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What are sterile neutrinos? · Daniel and Kelly’s Extraordinary Universe

0:08
Speaker 1: Ity, Daniel, you cook a lot. Are you the kind
0:11
Speaker 1: of person who keeps their kitchen clean? I don't think
0:13
Speaker 1: I'm excessive, you know, clean, but in a normal way.
0:16
Speaker 1: I think usually your definition of normal is different than
0:19
Speaker 1: most normal people. As a picture, they leave the dishes
0:22
Speaker 1: in the sink. What dishes in the sink? This isn't
0:25
Speaker 1: the frat house? I guess that's it. What about the counter?
0:28
Speaker 1: Do you have any of you know, stuff in it?
0:30
Speaker 1: I mean, we have cabinets and drawers. There's no reason
0:32
Speaker 1: to leave stuff on the counter. That's another no, I guess.
0:36
Speaker 1: So I'm getting the sense that you keep a pretty
0:38
Speaker 1: tighty kitchen. I mean we do our best to keep
0:40
Speaker 1: it need and organized right. And your office, how need
0:43
Speaker 1: do you keep that one? I think I focus my
0:45
Speaker 1: energies on the kitchen and not your work. You know,
0:48
Speaker 1: my priorities are the kitchen is sterile and the office
0:51
Speaker 1: is a disaster. I am for him and cartoonists and
1:08
Speaker 1: the co author of frequently asked Questions about the universe. Hi,
1:12
Speaker 1: I'm Daniel. I'm a particle physicist and a professor you
1:15
Speaker 1: see Irvine, And my office is a mess, but I
1:18
Speaker 1: promise you my mind is organized. What is that saying
1:21
Speaker 1: you're a mess in the office but a neat freak
1:23
Speaker 1: in the kitchen. Barack Obama said, physical discipline, mental discipline,
1:27
Speaker 1: and he's pretty good at what he does, so we
1:29
Speaker 1: should all follow his example. I mean, I've never read
1:31
Speaker 1: one of his physics papers, but he does seem like
1:33
Speaker 1: a smart dude. I think he probably funded a lot
1:36
Speaker 1: of your research indirectly, so maybe you should be a
1:38
Speaker 1: little bit more thankful. That's true. He used to be
1:41
Speaker 1: my boss's boss's boss's bosses boss's boss's boss's boss's boss. Yeah,
1:45
Speaker 1: there you go. He used to be a part of
1:47
Speaker 1: the administration practically. I mean you were right there. I
1:50
Speaker 1: mean you weren't in the white House, but you were
1:52
Speaker 1: in the white sun House. That's true, which, because I
1:55
Speaker 1: live in Irvine, is actually beige. But welcome to our podcast.
1:58
Speaker 1: Daniel and Jorge explained the Uni Verse, a production of
2:01
Speaker 1: I Heart Radio, in which we want to defeat all
2:03
Speaker 1: of the levels of the universe. We want to kill
2:06
Speaker 1: the lower minions and we want to work our way
2:08
Speaker 1: up to the boss levels of understanding of the universe.
2:11
Speaker 1: We want to break down all of those barriers. Talk
2:13
Speaker 1: about the deepest hardest, most confusing, trickiest questions about the
2:18
Speaker 1: nature of reality and show you what science does and
2:22
Speaker 1: does not understand about all of it. Yeah, because it
2:24
Speaker 1: is a pretty big universe, a pretty complicated with a
2:27
Speaker 1: kind of a complicated or chart. I guess a lot
2:29
Speaker 1: of middle management levels here in the universe that you
2:31
Speaker 1: have to go through, Daniel in in order to understand it.
2:34
Speaker 1: We are trying to make sense of it, and we're
2:36
Speaker 1: hoping that if we have a complete picture of everything
2:38
Speaker 1: that's out there and how they all talk to each other,
2:41
Speaker 1: that some patterns will emerge, that some sense will be
2:44
Speaker 1: a parent, that we will gain some understanding about how
2:47
Speaker 1: it all makes sense, or maybe the whole thing is
2:49
Speaker 1: just a big mess. What do you think is the
2:51
Speaker 1: universe's management style? Do you think it's pretty flat or
2:54
Speaker 1: is it kind of curved or is it multidimensional. It's
2:57
Speaker 1: definitely not Obama's management style. I mean, he was no
3:00
Speaker 1: drama Obama, but there's a lot of drama in the
3:02
Speaker 1: universe supernovas, that's pretty dramatic. Yeah, there's always those troublemakers
3:08
Speaker 1: in your organization, I guess who like to blow up exactly.
3:12
Speaker 1: But sometimes when things blow up they actually give us
3:14
Speaker 1: clues about how things work. When stars do go supernova,
3:18
Speaker 1: they give us a peek into the inside of those
3:20
Speaker 1: cosmic fusion reactors, so we can understand the crazy, intense
3:25
Speaker 1: forms of matter that are being created at their center.
3:28
Speaker 1: Because there are still a lot of incredible and tantalizing
3:31
Speaker 1: mysteries out there for us to discover. And that's just
3:34
Speaker 1: only on the things that we can see and detect.
3:36
Speaker 1: There's a lot about the universe that is actually hidden.
3:40
Speaker 1: You have these little windows into the universe through your senses.
3:43
Speaker 1: You can see some things, you can feel some things,
3:46
Speaker 1: you can taste a few things. But we know the
3:49
Speaker 1: universe is much richer and more complicated than the things
3:53
Speaker 1: we can detect directly with our senses. And over the years,
3:56
Speaker 1: we have built all sorts of technological eyeballs and ears
4:00
Speaker 1: and noses and tongues to experience the hidden parts of
4:04
Speaker 1: the universe. We can now see ultra violet and infrared.
4:07
Speaker 1: We can detect crazy particles that are invisible to our eyeballs.
4:11
Speaker 1: We know that there's a lot more going on out
4:13
Speaker 1: there than we can see directly. Wait. Wait, you're building
4:16
Speaker 1: like an electronic tongue. Is that what you're saying in
4:18
Speaker 1: a in a physics lab somewhere, or is this part
4:20
Speaker 1: of your kitchen endeavors. Well, the standard analogy is to
4:23
Speaker 1: talk about new technological eyeballs, like we are seeing the universe.
4:27
Speaker 1: I wonder sometimes why we always use the analogy of vision,
4:30
Speaker 1: because you could, in principle, translated to any of our senses.
4:33
Speaker 1: I don't know if people heard recently that the soonification
4:36
Speaker 1: of a black hole where they took like the oscillatory
4:39
Speaker 1: waves and the gas around the black hole and they
4:41
Speaker 1: turned it into sound. All these things are just an
4:43
Speaker 1: attempt to describe something that is unexperienceable into something that
4:49
Speaker 1: you know how to experience. Translate the unfamiliar into the familiar,
4:54
Speaker 1: so you can choose any of our senses, vision, hearing, taste, smell, right,
5:00
Speaker 1: what do electrons smell like? Are you trying to bring
5:02
Speaker 1: back to smell a vision? Are you trying to build
5:05
Speaker 1: the incredibly expensive smell atron. I think it's a whole
5:08
Speaker 1: new frontier for science communication. You know, people are making
5:11
Speaker 1: false color images of the James Webb pictures, people are
5:14
Speaker 1: making soonifications of black holes, but nobody's doing smellification of particles.
5:19
Speaker 1: Until we are today. I think if you made people
5:22
Speaker 1: smell physicists, they might be turned off by signs. So
5:25
Speaker 1: I'm not sure that's the best science communication strategy here. Yeah,
5:28
Speaker 1: I think we want to keep that in the office.
5:30
Speaker 1: But it is a pretty incredible universe because a lot
5:32
Speaker 1: of it is hidden. In fact, we've figured out that
5:36
Speaker 1: of the universe is totally invisible and we have no
5:39
Speaker 1: idea what it is. And stuff being invisible is not
5:42
Speaker 1: that unfamiliar. I mean, you are surrounded by air, but
5:45
Speaker 1: you can't see it, right, it's invisible to your eyes. Now,
5:49
Speaker 1: of course, you can touch it and you can tell
5:51
Speaker 1: that it's there, you can breathe it. But there are
5:54
Speaker 1: other things out there that are not just invisible. They're
5:57
Speaker 1: also intangible particles out there that fly right through your
6:00
Speaker 1: body and fly right through the Earth without even noticing.
6:04
Speaker 1: Does that mean they're also intasteable? Like? What does the
6:06
Speaker 1: neutrino taste like? I don't know, But we do have
6:09
Speaker 1: three flavors of neutrino, even though we have no idea
6:12
Speaker 1: which ones are delicious, So basically we can make up
6:14
Speaker 1: flavors for them, right, one one could be spicy, the
6:17
Speaker 1: other one could be sweet, the other one could be mommy, right.
6:21
Speaker 1: I was hoping you were gonna say, O, mommy neutrinos,
6:22
Speaker 1: because that's the most mysterious flavor. You know, it's the
6:26
Speaker 1: most delicious one, I guess if you're a savory kind
6:29
Speaker 1: of person. But there are a lot of mysterious things
6:31
Speaker 1: out there, some that are we are still trying to
6:33
Speaker 1: figure out, and so let's talk about one of them.
