right, This
• 49:53
  Speaker 2: would still leave ambiguity for which higherarchy we have, Like
• 49:56
  Speaker 2: are the two close ones the top? Were the two
• 49:58
  Speaker 2: close ones at the bottom. So this experiment's been running
• 50:01
  Speaker 2: for a couple of years and they have some preliminary results.
• 50:03
  Speaker 2: Their measurement says that this mass they're measuring is less
• 50:07
  Speaker 2: than eight hundred milli electron bolts. Now that's not much
• 50:11
  Speaker 2: information because we already know from the Big Bang that
• 50:13
  Speaker 2: it's less than one hundred and twenty. This is just
• 50:15
  Speaker 2: sort of like their first result. They're going to keep
• 50:18
  Speaker 2: running the collecting more data, and they hope they'll be
• 50:20
  Speaker 2: able to measure this thing more precisely.
• 50:22
  Speaker 1: Wait, so we know that they can be more than
• 50:24
  Speaker 1: one hundred and twenty, but the first measurements say it's
• 50:27
  Speaker 1: less than eight hundred.
• 50:28
  Speaker 2: Yeah, so this is not as sensitive as the Big
• 50:30
  Speaker 2: Bang measurement so far.
• 50:32
  Speaker 1: But it would be really weird if they found that
• 50:34
  Speaker 1: the mass in the try now is eight hundred million
• 50:36
  Speaker 1: electron bolts, because that's way too much.
• 50:38
  Speaker 2: Yeah, exactly, this sets an upper bound of less than
• 50:41
  Speaker 2: eight hundred. We already know they're less than one twenty,
• 50:43
  Speaker 2: so it'd be pretty weird to measure it at like
• 50:45
  Speaker 2: six hundred or five hundred, you know. But these are
• 50:48
  Speaker 2: very very different measurements, right, the Big Bang versus like
• 50:51
  Speaker 2: experiments we're doing here on Earth. So it's not always
• 50:53
  Speaker 2: the case that they're going to agree. There's a lot
• 50:55
  Speaker 2: of theoretical assumptions that go into both of them. But
• 50:57
  Speaker 2: the good thing about this one is we keep running,
• 50:59
  Speaker 2: and so we can keep getting more and more precise measurements,
• 51:02
  Speaker 2: and so they're hoping by twenty twenty four twenty twenty
• 51:05
  Speaker 2: five they can get their sensitivity down to like two
• 51:08
  Speaker 2: hundred MEB and then they can push even further.
• 51:11
  Speaker 1: Because I guess it's all statistical, right, and so just
• 51:14
  Speaker 1: the longer you run it, the more accurate you can
• 51:16
  Speaker 1: say what the minimum is.
• 51:18
  Speaker 2: And this experiment is also super fun because it involves
• 51:20
  Speaker 2: this huge metal container. They shoot these electrons into this
• 51:25
  Speaker 2: mammoth vacuum chamber to measure their energy super duper precisely
• 51:29
  Speaker 2: this spectrometer. It required a really specialized shop to build
• 51:32
  Speaker 2: this thing. You should go online and google a picture
• 51:35
  Speaker 2: of this thing. It's like a big steel blimp basically,
• 51:38
  Speaker 2: and it was so big that it was really hard
• 51:41
  Speaker 2: to transport from the factory where they built it, like
• 51:43
  Speaker 2: three hundred kilometers to the experimental site. They actually had
• 51:47
  Speaker 2: to put it on a boat and float it down
• 51:49
  Speaker 2: river through the Mediterranean, out through the Atlantic over to
• 51:53
  Speaker 2: the Netherlands, and then up another river to the experiment.
• 51:58
  Speaker 2: So it's only like three hundred fifty twolometers away, but
• 52:00
  Speaker 2: I have to take like a nine thousand kilometer long
• 52:03
  Speaker 2: detour because it was too big to like put on
• 52:06
  Speaker 2: a flatbed truck and drive around.
• 52:08
  Speaker 1: Wow, sounds like they should have thought about it before
• 52:10
  Speaker 1: they built it. I mean they have built it on site.
