Did we just discover dark matter?

Daniel and Kelly talk about the recent observation of a dark matter candidate See omnystudio.com/listener https://omnystudio.com/listener for privacy information.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Daniel Whiteson?

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• 4:21Kelly Weinersmith?

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• 5:00Daniel Whiteson?

  Most of the universe is dark. Not just because space is dark, vast stretches of emptiness illuminated by distant points of light, but because matter itself is dark. Most of the universe turns out to not be made out of our kind of matter, atoms, et cetera, but something else, something darker. Stars and gas and dust are like tracers that hint at a much more massive mystery underneath. For decades, we've been confident that dark matter is real. It's out there. It's massive. Its gravity is influencing the universe. But what is it? That's the deep question in the heart of a particle physicist.

• 5:44Daniel Whiteson?

  What is it made out of? Is it particles? Is it something else even weirder? Very recently, an experiment underground in South Dakota might have seen the first hint of an answer to that question. Welcome to Daniel and Kelly's Extraordinary Dark Universe.

• 6:16Kelly Weinersmith?

  Hello, I'm Kelly Weinersmith. I study parasites and space. And one of the great things about knowing a lot of scientists is that when something's in the news, you almost always have someone you can reach out to to be like, how excited should I be? And today, we all get to find out how excited we should be.

• 6:33Daniel Whiteson?

  Hi, I'm Daniel. I'm a particle physicist, and I like to think about aliens. And today, I'm quite excited about dark matter. The television show Dark Matter, the science, and the kind of dark matter we all produce daily.

• 6:46Kelly Weinersmith?

  Oh, well, I'm always excited about that, mostly because it often has parasites in it. But Daniel, my question for you is, not if you live in the United States, I don't want to get people worried about parasites. But, okay, my question for you is, today, we're talking about like, a big result that was announced relative to dark matter. What was your response when you heard the news?

• 7:04Daniel Whiteson?

  My response was, ooh, that's interesting. Because it's not something I expected. These experiments have been looking for dark matter forever, and it's been a parade of negative results of, we didn't see anything. Nope, we didn't find anything. Nope, nothing here, for like, more than a decade. So at some point, you start getting bored, and you stop paying attention to all their news releases because they're all the same. But I saw people posting on Twitter in advance of this one saying, y'all better show up for this one. This one's something different. So there was a lot of buzz before this one. And then the result itself was quite weird. So there's a lot to dig into here.

• 7:41Kelly Weinersmith?

  Yeah, I'm excited because I'm pretty sure that you and I have talked about this experiment on the show before. And we've been like, oh, but those giant vats haven't found anything. But today, we get to talk about how maybe, maybe they found something.

• 7:54Daniel Whiteson?

  Maybe the giant vats will tell us something about the universe.

• 7:58Kelly Weinersmith?

  Amazing. And we, you know, we did the thing that we always do when we have an interesting question. We see what the extraordinaries have to say about it. And so you wrote the extraordinaries and asked, did the LZ experiment just observe a particle of dark matter? That was a very specific question you sent them, Daniel.

• 8:18Daniel Whiteson?

  Based on experience, I tried to give them a few clues so they know at least a little bit about what we're talking about. Plus, I was curious about how much of this news had penetrated into the sort of global news cycle.

• 8:29Kelly Weinersmith?

  Yeah. All right, well, let's see what they had to say.

• 8:32Speaker 03

  We're not ready to say for sure that it's dark matter. It must be something to do with dark matter, so I hope it did. Well, if it did, that would be absolutely massive. So I feel very confident saying, nope, no dark matter particle is actually detected. Really curious to know your opinion about it. But my guess would be that they found something interesting, but not dark matter. It's possible that the LZ experiment discovered a dark matter particle. Okay, thanks, bye. Lazy experiment did not observe dark matter because...

• 9:02Daniel Whiteson?

  Dark matter is too lazy to interact with anything. That'd be amazing, but I think it's too good news to be true. Well, I love our extraordinaries. They are appropriately skeptical, but they are excited and they want to know what Daniel has to say. And they make silly jokes about the name of the experiment, which really is very mockable. I mean, it's called the LZ experiment, which is short for the Lux Zeppelin experiment because it's a merger of two previous experiments, both of which are their own acronyms. Oh, no. So it's an acronym of shortened versions of other acronyms, both of which are tortured.

• 9:42Daniel Whiteson?

  So it has nothing to do with lead zeppelin or heavy metal, but it does have to do with liquid xenon. Well, this wouldn't be the first time we discovered on the show that physicists aren't great at naming things. No, I think terrible acronyms should be applauded. I really think it's a good part of science. Terrible if they're like funny in some way, but if they're just terrible because they're terrible. No, I think it's like, it's like kitsch, you know, like the worse the acronym, the better the science. You and I can agree to disagree, but let's dig into the heart of the matter. A to D. What? Oh, agree to disagree. Oh, see, you don't always need an acronym. Sometimes it just makes things more confusing.

• 10:24Daniel Whiteson?

  All right, Daniel, let's get to the heart of the matter, the dark heart of the matter. What is dark matter? Let's start there. Yeah. So dark matter is a theory that explains a lot of different anomalies we notice about the universe. There's a lot of things we see in the universe that we can't explain without adding a lot of invisible matter that gravitates. So it seems like 85% of the matter that's out there in the universe is this kind of invisible matter. It creates gravity and we see its effects on like how galaxies rotate and other stuff, but it doesn't emit any light. It doesn't absorb any light. It doesn't scatter any light. So it doesn't interact electromagnetically or with a strong force,

• 11:07Daniel Whiteson?

  but it's out there and it has mass and so it creates gravity. And essentially we have all these various observations, things we see in the universe that we can't explain without adding huge amount of dark matter. And if you only saw one piece of evidence like that, you might think, hmm, well, maybe there's something wrong with your measurements or your understanding of physics or something, but we have like seven or eight completely independent lines of evidence, things that we cannot explain without adding this same amount of dark matter. And each one tells us the same story about how much dark matter there needs to be and roughly where it is and how it played a role in the history of the universe. So it's really a very nice coherent story to explain a lot of anomalies that we otherwise cannot explain.

• 11:53Daniel Whiteson?

