Quirks and Quarks - How starfish move their tiny tube feet, and more…

Episode Date: February 27, 2026

Starfish don't have brains, and yet they're able to mobilize hundreds of tiny hydraulic tube feet to get around. Now scientists are getting an understanding of just how they do that.PLUS:Atmospheric pollution from an individual rocket re-entry event measured for the first timeHow the Earth’s greenhouse age transitioned into a world with frozen polesWhat is dark matter? The contenders — from WIMPs to dark matter starsQuirks Question: why doesn’t flowing water freeze at the same temperature as still water?(Correction: A previous version of the dark matter story referred to a study published last fall that mapped the distribution of dark matter, but the study was published on Jan. 26, 2026.)

I am an actor, fresh out of theater school with big dreams and an even bigger drug habit. But things are pretty good. That is until my best friend is set up on a date with David Lee Roth. Yeah, from Van Halen. If you know, you know. From CBC's personally, this is Discount Dave and the Fix. The true-ish story about how a fake rock star led me to a real trial that held up a mirror to me. And okay, let's just say that not everyone in this story is who you think they are.

Personally, discount Dave and the Fix. Available now on CBC Listen or wherever you get your podcasts. This is a CBC podcast. Hi, I'm Bob McDonald. Welcome to Quarks and Quarks. On this week's show, incoming space debris may be polluting our upper atmosphere. It's the first time that we've made an observation connecting an event of metal pollution to a known re-entry trajectory of a rocket, in this case, Falcon 9.

And sea stars coordinate their legs without a brain. Together, they behave as if they have a brain that basically directs its collective movement. Plus, pushing the Earth's climate from one state to the next, and the hunt for dark matter. All is today on Quarks and Quarks. Two, one, ignition. Lift off of the Falcon 9. Go SpaceX, go Starlink. That was the 14th launch for the SpaceX.

Falcon 9 rocket last year. On February 1st, 2025, it blasted off from California, carrying another batch of Starlinked satellites into low Earth orbit. At first, everything seemed to be going according to plan. Stage one landing confirmed. But after successfully delivering the satellites to orbit, the rocket's second stage booster engine didn't fire up to de-orbit. So instead of splashing down on the Pacific Ocean like it was supposed to, it stayed in orbit. It stayed in orbit for another couple of weeks before finally descending back down to Earth in a blazing trail of of fire. This fireball was spotted hurtling through the sky in Lincolnshire in the early hours of Wednesday morning. It was also seen in Yorkshire and as far away as the Netherlands.

Well, that fireball also caught the attention of scientists over in Germany who had been working on a way to measure metal pollution in the upper atmosphere. I'll let the lead scientists pick up the story from here. Dr. Robert Wayne as a physicist at the Leibniz Institute for Atmospheric Physics in Kulansbond, Germany. Hello and welcome to Quirks and Quarks. Thank you for that introduction. That was a very nice summary of the current state of affairs. Well, thank you for being on our program. Let's start with the incoming fireball last year on that February morning. What specifically caught your interest about that?

So we've been interested in the impact of space debris on the atmosphere for a few years. There was a groundbreaking study in 2023 done by the Americans over Alaska that showed that about 10% of stratospheric aerosols are already contaminated by space debris. And that was a wake-up call for us. So we immediately started building an instrument, a LIDAR instrument, capable of measuring space debris in the upper atmosphere. So in terms of the Falcon 9 re-entry,

we had already been measuring lithium, which we are using as a... a tracer for space debris for several months. Well, tell me about LiDAR. How do you actually do the chemistry of the upper atmosphere? So the technique we're using is called resonance LIDAR. So what we're doing is we're sending out a specific frequency of laser light up into the sky. And that light hits an atomic target, an elemental target, in this case lithium, and it causes it to resonate, just like when you hit a tuning fork with a hammer. It resonates at a certain frequency. When we hit a moment,

