What is dark radiation?

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Speaker 1: Hey, Daniel, how do you like your coffee? Oh? I like it very dark, basically black. What about your chocolate? Also pretty dark? Maybe? Like? What about your wine? How do you drink your wine? Well? I like a pretty dark cabernet. Actually, now, are you optimistic about the fate of humanity? You know, I'll be honest until we start dismantling more nuclear weapons. I have a pretty dark view of our future. You kind of a dark physicist. Maybe you should lighten up a little, you know, try some dessert, wine, some rose, maybe maybe even some white chocolate. No, no, no, man, we live in a dark universe where dominated by dark matter, dark energy, and dark chocolate. You know what Yoda says, once you go into the dark side, forever your destiny well dominate. Do you think Yoda like dark chocolate? I always knew you were a dark fisicist. Daniel. Hi am poor handmade cartoonists and the creator of PhD comics. Hi, I'm Daniel. I'm a particle physicist and a professor UC Irvine. And white chocolate is not chocolate. It is chocolate. Then it's made from chocolate, made from chocolate, not the same as chocolate. You just don't like it. You still want to categorize this chocolate, But scientifically, I think if you asked a you know, chemist, they would say it's a it's chocolate. I don't know. I think it doesn't have any cocoa in it. It can't be chocolate. Doesn't taste like chocolate. You know, white chocolate is chocolate the same way deep dish pizza is pizza. But you do know it's made from chocolate, right, Like I think it uses like the cocoa butter from chocolate beans. Yeah, it's sort of like hangs around when chocolate is made, sort calls itself chocolate nearby it's chocolate and I'll give it that much. Oh man, how can it be a jacent if it comes from chocolated Daniel, I think you're stretching things a little bit here. My kids are coming from chocolate, but they're not chocolate, right. You should be more candy inclusive, Tenny, white chocolate is delicious. It's just not chocolate. But welcome to our podcast. Daniel and jorhead the Bait Chocolate apparently for an hour instead of talking about physics and explain the universe, in which we explore all the questions out there, the dark ones, the white ones, the milky ones, all the questions of the universe that are rattling around in your brain and in hours, the questions that make us human, the questions that make us wonder why the universe is this way and not some other way? Could it have been this way? Did it have to turn out in this particular manner? What are the rules of this universe? And is it possible for them to make sense to us? And is it possible for Daniel and Jorge to explain them to you in less than an hour while eating chocolates? Apparently, but only dark or milk? What about chocolate? Are you a fan of milk chocolate? Milk chocolate is chocolate. But I'm not a fan, I see, And there are different categories here of exclusion for you. Oh yeah, I got a whole matrix going here. But it is an amazing and beautiful and sometimes even delicious universe, full of amazing mysteries and tantalizing questions to answer and discover out there, and fortunately humans are here for all of it, to answer these questions and to wonder about the amazing things that are out there. That's right, and one of the most basic questions we ask about the universe is just what's in it? What's out there in the universe sharing this cosmos with us. There's this sense that if you could like make an accounting of all the stuff, all the forces in the matter, and all the particles in the universe, you'd get a sense for why the universe is the way that it is the way. For example, if you'd like looked at the ingredients of a pizza, you can get a sense for, like what makes a pizza a pizza. Yeah, because for thousands of years, we thought that, you know, the stuff that we were made out of was all there was too stuff, you know, the electron and corks and protons and neutrons that we're made out of. We thought that was the whole thing that was in the universe. But actually it turns out that most of the universe is made out of something else, something dark and mysterious. Yes, And the history of physics is these waves of realization, discovering that we are not at the center of the universe, we are not at the center of the Solar system, we are not really at the center of anything, and then discovering that our kind of matter is not even that special that we make up a tiny little fraction of this incredible universe pizza. Oh boy, Daniel, are you saying we are universe at Jason. We're not really part of the universe. We're not really universe material. We're sort of dark matter adjacent, right, We're around when the dark matter formed all this structure. We sort of like we're followers, but we're not dark matter. Yeah. Do you think there's some snob courmet I don't know, universe eater out there judging us talking down about the humanity and all the stars and planets and galaxies in the universe, saying that's not really universe. Yeah, I hope. So, I hope that there are some dark a adans out there made of dark matter, with their incredibly dark dark chocolate made of dark matter, and they're looking at our dark chocolate. They're like, that's ridiculous. Well, I guess the good news is they they're not gonna eat us, because if they're there's nobby about it, like you are white chocolate. That's good news for the white chocolate. Yeah. Here ago you can turn anything into positive news. I appreciate that. Yeah, but there is a lot of dark matter in the universe. In fact, it's most of the stuff in the universe is dark matter. Of the stuff in the universe. I mean, that's right. Even though we can't see it directly, we can tell that dark matter is there, that it's a thing that is affecting the whole shape of the universe. It affected their universe early on and its little wiggles and jiggles. It affected the formation of structure in the universe. It even shapes how galaxies spin today. So we know that it's there, we know that it's out there, and we know that it dominates the universe in terms of like the budget of the universe is mostly dark matter, not what we used to call normal matter. I e us. Yeah, it's something like, what's the exact budget percentage of dark matter in the universe compared to the regular stuff we're made out of. It's about five to one dark matter to normal matter, all right, So then that's like eight percent of the universe is sort of dark matter. Yeah, eighty percent of the universe. So you could almost neglect our part of the universe and you still got to be on your exam Befo, bueno, that's right, bueno. Barry on is exactly. Yeah, you could ignore all the atoms in the universe and that would only be five