Ideas - Perimeter Institute Public Lectures: The Physics of Jazz | Dark Matter Night

Episode Date: August 23, 2024

Physicist and jazz musician Stephon Alexander muses about the interplay of jazz, physics, and math. And cosmologist Katie Mack unpacks the latest thinking about the mysteries of dark matter, as part of the Perimeter Institute Public Lecture series. *This episode originally aired on Nov. 14, 2023.

Hey there, I'm Kathleen Goltar and I have a confession to make. I am a true crime fanatic. I devour books and films and most of all true crime podcasts. But sometimes I just want to know more. I want to go deeper. And that's where my podcast Crime Story comes in. Every week I go behind the scenes with the creators of the best in true crime. I chat with the host of Scamanda, Teacher's Pet, Bone Valley, the list goes on. For the insider scoop, find Crime Story in your podcast app. This is a CBC Podcast. Welcome to Ideas. I'm Nala Ayyad. Jazz and physics, both are about exploration and reaching, about mastering the rules, to see if those rules are really the last word.

Turns out, jazz and physics can shed light on each other too. So today what we're going to do is look at parallels between music with a focus on jazz music and my field of study, cosmology and particle physics. The important point here is that we're making analogies. By no way am I trying to say that everything you're going to see here, that the universe is jazz or that the jazz is universe. If you want to believe that now, you know, be my guest, okay? That's Stefan Alexander, a professor of physics at Brown University and a jazz musician. He was speaking at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, a hothouse of scientific curiosity, inquiry, creativity, and mystery. A place where some of the most imaginative intellects in science

let their ideas run wild and also engage with the public. In this episode of Ideas, excerpts from two recent public lectures hosted by the Perimeter Institute, excerpts from two recent public lectures hosted by the Perimeter Institute. Stephan Alexander is the author of The Jazz of Physics and Fear of a Black Universe, an Outsider's Guide to the Future of Physics. In his lecture version of The Jazz of Physics, he riffs on the connections between science, math, and music, from the ancient Greeks to hip-hop, and how music and jazz in particular can be a framework for understanding quantum physics and cosmology.

Well, I'm just so humbled to be here. I just remember when I was a postdoc, I used to come out here in the early days of PI and across the street in the middle of winter watching the hockey games with a nice pint. So it's good to be back and a great honor. So when I was younger, I grew up, immigrated from Trinidad and Tobago to the Bronx when I was eight, and I grew up in a musical family. It was expected. My grandmother wanted me to be a Broadway musician or something. I'm going to pay for piano lessons so that you can go do that.

But I hated practicing the piano. Then my father brought home a used saxophone, and it functioned more as a toy. So I didn't think I had to practice this thing. And it stuck with me for the rest of my life. But the point there was that when I was growing up, I was very inquisitive like a lot of young people. And after reading all these comic books and like a lot of young people and after reading all these comic books and seeing like a lot of these comic book superheroes and villains were once scientists I thought that might be a scientist and I grew up kind of with this tension or I don't know what a conflict should I be a musician should I be a scientist and that conflict has continued throughout my life and what you do to resolve a conflict is

that you write books about it and you give public talks and you bore your friends with this stuff, you know, so that's what I'm going to do today, okay? But the truth of the matter is, you know, I'm a kid of the 80s in the Bronx, that's where hip-hop music was being developed and some of my friends went on to become pretty famous people. They forgot all about me. They made a lot of money. At that time, though, hip-hop music was, they called it oftentimes drop in science. And oftentimes, a lot of rappers would actually speak about scientific concepts. They would make allusions to scientific concepts. And one of my favorites, and many people, including Eminem, claims this person is one of the greatest rappers of all time. This is Rakim. This song went number one. It kind of

took over the planet at the time, and all the young people were looking up to him. If you listen to that rhyme, it goes, that's what I'm saying. I drop signs like a scientist. My melodies are cold the very next episode. It has the mic off in the store and I'm ready to explode. I kick the mic at Fahrenheit, freeze MCs to make them colder. My listener system is kicking like solar. So rappers at that time, they would actually, they would have hidden codes in their rhymes. It wouldn't be literal. So he's like rapping about a supernova explosion, an exploding star. And so as a young person, we looked up to the hip-hop artists as actually scientific minds. They were referred to going to the recording studios,

going back to the lab, where they would explore and experiment and create and build new electronics, right, to make crossfaders and make turntables, do what they do. So this was what science looked like to me when I was growing up. And another person that influenced me in terms of this conflict was Albert Einstein, because Einstein wore his personality on his sleeve. He didn't fit into the tropes that we think scientists should be. And when he got stuck in a problem, he would go on a piano and improvise away, even though he didn't know how to play.