  And we recently did an episode called, is dark matter a fudge factor? Where we went through all of the evidence for dark matter being like a real thing. Yeah. Even though we have trouble measuring it. Can you go through and like remind us of just a couple of those? Cause I feel like it's important that we like have it in our head, how confident we are that this exists before we get on to the experiments. Yeah. And so there's a lot of reason for confidence, which we'll talk about. And there's also still big, open questions. One of the big open questions is like, we know it's there, we know it has mass, but what is it? And that's the central question that still remains open, but whether it exists at all is much less uncertain. And this sounds a little bit like a contradiction, like you know, it's there, you say there's a lot of evidence for it, but also you've never seen it.

• 12:37Daniel Whiteson?

  Right? And the reason is that we've seen it a lot. We have a lot of evidence for it gravitationally, but individual particles don't feel a lot of gravity, right? We can see the evidence for dark matter in like galaxy sized clumps of it or planet sized clumps of it, but an individual particle feels almost no gravity. And so if you have a particle out there and you're trying to detect it with gravity, it's almost hopeless. So that's why we have this juxtaposition of being very confident that it's out there and that it has mass and not a lot of confidence for what it is, which is what we're going to talk about today. But in terms of confidence that it exists, we see that it affects how galaxies spin. Right? Galaxies, when they spin, they need a lot of mass to hold themselves together because they spin really, really fast.

• 13:20Daniel Whiteson?

  Without dark matter, you can't explain why they hold themselves together. We see lensing where light passing through space gets bent because mass in that space is curving space, which changes how light moves through it. It's a really cool feature of Einstein's gravity that Newton didn't have. Light feels gravity, not because light has mass, but because mass bends space and light moves through that space. So you can use the bending of space to tell how much mass is there and that tells you about mass. You can also look at the very early universe and see the particles in the early universe when they're sloshing around in that plasma. Do we understand those dynamics? And it turns out not without dark matter. If you didn't have dark matter, it would slosh around very differently and we see those ripples. We can measure them very, very precisely.

• 14:05Daniel Whiteson?

  We can see the structure of the universe. Like if you run a simulation of the universe without dark matter, do you get the formation of stars and galaxies? No, you don't. You have to add dark matter. The universe needs that gravity to pull everything together to make the universe as we see it. And so these are all very strong, but exclusively gravitational pieces of information. And the thing to understand is like, we have no other theory that explains all of this. There are other theories to explain galaxy rotation curves or one or two other lines of evidence, but there's no other explanation that accommodates all of these things. That's why it's the mainstream theory. And that's why it should be the mainstream theory and we should be open minded to other ideas and we are.

• 14:48Daniel Whiteson?

  And if you came along with a new theory today of dark schmatter and it was a completely different theory, then you would have to also explain all of this data because science has egos and politics, but mostly it's about the data. If you can explain all this data with a better theory, you would be a star, but nobody can do that. That's why dark matter still reigns as the mainstream theory. Dark schmatter sounds like a more liquidy dark matter. And I feel like we're sort of back to biology. But so when we look at these different lines of evidence that suggest that dark matter is out there, do they tell us similar things about dark matter? Like give us similar estimates about how much of it is out there? Yes. They're consistent?

• 15:28Daniel Whiteson?

  Yes, absolutely. Each one tells us something different, like galaxy rotation curves tell us roughly how much dark matter there is in galaxies and also how it's distributed through the galaxy, because each star's motion tells you how much mass there is within its orbit. So as you go further and further out, you're looking for more and more distant stars. They tell you about the integrated mass in the whole galaxy. So because you can look at stars at the very center and stars at the very edge, you can tell roughly where the dark matter is. So that's very cool. The cosmic microwave background radiation tells you roughly what proportion of the universe is dark matter. And those numbers all add up. So they're all telling a consistent story. Each piece of evidence tells you a different subset of it,

• 16:09Daniel Whiteson?

  but there's lots of overlapping places where two measurements tell you the same fraction of the universe is dark matter or it's in the same place roughly. So we have lots of corroboration. Absolutely. How do you know it's not just normal matter, but we've been measuring it wrong or misunderstanding something about it? Yeah, exactly. So we've been treating dark matter as if it's something new, right? Like it's some new kind of particle. The question, what is it made out of assumes it's not made out of quarks and electrons like our kind of matter, right? The way like goats and sheep and kittens and stuff are. So why do we think it has to be something new? And the answer is that we have a pretty good accounting for what happened to all the quarks, which sounds crazy. Like, how could you possibly understand all the quarks from the early universe?

• 16:51Daniel Whiteson?

  And that's because we have a measurement that's very sensitive to the density of quarks in the early universe. So the early universe starts out, things are very hot and very dense, and then things are cooling and spreading out and getting colder and more dilute. And at one moment it went through this phase where it was about as hot and dense as the center of the sun. And so it did what the center of the sun does, which is it makes heavier stuff. So you have all these protons around. It was hot. It was dense. It fused them. It made some helium. And exactly how much helium got made and how much lithium got made and how much like trace amounts of even heavier stuff got made is very sensitive to the quark density. The quark density was a little higher, you get much more heavy elements.

• 17:32Daniel Whiteson?

  If the quark density was a little lower, you get much less heavy elements. So you measure the heavy elements produced in the early universe, which we can do very, very well. And that tells you what the quark density was. All right. So if you know what the quark density was in the early universe, you can say, well, is there enough quarks around to explain everything we see? And the answer is yes. The quark density maps pretty well to the atomic matter density, like stuff that's built out of quarks today lines up pretty well with the quark density in the early universe. So you can't use quarks to explain another factor of five of matter. Maybe we got that a little bit wrong. Maybe there were a little more quarks in the early universe than we think, but there's not a missing factor of five. So whatever this is, it can't be made out of quarks, which means it's something very new

• 18:15Daniel Whiteson?

  because everything we know, everything you've ever had for lunch is made out of quarks. Quarks can be delicious, but dark matter is not made out of them. But is dark matter, it's everywhere, right? So I've probably accidentally either. Ingested dark matter or had it pass through me? Not the biology kind, but the physics kind? Well, I hope you're passing dark matter and not dark splatter. Oh, I thought you called it schmatter. Oh. It should have been dark splatter, but anyway. Yeah, we think dark matter is mostly everywhere. So dark matter doesn't interact the way normal matter does. Normal matter is very clumpy, right? Two rocks in space hit each other, boom, they stick together. And that's how you get like formation of planets, et cetera. Dark matter doesn't have those interactions, we think.