6:35
Speaker 1: So to be on the podcast, we'll be tackling the
6:37
Speaker 1: question what are sterile neutrinos, or for our UK fans,
6:47
Speaker 1: what are sterile neutrinos? They're apparently pronounced differently, although Daniel,
6:51
Speaker 1: you I think you have to pronounce it with the
6:53
Speaker 1: British accent sterile neutrinos. You want the full phrase with
6:59
Speaker 1: with that's kind of like a cockney accident maybe, or
7:03
Speaker 1: I don't know. And it's embarrassing that my UK accent
7:05
Speaker 1: is so bad because I actually am a citizen of
7:07
Speaker 1: the UK and my father grew up in London and
7:10
Speaker 1: my grandmother, who recently passed lived in London for fifty years,
7:13
Speaker 1: so I should be able to do it, but I can't. Yeah,
7:15
Speaker 1: I'm sure you're a big disappointment to Brits. Everywhere other.
7:19
Speaker 1: If you are British, doesn't that mean that any accent
7:21
Speaker 1: you have is a British accent? There you go, Yes,
7:24
Speaker 1: I define Britishness in just existing. Yeah, you're a pretty
7:28
Speaker 1: dear fellow. But this is an interesting question. Sterile neutrinos.
7:32
Speaker 1: Does that mean there are fertile neutrinas. Yeah, I think
7:34
Speaker 1: we're gonna have a lot of fun talking about the
7:36
Speaker 1: meaning of sterile and what physicists intended it to mean
7:39
Speaker 1: and how people interpreted. But no, there are not neutrinos
7:43
Speaker 1: out there that are becoming parents. Or is it more
7:45
Speaker 1: like sterile like clean, like bacteria free? Does that mean
7:48
Speaker 1: there are dirty neutrinos? Dirty fertile neutrinos sort of in
7:52
Speaker 1: the clean sense. These are more neutrinos that are totally
7:55
Speaker 1: isolated from everything else. They don't interact. They're more like
7:58
Speaker 1: standoffish neutrin learn or neutrinos. There you go. See, you
8:03
Speaker 1: should just put me in charge of naming things. Things
8:05
Speaker 1: would go a lot easier example number four seventy seven.
8:08
Speaker 1: But anyways, as usually, we were wondering how many people
8:10
Speaker 1: out there had heard of sterile neutrinos or have any
8:13
Speaker 1: idea about what they could be. So thanks very much
8:16
Speaker 1: to everybody who volunteers to answer these questions for the podcast.
8:20
Speaker 1: We really appreciate it, and everybody out there loves hearing
8:22
Speaker 1: what you think. So if you would like to hear
8:25
Speaker 1: your voice on the podcast for a future episode, please
8:28
Speaker 1: don't be shy. Right to us two questions at Daniel
8:31
Speaker 1: and Horre dot com. So think about it for a second.
8:33
Speaker 1: What do you think a sterile neutrino is? Here's what
8:37
Speaker 1: people have to say sterile neutrinos. I don't know what
8:41
Speaker 1: they are. I feel like a neutrino can turn into
8:45
Speaker 1: a different type of particle, though, so maybe a sterile
8:49
Speaker 1: neutrino cannot for some reason. And so I think that's
8:53
Speaker 1: as a sterile environment is an environment way there is
8:56
Speaker 1: no interaction between sales and bacterias or other um micro organism.
9:01
Speaker 1: I think the right neutrinos don't want to interact with
9:04
Speaker 1: um other particles like they should do. From what I've
9:07
Speaker 1: heard about sterile neutrinos, and like the word itself, um,
9:11
Speaker 1: I think they're just neutrinos that don't really interact with anything,
9:14
Speaker 1: so they're just existing. But I don't really serve much
9:17
Speaker 1: of a purpose sterile little trinos. They are very clear,
9:23
Speaker 1: probably they washed their heads, very awful. Keep social distance well.
9:29
Speaker 1: A neutrino, as far as I know, is um a
9:33
Speaker 1: particle that doesn't interact with anything, so it flies straight
9:38
Speaker 1: through and apparently it's flying through me right now, So
9:41
Speaker 1: that already seems pretty sterile. So I'm not sure how
9:45
Speaker 1: it could get more sterile because it doesn't really interact
9:49
Speaker 1: with matter, and I've not heard of sterile neutrinos. I
9:52
Speaker 1: feel like I have heard that term before. I know
9:55
Speaker 1: neutrinos come in three flavors, but I'm not sure what
9:58
Speaker 1: would make one st or how we would know. I mean,
10:02
Speaker 1: who's checking anyway. Ordinary neutrinos can only interact with other
10:08
Speaker 1: particles by means of the weak interaction. Sterile neutrinos don't
10:14
Speaker 1: interact with other particles at all. I assume that sterile
10:19
Speaker 1: neutrinos are neutrinos that are even more neutral than regular neutrinos,
10:25
Speaker 1: but I have no idea, all right, I am totally
10:28
Speaker 1: with the person here who says that this combination of
10:31
Speaker 1: words doesn't make any sense to me. I think that's
10:34
Speaker 1: a direct message to you, Daniel and to all physicists
10:38
Speaker 1: to maybe take words that make more sense. There are
10:42
Speaker 1: a lot of people who were totally misled by this phrase,
10:44
Speaker 1: but a couple of people actually got it right on
10:47
Speaker 1: the nose. So this pair of words did lead a
10:50
Speaker 1: few people to the right idea. Who got it right
10:53
Speaker 1: The people who said it doesn't make any sense, the
10:57
Speaker 1: people who doubted the ability of physicists to name things accurately. No,
11:01
Speaker 1: it's all about the interactions, right. What can particles do?
11:04
Speaker 1: They can fly through the universe and they can interact,
11:06
Speaker 1: So sterility refers to whether or not they interact with
11:09
Speaker 1: other particles. Well, let's dig into it, and let's start
11:12
Speaker 1: with the basics. First of all, what is an netrino?
11:15
Speaker 1: So neutrino is a really fun, very weird little particle.
11:18
Speaker 1: It's unusual because it's one of the sort of base
11:21
Speaker 1: particles we think of in the universe. There are four
11:24
Speaker 1: of them, the up cork, the down cork, the electron,
11:28
Speaker 1: and then the fourth one is the neutrino. The weird
11:30
Speaker 1: thing is that the neutrino, unlike the other three, does
11:33
Speaker 1: not appear in the atom. It's not technically a sub
11:37
Speaker 1: atomic particle because it's not part of the atom. Take
11:40
Speaker 1: the atom apart has electrons around it, and inside our
11:43
Speaker 1: protons and neutrons. You take those apart, you get up
11:45
Speaker 1: corks and down corks. So to make everything in the universe,
11:48
Speaker 1: you have upcorks, down corks, and electrons. But then there
11:51
Speaker 1: is this other particle out there, the neutrino. It definitely exists.
11:56
Speaker 1: It's created in the heart of the sun during fusion.
11:59
Speaker 1: There's a lot of them out there, but they're not
12:01
Speaker 1: part of the atom, and they're especially weird because they
12:04
Speaker 1: don't have any electric charge and so they don't give
12:07
Speaker 1: off any light, they don't absorb any light. They're essentially invisible. Yeah. Well,
12:12
Speaker 1: I mean, there are a lot of other particles besides
12:14
Speaker 1: the electron and the corks, but this one, is it
12:17
Speaker 1: a different type of particles out there? Is it like
12:20
Speaker 1: a whole different quantum field and it's hold own classification
12:23
Speaker 1: or is it kind of like a cousin of one
12:25
Speaker 1: of the other particles, So it's its own different type
12:28
Speaker 1: of particle at the same level as the upcork, the
12:30
Speaker 1: down cork, and the electron. There are other particles out there,
12:33
Speaker 1: you're right, like the charm cork and the top cork,
12:36
Speaker 1: but those are like reflections of the up cork. And
12:38
Speaker 1: there are muons and towels, but those are reflections of
12:41
Speaker 1: the electron. They're really like four base matter particles, the upcork,
12:45
Speaker 1: the down cork, the electron, and then the new trino,
12:48
Speaker 1: of which there are also three kinds. Now we do
12:51
Speaker 1: group them together, like the two corks we grouped together,
12:54
Speaker 1: and the electron we grouped together with the new trino,
12:57
Speaker 1: because then trino is related to the electron like the electron.
13:00
Speaker 1: It also doesn't feel the strong force, So the neutrino
13:04
Speaker 1: is super weird because it doesn't feel the strong force
13:06
Speaker 1: like corks do. It also doesn't feel electromagnetism like electrons do.
13:11
Speaker 1: The only force that it feels is the weak interaction.
