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Did we just discover dark matter? · Daniel and Kelly’s Extraordinary Universe

• 0:07
  Speaker 1: Most of the universe is dark. Not just because space
• 0:12
  Speaker 1: is dark, vast stretches of emptiness illuminated by distant points
• 0:16
  Speaker 1: of light, but because matter itself is dark. Most of
• 0:21
  Speaker 1: the universe turns out to not be made out of
• 0:23
  Speaker 1: our kind of matter, atoms, etc., but something else, something darker.
• 0:29
  Speaker 1: Stars and gas and dust are like tracers that hint
• 0:33
  Speaker 1: at a much more massive mystery underneath. For decades, we've
• 0:38
  Speaker 1: been confident that dark matter is real. It's out there.
• 0:41
  Speaker 1: It's massive. Its gravity is influencing the universe. But what
• 0:47
  Speaker 1: is it? That's the deep question in the heart of
• 0:50
  Speaker 1: a particle physicist. What is it made out of? Is
• 0:53
  Speaker 1: it particles? Is it something else even weirder? Very recently,
• 0:58
  Speaker 1: an experiment underground in South Dakota might have seen the
• 1:02
  Speaker 1: first hint of an answer to that question. Welcome to
• 1:06
  Speaker 1: Daniel and Kelly's Extraordinary Dark Universe.
• 1:22
  Speaker 2: Hello, I'm Kelly Wiedersmith. I study parasites and space. And
• 1:25
  Speaker 2: one of the great things about knowing a lot of
• 1:27
  Speaker 2: scientists is that when something's in the news, you almost
• 1:30
  Speaker 2: always have someone you can reach out to to be like,
• 1:32
  Speaker 2: how excited should I be? And today we all get
• 1:35
  Speaker 2: to find out how excited we should be.
• 1:37
  Speaker 1: Hi, I'm Daniel. I'm a particle physicist and I like
• 1:42
  Speaker 1: to think about aliens. And today I'm quite excited about
• 1:46
  Speaker 1: Dark Matter, the television show, Dark Matter, the science and
• 1:50
  Speaker 1: the kind of Dark Matter we all produce daily.
• 1:53
  Speaker 2: Oh, well, I'm always excited about that, mostly because it
• 1:55
  Speaker 2: often has parasites in it. But Daniel, my question for
• 1:58
  Speaker 2: you is not if you live in the United States,
• 2:00
  Speaker 2: I don't want to get people worried about parasites. But OK,
• 2:02
  Speaker 2: my question for you is today we're talking about like
• 2:04
  Speaker 2: a big result that was announced relative to dark matter.
• 2:08
  Speaker 2: What was your response when you heard the news?
• 2:11
  Speaker 1: My response was, oh, that's interesting. OK, because it's not
• 2:16
  Speaker 1: something I expected. These experiments have been looking for dark
• 2:18
  Speaker 1: matter forever, and it's been a parade of negative results
• 2:22
  Speaker 1: of we didn't see anything. Nope, we didn't find anything. Nope,
• 2:25
  Speaker 1: nothing here for like more than a decade. So at
• 2:29
  Speaker 1: some point you start getting bored and you stop paying
• 2:31
  Speaker 1: attention to all their news releases because they're all the same.
• 2:35
  Speaker 1: But I saw people posting on Twitter in advance of
• 2:37
  Speaker 1: this one saying, y'all better show up for this one.
• 2:40
  Speaker 1: This one's something different. So there was a lot of
• 2:42
  Speaker 1: buzz before this one. And then the result itself was
• 2:45
  Speaker 1: quite weird. So there's a lot to dig into here.
• 2:47
  Speaker 2: Yeah, I'm excited because I'm pretty sure that you and
• 2:50
  Speaker 2: I have talked about this experiment on the show before.
• 2:53
  Speaker 2: And we've been like, oh, but those giant vats haven't
• 2:55
  Speaker 2: found anything. But today we get to talk about how
• 3:00
  Speaker 2: maybe they found something.
• 3:01
  Speaker 1: Maybe the giant vats will tell us something about the universe. Amazing.
• 3:06
  Speaker 2: Amazing. And we, you know, we did the thing that
• 3:08
  Speaker 2: we always do when we have an interesting question. We
• 3:11
  Speaker 2: see what the extraordinaries have to say about it. And
• 3:13
  Speaker 2: so you wrote the extraordinaries and asked, did the LZ
• 3:17
  Speaker 2: experiment just observe a particle of dark matter? That was
• 3:21
  Speaker 2: a very specific question you sent them, Daniel.
• 3:25
  Speaker 1: Based on experience, I try to give them a few
• 3:26
  Speaker 1: clues so they know at least a little bit about
• 3:29
  Speaker 1: what we're talking about. Plus, I was curious about how
• 3:32
  Speaker 1: much of this news had penetrated into the sort of
• 3:35
  Speaker 1: global news cycle.
• 3:36
  Speaker 2: Yeah.
• 3:37
  Speaker 1: All right.
• 3:37
  Speaker 2: Well, let's see what they had to say.
• 3:38
  Speaker 1: We're not ready to say for sure that it's dark matter.
• 3:42
  Speaker 1: It must be something to do with dark matter. So
• 3:44
  Speaker 1: I hope it did. Well, if it did, that would
• 3:46
  Speaker 1: be absolutely massive. So I feel very confident saying, nope,
• 3:51
  Speaker 1: no dark matter particles actually detected. Really curious to know
• 3:54
  Speaker 1: your opinion about it. But my guess would be that
• 3:56
  Speaker 1: they found something interesting, but not dark matter.
• 3:59
  Speaker 2: It's possible that the LZ experiment discovered a dark matter particle.
• 4:03
  Speaker 1: OK, thanks. Bye. lazy experiment did not observe dark matter
• 4:08
  Speaker 1: because dark matter is too lazy to interact with anything.
• 4:14
  Speaker 1: That'd be amazing, but I think it's too good news
• 4:16
  Speaker 1: to be true.
• 4:17
  Speaker 2: Well, I love our extraordinaries. They are appropriately skeptical, but
• 4:22
  Speaker 2: they are excited and they want to know what Daniel
• 4:24
  Speaker 2: has to say.
• 4:26
  Speaker 1: And they make silly jokes about the name of the experiment,
• 4:29
  Speaker 1: which really is very mockable. I mean, It's called the
• 4:33
  Speaker 1: LZ experiment, which is short for the Lux Zeppelin experiment
• 4:37
  Speaker 1: because it's a merger of two previous experiments, both of
• 4:40
  Speaker 1: which are their own acronyms.
• 4:42
  Speaker 2: Oh, no.
• 4:42
  Speaker 1: So it's an acronym of shortened versions of other acronyms,
• 4:46
  Speaker 1: both of which are tortured. So it has nothing to
• 4:50
  Speaker 1: do with Led Zeppelin or heavy metal, but it does
• 4:53
  Speaker 1: have to do with liquid xenon.
• 4:55
  Speaker 2: Well, this wouldn't be the first time we discovered on
• 4:57
  Speaker 2: the show that physicists aren't great at naming things.
• 5:01
  Speaker 1: No, I think terrible acronyms should be applauded. I really
• 5:04
  Speaker 1: think it's a good part of science.
• 5:06
  Speaker 2: Terrible if they're like funny in some way, but if
• 5:09
  Speaker 2: they're just terrible because they're terrible.
• 5:11
  Speaker 1: No, I think it's like, it's like kitsch, you know,
• 5:14
  Speaker 1: like the worse the acronym, the better the science.
• 5:17
  Speaker 2: You and I can agree to disagree, but let's dig
• 5:20
  Speaker 2: into the heart of the matter.
• 5:22
  Speaker 1: A to D. What?
• 5:24
  Speaker 2: Oh, agree to disagree. See, you don't always need an acronym.
• 5:28
  Speaker 2: Sometimes it just makes things more confusing. All right, Daniel,
• 5:32
  Speaker 2: let's get to the heart of the matter.
• 5:33
  Speaker 1: Yes.
• 5:34
  Speaker 2: The dark heart of the matter. What is dark matter?
• 5:37
  Speaker 2: Let's start there.
• 5:38
  Speaker 1: Yeah, so dark matter is a theory that explains a
• 5:42
  Speaker 1: lot of different anomalies we notice about the universe. There's
• 5:46
  Speaker 1: a lot of things we see in the universe that
• 5:47
  Speaker 1: we can't explain without adding a lot of invisible matter
• 5:51
  Speaker 1: that gravitates. So it seems like 85% of the matter
• 5:55
  Speaker 1: that's out there in the universe is this kind of
• 5:58
  Speaker 1: invisible matter. It creates gravity and we see its effects
• 6:02
  Speaker 1: on like how galaxies rotate and other stuff, but it
• 6:05
  Speaker 1: doesn't emit any light. It doesn't absorb any light. It
• 6:08
  Speaker 1: doesn't scatter any light. So it doesn't interact electromagnetically or
• 6:12
  Speaker 1: with a strong force. but it's out there and it
• 6:15
  Speaker 1: has mass, and so it creates gravity. And essentially, we
• 6:19
  Speaker 1: have all these various observations, things we see in the
• 6:22
  Speaker 1: universe that we can't explain without adding a huge amount
• 6:27
  Speaker 1: of dark matter. And if you only saw one piece
• 6:29
  Speaker 1: of evidence like that, you might think, well, maybe there's
• 6:31
  Speaker 1: something wrong with your measurements or your understanding of physics
• 6:34
  Speaker 1: or something. But we have like seven or eight completely
• 6:37
  Speaker 1: independent lines of evidence, things that we cannot explain without
• 6:41
  Speaker 1: adding this same amount of dark matter. And each one
• 6:45
  Speaker 1: tells us the same story about how much dark matter
• 6:48
  Speaker 1: there needs to be and roughly where it is and
• 6:50
  Speaker 1: how it played a role in the history of the universe.
• 6:53
  Speaker 1: So it's really a very nice coherent story to explain
• 6:56
  Speaker 1: a lot of anomalies that we otherwise cannot explain.
• 6:59
  Speaker 2: And we recently did an episode called Is Dark Matter
• 7:02
  Speaker 2: a Fudge Factor? Where we went through all of the
• 7:05
  Speaker 2: evidence for dark matter being like a real thing. Yeah.
• 7:09
  Speaker 2: Even though we have trouble measuring it. Can you go
• 7:12
  Speaker 2: through and like remind us of just a couple of those?
