Latest / Daniel and Kelly’s Extraordinary Universe / Are neutrinos hiding the dark matter?

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Are neutrinos hiding the dark matter? · Daniel and Kelly’s Extraordinary Universe

0:01
Speaker 1: Hey, Jorhan Daniel here, and we want to tell you
0:03
Speaker 1: about our new book. It's called Frequently Asked Questions about
0:07
Speaker 1: the Universe because you have questions about the universe, and
0:09
Speaker 1: so we decided to write a book all about them.
0:12
Speaker 1: We talk about your questions, we give some answers, we
0:14
Speaker 1: make a bunch of silly jokes as usual, and we
0:17
Speaker 1: tackle all kinds of questions, including what happens if I
0:20
Speaker 1: fall into a black hole? Or is there another version
0:22
Speaker 1: of you out there that's right? Like usual, we tackle
0:25
Speaker 1: the deepest, darkest, biggest, craziest questions about this incredible cosmos.
0:29
Speaker 1: If you want to support the podcast, please get the
0:31
Speaker 1: book and get a copy, not just for yourself, but
0:34
Speaker 1: you know, for your nieces and nephews, cousins, friends, parents, dogs, hamsters,
0:39
Speaker 1: and for the aliens. So get your copy of Frequently
0:42
Speaker 1: Asked Questions about the Universe is available for pre order now,
0:46
Speaker 1: coming out November two. You can find more details at
0:49
Speaker 1: the book's website, Universe f a Q dot com. Thanks
0:53
Speaker 1: for your support, and if you have a hamster that
0:55
Speaker 1: can read, please let us know. We'd love to have
0:57
Speaker 1: them on the podcast. Hey Daniel, what kind of floors
1:10
Speaker 1: do you have in your office. Actually, I'm glad you asked,
1:13
Speaker 1: because I'm kind of proud of them. I put wood
1:15
Speaker 1: floors into my office last year. What you like, you
1:18
Speaker 1: did it yourself in your university office? Yeah, I did it,
1:21
Speaker 1: just snuck in and did it on the weekend. Did
1:24
Speaker 1: you ask permission to do it? I figured better to
1:27
Speaker 1: ask forgivenness than permission. Oh man, well, I guess you.
1:31
Speaker 1: You better not mention it on a podcast or anything.
1:33
Speaker 1: I would never be so shortsighted. And let me guys,
1:37
Speaker 1: did the floors you put in? Are they made out
1:39
Speaker 1: of particle boards since you're a particle physicists? But they
1:43
Speaker 1: are particularly snazzy? Are they made out of florine florinos?
1:48
Speaker 1: Or are they fluorescent? They are scintillating. I think reached
1:51
Speaker 1: the floor of the pun space. They're glad to know
1:54
Speaker 1: this a bottom. Hi am or hammy cartoonists and the
2:12
Speaker 1: creator of PhD comics. Hi. I'm Daniel. I'm a particle
2:15
Speaker 1: physicist and a professor UC Irvine, and technically I'm an experimentalist,
2:19
Speaker 1: but I don't actually like to build things. But you
2:21
Speaker 1: are an amateur carpenter, would you say, or a florist
2:25
Speaker 1: I'm definitely an amateur. Nobody is paying me to put
2:28
Speaker 1: in their floors. Welcome to our podcast Daniel and Jorge
2:30
Speaker 1: Explain the Universe, a production of I Heart Radio, in
2:33
Speaker 1: which we dig through the floor of your understanding and
2:36
Speaker 1: into all of the mysteries of the universe. We try
2:40
Speaker 1: to understand everything that's out there above our heads and
2:43
Speaker 1: below our feet and all around us, from the crazy
2:46
Speaker 1: mysteries inside black holes, to the swirling insanity that is
2:50
Speaker 1: our galaxy, to the tiniest particles that are flying through
2:53
Speaker 1: our bodies. That's right. We try to break through the
2:55
Speaker 1: ceilings and the floors of our brains and our human
2:58
Speaker 1: understanding of how the verse works, because there is a
3:01
Speaker 1: lot more than what is immediately around us, beyond our
3:05
Speaker 1: floors and ceilings. Do you think it's possible to hear
3:07
Speaker 1: an idea which would break your mind, which would like
3:09
Speaker 1: literally make you go insane. You mean, like it makes
3:12
Speaker 1: you insane and then you hear it, or you hear
3:13
Speaker 1: it right before you go insane. Yeah, it makes you
3:16
Speaker 1: go insane. Like if I reveal to you the true
3:19
Speaker 1: nature of the universe and it blew your mind in
3:21
Speaker 1: such a way that you just couldn't function afterwards. Oh boy, Well,
3:25
Speaker 1: let's hope it doesn't happen today because we got to
3:26
Speaker 1: record this podcast live on the air. We blow over
3:32
Speaker 1: his mind. It could be like a weapon of mass destruction,
3:35
Speaker 1: almost like an earworm that disables your brain somehow. I
3:38
Speaker 1: think I read a book like that. It was called lexicon.
3:40
Speaker 1: Actually there was a word that if you heard it
3:43
Speaker 1: or if you saw it written, it would blow your mind.
3:46
Speaker 1: Was it particle wood floors. I don't know what it
3:48
Speaker 1: was because if I had read it then it would
3:50
Speaker 1: have blown my mind. So I didn't finish the book. Really,
3:52
Speaker 1: it was like a warning doesn't tell you what the
3:54
Speaker 1: word is. It can't, man, it can't. Oh boy. But anyways,
3:58
Speaker 1: we do try to talk about everything in the universe
4:00
Speaker 1: that's interesting and that might be hiding out there for
4:02
Speaker 1: humans to discover and to blow our minds with. Once
4:05
Speaker 1: we sort of figure out how everything works, because it
4:07
Speaker 1: is a pretty mysterious and interesting universe, that's right, and
4:11
Speaker 1: we want to know how it works, and more specifically,
4:14
Speaker 1: we want to know what's in it, what's it made
4:16
Speaker 1: out of. We look at this universe with wonder and admiration,
4:20
Speaker 1: but we also look at it with curiosity. We wonder
4:22
Speaker 1: what makes it up and what makes it tick, And
4:26
Speaker 1: as a particle physicist, I'm always trying to break it
4:28
Speaker 1: down into the smallest pieces to understand, like what are
4:31
Speaker 1: the lego bricks of the universe and how do they work? Yeah,
4:35
Speaker 1: and do they hurt when you step on them as well?
4:38
Speaker 1: But it is an interesting question, this question of what
4:40
Speaker 1: is the universe made out of? And it turns out
4:43
Speaker 1: that it's made out of things we have no idea about.
4:46
Speaker 1: It turns out most of the universes is made out
4:48
Speaker 1: of mysterious things and mysterious energies. So there's still a
4:51
Speaker 1: lot of work left for particle physicists. Yeah, the mystery
4:54
Speaker 1: is not solved. You can never retire, Daniel, I don't
4:57
Speaker 1: know why you're celebrating. Well. On one hand, is disappointing
5:01
Speaker 1: and frustrating to not know the answer to this great
5:04
Speaker 1: mystery of the universe. On the other hand, it's exciting.
5:07
Speaker 1: It's an opportunity because it means that we have crazy
5:10
Speaker 1: discoveries ahead of us. I think that one day humans
5:13
Speaker 1: will know what the universe is made out of, and
5:16
Speaker 1: they will hold that knowledge in their little brains. I
5:18
Speaker 1: hope it doesn't blow their minds and melt their brains
5:20
Speaker 1: out of their noses. Yeah. So, when you think about
5:23
Speaker 1: what the universe is made out of, it turns out
5:25
Speaker 1: that physicists have figured out that most of the stuff
5:27
Speaker 1: in the universe, like the stuff that the matter in
5:30
Speaker 1: the universe, is apparently not the regular kind of stuff
5:33
Speaker 1: that we're used to, like the atoms and all of
5:36
Speaker 1: the corks and electrons that you and I are made
5:38
Speaker 1: out of, and that cats and bananas are made out of.
5:42
Speaker 1: It turns out most of the stuff in the universe
5:44
Speaker 1: is something completely different and mysterious. That's right, And we
5:48
Speaker 1: make this mistake all the time in science and in physics,
5:52
Speaker 1: that we see one kind of thing around us or
5:54
Speaker 1: one kind of behavior around us, and we assume that
5:57
Speaker 1: that's it, that everything follows these rules, that the whole
6:00
Speaker 1: universe obeys these principles that we see around us, and
6:03
Speaker 1: then we discover, Oops, it turns out this is unusual.
6:06
Speaker 1: It doesn't hold generally, and there are lots of circumstances
6:09
Speaker 1: where the rules are totally different, from relativity to quantum mechanics.
6:14
Speaker 1: And here's another scenario where we have spent hundreds or
6:17
Speaker 1: thousands of years studying the nature of matter, only to
6:20
Speaker 1: learn that the kind of stuff we've been studying that
6:23
Speaker 1: makes up me and you and hamsters and ice cream
6:25
Speaker 1: and lava is only five per cent of the stuff
6:28
Speaker 1: in the universe, and that most of the universe is
6:31
Speaker 1: something totally different. Yeah, well, most of the universe, it
6:33
Speaker 1: turns out, is it's something called dark energy. But about
6:36
Speaker 1: twenty of the rest of the things in the universe,
6:39
Speaker 1: the energy and matter is something that business is called
6:42
Speaker 1: dark matter, right, Daniel, that's right. It's a kind of
6:45
Speaker 1: matter that we know is out there. We know it's matter.