13:14
Speaker 1: So like a neutrino can fly through a magnetic field
13:16
Speaker 1: and not get bent. If you've applied an electric field
13:19
Speaker 1: to a neutrino, it doesn't pull on it. A neutrino
13:22
Speaker 1: can fly through the heart of an atom without interacting
13:25
Speaker 1: with any of the corks and luons inside of it
13:28
Speaker 1: ignores almost all the interactions that make the universe the
13:31
Speaker 1: way that it is. Right, it doesn't feel any charge
13:34
Speaker 1: because I guess the corks feel a positive charge, right,
13:36
Speaker 1: and the electrons feel a negative charge, but the nutrina
13:40
Speaker 1: doesn't feel any charge at all. Right, Yeah, the corks
13:42
Speaker 1: actually do. The corks have really weird charges. The upcork
13:45
Speaker 1: has a charge of plus two thirds it's a fraction,
13:49
Speaker 1: and the down cork has a charge of minus one third.
13:52
Speaker 1: So the upcork is a positive charge to down cork
13:54
Speaker 1: is a negative charge. Really interesting is that their charge
13:57
Speaker 1: difference is one, is exactly one, between two thirds and
14:01
Speaker 1: negative one third. It's the same for the electron and
14:05
Speaker 1: the neutrino. They also have a charge difference of one.
14:08
Speaker 1: In this case, the electron has charged negative one and
14:11
Speaker 1: the neutrino has charged zero. So then trino is the
14:14
Speaker 1: only fundamental matter particle that has no electric charge. And
14:17
Speaker 1: you're pretty sure about that. I guess could it be
14:20
Speaker 1: that it just feels it, but just super weakly or something.
14:22
Speaker 1: That's a super good question. How well do we know it?
14:25
Speaker 1: It's definitely part of the theory that it's exactly zero.
14:29
Speaker 1: How well do we know that experimentally? In the end,
14:32
Speaker 1: we never know anything perfectly well, Right, there's some limit
14:35
Speaker 1: on the charge of the neutrino. It's got to be
14:38
Speaker 1: something super duper duper tiny. Even a very small charge
14:41
Speaker 1: in the neutrino would upset all sorts of things because
14:43
Speaker 1: charge is conserved in the universe. That would mean, for example,
14:46
Speaker 1: if the neutrino had even a little bit of charge,
14:48
Speaker 1: would make it more complicated to keep charge conserved when
14:52
Speaker 1: you produce it together with an electron. For example, like
14:54
Speaker 1: we have a W boson that has charged positive one
14:57
Speaker 1: elects to decay into an electron and a neutrino. But
15:00
Speaker 1: if the neutrino has a little bit of charge, that
15:03
Speaker 1: has to come from somewhere, and that W only has
15:05
Speaker 1: charged one, so it can't create an electron and a
15:07
Speaker 1: neutrino if the neutrino has charged so we're pretty confident
15:11
Speaker 1: has zero charge, but you can never know absolutely for sure.
15:15
Speaker 1: But it does feel the weak force, which is part
15:17
Speaker 1: of the electromagnetic force, but it's kind of not it's
15:20
Speaker 1: like a sister force kind of yeah, it's not part
15:22
Speaker 1: of electromagnetism. It's part of a combined idea called electroweak,
15:26
Speaker 1: which puts electromagnetism and the weak force together into a
15:30
Speaker 1: single concept. It shows that mathematically, they obey some larger symmetries,
15:35
Speaker 1: some rules that tie them together, the same way that
15:37
Speaker 1: we think electricity and magnetism don't make as much sense
15:40
Speaker 1: separate as they do together, because we see that magnetic
15:44
Speaker 1: fields create electric fields and electric fields create magnetic fields.
15:47
Speaker 1: In that same way, it makes more sense to put
15:49
Speaker 1: the weak force together with electromagnetism into one holistic thing
15:53
Speaker 1: called electroweak. Right, and you said neutrinos are matter particles,
15:56
Speaker 1: which means they do actually have mass. Right, They do
15:59
Speaker 1: feel it, right, and they it does take them some
16:02
Speaker 1: time to accelerate. Right. That's a really interesting topic because
16:06
Speaker 1: it's actually one of the few hints we have that
16:08
Speaker 1: the standard model of particle physics is wrong. Neutrinos in
16:12
Speaker 1: our theory are perfectly massless. O. Our theory does not
16:17
Speaker 1: allow for them to have any mass, and yet in
16:19
Speaker 1: our experiments we measure them to have some mass. We
16:22
Speaker 1: know that they do have very tiny little masses, so
16:26
Speaker 1: you're right, they don't travel at the speed of light,
16:28
Speaker 1: but it's not something that we yet understand the masses
16:31
Speaker 1: of neutrinos. Wait, we've measured that they have mass, but
16:33
Speaker 1: in our theory they don't have mass. Don't you have
16:36
Speaker 1: to revise your theory. We do have to revise our theory.
16:38
Speaker 1: We know that the standard model is not correct and
16:40
Speaker 1: we need to somehow modify it to accommodate neutrino masses.
16:44
Speaker 1: But it's pretty tricky, and one way to do that
16:46
Speaker 1: actually is to add a new kind of neutrino called
16:49
Speaker 1: a sterile neutrino, which helps solve the problem, because that's
16:53
Speaker 1: a big question of the day. And just to clarifying,
16:56
Speaker 1: nutrients are not rare in the universe, right There's like
16:58
Speaker 1: a whole bunch of them going through us right now. Yeah,
17:01
Speaker 1: Our Sun is a huge neutrino factory. Fusion produces an
17:05
Speaker 1: enormous number of neutrinos, so many that they carry like
17:09
Speaker 1: one percent of the Sun's energy output in neutrinos. So
17:13
Speaker 1: the Sun isn't just a star that pumps out photons
17:15
Speaker 1: and light. It also pumps out neutrinos. Like you could
17:18
Speaker 1: use neutrinos to navigate around the Solar system because you
17:21
Speaker 1: can use it to see the Sun. There's so many
17:24
Speaker 1: that by the time they get to Earth, there's a
17:26
Speaker 1: hundred billion passing through a square centimeter per second. So
17:30
Speaker 1: if you like, hold out your hands and you count
17:32
Speaker 1: one Mississippi, a trillion neutrinos have passed through your fingernails. Wow.
17:38
Speaker 1: Unfortunately they don't interact with the electrons and the corks
17:41
Speaker 1: in my body. Otherwise would be a toast. Right, Yeah,
17:44
Speaker 1: you'd all get neutrino cancer or we'd be putting on
17:46
Speaker 1: neutrinos screen every time we go outside. All right, Well,
17:49
Speaker 1: those are neutrinos, and it seems like there's a different
17:52
Speaker 1: kind of neutrino called the sterile neutrino, which it sounds
17:55
Speaker 1: like solves a lot of problems in our theory of
17:58
Speaker 1: the universe. So let's get into that. But first let's
18:01
Speaker 1: take a quick break. All right, we're talking about neutrinos,
18:16
Speaker 1: which are plentiful in the universe. They're made in the Sun.
18:20
Speaker 1: The account for one percent of the Sun's output. So
18:22
Speaker 1: it's kind of a big deal in the universe, right,
18:25
Speaker 1: I mean, the universe seems to like making them. Yeah,
18:27
Speaker 1: the universe certainly pumps out a lot of them. They
18:29
Speaker 1: produced also in supernovas. We have a whole episode about
18:31
Speaker 1: how neutrinos let us see what's going on inside Supernova's
18:35
Speaker 1: one of My favorite applications of neutrinos, though, is that
18:38
Speaker 1: it lets us see that the sun is still there
18:41
Speaker 1: at nighttime because neutrinos hardly interact with anything, so they
18:45
Speaker 1: can pass all the way through the Earth. So if
18:47
Speaker 1: you're like up at three am and you're worried that
18:49
Speaker 1: the sun has disappeared, we can actually verify that the
18:52
Speaker 1: sun is still there on the other side of the
18:54
Speaker 1: Earth by looking for its neutrino signal, because that neutrino
18:57
Speaker 1: signal will pass all the way through the Earth to
18:59
Speaker 1: your neutrino detector. You could also just like call your
19:02
Speaker 1: friend in Japan, but that's less cool, or you could
19:04
Speaker 1: wait a few hours. I mean, the sun will rise tomorrow,
19:08
Speaker 1: isn't that what the song said, unless it's like disappeared
19:10
Speaker 1: or been blown up by aliens. And how would you
19:12
Speaker 1: even know if it's three am? Right? This keeps some
19:15
Speaker 1: people up at night, so this is a way for
19:17
Speaker 1: them to feel comfortable, or how how would it matter?
19:19
Speaker 1: You know, we'd all be toast if the sun disappeared,
19:22
Speaker 1: so you might as well sleep peacefully for the last
19:24
Speaker 1: couple of hours of life that you have. If I
19:27
Speaker 1: told you that aliens had blown up the sun, you'd
19:28
Speaker 1: go take a nap. Is that what you're saying? Yeah, basically,
19:33
Speaker 1: I mean what else would you do? Worry about it?