• 7:14
  Speaker 2: Because I feel like it's important that we like have
• 7:16
  Speaker 2: it in our head how confident we are that this
• 7:18
  Speaker 2: exists before we get on to the experiments. Yeah.
• 7:21
  Speaker 1: And so there's a lot of reason for confidence, which
• 7:23
  Speaker 1: we'll talk about. And there's also still big open questions. Yeah.
• 7:25
  Speaker 1: One of the big open questions is like, we know
• 7:27
  Speaker 1: it's there, we know it has mass, but what is it?
• 7:30
  Speaker 1: And that's the central question that still remains open. But
• 7:33
  Speaker 1: whether it exists at all is much less uncertain. And
• 7:37
  Speaker 1: this sounds a little bit like a contradiction. Like, you
• 7:40
  Speaker 1: know it's there, you say there's a lot of evidence
• 7:41
  Speaker 1: for it, but also you've never seen it, right? And
• 7:44
  Speaker 1: the reason is that we've seen it a lot. We
• 7:47
  Speaker 1: have a lot of evidence for it gravitationally, but individual
• 7:50
  Speaker 1: particles don't feel a lot of gravity, right? We can
• 7:53
  Speaker 1: see evidence for dark matter in like galaxy-sized clumps of
• 7:57
  Speaker 1: it or planet-sized clumps of it, but an individual particle
• 8:00
  Speaker 1: feels almost no gravity. So if you have a particle
• 8:02
  Speaker 1: out there and you're trying to detect it with gravity,
• 8:04
  Speaker 1: it's almost hopeless. So that's why we have this juxtaposition
• 8:08
  Speaker 1: of being very confident that it's out there and that
• 8:11
  Speaker 1: it has mass and not a lot of confidence for
• 8:14
  Speaker 1: what it is, which is what we're going to talk
• 8:15
  Speaker 1: about today. But in terms of confidence that it exists,
• 8:19
  Speaker 1: we see that it affects how galaxies spin, right? Galaxies,
• 8:23
  Speaker 1: when they spin, they need a lot of mass to
• 8:25
  Speaker 1: hold themselves together because they spin really, really fast. Without
• 8:28
  Speaker 1: dark matter, you can't explain why they hold themselves together.
• 8:31
  Speaker 1: We see lensing, where light passing through space gets bent
• 8:34
  Speaker 1: because mass in that space is curving space, which changes
• 8:37
  Speaker 1: how light moves through it. It's a really cool feature
• 8:40
  Speaker 1: of Einstein's gravity that Newton didn't have. Light feels gravity,
• 8:44
  Speaker 1: Not because light has mass, but because mass bends space
• 8:48
  Speaker 1: and light moves through that space. So you can use
• 8:50
  Speaker 1: the bending of space to tell how much mass is there.
• 8:53
  Speaker 1: And that tells you about mass. You can also look
• 8:56
  Speaker 1: at the very early universe and see the particles in
• 8:59
  Speaker 1: the early universe when they're sloshing around in that plasma.
• 9:01
  Speaker 1: Do we understand those dynamics? And it turns out, not
• 9:04
  Speaker 1: without dark matter. If you didn't have dark matter, it
• 9:07
  Speaker 1: would slosh around very differently. And we see those ripples.
• 9:10
  Speaker 1: We can measure them very, very precisely. We can see
• 9:12
  Speaker 1: the structure of the universe. Like if you run a
• 9:14
  Speaker 1: simulation of the universe without dark matter, do you get
• 9:17
  Speaker 1: the formation of stars and galaxies? No, you don't. You
• 9:20
  Speaker 1: have to add dark matter. The universe needs that gravity
• 9:24
  Speaker 1: to pull everything together to make the universe as we
• 9:27
  Speaker 1: see it. And so these are all very strong but
• 9:30
  Speaker 1: exclusively gravitational pieces of information. And the thing to understand
• 9:35
  Speaker 1: is like, We have no other theory that explains all
• 9:37
  Speaker 1: of this. There are other theories to explain galaxy rotation
• 9:41
  Speaker 1: curves or one or two other lines of evidence, but
• 9:44
  Speaker 1: there's no other explanation that accommodates all of these things.
• 9:48
  Speaker 1: That's why it's the mainstream theory. And that's why it
• 9:50
  Speaker 1: should be the mainstream theory. And we should be open-minded
• 9:53
  Speaker 1: to other ideas, and we are. And if you came
• 9:55
  Speaker 1: along with a new theory today of dark shmatter, and
• 9:58
  Speaker 1: it was a completely different theory, then you would have
• 10:00
  Speaker 1: to also explain all of this data. Because science has
• 10:03
  Speaker 1: egos and politics, but mostly it's about the data. If
• 10:06
  Speaker 1: you can explain all this data with a better theory,
• 10:09
  Speaker 1: you would be a star. But nobody can do that.
• 10:12
  Speaker 1: That's why dark matter still reigns as the mainstream theory.
• 10:15
  Speaker 2: Dark schmatter sounds like a more liquidy dark matter. And
• 10:19
  Speaker 2: I feel like we're sort of back to biology. But
• 10:21
  Speaker 2: so when we look at these different lines of evidence
• 10:25
  Speaker 2: that suggest that dark matter is out there, do they
• 10:27
  Speaker 2: tell us similar things about dark matter? Like give us
• 10:30
  Speaker 2: similar estimates about how much of it is out there? Yes.
• 10:34
  Speaker 2: They're consistent?
• 10:35
  Speaker 1: Yes, absolutely. Each one tells us something different. Like galaxy
• 10:39
  Speaker 1: rotation curves tell us roughly how much dark matter there
• 10:42
  Speaker 1: is in galaxies, and also how it's distributed through the galaxy.
• 10:46
  Speaker 1: Because each star's motion tells you how much mass there
• 10:49
  Speaker 1: is within its orbit. So as you go further and
• 10:52
  Speaker 1: further out, you're looking for more and more distant stars.
• 10:54
  Speaker 1: They tell you about the integrated mass in the whole galaxy.
• 10:57
  Speaker 1: So because you can look at stars at the very
• 10:59
  Speaker 1: center and stars at the very edge, you can tell
• 11:00
  Speaker 1: roughly where the dark matter is. So that's very cool.
• 11:04
  Speaker 1: The cosmic microwave background radiation tells you roughly what proportion
• 11:07
  Speaker 1: of the universe is dark matter. And those numbers all
• 11:10
  Speaker 1: add up. So they're all telling a consistent story. Each
• 11:13
  Speaker 1: piece of evidence tells you a different subset of it,
• 11:16
  Speaker 1: but there's lots of overlapping places where two measurements tell
• 11:19
  Speaker 1: you the same fraction of the universe is dark matter,
• 11:22
  Speaker 1: or it's in the same place, roughly. So we have
• 11:24
  Speaker 1: lots of corroboration, absolutely.
• 11:27
  Speaker 2: How do you know it's not just normal matter, but
• 11:28
  Speaker 2: we've been measuring it wrong or misunderstanding something about it?
• 11:31
  Speaker 1: Yeah, exactly. So we've been treating dark matter as if
• 11:34
  Speaker 1: it's something new, right? Like it's some new kind of particle.
• 11:37
  Speaker 1: The question, what is it made out of, assumes it's
• 11:40
  Speaker 1: not made out of quarks and electrons like our kind
• 11:42
  Speaker 1: of matter, right? The way like goats and sheep and
• 11:44
  Speaker 1: kittens and stuff are. So why do we think it
• 11:47
  Speaker 1: has to be something new? And the answer is that
• 11:50
  Speaker 1: we have a pretty good accounting for what happened to
• 11:52
  Speaker 1: all the quarks. Which sounds crazy. Like, how could you
• 11:54
  Speaker 1: possibly understand all the quarks from the early universe? And
• 11:58
  Speaker 1: that's because we have a measurement that's very sensitive to
• 12:01
  Speaker 1: the density of quarks in the early universe. So the
• 12:04
  Speaker 1: early universe starts out, things are very hot and very dense,
• 12:08
  Speaker 1: and then things are cooling and spreading out and getting
• 12:10
  Speaker 1: colder and more dilute. And at one moment, it went
• 12:14
  Speaker 1: through this phase where it was about as hot and
• 12:16
  Speaker 1: dense as the center of the sun. And so it
• 12:19
  Speaker 1: did what the center of the sun does, which is
• 12:21
  Speaker 1: it makes heavier stuff. So you have all these protons around.
• 12:24
  Speaker 1: It was hot. It was dense. It fused them. It
• 12:26
  Speaker 1: made some helium. And exactly how much helium got made
• 12:29
  Speaker 1: and how much lithium got made and how much trace
• 12:32
  Speaker 1: amounts of even heavier stuff got made is very sensitive
• 12:35
  Speaker 1: to the quark density. If the quark density was a
• 12:37
  Speaker 1: little higher, you get much more heavy elements. If the
• 12:39
  Speaker 1: quark density was a little lower, you get much less
• 12:42
  Speaker 1: heavy elements. So you measure the heavy elements produced in
• 12:45
  Speaker 1: the early universe, which we can do very, very well.
• 12:48
  Speaker 1: And that tells you what the quark density was. All right.
• 12:50
  Speaker 1: So if you know what the quark density was in
• 12:52
  Speaker 1: the early universe, you can say, well, is there enough
• 12:55
  Speaker 1: quarks around to explain everything we see? And the answer
• 12:57
  Speaker 1: is yes. The quark density maps pretty well to the
• 13:01
  Speaker 1: atomic matter density. Like stuff that's built out of quarks
• 13:03
  Speaker 1: today lines up pretty well with the quark density in
• 13:06
  Speaker 1: the early universe. So you can't use quarks to explain
• 13:10
  Speaker 1: another factor of five of matter. Maybe we got that
• 13:13
  Speaker 1: a little bit wrong. Maybe there are a little more
• 13:14
  Speaker 1: quarks in the early universe than we think. There's not
• 13:17
  Speaker 1: a missing factor of five. So whatever this is, it
• 13:19
  Speaker 1: can't be made out of quarks, which means it's something
• 13:22
  Speaker 1: very new because everything we know, everything you've ever had
• 13:25
  Speaker 1: for lunch is made out of quarks. Quarks can be delicious,
• 13:29
  Speaker 1: but dark matter is not made out of them.
• 13:31
  Speaker 2: But is dark matter- It's everywhere, right? So I've probably
• 13:35
  Speaker 2: accidentally either ingested dark matter or had it pass through me.