6:48
Speaker 1: We know it's some kind of stuff. It feels gravity,
6:50
Speaker 1: it causes gravity, it's shaped the structure of the universe.
6:54
Speaker 1: We know it's there, but we don't know what it is.
6:56
Speaker 1: We don't know what is made out of. We don't
6:58
Speaker 1: know how it's made or what rules it follows. Yeah,
7:01
Speaker 1: it's kind of like a giant, cosmic elephant in the room. Right.
7:05
Speaker 1: That's that's invisible at the same time, like the whole
7:08
Speaker 1: universe knows it's there. It feels its presence, it's it's
7:11
Speaker 1: being affected by it. It's definitely there, but nobody can
7:14
Speaker 1: see it. So it's like having a giant elephant invisible
7:17
Speaker 1: elephant in the room. Yeah, and it's a reminder of
7:19
Speaker 1: a really important point that the universe that you see
7:22
Speaker 1: in the universe that we know is just one slice
7:25
Speaker 1: of the universe. That there's a lot going on around
7:27
Speaker 1: us that is invisible to us. That doesn't mean it
7:30
Speaker 1: isn't there, And in fact, what we see is a
7:33
Speaker 1: little tiny fraction of what's out there in the universe.
7:36
Speaker 1: So it's not just dark matter this out there. There's
7:38
Speaker 1: lots of particles out there in the universe that are
7:40
Speaker 1: invisible or almost invisible to us. For example, new trinos
7:44
Speaker 1: are these tiny little particles that are flying everywhere produced
7:47
Speaker 1: from the Sun, but you don't see them even though
7:50
Speaker 1: there are billions flying through your fingers. So the universe
7:53
Speaker 1: is mostly dark and invisible to us. Yeah, it seems
7:55
Speaker 1: to be hiding for most. But do you think it's
7:57
Speaker 1: hiding then? What? What doesn't it want us to see?
8:00
Speaker 1: Why are you so suspicious? Man? Maybe it's just like
8:02
Speaker 1: gently walking us down the garden path to reveal its
8:05
Speaker 1: beautiful secrets. It's just leading us down the path to
8:09
Speaker 1: hopefully it is not going to blow our minds and
8:11
Speaker 1: kill us at the end. I trust the universe alright.
8:15
Speaker 1: So dark matter is something that is big and mysterious,
8:18
Speaker 1: a big part of the universe, but we actually don't
8:20
Speaker 1: know what it is. The name dark matter is just
8:22
Speaker 1: kind of a placeholder, right, Like dark just means it's invisible,
8:25
Speaker 1: and matter just means you know, we feel it's gravity,
8:27
Speaker 1: but we don't actually know what it is, even though
8:30
Speaker 1: we are sort of actively wondering what it is and
8:33
Speaker 1: we're trying to find ways to study it. Yeah, it's
8:35
Speaker 1: a bit of a fine point. Some people say dark
8:38
Speaker 1: matter exists, other people say, we haven't discovered dark matter yet.
8:41
Speaker 1: How can both of those things be possible? Well, we
8:43
Speaker 1: know that dark matter exists, but we don't really understand
8:46
Speaker 1: what it's made out of or what it is. So
8:48
Speaker 1: what we're looking to do is understand, like, is dark
8:51
Speaker 1: matter made of a new kind of particle, and if so,
8:53
Speaker 1: what we're dark matter made out of something else entirely.
8:56
Speaker 1: So we know it exists, but we have been able
8:59
Speaker 1: to isolate it. Yeah, we've been looking for a while now.
9:01
Speaker 1: I mean dark matter was discovered kind of in the nineties, right,
9:04
Speaker 1: and people have been thinking about it looking for devising experiments,
9:08
Speaker 1: and without success, we haven't sort of seen anything that
9:12
Speaker 1: can tell is more concretely like, hey, that was made
9:15
Speaker 1: by a little bit of dark matter. We actually have
9:18
Speaker 1: clues about the existence of dark matter dating all the
9:20
Speaker 1: way back to the nineteen thirties, and then in the
9:22
Speaker 1: nineteen seventies it really became mainstream when VERA. Reubens saw
9:26
Speaker 1: how galaxies were rotating, and since then there's been a
9:29
Speaker 1: very exciting and active program to try to understand what
9:32
Speaker 1: dark matter is made out of. But yeah, we haven't
9:35
Speaker 1: seen it yet. Yeah, and part of the problem is
9:37
Speaker 1: that it's invisible. Right, It's invisible and you can't see
9:40
Speaker 1: it or touch it, and so it's really hard to
9:42
Speaker 1: study something that you know, you can't see or touch.
9:45
Speaker 1: That's right. So far, the only way we know how
9:47
Speaker 1: to interact with dark matter is through gravity, and gravity
9:50
Speaker 1: is the weakest force in the universe, which makes it
9:54
Speaker 1: a very very bad way to discover particles because particles
9:58
Speaker 1: have almost no mass and so they have very very
10:00
Speaker 1: weak gravity. Yeah, that gravity is really weak. And but
10:04
Speaker 1: also I think one of the things that might be
10:06
Speaker 1: preventing us from seeing and studying dark matter better is
10:09
Speaker 1: this concept that we're going to talk about today. That
10:11
Speaker 1: has to do with neutrinos, which are also kind of
10:14
Speaker 1: elusive in invisible particles. That's right. So we're looking for
10:17
Speaker 1: something elusive and almost invisible, and it might be hiding
10:20
Speaker 1: behind something else, also elusive and also almost invisible. So
10:24
Speaker 1: today on the program, we'll be tackling the question our
10:32
Speaker 1: neutrinos hiding dark matter. Oh man, this is such a
10:37
Speaker 1: mistrustful episode exactly, so much suspicion, like who's hiding one,
10:42
Speaker 1: who's keeping the truth from us? Man, Let's just have
10:45
Speaker 1: like a universe truthtelling commission where everybody comes to table
10:48
Speaker 1: and says what they got and what they know, and
10:51
Speaker 1: everybody can just share all the information. Maybe a better
10:53
Speaker 1: title would have been our Neutrinos walking us down the
10:56
Speaker 1: poorest path of knowledge and enlightenment. Maybe neutrinos just have
10:59
Speaker 1: a brand plan and this is just part of the plan,
11:02
Speaker 1: you know, keep us in darkness for a while and
11:04
Speaker 1: then boom, reveal the truth that blows our minds without
11:08
Speaker 1: actually melting our brains. Wow, these neutrinos man, all right.
11:12
Speaker 1: And this has to do with this concept called the
11:14
Speaker 1: neutrino floor that physicists talk about, and that might be
11:19
Speaker 1: the thing that is hiding or kind of preventing us
11:22
Speaker 1: from seeing or experimenting with dark matter. Yeah, exactly, we
11:26
Speaker 1: have these experiments to look for a dark matter, and
11:29
Speaker 1: they're worried that dark matter might be hiding for them
11:31
Speaker 1: by hiding under the neutrino floor in the what the
11:36
Speaker 1: universe basement or what's under the trino floor? I guess
11:40
Speaker 1: that's the question we'll be tackling today. All right, Well,
11:43
Speaker 1: we were, as usual, wondering how many people out there
11:45
Speaker 1: had heard of the neutrino floor or had any guesses
11:48
Speaker 1: as to what it could be. So Daniel went out
11:51
Speaker 1: there and asked people on the internet this question. That's right.
11:54
Speaker 1: We are still in pandemic mode. So if you'd like
11:57
Speaker 1: to participate in euro out there on the internet, please
12:00
Speaker 1: don't hesitate to right to me two questions at Daniel
12:02
Speaker 1: and Jorge dot com. I know you want to hear
12:05
Speaker 1: your voice in the podcast. You desperately want to participate,
12:08
Speaker 1: you just haven't yet, so send us that email. It's easy,
12:11
Speaker 1: it's fun. Yeah, So think about it for a second.
12:13
Speaker 1: If I say the words neutrino floor, what does it
12:16
Speaker 1: make you think of? Here's what people have to say.
12:18
Speaker 1: I know what a neutrino is, and I know what
12:21
Speaker 1: a floor is. But putting those two things together, neutrino
12:26
Speaker 1: and floor, and assuming there's only one of them, makes
12:29
Speaker 1: it feel like we're talking about some kind of minimum,
12:34
Speaker 1: a global minimum in something to do with neutrinos. So
12:38
Speaker 1: I don't know that like the lowest energy that a
12:41
Speaker 1: neutrino could possibly have, or in the distribution of neutrinos
12:46
Speaker 1: energy which we observe, what is the smallest one, or like,
12:50
Speaker 1: what is the lightest neutrino that can possibly exist? Well,
12:53
Speaker 1: we know what that is. The neutrino floor I don't know,
12:56
Speaker 1: so probably trinos is the floor of neutrinos. Floor is
13:04
Speaker 1: the bottom of something, right, Yes, the bottom of what
13:09
Speaker 1: they're composing. And okay, but what are they composing? Particles?