He was a very good violinist. He was known for his sound, not his technique. And that's something we look up to in the jazz world. Sound is about what that's what it's about. And of course, many people know John Coltrane as probably one of the greatest musicians of the century, of last century, regardless of the genre of music. But how many of us know that John Coltrane read everything about Albert Einstein that he can get his hands on? You know, he approaches music like a scientist and a mathematician. Him and Wayne Shorter, for example, they saw music not simply as a form of entertainment, but something that can change the world, that can change people's minds. They felt like they could bend the fabric of space-time with their sound and their music.

And Coltrane was going after a system similar to what Albert Einstein did with symmetry and invariance. And this is his own handwriting. This is a diagram that he drew. But the conflict that I had as a young person didn't have to be so, okay? It turns out that this is an age-old thing. I wish that my teachers would have told me that from the very birth of what we now call Western science, and even science from Africa, because Pythagoras spent about 18 years in Egypt, that actually the Pythagoreans believed in something called music of the spheres, that the planets played a harmony in proportion to the distance from the Earth. It was an Earth-centric universe back then.

And in fact, the Pythagoreans created the word cosmos, which referred to the order, that which has order in the universe and harmony. So Pythagoras, when I say Pythagoras, what's the first thing that comes to mind? That's right, the Pythagoras. Give the young man a hand. All right. I'm going to ask you a question. Given that Pythagoras did the Pythagorean theorem, but he believed in music of the spheres, do you think that Pythagoras played around with music? Okay, you're completely correct. Okay, that was a nice setup. That's Stefan Alexander and his public lecture, The Jazz of Physics,

delivered at the Perimeter Institute in April 2023. He brought along his saxophone and was accompanied by bassist Dennis Rondon. So I'm now going to call up Dennis. We're going to do a little demonstration. This is an invention. Pythagoras, he basically had one of these, what do you call those moments, aha moments. And he said, aha, and he went on to make this instrument called a monochord. And what you're looking at here is basically a bridge that you have a string suspended, clipped at the end, and you could slide this bridge and then pluck the string. This is a modern version of a monochord, except he's using his finger to pluck the string. So the first thing we're going to do is play the lowest note on one of the strings.

Okay, that's this vibration. So the Pythagoreans believed in whole numbers were sacred, going up to number 10. You would get assassinated, actually, if you worked with irrational numbers. One half is the other perfect integer. So play that. Does anybody recognize the similarity between the first one? Half. It's an octave. Okay. Let's do a third, the

other ratio. So play, just play them in sequence. Okay, that's a perfect fifth. That's what we call a dominant. One quarter, please. So you see where this is going. Basically, the discovery of what we call the Western tonal system was discovered by Pythagoras. And he discovered it from the intuition, this belief in music of the spheres. He was trying to make a replica of what he thought was going on out there and out of space with these ratios. So basically the entire diatonic system,

so let me show you something that's really cool. This is like, I noticed that Brian Eno was out here not too long ago and he's really hip to this. So watch this. I'm going to play something that the music experts should know about. That's a minor scale, right? But you know this one. Right?

I just played the same notes. So all of the qualities, all of the different scales that we play with, you know, diminish all these scales, are contained in what Pythagoras did, coming from the ratios. So that's, but it didn't end there. People were still trying to understand the motion of the planets for like 2,000 years after five, roughly 500 BC, when the Pythagoreans came on the scene. And his followers like Plato and Aristotle, they all, all these people whose name I still can't pronounce. At this point, Stefan Alexander puts up a slide

of the 17th century German astronomer Johannes Kepler. But here's the point. They were getting it all wrong. You know, you probably heard about Copernicus and the sun-centered universe, right? Heliocentrism. But this person came on the scene in the early 1600s. He had an interest in life. You want a good read, understand somebody with a crazy life? There's this guy. There's some rumor that he only took three showers interest in life. You want a good read, understand somebody with a crazy life? There's this guy. There's some rumor that he only took three showers in his life.

Anyway, you know, he had these models that were based on platonic solids because Plato was one of his heroes. He tried to understand the motion of the planets using these geometric models because this was coming from the Pythagoreans and Plato, and failed. And then he returned back to Pythagoras, the idea of music of the spheres. And what he did was, he had data from Tycho Brahe, this guy that got his nose cut off in a sword fight. He had a metal nose. I usually have that picture there. So what he did was, in trying to figure out, you know, the motion of the planets, the underlying astrophysics, the first person to write down equations for astrophysics called Kepler's laws.