percent of the energy in the universe or twenty percent of the matter in the universe. So in some sense, the question what is the universe made out of? What is all this crazy stuff? Tells you that what we're made out of is not a big part of it is not a central element in the structure of the universe. Right, And just to clarify, most of this stuff and energy in the universe is dark energy, which is something totally different. But if we're just talking about stuff like matter, things that feel gravity, then most of it is dark matter. That's right. The matter portion of the universe is about and of that thirty percent, eight percent that is dark matter. So if the matter portion of the universe it is dark matter, well that's pretty amazing. It's almost like most of the stuff in the universe is this other stuff called dark matter, but we don't know actually what it is, right, It's pretty mysterious. We've only know it's there, we haven't actually seen it directly because it's dark. Yeah, but it's a great story. You know. I got into physics for exactly these kinds of realizations, discovering that the universe out there is not the one that you thought it was, or that scientists and deep thinkers for thousands of years hadn't known the true picture of the universe. You know. Physics is this incredible technique to like reveal the truth about the universe, to methodically build up knowledge and tell us what's actually out there. So for it to discover something shocking, like an enormous plot twist, like wow, it turns out most of the stuff in the universe out there is something you never heard about or thought about or felt before. That's fantastic and it's exactly the kind of thing that I hope happens, you know, again and again, overturning everything we thought we knew about the universe. Yeah, we know you went into physics for dark reasons. Daniels that it's pretty close to home, given that a group in Los Almos where they invented nuclear weapons which are now being used to threaten genocide against civilian populations. So yeah, that one stings a little bit, but it might say humanity even if the media ever comes towards Earth and we you know, break it up with nuclear weapons, then then your parents would be heroes. There you go. All right, So in some scenario we are saving the Earth instead of destroying it. You know, you apply the quantum physics principle. You know, it's all true. It's just the different probabilities. There's a little white chocolate lining to that dark chocolate cloud. Oh no, that ruins it for you though. It doesn't taste as good, but it goes down easier. Yeah. Yeah, So most of the stuff in the universe is made out of dark matter, and so it kind of makes you wonder if that's all that's dark in the universe. Could there be other things in the universe that we're not seeing or that we can't see. And it's certainly true that most of the universe are things that you cannot see directly. If you look out in front of you, the information that's coming to you is just from photons, and photons can only interact with things that have electric charges. For example, there are particles flying right in front of you right now that you cannot see. Billions of neutrinos coming from the Sun raining down constantly in every square centimeter per second, but they are invisible to you. So the true universe out there. The actual reality of the universe outside your skull is vastly different from the tiny slice of it that you can see. Yeah, because if we can't see of the stuff in the universe, it kind of makes me wonder what else it's doing or what else it can do, like is it maybe radiating dark forces or dark light? Exactly? Because our kind of matter does all sorts of complicated things. It doesn't just sit there right. It shoots off photons at each other. It has electromagnetic forces and strong forces and weak forces, and forms complicated things like ice cream and pizza and chocolate, and so a natural question is what's going on with the dark matter? What else can it do? Can it interact with itself in some way? So today on the podcast, we'll be asking the question what is dark radiation? Now, Daniel, this sort of sounds like an oxy moron, you know, you're almost asking like, what is not light light? Exactly? But this is the kind of game we play in physics all the time. We don't really know how to grapple with the unknown, so we tend to explore it in terms of the known. Like when we think about photons, we don't really know how to deal with the quantum objects. We say, is it a particle? Is it a wave? It's kind of both. That's kind of the same. It's a contradiction there, right, and so here that's what we're doing. We're saying, well, we know about photons, is there like another kind of like dark matter version of the photon. Were extrapolating from what we know into what we don't know. Interesting, well, it sounds like the plot device for a great science fic to novel or the next Marvel movie. You know, dark radiation. That's how you know Stephen Strange gets his powers or something. That's right. And if they are like what ten avengers made of normal matter, there should be like, you know, forty avengers made of dark matter, the dark avengers, Oh many, they can make another gazillion dark billions of dollars. That's right, and my one percent cut of that is going to be pretty sweet. Hopefully not you don't like sweet things exactly, it'll be pretty dark. Pile up those dark dollars in my dark bank account. Only pay Daniel with bitter dollars. We should start some sort of dark matter currency, that's right, dark chocolate coin. But it is a real question in physics, what is going on with the dark matter? Is it interacting with itself? Are dark matter particles shooting dark photons at each other? We just don't know. It is a dark question. And so as usual, we were wondering how many people out there had thought about this question or even know what these two words put together could mean. So, as usual, Daniel went out there into the Internet to ask people what is dark radiation? And I'm very grateful to those of you who volunteered to try to answer this question without having the opportunity to look it up or Google or get a PhD in physics first, And so thank you to all of those of you who volunteered. And if you would like to hear your voice speculating baselessly on the podcast, please don't be shy right to me. Two questions at Daniel and Jorge dot com. Has anyone tried that? They're like, oh, that's a great question, give me a seven years, I'll come back with a PhD and answer. Or is that what you do to your grad students every day? Yeah? I take the answers people give me. I'm like, oh, that's a good idea for a project, and then I go write a grand proposal based on it nice, and then you get dark bitter money for it, and my grad