13:14
Speaker 1: This is a kind of particle. But the neutrino floor
13:17
Speaker 1: means probably something like the bottom of something to do
13:20
Speaker 1: with neutrinos. Let's say, the smallest possible neutrino or the
13:25
Speaker 1: lowest energy neutrino or something like that. I think the
13:27
Speaker 1: neutrino floor is the baseline saturation of neutrinos. Throughout the universe,
13:35
Speaker 1: and perturbations in that floor would indicate something weird going
13:38
Speaker 1: on if you detect extra neutrinos. Maybe this is the
13:42
Speaker 1: absolute lowest state of energy that the neutrino can't be in.
13:45
Speaker 1: Is there like something weird about neutrinos where they have
13:48
Speaker 1: to have some energy and they just approach the floor
13:51
Speaker 1: where they don't get any less energy? I don't know, Uh,
13:54
Speaker 1: neutrino floor, I'm guessing is the the lowest amount of
14:03
Speaker 1: neutrinos that could exist in the universe, so on that
14:12
Speaker 1: that would be the theoretical lower limit of of how
14:19
Speaker 1: many would have to exist in order for the universe
14:24
Speaker 1: to be stable. The first thing that comes to mind
14:28
Speaker 1: is neutrinos are small, and perhaps that's the floor is
14:33
Speaker 1: defining a smallest particle that we can know of. So
14:38
Speaker 1: maybe the neutrino floor is the smallest particle that we
14:43
Speaker 1: know of at the moment. Maybe that's UM basically maybe
14:50
Speaker 1: a base like a like a baseplate of neutrinos where
14:54
Speaker 1: UM we can basically detected measurements according to what neutrinos
14:58
Speaker 1: hit that area. Neutron e knows I've learned in the
15:01
Speaker 1: podcast are pretty much everywhere, and many, many of them.
15:07
Speaker 1: So I guess if you don't have another source of neutrinos,
15:12
Speaker 1: the neutrinow flaw would be the sort of normal or
15:16
Speaker 1: median neutrino density um that that you would find. All Right,
15:23
Speaker 1: a lot of great answers. I feel like any of
15:25
Speaker 1: these could be the real answer. These are great speculative answers.
15:28
Speaker 1: None of them are right, but they are great examples
15:31
Speaker 1: of people brainstorming and using their physics knowledge and coming
15:35
Speaker 1: up with totally reasonable ideas for what this crazy concept
15:39
Speaker 1: could be. Well, that these all seem plausible, and I
15:42
Speaker 1: think everyone sort of latching onto the word floor as
15:45
Speaker 1: it's kind of like the bottom of something, or like
15:47
Speaker 1: the minimum of something, or like the point at which
15:49
Speaker 1: you can't go below exactly. And that's reasonable, and that's
15:52
Speaker 1: why floor is in there. But as you'll hear us
15:55
Speaker 1: talk about today, that's not exactly what we're talking about.
15:57
Speaker 1: We're not talking about the lowest energy date of neutrinos
16:01
Speaker 1: or literal floors made out of neutrinos. Do you have
16:04
Speaker 1: a neutrino house? Is that possible? Well, it wouldn't give
16:07
Speaker 1: you much privacy, and it wouldn't shield you from the weather,
16:10
Speaker 1: but you still have to pay taxes on it. So
16:12
Speaker 1: I don't think it's a great idea. Oh boy, but
16:14
Speaker 1: it's neutral taxes though, right is weak taxes. You have
16:18
Speaker 1: to pay it in dark matter dollars. Maybe maybe you
16:20
Speaker 1: could pay in weak dollars as the US dollar might
16:23
Speaker 1: be these things all right, Well, the topic is to
16:25
Speaker 1: the general topic though, is dark matter and why we
16:28
Speaker 1: can't measure it as well as we should be able to.
16:31
Speaker 1: And so let's maybe recap for folks what is dark
16:34
Speaker 1: matter and how we know it's there? Yeah, so, very briefly,
16:37
Speaker 1: dark matter is most of the stuff in the universe.
16:41
Speaker 1: We know that out there, deep in space and also
16:44
Speaker 1: around us here there is some kind of matter which
16:47
Speaker 1: has gravity, and so it affects the way things move
16:49
Speaker 1: and the way the universe has been shaped and evolved.
16:52
Speaker 1: But we don't know exactly where it is and what
16:55
Speaker 1: it is, but we are sure that it's there and
16:57
Speaker 1: that it's here. It was discovered initially because we were
17:00
Speaker 1: looking at how galaxies rotate. When galaxy spin, you might wonder,
17:04
Speaker 1: like why don't the stars fly off into intergalactic space
17:08
Speaker 1: like ping pong balls would on a Merry go Round,
17:10
Speaker 1: and the answer is gravity. Gravity holds them into the galaxy.
17:15
Speaker 1: But you can ask is there enough gravity to hold
17:18
Speaker 1: the galaxy together, because we can measure how fast the
17:21
Speaker 1: galaxy is spinning, and we can count up how many
17:23
Speaker 1: stars there are to estimate how much gravity there is,
17:25
Speaker 1: so you can do this kind of cross check. This
17:28
Speaker 1: is a great kind of thing to do in science.
17:29
Speaker 1: It's just like double check that things make sense and
17:32
Speaker 1: nine at the time they do and you move on,
17:34
Speaker 1: and it's really boring. But sometimes they reveal, like a
17:37
Speaker 1: cosmic mystery, a clue that there's something missing in our understanding.
17:41
Speaker 1: And that's what happened here. Because the galaxies are spinning
17:44
Speaker 1: way too fast, there's not nearly enough gravity to hold
17:47
Speaker 1: them together if you just count the mass of the
17:50
Speaker 1: stars that we can see. So people thought, well, there
17:53
Speaker 1: must be some other kind of stuff out there in
17:55
Speaker 1: the galaxy. And people had other ideas to like maybe
17:58
Speaker 1: we just don't understand gravity or something wrong with our
18:01
Speaker 1: theory of gravity. But pretty soon we had other evidence
18:04
Speaker 1: of dark matter that convinced us it was real and
18:06
Speaker 1: it was matter. For example, we understand that dark matter
18:10
Speaker 1: has shaped the whole evolution of the universe, since it's
18:14
Speaker 1: most of the matter of the universe, it's most of
18:16
Speaker 1: the gravity, which changes how the stuff in the universe flows.
18:20
Speaker 1: You know that there are galaxies and filaments of galaxies
18:23
Speaker 1: and superclusters and this all this structure in the universe
18:26
Speaker 1: that's controlled by dark matter. If dark matter hadn't been around,
18:30
Speaker 1: we wouldn't have had that structure. We wouldn't have galaxies
18:32
Speaker 1: and stars today. So we have lots of evidence for
18:35
Speaker 1: dark matter in the universe, but we don't yet know
18:37
Speaker 1: exactly what it's made out of. Yeah, we definitely know
18:40
Speaker 1: it's there. Like, like we've mentioned before, it's like having
18:43
Speaker 1: an elephant in the room. It's like you can feel
18:45
Speaker 1: its presence in the universe, and like you're saying, like,
18:48
Speaker 1: we know it's there because it explains to the rotation
18:50
Speaker 1: of galaxies. And also, like I think we've covered this
18:53
Speaker 1: in another episode that it's sort of the only way
18:55
Speaker 1: to explain how the universe is the way it is now.
18:58
Speaker 1: Like if you took out dark matter from the beginning
19:01
Speaker 1: of the universe, like we wouldn't end up with the
19:03
Speaker 1: same universe. Yeah, you wouldn't at all. The universe wouldn't
19:06
Speaker 1: have galaxies and stars form as early as they did
19:09
Speaker 1: in our universe. So we're pretty sure that that gravity
19:12
Speaker 1: is there. We just don't know what it is that's
19:14
Speaker 1: generating that gravity, all right. So we know the elephant
19:17
Speaker 1: is there in the room. We can smell the you know,
19:19
Speaker 1: elephant done, and we know we can feel its attraction
19:24
Speaker 1: or repulsion. But what do we actually know about dark matter? Like,
19:26
Speaker 1: do we know anything about it except other than its
19:29
Speaker 1: effects in terms of it's matter and gravity. We do
19:32
Speaker 1: know some things because we've been looking for it, and
19:35
Speaker 1: we know, for example, that it doesn't feel some of
19:37
Speaker 1: the forces like electromagnetism, the force that makes things reflect
19:41
Speaker 1: light or it makes things glow. Doesn't feel that force
19:44
Speaker 1: at all. It has no electric charge. If it did,
19:47
Speaker 1: it would reflect light or it would glow because it
19:49
Speaker 1: has some temperature. But it doesn't, so it doesn't feel electromagnetism.