• 13:39
  Speaker 2: Not the biology kind, but the physics kind.
• 13:42
  Speaker 1: Well, I hope you're passing dark matter and not dark splatter.
• 13:44
  Speaker 2: Oh, I thought you called it schmatter.
• 13:46
  Speaker 1: Oh, dark splatter.
• 13:46
  Speaker 2: It should have been dark splatter. But anyway.
• 13:49
  Speaker 1: Yeah, we think dark matter is mostly everywhere. So dark
• 13:53
  Speaker 1: matter doesn't interact the way normal matter does. Normal matter
• 13:56
  Speaker 1: is very clumpy, right? Two rocks in space hit each other. Boom,
• 13:59
  Speaker 1: they stick together. And that's how you get like formation
• 14:03
  Speaker 1: of planets, et cetera. Dark matter doesn't have those interactions,
• 14:05
  Speaker 1: we think. So it mostly passes through itself and normal matter.
• 14:09
  Speaker 1: So it remains in these big fluffy clouds. So it
• 14:12
  Speaker 1: does clump together because of gravity, but it's not as
• 14:14
  Speaker 1: clumpy as normal matter. And you know, we're big fans
• 14:17
  Speaker 1: of clumpiness here.
• 14:18
  Speaker 2: One of us.
• 14:19
  Speaker 1: I mean, I don't know if you prefer things more
• 14:23
  Speaker 1: loose and watery, but I like clumps. So dark matter
• 14:26
  Speaker 1: is less clumpy, which means that like, The galaxy is
• 14:29
  Speaker 1: in a big dark matter halo. It's not the dark
• 14:31
  Speaker 1: matter spread evenly through the universe. Like we're in a
• 14:34
  Speaker 1: big dark matter halo, our galaxy. It's probably spread more
• 14:37
  Speaker 1: evenly than normal matter. So there's a less local density,
• 14:41
  Speaker 1: but it's everywhere. So yeah, it's passing through us. We're
• 14:43
  Speaker 1: passing through it. There's like a dark matter wind passing
• 14:46
  Speaker 1: through us right now. So have you eaten some? Well,
• 14:49
  Speaker 1: probably some has gone through your open mouth, but then
• 14:52
  Speaker 1: it just goes out the back of your head. So
• 14:53
  Speaker 1: if you're not metabolizing it, I don't think you're really
• 14:55
  Speaker 1: eating it.
• 14:56
  Speaker 2: Yeah, that's a fine definition of Eden, I think. I
• 14:59
  Speaker 2: can get behind that. Are there other theories for this?
• 15:02
  Speaker 1: So there are a few alternative ideas to explain these
• 15:05
  Speaker 1: gravitational anomalies. One is called modified Newtonian dynamics. Maybe you
• 15:10
  Speaker 1: misunderstand how gravity works, and that would explain it. You
• 15:12
  Speaker 1: don't need to add this crazy amount of matter. And
• 15:15
  Speaker 1: that works to explain like galactic rotation curves, and one
• 15:18
  Speaker 1: or two other lines of evidence can be made to accommodate,
• 15:21
  Speaker 1: but not all of them. It just doesn't work. That
• 15:24
  Speaker 1: doesn't mean it's crazy talk where people shouldn't think about it.
• 15:26
  Speaker 1: Like absolutely keep thinking about it, work on it, but
• 15:29
  Speaker 1: it's not really a full alternative to dark matter. The
• 15:33
  Speaker 1: one that's the most surprising to me is very popular online.
• 15:36
  Speaker 1: You see people commenting on videos is this electric universe
• 15:39
  Speaker 1: theory that says like there is no dark matter. It's
• 15:42
  Speaker 1: just electricity everywhere because look, electricity is much more powerful
• 15:46
  Speaker 1: than gravity, right? And so maybe you're just not thinking
• 15:50
  Speaker 1: about electricity, right? And they think the whole universe is
• 15:53
  Speaker 1: like powered by electricity. Like the sun is powered by
• 15:56
  Speaker 1: galactic electric currents instead of nuclear fusion and all sorts
• 15:59
  Speaker 1: of crazy stuff. And it's a fun theory. It's like
• 16:04
  Speaker 1: has lots of nice visual analogies and connects to like
• 16:07
  Speaker 1: ancient mythology, et cetera, et cetera. But it doesn't really
• 16:09
  Speaker 1: have like a solid mathematical framework. And it's also violently
• 16:13
  Speaker 1: in tension with what we see in the universe. Like
• 16:16
  Speaker 1: we know the sun is powered by nuclear fusion. We
• 16:18
  Speaker 1: measure neutrinos from the sun. And we can very accurately
• 16:22
  Speaker 1: measure the energies of those neutrinos. And it's just what
• 16:24
  Speaker 1: you would expect if there was nuclear fusion happening in
• 16:27
  Speaker 1: the sun. So not a big mystery about that kind
• 16:29
  Speaker 1: of thing. We know that most of space is electrically neutral,
• 16:32
  Speaker 1: which is why electromagnetism, despite being much more powerful than gravity,
• 16:37
  Speaker 1: is essentially irrelevant on cosmic scales because there's two electric charges.
• 16:42
  Speaker 1: And so things can balance each other out. Whereas there's
• 16:44
  Speaker 1: not two gravitational charges. There's no negative mass to balance
• 16:48
  Speaker 1: the positive mass. So gravity will always be around, whereas
• 16:52
  Speaker 1: electromagnetism can neutralize itself. So the problem with the Electric
• 16:56
  Speaker 1: Universe Theory, despite its incredible surprising popularity online, is that
• 17:00
  Speaker 1: it doesn't describe the universe. And, you know, you got
• 17:03
  Speaker 1: to follow the data. It doesn't matter if your theory
• 17:05
  Speaker 1: is pretty or if you think Ramses II wrote about
• 17:08
  Speaker 1: it or whatever. Like, it's got to follow the data.
• 17:11
  Speaker 1: And Electric Universe Theory just doesn't.
• 17:14
  Speaker 2: Was Ramses II a particularly astute physicist?
• 17:17
  Speaker 1: No, not at all. But there's also these crazy theories
• 17:19
  Speaker 1: that the pyramids were like energy generating devices instead of tombs. Like,
• 17:24
  Speaker 1: maybe I've done too much reading of crazy conspiracy theories online.
• 17:29
  Speaker 2: All right. Well, I actually feel like I'd like to
• 17:31
  Speaker 2: hear more about that at some point. But today I
• 17:33
  Speaker 2: want to get to this big, exciting experimental result. So
• 17:36
  Speaker 2: let's take a break. And when we get back, we'll
• 17:38
  Speaker 2: talk about some ways that we've started looking for dark matter.
• 17:41
  Speaker 1: Thank you.
• 18:01
  Speaker 2: And we're back. And Daniel has just told us about
• 18:03
  Speaker 2: the evidence we have for the existence of dark matter.
• 18:06
  Speaker 2: But it's hard to find. So Daniel, just because something
• 18:09
  Speaker 2: is hard doesn't mean scientists throw up their hands and
• 18:11
  Speaker 2: give up.
• 18:12
  Speaker 1: Exactly.
• 18:12
  Speaker 2: What have we tried to look at it?
• 18:15
  Speaker 1: So we know that dark matter is out there. We
• 18:17
  Speaker 1: know how much of it there is in terms of mass.
• 18:21
  Speaker 1: We know roughly where it is. But we don't know
• 18:23
  Speaker 1: what it's made out of. Like everything we've ever seen
• 18:26
  Speaker 1: has been made out of particles. So we think, well,
• 18:28
  Speaker 1: maybe dark matter is also made out of particles. And
• 18:31
  Speaker 1: that's a maybe, right? Let's say that honestly and upfront,
• 18:33
  Speaker 1: that's a guess. And it's not necessarily a well-justified guess.
• 18:38
  Speaker 1: We're extrapolating from an unrepresentative corner of the universe, normal matter, atoms,
• 18:43
  Speaker 1: et cetera, which make up 5% of the universe and saying,
• 18:46
  Speaker 1: maybe the rules we learned here apply everywhere. And that
• 18:49
  Speaker 1: hasn't gone well historically. Right. The rules of motion on
• 18:54
  Speaker 1: Earth don't apply everywhere because it turns out the rules
• 18:56
  Speaker 1: are different at high speeds or near black holes or whatever.
• 18:59
  Speaker 1: But it's also a reasonable starting place. Physics is usually,
• 19:03
  Speaker 1: let's try the dumbest thing first. That didn't work? Okay,
• 19:05
  Speaker 1: let's try the second dumbest thing. Let's be lazy about
• 19:08
  Speaker 1: it and build up. Because you don't want to start
• 19:10
  Speaker 1: with complexity. Start with the simplest thing. So the simplest
• 19:12
  Speaker 1: idea is maybe it's a particle and maybe it has
• 19:16
  Speaker 1: some kind of interaction with our matter. Again, an assumption
• 19:19
  Speaker 1: because earlier we said, We know dark matter feels gravity.
• 19:23
  Speaker 1: We don't know that it feels anything else. So it
• 19:25
  Speaker 1: might be that dark matter is a particle, but it
• 19:28
  Speaker 1: only feels gravity, in which case we probably will never
• 19:31
  Speaker 1: figure out what it's made out of because an individual
• 19:34
  Speaker 1: particle's gravity is basically zero. And here, again, I'm saying
• 19:37
  Speaker 1: with a lot of confidence that dark matter is a
• 19:39
  Speaker 1: thing and that it's out there and has mass, but
• 19:41
  Speaker 1: we've never seen it and we might not be able to.
• 19:44
  Speaker 1: Those two things are not in conflict. What I mean
• 19:47
  Speaker 1: is that we know it's out there in a bulk sense,
• 19:49
  Speaker 1: but we don't know what it's made out of, and
• 19:50
  Speaker 1: we may not figure that out. But you don't just
• 19:52
  Speaker 1: throw up your hands. You say, well, let's assume that
• 19:55
  Speaker 1: it does have some kind of interaction. What if it
• 19:57
  Speaker 1: has some kind of interaction with normal matter that's not gravity,
• 20:01
  Speaker 1: some new kind of dark force? So now we're adding complexity.
• 20:04
  Speaker 1: We're saying not just that there's dark matter out there,
• 20:07
  Speaker 1: what if there's some new force we've never observed before
• 20:09
  Speaker 1: that lets dark matter interact with normal matter?