When we hit lithium or any other atom with a certain frequency of light, it will resonate and give us a chemical fingerprint of that atom in the mesosphere. And the light then scatters back and we detect it in our telescopes and process the data. Wow. Now, why are you interested in lithium? Most spacecraft are made with lithium aluminum hulls. The principal concern for space debris re-entry into the atmosphere is not lithium. it's the aluminum. As the aluminum enters into the atmosphere, it forms aluminum oxides,

which are effective at catalyzing radical chlorine, which leads to the catalytic destruction of ozone. Wow. Well, take me back to this rocket plume that you saw with the Falcon 9. How did you know where to look to see the plume as it passed over? So in addition to our LIDARs, we operate a network of cameras all across, across the world and we had seen the re-entry and the following morning we saw the news article

showing that it crashed into Poland some of the debris. So we thought, oh, this is really lucky for us. We might have a chance to test our instruments and really see the debris plume by this particular re-entry event. So I prepared the laser, mixed up the dye and set everything up. We did the measurement the next morning, we looked at the data, we checked the mesospheric winds with our local radar. They looked promising. So then we went to our modeling colleagues upstairs and asked if they could run a global atmospheric circulation, a model for us. They did, and we used that model data to back trace the observed lithium plume from above our observatory, back to a re-entry point of the Falcon 9 rocket,

and that was just west of Ireland at an altitude of about 100 kilometers. Boy. And the two trajectories intersect very nicely. So how much lithium did you detect? About 31 atoms per cubic centimeter, which sounds particularly low, but the natural abundance of lithium in the atmosphere

is near zero. There is very little lithium that comes in through natural meteoric sources, almost all of it that comes in comes in through rocket bodies and satellites. Oh, really? We've estimated the natural lithium influx as something on the order of 80 grams per day globally. But in a single Falcon 9 upper stage, there's approximately 30 kilograms of lithium in the lithium aluminum alloys.

So what does this all mean then if all this lithium is being injected into the upper atmosphere? So I don't have a definite answer to give you. This is a question we are asking ourselves as well. What is the final impact of the reentry of space debris on the atmosphere? There's a wide concern among scientists that the aluminium oxides might destroy ozone in the stratosphere and reverse our trends that we've made in restoring the global ozone layer. We started having problems, as you might know, in the night. 1980s with the release of chloroflora carbons, and it's taken us many decades to slowly start

recovering the ozone layer. And the introduction of this space debris into the middle atmosphere may slow that progress or indeed possibly reverse it. Boy, we were worried about losing the ozone layer because it's a natural filter of ultraviolet light from the sun. We just start repairing it. Now we're destroying it again. That is the worry. The middle and upper atmosphere are extremely fragile. They're very delicate to even small amounts of chemicals that we put into the atmosphere. It didn't take many thousands of kilograms of CFCs. It's actually a very small amount in the 1980s, and it made a very large impact.

And we have a similar worry now with space debris. Now, you mentioned that you saw this plume 20 hours after the rocket had actually crashed. How long does that metal stay up in the upper atmosphere? We simply don't know. Ultimately, every plume that exists in the upper atmosphere from a reentering rocket stage or a small satellite will eventually be mixed in the mesosphere, so between 50 and 80 kilometers. And every winter, that air goes from the southern hemisphere to the northern hemisphere and comes down over the poles through the polar vortex. So this is another aspect to this problem is that the poles will really be the epicenter of the effect of space debris on. Earth's atmosphere. And they're already the epicenter of climate change. So we're really putting our

space debris thumb on the hot spot of climate change in the Arctic and the Antarctic, which is just another dimension to this problem. What other metals are coming from satellites besides lithium? So this year we're building a new instrument, a new LIDAR that will measure three metals per night. So we intend to go through the periodic table and test each metal and see what we see. Our first target this year is copper and will likely also try titanium and ribidium, also potentially hafnium and lead. Boy, it sounds like we're going to have metals raining down on the poles in the future. Yes, that is a concern. Well, how concerning is this?

So very little is known for certain. Most of our understanding comes from model runs using simulated atmospheres. There are very few observations in the middle atmosphere. That's what makes our study quite unique. It's the first time that we've made an observation connecting an event of metal pollution to a known re-entry trajectory of a rocket, in this case Falcon 9. So obviously we need more eyes on the middle atmosphere. Yes, and global cooperation.