• 18:59Daniel Whiteson?

  So it mostly passes through itself and normal matter. So it remains in these big fluffy clouds. So it does clump together because of gravity, but it's not as clumpy as normal matter. And you know, we're big fans of clumpiness here. One of us.

• 19:15Daniel Whiteson?

  I mean, I don't know if you prefer things more loose and watery, but I like clumps. So dark matter is less clumpy, which means that like the galaxy is in a big dark matter halo. Not the dark matter spread evenly through the universe. Like we're in a big dark matter halo, our galaxy. It's probably spread more evenly than normal matter. So there's a less local density, but it's everywhere. So yeah, it's passing through us, we're passing through it. There's like a dark matter wind passing through us right now. So have you eaten some? Well, probably some has gone through your open mouth, but then it just goes out the back of your head. So if you're not metabolizing it, I don't think you're really eating it. Yeah, I'll give, that's a fine definition of eating, I think. I can get behind that. Are there other theories for this? So there are a few alternative ideas to explain these gravitational anomalies.

• 19:59Daniel Whiteson?

  One is like called modified Newtonian dynamics is maybe you misunderstand how gravity works. And that would explain it. You don't need to add this crazy amount of matter. And that works to explain like galactic rotation curves. And one or two other lines of evidence can be made to accommodate, but not all of them. It just doesn't work. That doesn't mean it's crazy talk or people shouldn't think about it. Like absolutely keep thinking about it, work on it. But it's not really a full alternative to dark matter. The one that's the most surprising to me is very popular online. You see people commenting on videos is this electric universe theory that says like, there is no dark matter, it's just electricity everywhere. Because look, electricity is much more powerful than gravity, right? And so maybe you're just not thinking about electricity right.

• 20:44Daniel Whiteson?

  And they think the whole universe is like powered by electricity, like the sun is powered by galactic electric currents instead of nuclear fusion and all sorts of crazy stuff. And it's a fun theory. It's like, has lots of nice visual analogies and connects to like ancient mythology, et cetera, et cetera. But it doesn't really have like a solid mathematical framework. And it's also violently in tension with what we see in the universe. Like we know the sun is powered by nuclear fusion. We measure neutrinos from the sun and we can very accurately measure the energies of those neutrinos. And it's just what you would expect if there was nuclear fusion happening in the sun. So not a big mystery about that kind of thing. We know that most of space is electrically neutral, which is why electromagnetism,

• 21:28Daniel Whiteson?

  despite being much more powerful than gravity, is essentially irrelevant on cosmic scales because there's two electric charges. And so things can balance each other out, whereas there's not two gravitational charges. There's no like negative mass to balance the positive mass. So gravity will always be around, whereas electromagnetism can neutralize itself. So the problem with the electric universe theory, despite its incredible surprising popularity online, is that it doesn't describe the universe. And you know, you gotta follow the data. Doesn't matter if your theory is pretty, or if you think Ramses the second wrote about it, or whatever, like it's gotta follow the data and electric universe theory just doesn't. Was Ramses the second a particularly astute physicist? No, not at all.

• 22:11Daniel Whiteson?

  But there's also these crazy theories that the pyramids were like energy generating devices instead of tombs, like. Oh my. Maybe I've done too much reading of crazy conspiracy theories.

• 22:22Daniel Whiteson?

  All right, well, I actually feel like I'd like to hear more about that at some point. But today I want to get to this big, exciting experimental result. So let's take a break. And when we get back, we'll talk about some ways that we've started looking for dark matter.

• 22:43Speaker 03

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• 27:57Kelly Weinersmith?

  And we're back. And Daniel has just told us about the evidence we have for the existence of dark matter, but it's hard to find. So Daniel, just because something is hard doesn't mean scientists throw up their hands and give up.

• 28:08Daniel Whiteson?

  Exactly.

• 28:09Kelly Weinersmith?

  What have we tried to look at it?

• 28:11Daniel Whiteson?

  So we know that dark matter is out there. We know how much of it there is in terms of mass. We know roughly where it is, but we don't know what it's made out of. Like, everything we've ever seen has been made out of particles. So we think, well, maybe dark matter is also made out of particles. And that's a maybe, right? Let's say that honestly and upfront, that's a guess. And it's not necessarily a well justified guess. We're extrapolating from an unrepresentative corner of the universe, normal matter, atoms, et cetera, which make up 5% of the universe and saying, maybe the rules we learned here apply everywhere. And like, that hasn't gone well historically. You know, the rules of like, motion on Earth don't apply everywhere because it turns out the rules are different

• 28:52Daniel Whiteson?

  at high speeds or near black holes or whatever. But it's also a reasonable starting place. Like physics is usually, let's try the dumbest thing first. If that didn't work, okay, let's try the second dumbest thing. Let's be lazy about it and build up because you don't want to start with complexity. Start with the simplest thing. So the simplest idea is maybe it's a particle and maybe it has some kind of interaction with our matter. Again, an assumption, because earlier we said, we know dark matter feels gravity. We don't know that it feels anything else. So it might be that dark matter is a particle, but it only feels gravity, in which case we probably will never figure out what it's made out of because an individual particle's gravity is basically zero. And here, again, I'm saying we have a lot of confidence that dark matter is a thing and that it's out there and has

• 29:37Daniel Whiteson?

  mass, but we've never seen it and we might not be able to. Those two things are not in conflict. What I mean is that we know it's out there in a bulk sense, but we don't know what it's made out of and we may not figure that out. But you don't just throw up your hands. You say, well, let's assume that it does have some kind of interaction. What if it has some kind of interaction with normal matter that's not gravity, some new kind of dark force? So now we're adding complexity. We're saying, not just that there's dark matter out there, but what if there's some new force we've never observed before that lets dark matter interact with normal matter?

• 30:08Kelly Weinersmith?

  Gosh, where do you even start to like, figure out how to measure that if it's happening?

• 30:13Daniel Whiteson?