19:36
Speaker 1: Panic called Bruce Willis. So there's something called that's a
19:40
Speaker 1: sterile neutrino, which I guess it's a different kind of
19:43
Speaker 1: new trino. I mean you said there are different kinds
19:45
Speaker 1: of nutrinos already, right, there's three flavors of neutrinos, but
19:49
Speaker 1: this is something different. Yeah, it is something very different.
19:52
Speaker 1: We have three flavors of neutrinos. We call them electron,
19:55
Speaker 1: muan in town neutrinos because they are produced with electrons,
19:59
Speaker 1: muans and towns. Like when a W boson decays into
20:02
Speaker 1: an electron and a neutrino, it doesn't just decay into
20:05
Speaker 1: any random neutrino decays into an electron neutrino, which is
20:08
Speaker 1: different from a muon neutrino. Muon neutrinos the kind produced
20:12
Speaker 1: when a W boson decays into a muon and a neutrino.
20:15
Speaker 1: In that case it produces a muon neutrino. So there's
20:18
Speaker 1: three different kinds electron, nuon, and town neutrinos. Those we
20:22
Speaker 1: know exist. We've definitely seen them. They interact with the
20:25
Speaker 1: weak force. We've measured each of their various kinds. We're
20:29
Speaker 1: sure that they exist. But the idea is maybe there's
20:31
Speaker 1: a fourth kind of neutrino, a sterile neutrino, which doesn't
20:36
Speaker 1: even feel the weak interaction. It doesn't have the strong force,
20:40
Speaker 1: it doesn't have electromagnetism, and it also doesn't have the
20:43
Speaker 1: weak force. It's totally without interactions, which is why they
20:47
Speaker 1: call it sterile. So it's kind of like a fourth
20:49
Speaker 1: kind of nutrio. Maybe step us through this first, though,
20:53
Speaker 1: like the three other kinds of neutrinos, how are they different?
20:56
Speaker 1: Because I think I've heard that we don't actually know
20:59
Speaker 1: their masses or we're not sure what their masses are,
21:02
Speaker 1: So how do we know they're different? So we know
21:04
Speaker 1: that they are different because we know that the universe
21:06
Speaker 1: keeps count of how many electrons there are, how many
21:10
Speaker 1: muons there are, and how many towels there are. Particle physics,
21:13
Speaker 1: we do this all we look for rules that the
21:15
Speaker 1: universe seems to follow, like conservation of momentum. We notice
21:19
Speaker 1: that in particle interactions, the amount of momentum afterwards is
21:22
Speaker 1: the same as the momentum beforehand and we go, oh,
21:25
Speaker 1: that's cool, what does that mean. We have a whole
21:27
Speaker 1: podcast episode about what that means about the universe. So
21:30
Speaker 1: we're always on the lookout for are the rules that
21:33
Speaker 1: are being followed, because then we can try to figure
21:35
Speaker 1: out what that means. And one thing we've noticed is
21:38
Speaker 1: that the universe keeps track of the number of electrons,
21:42
Speaker 1: so you can't just create one electron. If you do so,
21:45
Speaker 1: you have to also create an anti electron. But the
21:47
Speaker 1: electron neutrino is sort of in the same column. So
21:50
Speaker 1: when you produce an electron, if you produce an anti
21:52
Speaker 1: electron neutrino, then the universe is happy and all the
21:56
Speaker 1: accounts balance. That's why we think that there are different neutrinos.
21:59
Speaker 1: That's what it means. Means for these to be different neutrinos.
22:02
Speaker 1: There's the one that balances the electron book, another one
22:05
Speaker 1: that bounces the muon book, another one that balances the
22:07
Speaker 1: tao's books, which so you only think there are three
22:11
Speaker 1: different neutrinos. I mean, it sounds like what if just
22:15
Speaker 1: the one neutrino helps balance all the other electron, muon
22:18
Speaker 1: and tow particles, and it's not a universal currency. Well,
22:22
Speaker 1: if we produce just one kind of neutrino, for example,
22:25
Speaker 1: you start with muans. You can produce muon neutrinos from
22:29
Speaker 1: their decays, and then we smash it into a bunch
22:32
Speaker 1: of stuff. It tends to only interact with muans, and
22:35
Speaker 1: the same with electrons. You produce an electron neutrino beam
22:38
Speaker 1: and you smash it into a bunch of stuff. It
22:40
Speaker 1: tends to bounce off only electrons because those like to
22:43
Speaker 1: talk to each other. So we're pretty sure that those
22:46
Speaker 1: different flavors exist. Also, it would be very very weird
22:49
Speaker 1: in our theory if we had three kinds of charge
22:52
Speaker 1: lectons e. Mutao and only one kind of neutral lecton.
22:55
Speaker 1: That's not an argument for it not existing it being weird,
22:58
Speaker 1: but theoretically that to be very strange. But we do
23:01
Speaker 1: have experiments where we can isolate each different kind of
23:04
Speaker 1: neutrino and see that they really do have different properties.
23:07
Speaker 1: And we also know that there are three kinds of
23:09
Speaker 1: neutrinos because we have made measurements of their masses. We
23:13
Speaker 1: know that there are three different neutrino masses, all right,
23:18
Speaker 1: But I think you don't know exactly what those masses
23:20
Speaker 1: are right, we don't know what those masses are. In fact,
23:22
Speaker 1: we've only ever measured the mass differences. We know that
23:25
Speaker 1: there are differences between the masses of neutrinos because those
23:28
Speaker 1: mass differences help the neutrinos change flavor. So, for example,
23:32
Speaker 1: neutrinos produced in the Sun are all electron neutrinos because
23:36
Speaker 1: the Sun has electrons in it and almost no muans.
23:38
Speaker 1: By the time those electron neutrinos get to the Earth,
23:41
Speaker 1: a huge fraction of them have turned into something else
23:44
Speaker 1: like muon neutrinos, and that comes directly from the mass
23:47
Speaker 1: differences in the neutrinos. But it sounds like maybe it's
23:50
Speaker 1: a little bit hypothetical. Still, there's a lot we don't
23:53
Speaker 1: understand about neutrino mixing and oscillation and how this all works,
23:57
Speaker 1: and there's a bunch of things in the theory that
23:58
Speaker 1: don't quite fit together. Yeah, and there's also a bunch
24:00
Speaker 1: of experiments that all contradict each other and have had
24:03
Speaker 1: people arguing for about two decades. That's a physicist favorite activity,
24:06
Speaker 1: so they're pretty happy about that. But you said there's
24:09
Speaker 1: maybe a fourth kind of neutrino, or we know for
24:12
Speaker 1: sure there is a fourth kind of matrino called a
24:14
Speaker 1: sterile neutrino, or as they say in England, sterile neutrino.
24:18
Speaker 1: So we don't know for sure if there is a
24:20
Speaker 1: sterile neutrino. We have good reasons to suspect it, both
24:24
Speaker 1: theoretically and experimentally. We have a lot of hints from
24:28
Speaker 1: both directions. We definitely do not know for sure. Sterile
24:31
Speaker 1: neutrinos are still hypothetical, and there's a lot of very strong,
24:34
Speaker 1: very different opinions about the strength of the evidence for them.
24:38
Speaker 1: They're more hypothetically the other nutrina. Yeah, exactly, they're more hypothetical.
24:44
Speaker 1: And these things are really hard to observe because the
24:46
Speaker 1: only interaction they have is gravity, right, which, as we know,
24:50
Speaker 1: is the weakest force, And so that makes it very
24:52
Speaker 1: very difficult to like study sterile neutrinos directly or to
24:55
Speaker 1: discover them. All Right, well, it sounds like they're double
24:58
Speaker 1: they're hypothetical. Well, what makes us think they might exist
25:01
Speaker 1: or why did we give him a name and think
25:03
Speaker 1: that they are out there? So they started out in
25:06
Speaker 1: the theory, it's an idea of people were playing with.
25:08
Speaker 1: One of the central mysteries is why neutrino masses are
25:12
Speaker 1: so small, and actually how they get mass in the
25:14
Speaker 1: first place. Like most of the particles that are out
25:17
Speaker 1: there in the standard model, the electron, the corks, we
25:20
Speaker 1: think they get their masses by interacting with the Higgs boson.
25:23
Speaker 1: The electron flies through this field which fills the universe,
25:27
Speaker 1: the Higgs field, and it interacts with it a little bit,
25:29
Speaker 1: and that changes how it moves. The pure theoretical electron
25:32
Speaker 1: by itself has no mass, but the effective electron, the
25:35
Speaker 1: one that moved through the Higgs field, moves as if
25:38
Speaker 1: it had mass, and that's actually what we call mass.
25:41
Speaker 1: So that's how an electron gets mass, and that's how
25:43
Speaker 1: an upcork gets mass. But we don't think that happens
25:46
Speaker 1: for neutrinos. What do you mean you don't think neutrinos
25:48
Speaker 1: interact with the Higgs field. I thought we knew for sure,
25:52
Speaker 1: or that they have mass, or you're just talking about
25:54
Speaker 1: stole neutrinos. We're just talking about the standard model of
25:56
Speaker 1: neutrinos right now, we don't understand how they get mass.