• 20:12
  Speaker 2: Gosh, where do you even start to figure out how
• 20:15
  Speaker 2: to measure that if it's happening?
• 20:16
  Speaker 1: Yeah, well, what you do is you look to see
• 20:18
  Speaker 1: if dark matter is interacting with normal matter. And one
• 20:21
  Speaker 1: way to do that is to try to make dark matter.
• 20:24
  Speaker 1: So that's the kind of thing that I worked on
• 20:26
  Speaker 1: for like 10 years. If dark matter interacts with normal matter,
• 20:30
  Speaker 1: that means that sometimes when you smash normal matter together,
• 20:33
  Speaker 1: you will produce dark matter. Because what happens at a
• 20:36
  Speaker 1: collider is you don't have to know in advance what
• 20:39
  Speaker 1: you're trying to make. It's not like chemistry, like you
• 20:41
  Speaker 1: have a cookbook and you're rearranging stuff from ingredients into cookies, right?
• 20:46
  Speaker 1: It's alchemy. At the collider, we do alchemy. We create
• 20:49
  Speaker 1: new kinds of stuff and it doesn't have to have
• 20:51
  Speaker 1: the remnants of the old kind of stuff. We annihilate
• 20:54
  Speaker 1: quarks and anti-quarks. It turns into a gluon and then
• 20:56
  Speaker 1: it can turn into anything that gluons couple to, or
• 20:59
  Speaker 1: it turns into a photon and then it can turn
• 21:01
  Speaker 1: into anything that photons couple to, or anything Z bosons
• 21:04
  Speaker 1: couple to. So, if there's some new kind of particle
• 21:08
  Speaker 1: out there, like a dark photon, maybe you annihilate quarks
• 21:11
  Speaker 1: and antiquarks and it turns into a dark photon and
• 21:14
  Speaker 1: then that turns into dark matter. So that's the kind
• 21:16
  Speaker 1: of thing we were hoping to see at the Large
• 21:18
  Speaker 1: Hadron Collider. And, you know, I have a great track
• 21:20
  Speaker 1: record of picking experiments that don't win Nobel Prizes. And
• 21:23
  Speaker 1: so we didn't see anything and I didn't win a
• 21:26
  Speaker 1: Nobel Prize. It would have been so awesome to make
• 21:29
  Speaker 1: it in the lab and be able to study it
• 21:31
  Speaker 1: and produce it at will. So awesome. Because then you're
• 21:33
  Speaker 1: not dependent on things like, well, I think there's a
• 21:35
  Speaker 1: dark matter wind. Let me try to spot it. You're like,
• 21:38
  Speaker 1: I'm just going to make it here in the lab.
• 21:40
  Speaker 1: And that's the awesome power of the collider. But we
• 21:42
  Speaker 1: haven't seen anything. And so that's been disappointing. Another thing
• 21:46
  Speaker 1: you can do is try to reverse that and say, well, look,
• 21:49
  Speaker 1: if you could smash quarks and anti-quarks together and make
• 21:52
  Speaker 1: a dark photon, which turns into dark matter, what about
• 21:55
  Speaker 1: the opposite? What happens when two dark matter particles smash
• 21:58
  Speaker 1: into each other? Sometimes they should produce a dark photon,
• 22:02
  Speaker 1: which should turn into quarks and anti-quarks. Basically, the inverse
• 22:05
  Speaker 1: of what we're hoping to happen at the collider should
• 22:08
  Speaker 1: happen out there in the universe if there's dark matter
• 22:11
  Speaker 1: banging into other dark matter.
• 22:13
  Speaker 2: And if dark matter is anything like, I mean, so
• 22:15
  Speaker 2: like assuming that it would make anti-quarks is an assumption.
• 22:18
  Speaker 1: Yes, exactly. So we got a couple of assumptions here. One,
• 22:21
  Speaker 1: it's a particle. Two, it has some kind of interaction
• 22:24
  Speaker 1: with normal matter that couples to quarks or maybe electrons
• 22:27
  Speaker 1: or something we can see. Okay. Both of those are
• 22:30
  Speaker 1: just guesses. Guesses that we have to make to make
• 22:32
  Speaker 1: this whole experimental program feasible. And so you might think, well,
• 22:35
  Speaker 1: you're just guessing. Well, that's true. But again, it's a
• 22:38
  Speaker 1: reasonable guess and it's all we can do. Yeah. let's
• 22:40
  Speaker 1: hit those nails with a hammer because the hammer's all
• 22:43
  Speaker 1: we got. Yeah, sure.
• 22:45
  Speaker 2: Reasonable place to start.
• 22:46
  Speaker 1: And so where might this happen? Well, most of the
• 22:48
  Speaker 1: dark matter in the galaxy is in the center of
• 22:51
  Speaker 1: the galaxy because that's where most of the gravity is.
• 22:54
  Speaker 1: And so that's a big blob in the center of
• 22:55
  Speaker 1: the galaxy. So if sometimes dark matter bumps into other
• 22:59
  Speaker 1: dark matter to make normal matter, you expect it to
• 23:01
  Speaker 1: happen mostly in the center of the galaxy. And we
• 23:03
  Speaker 1: have special space telescopes trained on the center of the
• 23:06
  Speaker 1: galaxy looking for just this. And one of them is
• 23:09
  Speaker 1: called the Fermi LAT telescope. It's basically a particle detector
• 23:12
  Speaker 1: in space that looks for high energy particles from the
• 23:15
  Speaker 1: center of the galaxy. We also look for it in Antarctica.
• 23:19
  Speaker 1: So we talked about ice cubes sometimes, which is a
• 23:21
  Speaker 1: cubic kilometer of ice in Antarctica that's been instrumented with cameras.
• 23:26
  Speaker 1: And if you have dark matter that comes through the
• 23:28
  Speaker 1: earth and interacts and makes a muon, you could see
• 23:31
  Speaker 1: that in the ice. All sorts of ways to look
• 23:34
  Speaker 1: for this. This is really, really hard because we don't
• 23:38
  Speaker 1: understand the center of the galaxy very well. It's filled
• 23:40
  Speaker 1: with all sorts of mysteries, black holes and radiation and
• 23:43
  Speaker 1: all sorts of crazy stuff happening. So if you see
• 23:46
  Speaker 1: a signal from the center of the galaxy, it might
• 23:48
  Speaker 1: be dark matter, but it might just be like, oh,
• 23:50
  Speaker 1: it turns out you don't understand the center of the galaxy.
• 23:53
  Speaker 1: So it's a very tough thing to do. Worth doing,
• 23:55
  Speaker 1: but a tough thing to do. And they've seen all
• 23:57
  Speaker 1: sorts of signals in the last 10 years. And then
• 23:59
  Speaker 1: each time they're like, oh, This turns out to be
• 24:01
  Speaker 1: a quasar, or this turns out to be a pulsar,
• 24:04
  Speaker 1: or this turns out to be something else. Because the
• 24:06
  Speaker 1: universe is filled with super weird stuff, which is fun.
• 24:08
  Speaker 1: And you're going to learn a lot of astrophysics. but
• 24:11
  Speaker 1: they've never seen any dark matter.
• 24:12
  Speaker 2: And one thing that's presumably nice about these experiments is
• 24:15
  Speaker 2: that even if you don't get the answer you were
• 24:16
  Speaker 2: looking for, you've learned something.
• 24:18
  Speaker 1: Oh, yeah.
• 24:19
  Speaker 2: Right?
• 24:19
  Speaker 1: Absolutely.
• 24:20
  Speaker 2: So these are all designed to teach you something, maybe
• 24:23
  Speaker 2: not what you were expecting, but at the end, the
• 24:24
  Speaker 2: money is spent. You've learned more about the universe, even
• 24:27
  Speaker 2: if it wasn't what you thought.
• 24:28
  Speaker 1: For sure. And both of these directions are very general. Like,
• 24:31
  Speaker 1: we don't just build the LHC to look for dark matter.
• 24:34
  Speaker 1: We use it to look for all sorts of stuff.
• 24:35
  Speaker 1: It's done great physics. We build these telescopes to look
• 24:38
  Speaker 1: at the center of the galaxy to look for dark
• 24:40
  Speaker 1: matter and other stuff. So we've learned a lot from Fermilab.
• 24:43
  Speaker 1: It's been a fantastic success, despite not finding dark matter. Now,
• 24:47
  Speaker 1: the third category are much more specialized. This we call
• 24:51
  Speaker 1: direct detection. And this is where we got the really
• 24:53
  Speaker 1: exciting result from. These are experiments designed expressly to see
• 24:57
  Speaker 1: dark matter. And their goal is to detect this dark
• 25:00
  Speaker 1: matter wind. They're saying, if we're flying through a wind
• 25:03
  Speaker 1: of dark matter... and that dark matter can interact with
• 25:06
  Speaker 1: normal matter in some way, then eventually we should see it.
• 25:09
  Speaker 1: You should see dark matter bumping into normal matter, right?
• 25:13
  Speaker 1: If the two interact and you have a wind, you
• 25:15
  Speaker 1: should literally feel it.
• 25:16
  Speaker 2: Should you smell dark matter wind as well?
• 25:18
  Speaker 1: Sorry. She who smelt it dealt it. You know, that's
• 25:24
  Speaker 1: the rule in physics.
• 25:26
  Speaker 2: Oh, that's also the rule in biology. I guess we're
• 25:28
  Speaker 2: not so different after all.
• 25:29
  Speaker 1: There's some common ground. And the problem is that it's
• 25:32
  Speaker 1: hard to feel it because it's very weak, right? The
• 25:35
  Speaker 1: way that like you can't really feel neutrinos, there's trillions
• 25:39
  Speaker 1: of neutrinos passing through your body all the time, but
• 25:42
  Speaker 1: you don't really notice. You're not like beaten down by
• 25:44
  Speaker 1: the neutrino radiation because they mostly pass right through you
• 25:47
  Speaker 1: because the interaction between a neutrino and your matter is very,
• 25:50
  Speaker 1: very weak. And I sometimes hear people say it's because
• 25:53
  Speaker 1: neutrinos have low mass. It's not. Or because neutrinos are
• 25:57
  Speaker 1: super duper small. It's not. It's about the strength of
• 26:00
  Speaker 1: the interaction. When two particles come near each other, there's
• 26:03
  Speaker 1: a chance that they interact, and that depends on how
• 26:05
  Speaker 1: strong the interaction is. If it's very strong, like electromagnetism,
• 26:08
  Speaker 1: you know it's pretty much going to happen. If it's very,
• 26:11
  Speaker 1: very weak, like the weak force, then it happens 1
• 26:13
  Speaker 1: in 10 to the 20 times. So it's a quantum
• 26:16
  Speaker 1: mechanical probability that depends on the strength of the interaction.