19:53
Speaker 1: We know that it doesn't feel the weak force. We've
19:55
Speaker 1: also looked for using the weak force, and that's actually
19:57
Speaker 1: what we'll talk about today, experiments that tried to look
20:00
Speaker 1: for it using the weak force, and it also doesn't
20:02
Speaker 1: feel a strong force. Similarly, if it felt a strong force,
20:05
Speaker 1: it would have crazy interactions with our matter that we
20:08
Speaker 1: could see. But it does feel gravity, which means that
20:10
Speaker 1: it has mass. Right, it changes the shape of space,
20:14
Speaker 1: means it has mass. It has some energy to it,
20:16
Speaker 1: but we also know a little bit more about how
20:18
Speaker 1: much energy it has. We actually know something about the
20:21
Speaker 1: temperature of dark matter. You might think like, well, how
20:24
Speaker 1: could you measure the temperature of dark matter. You can't
20:26
Speaker 1: like take a thermometer and stick it in there because
20:29
Speaker 1: it can't interact with the dark matter. What we do
20:31
Speaker 1: is we look at how dark matter is clumped and
20:34
Speaker 1: spread through the universe. If dark matter was hot, if
20:37
Speaker 1: it's particles were moving really really fast, it would spread
20:39
Speaker 1: out a lot, and that would change the shape of
20:42
Speaker 1: the universe how things were distributed, like the galaxies and
20:44
Speaker 1: where they are. And if dark matter was cold, if
20:46
Speaker 1: it was slow moving, you would tend to clump a
20:48
Speaker 1: little bit more and that would give you a different
20:50
Speaker 1: shape for the universe. And what we see is that
20:53
Speaker 1: dark matter is pretty cold. It's not like some fast
20:55
Speaker 1: moving little particle that spreads out a lot tends to
20:58
Speaker 1: be cold and clumpy. Dark matter is pretty cool, for sure,
21:02
Speaker 1: But we also sort of know kind of where it
21:05
Speaker 1: is right, or the kind of the distribution of it
21:07
Speaker 1: or the shape of it throughout the universe, Like we
21:10
Speaker 1: know it's sort of like a giant cloud, or some
21:12
Speaker 1: people call it a halo around our galaxy. And when
21:15
Speaker 1: you look out into the the universe, you can see
21:18
Speaker 1: that there's sort of ripples and clumps of it here,
21:21
Speaker 1: and they're like, it's not evenly distributed throughout the universe.
21:24
Speaker 1: It seems to clump to things, and there's clumps of
21:27
Speaker 1: it out there in space. That's right, and describing it
21:29
Speaker 1: as a halo is accurate, but it suggests sort of
21:31
Speaker 1: that dark matter follows the normal matter that you know
21:34
Speaker 1: dark matter is there where normal matter is, when actually
21:37
Speaker 1: it's the opposite. Normal matter follows the dark matter because
21:41
Speaker 1: dark matter it's much more gravity. The reasons in galaxy
21:44
Speaker 1: right here is because there was a big clump of
21:46
Speaker 1: dark matter and so all the normal matter fell into
21:49
Speaker 1: that gravitational well and was then squeezed together into a galaxy.
21:53
Speaker 1: So yes, it's not everywhere through space. It's in these
21:55
Speaker 1: big clumps. And we can tell where the really big
21:57
Speaker 1: clumps are because they have enough gravity, but we can't
21:59
Speaker 1: tell example, is there a blob of it right here
22:02
Speaker 1: in the room with me, because it's really hard to
22:04
Speaker 1: measure the gravity of smaller objects, so we can't tell
22:06
Speaker 1: where the dark matter is with like really fine resolution,
22:09
Speaker 1: smaller than like, you know, a chunk of the galaxy.
22:12
Speaker 1: We can't even tell, like down to the Solar System
22:14
Speaker 1: where the dark matter is. Right But the big mystery
22:16
Speaker 1: of dark matter is that we sort of don't know
22:19
Speaker 1: what it is, right like, we know it's there. We
22:22
Speaker 1: know there's something or some something we I don't know
22:25
Speaker 1: if the thing is even the right word, but there's
22:27
Speaker 1: something there that's pulling us gravitationally. But it could be
22:30
Speaker 1: like a particle, or could be something non particle. It
22:33
Speaker 1: could be, like I don't know, some kind of like
22:35
Speaker 1: new kind of stuff. Right, absolutely it could, and we
22:38
Speaker 1: need to keep an open mind because all of our
22:40
Speaker 1: ideas about what matter is come from studying the kind
22:43
Speaker 1: of matter we are made out of. But we know
22:45
Speaker 1: that dark matter is different. It has to be different.
22:48
Speaker 1: It's not made out of atoms, and so it might
22:50
Speaker 1: follow very different rules and whole different concepts. So on
22:54
Speaker 1: one hand, we could try to extrapolate from what we
22:56
Speaker 1: know and say everything out here is made a particle.
22:59
Speaker 1: So maybe dark matter is also, but maybe dark matters
23:02
Speaker 1: made of lots of different kind of particles or as
23:04
Speaker 1: you say, even something weirder, something that's not a particle.
23:07
Speaker 1: And we did a whole fun podcast episode about what
23:09
Speaker 1: an unparticle might be. It's weird new kind of matter
23:13
Speaker 1: that people are playing with conceptually. But it also could
23:16
Speaker 1: be something that nobody has thought of at all, something
23:19
Speaker 1: totally new and weird that blows our minds. Right, it
23:22
Speaker 1: could be an unparticle, or maybe it's a fun particle,
23:24
Speaker 1: who know, Or it's an unidea waiting to join the party.
23:29
Speaker 1: All right, That's what dark matter is. It's something that's there.
23:31
Speaker 1: We know it's there, but we don't know what it is.
23:33
Speaker 1: And because it's invisible, it doesn't interact with the electromagnetic force,
23:37
Speaker 1: And so how do you study something like that? How
23:39
Speaker 1: do you look for something that's invisible and doesn't want
23:41
Speaker 1: to be touched. So let's get into that, but first
23:44
Speaker 1: let's take a quick break. Alright, we're talking about the
24:00
Speaker 1: neutrino floor and how somehow that floor is preventing us
24:04
Speaker 1: from or hiding maybe dark matter and our understanding of
24:09
Speaker 1: what it's made out of So we talked about what
24:11
Speaker 1: dark matter is. And now, Daniel, how are people, I guess,
24:15
Speaker 1: trying to study dark matter. I mean, it's invisible and
24:17
Speaker 1: you can't touch it with your finger or any kind
24:19
Speaker 1: of instrument really, because it only feels certain forces. How
24:23
Speaker 1: do you look for something that elusive? It's overwhelming, you know,
24:27
Speaker 1: to say, I want to look for something I don't
24:28
Speaker 1: really know what it is, if it's even a particle,
24:30
Speaker 1: if it's something I haven't imagined yet, I don't know
24:33
Speaker 1: how to interact with it. In general, that's an unsolvable problem.
24:36
Speaker 1: We just don't know in science how to tackle such
24:39
Speaker 1: a big problem. So what we do is we break
24:41
Speaker 1: it into pieces and we say, well, let's assume that
24:44
Speaker 1: we're lucky. Let's assume that dark matter is something that
24:47
Speaker 1: we do know how to discover, and that we're lucky
24:50
Speaker 1: and it can talk to us. So we make a
24:52
Speaker 1: bunch of totally unjustified assumptions and hope that they're correct. So,
24:56
Speaker 1: for example, we assume that dark matter is made of
24:59
Speaker 1: particle because that's basically all we know how to do.
25:03
Speaker 1: And we assume that dark matter is made of one
25:05
Speaker 1: kind of particle because that's just simpler, And as you
25:08
Speaker 1: said earlier, there's no justification for assuming that dark matter
25:11
Speaker 1: is made of particles other than everything so far has
25:13
Speaker 1: been made of particles, And there's no justification for assuming
25:16
Speaker 1: it's one kind of particle, because you know, our matters
25:19
Speaker 1: made of lots of different kinds of particles. But it's
25:21
Speaker 1: just sort of like the simplest place to start, I see,
25:24
Speaker 1: because you know, most of the stuff we are familiar
25:26
Speaker 1: with and that we that we have at our disposal
25:28
Speaker 1: and that we know about is made out of particles, right,
25:31
Speaker 1: and not just sort of like any particles, but sort
25:33
Speaker 1: of like a certain range of sizes of particles, right, yeah, exactly.
25:37
Speaker 1: And we think that dark matter is heavy. We do
25:39
Speaker 1: have some evidence there because we know that it's cold.
25:42
Speaker 1: If dark matter was really really low mass, then it
25:45
Speaker 1: would have enough energy to be zipping around at higher speed.
25:48
Speaker 1: So we have some evidence that dark matter, if it
25:50
Speaker 1: is a particle, is probably on the heavier side. But
25:53
Speaker 1: we don't know if that particle feels any forces other
25:57
Speaker 1: than gravity, Like we know it feels gravity, but if
25:59
Speaker 1: it only feels gravity, it would make it very very
26:02
Speaker 1: difficult to ever discover a particle of dark matter. Because
26:06
Speaker 1: even if it's heavy, it might weigh, you know, like
26:09
Speaker 1: as much as a gold nucleus. But how much gravity
26:13
Speaker 1: does a gold nucleus have? Almost zero. It's very difficult
26:17
Speaker 1: to measure the gravity of a single particle. So in
26:20
Speaker 1: order to discover dark matter, we also make another almost
26:23
Speaker 1: totally unjustified assumption, which is that there's some new force
26:27
Speaker 1: out there that dark matter feels and we also feel,
26:31
Speaker 1: and we can use that to discover dark matter. What Okay,
26:35
Speaker 1: I got stuck a little bit while ago because you
26:36
Speaker 1: said that it has to be heavy because it's cold.