  Yeah. Well, what you do is you look to see if dark matter is interacting with normal matter. And one way to do that is to try to make dark matter. So that's the kind of thing that I worked on for like 10 years. If dark matter interacts with normal matter, that means that sometimes when you smash normal matter together, you will produce dark matter. Because what happens at a collider is you don't have to know in advance what you're trying to make. It's not like chemistry, like you have a cookbook and you're rearranging stuff from ingredients into cookies, right? It's alchemy. At the collider, we do alchemy. We create new kinds of stuff and doesn't have to have the remnants of the old kind of stuff. We annihilate quarks and antiquarks. It turns into a gluon and then it can turn into anything that gluons coupled to, or it turns into a photon and then it can turn

• 30:57Daniel Whiteson?

  into anything the photons coupled to, or anything Z bosons coupled to, et cetera. So if there's some new kind of particle out there, like a dark photon, maybe you annihilate quarks and antiquarks and it turns into a dark photon and then that turns into dark matter. So that's the kind of thing we were hoping to see at the Large Hadron Collider. And, you know, I have a great track record of picking experiments that don't win Nobel Prizes. And so we didn't see anything and I didn't win a Nobel Prize.

• 31:23Kelly Weinersmith?

  Oh.

• 31:23Daniel Whiteson?

  It would have been so awesome to make it in the lab and be able to study it and produce it at will. So awesome. Because then you're not dependent on things like, well, I think there's a dark matter wind. Let me try to spot it. You're like, I'm just going to make it here in the lab. And that's the awesome power of the collider, but we haven't seen anything. And so that's been disappointing. Another thing you can do is try to reverse that and say, well, look, if you could smash quarks and antiquarks together and make a dark photon, which turns into dark matter, what about the opposite? What happens when two dark matter particles smash into each other? Sometimes they should produce a dark photon, which should turn into quarks and antiquarks. Basically, the inverse of what we're hoping to happen at the collider should happen out there in the universe if there's dark matter banging into other dark matter.

• 32:09Kelly Weinersmith?

  And if dark matter is anything like, I mean, so like, assuming that it would make antiquarks is an assumption?

• 32:15Daniel Whiteson?

  Yes, exactly.

• 32:16Kelly Weinersmith?

  Yeah, okay.

• 32:16Daniel Whiteson?

  So we got a couple of assumptions here. One, it's a particle. Two, it has some kind of interaction with normal matter that couples to quarks or maybe electrons or something we can see.

• 32:25Kelly Weinersmith?

  Okay.

• 32:25Daniel Whiteson?

  Both of those are just guesses, guesses that we have to make to make this whole experimental program feasible. And so you might think, well, you're just guessing. Well, that's true. But again, it's a reasonable guess and it's all we can do. So let's hit those nails with a hammer because the hammer's all we got.

• 32:40Kelly Weinersmith?

  Yeah, sure. Reasonable place to start.

• 32:42Daniel Whiteson?

  And so where might this happen? Well, most of the dark matter in the galaxy is in the center of the galaxy, because that's where most of the gravity is. And so it's a big blob in the center of the galaxy. So if sometimes dark matter bumps into other dark matter to make normal matter, you expect it to happen mostly in the center of the galaxy. And we have special space telescopes trained on the center of the galaxy, looking for just this. And then one of them is called the Fermilat Telescope. It's basically a particle detector in space that looks for high energy particles from the center of the galaxy. We also look for it in Antarctica. So we talked about ice cubes sometimes, which is a cubic kilometer of ice in Antarctica.

• 33:21Kelly Weinersmith?

  Wow.

• 33:21Daniel Whiteson?

  That's been instrumented with cameras. And if you have dark matter that comes through the earth and interacts and makes a muon, you could see that in the ice. All sorts of ways to look for this. This is really, really hard because we don't understand the center of the galaxy very well. It's filled with all sorts of mysteries, black holes and radiation and all sorts of crazy stuff happening. So if you see a signal from the center of the galaxy, it might be dark matter, but it might just be like, oh, it turns out you don't understand the center of the galaxy. So it's a very tough thing to do. Worth doing, but a tough thing to do. And they've seen all sorts of signals in the last 10 years. And then each time they're like, oh, this turns out to be a quasar, or this turns out to be a pulsar, or this turns out to be something else. Because the universe is filled with

• 34:03Daniel Whiteson?

  super weird stuff, which is fun. And you're going to learn a lot of astrophysics, but they've never seen any dark matter.

• 34:09Kelly Weinersmith?

  And one thing that's presumably nice about these experiments is that even if you don't get the answer you were looking for, you've learned something.

• 34:15Daniel Whiteson?

  Oh, yeah.

• 34:15Kelly Weinersmith?

  Right? Yeah. So these are all designed to teach you something, maybe not what you were expecting, but at the end, the money is spent. You've learned more about the universe, even if it wasn't what you thought.

• 34:24Daniel Whiteson?

  For sure. And both of these directions are very general. Like, we don't just build the LHC to look for dark matter. We use it to look for all sorts of stuff. It's done great physics. We build these telescopes to look at the center of the galaxy and to look for dark matter and other stuff. So we've learned a lot from Fermilat. It's been a fantastic success despite not finding dark matter. Now, the third category are much more specialized. This we call direct detection. And this is where we got the really exciting result from. These are experiments designed expressly to see dark matter. And their goal is to detect this dark matter wind. They're saying, if we're flying through a wind of dark matter, and that dark matter can interact with normal matter in some way, then eventually we should see it. You should see dark matter bumping into normal matter,

• 35:09Daniel Whiteson?

  right? If the two interact and you have a wind, you should literally feel it.

• 35:12Kelly Weinersmith?

  Should you smell dark matter wind as well? Sorry.

• 35:18Daniel Whiteson?

  She who smelt it dealt it. You know, that's the rule in physics.

• 35:22Kelly Weinersmith?

  Oh, that's also the rule in biology. I guess we're not so different after all.

• 35:25Daniel Whiteson?