26:00
Speaker 1: In our theory, they shouldn't have any mass, and that's
26:03
Speaker 1: because in order to interact with the Higgs field, and
26:05
Speaker 1: to get mass the way that electrons do and corks do,
26:08
Speaker 1: you have to have two different kinds of the particle.
26:10
Speaker 1: One time we talked about how the Higgs field can't
26:12
Speaker 1: give masses to particles that are their own antiparticle, because
26:15
Speaker 1: the Higgs field has to talk to two different sides
26:18
Speaker 1: of the particle, has to talk to the particle and
26:20
Speaker 1: the anti particle. But even more than that, particles have
26:23
Speaker 1: another quality to them. It's called holicity or chirality. Particles
26:26
Speaker 1: can be left handed or right handed, and to talk
26:29
Speaker 1: to the Higgs field, to get your mass from the
26:30
Speaker 1: Higgs field, you have to have both right and left
26:33
Speaker 1: handed versions of the particle. And that's true for the
26:36
Speaker 1: electron and for the up cork and for the down cork,
26:38
Speaker 1: but it's not true for the neutrino. Neutrinos only ever
26:41
Speaker 1: exist as left handed versions of the particle. Well, it's long,
26:45
Speaker 1: I'm not underhanded, it's I guess it's all right, But
26:48
Speaker 1: I think I'm getting a little loss in all this
26:50
Speaker 1: term anology. Here. You're saying that in neutrinos are do
26:54
Speaker 1: have mass, right, you confirmed that with experiments, it seems
26:57
Speaker 1: like and but it's very little mass. But that's weird
27:00
Speaker 1: because your theory says that they should have zero mass.
27:04
Speaker 1: Why does the theory say they should have zero mass,
27:06
Speaker 1: because neutrinos only interact via the weak force, and the
27:09
Speaker 1: weak force only talks to left handed particles. We have
27:12
Speaker 1: an episode about parody violation, how the weak force doesn't
27:15
Speaker 1: look the same in the mirror, and that's because the
27:18
Speaker 1: weak force has this really weird property where it ignores
27:21
Speaker 1: right handed particles and only talks to left handed particles.
27:24
Speaker 1: And this handedness refers to like the direction of the
27:27
Speaker 1: particle's motion relative to the direction of its quantum spin,
27:31
Speaker 1: and those two things translate differently in the mirror, so
27:33
Speaker 1: like a left handed particle in the mirror becomes a
27:35
Speaker 1: right handed particle. And so neutrinos are only left handed,
27:38
Speaker 1: and the weak force only talks too left handed particles,
27:42
Speaker 1: and so in our theory, only left handed neutrinos should exist,
27:45
Speaker 1: which means there should be no right handed neutrinos, which
27:48
Speaker 1: means they don't talk to the Higgs field, so they
27:50
Speaker 1: should have zero mass, and yet we see that they
27:53
Speaker 1: do have mass. So there's definitely some gap in the theory.
27:56
Speaker 1: Right the theory is not describing the universe that we see. Well,
28:00
Speaker 1: maybe you're just you know, wrong about that. Maybe you
28:02
Speaker 1: can interact with the Higgs field with the other kind
28:05
Speaker 1: of particle, you know, like what made you think that
28:07
Speaker 1: it can only the Higgs can only interact with one
28:09
Speaker 1: type of spinning particle and not the other if you
28:12
Speaker 1: have experimental evidence that it does. Well, we know that
28:15
Speaker 1: it has mass, but the Higgs field really mathematically just
28:17
Speaker 1: cannot give mass to particles that don't have a right
28:20
Speaker 1: handed component. But it does right well. We don't know
28:23
Speaker 1: that it gets its mass from the Higgs field. We
28:24
Speaker 1: know that it has mass. There are other ways for
28:27
Speaker 1: particles to get mass. We talked once about whether the
28:29
Speaker 1: neutrino is a myorona particle. Maybe it is its own antiparticle,
28:33
Speaker 1: so that's a different way to get mass. That's one
28:35
Speaker 1: possibility that maybe the neutrino gets mass but not from
28:38
Speaker 1: the Higgs field. Or maybe we're wrong about the neutrino
28:41
Speaker 1: not having a right handed component. Maybe there is a
28:44
Speaker 1: right handed neutrino out there. The weak force doesn't create
28:47
Speaker 1: it or talk to it, but maybe it still exists,
28:50
Speaker 1: and so that allows the neutrino to get mass from
28:52
Speaker 1: the Higgs field, all right, So that's one mystery how
28:55
Speaker 1: does the neutrino get its mass? And it's also kind
28:57
Speaker 1: of a mystery why that mass is so small, right,
29:00
Speaker 1: because it's a lot lighter than all the other particles.
29:03
Speaker 1: It would be really weird if the neutrino got its
29:06
Speaker 1: mass from the Higgs field and then it got such
29:08
Speaker 1: a tiny little serving of mass. Right, The neutrinos are
29:11
Speaker 1: like less than one millions the mass of the electron,
29:14
Speaker 1: which is already very very light compared to like the proton.
29:17
Speaker 1: So we have this really deep question in particle physics
29:19
Speaker 1: about why particles are interacting with the Higgs field at
29:22
Speaker 1: different strengths. And the neutrino would be like the outlier,
29:26
Speaker 1: be even crazier than all the other particles. So to
29:29
Speaker 1: make it sort of hang together more crisply mathematically, people
29:32
Speaker 1: like adding another neutrino. They say, what if there's another
29:36
Speaker 1: kind of neutrino and this one is only right handed,
29:39
Speaker 1: so it doesn't interact with the weak force at all.
29:42
Speaker 1: So this idea that maybe they're right handed neutrinos out
29:44
Speaker 1: there gets extended to like creating a pure right handed neutrino,
29:48
Speaker 1: a sterile neutrino. If you put that in and calculate
29:52
Speaker 1: all your equations, then it tends to pull on the
29:54
Speaker 1: masses of the other neutrinos and make them small. So
29:57
Speaker 1: the idea of a stero neutrino would explain how new
30:00
Speaker 1: trinos get mass and why they are so small. All right,
30:03
Speaker 1: So we have those two mysteries, why it neutrinos have
30:06
Speaker 1: mass and why they are so light. But you're saying
30:09
Speaker 1: the solution is to create a whole different kind of neutrinos.
30:11
Speaker 1: How would it hold different kind of natrino explain the
30:14
Speaker 1: mysteries and the other kinds of neutrinos. Well, it would
30:16
Speaker 1: demonstrate that right handed neutrinos exist, and that helps us
30:19
Speaker 1: understand how neutrinos get mass, because maybe we were wrong
30:23
Speaker 1: that neutrinos are only left handed. Maybe they all do
30:25
Speaker 1: have a little bit of a right handed component. It's
30:28
Speaker 1: possible for them to then talk to the Higgs field
30:30
Speaker 1: and to get mass. Then if you have a special
30:32
Speaker 1: fourth kind of neutrino through some mathematical mixing, it turns
30:36
Speaker 1: out that it gets most of the mass and it
30:38
Speaker 1: becomes very very heavy. And because the seesaw mechanism with
30:42
Speaker 1: like it sits on one side very very heavy, it
30:43
Speaker 1: makes the other particles very very light. Oh, I see,
30:47
Speaker 1: you've made up this new kind of neutrino because just
30:50
Speaker 1: to like balance out the weirdness of the other ones.
30:53
Speaker 1: It's like, all the other ones seem to be left handed.
30:55
Speaker 1: Maybe there's a huge ginormous right hand in the trin
30:58
Speaker 1: on the other side that's uh, you know, kind of
31:01
Speaker 1: taking up all of the neutrino mass in the universe.
31:04
Speaker 1: Is that kind of what you mean? Yeah, that's exactly it.
31:06
Speaker 1: And it's sort of a weird idea, but it also
31:09
Speaker 1: kind of works. Like this is part of the process
31:11
Speaker 1: of physics. It's just being creative and being like, well,
31:14
Speaker 1: what about this crazy idea? Now that doesn't work, And
31:16
Speaker 1: this is a crazy idea somebody had one day and
31:18
Speaker 1: they cranked it through the math and dot com. This
31:20
Speaker 1: actually kind of works. It's not that complicated, and it
31:23
Speaker 1: would explain what we're seeing. So it became sort of
31:26
Speaker 1: attractive theoretically for that reason. But I guess if there
31:31
Speaker 1: does exist humongous right handed netrino out there and tun
31:36
Speaker 1: when you have seen it already, wouldn't be like extra obvious.