It seems like we've been using the upper atmosphere as a garbage can in the space program since the very, very beginning back in the late 1950s. So what do you think policymakers need to do to get a handle on this situation before it gets out of hand? Yeah, so I think we need to realize that we're now entering a new era of how we interact with our space environment and our upper atmosphere. The early space programs up until the modern age, they were launching, one, two, three, a dozen rockets a year. So the impact on the atmosphere, as we were throwing that garbage in, was really negligible. But since 2020, we now have large amount of commercial activity in low Earth orbit. So all of these mega constellations of satellites put up for

commercial purposes, the most notable is Starlink. So this is orders of magnitude more material up in Earth's atmosphere than was previous. At the same time, we're depending more and more on resources in space for communication and whatnot? Yes, we rely on the satellite communication infrastructure, but just like every other industry and infrastructure on Earth, airplanes, industrial factories, anything that pollutes the atmosphere, they all have their associated regulations and standards. And we need to seriously consider which materials

and which regulation should apply to our commercial activity in space. Dr. Wing, thank you so much for your time. Oh, thank you very much. It was a very enjoyable interview. Dr. Robin Wing is a physicist at the Leibniz Institute for Atmospheric Physics in Germany. Have you ever seen a sea star, otherwise known as a starfish in person? They're fascinating to watch, partly because they're such unusual animals. They move around on thousands of little tube-like feet that function like suction cups. You can see these tiny tubes attaching and detaching to whatever they're walking on as they move around their environment.

Now, I say walk, but it's actually more like a creep because of how slowly they move. And the really striking thing about it is that they control their movements without a brain. Dr. Ava Koso and her team wanted to take a closer look at the mechanics of how the C-stars pull off this locomotive feet. She's a professor of aerospace and mechanical engineering and the founder of the bio-inspired motion lab at the University of California in Los Angeles. Hello and welcome to our program. Hi, well, thank you for having me.

Well, when you watch a C-Star move, what is it about its motion that you see that maybe my untrained eyes might miss? They don't move very often. Most often, they like to attach to surfaces. And just the notion of C-star moving is sometimes a bit surprising. So what I first learned that was fascinating about sea stars

is not just that they move. They actually have different gates of motion. They can move in like a crawling way. That would be equivalent to us walking. But they can also transition to a bounce mode that would be equivalent to us running. So they have different gates of movement. So how do you study how the sea stars can move like that without a brain?

The first experiments that we did when we were looking at the transition to bouncing, we saw that it actually recruits more two feet when it transitions to bouncing because it wants to move faster. It's recruiting more feet. And we're actually looking at how this transition happens when you add extra weight on the back of the animal to see now it has extra weight on it. How does it respond to that extra weight? Does it have to process centrally that it has extra weight? Or does it basically every tube foot responds locally and then recruits more feet because it feels an extra weight on it? And it seems that it's recruiting the tube feet completely mechanically. They all feel something locally and respond to it where they are.

And they do all this, as we mentioned, with, hundreds of tube feet, but with a very distributed nervous system without localized control. So what do you mean when you say distributed nervous system? What I mean, I mean the nervous system is distributed everywhere. Every tube foot has its own way to gather information about itself, to know if it's being stretched or it's compressed. It has mechanosensation, and it can make decisions in the sense that it can extend, or contract, and that is local at the tube foot level.

And then you have many, many of them that are all having this ability to sense and respond. And that's distributed in the entire body of the organism. Wow. So each little foot has its own decision-making process, and there's no central brain to make a decision about where they're all going to go. So we think that it has its own ability to make decisions because if you detach a tube foot or if you detach an entire arm, it can continue doing what it does. So we know from these types of experiments that they can continue their behavior. So it's possible that they are responding locally.

But just to say that there is a nervous system, there is a nerve net in the entire animal that connects all the arms and connects all the, the two feet. So you can see it as you wish. You can see it as no brain or you can see the entire organism as a brain because the brain and the body are so enmeshed together. It's like a brain with feet. Yes, yes, if you like, yes. I'm thinking about where we see large groups of animals like a school of fish or a flock of birds where they all move like one group. You know, they all have their own little brains, but none of them are making a decision or where they're going to go. That is an excellent analogy. That's always the way I start my talk. I show schools of fish or flocks of birds and I say here you have individual entities that each one has the ability

to sense its environment and its neighbors and respond, but together they behave as if they have a brain that basically directs its collective movement. And that's a great analogy to think about how those two feet in the C-star coordinate their motion because they are coupled together through the body of the C-star itself. Well, take me right down to one foot. What does it do? How does it attach and then detach itself from a surface? It's a muscular membrane. You have the podium, that's the part that extends out of the C-star body. There is another muscle that looks like a balloon at the other end embedded in the C-Star body.

And when the balloon muscle, when it contracts, it pushes the fluid into the podium, and the podium extends. And that would be an extension of the tube foot until it attaches to the surface. And by the way, the attachment is not a suction-driven attachment. It's an adhesion. It's an adhesion-driven.