  There's some common ground. And the problem is that it's hard to feel it because it's very weak, right? The way that like, you can't really feel neutrinos. There's trillions of neutrinos passing through your body all the time, but you don't really notice. You're not like beaten down by the neutrino radiation because they mostly pass right through you because the interaction between a neutrino and your matter is very, very weak. And I sometimes hear people say, it's because neutrinos have low mass. It's not. Or because neutrinos are super duper small. It's not. It's about the strength of the interaction. When two particles come near each other, there's a chance that they interact and that depends on how strong the interaction is. If it's very strong, like electromagnetism, you know it's pretty much going to happen. If it's very, very weak, like the weak force,

• 36:08Daniel Whiteson?

  then like it happens one in 10 to 20 times. So it's a quantum mechanical probability that depends on the strength of the interaction. So the idea is, well, how do you see something that's very, very gentle and very, very rare? And the strategy is to filter out everything else. So you get a really big tank of like liquid xenon, something which is already very quiet. So it's not making a lot of noise. It should just sit there inert and do nothing. And you put it like a mile underground. So it's not getting bombarded by cosmic radiation from space, right? Cosmic rays and high energy particles. So in principle, it should just sit there. It should be totally quiet. It should be totally inert. You should never

• 36:49Daniel Whiteson?

  see anything. But if dark matter passes through all that rock and gets to your detector, it should occasionally bump into one of those particles and you should see it. So if you build a huge vat of quiet liquid underground, it's sort of like a camera for seeing really rare particles that really weakly interact.

• 37:10Kelly Weinersmith?

  And so what would a weak interaction between dark matter and xenon look like?

• 37:16Daniel Whiteson?

  Yeah. So it would just bump a xenon nucleus, right? It would be, we call it a nuclear recoil. It would just bang against it and it would give off a little flash of light because that nuclear recoil is getting some energy and then it gives up that energy and also can lose electrons. And so you look for that flash of light and you look for those electrons. And that's how you tell that one of your xenons got bumped. Now, the chances of a xenon getting bumped is really small because the dark matter coming through like rolls a die that has like 10 to the 20 sides on it and it has to get a one. But you got lots of xenon. So the bigger your tank of xenon, the more rolls you get on that huge die. And eventually you might get one.

• 37:56Kelly Weinersmith?

  Okay. And what else could get through?

• 37:59Daniel Whiteson?

  All of that earth. Do we think that only dark matter could get there? Great question, because it's not like you see one, you know you found dark matter. There are other ways to see xenon get bumped. So we're pretty sure that the cosmic ray background is almost zero, right? Because you've got like a mile of shielding of rock, but not exactly zero, right? Everything is a probability. Also, the rock is radioactive, like there's some trace uranium and whatever, and that emits particles. And sometimes you're going to see that. You're going to get neutrons or whatever, or some other particle decays radioactively and gives you a gamma ray. So they've thought very carefully about that, and they've designed the detector specifically so they can try to identify whether a flash came from one of these other sources or whether it looks like it came from dark matter.

• 38:43Daniel Whiteson?

  Tell me more, Daniel. And so the experiment we're talking about today is called the LZ experiment, and it has seven tons of ultra pure liquid xenon. And it's a mile underground in an old gold mine in Lead, South Dakota. It's called the Surf Lab, which is some other tortured acronym. I don't remember. And it's a really cool device. Finally, there's a fun acronym, and you don't know what it stands for.

• 39:08Daniel Whiteson?

  Well, I'm pretty sure they're not doing any surfing in South Dakota. And landlocked South Dakota. Yeah, probably not. So it definitely qualifies for tortured acronym. Okay. And they've designed this experiment specifically to give them handles so they can answer questions like, is this likely to have been a neutron? Is this likely to have been a radioactive decay? Or does this look like dark matter? And so it's a massive tank of xenon, but it's also surrounded by a tank of water. And the tank of water is there to suppress cosmic rays. So like any muon that makes it through that rock is going to go through that water and leave a flash. Cosmic ray muons are moving really, really fast. They'll leave a little flash in that water, and you'll see it. So it's like a veto. If you see something give a flash in your outside water tank,

• 39:53Daniel Whiteson?

  then you're pretty sure it's not going to be dark matter. You just throw that away. Okay. And then they have a layer of something called gadolinium, which is really good at seeing neutrons. It's like super duper reactive to neutrons. So if you have neutrons that come in and bump your xenon atom, you'll also see a flash in your gadolinium. And she'd be like, oh, no, that was just a neutron. Clever. We also have two ways to measure the flash. They call them S1 and S2. Brilliant names, scientists. Way to go, physicist. Yeah, S1 is that flash of light you get when the xenon bumps, right? So the xenon gets some energy, then it dumps some energy into a photon. It's a flash of scintillation light. And they have the whole tank of xenon surrounded by cameras to capture those flashes.

• 40:35Daniel Whiteson?

  Then also, the electrons that have gotten stripped off the xenon by the nuclear recoil, they drift upwards because there's a whole electric field in the thing. And the time it takes the electron to drift upward tells you how far it drifted, which tells you roughly where it happened. Wow. Plus. This is awesome. Yeah, it's super cool. This is like really clever, multiple levels of cleverness. And this is many generations in. They started out with one idea, and it worked, but they're like, oh, this is really hard to tell this apart from neutrons. It's added its other layer. So this is the benefit of lots of experience of knowing what goes wrong and designing exactly an experiment the next time to make it more precise. Awesome. And another really cool trick is that you can look at the ratio of S1,

• 41:17Daniel Whiteson?

  which is that first flash of light, and S2, which is from the electrons that drift upwards. And if you get a lot more energy in S2 from the electrons, then you know that probably what was hit was the electron cloud around your xenon atom and not the nucleus itself. Okay. Because then you're getting a lot more energy in those electrons. And that's really useful information because things like gamma rays or anything that has electric charge, if it hits a xenon atom, it's going to hit those electrons. Whereas a wimp, like a dark matter particle, it's not going to hit those electrons because it doesn't have electric charge. It's going to hit the nucleus, and it's going to bang against those quarks. And so what you're looking for, if you want to find something that hit the nucleus, is more energy in that first flash and less in that electron splash. Wow.

• 42:02Daniel Whiteson?

  So you've got lots of handles here to figure out what's dark matter? What's your typical background, et cetera, et cetera? So they did a lot of work to try to understand this. So you said there's like literal tons of xenon? Yeah. Where does one get literally tons of xenon? This turns out to be one of the limiting factors because xenon is really rare. It's like 0.00001% of Earth's atmosphere. And the whole global production of xenon is 40 tons per year. Oh, wow. So if you want seven tons of xenon, it's like a significant fraction of all the xenon available to all industry on Earth every year. And that's like a huge fraction of the budget.

• 42:45Daniel Whiteson?