31:39
Speaker 1: So that's the other fun clue. But Darren neutrinos is
31:41
Speaker 1: that maybe they are the dark matter. Maybe we have
31:45
Speaker 1: been seeing them right. We know that there is some
31:49
Speaker 1: sort of particle out there that only feels graft. It
31:52
Speaker 1: has no other kinds of interactions. We've been calling it
31:55
Speaker 1: dark matter on the podcast for years, and people have
31:58
Speaker 1: been talking about in physics for decades. We know that
32:00
Speaker 1: most of the mass in the universe is this new,
32:02
Speaker 1: weird kind of thing, and so sterile neutrino really fits
32:05
Speaker 1: that bill right. One reason we don't think that normal
32:09
Speaker 1: neutrinos e. Mute town neutrinos are the dark matter is
32:13
Speaker 1: because they have very very low mass because they fly
32:16
Speaker 1: through the universe very very fast, and so they would
32:19
Speaker 1: have given a different shape to the universe. It wouldn't
32:21
Speaker 1: get like the clumpiness that we see in the universe today.
32:23
Speaker 1: They're too hot, is what we say. But this could
32:26
Speaker 1: be cold dark matter. They could be very massive and
32:29
Speaker 1: just sort of like hanging out there. So the universe
32:31
Speaker 1: could be filled with these sterile neutrinos. You ask, why
32:34
Speaker 1: wouldn't we have seen them, Well, maybe we have been
32:36
Speaker 1: Maybe they are the dark matter. What it's like a
32:39
Speaker 1: huge plot twist if it turns out the dark matter
32:42
Speaker 1: was just a big neutrino in the universe. So let's
32:45
Speaker 1: get into this idea a little bit more and whether
32:48
Speaker 1: or not we've seen these in experiments. But first let's
32:51
Speaker 1: take another quick break. We're in the third act of
33:06
Speaker 1: the movie here, which just revealed the big plot twist
33:09
Speaker 1: in our story here. It turns out the dark matter
33:13
Speaker 1: of the universe that we have no idea what it is.
33:15
Speaker 1: Maybe we do know what it is, and it could
33:17
Speaker 1: be maybe a fourth kind of neutrino, which kind of
33:20
Speaker 1: totally fits the bill right. It's like an almost invisible
33:23
Speaker 1: type of matter. It's heavy, and that white did means
33:25
Speaker 1: you think of this before. It's been a candidate for
33:31
Speaker 1: dark matter for a while, but it's become more popular
33:34
Speaker 1: in the last couple of decades because we have a
33:36
Speaker 1: bunch of really confusing experimental results in neutrino physics that
33:41
Speaker 1: have been hinting towards a sterile neutrino, but then also
33:44
Speaker 1: kind of contradicting other experiments. So it's a bit of
33:47
Speaker 1: a confusing landscape right now about whether the experiments are
33:49
Speaker 1: consistent with a sterile neutrino or not? Wait, which experiments?
33:53
Speaker 1: The dark matter experiments tell us there is dark matter
33:55
Speaker 1: out there. We know that it's out there, we know
33:57
Speaker 1: how much it is, but we don't know what particle
33:59
Speaker 1: it's made out of, or if it is even made
34:01
Speaker 1: out of a particle. The dark matter experiments have never
34:03
Speaker 1: seen a particle of dark matter. But there are people
34:06
Speaker 1: studying neutrinos in great detail, watching them change from one
34:11
Speaker 1: kind to another, electrons to muans, two towers to back,
34:14
Speaker 1: and they've been seeing some weird things that suggest maybe
34:17
Speaker 1: there's another kind of neutrino out there that would help
34:20
Speaker 1: explain these weird neutrino experiments. And that could also maybe
34:25
Speaker 1: be the dark matter that we're seeing. Right, that's the
34:27
Speaker 1: idea exactly. Maybe it all come together into a beautiful
34:30
Speaker 1: resolution at the end of the movie, right, Yeah, bring
34:33
Speaker 1: together the A B storylines. That's that's always the coal.
34:36
Speaker 1: But um, I guess one question is why are they
34:38
Speaker 1: called sterile matrinos or sterile if you're in England, are
34:41
Speaker 1: there neutrinos who are not sterile or reproductive or fertile
34:45
Speaker 1: or dirty? Well, there are neutrinos that interact electron muan
34:49
Speaker 1: in town neutrinos do feel the weak force, so we
34:53
Speaker 1: are capable of detecting them. If you send a bunch
34:56
Speaker 1: of electron neutrinos into a huge vat of liquid, one
35:00
Speaker 1: out of a billion of them will bang into an
35:02
Speaker 1: electron and will give it some recoil. So you can
35:04
Speaker 1: see that if you send muon neutrinos into a huge
35:08
Speaker 1: vat of liquid, one of them will bang into a
35:10
Speaker 1: muan if it happens to be created as a virtual
35:12
Speaker 1: particle temporarily, so we can see electron muan and town
35:16
Speaker 1: neutrinos sort of indirectly. But sterile neutrinos don't have any
35:19
Speaker 1: interaction at all other than gravity, so we can't do
35:22
Speaker 1: particle physics experiments with them. But what we can do
35:25
Speaker 1: is try to see if there's a missing part of
35:27
Speaker 1: the story. We watch electron Muan and town and Trina
35:30
Speaker 1: sort of turn into each other back and forth, and
35:33
Speaker 1: we try to notice if somebody accounting doesn't seem to
35:35
Speaker 1: be adding up, is like a missing person. It sounds
35:38
Speaker 1: sort of like the definition of a sterile metrino is
35:40
Speaker 1: the same definition is dark matter. I mean you're saying
35:43
Speaker 1: asteria lutrino is massive, it has mass, it feels gravity,
35:46
Speaker 1: but it doesn't feel the weak force, which is a
35:50
Speaker 1: sort of kind of what devolves. It's turn about about
35:52
Speaker 1: dark matter, and so you can't see this potential type
35:55
Speaker 1: of neutrino, and so you're saying there might be other
35:58
Speaker 1: ways to see it. Yeah, there might be other ways
36:00
Speaker 1: to see it. There are various candidates for dark matter.
36:02
Speaker 1: You know, some generic particle out there that only feels gravity.
36:05
Speaker 1: This is a special type of dark matter candidate because
36:08
Speaker 1: it's not just a generic particle. It's a neutrino, and
36:11
Speaker 1: it actually does mix with the other kinds of neutrinos.
36:14
Speaker 1: See neutrinos do this really weird thing, which is that
36:16
Speaker 1: they mix with each other. We've been talking about neutrinos
36:19
Speaker 1: in two different ways. There's different flavors of neutrinos Electron, Muan,
36:23
Speaker 1: and tao. And then this is the different masses of neutrinos,
36:26
Speaker 1: like call those one, two, and three. The super duper
36:29
Speaker 1: confusing thing about neutrinos is that those are not the
36:31
Speaker 1: same things. Like we know there are three different mass neutrinos,
36:34
Speaker 1: and we know there are three different flavor neutrinos. But
36:37
Speaker 1: it's not the case that electron neutrino has one specific
36:40
Speaker 1: mass and the muon neutrino has another specific mass and
36:43
Speaker 1: the town neutrino has a third specific mass. Instead, they're
36:46
Speaker 1: all mixtures of each other. So the electron neutrino is
36:49
Speaker 1: a mixture of the three different neutrino masses. Wait, what
36:52
Speaker 1: do you mean? What? How can something have different masses?
36:54
Speaker 1: What even the words different masses have mean? Make some
36:59
Speaker 1: one thing having three different masses? What does that mean? Yeah, So,
37:01
Speaker 1: as you fly through the universe, your mass determines how
37:05
Speaker 1: you move, right, So that's the property of a particle.
37:07
Speaker 1: It doesn't determine how you move. It determs how you
37:10
Speaker 1: speed up or slow down. Right, If you're just moving
37:11
Speaker 1: through the universe, your mass is kind of irrelevant. Yeah,
37:15
Speaker 1: that's what I mean. It determines how you're creative, determines
37:17
Speaker 1: how fast you go based on your energy. This kind
37:20
Speaker 1: of stuff. The mass is part of the property determines
37:23
Speaker 1: how you move through the universe and includes acceleration and
37:26
Speaker 1: momentum and all that kind of stuff. So neutrinos have
37:28
Speaker 1: different kinds of masses. Yeah, normally particles are created with
37:31
Speaker 1: a definite mass. You create an electron, it has a
37:33
Speaker 1: certain mass. It flies through the universe. Happy you create
37:36
Speaker 1: an electron neutrino, it's a combination of three different masses.
37:40
Speaker 1: This is super duper weird, and so as it flies
37:43
Speaker 1: through the universe, those masses do move differently. And because
37:47
Speaker 1: the muon neutrino is a different combination of those masses,
37:51
Speaker 1: that's how electron neutrinos can turn into muon neutrinos as
37:55
Speaker 1: they fly through space. It's sort of like two different
37:58
Speaker 1: ways to see the neutrinos. Imagine you have like you know,
38:01
Speaker 1: an X and Y axis that's like the masses, and
38:04
Speaker 1: now you have like a rotated version like X prime
38:07
Speaker 1: and y prime. You create something along the y prime axis.
38:10
Speaker 1: It's like a mixture of X and y and so
38:13
Speaker 1: in the same way, electron neutrinos are a mixture of
38:16
Speaker 1: the three different masses. Muon neutrinos are a different mixture
38:19
Speaker 1: of those masses. He lost me a little bit there
38:21
Speaker 1: with the rotating xs that, but I guess I'm trying
38:24
Speaker 1: to understand what you mean by like having a combination
38:27
Speaker 1: of different masses. What does it mean like I'm heavy
38:29
Speaker 1: and I'm light at the same time. Or are these
38:30
Speaker 1: like types of masses. No, that's exactly what it means.