It attaches for a while. And then it detaches, lifts off, and go through a recovery stroke cycle. and then it comes back. Every two foot does this for however amount of time. There is a lot of variability in the attachment time or whether it actually engages with the surface or not. Only 10% of the tube feet need to be engaged with the surface in order for the animal to move. And that's also fascinating. It's not using all its hundreds of two feet to move. Wow. It's just astounding that there's all that coordination going on and yet there's no central brain. To coordinate, they're just each one of them acting independently, but together at the same time. That's right. So if all of these little tiny feet under the C-star moving independently, but it turns into a coordinated motion,

what decides the direction the C-star is going to move if there's no central brain? There may be a tug-of-war between those individual two feet. Usually one of the arms takes the leads and decides on the direction of movement. We know mechanically that you could take the lead if you're producing a stronger force. There are also some sensory information that could be coming from eye spots at the tip of each arm and sensory information. Some of the two feet at the tip of the arm act as sensors. They sense the chemical composition of the environment or mechanical sensors. So even with very little communication of information in the classic sense of

communicating a piece of information through the nervous system, but only through mechanical interaction between those tube feet, you can produce a lot of useful movement. Also, the nervous system also allows for sharing of information through the nervous system, electrical information, not only mechanical. So what insights can you take away from this as a mechanical engineer for robotics? So the main thing here is that we can have a system with many, many components where each one has the ability to sense and respond locally. But because they are coupled together, they can produce an overall function that is beneficial for the entire team, if you like. So we're hoping that in robotic systems, if they are made of many components that are easy to fabricate, easy to,

put together. And then when they are deployed in difficult places where we cannot control the entire organism, they can make decisions locally and produce this global function. But also, if some of them fail, it wouldn't be disasters to the entire robot. Because if some items fail, you could still have the overall function intact. It just shows you don't need a big brain to succeed, right? No, you don't need more than what's necessary for that environment. Dr. Kansel, thank you so much for your time. Thank you for having me. Dr. Ava Kansso is a professor of aerospace and mechanical engineering

and the founder of the Bio-inspired Motion Lab at the University of Southern California in Los Angeles. Since the time of the dinosaurs, Earth's climate has undergone a major transformation. Many tens of millions of years ago, our climate was 15 to 20 degrees warmer than it is today. But how Earth transition from that tropical greenhouse environment to the ice-capped world we live in today has been a mystery? Well, now we have an answer. Thanks to scientists who created the most detailed record ever of how ocean chemistry has changed over time. Dr. Dave Evans is a geochemist and is the lead scientist on this study. He's a research fellow in the School of Ocean and Earth Sciences at the University of Southampton in England.

welcome to our program. Thank you very much for having me. So first of all, what was the earth like before during this greenhouse age? Can you paint me a picture of what the world was like that? Yeah, absolutely. So it would have been a world in many respects that would look very unfamiliar to us if we were able to travel back in time and view it for ourselves. So to give you a few examples, here in northwest Europe, we know that there would have been a subtropical or tropical forest at the time. And in the polar regions, there would have also been forests and very different fauna as well. So to give you a more local example in the northernmost Canadian Arctic on Ellesmere Island, a fossil assemblage has been recovered that includes alligators, lizards, cold-blooded reptiles.

So very different to today. Wow, a much warmer and greener and wetter world than we have today. Absolutely, yeah. Well, how do you go about studying what was behind the shift in climate from the, that world to what we see now. People like me work on fossil samples that we recover in a number of different ways, so we can go out and find geological sections that contains exceptionally well-preserved fossil material, or we can recover what are called sediment cores from the bottom of the ocean,

essentially sending out a large drill ship to the ocean floor. And then geochemists like myself study the fine details of the chemical composition of these fossils. So shells and skeletons of marine organisms are principally composed of calcium carbonate. The corals that your listeners will be familiar with, a composed of calcium carbonate, for example. But in detail, there are trace quantities of lots of other different elements incorporated almost by accident as those shells and skeletons are forming. And that has turned out to be really, really useful because the sort of fine details in the changes in chemical composition of these biominerals can be related to the conditions that prevailed when those organisms were alive.

So when the temperature was a little bit warmer, you would expect the concentration of some elements to be present at a little bit of a higher level compared to when the climate is cooler. And likewise, when the chemistry of the ocean was a little bit different, you would expect that to have some impact on the fine details of the composition. of the remains of these organisms. Well, take me through that. How can studying seashells and skeletons over this long period of time

tell you about what's going on with the climate? Organisms living in a slightly warmer ocean will incorporate some elements in trace quantities to a slightly greater degree than they would if they were living in a colder ocean. And so we can use this as a quantitative tool to reconstruct, for example, what the temperature was when a fossil specimen was alive.