26:39
Speaker 1: Can't it also be a small or light particle and
26:41
Speaker 1: also be cold. It can be. It's possible. We think
26:44
Speaker 1: it's more likely because it's cold that it's heavy. That
26:46
Speaker 1: would make it easier for it to be slow moving,
26:48
Speaker 1: right when we know something about its velocity, And if
26:51
Speaker 1: it's light, then it would have to be even low
26:53
Speaker 1: energy to be cold. So yeah, it's possible to have
26:56
Speaker 1: cold light particles, but they're less likely, we think, all right,
26:59
Speaker 1: And so then you said you have to assume it
27:00
Speaker 1: interacts through a new kind of force that we have
27:03
Speaker 1: no idea about, and I guess maybe explain why that
27:06
Speaker 1: is is it. Couldn't it also be interacting through the
27:09
Speaker 1: weak or strong forces, maybe in a different way than
27:12
Speaker 1: you thought, but it could also be interacting through those forces. Yeah,
27:15
Speaker 1: And we've looked for it interacting through those forces, and
27:17
Speaker 1: we haven't seen it. You know, if dark matter interacted
27:20
Speaker 1: with the strong force, then everything that feels a strong
27:23
Speaker 1: force would interact with dark matter, and it should be
27:25
Speaker 1: pretty easy to find because the strong force is very strong.
27:28
Speaker 1: Like any blob of matters out there, every rock, for example,
27:32
Speaker 1: should be hit by dark matter, should be interacting with
27:35
Speaker 1: dark matter. But we don't see that, you know, we
27:36
Speaker 1: don't see those things. We study particles really carefully. And
27:40
Speaker 1: the strong force, again is very very strong. So if
27:42
Speaker 1: dark matter was interacting with matter using the strong force,
27:46
Speaker 1: we would see weird, unexplained effects. We would discover dark
27:49
Speaker 1: matter years and years ago. The same thing for the
27:52
Speaker 1: weak force. We have looked for dark matter interacting with
27:54
Speaker 1: our particles using the weak force, and we haven't seen it.
27:58
Speaker 1: And we know these forces really really well, like very
28:00
Speaker 1: precise measurements of these forces and colliders and other experiments,
28:04
Speaker 1: So any deviations from our theories of these forces would
28:07
Speaker 1: be hints of dark matter and we've looked for that
28:09
Speaker 1: and we just haven't seen it. I see, there's no
28:11
Speaker 1: evidence that it feels a weak or strong force so far.
28:14
Speaker 1: That's right, And we've studied those in the wazoo, out
28:16
Speaker 1: the wazoo, and around the wazoo. I don't want to
28:18
Speaker 1: know where the wazoo of an elephant is. So then
28:22
Speaker 1: is that where we're at now? Like we've given up
28:24
Speaker 1: on the weak force and the strong force for dark matter,
28:28
Speaker 1: we've given up on those, and so we said, well,
28:30
Speaker 1: what if there's another force? Right? What if there's another
28:34
Speaker 1: force that dark matter can use to interact with our matter?
28:37
Speaker 1: And if there is, then we can use that to
28:39
Speaker 1: discover it. The argument for that existing is mostly boy,
28:43
Speaker 1: I hope it exists because that would make it possible
28:46
Speaker 1: to discover dark matter. There are some other hand wavy
28:49
Speaker 1: arguments to suggest that maybe it's reasonable, but mostly it's
28:53
Speaker 1: the first one. It's just out of desperation. It's the
28:55
Speaker 1: only way we would be able to see it. So
28:56
Speaker 1: let's hope it's true. Yeah, exactly, let's start there. And
28:59
Speaker 1: you know, if we don't find it, then we need
29:01
Speaker 1: to re examine these assumptions and go back and think well,
29:04
Speaker 1: what if there's another way, let's be more clever about it.
29:06
Speaker 1: But it also makes sense in science to try the
29:09
Speaker 1: simplest thing first. Hey, maybe we'll get lucky and dark
29:12
Speaker 1: matter will be some heavy kind of particle that has
29:15
Speaker 1: a new force that interacts with our detectors and we
29:17
Speaker 1: can discover it that way. It would be silly not
29:19
Speaker 1: to try the simplest thing first, right, But I guess
29:22
Speaker 1: it doesn't sound that simple, right, like making up a
29:24
Speaker 1: whole new we're assuming a whole new force in the
29:26
Speaker 1: universe that nobody has ever seen or or felt before.
29:29
Speaker 1: It seems a bit of a stretch because like, if
29:32
Speaker 1: such a force existed, then our matter can interact through it,
29:36
Speaker 1: wouldn't we have noticed by now? Yes, And so it
29:38
Speaker 1: has to be a weak force, not weak with a
29:40
Speaker 1: capital W, like the weak force, but it has to
29:43
Speaker 1: be like a feeble force. So you're right, exactly, if
29:47
Speaker 1: there is this other force and our particles can feel it,
29:51
Speaker 1: then we should see it in experiments. We have other
29:53
Speaker 1: experiments also that are looking for these kinds of forces,
29:56
Speaker 1: and so we hope it's out there. But if it's true,
29:58
Speaker 1: it would have to be very, very feeble and that's
30:01
Speaker 1: why these experiments are very difficult. Is that the official name.
30:04
Speaker 1: Have you guys christened the feeble force? No, we haven't
30:07
Speaker 1: christened it fantastic force. And it's very confusing, of course,
30:11
Speaker 1: because it's particle physics. Naming. This particle we're talking about
30:14
Speaker 1: is called the WHIMP weakly interacting massive particle. But weakly
30:19
Speaker 1: interacting there does not mean interacting through the weak force.
30:22
Speaker 1: It means interacting through some new, not very powerful force.
30:26
Speaker 1: So they should have called it, like the FIMP, the
30:28
Speaker 1: feebly interacting massive particle. Yeah, because you don't want to
30:31
Speaker 1: call it the powerful interacting massive particle. That would be
30:35
Speaker 1: inappropriate for any podcast, that's right, or the limp or something, right,
30:40
Speaker 1: the lightly interacting massive particle. There you go, all right, Well,
30:43
Speaker 1: so if dark matter is interactive through this new imaginary
30:47
Speaker 1: force we're hoping to see, um, how would we see it?
30:50
Speaker 1: It's not imaginary? Is hypothetical? I see? There's a difference
30:56
Speaker 1: right between hypothetical and imaginary. It's all about spin. I mean,
30:59
Speaker 1: you say, it's like making it very complicated. On the
31:02
Speaker 1: other hand, it's sort of ambitious, right, Like this way,
31:05
Speaker 1: if we discover dark matter, we get two discoveries for
31:08
Speaker 1: the price of one. We find a new particle and boom,
31:11
Speaker 1: new force. At the same time. It's like Nobel prize
31:14
Speaker 1: with the side of another Nobel prize. I see. It's like,
31:18
Speaker 1: how are we going to find the big invisible elephant
31:20
Speaker 1: in the room? Well, I'm glad you asked. We're going
31:22
Speaker 1: to use invisible camels. Yes, exactly, exactly, And in this
31:27
Speaker 1: case you need the invisible camels to show you where
31:30
Speaker 1: the invisible elephant is. And so you assume both exist
31:32
Speaker 1: and then when you discover them, boom, two discoveries, two
31:37
Speaker 1: animals for the price of one. That's right. So we're
31:39
Speaker 1: looking for the zimp, the zoo interacting massive particle. All right,
31:42
Speaker 1: So then what are these experiments? What do they look like?
31:45
Speaker 1: How do you look for a force you don't even
31:46
Speaker 1: know it exists? So there's actually three different ways that
31:50
Speaker 1: we look for the whimp. They're called make it, shake it,
31:52
Speaker 1: or break it. But today we're gonna talk about just
31:54
Speaker 1: one of them because it's the one that's related to
31:56
Speaker 1: neutrinos and the neutrino floor, and that's the shake it.