38:33
Speaker 1: Electron neutrinos. These are quantum particles, right, so they are
38:36
Speaker 1: a superposition of three different masses. I'm heavy and light
38:40
Speaker 1: at the same time. What does that mean? Doesn't energy
38:42
Speaker 1: need to be conserved? How can I switch back and forth?
38:44
Speaker 1: Willy Nili, Well, the different states have different energies, right,
38:48
Speaker 1: They have different masses and different energies in the same
38:50
Speaker 1: way the particle can be like, Oh, I'm a combination
38:52
Speaker 1: of spin up and spin down. Electron neutrinos are a
38:55
Speaker 1: combination of three different masses, and muon neutrinos are a
38:58
Speaker 1: different combination of those masses. Oh, you mean quantum mechanically combined, right,
39:04
Speaker 1: like probabilistically combined. Like it could be when I measure
39:07
Speaker 1: it could be heavy, light, or medium. But since it's
39:10
Speaker 1: quantum and I haven't measured it, you say that it's
39:12
Speaker 1: a combination of the three. Yeah, But there's another layer
39:15
Speaker 1: of nuance, which is about how you measure it. If
39:18
Speaker 1: you measure it, if you try to measure its mass,
39:20
Speaker 1: then you get either mass one, two, or three. If
39:22
Speaker 1: you're trying to measure using the weak force, then you
39:24
Speaker 1: get electron, muan or tow. Right, So the weak force
39:28
Speaker 1: looks at these neutrinos differently than mass looks at these neutrinos.
39:32
Speaker 1: There's like a rotation between them. There's like offset. The
39:35
Speaker 1: weak forces like twisted in this strange way, so the
39:38
Speaker 1: neutrinos it likes to create are not lined up with
39:41
Speaker 1: the masses of the neutrinos. There this weird twist you mean,
39:44
Speaker 1: like they're not correlated. It's not like the tow is
39:47
Speaker 1: always one type of mass and the men is another
39:51
Speaker 1: type of mass. It's like each of the three kinds
39:53
Speaker 1: have different you know, probabilities of being three different masses exactly.
39:57
Speaker 1: So we do these experiments where we see one count
40:00
Speaker 1: of neutrino turning into another one. What I mean, it
40:02
Speaker 1: turns into another one, like it flies through the air
40:04
Speaker 1: and it turns into a muon or a tower or
40:06
Speaker 1: something like that. That's exactly right. An electron neutrino produced
40:10
Speaker 1: in the Sun, by the time it gets to the Earth,
40:12
Speaker 1: a big fraction them are now muon neutrinos. Why, because
40:16
Speaker 1: they're created with these superposition of different masses and those
40:19
Speaker 1: different masses moved through the universe differently, right, because they
40:22
Speaker 1: have different masses, and so by the time it gets
40:25
Speaker 1: to the by the time it gets to the Earth,
40:26
Speaker 1: it's a different combination of those masses which has a
40:30
Speaker 1: good chance to line up with the Muon kind of neutrino.
40:32
Speaker 1: So that's how they can change. They can change from
40:34
Speaker 1: one flavor into another because the masses are not all
40:37
Speaker 1: the same, so they fly through the universe differently. Wait
40:40
Speaker 1: do they actually change or is it again one of
40:42
Speaker 1: these quantum mechanical tricks where it's like it has the
40:45
Speaker 1: probability of being one of the three, but it's one
40:49
Speaker 1: depending on how you measure it. Well, it's created, it
40:51
Speaker 1: has chance to being an electron neutrino as it flies
40:54
Speaker 1: through the universe. That probability changes when you measure it
40:58
Speaker 1: after one a U it has like a sixty chance
41:01
Speaker 1: of being a Muan neutrino or in a five chance
41:04
Speaker 1: of being a town neutrino. And so yes, it's a
41:06
Speaker 1: quantum mechanical trick, but also that's the trick that describes
41:09
Speaker 1: our universe. But then you say, these things have to
41:11
Speaker 1: be conserved, so like if one of them changes from
41:14
Speaker 1: a meal into a tower or something. Doesn't that, you know,
41:17
Speaker 1: throw the balance sheet of the universe off, like if
41:19
Speaker 1: a dollar turned into a British pound exactly. And that's
41:22
Speaker 1: why it was such a huge discovery because it breaks
41:25
Speaker 1: the standard model. In the standard model, these things are
41:27
Speaker 1: totally conserved, but in the universe they are not. This
41:31
Speaker 1: breaks that rule. You're exactly right, very sharp point. And
41:34
Speaker 1: so we know that the standard model is violated, right,
41:37
Speaker 1: because neutrino oscillations, that's what this is called, breaks the
41:41
Speaker 1: conservation of lefton number. And that's why people won several
41:44
Speaker 1: Nobel Prizes for these discoveries because it's a hint that
41:47
Speaker 1: the standard model is wrong. I feel like Daniel, this
41:50
Speaker 1: whole episode has been us saying there's a rule, but
41:53
Speaker 1: you know, it turns out that rule doesn't work. So
41:56
Speaker 1: why even have rules? Maybe you shouldn't call them rules
41:59
Speaker 1: such a questions. Yeah, there you go. We were looking
42:02
Speaker 1: for the suggestions of the universe, not the laws of
42:04
Speaker 1: the And to me, the really fascinating part experimentally is
42:07
Speaker 1: that we look at all these experiments that see one
42:10
Speaker 1: kind of neutrino changing into another, and they don't all
42:12
Speaker 1: make sense. Like there was one experiment in Los Almos
42:15
Speaker 1: in the nine nineties they trying to see muon neutrinos
42:18
Speaker 1: turning into electron neutrinos, and they saw way too many.
42:21
Speaker 1: We couldn't explain them even with a modified version of
42:23
Speaker 1: the standard model that had three neutrino types in it.
42:26
Speaker 1: But you can explain it if you add a fourth
42:28
Speaker 1: type of neutrino, a sterile neutrino, would explain that experiment.
42:33
Speaker 1: It like helps participate in this weird mixing that neutrinos
42:36
Speaker 1: do with each other and would change those rates of
42:39
Speaker 1: mixing and would explain that experiment. Wait, so I guess
42:42
Speaker 1: this helps us get back on the topic of sterile
42:45
Speaker 1: neutrinos because we think it's maybe an interesting possibility that
42:49
Speaker 1: could explain maybe dark matter. But I guess then the
42:51
Speaker 1: question is if they're so hard to see, just like
42:53
Speaker 1: dark matter is hard to see, how are we trying
42:55
Speaker 1: to see this kind of neutrino. And you're saying that
42:58
Speaker 1: maybe you can see it by looking at experiments with
43:00
Speaker 1: regular neutrinos, right, I think that's what you're saying, because
43:04
Speaker 1: this fourth potential kind of neutrino can also turn into
43:07
Speaker 1: the other three kinds of neutrinos you can't directly, but
43:10
Speaker 1: it's also a weird mixture of the mass states. If
43:13
Speaker 1: it turns out there's four different masses of neutrinos, and
43:16
Speaker 1: stero neutrinos are a weird mixture of those four mass states.
43:20
Speaker 1: So the short version is it changes how the other
43:22
Speaker 1: two neutrinos turned into each other. Like stero neutrinos can
43:25
Speaker 1: turn sometimes into electron neutrinos, muon neutrinos could be turning
43:29
Speaker 1: into stero neutrinos because of this weird mixing that neutrinos
43:32
Speaker 1: can do with each other. So it's sort of like
43:34
Speaker 1: discovering that there's a fourth person in your conversation but
43:37
Speaker 1: you haven't been hearing them, but it explains what everybody
43:39
Speaker 1: else has been doing, you know, why they've been acting
43:42
Speaker 1: sort of strangely. So the muon, electron and town atrinos
43:45
Speaker 1: have been acting kind of weird. We can't explain how
43:47
Speaker 1: they're turning into each other and back and forth without
43:50
Speaker 1: invoking this other fourth neutrino I see. So in the
43:54
Speaker 1: experiments that you've have been doing with neutrinos, there's something
43:57
Speaker 1: maybe missing some unexplainable things, and you think that me me,
44:00
Speaker 1: this new neutrino would fix it. But I mean, if
44:02
Speaker 1: it is kind of part of dark matter, wouldn't that
44:04
Speaker 1: mean there are there's a lot of this fourth kind
44:07
Speaker 1: of a netrino, Like, wouldn't that just totally throw the
44:09
Speaker 1: balances off because there is of the universe that is
44:12
Speaker 1: supposed to be dark matter. Wouldn't that be a huge
44:15
Speaker 1: amount of sterile neutrinos. Yes, absolutely, it would be a
44:18
Speaker 1: huge amount of steril neutrinos, and it would throw a
44:20
Speaker 1: lot of other things off. Like people who study the
44:22
Speaker 1: early universe, they can actually measure how many different kinds
44:26
Speaker 1: of neutrinos there are because it would affect like how
44:29
Speaker 1: the universe expanded very very early on, and those measurements
44:33
Speaker 1: are very conclusive. There are exactly three kinds of neutrinos
44:36
Speaker 1: and no more and no room for any other kind,
44:38
Speaker 1: thank you very much. So the very early universe picture
44:41
Speaker 1: from like studying the cosmic microwave background says there's absolutely
44:44
Speaker 1: only three kinds of neutrinos. But then we have this
44:47
Speaker 1: weird experiment from Los Almos that can only be explained
44:50
Speaker 1: using a sterile neutrino or maybe they messed it up.