  So they spent $70 million on this liquid xenon for LZ. And you know, they couldn't buy it all in one year. They had to stage it over several years. They have special contracts with these producers, like, please ramp up your production. We're going to need this amount because you can't just like enter the market and be like, oh, we need 25% of the whole market this year. Yeah. So they have these contracts. It's a whole thing. Liquid xenon is excellent for dark matter detection because it's massive and it's inert. It also turns out to be really good for like aerospace and electronics and semiconductor industry. They use xenon gas for plasma etching to precisely etch nanoscale features on the silicon wafers. In aerospace, they like it because it's a great fuel.

• 43:28Daniel Whiteson?

  Like we talked about ion thrusters that use liquid xenon, right? You want these big, massive ions. So there are other places that need liquid xenon, very reasonable. And they don't like it when the scientists come in and be like, we're taking all of it this year, by the way. So that turns out to be quite complicated. Yeah. But once you've got it, do you need to replenish it? Or you, you just, you've got it, and then you're good for forever? You've got it, and you're mostly good. This is going to be a little bit of leakage, so you've got to top up occasionally. Okay. But no, you don't need to have a continuous budget for buying liquid xenon. And that's the challenge for this industry is that they're like, we're going to come in with a huge order, and then we're going to disappear. Yeah. So they're like, well, we're not going to ramp up our production, and then we're going to be left with all the xenon nobody wants. Right. So it's a challenge for the industry to respond to that. But this, this has been done.

• 44:13Daniel Whiteson?

  It has been done. It does exist. Yeah. And it's this escalation, like they started out these detectors with a fraction of a ton. Then the next generation was like the one ton, and this is the many ton. And people are talking about like 100 ton versions of this stuff. Oh, wow. And there's several of these detectors around the world. The one we're talking about today is the LZ. But there's a competing experiment called the Xenon Experiment. And there's another one in China called Panda X. And they all need like their own five to 10 tons. And they're all talking about 100 ton version of this thing. Wow. And so it's complicated. It sounds like anybody who's looking for a business proposition should get into xenon production.

• 44:51Daniel Whiteson?

  I'm not pumping shares in xenon stock yet, but yeah, it's important stuff and expensive. All right. Well, let's take a break. And when we get back, we will talk about the data set and what we saw.

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• 50:23Kelly Weinersmith?

  Okay, so the LZ, which is part of the SURF project, if I'm getting all of these connections correct, saw something exciting.

• 50:30Daniel Whiteson?

  Yeah.

• 50:30Kelly Weinersmith?

  First, how long did they have to make measurements before they saw something possibly exciting?

• 50:36Daniel Whiteson?

  Yeah, so it's like the LHC, where you build this thing, but you're not running it all the time, because sometimes you're working on it, and you want it to be operating really, really well. So they have various data sets. And this data set we're talking about today was taken between March of 2023 and April of 2024. So that's already two years ago.

• 50:53Kelly Weinersmith?

  Yeah.

• 50:53Daniel Whiteson?

  And they spent two years analyzing this data, understanding this data, developing their models of the background, figuring out how to select events to make sure they get dark matter, they don't cut out any dark matter events, they don't get drowned in the background. It's really important. And what's really important about it is designing it before you look at the data.

• 51:12Kelly Weinersmith?

  Yeah.

• 51:13Daniel Whiteson?

  In the end, we're going to do statistics on like a handful of numbers. Like, we see one, we expect zero. We see two, we expect one. You really have to make sure you're not biasing yourself. It's very, very easy to design an experiment to get the result you want if you already know where the data are.

• 51:29Kelly Weinersmith?

  Yeah.

• 51:30Daniel Whiteson?

  Right? It's like predicting the stock market after already looking at it. It's not very impressive, right? It's not a measure of your ability to predict the stock market. So what they do is they call salt the data. They put a bunch of fake data in with the real data.

• 51:42Kelly Weinersmith?

  Oh.

• 51:42Daniel Whiteson?

  Like fake dark matter and fake this and fake the other things, so that nobody is biased by what they're seeing. Because nobody knows which is real. And they design the analysis techniques without like, looking at the actual clean data set until the very, very end, until they're very, very sure. Because you only get one try.

• 51:57Kelly Weinersmith?

  Wow.

• 51:58Daniel Whiteson?

  Once you've seen the data set, you can't change how you're analyzing it. You can't say like, anything in this box, we're going to call a dark matter candidate, and we're going to predict the number of other things that might fall in that box. And then see a candidate just outside the box, be like, oh, we're going to change the box. We're going to grow the box to get that one thing, right?

• 52:15Kelly Weinersmith?

  Yeah.

• 52:16Daniel Whiteson?

  You cannot sculpt your analysis technique after you've seen the data. So that's why it takes two years, because they have a single shot. They've worked on this experiment for like a decade. They're not wasting their one chance at the data to have a fresh statistical look.

• 52:29Kelly Weinersmith?

  Nice.

• 52:29Daniel Whiteson?

  So this data set that we're talking about today is like 220 live days. And the exciting thing is they have more data. They haven't yet analyzed. So that's always cool.

• 52:39Kelly Weinersmith?

  Yeah, because then you can run your analysis multiple times on the data and like, increase your confidence.

• 52:44Daniel Whiteson?

  Well, you can get fresh data. That's the most important thing. You don't want to reanalyze old data.

• 52:48Kelly Weinersmith?

  Yeah, yeah.

• 52:48Daniel Whiteson?

  You want to answer questions in new data. And that's going to be very exciting. Because what happened when they opened up the data was very exciting for two reasons. One is, they saw something. So a lot of these experiments they run, they see nothing. Like, not anything. Everything they see, their confidence is background. So they have these very restrictive selection requirements to remove all of that. And left in what they call their signal region, candidates for dark matter is zero. That's like, very compelling, but also disappointing. It's like,

• 53:18Kelly Weinersmith?

  Darn.

• 53:18Daniel Whiteson?

  Very underwhelming to go to a whole presentation, to hear about this incredible engineering, and then be like, we saw nothing. You know?

• 53:25Kelly Weinersmith?

  Yeah.

• 53:25Daniel Whiteson?

  It's like spaceballs. We ain't found shit, you know?

• 53:28Kelly Weinersmith?

  Yeah.

• 53:29Daniel Whiteson?

  But it is an answer. But it's not the science answer you want, right?

• 53:32Kelly Weinersmith?

  Yeah.

• 53:32Daniel Whiteson?