And what you're looking up here is basically the idea now, or the fact, that planets are going around the sun with an elliptical orbit. Back then, astronomers or natural scientists were trained in music theory. And what he did was, just like how Pythagoras took the ratios of strings, he took the ratios with the belief that the planets were playing a harmony, like Pythagoras said, he took the ratios of the velocities of the planets, of a planet as it's moving slowest from the sun and fastest relative to the sun. And for example, the earth had basically a perfect fifth, okay? So he really

believed this and then he wrote down the notes and then through that he got to this, all right? So this idea of making parallels, okay, between music and astronomy and physics is nothing new. So, you know, we use Kepler's laws to find gazillions of exoplanets outside of our solar system. All right, so what we're going to do now, I'm going to elucidate something about cosmology, the cosmology that we deal with today, and then from there go into the quantum world. And in doing so, I'm going to use music. I'm going to use some of the language of music I hoped to inspire, to clarify maybe, this type of physics. Because as humans, we do have a natural, I don't have to tell you about the ratios. When you hear a sound, you hear a sound, right? It's

kind of embedded in us. So we just demonstrated that waves can come in these harmonic forms, meaning that we can understand certain types of sound waves. We call them standing waves because they appear to just be standing still. They're not really standing still, but just, you know, the endpoints are usually fixed. They're really vibrating up and down, but these things we call nodes, they stand fixed. And this gives us basically the different tones, the different pitches that we hear. Do, re, mi, fa, sol, la, ti, do. And these things are mechanically generated in instruments.

So we're looking out here, a flute, and we have the pressure waves of air molecules that's generating a wave pattern as I blow through the flute, for example. But waves are actually much more interesting than that. These types of waves are what we call simple harmonic waves, because they're very periodic and simple. But they have a special property, which is when I add them together, it's just important that you know that when I add simple waves, I get complicated waves. And the magic is, if I present to you, or a physicist, or an astronomer, or a computer scientist, a complicated wave, or an engineer, they can decompose it into the simple

waves. It's just like I could take a word and decompose it into the letters of an alphabet. And this is a very important technique. It's called the Fourier transform. This is like one of the mother tools that we use in scientific research. And in fact, what we're going to now do is try to take what Kepler did for planets and apply that to the universe. What we're looking at here is a computer simulation which matches the data really well of the large scale distribution of what. of the large-scale distribution of what. So if you look at this, it looks like, you know, the glimmering of waves on a lake and a bright-lit moonlight evening, summer evening.

It is a large-scale distribution of galaxies that we're looking at here. And these days, we can actually, we have measured on the order of a billion galaxies, our own Milky Way being one of those galaxies. And so this is a large-scale distribution of structure in our universe. And one of the big research projects that actually happens here at the Pyramid Institute is to understand the physics that led to this. This is one of the holy grails of research.

What is the physics that led to the formation of the galaxies? Well, what do we mean by that? Well, what we mean by that is that it's a picture today, the here and now, and this is a picture of the universe at the largest distance scales that we can see, going back about 14 billion years ago. And in fact, by measuring the properties, this is a baby picture of the universe. It's called a cosmic microwave background radiation. Actually, there are no galaxies, no nothing. It's just a seething, vibrating ocean of plasma, which is basically something similar to the surface of the sun.

And it's waving with tiny little ripples. We can compare the physics of this to later on and realize a very interesting picture, that these tiny little undulations, tiny little waves, underwent gravitational collapse with some mysterious form of matter. We call it dark matter, but we don't know what it really is. We know it exists because we need it to form those galaxies. But what I want to do is to actually understand this through the lens of music.

This is a nice little animation of how the early universe, the cosmic ray background, those undulations, how they can grow and develop. And to understand that, what we're going to do is look and study the physics of an instrument. So I'm going to call Dennis back. So Dennis, play me an A. It's a good time for us to be in tune with each other, too. Okay. I'm going to play an A. Play an A. Okay. You all agree that's an A, right? But what makes a bass sound like a bass and a sax sound like a sax? Because if it was only just one vibration, right, doing doing that then that would be it but what we actually see going on when an instrument actually generates a note one note

so actually when an instrument plays it generates a range of vibrations this instrument is generating a whole set of vibrations a spectrum of, but at different volumes or amplitude. And the one that you hear the most, the one that you hear as the A, is the loudest one. And these others, your brain perceives that as the timbre or the color or the saxophoneness or the bassiness of the bass. It is the signature of what gives the instrument its personality. So this just represents the physical vibration of the instrument. It's going to represent the material makeup of the instrument, the geometry and the shape of the instrument, right?

And so we can actually look at this spectrum and by using this four-way transform idea, decompose it and figure out what instrument we're playing. All right? When people make synthesizers, that's basically the physics going on there. Thank you, Dennis. So this is a very important piece of physics

because we're going to use this idea that objects vibrate, and they generate a whole set of vibrations, including instruments, and we see basically different resonances that reveal the geometry and the material makeup. So let's do the same thing for what appears to be this very chaotic cosmic microwave background radiation. We can decompose the waves of the universe that's vibrating in a similar way that we did for the instrument, can't we? We're allowed to do this. We're scientists, right? We can just decompose these waves, they look very complicated, and say how much of what frequency is contained and how loud is it for this thing, right?