So we collect exceptionally well-preserved samples of this fossil material. We take it into the laboratory, and we used advanced analytical techniques, in particular mass spectrometry, to determine the fine details of the chemical composition of these fossilized remains and relate that back to the conditions when that organism was alive. So what did you find when you looked over long periods of time?

Ultimately, what we found is the very low, Large changes in the composition of seawater have taken place over this 50 million year interval that we're talking about since Earth was last in a greenhouse state. So we all know that the sea is salty, of course, but in detail, the composition of seawater is quite complicated. It's not as simple as the table salt that we put on our food. It's not simply sodium chloride. There are lots of other elements in seawater that go into making its saltiness. Key amongst those is that we identified very large changes in the concentration of calcium in seawater that have taken place over this interval, and we suggest that this played a key role in the transition from this greenhouse climate state into the ice house world that we now inhabit.

Well, how does calcium concentration affect the climate? That's a great question. So the way in which calcium in seawater can drive the carbon. cycle is via the formation of the shells and skeletons of these sea creatures that we've been talking about. So they're principally composed of calcium carbonate, as we've said. And if you decrease the amount of calcium in seawater, it makes it a little bit harder for those organisms to produce their shells or skeletons, and it makes it a little bit harder to bury those shells or skeletons in ocean sediments at the seafloor. And so as the concentration in calcium

decreases the amount of these organisms buried in ocean sediments decreases, and that impacts the carbon cycle by shifting the balance between how much carbon is stored in the ocean and how much carbon is stored in the atmosphere. Okay, so take me through the change that happened then, that caused cooling. We identify a two-fold decrease in the concentration of calcium in seawater, And we think that that was one of the key drivers of atmospheric CO2 over this 50 million year interval that we're talking about. So a large portion of that CO2 change may have been driven by this process. Okay. So essentially CO2 gets locked into these organisms and then they die and fall to the ocean floor and it remains there.

Absolutely, yes. Boy. Well, how does this climate change compare to the changes in climate worries? today? I think this is one of the key contributions that information like this can make to our understanding of present-day climate change. So what we now know to have taken place are very large natural climate changes over the course of, for example, the last 50 million years, but throughout Earth's history. And we're talking about a CO2 decrease over this time interval of around a factor of six or so, so a decrease from around 1,500 PPM back 50 million years ago to 280 before

the pre-industrial revolution. So hopefully a much larger change than we're likely to experience over the coming centuries. But of course, the key difference here is the rate of change. So the changes that we're talking about have taken place over millions of years, whereas we are now faced with an overall similar magnitude of change that is. is taking place over just a few decades or a few centuries. And obviously the very worrying thing about that is that it's very easy to adapt when you have a million years to do so. It's very difficult to adapt over a few decades.

Dr. Evans, thank you so much for your time. It's an absolute pleasure. Thank you very much for having me. Dr. Dave Evans is a research fellow in the School of Ocean and Earth Sciences at the University of Southampton in England. I'm Bob McDonald and you're listening to Quarks and Quarks on CBC Radio 1 and streaming live on the CBC News app. Just go to the local tab and press play wherever you are. Coming up later in the program, how the hunt for dark matter may reveal a new universal force.

Well, maybe dark matter is not just gravity only, but it may have other forces that are unique to dark matter that maybe the regular matter doesn't experience. I am an actor, fresh out of theater school with big dreams and an even bigger drug habit. But things are pretty good. That is until my best friend is set up on a date with David Lee Roth. Yeah, from Van Halen. If you know, you know. From CBC's personally, this is Discount Dave in the Fix. The trueish story about how a fake rock star led me to a real trial that held up a mirror to me.

And okay, let's just say that not everyone in this story is who you think they are. personally, discount Dave and the Fix. Available now on CBC Listen or wherever you get your podcasts. It's one of the great puzzles in astrophysics. Why, when we look through our biggest telescopes, we only see a small fraction of the matter that we know is out there. We know that because of the gravitational tug it exerts on what we can see.