students spend dark bitter years working on it. Yeah. So here's what people had to say. When I figure of radiation, I think of waves being admitted from something, so that could be heat radiation, or it could be light radiation. And even when something is invisible light to humans radiation, UM, it's still quote unquote light. So dark radiation I imagine. I think it has something to do with dark energy. Probably I have to give all credit to no Degress Tyson for why I know this. But dark radiation is the mediator of dark matter particle interaction. Say that five times fast. The best analogy for it is, um, it's the dark matter equivalent of a photon. I think dark radiation is radiation coming from dark matter right away. It makes me think of dark matter and dark energy, and I know they both interact with regular matter only through gravity. So I would guess that it has something to do with how dark matter radiates away or evaporates in the same manner regular mass does with like a black hole. All right, some pretty good answers here, mostly questions. I feel like people answered back with some questions too. Yeah, and people get the sense that it has something to do with dark matter or dark inner g right, because people have this concept that radiation energy, and so maybe it falls into the like energy category rather than the dark matter category. So yeah, some great answers. Yeah, I feel like you've really sort of branded that word dark, Like people automatically anything you associated with the word dark, people know it's oh dark matter or dark energy. Yeah. The funny thing is that the word dark and physics just means we don't know anything about it. It's like hidden from us, something we can't see, and so like the only relationship between dark matter and dark energy really is that we're pretty clueless about both of them. Well, my question is, what's gonna happen when you do discover what it is and when you can see it. Do you need to change the name. Oh, that's a great question. It depends, I suppose, on whether it remains dark or not. You know, if we find some way to communicate with the dark sector with these dark particles, then they won't any longer be totally dark. So yeah, because it depends specifically on what we figure out about it. Well, you just said that it's you call it dark when you don't know anything about it. So if you do know something about it and can't see it, maybe think about changing the name to like, you know, Horhi matter or cam matter, just saying, you know, some suggestions. We'll put that on the list, right next to white chocolate matter. Yeah, I just realized its kind of running for a guy named white sun. It sounds like you have some father issues. Yeah, self loathing matter, that's what it is. That's that's pretty dark. That's pretty dark. I'm gonna go in some more dark chocolate and now feel better. Alright, So Daniel, we're going to answer this question. What is dark radiation? Which is sounds very tantalizing, it sounds cool. It sounds maybe dangerous dark radiation, or maybe dark radiation is good for you because it's the opposite of regular radiation. It's not like entire radiation like that. What you need is a dose of dark radiation that'll cure Peter Parker, right, he'll no longer be Spider Man. The you know, damage right out of you. No, dark radiation is an analog to radiation, but in dark matter. So in our kind of matter, we have you know, electrons and quarks, and they can radiate things like photons because there are forces between these particles, and so dark radiation would imagine that between dark matter particles there might be some new dark force, and that new dark force would be capable of generating dark radiation. But you know, I think technically in physics you use the word radiation for basically any particle, any quantum particle moving in a quantum field, right, I think that's isn't that the technical definition? Unfortunately, we're not totally consistent that when we talk about radiation. When we talk about radiation sort of chemically like what is producing radiation, that can include things like photons, but also, as you say, high energy particles. Here, when we talk about radiation and we distinguish it from matter, we're talking about the particles that are associated with forces rather than the particles that are associated with matter. So for example, a photon would be radiation, whereas a cork would be matter. Even if a cork moving at high speed, a chemist would call it radiation. Oh I see, So if we're going with the chemist definition, then radiation is just any particle moving kind of what if you're going, if you're going with the day annual interpretation. Here, radiation just means a force particle or like a particle associated with a force moving through space. The idea is that maybe dark matter also produces dark radiation. That's the idea, and it's a really interesting area to study because you know, we don't know what's going on with dark matter. We just know that there's a lot of it in the universe. But it's tempting to extrapolate from the kind of matter that we have, right, the kind of matter we have does all sorts of interesting crazy stuff. It feels forces, it forms complex objects like ice cream and chocolate and pizza. So we don't know what's going on with the dark matter. There's various possibilities all the way from like it's totally inert it feels no forces. It just sort of sits there and feels gravity too. It has like seventy five new forces we haven't even imagined. They're capable of forming intricate, complex things that would blow our minds. And in that scenario, those forces would be mediated by dark particles, and so those would be dark radiation, right, Because I think all we know about dark matter is that it it sort of looks like it's a big giant, fuzzy clump. But that's just what we can see of it. You know, it could be something really detailed and organized. We just have a very unfocused view of it. That's right, And that's because so far, our only way to learn things about dark matter, to interact with dark matter so as to figure out like where it is and what it's doing, is through gravity, and gravity is the weakest force in the universe we've discovered, and not by a little bit, but by like ten to the thirty six. It's like super duper crazy week. So it's like trying to look at dark matter, but you're looking through very very dark glasses, so you can just barely see it. You can only see huge clumps of it, and that gives us a very fuzzy view of what's going on. The glasses are made out of dark matter. I'm a little confused here. They filter dark radiation. I think you mean like fuzzy glasses. Yeah, yeah, all right, Well maybe for the people who are not familiar with dark matter, maybe give us a quick refresher of what it is and