32:00
Speaker 1: And this is a strategy of looking for dark matter
32:03
Speaker 1: by basically making a really big chunk of stuff and
32:07
Speaker 1: putting in a really quiet place so you don't expect
32:10
Speaker 1: anything to happen, and then looking to see if dark
32:12
Speaker 1: matter ever bumps into part of your really big chunk
32:16
Speaker 1: of stuff. This is one of those like xenon detectors, right,
32:20
Speaker 1: so it's a huge tank of liquid and actually gashes
32:23
Speaker 1: zenon that's deep deep underground and zenon because Zenon is
32:27
Speaker 1: one of the noble elements. So if you have a
32:29
Speaker 1: big tank of xenon, it basically does nothing, doesn't like
32:32
Speaker 1: give off flashes of light or interact. And so if
32:35
Speaker 1: something does cause the zenon to do something, it means
32:39
Speaker 1: something has penetrated somethings like kicked the zenon or giving
32:42
Speaker 1: it a little boost or something. Do you take your
32:43
Speaker 1: big vast tank of xenon, you bury deep deep underground
32:47
Speaker 1: so that particles from space don't hit it, and then
32:50
Speaker 1: you put basically cameras they'll photo multiplier tubes to watch
32:53
Speaker 1: it and ask like did any of my xenon atoms
32:55
Speaker 1: get bumped? Right? But doesn't Zenon sort of move or
32:59
Speaker 1: does thing by itself? Like how do you know? You
33:02
Speaker 1: know what I mean? Like if you put something in
33:03
Speaker 1: a room. How do you know it's something moved it
33:05
Speaker 1: or if it moved by itself. Yeah, So it's cryogenic,
33:08
Speaker 1: so it's very very cold liquid xenon. And they do
33:11
Speaker 1: a lot of work to try to isolate other sources
33:14
Speaker 1: of noise. This is the kind of detector where they're
33:16
Speaker 1: hoping to see one dark matter particle come through over
33:20
Speaker 1: like several years. Right. So in order to claim discovery
33:23
Speaker 1: when you see like one example, you need to make
33:26
Speaker 1: sure there are no other ways for this kind of
33:29
Speaker 1: thing to happen, no other things that might look like
33:31
Speaker 1: you're invisible camel. And so they do a lot of
33:34
Speaker 1: work to shield this thing from particles from the outside,
33:37
Speaker 1: from radiation from the rock, and also to cool this
33:40
Speaker 1: thing down so there's no internal noise. Right. And I
33:43
Speaker 1: think the idea is not that there's only one particle
33:46
Speaker 1: dark matter there per year. It's more like it's full
33:49
Speaker 1: of dark matter particles, but only like once in a
33:51
Speaker 1: blue moon do they sort of interact with the zenon
33:55
Speaker 1: particles exactly. We think that the Earth is moving through
33:58
Speaker 1: a dark matter wind, like we think that dark matter
34:01
Speaker 1: is this big halo that surrounds the galaxy, or the
34:05
Speaker 1: galaxy is embedded in the halo of dark matter. So
34:08
Speaker 1: we think that dark matter is probably everywhere, and so
34:11
Speaker 1: we have dark matter in our room with us right now,
34:14
Speaker 1: probably a good bit of it. And so this is
34:16
Speaker 1: this wind of dark matter. But as you say, we
34:19
Speaker 1: don't have a high probability to interact with that dark matter.
34:22
Speaker 1: Most of the time when dark matter passes through normal matter,
34:24
Speaker 1: nothing happens. So you need to do a lot of that.
34:27
Speaker 1: You need to shoot dark matter through your detector a
34:29
Speaker 1: lot of times, and that's why you wait a long
34:32
Speaker 1: time and you have really big detector, and you're exactly right.
34:34
Speaker 1: We expect like thousands and millions and billions of dark
34:37
Speaker 1: matter particles to pass through. But this is a very
34:40
Speaker 1: weak interaction. It's very feeble, and so only occasionally will
34:44
Speaker 1: the dark matter particle bump into Zenon in a way
34:46
Speaker 1: that we can see, right, Because I think when you
34:48
Speaker 1: say like weak or feeble, you don't necessarily mean like
34:51
Speaker 1: it hits it and it doesn't create an effect. It's
34:54
Speaker 1: more like the probability of this interaction is very low
34:58
Speaker 1: because it is, after all, presumably a quantum interaction. It
35:02
Speaker 1: is a quantum interaction, and we talk about it in
35:05
Speaker 1: terms of cross sections, because we'd like to use like
35:07
Speaker 1: a classical physical analogy. Imagine that you're shooting two particles
35:11
Speaker 1: at each other, or two balls at each other, right,
35:14
Speaker 1: the chances that those balls will hit each other depends
35:17
Speaker 1: on their cross sectional areas. If you shoot two like
35:19
Speaker 1: basketballs at each other, it's much easier for them to
35:22
Speaker 1: hit than if you shoot two ping pong balls at
35:24
Speaker 1: each other. And so we think that dark matter has
35:27
Speaker 1: a very low cross section with the zenon nucleus, which
35:30
Speaker 1: means basically, it sees the zeno nucleus as a tiny,
35:33
Speaker 1: tiny little dot and most of the time just flies
35:35
Speaker 1: right by, whereas other particles, you know, like corks, have
35:38
Speaker 1: a very large cross section with the zenon nucleus. If
35:41
Speaker 1: you shout a cork through a tank of zenon, it
35:43
Speaker 1: would interact with basically everything, right, So that that's the
35:46
Speaker 1: basicic experiment that you're saying. You take a block of
35:48
Speaker 1: really cold zenon, you put it under a lot of
35:51
Speaker 1: installations so nothing else can reach it, and then you
35:54
Speaker 1: just wait. As we're moving through the universe, we're moving
35:57
Speaker 1: through a cloud or haze of dark matter, and hopefully eventually,
36:01
Speaker 1: maybe one day this dark matter will interact with your
36:04
Speaker 1: zene on in some way that you can detect, and
36:06
Speaker 1: then you'll be like, hey, there is the dark matter,
36:08
Speaker 1: that's right. And we look for these little flashes of
36:11
Speaker 1: light that indicate that dark matter might have come through
36:14
Speaker 1: and kicked one of these senon atoms which caused it
36:17
Speaker 1: to either like lose an electron or to give off
36:19
Speaker 1: a little scintillation flash of light. So that's the signal
36:22
Speaker 1: that we're looking for. All right, Well, it sounds simple enough,
36:25
Speaker 1: but there is apparently a sort of a hiccup in
36:27
Speaker 1: this schematic, in this idea, and it has to do
36:30
Speaker 1: with the nutrino floor. So let's get into that or
36:33
Speaker 1: under that. But first let's take a quick break. All right.
36:49
Speaker 1: We're trying to find dark matter, Daniel, and it's invisible
36:52
Speaker 1: and it doesn't want us to a theater or touch
36:54
Speaker 1: it or bump into it. But it might have a weakness,
36:57
Speaker 1: which is a totally new kind of for yeah, exactly,
37:01
Speaker 1: and so we are hoping that it will reveal itself
37:04
Speaker 1: in this very quiet tank of liquid zenon deep underground.
37:08
Speaker 1: There's actually several of them. There's one underground in Canada,
37:10
Speaker 1: there's one underground in China. And there's one underground in Italy,
37:14
Speaker 1: and these are all competing tanks of liquid zenon. M interesting.
37:18
Speaker 1: It's interesting to think there are three places in the
37:21
Speaker 1: world where there's a block of zenon just waiting for
37:24
Speaker 1: something to happen and trying to ignore everything else that's
37:27
Speaker 1: going on in the universe basically, right, Yeah, yeah, it's
37:30
Speaker 1: not exactly a block. It's liquid zenon. Mostly there's a
37:33
Speaker 1: gaseous face on the top. But yes, this tank of zenon.
37:36
Speaker 1: And it must be interesting to be sitting there and
37:38
Speaker 1: babysitting at you know, because it could be that one
37:41
Speaker 1: day when you're on shift, that's the day you see
37:44
Speaker 1: the signal that dark matter makes itself obvious. Most days
37:48
Speaker 1: nothing happens, so most days you hope nothing happens. You
37:51
Speaker 1: just sit there, and the end result of sitting there
37:53
Speaker 1: for eight hours is yet we saw nothing today. So
37:55
Speaker 1: it's very strange, like gambling with the universe, wondering if
37:59
Speaker 1: today is gonna be the day you see something. It's
38:01
Speaker 1: like you're staring down dark matter. It's like who's gonna
38:04
Speaker 1: blink first. It's like you're staking out dark matter. You're
38:07
Speaker 1: sitting outside of dark Matter's house and it's never left
38:10
Speaker 1: to buy groceries, and you've been there for years and
38:12
Speaker 1: you're wondering if it's ever going to come out. You're
38:14
Speaker 1: waiting for it to peek out of its blind and
38:15
Speaker 1: you're like, I knew somebody was home, all right. So
38:18
Speaker 1: we're waiting for a dark matter to bump into a
38:20
Speaker 1: zenon atom in one of these tanks around the world.
38:23
Speaker 1: But there's sort of a confusing factor here. There's something
38:26
Speaker 1: that might prevent us forever maybe finding in any of
38:31
Speaker 1: these dark matter bumps exactly, or at least a complication.
38:34
Speaker 1: And you brought up one of them earlier, which is
38:36
Speaker 1: aren't there other ways for zeno and to get bumped
38:38
Speaker 1: over to bump into itself? You know, this is an
38:42
Speaker 1: indirect experiment. It's not like we capture the dark matter
38:44
Speaker 1: and we can like see it and look at it
38:46
Speaker 1: and study it and parade it around the world in
38:48
Speaker 1: a dark matter zoo. Right. All we see is that
38:51
Speaker 1: the xenon atom got bumped. We don't ever directly see
38:55
Speaker 1: what bumped it, right, And so there are other things
38:58
Speaker 1: that might be able to bump zenon, and we try
39:01
Speaker 1: to shield the detector from that by having a deep
39:04
Speaker 1: underground under many layers of rock. So for example, muans
39:08
Speaker 1: and other cosmic ray particles from space don't bump it.