And if we do this, lo and behold, this is what we see. We actually see that the universe is functioning like a very simple instrument. The first peak corresponds to the loudest note, which is actually the A note, believe it or not, many octaves lower. The other peaks represent, if we believe in this hypothesis, should represent something about the geometry

and the material make-up of the universe. And even back then, in the early 90s, we knew that there had to be dark matter in the universe because of this peak. We kind of knew that already from Vera Rubin and Zwicky. But this already, looking at the early universe and how it vibrated, the material makeup of the universe is revealed by these other peaks. And that's basically the physics. That was the Canadian Jim Peebles Nobel Prize. All right, now we're going to move into the quantum world. So when I was writing my book, The Jazz of Physics, back, I don't know, 10 years ago,

world. So when I was writing my book, The Giles of Physics, back, I don't know, 10 years ago, I got an email and I, you know, we all get emails from people and it was from, oh, I have a quantum theory of music. I'm like, okay, junk box. The problem is it was this person that wrote me the email. Okay. Do y'all know who this guy is? You know Miles Davis is? Okay, you know Art Blakey is? Okay, he played with those people. Okay, and then some, right? New Orleans, Wynton Marsalis, this is the NEA Jazzmaster. Okay, Donald Harrison from New Orleans. It was him that sent me that email. Well, I had to go back and do some apologizing years later. And we continued talking. Donald Harrison had an idea. Oh, let me tell you another thing. The reason why I get involved in

the whole music science thing is because I just want to become a better musician. And I have to use whatever crutch I have. You know what I mean? Like, you know, this is my end. This is how I get to talk to the musicians. They want to talk to a physicist, right? So I'm like, hey, I'm available. I can talk quantum. So that's what's going on with me and Donald, right? I tell him about my quantum stuff, and he's like, I can't believe this is happening to me.

I'm like, you can't believe this is happening to me. I can't believe I'm talking to Donald Harrison, right? So by the way, Donald Harrison, if you are a sax player, his stuff is incredible. I mean, he could just play seamless through the chord. He's just so good. And I was like, how does he do this? So I said, Donald, how do you do it? He goes, well, I don't play, you know, I don't play the changes, right? You don't play the changes. By the way, in jazz, we have chord changes that changed over time. And, job is to make sure you're playing within the changes. And if you're going to play outside the chord changes, it better sound hip. So Donald Harrison, he goes, yeah, man,

it's like quantum mechanics. I was like, what do you mean? He goes, I don't play in the changes. I play through the changes. I said, oh, let's talk more about this. There are many strategies, many ways you can maybe artfully improvise through what have you in jazz, chord changes, rhythm changes, yada, yada, yada. But he had another strategy, and it lined up with how we think about quantum mechanics. So let's actually use jazz improvisation to understand a mystery about how to interpret quantum mechanics. Let's just do this. In classical physics, if I threw a ball from point A to point B in a straight line, it's obviously going to do something very intuitively, right, straightforward. It's going to traverse a unique path. No matter how complicated that path is, it will be unique. But what Richard Feynman taught us how to think about what quantum particles do is that when they move from point A to point B, according to the equations that we have to trust to make all those devices

that's gonna come out of the PI, the one particle has to traverse every possible path from point A to point B. And that is completely bananas, okay? If you really sit and think about this. All right, but the point here is that this is actually what the equations are saying. And so that's kind of what we're going to, the game we're going to play now is use Donald Harrison's idea of playing through the changes.

Yeah, so we developed a theory. So the idea is the same way a quantum particle starts off at a given position. So I want you to analogize a note as a particle. And it moves, then I could think of many notes, and then an endpoint. So if I go from, for example, the G to C, right, I might be able to do something like, That's one line.

Or I could do something like, That's one line. Or I could do something like... And all of these possibilities are sort of improvisations or considerations of a whole set of infinite possibilities as I go from point A to point B. So an improvised jazz line considers all possibilities. So the interpretation is quantum particles improvise their way from point A to point B. And that's kind of interesting if there are some interpretations of quantum mechanics that actually say something close to that. So that's an interesting thing I wanted. That's another kind of parallel. And so this is the path and the rule now for you're playing through the changes. You're not playing. Let me say it another way. You know, as music is happening, it's happening too quickly for you to think about it

if you're improvising. And the minute you stop to think, you may stumble and fall, right? So the point here is you have many different paths that you can start, and you're playing through the changes. So it really doesn't matter what path you might take. It's where you end and where you land. And that takes a lot of practice, by the way. It's not anyone can get up there and just do that. One has to practice the strategy. All right? So instead of thinking about, oh, 2, 5, 1, dominant, 7, da, da, da, that's not happening here. What you want to do is now internalize the beginning and end points of chord changes. And you want to practice that strategy. Okay, just like a gymnast is practicing and tripping and doing all these things, right? So that's kind of an interesting lesson. And it's kind of interesting that one of the things that

I've been playing with as a person that, you know, tries to use my music to inform my science, and I could just simply be listening to music while I'm doing my calculation, right, or falling asleep, because you never know, right? The idea may come in a dream. Or using physics to say something to understand or clarify my music. Now, it doesn't, I mean, one thing that you can say is that there are many musical systems and many cultures play da-da-da-da-da. That's not actually quite true. Most cultures, regardless of their musical system, recognize the perfect fifth, right? It is something that is actually sort of registered. I mean, It is something that is actually sort of registered.