how we know it's there. So we don't know that much about dark matter, but we do know that it makes up most of the stuff in the universe, and it's some kind of matter by which we mean that it provides gravity. And you know, we invented the idea of dark matter to explain why there was gravity in the universe that we couldn't otherwise explain. So, for example, we see that galaxies are rotating really really fast, and there's apparently enough gravity to hold them together to keep the stars from going out into interstellar space being tossed out. But we can't explain where that gravity is coming from, so we say probably dark matter. And if we want to understand the structure of the universe, how it got to have these galaxies and these superclusters and all of this stuff. If you run a simulation of the universe without dark matter, then it just doesn't form galaxies and stars this quickly. You need more gravity to pull that stuff together. So we need dark matter not just to explain how galaxies are spinning, but also the whole structure of the universe. And if we look in the very early ever beginnings of the universe and we see like how wiggles in the first plasma of the universe formed, and how those wiggles propagated, they don't really make sense unless you add dark matter to our calculations. So we have a lot of evidence that dark matter is a thing, that it's out there, that it clumps together in these big structures that shaped our whole universe. We just don't really know what it's made out of other than something that gives gravity. Yeah, we we know it's there and it has to be there to explain what we see. But I think that's the crazy things that we don't know what it is, Like, we really have no idea, Like it could be a totally new kind of thing that's not even a particle, or even something sort of like cohesive. It could be anything, right, Yeah, And we've done a lot of podcast episodes diving into some of those examples. Sort Of the mainstream idea is a whim weakly interacting massive particle that's sort of like, you know, a generic idea like what's the simplest possible explanation for dark matter? But there's lots of other more exotic and fascinating ideas, from things like axions, these weird heavy photons, to things like primordial black holes formed very very early in the universe to things like super duper long wave photons generated by dark stars. So we just don't really know what it is. And as you say, it might not even be a particle. It might be that this whole idea of particles and fields and quantum forces only applies to the little bit of the universe that we've been studying for the last few hundred years, and that you can't extrapolate to the rest of the universe the same way you can't, like look at an elephant's tail and assume the rest of the elephant is like the tail. So it might be that we have dramatic mind blowing lessons to be learned about dark matter, or it could be super boring and just be one particle that doesn't do anything. Some pretty extreme possibilities there. But but are you saying that maybe, like maybe even quantum mechanics only applies to a small part of the universe. I think that's true. Yeah, All of the tests that we've done of quantum mechanics use our kind of matter and our kind of forces, and so we have a pretty good quantum description of quantum particles, but we don't really have a quantum description of gravity and gravity is our poor to dark matter. The only thing we know about dark matter is that it does feel gravity, and so it could be some other new, weird kind of thing that generates gravity but doesn't otherwise follow the rules of quantum mechanics. Well, all right, so that's a dark matter. Now, what would you say is a dark force? Besides the obvious Star Wars reference, a dark force would be anybody who tries to force me to eat white chocolate, the dark force in my life. Now, a dark force would be in the assumption that dark matter is made of particles that follow the rules of quantum mechanics like our particles, then you might imagine that they also feel some dark forces between them. You know that too. Dark matter particles can exchange some like dark photon or dark z boson or dark higgs boson that they can like push or pull on each other. They can do more than just feel each other's gravity. Maybe there's some additional dark forces and they're passing particles back and forth as a way to mediate those forces. Rights. Even the matter in our universe is sort of selective about which forces it feels or which forces they give out, right, Like some particles in our type of matter only feel like the magnetic forces, or they don't feel the strong force and things like that. Right, that's exactly right. Neutrinos, for example, only feel the weak force. They ignore the strong force, they ignore electromagnetism. Electrons they do feel electromagnetism and the weak force, but they ignore the strong force. Only the corks feel all of the forces, including the strong force. And so for an individual particle, you can ask, like whether it has a charge for that force. So electromagnetism only interact with particles that have electric charges. For example, we know that dark matter doesn't have electric charges, and it doesn't have weak charges, or it doesn't have strong charges, but it might have charges for other forces we haven't yet discovered. Wow, like there could be a whole different category of forces that we just don't happen to feel in our kind of matter, but that maybe dark matter it does feel exactly So there could be like one or forty new forces that we haven't yet discovered because our particles don't feel those, or maybe more, not just forty, maybe four thousand exactly. All right, Well, let's dig into what this new kind of force might be, this dark force and what it could all mean and whether or not we can never hope to see it or feel it. But first let's take a quick break. All right, we're talking about dark radiation, and in this case, the word radiation sort of is associated with the word force. So really we're asking does dark matter feel dark forces that maybe are out there, but that we in our kind of matter can feel or touch or detect. But then maybe it is maybe organizing dark matter into structures or maybe you know, even making dark matter particles and Adams exactly. And it's part of this imagination game we play, you know, we wonder what's going on out there in the dark universe. Is it simple, is it fascinating, is it complex? What is going on? What is they're out there to learn? And usually when we go out and explore the universe, we find that things are crazier and more fascinating and richer than we ever imagined. And so we're at this point where we really don't know about most of the universe. But at some point in the future, I hope a hundred