39:10
Speaker 1: But there is one kind of cosmic ray, a particle
39:14
Speaker 1: from space, which can get through all of that rock
39:16
Speaker 1: and can bump Zenon and can mimic this dark matter signal. Yeah,
39:20
Speaker 1: because dark matter is not the only elusive and invisible
39:23
Speaker 1: particle or stuff in universe. There are also neutrinos, which
39:26
Speaker 1: we know are invisible and don't interact with the electromagnetic force,
39:31
Speaker 1: that's right, And they can penetrate through the entire Earth
39:34
Speaker 1: without thinking twice, and so it's no big deal for
39:37
Speaker 1: them that we shield these tanks of Xenon with layers
39:39
Speaker 1: and layers and layers of rock. They just fly right through.
39:42
Speaker 1: And most of the time they also fly right through
39:44
Speaker 1: that tank of Zenon without interacting or causing anybody any trouble. Right,
39:49
Speaker 1: But neutrinos do feel the weak force, and the nucleus
39:53
Speaker 1: of the atom also feels the weak force, and so sometimes,
39:56
Speaker 1: very rarely neutrino will bump into Zenon and it will
39:59
Speaker 1: look a lot like what would happen if dark matter
40:02
Speaker 1: bumped into Zenon. It would bump the zenon through the
40:05
Speaker 1: weak force or through maybe this new hypothetical I imaginary
40:10
Speaker 1: force that you guys are hypothesizing. It might bump it
40:12
Speaker 1: through the imaginary camel force. I don't know, but it
40:15
Speaker 1: would definitely bump it through the weak force. Neutrinos feel
40:18
Speaker 1: the weak force, and so do the components of a
40:20
Speaker 1: zeno atom, and so we definitely expect these neutrinos to
40:24
Speaker 1: bump the zenon at some rate. And you can't tell
40:26
Speaker 1: the difference between a neutrino bump and a potential dark
40:29
Speaker 1: matter bump. We can't for an individual bump. We can't
40:32
Speaker 1: look at an individual bump and say, oh, this one
40:33
Speaker 1: was a neutrino, this one was a dark matter. But
40:35
Speaker 1: we do have some handles for telling, like is it
40:38
Speaker 1: more likely to have been dark matter or is it
40:40
Speaker 1: more likely to have been a neutrino based on what
40:42
Speaker 1: time of year it was? So interesting it's seasonal. It's
40:45
Speaker 1: seasonal because you know, the Earth goes around the Sun,
40:48
Speaker 1: which changes, for example, how much dark matter we are
40:51
Speaker 1: flowing through. Our velocity through the dark matter halo of
40:55
Speaker 1: the galaxy changes during the year, right, and so dark
40:59
Speaker 1: matters natural flux which comes from the motion of the
41:02
Speaker 1: actual the Sun, the whole Solar System to the Milky
41:04
Speaker 1: Way peaks in June and then it reaches its lowest
41:08
Speaker 1: point in December. But neutrinos have a different pattern because
41:11
Speaker 1: neutrinos are produced by the Sun and so they have
41:14
Speaker 1: a different pattern. Oh that's interesting. But I guess I'm
41:17
Speaker 1: confused because I thought we didn't know that much about
41:19
Speaker 1: where dark matter was. How do we know that dark
41:21
Speaker 1: matter is even seasonal if we can really see or touchet.
41:24
Speaker 1: We're assuming the dark matter is distributed through the galaxy
41:27
Speaker 1: fairly evenly, and we know something about its rotation because
41:30
Speaker 1: we know something about it's like distribution, and that requires
41:34
Speaker 1: it to be rotating at some rate, And so we
41:36
Speaker 1: know something about where the dark matter is. We can't
41:38
Speaker 1: resolve it down to like here's a piece of dark matter,
41:40
Speaker 1: or even there's a clumpier piece of space that has
41:43
Speaker 1: more dark matter in it. But we just assumed the
41:45
Speaker 1: dark matter is like evenly distributed through the galaxy. And
41:48
Speaker 1: you're also assuming that it's spinning. I hadn't heard that
41:51
Speaker 1: one before. You're assuming that the dark matter and the
41:53
Speaker 1: galaxy is spinning with the galaxy. It has to be,
41:55
Speaker 1: because if it wasn't spinning, it would just fall towards
41:58
Speaker 1: the center. And the reason that the dark matter is
42:00
Speaker 1: in a halo is because it's spinning. If it wasn't spinning,
42:03
Speaker 1: it would just fall into the black hole at the
42:05
Speaker 1: center of the galaxy. I see. So when you say
42:07
Speaker 1: it's seasonal, I guess what you mean is like, you know,
42:10
Speaker 1: as this Earth goes around the Sun, sometimes we're kind
42:13
Speaker 1: of swimming upstream of the dark matter currents, and sometimes
42:17
Speaker 1: we were swimming downstream of the dark matter wind or
42:19
Speaker 1: current exactly. And just like on a boat, if you're
42:22
Speaker 1: moving with the wind, you don't feel it, and if
42:24
Speaker 1: you're moving up wind, you do feel it. Or maybe
42:26
Speaker 1: like more like on a bicycle, the wind is at
42:29
Speaker 1: your back, you don't really feel the wind. And when
42:30
Speaker 1: you're going uphill and the wind is down your face,
42:33
Speaker 1: you feel it more strongly, and so you have a
42:35
Speaker 1: higher flux of wind. And so if more dark matter
42:38
Speaker 1: particles are passing through our detectives, we expect to see
42:41
Speaker 1: more of them. Remember we had this podcast episode about
42:43
Speaker 1: the Dama experiment. This is another dark matter experiment that
42:46
Speaker 1: actually claimed to have discovered dark matter and saw a
42:49
Speaker 1: seasonal variation, just like you would expect. Turns out there
42:53
Speaker 1: other reasons not to believe that experiment, but that's the
42:55
Speaker 1: kind of thing we see. If we ever do see
42:57
Speaker 1: a lot of interactions, then we would expect them, if
43:00
Speaker 1: they are a dark matter, to come in this pattern
43:02
Speaker 1: where there's more in one part of the year and
43:04
Speaker 1: fewer in another part of the year. I see, But
43:07
Speaker 1: generally speaking, that's what the Nutrina floor is. It's like,
43:11
Speaker 1: you know, we're trying to look for dark matter and
43:14
Speaker 1: we're trying to find it when it bumps into zeno tanks.
43:16
Speaker 1: But the neutrinos could also be they're bumping into zenon,
43:19
Speaker 1: and so that's when you say like the neutrina floor,
43:21
Speaker 1: it's it's more like you're saying sort of like there's
43:24
Speaker 1: a base level of noise that we expect from neutrinos. Yeah,
43:28
Speaker 1: and if dark matter interacted with zenon more than this
43:32
Speaker 1: neutrino floor right at a higher rate than this neutrina floor,
43:35
Speaker 1: it wouldn't be a big deal. The rate would be
43:37
Speaker 1: above the floor. We could see dark matter interaction with
43:40
Speaker 1: zenon be all great. We collect our Nobel Prize. If
43:42
Speaker 1: the dark matter rate is low is small. The probability
43:46
Speaker 1: for dark matter to hit Xenon is small enough, then
43:48
Speaker 1: it's going to be below the neutrina floor, meaning that
43:50
Speaker 1: we expect to get more interactions from the neutrino interacting
43:54
Speaker 1: with Xenon than we do from dark matter, and that's
43:56
Speaker 1: going to make it very difficult to disentangle this dark
43:59
Speaker 1: matter signal from the new trino floor. That looks just
44:01
Speaker 1: like it, right, it's like a base level of noise
44:04
Speaker 1: or like you know, it's like there's a little bit
44:06
Speaker 1: of fog on the floor and you're trying to find
44:08
Speaker 1: your keys that you dropped, or like, if you're trying
44:10
Speaker 1: to find your keys, it's hard on a foggy floor,
44:12
Speaker 1: But if you're trying to find a basketball, maybe that
44:14
Speaker 1: would stick more. Yeah, or if your keys are floating
44:18
Speaker 1: or something. All right, So then that's the netrino floor.
44:20
Speaker 1: It's sort of like fuzzing up our view of dark
44:23
Speaker 1: matter through this hypothetical new fourth But you're saying the
44:26
Speaker 1: seasonality of these two things, maybe we'll let us kind
44:29
Speaker 1: of break through that fog. It's gonna be tricky, right,
44:31
Speaker 1: because in order to make these arguments, you need to
44:33
Speaker 1: see more than one interaction, Like if you just see
44:36
Speaker 1: one and it comes in June, you might say, well,
44:39
Speaker 1: dark matter is most likely to come in June. So
44:41
Speaker 1: I guess this is more likely dark matter than new trinos,
44:44
Speaker 1: which tend to peek in January when the Earth is
44:47
Speaker 1: closest to the Sun. But it's not a great argument.