We call it dark matter, because even though it's estimated to account for something like four-fifths of matter in the universe, we can't see it. But that doesn't mean we can't study it. In fact, we now know more about how much dark matter there is and where it might be found than ever before, but we still have no idea what it is. Science journalist Dan Falk is with us now to help us make sense of the dark matter mystery and to bring us up to date on our search for it. Dan, welcome back to the program. I'm glad to be here. Let's start with laying out the case for dark matter. Why Your scientist is so sure it's out there.

So the story goes back nearly a century to the 1930s when a Swiss astronomer named Fritz Zwicki was studying galaxy clusters, and he noticed that the galaxies were moving faster than we'd expect based on the gravitational influences of the matter we can see, so fast that the galaxies ought to just fly off into space, but they weren't flying off, so Zwicki postulated that there must be some additional unseen matter hiding in those galaxy clusters holding them together.

And then in the 1960s and 70s, an American astronomer named Vera Rubin found that the motion of stars and gas and dust within individual galaxies also doesn't quite add up. Again, the material was moving faster than it ought to based on what we can actually see. Think of a chef spinning a pizza. If the chef spins the dough too fast, it just flies apart.

So if it doesn't fly apart, that means something is holding it together, and for galaxies, that something is gravity, and if there's more gravity, there must be more matter. So with Vera Rubin's work, astronomers finally said, okay, this dark matter is real, and that kicked off what is now a decades-long quest to figure out how much of it there is, and where it is, and of course what it is. Okay, so we can infer the existence of this dark matter based on its gravitational effects, But how much can we really learn about it, given that we can't actually see it? That's right. We can't see it, but we can see what it's doing, thanks to gravity,

and that gravitational influence can even affect the appearance of distant objects. So, for example, if we're looking at a cluster of galaxies and there's dark matter in between us and the cluster, then we'll end up seeing a distorted image of those galaxies. Is this the famous bending of starlight that Einstein talked about? Exactly. I'll let Dr. Richard Massey explain. He's a physicist at Durham University in the UK. Dark matter is completely invisible, so we can't see it directly. It doesn't emit light, it doesn't even absorb light, so we can't even see it in silhouette. But we can see the effect it has on things around it. So to see some dark matter, we look at galaxies behind the dark matter, and their light rays

have had to travel all the way across the universe from there through some dark matter to us, and in their weird long journey through that patch of universe with some dark matter in it, they end up getting bent and deflected. Like we're looking at them through a bathroom window pane or something. So they end up being distorted into characteristic patterns. Okay, so we can see the effects of dark matter by observing distant galaxies and we see these distorted images. So what have astronomers learned from making those kinds of observations?

Well, they've learned a lot. For example, Dr. Massey and his colleagues used the James' Webb Space Telescope to map the distribution of dark matter in more detail than ever before. And the results actually tell us more than just where the dark matter is. They also tell us something about the crucial role that dark matter has played over the history of the universe. In fact, astronomers often describe the dark matter as providing a kind of scaffolding that holds the regular matter together, so you could say the dark matter is really shaping the universe. Here's Dr. Massey again.

go back in time to the Big Bang. And there was a primordial soup where all of the ingredients of the universe were spread out super thin. So there wasn't very much of anything in any one place. Certainly wasn't enough of anything to build a galaxy. Fortunately, in this primordial soup, there were a few lumps. And there were the lumps of dark matter that, because they don't care about anything else, they just separated out and congealed quite quickly. But then they have gravity. So they pull in more dark matter and they get bigger and they pull in ordinary matter and more and more ordinary matter gets sucked into these big lumps of dark matter and eventually there's enough of that ordinary material to to fuse together and form stars and form galaxies and burst the universe into into life and

you know so eventually there actually becomes life that all the stuff that is attracted into that scaffolding of dark matter is everything that the stars the worlds around us the planets the people Everything that we can see is all made of stuff that was built inside that dark matter scaffolding because of its gravity. Wow. So it sounds like dark matter isn't just some sort of add-on. It's really playing a fundamental role in guiding the evolution of the visible matter that we can actually see, like galaxies and, well, everything else. Exactly. So we know that dark matter is out there, and we know it's playing this hugely important role in cosmic evolution, but we still have no idea what the dark matter actually. is. Okay. So, what are physicists doing to try to figure this out? Well, the Holy Grail of dark matter discoveries is what they call direct detection, which is exactly what it sounds like, actually snagging a dark matter particle in a detector. Now, Bob, I know we try to avoid