I mean, the work of Carol Kramansky at Cornell University, experimental psychology, we humans recognize the perfect fifth. And the minute you establish the dominant or this perfect fifth, and that's what we're looking at here, like if you go across and you project down this hexagonal system, these are all the perfect fifths. Like the minute you do that, you have it. And I find it to be very interesting. You have this structure. And on that note, the journey continues. Stefan Alexander is a musician, a professor of physics at Brown University, and the author of The Jazz of Physics.

Ideas is a podcast and a broadcast heard on CBC Radio 1 in Canada, on US Public Radio, across North America on Sirius XM, in Australia on ABC Radio National, and around the world at cbc.ca slash ideas. Find us on the CBC Listen app and wherever you get your podcasts. I'm Nala Ayyad. Hey there, I'm David Common. If you're like me, there are things you love about living in the GTA and things that drive you absolutely crazy. Every day on This Is Toronto, we connect you to what matters most about life in the GTA and things that drive you absolutely crazy. Every day on This is Toronto, we connect you to what matters most about life in the GTA, the news you got to know, and the conversations your friends will be talking about. Whether you listen on a run

through your neighbourhood or while sitting in the parking lot that is the 401, check out This is Toronto wherever you get your podcasts. Check out This Is Toronto wherever you get your podcasts. Stefan Alexander mentioned one of the deepest mysteries of the universe, dark matter. Something which seems to have been essential to how the universe evolved and galaxies formed. Observations show that there is something out there besides the matter we know of, the things included in the standard model of particle physics, from quarks to electrons to the Higgs boson.

In fact, there's five times as much dark matter as known matter. That's way too much to ignore. So scientists have been racking their brains for decades to figure out just what it is. Katie Mack is one of those scientists. She's a theoretical astrophysicist and the Hawking Chair in Cosmology and Science Communication at the Perimeter Institute. She's also the author of The End of Everything, astrophysically speaking. astrophysically speaking. Thank you so much. Thank you so much for coming out on this dark and spooky evening. We're going to be talking about dark matter, and dark matter is very close to my heart because it's one of the main things that I work on as a researcher. And I'll also talk about how it connects to our understanding of particle physics

and the universe. So just to get us oriented here, we live in the Milky Way galaxy. Because we are inside the galaxy, we can't see the whole thing. If we could, it would look something like this. This is our nearest neighbor large galaxy, the Andromeda galaxy. So we live in this spiral galaxy with stars and gas and dust and black holes and all of that and when we look out into the cosmos we see galaxies everywhere we see an amazing assortment of galaxies throughout the universe but when we look at those galaxies when we look at all that structure in the universe that's really just window dressing the majority of the universe is entirely invisible. So I'm going to be talking about one aspect of that dark matter. And I like to use analogies

and metaphors when I talk about dark matter and compare it to things that people have a personal connection to. So in this case, I will talk about the comparison between dark matter and the force. So if you're familiar with Star Wars, then you know that the force is what gives a Jedi his power. It's an energy field created by all living things. It surrounds us and penetrates us. It binds the galaxy together. So we can go through each of these characteristics of the force and compare it to what we know about dark matter. Okay, let's start at the beginning. What gives a Jedi his power? Well, as far as we know, there's no connection between Jedi and dark matter. Also, of course,

we know that not all Jedi are male, so that clearly doesn't work. What about the next bit? An energy field. So dark matter is a kind of matter, which means it's something that has mass. It has gravity. There is a connection between matter and energy, as we know from Einstein. So it does have energy, but really it's a kind of matter. It's something that has mass, like the other particles that we deal with day to day. Next part, created by all living things. Now, as far as we know, there's no connection between dark matter and life. Whatever Philip Pullman says in his dark material series, it is true that without dark matter, it would be very difficult for living things on Earth, at least, to exist because of its role in how galaxies

formed in the very early universe. So that's one of the things that I'm studying is the connection between dark matter and galaxies in the early universe. But really, dark matter is something that helped galaxies to form in the first place in the universe. Surrounds us? Yes. So dark matter is something that seems to be around us all the time. It's something that galaxies are kind of embedded in. So you can imagine galaxies sort of inside these giant clouds or blobs of dark matter, where the galaxy is just a small piece and the dark matter cloud or blob, or we call them dark matter halos, are kind of engulfing the galaxies. Penetrates us. Yes. So it turns out that dark matter is,

it's invisible, as far as we know, it's invisible because it doesn't interact with light, which means it doesn't interact with the electromagnetic force. So something that doesn't interact with light is invisible because light passes through it. It doesn't reflect or absorb light. It doesn't emit light, but if something's invisible, it's also untouchable. Because when you touch something, what's really happening there is the electrons in your hand are pushing against the electrons in that object, and that's the force that you're feeling. So if you don't have electromagnetism, if those electrons don't feel anything, then your hand would pass right through objects. And we think that dark

matter is something that doesn't experience electromagnetism, and so it's something that can pass through other matter. So as far as we know, there is dark matter passing through this room right now. It passes through the Earth. It's going through us all. Binds the galaxy together. Yes. This is one of the ways that we first understood how dark matter works, is it's something that seems to be providing the extra gravity that keeps galaxies from spinning apart. Dark matter is something that brings other matter together and holds it together. So when you think, when you see a galaxy like this in the sky, you should imagine that it's really just a small part embedded in this sort of blob, this halo of dark matter.