44:50
Speaker 1: When you need to do is see like a hundred
44:51
Speaker 1: of these things and show that they tend to cluster
44:54
Speaker 1: in June, that there are more in June than there
44:56
Speaker 1: are in January in December, because otherwise you could just
44:59
Speaker 1: be seeing like a rare newtrino interaction in June. So
45:03
Speaker 1: it means that we need to see a lot more
45:04
Speaker 1: than to really be confident. I see, But the problem
45:07
Speaker 1: is that the experiment is really feeble, and so you
45:10
Speaker 1: don't get hundreds of him, or you would at least
45:12
Speaker 1: have to wait hundreds of years maybe to get a
45:14
Speaker 1: hundred of them. Yeah, and so far we haven't seen any.
45:17
Speaker 1: So we've been running these tanks and the plan is
45:19
Speaker 1: to make them bigger and bigger and bigger. So they
45:22
Speaker 1: got the technology to work on a smaller tank, and
45:24
Speaker 1: then they scaled it up and up and up, and
45:26
Speaker 1: right now they're running these tanks with many tons of
45:29
Speaker 1: liquid zen on underground, and you're right, the idea is
45:32
Speaker 1: to either run them for years or decades or make
45:36
Speaker 1: them bigger. So we're impatient to see dark matter, and
45:39
Speaker 1: so we're making them bigger and bigger and bigger. And
45:42
Speaker 1: right now we're at the point where we should start
45:45
Speaker 1: to see the new trino floor very very soon. We
45:48
Speaker 1: were all hoping to discover dark matter before we hit
45:51
Speaker 1: the floor, but we didn't see anything. And so now
45:53
Speaker 1: we're like right at the new trino floor and we
45:56
Speaker 1: might spot it just before we get there, but we
45:59
Speaker 1: might also have to dig into the neutrino floor. I
46:01
Speaker 1: guess what do you mean? Like hitting the floor? That
46:03
Speaker 1: means that you have enough zenon out there that if
46:07
Speaker 1: there was dark matter, you would have seen it by now.
46:09
Speaker 1: And we know how neutrinos interact, so we know when
46:12
Speaker 1: we should expect to see neutrinos interacting with our zenon,
46:15
Speaker 1: and neutrinos don't interact very often, and so far tanks
46:18
Speaker 1: have been smaller, but now we have these much bigger tanks,
46:21
Speaker 1: and so we expect to see neutrinos interacting with them
46:24
Speaker 1: pretty soon, meaning like we're out there, we're running these detectors,
46:28
Speaker 1: were gathering the data, we're analyzing it. You know, the
46:31
Speaker 1: year or two these experiments will come out with the
46:33
Speaker 1: results and they will be sensitive to neutrinos like they
46:36
Speaker 1: should have seen neutrinos. Interesting. All right, Well, then I
46:39
Speaker 1: guess what are the possible outcomes there? It could be
46:42
Speaker 1: that how you build these giant tanks and then you
46:44
Speaker 1: find nothing except neutrinos? What would that mean about dark matter?
46:48
Speaker 1: What would that mean about the darkroa matter? That would
46:49
Speaker 1: be disappointing. It would mean either that dark matter does
46:53
Speaker 1: exist and is a whimp and does interact with denon,
46:57
Speaker 1: just at a lower level than we're capable of seeing, right,
47:00
Speaker 1: just like even more feeble than the interactions with neutrinos.
47:04
Speaker 1: So that's one possibility. Another possibility is dark matter is
47:07
Speaker 1: real and it's a particle, but it doesn't have this
47:10
Speaker 1: invisible camel force that we made up to give us
47:13
Speaker 1: a window to interact with it. That the only thing
47:15
Speaker 1: it feels is gravity, And that would mean it would
47:17
Speaker 1: be very, very difficult to ever discover the particle nature
47:21
Speaker 1: of dark matter, because particles don't feel a lot of gravity.
47:24
Speaker 1: But I mean we definitely know it's there it which
47:26
Speaker 1: is maybe confirmed that we could maybe never see it
47:29
Speaker 1: or really experiment with it, at least from using these ideas,
47:33
Speaker 1: using these ideas, and so we'll have to be more clever,
47:36
Speaker 1: and people are working on other ideas. They're working on
47:38
Speaker 1: other ways to detect dark matter that would be more sensitive.
47:41
Speaker 1: One of my favorites is an idea for directional dark
47:44
Speaker 1: matter that can see dark matter if only if it's
47:47
Speaker 1: moving in one direction and not in the other. And
47:49
Speaker 1: that helps like remove the neutrino background, because neutrinos tend
47:53
Speaker 1: to be moving from the Sun. So if you can
47:55
Speaker 1: tell the direction that something bumped your zenon or zenion equivalent,
47:59
Speaker 1: you can help understand if it's neutrino or something else.
48:01
Speaker 1: And we have an idea for the direction we think
48:03
Speaker 1: that dark matter wind is going, so that's one way forward.
48:06
Speaker 1: And then there are other crazier ideas like maybe dark
48:09
Speaker 1: matters not a wind, Maybe it's an axon and go
48:12
Speaker 1: check out our podcast episode about how to discover axons.
48:15
Speaker 1: Or maybe it's made of primordial black holes, or maybe
48:18
Speaker 1: it's made of weird crazy elephants. We just don't know.
48:21
Speaker 1: So we've got to be more creative. But this is
48:23
Speaker 1: the first thing to try, is this heavy particle that
48:25
Speaker 1: has a new, weak kind of interaction, And so it
48:29
Speaker 1: makes sense to try. But if we don't see it,
48:31
Speaker 1: it just means we need to sort of broaden our
48:33
Speaker 1: ideas a little bit. It could be an invisible rhino,
48:35
Speaker 1: not an elephant this whole time. You've been holding onto
48:38
Speaker 1: this idea this whole time. You just now drop it
48:41
Speaker 1: on us. But it seems I guess kind of crazy
48:44
Speaker 1: that you would build all these I mean, I'm guessing
48:46
Speaker 1: billions of dollars in experiments based on an idea that
48:49
Speaker 1: is so hypothetical, you know what I mean. But I
48:51
Speaker 1: guess you have to. You have to do it because
48:53
Speaker 1: you have to check off that box. Yeah. Well, there
48:55
Speaker 1: are people in the theoretical physics community who say it's
48:57
Speaker 1: ridiculous and it's a huge waste of money. These rants
49:00
Speaker 1: don't cost billions of dollars. They do cost tens of
49:02
Speaker 1: millions of dollars, and I think the idea is strong
49:05
Speaker 1: enough that it's worth checking. Other people do think that
49:08
Speaker 1: it's a very narrow idea, has a lot of assumptions
49:11
Speaker 1: that are not justified, and it's a lot of money
49:14
Speaker 1: to spend on those kinds of experiments. So there's some
49:16
Speaker 1: very vocal people out there who think that the whole
49:18
Speaker 1: thing is a waste of time. I am very curious.
49:21
Speaker 1: My judgment is, do I want to go to this
49:23
Speaker 1: seminar and see the results. Every time these experiments xenon
49:27
Speaker 1: Lux and Panda x put out new results, I'm desperately
49:30
Speaker 1: curious to see what they say. So I'm willing to
49:32
Speaker 1: spend my taxpayer dollars on funding these experiments on the camels.
49:38
Speaker 1: I want to know if the camels are out there,
49:40
Speaker 1: yes or no, will be a whole new particle zoo
49:43
Speaker 1: for the universe to lead us to and maybe we can,
49:45
Speaker 1: you know, recover some of the expenses by charging admission.
49:48
Speaker 1: There you go. You can pick up those dark, battered
49:50
Speaker 1: dollars from the floor. So yeah, I'm learning to monetize. Monetized, monetize.
49:54
Speaker 1: Man become a patreon of our invisible camels. Please, there
49:58
Speaker 1: you go. As physicists ever using Catron to fund the research,
50:02
Speaker 1: I don't know, and I don't think anybody's ever kick
50:04
Speaker 1: started a particle physics experiment either. Zero out of forty
50:07
Speaker 1: million dollars pledged, Well it's not too late, all right. Well,
50:12
Speaker 1: that gives us an answer. Here are neutrinos hiding dark
50:15
Speaker 1: matter potentially, It kind of sounds like maybe the answer
50:18
Speaker 1: is yes. I mean, we're getting pretty close to the floor,
50:20
Speaker 1: and it sounds like if dark matter is there and
50:23
Speaker 1: we'll never see it because of these neutrinos. Neutrinos are
50:26
Speaker 1: definitely clouding the issue and can make it harder to
50:29
Speaker 1: find dark matter. But I'm still holding out hope. We're
50:31
Speaker 1: not at the floor yet, and it could be that
50:33
Speaker 1: one of these detectors reveals results that are above the floor,
50:37
Speaker 1: that are at a rate the neutrinos couldn't reproduce and
50:40
Speaker 1: is a much more convincing sign of dark matter. But
50:42
Speaker 1: we'll find out in a year or so. Al Right,
50:45
Speaker 1: stay tuned in the meantime, Thanks for joining us. We
50:48
Speaker 1: hope you enjoyed that. See you next time. Thanks for listening,
50:58
Speaker 1: and remember that Dan lan Jorge Explain the Universe is
51:01
Speaker 1: a production of I Heart Radio. For more podcast from
51:05
Speaker 1: my Heart Radio, visit the I Heart Radio Apple Apple Podcasts,
51:09
Speaker 1: or wherever you listen to your favorite shows. Yeah,