years, five hundred years from now, we will know what's going on in the dark sector and those people will look back at our ideas as ridiculous. Probably you might think of us as the Dark Ages physics. They like the really dark ages, not that the trial run in the Middle Ages, but more serious. Yes, back when people ate white chocolate, that's right. Maybe we're looking forward to an age of dark enlightenment, in the age of oxymorons. But yeah, So in our kind of matter, we feel the electromagnetic force, which is what basically holds our atoms together. And what you know when you touch something, when you touch the table or you hold the banana, that's the force you're feeling. And that force is transmitted by a particle called the photon. So the photon is not just lighted Tosso what transmits the electromagnetic force? And so the question daniel I guess is does dark matter have the equivalent of a photon or at least an electromagnetic force field. We just don't know, and so we're playing this game of trying to extrapolate from what we do know. And in the same way that we imagine maybe dark matter is made of a particle, because we're made of particles, we wonder if there are dark forces that look like our forces. And so one of the simplest forces to understand is electromagnetism because it just has like one particle and it interacts with lots of stuff, and so a simple proposals to say, well, what if dark matter has a dark electromagnetism, meaning a dark photon. This would be a particle that has a lot of properties similar to our photon, but it would be different. You know, those dark matter particles would have like a dark electric charge and the dark photon, but only interact with particles with a dark electric charge, not ones with our electric charge and our normal vanilla photons wouldn't interact with dark matter because they don't have an electric charge, only a dark electric charge. But it wouldn't necessarily be the same or the sort of the analog of light. Right you're just saying like, maybe it's a force, and let's why not let's qual it the dark electromagnetic force. It doesn't have to be, and it could be the dark matter fields forces that are more similar to like our weak force, or forces that are more similar to our strong force, or forces that are totally different from these forces and have other new weird capacities. Right before we discovered the strong force, who could have imagined this crazy force, you know, where it gets stronger as things get further apart, and it has like eight different kinds of gluons pulling it together. It's bonkers. Nobody would have written that into the script. So it's possible that there are other kinds of forces. But if there is a dark force that's very similar to electromagnetism, then that dark photon would have properties very similar to our photon, because those properties come from the structure of the force, all right. And also it makes for a cool name, dark photon, because it kind of it trips up your brain, right, Like, how can a photon which transmits light be dark? That's right, because it transmits dark light into dark eyeballs. Oh yeah, I feel like you should maybe just call it something different. What would you call it the dark on the invisible photon? I don't know, dark on is good, But then what do you find another force? Yeah, we'll have to be even more creative with our names. We'll just have, you know, a brainstorming session with lots of chocolate, and we'll come up with something good and then and then who knows what you'll get out of it? Maybe a heart attack, all right? So dark matter might feel this new kind of or strange new kind of force, a dark force. So how would we ever detected Daniel, would we ever hope to feel it or you know, know it's there if we can't touch it or see it. So in the end, this question is really asking what dark matter does to itself? You know, can we speculate about what's going on in the dark sector beyond these particles existing can like touch each other and become sticky and do stuff, And so we have sort of two ways to probe this. The first category is sort of like indirect ways to look at it, and broadly speaking, we can tell that dark matter is doing things because of its gravity. And even though it's not a great way to study dark matter, we can use its gravitational effects to ask questions like is it clumping? Is it interacting with itself? Because if it interacts with itself, it forms sort of different structures that have different gravity. So we can sort of ask questions about whether dark matter is feeling itself by trying to look to see how it organizes itself in the universe. I see, we can't maybe see it directly or touch dark matter, but if we can somehow kind of know what it's doing, And if we see it doing stuff to itself, then we know that it's there must be some kind of force that it does interact with exactly. For example, if dark matter felt a version of the strong force, if it interacted with itself very very strongly and could like clump together very very very sticky ways, then it would form much denser objects than we see currently in the universe. Currently, dark matter looks really big and fluffy, like the dark matter in our galaxy isn't nearly as compact as the normal matter in our galaxy. It spreads out much further out. It's a big, fluffy cloud hasn't collapsed nearly as much, and we think that's because it's not as sticky as our matter. We think that it doesn't feel like a super powerful force that sticks it together, because that would help it like form denser objects like dark planets and dark stars. So we already know very broadly something about how dark matter can't feel itself because it stays sort of fluffy and doesn't seem to clump as much as normal matter. Right, But I wonder you know it sort of looks fluffy to us because we don't have a good way of seeing it. You know, we talked earlier about having we kind of have a fuzzy lens when we look at dark matter because we can only see it through gravity. You know, is it maybe even possible that there are dark matter stars out there, which is don't have the resolution to see them. You know, absolutely, there's a very big loophole in this argument that you allude to, which is that you can see sort of the bulk of dark matter, but we can't tell what it's doing in detail. In an episode recently about where is the dark matter, and we talked about how we can tell sort of the dark matters in these big fluffy clouds, but we also can't really tell if it is forming clumps. So you could imagine that dark matter maybe it's made out of a few different kinds of things. So most of it, maybe like you know, half of the dark matter or three fourths of the dark matter, is some big, fluffy stuff that hardly interacts with itself. But there could be a component of the dark