That's Katie Mack delivering a public lecture as part of Dark Matter Night, co-presented last year by the Perimeter Institute in Waterloo, Ontario, and the MacDonald Institute at Queen's University in Kingston. Okay, so I've just told you it's invisible. I've just told you we can't touch it. So how do we know it's there? How do we see it? So the main way that we learn about dark matter,

about its existence in the universe, is by using the fact that it affects how things that do emit light move around. And that's one of the ways that we first learned about the existence of dark matter. So this person here, Vera Rubin, she was one of the people who was studying how spiral galaxies rotate in the 1970s. And she was one of the people who gave us some of the most convincing evidence of dark matter's existence. She wasn't the only person working on this stuff. She wasn't the first person to hypothesize it, but she did play a large role in getting astronomers to understand that dark matter is a real thing that's out there. So she was looking at galaxies like this one, spiral galaxies,

and she was looking at how they rotate. So spiral galaxies do rotate, not really like this, but the stars on the outside, the stars in the galaxy rotate around the central region. And by looking at how those stars are rotating around the central regions of the galaxy, you can learn something about the gravity that's holding them in. What is it, what it is that they're orbiting around. So for example, if you have something like a solar system or asteroid belt, you have most of the matter concentrated at the center and the objects orbiting around go more quickly in the center and more slowly toward the outside because toward the center they feel the gravity more strongly. They're closer in so they can go really quickly without

flying away and on the outside they go more slowly. But when Vera Rubin and others looked at the way that stars moved around in galaxies, they saw something more like this, where the stars at the center and the stars at the outside are actually moving the same speed. The ones on the outside take longer to get around because they have farther to go, but everything's moving at the same speed, which doesn't make sense if all the matter is concentrated in the center or even if most of the matter is concentrated right there. or even if most of the matter is concentrated right there. And so it's kind of analogous to like, if you see a merry-go-round, and there's a big kid pushing this merry-go-round, there's this little kid kind of holding on to the edge,

and that merry-go-round goes really, really fast, then that kid, his arms are not strong enough to hold him in. He would just fly off into the dirt, right? But if it's going really fast and he's still sitting there, like he must have a little seatbelt or something holding him into that merry-go-round. And so dark matter seems to be that invisible force holding the stars within galaxies as they're moving around really, really quickly. And our understanding is that it's more concentrated in the center and it gets less concentrated on the outside. It's kind of this spherical distribution-ish of matter.

But you might ask if there's other evidence, because there are other ways to explain the orbits of stars around galaxies. And there is other evidence. There's a lot of other evidence in other areas of astrophysics and cosmology. I don't have time to go into all of them. I will tell you something about gravitational lensing, though. So Einstein's theory of relativity, of general relativity, his theory of gravity. So the idea there is that massive objects kind of make dents in space. They kind of bend space around them. What's really happening is that massive objects are kind of pulling space

toward them in all directions. The idea is that that massive objects curve space and then other objects if they're moving around nearby respond to the curvature of the space. They move through the space along its its curvature. So if you have a massive object and another object moving past then the path of that object will be curved because the space is curving due to the presence of that mass. And if you, instead of having a massive object moving past, if you just sort of shine light past something massive,

then the light also follows the curve of space. And so you end up with light curving around in that curved space as well. And the cool thing about gravitational lensing is it doesn't care what the matter is made of. If there's matter that's bending space, then space is bent and everything responds to that, even if that matter is invisible, even if it doesn't interact with light. So imagine an invisible object is kind of moving between you and some background stars or galaxies or whatever. Now, you won't be able to see the object, but you'll know it's there.

So when I start the animation, you can see that the background is being warped by the presence of this mass. And as this mass moves along, it's distorting the images of those background objects because it's bending the space so much that the light from those background objects because the space is bending the space so much that the light from those background objects is being warped. You're getting multiple images of some of these background objects. You're getting these rings if the alignment is just right. And that's called gravitational lensing. And we actually see that in the sky all the time. So this is a cluster of galaxies here and it it's bending the space, and you can see

these arcs of where the background galaxies are distorted by the bending of that space. And if you actually counted up how much mass was in all the stuff that's shining, it's not enough to create that much bending, right? So when you look at an image of gravitational lensing, it's measuring all of the mass, and most of that mass is invisible mass. And we have tons of images of these where you have galaxies or clusters of galaxies and they have these really amazing distortions of these background galaxies and multiple images of background galaxies. And if the alignment is really good, you get these giant arcs. And in all of these cases, just by looking at the placement and distribution of those arcs and those distortions, you're really weighing the total amount of matter there. And you find

that most of it is stuff you cannot see. Okay, so then what is it? We don't know. Okay, so then what is it? We don't know. The usual starting point is something called weakly interacting massive particles. What we're saying here, so weakly means it interacts with other matter either weakly or not at all. So weakly being either sort of feebly

or just via the weak nuclear force. There are a couple of different ways to interpret that. But we haven't seen any interactions with other matter. I'll talk more about that in a bit. Massive in the sense just that it has mass, not that it's really heavy. And particle in the sense that it acts like a collection of particles. It acts like a collection of particles that just don't have electromagnetism.