• 26:20
  Speaker 1: So the idea is, well, how do you see something
• 26:23
  Speaker 1: that's very, very gentle and very, very rare? And the
• 26:26
  Speaker 1: strategy is to filter out everything else. So you get
• 26:29
  Speaker 1: a really big tank of liquid xenon, something which is
• 26:33
  Speaker 1: already very quiet, so it's not making a lot of noise.
• 26:35
  Speaker 1: It should just sit there inert and do nothing. And
• 26:38
  Speaker 1: you put it like a mile underground, so it's not
• 26:42
  Speaker 1: getting bombarded by cosmic radiation from space, cosmic rays and
• 26:46
  Speaker 1: high energy particles. So in principle, it should just sit there.
• 26:49
  Speaker 1: It should be totally quiet. It should be totally inert.
• 26:52
  Speaker 1: You should never see anything.
• 26:54
  Speaker 2: But.
• 26:55
  Speaker 1: If dark matter passes through all that rock and gets
• 26:58
  Speaker 1: to your detector, it should occasionally bump into one of
• 27:01
  Speaker 1: those particles and you should see it. So if you
• 27:04
  Speaker 1: build a huge vat of quiet liquid underground, it's sort
• 27:07
  Speaker 1: of like a camera for seeing really rare particles that
• 27:12
  Speaker 1: really weakly interact.
• 27:14
  Speaker 2: And so what would a weak interaction between dark matter
• 27:18
  Speaker 2: and xenon look like? Yeah.
• 27:21
  Speaker 1: So it would just bump a xenon nucleus, right? We
• 27:23
  Speaker 1: call it a nuclear recoil. It would just bang against
• 27:26
  Speaker 1: it and it would give off a little flash of
• 27:28
  Speaker 1: light because that nuclear recoil is getting some energy and
• 27:31
  Speaker 1: then it gives off that energy and also can lose electrons.
• 27:35
  Speaker 1: And so you look for that flash of light and
• 27:37
  Speaker 1: you look for those electrons and that's how you tell
• 27:40
  Speaker 1: that one of your xenons got bumped. Now the chances
• 27:43
  Speaker 1: of a xenon getting bumped is really small because the
• 27:46
  Speaker 1: dark matter coming through like rolls a dye that has
• 27:48
  Speaker 1: like 10 to the 20 sides on it, and it
• 27:51
  Speaker 1: has to get a one, but you got lots of xenon.
• 27:53
  Speaker 1: So the bigger your tank of xenon, the more rolls
• 27:56
  Speaker 1: you get on that huge die. And eventually you might
• 27:59
  Speaker 1: get one.
• 28:00
  Speaker 2: Okay, and what else could get through all of that Earth?
• 28:03
  Speaker 2: Do we think that only dark matter could get there?
• 28:06
  Speaker 1: Great question, because it's not like you see one, you
• 28:08
  Speaker 1: know you found dark matter. There are other ways to
• 28:10
  Speaker 1: see Xenon get bumped. So we're pretty sure that the
• 28:13
  Speaker 1: cosmic ray background is almost zero, right? Because you've got
• 28:17
  Speaker 1: like a mile of shielding of rock, but not exactly zero, right?
• 28:21
  Speaker 1: Everything is a probability. Also, the rock is radioactive. Like
• 28:25
  Speaker 1: there's some trace, uranium and whatever, and that emits particles.
• 28:28
  Speaker 1: And sometimes you're going to see that. You're going to
• 28:30
  Speaker 1: get neutrons or whatever. Or some other particle decays radioactively
• 28:34
  Speaker 1: and gives you a gamma ray. So they've thought very
• 28:36
  Speaker 1: carefully about that. And they've designed the detector specifically so
• 28:40
  Speaker 1: they can try to identify whether a flash came from
• 28:43
  Speaker 1: one of these other sources or whether it looks like
• 28:46
  Speaker 1: it came from dark matter.
• 28:47
  Speaker 2: Tell me more, Daniel.
• 28:49
  Speaker 1: And so the experiment we're talking about today is called
• 28:50
  Speaker 1: the LZ experiment. And it has seven tons of ultra-pure
• 28:55
  Speaker 1: liquid xenon. And it's a mile underground in an old
• 28:59
  Speaker 1: gold mine in Lead, South Dakota. It's called the SURF Lab,
• 29:04
  Speaker 1: which is some other tortured acronym I don't remember. And
• 29:06
  Speaker 1: it's a really cool device.
• 29:07
  Speaker 2: Finally, there's a fun acronym and you don't know what
• 29:09
  Speaker 2: it stands for.
• 29:10
  Speaker 1: Yeah. Well, I'm pretty sure they're not doing any surfing
• 29:14
  Speaker 1: in South Dakota.
• 29:15
  Speaker 2: In landlocked South Dakota. Yeah, probably not.
• 29:17
  Speaker 1: So it definitely qualifies for a tortured acronym.
• 29:21
  Speaker 2: Okay.
• 29:22
  Speaker 1: And they've designed this experiment specifically to give them handles
• 29:26
  Speaker 1: so they can answer questions like, is this likely to
• 29:28
  Speaker 1: have been a neutron? Is this likely to have been
• 29:30
  Speaker 1: a radioactive decay? Or does this look like dark matter?
• 29:34
  Speaker 1: And so it's a massive tank of xenon, but it's
• 29:38
  Speaker 1: also surrounded by a tank of water. And the tank
• 29:41
  Speaker 1: of water is there to suppress cosmic rays. So like
• 29:43
  Speaker 1: any muon that makes it through that rock is going
• 29:45
  Speaker 1: to go through that water and leave a flash. Cosmic
• 29:48
  Speaker 1: ray muons are moving really, really fast. They'll leave a
• 29:50
  Speaker 1: little flash in that water and you'll see it. So
• 29:53
  Speaker 1: it's like a veto. If you see something give a
• 29:55
  Speaker 1: flash in your outside water tank, then you're pretty sure
• 29:57
  Speaker 1: it's not going to be dark matter. You just throw
• 29:59
  Speaker 1: that away. And then they have a layer of something
• 30:02
  Speaker 1: called gadolinium, which is really good at seeing neutrons. It's
• 30:05
  Speaker 1: like super duper reactive to neutrons. So if you have
• 30:09
  Speaker 1: neutrons that come in and bump your xenon atom, You'll
• 30:12
  Speaker 1: also see a flash in your gadolinium. And she'd be like, oh, no,
• 30:14
  Speaker 1: that was just a neutron. Clever. We also have two
• 30:17
  Speaker 1: ways to measure the flash. They call them S1 and S2.
• 30:21
  Speaker 1: Brilliant names, scientists.
• 30:24
  Speaker 2: Way to go, physicist. Yeah.
• 30:25
  Speaker 1: S1 is that flash of light you get when the
• 30:28
  Speaker 1: xenon bumps, right? So the xenon gets some energy, then
• 30:31
  Speaker 1: it dumps some energy into a photon. It's a flash
• 30:33
  Speaker 1: of scintillation light. And they have the whole tank of
• 30:35
  Speaker 1: xenon surrounded by cameras to capture those flashes. Then also
• 30:40
  Speaker 1: the electrons that have gotten stripped off the xenon by
• 30:42
  Speaker 1: the nuclear recoil, they drift upwards because there's a whole
• 30:46
  Speaker 1: electric field in the thing. And the time it takes
• 30:48
  Speaker 1: the electron to drift upward tells you how far it drifted,
• 30:52
  Speaker 1: which tells you roughly where it happened.
• 30:54
  Speaker 2: This is awesome.
• 30:55
  Speaker 1: Yeah, it's super cool.
• 30:56
  Speaker 2: This is like really clever, multiple levels of cleverness.
• 30:59
  Speaker 1: And this is many generations in. They started out with
• 31:02
  Speaker 1: one idea and it worked, but they're like, ooh, this
• 31:04
  Speaker 1: is really hard to tell apart from neutrons. It's adding
• 31:06
  Speaker 1: this other layer. So this is the benefit of lots
• 31:09
  Speaker 1: of experience of knowing what goes wrong and designing exactly
• 31:13
  Speaker 1: an experiment the next time to make it more precise.
• 31:15
  Speaker 2: Awesome.
• 31:16
  Speaker 1: And another really cool trick is that you can look
• 31:19
  Speaker 1: at the ratio of S1, which is that first flash
• 31:21
  Speaker 1: of light, and S2, which is from the electrons that
• 31:25
  Speaker 1: drift upwards. And if you get a lot more energy
• 31:27
  Speaker 1: in S2 from the electrons, then you know that probably
• 31:30
  Speaker 1: what was hit was the electron cloud around your xenon
• 31:34
  Speaker 1: atom and not the nucleus itself.
• 31:36
  Speaker 2: Okay.
• 31:36
  Speaker 1: Because then you're getting a lot more energy in those electrons.
• 31:39
  Speaker 1: And that's really useful information because things like gamma rays
• 31:43
  Speaker 1: or anything that has electric charge, if it hits a
• 31:45
  Speaker 1: xenon atom, it's going to hit those electrons. Whereas a WIMP,
• 31:49
  Speaker 1: like a dark matter particle... It's not going to hit
• 31:52
  Speaker 1: those electrons because it doesn't have electric charge. It's going
• 31:54
  Speaker 1: to hit the nucleus, and it's going to bang against
• 31:56
  Speaker 1: those quarks. And so what you're looking for, if you
• 31:59
  Speaker 1: want to find something that hit the nucleus, is more
• 32:02
  Speaker 1: energy in that first flash and less in that electron splash.
• 32:06
  Speaker 2: Wow.
• 32:06
  Speaker 1: So you've got lots of handles here to figure out
• 32:09
  Speaker 1: what's dark matter, what's your typical background, et cetera, et cetera.
• 32:13
  Speaker 1: So they did a lot of work to try to
• 32:15
  Speaker 1: understand this.
• 32:16
  Speaker 2: So you said there's like literal tons of xenon.
• 32:18
  Speaker 1: Yeah.
• 32:19
  Speaker 2: Where does one get literally tons of xenon?