matter that does do interesting, complicated things and has crazy interactions and forms dark stars and dark planets and dark life, and that would not mess up the distribution of dark matter, because remember, there's like so much dark matter out there that even like if a quarter of it does more interesting stuff, that's still more than all of our kind of matter. So there's plenty of room to have like a component that has complex interactions without missing up these constraints about the big picture of dark matter. So I feel like dark matter could be in the shape of I don't know, like rubber duckies. It's just that they're sort of distributed out in space, these rubber duckies, and to us it just seems like a big fluffy cloud. Yeah, most of it has to be pretty big and fluffy to form that big fluffy cloud. It can't stick to itself. But it could be that inside that big fluffy cloud there are like rubber duckies made out of like a special kind of dark matter, you know, the way they like our matter. There's lots of different kinds is electrons, is different kinds of quarks. Right, we have twelve different matter particles. Dark matter could have like fifty different kinds of particles, and maybe most of them don't interact and make a big fluffy cloud, but one or two of them form dark rubber duckies that are floating out there in the universe, and we couldn't tell the difference because the only way we can see them is through their overall gravitational interaction. I'm pretty sure that's what dark Vader uses in his bathtuff. But I guess the question is, you know, isn't the fact that dark matter does seem to be sort of fluffy and doesn't seem to stick to itself. Is that evidence that maybe it doesn't interact with itself other than with gravity. It's evidence that it doesn't interact with itself very very strongly. But again, the loophole is that some component of it might be able to Now. The way to probe that a little bit more deeply is to do experiments. Is to take like two big clouds of dark matter and throw them against each other and see if they pass right through each other, or if maybe they stick together and make them like dark explosions. That kind of experiment is pretty tough to do, but we were lucky and the universe did it for us. It collided two huge clusters of galaxies, each of which have their own dark matter associated with them, smashed them together millions of years ago, and we got to see what happened and what did happen, and so this is called the bullet cluster. You can go ahead and look at these pictures online if you're interested. It's really beautiful. What happened is that the stars in these galaxy clusters mostly passed through themselves because stars are pretty sparse, so it's like two clouds of sand passing through each other. The gas and the dust that were in these galaxies smashed together and admitted a lot of light and radiation. But the dark matter looks like it basically passed right through itself. You might ask school like, how could we know where the dark matter is? We can see the dark matter because it distorts the stuff behind it. From the dark matter is matter, it feels gravity, so it bends space a little bit, which creates like gravitational distortions in the light that passes through it. So from this bullet cluster, it looks like these two clouds mostly passed right through each other. So not it doesn't have any strong interactions with itself, but it could still have some interactions with itself, which would maybe give rise to something like a dark photon. That's right, and so people often cite the bullet cluster is evidence that dark matter can't feel anything with itself, and that's not exactly true, because, as you say, there's still room in there for some kind of interaction. Because remember that the stars, which do definitely feel each other, also passed right through themselves. Right. The star is hardly collided, and that's just because it's pretty sparse, and so could be that dark matter forms the structures, but they didn't collide with each other because just like the stars, they're pretty sparse and space is pretty big. And it could also be that some components of the dark matter did collide with itself, did get stuck sort of in the middle there. But we don't really have the resolution to tell. We can't very precisely measure how much dark matter passed through and how much stuck. We can just tell that a lot of it passed through, but some component could have gotten stuck and could be doing like crazy stuff there in the middle. I see, like maybe the Rubert duckies didn't all crash into each other, but some just kept going. I think the key point to understand is that there's so much dark matter out there that even if a tiny fraction of it is doing interesting things we couldn't tell, and that tiny fraction would still be more stuff than all of the stars and gas and dust in the universe that we know. All right, well, let's get into other ways that we might be able to detect this dark radiation and maybe even see these dark or not see these dark photons. I'm a little confused about how he might see are not photon But first let's take another quick break. All right, we're talking abot dark gradiation and dark photons, which it might be what dark matter sees and feels with itself when dark matter looks in the mirror. That's what they would see dark photons. Yeah, exactly. I hope they haven't eating too much dark chocolate. But to us it's pretty elusive because we can see or feel the dark matter or maybe even these forces. So, Daniel, what are some of the other ways that we might hope to detect whether or not dark matter has these dark interactions. There are a couple other indirect ways, basically again using gravity. One of them is looking at neutron stars. Neutron stars are these crazy dense objects that are the remnants of stars that have blown up and left this very very compact core, and sometimes neutron stars come in pairs and then those pairs spin around each other in this sort of like death spiral before they collide, and when they do that, they create gravitational waves that we can see here from Earth. This is like the shaking of space itself. If you're interested in learning more about that, we have a couple of episodes about gravitational waves. It's really cool because the gravitational waves reveal in detail how these stars are spiraling towards the center, how they're losing energy, and we can calculate with great precision how fast they should be going and how much energy they're losing. And so this is a really nice test of sort of like what's going on with this kind of matter, Because if these neutron stars were capable, for example, of feeling this dark radiation at some level, if the particles in there could feel this force at all, then they would radiate