  So the exciting thing is, they saw something. Oh, my gosh, there's an event in there. And it's very hard to explain how they would see anything but dark matter. Because they're very, very good at removing radioactive background sources, at vetoing neutrons, at vetoing gamma rays, you know? What this looks like is, looks very, very, very much like a dark matter. Like, the nuclear recoil energy is very large, and the electron flash is much smaller. So it's exactly in the place you would expect dark matter. So it's a very interesting, very compelling event. And they went through in their presentation and talked about like, all the possible sources of background, and calculated, how likely is it that this is this source of background? Or, we're seeing a neutrino, or we're seeing a neutron, or

• 54:17Daniel Whiteson?

  we're seeing a radioactive decay. And all of those are like, very, very tiny, but non-zero probabilities. And that's the crux of the issue, is we have a single event. All we know about it are these two numbers, S1 and S2. And we can calculate the probability that it came from something boring, some prosaic explanation, and that's never zero. And anytime you have one event, you're like, well, I mean, what does it mean, right? One event, it could just be good luck, bad luck, it could be anything. But the other thing about this is that it's a very weird event. It's very, very high energy. If we were seeing the dark matter wind, you expect to see a lot of events at lower energy, and very few at these very

• 54:57Daniel Whiteson?

  high energies. And so to see one at very high energy and none at low energy, that's weird. It's not what we expected to see from dark matter.

• 55:07Kelly Weinersmith?

  What does that mean?

• 55:08Daniel Whiteson?

  So of course, the first thing that happened when they put this result out, was that the theory community came out with like, a flurry of papers, all trying to explain what this is. And I was amazed. I mean, even in these days of AI, like, some of those papers were posted hours after the result came out. So like, maybe there was some leaks, or maybe people like, had their models ready for no matter what they posted, and they're like, oh, okay, it's this one. Or maybe they generated it with AI in five hours. I don't know. But one possibility that people initially were attracted to is a theory of dark matter called a Higgsino.

• 55:44Kelly Weinersmith?

  Well, that's cute. I like that.

• 55:47Daniel Whiteson?

  Yeah, it is kind of cute. Yeah. Well, eno, you know, is like, cute and a little. And the Higgsino is a hypothetical particle that's related to the Higgs. And you might remember, we talked about supersymmetry. That's this idea that every particle we know about has a partner particle, the way that like, every charged particle has an antiparticle. This is like a different symmetry, and different kind of reflection. And every particle we know about that's a boson, has a partner particle where you add an eno to it. So like, photon is a photino, and the Z has a zeno, and the W is a wino particle. And the Higgs is a Higgsino particle. And so the idea is, maybe this is a Higgsino. Maybe this is a

• 56:28Daniel Whiteson?

  supersymmetric version of a Higgs boson. And there's a whole theory about like, the spectrum of dark matter masses that would explain this particular event.

• 56:38Kelly Weinersmith?

  So a Higgsino would give the same S1 and S2 signal as dark matter? And is this, this wasn't something that we anticipated before we got the results?

• 56:48Daniel Whiteson?

  Yes, it would give this signature. And it wasn't the leading candidate. It was something that was out there. It's like, on the spectrum of ideas. But it wasn't the most expected thing. It wasn't like, the most vanilla. You know, this is like, the slightly spicy jalapeno version of dark matter.

• 57:04Kelly Weinersmith?

  But this would make more sense with the, you said there's no wind that came along

• 57:09Daniel Whiteson?

  With it. And so dark matter would have the wind, but the Higgsino would be windless? It would just explain why we started at high energy rather than seeing it at low energy first. Okay. Particular kind of dark matter that interacts in this way. So that's really exciting, but that only lasted for like a couple of days because then people were thinking, well, if it is a Higgsino, you should also see this other kind of events at even higher energy and they don't see any of that. And so that theory like lasted 24 hours. And now there's other ideas people have and like in the hallways here at Irvine, there's a lot of talk every day about what the latest theories are and what the papers are. There's like a proliferation of theories out there to explain this, which is a lot

• 57:51Daniel Whiteson?

  of fun. And this is how science should work. You know, you get ideas, you get competing ideas, people are, it's a scientific conversation like live. It's out there for anybody to participate in, which I think is cool. But also we should remember like it's a single observation. We saw one thing. It's like, you know, the wow signal. We saw this one burp from space and never repeated. And it makes it really hard to know, was it aliens? Was it some weird hiccup in hydrogen somewhere? You really need something to repeat to order to understand it. If you just get one weird blip, it's like when you're walking down the street and you have like a sudden pain in one joint and you're like, oh, but then it never happens again. You forget about it. You're like, well, I'm just going to

• 58:32Daniel Whiteson?

  move on with my life. You know, single things can just be weird or random outliers. So that's why it's really exciting that they have more data they haven't looked at yet. If this is dark matter and it's real, then it should happen regularly. Right? And so if you have four times as much data and you look at your new data, you should see it four times as often. Right? There's going to be fluctuations, but if this is dark matter, when they look at their new data, they should see more examples. It should start to add up. It should start to pile up, right? As you get more and more data. Plus we have experiments around the world. We have one in Italy. We have one in China. All are sensitive to the same things using very similar technology, but not exactly.

• 59:15Daniel Whiteson?

  So they can cross check each other. We should start to see it in those data sets. So if this is real, then in the next couple of years, it'll become overwhelming evidence that we're seeing something new, something massive, something that interact like dark matter. And we'll be able to measure its mass. We'll know something about how much of it there is. We'll be able to cross check that against our calculations for the thing we think is explaining all the gravity that we don't understand. And if that all agrees, then we'd be like, wow, look, we found an example of dark matter interacting. So it would be a new particle and a new force at the same time. Oh my gosh, it would be super exciting. All right. So I have two questions. One, this sounds very exciting. And given your history, this suggests that you were invited to be part of one of these projects

• 1:00:00Daniel Whiteson?

  and you must have turned it down. And that's why it's heading towards a Nobel. Is that true? Did you turn down this project at some point? You know, your historical reenactment is actually correct. Yes. Oh, no, Daniel.

• 1:00:12Daniel Whiteson?