• 32:22
  Speaker 1: This turns out to be one of the limiting factors
• 32:25
  Speaker 1: because xenon is really rare. It's like 0.00001% of Earth's atmosphere.
• 32:32
  Speaker 1: And the whole global production of xenon is 40 tons
• 32:36
  Speaker 1: per year. Oh, wow. So if you want seven tons
• 32:39
  Speaker 1: of xenon, it's like a significant fraction of all the
• 32:42
  Speaker 1: xenon available to all industry on Earth every year. And
• 32:46
  Speaker 1: that's like a huge fraction of the budget. So they
• 32:49
  Speaker 1: spent $ 70 million on this liquid xenon for LZ. And
• 32:55
  Speaker 1: they couldn't buy it all in one year. They had
• 32:57
  Speaker 1: to stage it over several years. They have special contracts
• 33:00
  Speaker 1: with these producers. Like, please ramp up your production. We're
• 33:02
  Speaker 1: going to need this amount of Because you can't just
• 33:05
  Speaker 1: enter the market and be like, oh, we need 25%
• 33:08
  Speaker 1: of the whole market this year. So they have these contracts.
• 33:11
  Speaker 1: It's a whole thing. Liquid xenon is excellent for dark
• 33:14
  Speaker 1: matter detection because it's massive and it's inert. It also
• 33:18
  Speaker 1: turns out to be really good for aerospace and electronics
• 33:21
  Speaker 1: and semiconductor industry. They use xenon gas for plasma etching
• 33:26
  Speaker 1: to precisely etch nanoscale features on the silicon wafers. In aerospace,
• 33:30
  Speaker 1: they like it because it's a great fuel. Like we
• 33:32
  Speaker 1: talked about ion thrusters, they use liquid xenon, right? You
• 33:35
  Speaker 1: want these big, massive ions. So there are other places
• 33:38
  Speaker 1: that need liquid xenon, very reasonable, and they don't like
• 33:41
  Speaker 1: it when the scientists come in and be like, we're
• 33:43
  Speaker 1: taking all of it this year, by the way. So
• 33:45
  Speaker 1: that turns out to be quite complicated, yeah.
• 33:48
  Speaker 2: But once you've got it... Do you need to replenish
• 33:51
  Speaker 2: it or are you just, you've got it and then
• 33:53
  Speaker 2: you're good for forever?
• 33:54
  Speaker 1: You've got it and you're mostly good. This is going
• 33:56
  Speaker 1: to be a little bit of leakage, so you've got
• 33:57
  Speaker 1: to top up occasionally. But no, you don't need to
• 34:00
  Speaker 1: have a continuous budget for buying liquid xenon. And that's
• 34:03
  Speaker 1: the challenge for this industry is that they're like, we're
• 34:05
  Speaker 1: going to come in with a huge order and then
• 34:06
  Speaker 1: we're going to disappear. So they're like, well, we're not
• 34:08
  Speaker 1: going to ramp up our production and then we're going
• 34:10
  Speaker 1: to be left with all this xenon nobody wants. So
• 34:13
  Speaker 1: it's a challenge for the industry to respond to that.
• 34:16
  Speaker 2: But this has been done.
• 34:17
  Speaker 1: It has been done.
• 34:18
  Speaker 2: It does exist.
• 34:18
  Speaker 1: Yeah. And it's this escalation. Like they started out these
• 34:21
  Speaker 1: detectors with a fraction of a ton. Then the next
• 34:24
  Speaker 1: generation was like the one ton. And this is the
• 34:26
  Speaker 1: many ton. And people are talking about like 100 ton
• 34:29
  Speaker 1: versions of this stuff.
• 34:30
  Speaker 2: Oh, wow.
• 34:30
  Speaker 1: And there's several of these detectors around the world. The
• 34:33
  Speaker 1: one we're talking about today is the LZ. But there's
• 34:35
  Speaker 1: a competing experiment called the Xenon experiment. And there's another
• 34:39
  Speaker 1: one in China called Panda X. And they all need
• 34:42
  Speaker 1: like their own five to 10 tons. And they're all
• 34:45
  Speaker 1: talking about 100 ton version of this thing.
• 34:47
  Speaker 2: Wow.
• 34:48
  Speaker 1: And so it's complicated.
• 34:49
  Speaker 2: It sounds like anybody who's looking for a business proposition
• 34:52
  Speaker 2: should get into Xenon production.
• 34:55
  Speaker 1: I'm not pumping shares in Xenon stock yet. But yeah,
• 34:58
  Speaker 1: it's important stuff and expensive.
• 35:00
  Speaker 2: All right. Well, let's take a break. And when we
• 35:01
  Speaker 2: get back, we will talk about the data set and
• 35:04
  Speaker 2: what we saw.
• 35:25
  Speaker 1: Okay.
• 35:25
  Speaker 2: So the LZ, which is part of the surf project,
• 35:28
  Speaker 2: if I'm getting all of these connections, correct. Saw something exciting. Yeah. First,
• 35:33
  Speaker 2: how long did they have to make measurements before they
• 35:36
  Speaker 2: saw something possibly exciting?
• 35:38
  Speaker 1: Yeah, so it's like the LHC where you build this thing,
• 35:40
  Speaker 1: but you're not running it all the time because sometimes
• 35:42
  Speaker 1: you're working on it and you want it to be
• 35:44
  Speaker 1: operating really, really well. So they have various data sets.
• 35:47
  Speaker 1: And this data set we're talking about today was taken
• 35:49
  Speaker 1: between March of 2023 and April of 2024. So that's
• 35:53
  Speaker 1: already two years ago. And they spent two years analyzing
• 35:57
  Speaker 1: this data. understanding this data, developing their models of the background,
• 36:02
  Speaker 1: figuring out how to select events to make sure they
• 36:04
  Speaker 1: get dark matter or they don't cut out any dark
• 36:07
  Speaker 1: matter events and they don't get drowned in the background.
• 36:10
  Speaker 1: It's really important. And what's really important about it is
• 36:12
  Speaker 1: designing it before you look at the data. In the end,
• 36:15
  Speaker 1: we're going to do statistics on like a handful of numbers.
• 36:19
  Speaker 1: Like we see one, we expect zero. We see two,
• 36:21
  Speaker 1: we expect one. You really have to make sure you're
• 36:24
  Speaker 1: not biasing yourself. It's very, very easy. to design an
• 36:27
  Speaker 1: experiment to get the result you want if you already
• 36:30
  Speaker 1: know where the data are. It's like predicting the stock
• 36:33
  Speaker 1: market after already looking at it. It's not very impressive.
• 36:36
  Speaker 1: It's not a measure of your ability to predict the
• 36:38
  Speaker 1: stock market. So what they do is they call salt
• 36:41
  Speaker 1: the data. They put a bunch of fake data in
• 36:43
  Speaker 1: with the real data, like fake dark matter and fake
• 36:46
  Speaker 1: this and fake the other things, so that nobody is
• 36:48
  Speaker 1: biased by what they're seeing. Because nobody knows which is real.
• 36:51
  Speaker 1: And they design the analysis techniques without like looking at
• 36:54
  Speaker 1: the actual clean data set until the very, very end,
• 36:57
  Speaker 1: until they're very, very short, because you only get one try.
• 37:00
  Speaker 1: Once you've seen the data set, you can't change how
• 37:03
  Speaker 1: you're analyzing it. You can't say like anything in this box,
• 37:06
  Speaker 1: we're going to call a dark matter candidate and we're
• 37:08
  Speaker 1: going to predict the number of other things that might
• 37:10
  Speaker 1: fall in that box and then see a candidate just
• 37:13
  Speaker 1: outside the box and be like, oh, we're going to
• 37:14
  Speaker 1: change the box. We're going to grow the box to
• 37:16
  Speaker 1: get that one thing, right? You cannot sculpt your analysis
• 37:20
  Speaker 1: technique after you've seen the data. So that's why it
• 37:22
  Speaker 1: takes two years, because they have a single shot. They've
• 37:25
  Speaker 1: worked on this experiment for like a decade. They're not
• 37:27
  Speaker 1: wasting their one chance at the data to have a
• 37:29
  Speaker 1: fresh statistical look. Nice. So this data set that we're
• 37:32
  Speaker 1: talking about today is like 220 live days. And the
• 37:36
  Speaker 1: exciting thing is they have more data. They haven't yet analyzed.
• 37:40
  Speaker 1: So that's always cool.
• 37:41
  Speaker 2: Yeah, because then you can run your analysis multiple times
• 37:44
  Speaker 2: on the data and like increase your confidence.
• 37:46
  Speaker 1: Well, you can get fresh data. That's the most important thing.
• 37:48
  Speaker 1: You don't want to reanalyze old data. Yeah, yeah. You
• 37:51
  Speaker 1: want to answer questions in new data. And that's going
• 37:53
  Speaker 1: to be very exciting because what happened when they opened
• 37:55
  Speaker 1: up the data was very exciting for two reasons. One
• 37:58
  Speaker 1: is they saw something. So a lot of these experiments
• 38:01
  Speaker 1: they run, they see nothing, like not anything. Everything they see,
• 38:06
  Speaker 1: they're confident is background. So they have these very restrictive
• 38:09
  Speaker 1: selection requirements to remove all of that. And left in
• 38:12
  Speaker 1: what they call their signal region, candidates for dark matter
• 38:15
  Speaker 1: is zero. That's like very compelling, but also disappointing. It's
• 38:20
  Speaker 1: like very underwhelming to go to a whole presentation to
• 38:23
  Speaker 1: hear about this incredible engineering and then be like, we
• 38:25
  Speaker 1: saw nothing. It's like space balls. We ain't found shit,
• 38:30
  Speaker 1: you know? But it is an answer, but it's not
• 38:32
  Speaker 1: the science answer you want, right? So the exciting thing
• 38:35
  Speaker 1: is they saw something. Oh my gosh, there's an event
• 38:37
  Speaker 1: in there. And it's very hard to explain how they
• 38:41
  Speaker 1: would see anything but dark matter because they're very, very
• 38:44
  Speaker 1: good at removing radioactive background sources, at vetoing neutrons, at
• 38:49
  Speaker 1: vetoing gamma rays. What this looks like is looks very, very,
• 38:54
  Speaker 1: very much like a dark matter. Like the nuclear recoil
• 38:57
  Speaker 1: energy is very large and the electron flash is much smaller.
• 39:01
  Speaker 1: So it's exactly in the place you would expect dark matter.