a little bit of those dark photons and it would change the way this inspiral happens, and we would be able to see that by looking at the gravitational waves. Wait, so these neutron stars would be made out of regular matter, the kind we're made out of. But you're saying that even though the regular matter it might admit this dark radiation, how is that possible. It might be that our matter can feel these sort of like new forces, just at a very very low level. We don't know, and so this is like, you know, one way to look for it under the assumption that our kind of matter could feel these dark forces, which is not something we know. It's just sort of like a guess. We do this in physics a lot. We're like, we don't know if it's possible to see it this way, but let's check, because if it does feel this force, if neutron stars can make these dark photons, we would be able to see that. I see neutron stars are so it's such an extreme event when they crash into each other that that's one possible, you know, crazy scenario where it might make dark gradiations exactly. And it's a place where we can make very very precise calculations about the gravitational waves and compare them to very very detailed data. So it's an opportunity to look for like very small deviations, you know, things that the neutron stars are doing that we can't otherwise explain that might be explainable through dark photons. I see, all right, what are some other ways we could maybe feel this dark force? All the most generic ways you just use gravity the kind of tests that we did before, But there are some other approaches to look for these dark photons, hoping that there's some connection between our kind of matter and the dark matter, Like hoping that somehow maybe these dark photons can turn into normal photon very very rarely, and then we could detect that what how can a dark photon turn into a regular photon. It turns out that if you have another kind of force that's very similar to the photon, has sort of like a similar mathematical structure, that these forces like to talk to each other, that there's like it's very easy to build a physics model in which another kind of photon can turn into our photon wha like spontaneously or only in like high energy completions or you know, moments where you have kind of this a state of pure energy. It would be spontaneous, and in order for that to happen, they would have to be some kind of particle in the universe that has both kinds of charges. So imagine some new kind, very very rare dark matter that does have electric charges and also has some new kind of dark electric charge. That particle could effectively be like a portal that connects our photons to these other photons, and it might be that these particles never really exist in the universe, the way like top corks almost never exist in the universe on their own, but they're sort of like on the list of possibilities. As long as it's on the list of possibilities, then dark photons can use that as a portal to become normal photons and vice versa. This allows them to spontaneously turn from one into the other. Whoa, So that wouldn't make dark matter not dark, right, It would mean that you can sort of see them. It would, but these particles again wouldn't actually have to exist in the universe. So imagine now the dark sector is some kind of particle that's actually out there, that most of the dark matter is made out of this particle. And then there's the possibility for this other particle that has electric charges and dark charges, even though it's never really out there in the universe, as long as the possibility for it to exist is part of sort of nature's menu. That possibility allows dark photons to turn into photons. It's called kinetic mixing because we see it happening with regular matter, right, Like our photons turned into other kinds of stuff all the time, yes, exactly, Like photons turned into pairs of particles electrons and positrons, and then those can turn into z bosons, right because z's also interact with electrons impositron. So even if there weren't a single electron or positron in the universe, a photon could still turn into a z boson. And in the same way, as long as there's the possibility for some virtual particle that connects these two forces to exist, then photons could turn into dark photons and vice versa. And that's kind of our only hope of seeing this dark force directly. That's the best way to see these dark forces directly, exactly, And so people have built really cool experiments to try to like make this happen more often, try to like create the scenarios that would help induce dark photons to spontaneously turn into photons. They have these resonant cavities at Fermilab, for example, that should enhance the rate of this happening. If you like build something with the right shape and size, it creates a situation where this kind of spontaneous transformation is more likely to happen. And so they look for this kind of signature. Wait, that's being built right now, Like there are people building basically dark forest boxes. These things exist already and they are being run and they're just building them bigger and bigger. It's sort of like the way people are looking for a dark matter. They look in these containers underground, and they started with small ones, and then they're making bigger and bigger and bigger ones. Nobody's seeing a dark photon yet, but they're hoping as they make these cavities more precise and larger to induce a dark photon to turn into a normal photon. Cool, and how else can you look for these? Another way to look for these is to look for particles appearing where they shouldn't be. So one of my favorite kind of experiments, it's called a beam dump experiment, where you take a particle beam you were using for something else and you dump it, meaning you just like shoot it into a huge block of concrete or into a mountain side, just because you know, it's got to go somewhere. You don't want to like sprayed over the neighborhood because it's dangerous, right, just shoot it through the earth at people on the other side of the earth. Is that what you're saying, Yeah, exactly. And so Sho didn't do a mountain. It was on the other side of Daniel. It's not California, is I'm to have to look that up. But if you shoot this particle beam into a mountain, for example, and then you put a particle detector on the other side of the mountain, none of the particles should make it through the mountain, but you might see particles appearing in your detector on the other side of the mountain, because dark photons might make it through the mountain because they basically see the mountain is invisible as transparent. The ideas if your beam sometimes occasionally produces dark photons, those dark photons would make it through the mountain. And