So we usually abbreviate this to WIMP. The name came about back in the day when the other major alternative was massive compact halo objects, machos. So, okay, so what is a WIMP? Well, we know we have a bunch of particles in the standard model of particle physics. We actually found a new one recently, the Higgs boson.

So these are the particles, these are all the particles we've ever detected in a particle detector in an experiment of any kind. Okay, so the blue here, these are the quarks. They've come in six different flavors, up, down, charm, strange, top, bottom. They're named in the 60s and 70s. The quarks are the things that make up the protons and the neutrons, the particles that sit in the centers of atoms. So protons are made of two up quarks and a down quark. Neutrons two down quarks and an up quark. Then in the green, these are the leptons.

There's the neutrinos, which are these sort of ghostly particles that come from the sun and other stars. Then there's the electron and its heavier cousins, the muon and the tau. The electron is the thing that goes around the atom. You know, usually it's depicted as a kind of orbit, but when you get into more physics, you find that it's not really orbiting. It's kind of a cloud of electron-ness around the center of the atom. And then in the red here, these are the gauge bosons, which are the force carriers. They're the particles that mediate the forces of nature. So there's the photon that does the electromagnetism.

So photons are responsible not just for light, but for electromagnetic interactions. There's the gluon. That's the thing that kind of holds everything together inside the nucleus. It's the mediator of the strong nuclear force. And then the W and the Z bosons have to do with the weak nuclear force.

So they have to do with things like radioactive decay, weak nuclear interaction. Okay, so which of these could be the dark matter? So we can look at what the properties we need to have for dark matter are. So we need it to be massive. It has to have some mass. So that rules out the massless particles in this model. It needs to be long-lived in the sense that it can't be decaying into

other particles. It needs to be something that holds its own. So that gets rid of the ones that have short decayed lifetimes. It has to have no electric charge because we already said it doesn't interact with light and it doesn't interact with electromagnetism. So that rules out the electron and the remaining two quarks. And the final thing is it has to be slow moving in the sense that if it's just zipping through the universe all the time really fast, it doesn't collect into those halos. It doesn't bring matter together. So we'd have to, it's called cold dark matter is the usual paradigm. It has to be slow moving. It has to be kind of sluggish. And that rules out all of the neutrinos that we know about. So we're left with a bit of a quandary,

which is that we have this standard model particle physics. It's everything we've ever seen, touched, interacted with, detected, and it's none of those. And that means that whatever it is, it has to be something that's in addition to the standard model. It may be one thing. It might be several things. But it's something beyond the standard model particle physics,

which is part of why it's exciting, because if we figure out what it is, then we've automatically improved our understanding of particle physics. Okay, so how do we find it? Now, there are a number of different things that we can look for, and I'll kind of talk through a few of those. So the problem is that, you know, I said it's sort of untouchable. That means that when dark matter and regular matter kind of go past each other, they tend not to interact.

But there's a chance that every once in a while, there will be some interaction between dark matter and regular matter. Maybe they have some interaction via the weak nuclear force. Maybe it's really, really rare, so much that it doesn't show up in our sort of astrophysical observations. In which case, there's got to be some kind of interaction that could happen between these particles, right? So if you want to look for dark matter through direct detection, you kind of wait for a dark matter particle to bump into a regular matter particle, and the regular matter particle comes out.

So what we'd actually see, since we can't see the dark matter, is we'd see a regular matter particle bounce off something invisible or be bumped by something invisible. That's kind of what they're looking for with direct detection. Then you can think of maybe reading this diagram a different way, right? So if there's some interaction that connects these things, maybe it can connect them in a different way. Maybe you can have a situation where two dark matter particles come together, annihilate with each other, and regular matter comes out the other side. And this is something that some theories of dark matter have in them, that dark matter can annihilate with other dark matter and create regular matter. And that gives us the option of indirect detection, which is

where we look into the sky and we look for the sudden appearance of high-energy particles from places where we don't expect them to come. Because what we because when this interaction happens, when the dark matter turns into regular matter, what we would see is just regular matter appearing high-energy particles or sometimes gamma rays because of the conversion of high-energy or sometimes gamma rays because of the conversion of high energy particles into gamma rays. But if that can happen, then if it can happen this way, then you can expect that it should also be able to happen the other way. So regular matter particles should be able to come together and create dark matter.