• 39:04
  Speaker 1: So it's a very interesting, very compelling event. And They
• 39:08
  Speaker 1: went through in their presentation and talked about like all
• 39:11
  Speaker 1: the possible sources of background and calculated how likely is
• 39:14
  Speaker 1: it that this is this source of background or we're
• 39:16
  Speaker 1: seeing a neutrino or we're seeing a neutron or we're
• 39:19
  Speaker 1: seeing a radioactive decay. And all of those are like very,
• 39:22
  Speaker 1: very tiny, but non-zero probabilities. And that's the crux of
• 39:26
  Speaker 1: the issue is we have a single event. All we
• 39:29
  Speaker 1: know about it. are these two numbers s1 and s2
• 39:32
  Speaker 1: and we can calculate the probability that it came from
• 39:34
  Speaker 1: something boring some prosaic explanation and that's never zero and
• 39:39
  Speaker 1: anytime you have one event you're like well i mean
• 39:41
  Speaker 1: what does it mean right one event it could just
• 39:43
  Speaker 1: be good luck bad luck it could be anything and
• 39:47
  Speaker 1: But the other thing about this is that it's a
• 39:48
  Speaker 1: very weird event. It's very, very high energy. If we
• 39:53
  Speaker 1: were seeing the dark matter wind, you expect to see
• 39:55
  Speaker 1: a lot of events at lower energy and very few
• 39:58
  Speaker 1: at these very high energies. And so to see one
• 40:02
  Speaker 1: at very high energy and none at low energy, that's weird.
• 40:06
  Speaker 1: It's not what we expected to see from dark matter.
• 40:09
  Speaker 2: What does that mean?
• 40:10
  Speaker 1: So, of course, the first thing that happened when they
• 40:13
  Speaker 1: put this result out was that the theory community came
• 40:16
  Speaker 1: out with like a flurry of papers all trying to
• 40:19
  Speaker 1: explain what this is. And I was amazed. I mean,
• 40:22
  Speaker 1: even in these days of AI, like some of those
• 40:24
  Speaker 1: papers were posted hours after the result came out. So
• 40:28
  Speaker 1: like maybe there was some leaks or maybe people like
• 40:31
  Speaker 1: had their models ready for no matter what they posted.
• 40:33
  Speaker 1: And they're like, oh, okay, it's this one. Or maybe
• 40:36
  Speaker 1: they generated it with AI in five hours. I don't know.
• 40:39
  Speaker 1: But one possibility that people initially were attracted to is
• 40:43
  Speaker 1: a theory of dark matter called a Higgseno.
• 40:46
  Speaker 2: That's cute. Yeah. I like that.
• 40:49
  Speaker 1: Yeah, it is kind of cute.
• 40:50
  Speaker 2: Yeah.
• 40:50
  Speaker 1: Well, Eno, you know, is like cute and little. And
• 40:53
  Speaker 1: the Higgs Eno is a hypothetical particle that's related to
• 40:58
  Speaker 1: the Higgs. And you might remember we talked about supersymmetry.
• 41:01
  Speaker 1: That's this idea that every particle we know about has
• 41:04
  Speaker 1: a partner particle, the way that like every charged particle
• 41:07
  Speaker 1: has an antiparticle. This is like a different symmetry and
• 41:10
  Speaker 1: different kind of reflection. And every particle we know about
• 41:13
  Speaker 1: that's a boson has a partner particle where you add
• 41:16
  Speaker 1: an eno to it. So like photon is a photeno,
• 41:18
  Speaker 1: and the Z has a zeno, and the W is
• 41:21
  Speaker 1: a wino particle, and the Higgs is a Higgseno particle.
• 41:26
  Speaker 1: And so the idea is maybe this is a Higgseno.
• 41:29
  Speaker 1: Maybe this is a supersymmetric version of a Higgs boson.
• 41:33
  Speaker 1: And there's a whole theory about like the spectrum of
• 41:36
  Speaker 1: dark matter masses that would explain this particular event.
• 41:40
  Speaker 2: So a Higgs Zeno would give the same S1 and
• 41:43
  Speaker 2: S2 signal as dark matter. And is this, this wasn't
• 41:47
  Speaker 2: something that we anticipated before we got the results?
• 41:50
  Speaker 1: Yes, it would give this signature and it wasn't the
• 41:53
  Speaker 1: leading candidate. It was something that was out there. It's
• 41:56
  Speaker 1: like on the spectrum of ideas, but it wasn't the
• 41:59
  Speaker 1: most expected thing. It wasn't like the most vanilla idea.
• 42:02
  Speaker 1: You know, this is like the slightly spicy jalapeno version
• 42:05
  Speaker 1: of dark matter.
• 42:06
  Speaker 2: But this would make more sense with the, you said
• 42:09
  Speaker 2: there's no wind that came along with it. And so
• 42:13
  Speaker 2: dark matter would have the wind, but the Higzino would
• 42:15
  Speaker 2: be windless?
• 42:17
  Speaker 1: It would just explain why we saw it at high
• 42:19
  Speaker 1: energy rather than seeing it at low energy first. A
• 42:22
  Speaker 1: particular kind of dark matter that interacts in this way.
• 42:26
  Speaker 1: So that's really exciting. But that only lasted for like
• 42:28
  Speaker 1: a couple of days because then people were thinking, well,
• 42:30
  Speaker 1: if it is a Higgs Zeno, you should also see
• 42:32
  Speaker 1: this other kind of events at even higher energy. And
• 42:34
  Speaker 1: they don't see any of that. And so that theory
• 42:37
  Speaker 1: like lasted 24 hours. And now there's other ideas people have.
• 42:42
  Speaker 1: And like in the hallways here at Irvine, there's a
• 42:45
  Speaker 1: lot of talk every day about what the latest theories
• 42:47
  Speaker 1: are and what the papers are. There's like a proliferation
• 42:50
  Speaker 1: of theories out there to explain this. Which is a
• 42:53
  Speaker 1: lot of fun. And this is how science should work.
• 42:55
  Speaker 1: You know, you get ideas, you get competing ideas. It's
• 42:58
  Speaker 1: a scientific conversation, like live. It's out there for anybody
• 43:02
  Speaker 1: to participate in, which I think is cool. But also
• 43:04
  Speaker 1: we should remember, like, it's a single observation. We saw
• 43:08
  Speaker 1: one thing. It's like, you know, the wow signal. We
• 43:11
  Speaker 1: saw this one burp from space and never repeated. And
• 43:14
  Speaker 1: it makes it really hard to know, was it aliens?
• 43:17
  Speaker 1: Was it some weird hiccup in hydrogen somewhere? You really
• 43:21
  Speaker 1: need something to repeat to order to understand it. If
• 43:24
  Speaker 1: you just get one weird blip, it's like when you're
• 43:27
  Speaker 1: walking down the street and you have like a sudden
• 43:29
  Speaker 1: pain in one joint and you're like, uh-oh, but then
• 43:31
  Speaker 1: it never happens again. You forget about it. You're like, well,
• 43:34
  Speaker 1: I'm just going to move on with my life. You know,
• 43:36
  Speaker 1: single things can just be weird or random outliers. So
• 43:41
  Speaker 1: that's why it's really exciting that they have more data
• 43:44
  Speaker 1: they haven't looked at yet. If this is dark matter
• 43:47
  Speaker 1: and it's real, then it should happen regularly. If you
• 43:52
  Speaker 1: have four times as much data and you look at
• 43:55
  Speaker 1: your new data, you should see it four times as often.
• 43:57
  Speaker 1: There's going to be fluctuations, but if this is dark matter,
• 44:01
  Speaker 1: when they look at their new data, they should see
• 44:02
  Speaker 1: more examples. It should start to add up, it should
• 44:05
  Speaker 1: start to pile up as you get more and more data. Plus,
• 44:08
  Speaker 1: we have experiments around the world. We have one in Italy,
• 44:11
  Speaker 1: we have one in China. All are sensitive to the
• 44:14
  Speaker 1: same things using very similar technology, but not exactly so
• 44:17
  Speaker 1: they can cross-check each other. We should start to see
• 44:19
  Speaker 1: it in those datasets. So if this is real, then
• 44:22
  Speaker 1: in the next couple of years, it'll become overwhelming evidence
• 44:26
  Speaker 1: that we're seeing something new, something massive, something that interacts
• 44:29
  Speaker 1: like dark matter. And we'll be able to measure its mass.
• 44:32
  Speaker 1: We'll know something about how much of it there is.
• 44:34
  Speaker 1: We'll be able to cross-check that against our calculations for
• 44:38
  Speaker 1: the thing we think is explaining all the gravity that
• 44:40
  Speaker 1: we don't understand. And if that all agrees, then we'd
• 44:43
  Speaker 1: be like, wow, look, we found something. an example of
• 44:46
  Speaker 1: dark matter interacting. So it would be a new particle
• 44:49
  Speaker 1: and a new force at the same time. Oh my gosh,
• 44:52
  Speaker 1: it would be super exciting. All right.
• 44:54
  Speaker 2: So I have two questions. One, this sounds very exciting.
• 44:58
  Speaker 2: And given your history, this suggests that you were invited
• 45:00
  Speaker 2: to be part of one of these projects and you
• 45:02
  Speaker 2: must have turned it down. And that's why it's heading
• 45:04
  Speaker 2: towards a Nobel. Is that true? Did you turn down
• 45:07
  Speaker 2: this project at some point?
• 45:08
  Speaker 1: You know, your historical reenactment is actually correct. Yes.
• 45:11
  Speaker 2: Oh, no, Daniel.
• 45:12
  Speaker 1: I know. I keep making this joke.
• 45:17
  Speaker 2: That's three Nobel prizes you've turned down.
• 45:19
  Speaker 1: I'm going to set a record or something. Yeah, well,
• 45:21
  Speaker 1: you know, when I went to grad school, there's a
• 45:23
  Speaker 1: whole set of options for what to pursue in particle physics.
• 45:26
  Speaker 1: Collider physics, neutrino physics, dark matter physics, gravitational wave physics.
• 45:31
  Speaker 1: And as a grad student, you got to choose. And
• 45:33
  Speaker 1: you don't really know. Like, you're a baby scientifically. You
• 45:36
  Speaker 1: don't know how to evaluate these things. You just trust
• 45:39
  Speaker 1: your gut and say, like, this seems cool. And I
• 45:41
  Speaker 1: thought colliders were pretty cool. Yeah. But yeah, dark matter.