44:56
Speaker 1: That's the other possibility. I guess maybe they just didn't
44:59
Speaker 1: care at the zero or something. Well, you know, these
45:01
Speaker 1: experiments are very very hard to do, and they were
45:04
Speaker 1: doing something nobody had ever done before, and it was
45:06
Speaker 1: a big puzzle. And so they actually did a follow
45:08
Speaker 1: up experiment like ten years later. It was called Mini Boon,
45:12
Speaker 1: where they used like a lot of the same devices,
45:14
Speaker 1: but they made it a little bit more clever to
45:15
Speaker 1: see if they could like nail this down. They changed
45:18
Speaker 1: it so they weren't just looking at muon neutrinos, they
45:20
Speaker 1: were looking also at anti muon neutrinos. And I remember
45:23
Speaker 1: being in the room at Fermilab when they unveiled the
45:26
Speaker 1: results of this experiment. It's like fifteen years ago, and
45:29
Speaker 1: everybody was wondering, like, are we going to see conclusive
45:31
Speaker 1: evidence us there on neutrinos, And unfortunately what they saw
45:34
Speaker 1: doesn't agree with the original Los Almos experiment, but it
45:38
Speaker 1: also doesn't agree with the standard model. So it's like
45:41
Speaker 1: they saw something else weird that they can't explain. So
45:44
Speaker 1: then we had like two mysteries that were inconsistent with
45:47
Speaker 1: each other. But from the same group of people. So yeah,
45:52
Speaker 1: it was a bit of a puzzle what was going
45:53
Speaker 1: on there, and they didn't carry the two this time. Yeah,
45:57
Speaker 1: And you know, sociologically it's fascinating because there's some very
46:00
Speaker 1: prominent neutrino physicists in the community, very fancy universities, so
46:05
Speaker 1: they have a lot of credibility. But you know, if
46:07
Speaker 1: you ask neutrino physicists, what do you think is the
46:09
Speaker 1: most likely explanation. Do you think it's star on neutrinos
46:11
Speaker 1: or do you think they've like misunderstood some of the
46:14
Speaker 1: background or mess something up. They start off talking about
46:16
Speaker 1: stering neutrinos, but in the end, I think a lot
46:18
Speaker 1: of people are convinced that these experiments there's something missing
46:21
Speaker 1: about our understanding of them. Well, I mean, the trinos
46:23
Speaker 1: are kind of a big deal right now in physics,
46:25
Speaker 1: and so there are a lot of ma trino experiments
46:27
Speaker 1: going on. Haven't have any of them found any indication
46:30
Speaker 1: that sterling neutrinos exist? Or have they all just been
46:34
Speaker 1: telling us the same picture. All the other experiments are
46:37
Speaker 1: consistent with each other and inconsistent with these two experiments
46:41
Speaker 1: that point to sting netrinos. So Without these two experiments,
46:44
Speaker 1: everything fits together very very nicely. Wait, without these two experiments,
46:47
Speaker 1: we wouldn't even be talking about sterl netrinos. Yeah, that's right.
46:51
Speaker 1: I mean, theoretically they're very attractive and they could explain
46:53
Speaker 1: the dark matter. But the early universe says no to
46:56
Speaker 1: star on the trinos. All the other neutrino experiments out there,
46:59
Speaker 1: not led by the group of people, are all consistent
47:01
Speaker 1: with each other and say no to stare on neutrinos,
47:03
Speaker 1: even very specifically cross checking the exact kind of stare
47:06
Speaker 1: on the trino predicted by this Los Almos experiment and
47:09
Speaker 1: the miniboon experiment. But these two other experiments, you know,
47:13
Speaker 1: we don't understand their results. Oh man, I feel a
47:16
Speaker 1: little bit let down. I feel like they shouldn't be
47:17
Speaker 1: called ster on lutrinos. Maybe they should just be called
47:19
Speaker 1: futile because it sounds like I'm not really believe that
47:24
Speaker 1: these exists, Right, You're not painting a very convincing picture
47:26
Speaker 1: from me here. Well, I'll say, you know, personally, I'm
47:29
Speaker 1: not convinced by these things at all. When I was
47:31
Speaker 1: in the room and they unveiled the results of this
47:33
Speaker 1: second experiment, the follow up one, I thought the results
47:35
Speaker 1: look pretty wonky. Not just the actual results sometimes you
47:38
Speaker 1: see a surprise from the universe, but they had a
47:40
Speaker 1: bunch of control regions where they were supposed to be
47:43
Speaker 1: able to understand the results if they knew what they
47:44
Speaker 1: were doing, and those also looked wonky. So immediately I
47:48
Speaker 1: was like, I'm just not sure that these folks understand
47:50
Speaker 1: the backgrounds. It's very personal, my personal opinion about these
47:54
Speaker 1: experimental results. And you know, these things are really really
47:57
Speaker 1: hard to do. I'm not saying I know how to
47:59
Speaker 1: do these things better than those experts. I'm just not
48:02
Speaker 1: sure that we really understand everything that's going on in
48:04
Speaker 1: those experiments. Well, I would hear about what happened, and
48:07
Speaker 1: I think that pretty much explains it. Yeah, I think
48:09
Speaker 1: their kitchens were just dirty and not though, and that
48:12
Speaker 1: probably uh you know, ended up correcting their data. Right, Yeah,
48:16
Speaker 1: maybe that's it. Actually, these experiments are amazing technological feats
48:20
Speaker 1: involved like a hundred and sixty tons of pure mineral
48:23
Speaker 1: oil in these vats underground. They're very difficult. You have
48:27
Speaker 1: to be very careful and exacting about your cleanliness to
48:30
Speaker 1: even do these experiments. So I'm pretty sure their kitchen,
48:32
Speaker 1: metaphorically speaking, is pretty clean. Maybe that you said the
48:36
Speaker 1: oven wrong, All right, Well, it sounds like it's another
48:39
Speaker 1: one of these interesting mysteries in physics, as as they
48:42
Speaker 1: seem to be everywhere and where you have some experiments
48:45
Speaker 1: that point to something interesting and some that don't, and
48:49
Speaker 1: if one of them is correct, and Mike tell us
48:52
Speaker 1: a lot about a huge portion of the universe, right Like,
48:54
Speaker 1: if something like this could explain dark matter, it would
48:57
Speaker 1: be a humongous deal. It would be twenty seven per
49:00
Speaker 1: end of the universe. It would be a humongous deal.
49:02
Speaker 1: And even without these experiments, even without sterio neutrinos, we
49:06
Speaker 1: know there's something weird going on with neutrinos, right because
49:09
Speaker 1: we can't explain why they have mass, why they seem
49:12
Speaker 1: to be right handed neutrinos, why they have such a
49:14
Speaker 1: little mass. Neutrinos themselves are very very strange objects. And
49:18
Speaker 1: in the physics community, a lot of people are persuaded
49:21
Speaker 1: that this is the next frontier, this is where we're
49:24
Speaker 1: going to make big discoveries. And that's actually why the
49:26
Speaker 1: United States has decided to make a big bet on
49:29
Speaker 1: neutrino physics. We're not going after like Energy Frontier super
49:33
Speaker 1: high energy colliders were letting the Europeans and the Chinese
49:35
Speaker 1: do that, but the U S community is betting big
49:37
Speaker 1: on neutrino experiments. Alright, well, stay tuned as we learn
49:41
Speaker 1: more about neutrinos, or maybe not learn much about neutrinos.
49:45
Speaker 1: Either way, it would point to some big mysteries in
49:47
Speaker 1: the universe. And we definitely know that the universe seems
49:50
Speaker 1: to be sort of a hot, delicious mess. Doesn't seem
49:52
Speaker 1: to make any sense to us, but maybe one day
49:54
Speaker 1: if we rotate our internal matrice is the right way,
49:57
Speaker 1: it'll all click into place and hopefully be delicious. That's
50:02
Speaker 1: the taste I'm going for in life. Let's open an
50:04
Speaker 1: ice cream shop called Neutrino Flavor. Well, we hope you
50:07
Speaker 1: enjoyed that. Thanks for joining us, See you next time.
50:17
Speaker 1: Thanks for listening, and remember that Daniel and Jorge explained.
50:20
Speaker 1: The Universe is a production of I Heart Radio. For
50:23
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