And that's the idea behind the production method where we're looking for the production of dark matter in particle colliders. And so what we would see there is that we would see that we'd smash our particles together and they would seem to disappear, that there'd be missing energy in the collision. So let's kind of sum up where we're at with all those things.

So direct detection, it's inconclusive. There have been a few hints, but so far there's nothing really convincing. Indirect detection, also inconclusive. We've seen a few places where there's extra stuff coming from things in the sky we didn't expect extra stuff to come in from, but there could just be astrophysics we don't understand.

And then production, there's been no signal. We haven't seen anything in those colliders. However, the astrophysical evidence for dark matter is really very, very strong in a lot of different regimes. And so where we sit is this. We can make a pie chart of what the universe is made of. And we can do this by studying the amount of dark matter we see out there, the evolution of the cosmos, a lot of different parts of cosmology, and it tells us that about 27% of the universe is dark matter. It tells

us that about 68% of the universe is dark energy, which I didn't get into. It acts kind of very differently from dark matter, but it's most of what the universe is made of. And then there's this little 5% slice that I've labeled atoms here. And that contains the entire sand model of particle physics. Everything we've ever seen, touched, interacted with, detected in an experiment. And that means we have this fantastic opportunity

by studying dark matter to massively increase our understanding of the universe and also massively increase our understanding of particle physics. Thank you. Katie Mack took questions from the audience, beginning with one that has a natural tie-in with her book, The End of Everything. What might happen to all that

dark matter when the universe comes to an end? Oh, that's an interesting question. What will happen to dark matter at the end of the universe? So I actually worked for a little while with a student thinking about this question about how dark matter would sort of evolve into the far, far future and what we would see. And my understanding, I mean, we don't know what dark matter is, so we can't be sure about this. But if dark matter is something that annihilates, then over time, that dark matter in halos would annihilate and turn into regular particles, turn into radiation through a cascade, and eventually sort of spread its energy out through the universe, just as everything else will. So as the universe evolves, you know, the galaxy, the stars will burn out,

the galaxies will sort of fade, black holes will evaporate, and dark matter, over time, the amount of dark matter will decrease if it is something that annihilates, because then it'll turn slowly into a sea of high energy particles and radiation. So that's our best guess at the moment for what dark matter will do. Earlier, you mentioned that there are certain fundamental particles that are short-lived and decomposed. What I'm wondering is, what do they decompose into? Is it just energy, or how are they short-lived in thatosed. What I'm wondering is, what do they decompose into? Is it just energy, or how are they short-lived in that way?

Yeah, that's a great question. So it depends on the particle. So in general, what they'll do is they'll decay into a lighter particle that they have some interaction with. There are other kinds of interactions that can happen. They can decay into different kinds of channels. So, for example, when the Higgs boson was detected at the large Hadron collider,

it wasn't the Higgs boson that they saw. It was the decay products of the Higgs boson that they saw. And they saw different kinds of decay products. So they saw, you know, gamma rays. They saw some quarks and things like that. In the case of the quarks,

the quarks themselves were also not really stable. They saw other things that came out like that. In the case of the quarks, the quarks themselves were also not really stable. They saw other things that came out of that. So it'll decay until it gets to something that doesn't decay. So we don't think that protons decay at any very long, for a very long time at least. So if you leave a neutron alone long enough, it'll decay into a proton. So it'll decay into whatever the most stable part of that part of the standard

model is. Is there a relationship between black holes and dark matter? Ah, that's a great question too. So there are some hypotheses for dark matter where it's made of tiny black holes. The idea is that there are some scenarios in which tiny black holes are formed in the very early universe. These are called primordial black holes. And if they're not so tiny that they would evaporate completely by now, then they might be around.

And if they're around, then depending on their mass, they might act a lot like the kind of dark matter we usually think of, just particles that don't interact that much with other things. If they're too big, then they can mess up galaxies. If they're too small, they evaporate. But there's a middle ground where they don't do a whole lot dynamically. They don't mess up galaxies too much. And they also don't evaporate.

And they don't pull in a lot of matter because they're just little. And in that scenario, in that sort of little mass range, kind of somewhere around an Earth mass where the size of the black hole would be like this big, we wouldn't be able to necessarily tell them apart from other kinds of other ideas about dark matter. So it's possible. It's not the favorite idea for what dark matter is made of, but we don't know for sure. Katie Mack is the Hawking Chair in Cosmology and Science Communication at the Perimeter Institute for Theoretical Physics. She's the author of The End of Everything, astrophysically speaking. This episode was produced by Chris Wadzkow.

Technical production, Danielle Duval. Our web producer is Lisa Ayuso. Senior producer, Nikola Lukšić. Greg Kelly is the executive Producer of Ideas. And I'm Nala Ayed. We're going to leave you now with a track called Dark Matter. Thank you. Thank you. For more CBC Podcasts, go to cbc.ca slash podcasts.