jargon and technical terms as much as we can, but I do have one acronym for you, and that is something called a W-I-M-P in capital letters. Now, that's not the little guy who never goes to the Jim or get sand kicked in his face on the beach? Not this time. In particle physics, a WIMP is a weekly interacting massive particle. These are hypothetical particles possibly created shortly after the Big Bang, and for a while now, they've been the leading contender for what dark matter is made of. Now, because they're weekly interacting, wimps are hard to detect, so most of the experiments

involve setting up some type of specially selected target material for the WIMP to interact with, and then you sit and wait and hope that you snag one. And actually, we have several such experiments underway in Canada at a facility in Northern Ontario that I know you've been to. That's Snow Lab, formerly the Sudbury Neutrino Observatory. Yes, I've been down there. It's an amazing laboratory. It's deep underground in a nickel mine in Sudbury.

Exactly. That's where they did the work on teeny subatomic particles called neutrinos that led to Art McDonald's Nobel Prize back in 2015. the lab has been greatly expanded over the years, and now many of the experiments underway there are focused on detecting dark matter. So even though the lab was originally built to catch neutrinos, Snow Lab turns out to be a great place to hunt for dark matter.

And in part, that's because you've got two kilometers of rock above your head to block out things like cosmic ray muons, which would otherwise make it hard to pick out the dark matter. Okay, so how do they actually detect a particle of dark matter? Well, let me describe one particular experiment at Snow Lab. It's called Super CDMS. You know, physicists love their acronyms. That stands for cryogenic dark matter search.

It looks like a giant metal cylinder. Picture an aluminum thermos scaled up to about the size of an elephant. And inside the cylinder, there are crystals that act as a target for dark matter particles. I had the privilege of visiting Snow Lab last fall, for the second time, actually. and I met with a number of scientists there, including the lab's director of research, Dr. Ray Bunker, here he is explaining why this experiment needs such bone-chilling temperatures. The reason has to be kept so cold is the approach for detecting dark matter here is to use large kilogram-scale germanium and silicon crystals.

Dark matter, we hope, comes along, interacts with an atom in the crystal, vibrates it, And in order to detect the small amount of vibration, you have to reject all the other vibration. And you do that by making them very cold. And so then you have a chance of seeing this very rare interaction of a dark matter particle pinging an atom in your crystal. And if you're wondering how cold it actually is inside the detector, it's about minus 273 degrees Celsius. That's even colder than a Canadian winter. Anyway, have they found anything yet? Well, this experiment is still pretty new.

At the time of my visit, they were just installing the final components, and they're hoping to start collecting data in a few months. Of course, since people have been looking for dark matter for decades, if they did find the dark matter particle, that would be a pretty huge deal. And we're talking about dark matter wimps in particular, right? That's right. But I want to emphasize that wimps are not the only game in town. In fact, although wimps have been the leading contender,

for a couple of decades now, there are other dark matter candidates like axions, for example, that physicists are also excited about. Axions. Okay, what's an axiom? So axions are another hypothetical particle that's also small and ridiculously hard to detect. In fact, scientists think axions are actually smaller and lighter than wimps. Wimpier than wimps. Wimpier than wimps. I'll let Dr. Sean Tulin tell us about axions.

He's a theoretical physicist at York University in Toronto. You've probably heard about this wave particle duality in quantum mechanics. Now, when people talk about wimps, that's a dark matter candidate that behaves like a particle. But axions are on the other extreme. These axions are so light that they behave as waves, not just in your experiment, but on the scale of, potentially on the scale of galaxies. Wow. So waves of dark matter. Would that be like a fog of dark matter? Something like that. When I asked Dr. Tulin for an analogy, he suggested it's like the water vapor in the atmosphere. It's there. It's important. But most of the time, we don't even notice it. Of course, the big difference is that we know water vapor is real, while axioms, for now, are hypothetical.

So are there experiments up and running that are looking for axions? There are. And I should mention that even some of the... the experiments at Snow Lab could conceivably detect axions if the axioms happen to have just the right properties. But the good news is that while we've been looking for Wimps for decades, we've only just begun to hunt for axioms in earnest. Here's Dr. Tulin again. The most exciting thing about axions that puts them in a different category of Wimp is not just their particle versus wave difference, but the fact that unlike Wimps, the most interesting theoretical range of parameter space for axions is only now just getting tested. So there's experiments on the horizon