  I know. I keep making. The three Nobel prizes you've turned down. I'm gonna set a record or something. Yeah. Well, you know, when I went to grad school, there's a whole set of options for what to pursue in particle physics, collider physics, neutrino physics, dark matter physics, gravitational wave physics. And as a grad student, you got to choose and you don't really know, like you're a baby scientifically. You don't know how to evaluate these things. You just trust your gut and say like, this seems cool. And I thought colliders were pretty cool. Yeah. But yeah, dark matter. I had the option to join these dark matter experiments and do a PhD on those. And I said, no. And the reason is that these experiments are really, really, really good at one thing. So they have their one measurement of dark matter

• 1:00:54Daniel Whiteson?

  and that's cool. But like 500 people all working on one thing felt claustrophobic to me. The thing about colliders is you can do lots of different stuff. You can look for dark matter. You can look for Higgs. You can look for anything. That's really cool and exciting. And so, but you know, we didn't find anything and they did. So they put all their money on black and it looks like they might be winning. Well, here's, here's my second question. So you said that their observation was consistent with dark matter, but it wasn't exactly what we expected to see. Yeah. If you get, I don't know, four events that all look exactly the same, which is to say consistent with dark matter, but not exactly what you expected, what does that tell you? Yeah. Great. So that's the next step. Number one is think about the statistics

• 1:01:38Daniel Whiteson?

  of it. Like we said, it's very unlikely to be anything else. It's not impossible. And we can't actually calculate very precisely what the chances are of seeing this if it wasn't dark matter. And that's really important. Every time we make an observation, we can never look and say, look, here's a Higgs boson. Here's a dark matter. Remember, we're always seeing indirect evidence for it. And there are other explanations that align with that evidence. So we have to calculate, what's the probability of seeing this if it wasn't this new exciting thing? That's the most important number. And in this case, it's about a half a percent. So the chances of this happening without dark matter are half a percent, which is like small enough to get exciting, but not small enough to say anything definitively. Yeah. In particle physics, we wait for like one in three and a half million before we say, okay, this is something real. Wow.

• 1:02:23Daniel Whiteson?

  And the way to get there is to see more examples, as you said. So if we see four of them and we expected point 001, then that number, the chances of seeing four events when you expect zero is close to a threshold for discovery. And then we can start answering your question, which is like, what does that tell us? If this is not the kind of dark matter we expected, it tells us something about what dark matter is, how it interacts, what mass it has. And then we start to get to play all sorts of fun games about, well, how many different kinds of particles of dark matter are there? How do they interact with each other? It's a whole new regime of dark matter particle physics that we can start to do. And remember that this is most of the universe. Everything we've ever studied

• 1:03:04Daniel Whiteson?

  is a little slice of the pie. And so it might be that the rest of it is explained by one boring particle, but it might be rich and complex and have all sorts of crazy stuff going on in there. And we are just beginning to dive into that pool. So there's so much exciting stuff happening in the next 10 years, 50 years in dark matter particle physics. If we keep funding this stuff, if we could decide, we're going to spend the tiniest fraction of our money on understanding the universe. And there is nothing that Daniel is more excited about than a result we didn't expect. Yes. And so I think you're probably excited that we found evidence of dark matter, but the fact that it wasn't exactly what we thought. Even better. Must really keep you up at night. Yeah.

• 1:03:47Daniel Whiteson?

  Oh, it's wonderful. It's so fun. You know, the worst case scenario is that they open the box on their next data sets and there's nothing there. Yeah. And that would mean that probably this was just a fluctuation. It was just a fluke. And that kind of stuff happens. I have seen crazy things in our data. Yeah. Which then just disappear like a mirage when you add more data. It's like if you flip a coin and you get four heads in a row and you're like, whoa, something weird is happening. And then the next hundred flips are balanced 50 50. And you're like, well, I guess it was just a fluctuation. Yeah. The problem is that if you look often enough, you will see fluctuations. And so you have to even account for that. So when we do our statistics and we say, how likely is this to have been something boring, which mimics an exciting signal? We also account

• 1:04:30Daniel Whiteson?

  for the number of places we've looked for a signal. Cause the more places you look for a signal, like the more monkeys you have banging on the typewriter, the more likely you are to get Shakespeare, not from Shakespeare. Yeah. So even that is accounted for in our statistics. So the data that they used for this observation were from March 2023 to April 2024. So that means it's been like two years since, like could next week, they tell us that result from the next two years? Cause they could just like clean it up and run the same stats package? You'd think that they should be able to do that. It's complicated because the detector performance is constantly changing. You know, oh, this thing didn't work that day. This thing didn't work that day, or it was hotter or it was colder. So your background models have to adapt to all of that. Okay.

• 1:05:13Daniel Whiteson?

  And your analysis machinery does. And then your software changes. And so like, it sounds straightforward, but it turns out this is zillion details. And because you only get one look, you better get it right. And this is the difference between discovering it, having conclusive evidence for dark matter, which is a guaranteed Nobel Prize, and messing it up and losing out to some other experiment, which is going to win the Nobel Prize. And so like, they're going to be cautious. Yeah. They can't be too cautious because if they wait too long, they're going to get scooped by the Chinese or the Italians. But if they go too fast, they're going to stumble and lose the race. So they're not taking a lot of vacations right now. All right. Well, it'll be exciting to keep an eye on what happens in the coming weeks, months or years. If you had to put your money on it, Daniel, do you think, do

• 1:05:58Daniel Whiteson?

  you think this is going to pan out? I think there's about half a percent chance that this is a fluctuation. Oh, you're a statistical. Yeah. All right. So that's a pretty good chance that it's something real. You can't say that probably that was dark matter. All sorts of Bayesian statistics complications there. But I have a hunch. This one feels good, but I'm famous for having bad hunches.

• 1:06:20Daniel Whiteson?

  Well, I think this is super exciting. I can't wait to see what the result is from the next two years of data. Me too. Really looking forward to it.

• 1:06:34Daniel Whiteson?

  Thanks, everybody for listening. Please go and do us a favor and rate the show on whatever podcast app you're using. It really helps people find us. Daniel and Kelly's extraordinary universe is edited by the amazing Matt Kesselman. He really is a wizard. You can also find us online on Blue Sky, Instagram and X, D and K Universe. Come engage with us. You can email us at [email protected]. We really do want to hear from you. And you can find our website, www.danielandkelly.org, where you'll also find an invitation to join our Discord where everybody comes and talks about the amazing universe. And we also have the most amazing moderators. This is an iHeart Podcast. Thanks for joining

• 1:07:19Daniel Whiteson?

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