• 52:14
  Speaker 2: Yeah exactly. But you know, you take specialized techniques just
• 52:17
  Speaker 2: to build this thing, and then specialized techniques just to
• 52:19
  Speaker 2: move this thing. There's some awesome videos of it making
• 52:22
  Speaker 2: its last seven kilometer journey across land from the docks
• 52:26
  Speaker 2: to the laboratory. They like squeezed it through these old villages,
• 52:30
  Speaker 2: you know, with like a centimeter to spare on each side.
• 52:32
  Speaker 2: It's pretty awesome, all right.
• 52:34
  Speaker 1: Well, again, a neutrino is part of our standard model
• 52:37
  Speaker 1: of denvers and so, and it's also kind of like
• 52:39
  Speaker 1: one of the last frontiers in terms of what we
• 52:42
  Speaker 1: know about the standard model, right, Like, once we found
• 52:44
  Speaker 1: the Higgs boson and we know about all the matter particles,
• 52:47
  Speaker 1: the neutrino is sort of one of the last big
• 52:50
  Speaker 1: questions we have about it, right, and which means it
• 52:52
  Speaker 1: sort of helps complete our understanding of matter particles in
• 52:56
  Speaker 1: the universe.
• 52:57
  Speaker 2: Yeah, you're absolutely right. It's the frontier particle physics, and
• 52:59
  Speaker 2: the US specifically has decided to double down on neutrinos.
• 53:03
  Speaker 2: We didn't build the next greatest best particle collider to
• 53:06
  Speaker 2: compete with CERN. Instead, the US has decided to build
• 53:09
  Speaker 2: big neutrino experiments to measure these masses, to measure the
• 53:12
  Speaker 2: neutrino interactions, to understand this weird sector of the universe
• 53:16
  Speaker 2: in more detail. We think there's probably a lot more
• 53:18
  Speaker 2: interesting hints.
• 53:19
  Speaker 1: There, and so learning more about the nutrino what would
• 53:22
  Speaker 1: that tell us about the universe.
• 53:23
  Speaker 2: Well, understanding the neutrino mass will help us understand the
• 53:26
  Speaker 2: Big Bang and like what was going on and the
• 53:27
  Speaker 2: neutrino contributions there. We also don't really know how the
• 53:30
  Speaker 2: neutrino gets mass, like does it get mass from the
• 53:33
  Speaker 2: Higgs boson the way other particles do, or does a
• 53:36
  Speaker 2: neutrino give itself mass? Like it might be that there
• 53:39
  Speaker 2: is no anti neutrino, that the neutrino is its own
• 53:42
  Speaker 2: anti particle. This is a fun story about a physicists
• 53:45
  Speaker 2: called Mayorana who thought about these Mayorana particles that might
• 53:48
  Speaker 2: be their own anti particles and give themselves mass in
• 53:51
  Speaker 2: this weird way. So it might even teach us about
• 53:53
  Speaker 2: what mass is for a particle.
• 53:55
  Speaker 1: HM cool, And that's very important because it it would
• 53:59
  Speaker 1: tell us why we have right.
• 54:00
  Speaker 2: Yeah, absolutely, it would tell us more about what the
• 54:02
  Speaker 2: meaning of mass is. They might also give us some
• 54:05
  Speaker 2: clues about the nature of dark matter. We know that
• 54:08
  Speaker 2: these three neutrinos are not the dark matter, but there
• 54:11
  Speaker 2: might be a fourth kind of neutrino, hysterile neutrino that
• 54:15
  Speaker 2: could be out there, and understanding the neutrino masses and
• 54:17
  Speaker 2: how they mix and interact with each other might clear
• 54:20
  Speaker 2: up some nagging questions about whether there are other flavors
• 54:23
  Speaker 2: of neutrinos out there.
• 54:25
  Speaker 1: That would be massive. All right, well, we hope you
• 54:30
  Speaker 1: enjoyed that. Thanks for joining us, see you next time.
• 54:41
  Speaker 2: Thanks for listening, and remember that Daniel and Jorge Explain
• 54:44
  Speaker 2: the Universe is a production of iHeartRadio. For more podcasts
• 54:48
  Speaker 2: from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever
• 54:52
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