so essentially it's like looking for light shining through walls or I see so many the ideas that you send this beam out into the mountain, some of it turns into dark photons, and then somehow it turns back into regular photons before it hits your detector. Yeah, that's exactly right. And so you're hoping that sometimes those dark photons turned back into normal photons. So you're assuming a lot of things here. You're assuming that maybe sometimes dark photons are created in your beam, and that sometimes they turn back into normal photons. Right, But if they turn back into regular photons, how would you know they turned into dark photons in the middle. Yeah, you wouldn't know for sure. You would just know that something passed through the mountain that typically shouldn't be able to. So you calculate, like how often should particles be able to pass through the mountain and turn into photons? And if that happens more often than you expect, then you know there's something else and new there, maybe a dark photon, maybe something else. But you know, that's often the case with particle physics. We find something new, we're not exactly sure what it is, and then we study it in detail, we try to characterize exactly what it is, just like with the Higgs boson. First thing, we saw some new particle decaying into two photons, and we weren't sure is this the Higgs boson or is it something else? We didn't expect, and over decades of study we sort of pinned down what it must be because of all of its behaviors. I see, it's like, the only way it could have made it through the mountain to and out into California to hit us it was if it turned into something invisible like dark photons somewhere in the middle. Otherwise it wouldn't have made it through. Exactly, I have to phase through the mountain by turning into something that doesn't interact with mountains. All right, cool, Well, what are some other ways we might be able to see these dark forces? A lot of the other experiments are very similar. There's one at CERN called Phaser Forward Search Experiment. It's a pretty tortured acronym, but a really cool idea for I don't even know what is there an A actually in the name, there's an A and forward and an R experiment. Oh man, it's a really cool idea because they're taking an already existing experiment Atlas where we collide protons together and they're wondering like maybe dark photons are created and shoot down the beam. So they built a detector like really far down along the beam past where the collisions happen to see if maybe dark photons are created in those collisions and then fly along the beam and then turn into something that they can see sort of downstream. So they added this little bit to the detector that can do something totally novel and new. It's actually led by a team here at you see Irvine, m M, I see. But we don't actually know if dark matter can switch back and forth that easily, right, We don't know. And again, it could be that dark matter is only some inert particle that feels gravity and nothing else and there are no dark forces and no way to interact. It could also be that dark matter does feel some new dark force, but can only interact with itself, and none of those dark forces can ever turn into normal matter or normal photons. Here, we're just guessing. We're trying to like study the various possibilities in the various ways that those possibilities might manifesto well, And so what does it all mean? Do you think we will find it has interactions with itself or do you think it just represents this whole part of the universe that we will never be able to see or touch or even confirm that it's there. You know, like we could be swimming in dark rubber duckies and never ever ever, Yeah, part of it comes down to what you think is more natural. Does it make sense to you to have a huge component of the universe be sort of simple and inert and not really doing any being interesting, or does this seem more natural for it to be complex and interactive the way that our matter is. Now, what I know is that the universe doesn't follow what we think is natural. Our conceptions of how the universe should work don't seem to be very well aligned with how it usually does work, and so I suspect there are some surprises out there. The other side of that question is what you asked, is might we ever be able to tell? And it could be that dark matter doesn't feel any of our forces, and none of these forces can talk to our forces, and we can never find these dark rubber duckies. But I have faith in physicists and engineers to come up with clever ways to probe these things, ways that we can't even today. Imagine kinds of experiments that people might think of in ten years or twenty years, experiments thought up by you know, clever Listeners to this podcast who aren't constrained by the kind of thinking that academic physicists have been trained to do. I think I know how you can do it, Daniel, Dark Chocolate collisions. You just gotta want it enough. You know, you have to really want it, because, as Yoda says, you know, wanting these to pain and pain needs to suffering, and suffering leads to the dark side, and so that's that's how you can get there faster. Maybe, Yeah, there is no try. I should just do it, that's right, Just I mean, do it? You should, yes, all right. I don't know why I didn't think of that before. I'm just gonna go do it right after we're done with this podcast, or not do it. I guess we're doing in the dark. Gosh, I'm so confused. But to me, it really touches on these deep mysteries of physics that we know the universe out there is telling stories that we haven't heard yet. We're desperate to hear those stories. We have hints that we know the shape of those stories, but we don't know any of the details yet, and I just hope that one day we'll be able to fill those in. Yeah, it's almost like there's a whole universe out there, sitting right in front of us that we have yet to discover, and that anyone listening to this could become part of that search. That's right. So there's plenty more of the universe to find out, and we desperately need new ideas. And whether the universe counts as dark chocolate or not, that's for other people to debate. That's right. And if you love white chocolate, take my apologies. I love all of them. You love listeners more than you love your chocolate, exactly, And if you eat white chocolate while listen to this podcast, I forgive you because he can't see your or feel you, so it doesn't matter to exactly. Please accept my dark apologies. All right, Well, we hope you enjoyed that. Thanks for joining us, See you next time. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of I Heart Radio. For more podcast for my heart Radio, visit the I heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows. Yeah,