• 45:45
  Speaker 1: I had the option to join these dark matter experiments
• 45:47
  Speaker 1: and do a PhD on those. And I said no.
• 45:50
  Speaker 1: And the reason is that these experiments are really, really,
• 45:52
  Speaker 1: really good at one thing. So they have their one
• 45:55
  Speaker 1: measurement of dark matter, and that's cool. But like 500
• 45:58
  Speaker 1: people all working on one thing felt claustrophobic to me.
• 46:01
  Speaker 1: The thing about colliders is you can do lots of
• 46:03
  Speaker 1: different stuff. You can look for dark matter. You can
• 46:05
  Speaker 1: look for Higgs. You can look for anything. That's really
• 46:07
  Speaker 1: cool and exciting. But we didn't find anything, and they did.
• 46:12
  Speaker 1: So they put all their money on black, and it
• 46:14
  Speaker 1: looks like they might be winning.
• 46:16
  Speaker 2: Well, here's my second question. So you said that their
• 46:18
  Speaker 2: observation was consistent with dark matter, but it wasn't exactly
• 46:21
  Speaker 2: what we expected to see. If you get, I don't know,
• 46:25
  Speaker 2: four events that all look exactly the same, which is
• 46:29
  Speaker 2: to say consistent with dark matter, but not exactly what
• 46:31
  Speaker 2: you expected, what does that tell you?
• 46:35
  Speaker 1: Yeah, great. So that's the next step. Number one is
• 46:39
  Speaker 1: think about the statistics of it. Like we said, it's
• 46:41
  Speaker 1: very unlikely to be anything else. It's not impossible. And
• 46:44
  Speaker 1: we can actually calculate very precisely what the chances are
• 46:47
  Speaker 1: of seeing this if it wasn't dark matter. And that's
• 46:49
  Speaker 1: really important. Every time we make an observation, we can
• 46:53
  Speaker 1: never look and say, look, here's a Higgs boson. Here's
• 46:55
  Speaker 1: a dark matter. Remember, we're always seeing indirect evidence for it.
• 46:58
  Speaker 1: And there are other explanations that align with that evidence.
• 47:01
  Speaker 1: So we have to calculate what's the probability of seeing
• 47:04
  Speaker 1: this if it wasn't this new exciting thing. That's the
• 47:07
  Speaker 1: most important number. And in this case, it's about a
• 47:09
  Speaker 1: half a percent. So the chances of this happening without
• 47:12
  Speaker 1: dark matter are half a percent, which is like small
• 47:15
  Speaker 1: enough to get exciting, but not small enough to say
• 47:17
  Speaker 1: anything definitively. In particle physics, we wait for like one
• 47:21
  Speaker 1: in three and a half million before we say, okay,
• 47:24
  Speaker 1: this is something real. And the way to get there
• 47:27
  Speaker 1: is to see more examples, as you said. So if
• 47:29
  Speaker 1: we see four of them and we expected 0.001, then
• 47:32
  Speaker 1: that number, the chances of seeing four events when you
• 47:35
  Speaker 1: expect zero is close to a threshold for discovery. And
• 47:39
  Speaker 1: then we can start answering your question, which is like,
• 47:41
  Speaker 1: what does that tell us? If this is not the
• 47:43
  Speaker 1: kind of dark matter we expected, it tells us something
• 47:45
  Speaker 1: about what dark matter is, how it interacts, what mass
• 47:48
  Speaker 1: it has. And then we start to get to play
• 47:50
  Speaker 1: all sorts of fun games about, well, how many different
• 47:53
  Speaker 1: kinds of particles of dark matter are there? How do
• 47:55
  Speaker 1: they interact with each other? It's a whole new regime
• 47:58
  Speaker 1: of dark matter particle physics that we can start to do.
• 48:01
  Speaker 1: And remember- that this is most of the universe. Everything
• 48:05
  Speaker 1: we've ever studied is a little slice of the pie.
• 48:08
  Speaker 1: And so it might be that the rest of it
• 48:10
  Speaker 1: explained by one boring particle, but it might be rich
• 48:14
  Speaker 1: and complex and have all sorts of crazy stuff going
• 48:17
  Speaker 1: on in there. And we are just beginning to dive
• 48:20
  Speaker 1: into that pool. So there's so much exciting stuff happening
• 48:24
  Speaker 1: in the next 10 years, 50 years, in dark matter
• 48:28
  Speaker 1: particle physics if we keep funding this stuff, if we
• 48:31
  Speaker 1: decide we're going to spend the tiniest fraction of our
• 48:34
  Speaker 1: money on understanding the universe.
• 48:36
  Speaker 2: And there is nothing that Daniel is more excited about
• 48:39
  Speaker 2: than a result we didn't expect.
• 48:41
  Speaker 1: Yes.
• 48:41
  Speaker 2: And so I think you're probably excited that we found
• 48:43
  Speaker 2: evidence of dark matter, but the fact that it wasn't
• 48:45
  Speaker 2: exactly what we thought must really keep you up at night.
• 48:49
  Speaker 1: Oh, it's wonderful. It's so fun. The worst case scenario
• 48:54
  Speaker 1: is that they open the box on their next data
• 48:56
  Speaker 1: sets and there's nothing there. And that would mean that
• 48:59
  Speaker 1: probably this was just a fluctuation. It was just a fluke.
• 49:01
  Speaker 1: And that kind of stuff happens. I've seen crazy things
• 49:04
  Speaker 1: in our data, which then just disappear like a mirage
• 49:08
  Speaker 1: when you add more data. It's like if you flip
• 49:10
  Speaker 1: a coin and you get four heads in a row
• 49:11
  Speaker 1: and you're like, whoa, something weird is happening. And then
• 49:14
  Speaker 1: the next hundred flips are balanced 50-50. And you're like, well,
• 49:18
  Speaker 1: I guess it was just a fluctuation. The problem is
• 49:20
  Speaker 1: that if you look often enough, you will see fluctuations.
• 49:23
  Speaker 1: And so you have to even account for that. So
• 49:25
  Speaker 1: when we do our statistics and we say, how likely
• 49:28
  Speaker 1: is this to have been something boring, which mimics an
• 49:30
  Speaker 1: exciting signal, we also account for the number of places
• 49:33
  Speaker 1: we've looked for a signal. Because the more places you
• 49:36
  Speaker 1: look for a signal, like the more monkeys you have
• 49:37
  Speaker 1: banging on the typewriter, the more likely you are to
• 49:40
  Speaker 1: get Shakespeare not from Shakespeare. So even that is accounted
• 49:44
  Speaker 1: for in our statistics.
• 49:46
  Speaker 2: So the data that they used for this observation were
• 49:49
  Speaker 2: from March 2023 to April 2024. So that means it's
• 49:52
  Speaker 2: been like two years since. Like could next week, they
• 49:56
  Speaker 2: tell us, result from the next two years? Because they
• 49:58
  Speaker 2: could just like clean it up and run the same
• 50:01
  Speaker 2: stats package?
• 50:02
  Speaker 1: You'd think that they should be able to do that.
• 50:04
  Speaker 1: It's complicated because the detector performance is constantly changing. You know, oh,
• 50:08
  Speaker 1: this thing didn't work that day. This thing didn't work
• 50:10
  Speaker 1: that day. Or it was hotter or it was colder.
• 50:12
  Speaker 1: So your background models have to adapt to all of that.
• 50:15
  Speaker 1: And your analysis machinery does, and then your software changes.
• 50:19
  Speaker 1: And so, like, it sounds straightforward, but it turns out
• 50:21
  Speaker 1: there's a zillion details. And because you only get one look,
• 50:24
  Speaker 1: you better get it right. And this is the difference
• 50:27
  Speaker 1: between discovering it, having conclusive evidence for dark matter, which
• 50:31
  Speaker 1: is a guaranteed Nobel Prize, and messing it up and
• 50:35
  Speaker 1: losing out to some other experiment, which is going to
• 50:37
  Speaker 1: win the Nobel Prize. And so, like, they're going to
• 50:40
  Speaker 1: be cautious. They can't be too cautious because they wait
• 50:42
  Speaker 1: too long. they're going to get scooped by the Chinese
• 50:45
  Speaker 1: or the Italians. But if they go too fast, they're
• 50:48
  Speaker 1: going to stumble and lose the race. So they're not
• 50:51
  Speaker 1: taking a lot of vacations right now.
• 50:53
  Speaker 2: All right. Well, it'll be exciting to keep an eye
• 50:55
  Speaker 2: on what happens in the coming weeks, months, or years.
• 50:58
  Speaker 2: If you had to put your money on it, Daniel,
• 51:00
  Speaker 2: do you think this is going to pan out?
• 51:02
  Speaker 1: I think there's about half a percent chance that this
• 51:04
  Speaker 1: is a fluctuation.
• 51:06
  Speaker 2: Oh, you're statistical. Yeah, all right.
• 51:09
  Speaker 1: So that's a pretty good chance that it's something real.
• 51:11
  Speaker 1: You can't say that probably that was dark matter. All
• 51:13
  Speaker 1: sorts of Bayesian statistics complications there. But I have a
• 51:17
  Speaker 1: hunch this one feels good. But I'm famous for having
• 51:20
  Speaker 1: bad hunches.
• 51:22
  Speaker 2: Well, I think this is super exciting. I can't wait
• 51:24
  Speaker 2: to see what the result is from the next two
• 51:27
  Speaker 2: years of data.
• 51:27
  Speaker 1: Me too. Really looking forward to it. Thanks, everybody, for listening.
• 51:37
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• 51:39
  Speaker 1: show on whatever podcast app you're using. It really helps
• 51:42
  Speaker 1: people find us.
• 51:44
  Speaker 2: Daniel and Kelly's Extraordinary Universe is edited by the amazing
• 51:48
  Speaker 2: Matt Kesselman.
• 51:49
  Speaker 1: He really is a wizard. You can also find us
• 51:52
  Speaker 1: online on Blue Sky, Instagram, and XDNK Universe. Come engage
• 51:58
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• 51:59
  Speaker 2: You can email us at questions at danielandkelly.org. We really
• 52:03
  Speaker 2: do want to hear from you.
• 52:04
  Speaker 1: And you can find our website, www.danielandkelly.org, where you'll also
• 52:09
  Speaker 1: find an invitation to join our Discord, where everybody comes
• 52:13
  Speaker 1: and talks about the amazing universe.
• 52:15
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• 52:19
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