over the next decade or few decades that are really going to help cover that theoretically well-motivated space for axioms. So the hunt for axioms, I would say, is only beginning, unlike the hunt for wimps, which is drawing towards its natural conclusion. And Bob, I'll just add that I don't want listeners to think that it's wimps or axions because it could be wimps and axions. In fact, there could be an array of different particles that constitute the dark matter. It might even include exotic entities like primordial black holes that may have formed in the early universe and could still be around today. And for that matter, pun intended, since neutrinos have a tiny amount of mass, they could account for some of the dark matter, but according to the experts,

probably just a small fraction of it. So it would be nice if dark matter had a simple explanation, but it might not be simple, and so physicists sometimes talk about a dark sector because it could be an array of different kinds of particles. Okay, so there are a number of particle contenders for what dark matter might be and various searches underway to try to detect them. But we started off by talking about the clues from astronomy when we aim our best telescopes to the sky. So what else are astronomers excited about when it comes to what might be behind dark matter?

Well, one really interesting idea is something called self-interacting dark matter. I'll let Dr. Tulin explain what that is. A lot of my research is focused on another candidate known as self-interacting dark matter. Now, the idea of this is easy to understand. We know that for regular matter, there are four fundamental forces. There's the force of gravity, but there's also three other forces. There's electromagnetism, the strong nuclear force, and the weak nuclear force. nuclear force. Now, as far as we can tell, dark matter for sure experience and exerts the force of

gravity, but it seems not to experience any of the other three forces. Now, the idea of self-interacting dark matter is saying that, well, maybe dark matter is not just gravity only, but it may have other forces that are unique to dark matter that maybe the regular matter doesn't experience. Wow. If we can't directly see ordinary dark matter, how would we see this self-interacting dark matter? So, as usual, astronomers have to observe the galaxies and clusters of galaxies that we can see with our telescopes, and then figure out which theoretical model best explains the observations. This work is ongoing, so we don't yet know which model will best fit what we observe, and whether or not it would include this self-interacting dark matter.

Okay, but if you have dark matter experiencing its own dark forces, it sounds like there could potentially be a lot of new physics to discover. Well, for sure. So while dark matter doesn't interact with ordinary matter via, say, electromagnetism, maybe it interacts with itself by some weird dark analog of electromagnetism. And this raises an even weirder possibility that dark matter might come together via gravity to form dark matter structures that are, in some sense equivalent to the structures in ordinary matter that we see. Here's Dr. Miriam Diamond, an astroparticle physicist at the University of Toronto. So actually there are some recently published papers about the possibility of, let's say, dark matter stars. So an entire conglomeration of dark matter where there is some additional dark force that acts between the dark matter particles and then you have some dark equivalents.

of nuclear fusion happening in that you have particles interacting with each other very strongly under this additional force and actually maybe changing from one particle type to another and maybe giving off dark radiation. And then maybe could you get something analogous to a dark matter planet? It would not be a hard packed solid thing. It would be a much looser agglomeration of matter. And then could you have some sort of life form that exists in sort of a cloud or a star or a conglomeration of dark matter? It would not be life as we typically think of it, but some sort of a coherent energy pattern that maybe has something to do with this dark force or dark radiation. It's not completely impossible. This is amazing. I mean, dark matter

stars, dark matter planets, maybe some kind of dark life. It's all really fascinating, but it also sounds pretty speculative, almost like science fiction. Yeah, I mean, I guess it just depends how far you go. There's a very broad consensus that dark matter is out there, but before we start theorizing about dark matter creatures living on dark matter planets, it would be nice to pin down what dark matter actually is. So are the scientists that you spoke with optimistic about our chances of identifying what dark matter actually is? Well, I've heard a spectrum of answers to that question. Here's Dr. Diamond.

I do have an expectation that the progress will continue to accelerate because everything builds on what we've had previously. And so I am very hopeful that within the next couple of decades, we will have a dark matter detection, and that then we will move into the phase of what I would call more precision physics in terms of not just discovering that this particle exists, but actually looking to probe the properties of the particle. Well, okay, she sounds fairly optimistic.

But what about scientists on the other end of that spectrum? Yeah, well, after spending the better part of a day, two kilometers underground at Snow Lab, here's what Dr. Ray Bunker had to say about the 30 years that he's already devoted to trying to pin this down. I can tell you, when I first started, it felt like a discovery was imminent.

It was right around the corner. We thought it was a wimp, and we were about to detect it. That didn't happen. We didn't detect it with the previous generation of experiments, and so we had to rethink things. The theorists also rethought things, and they thought, okay, maybe it's not the Wimp.

What else could it be? And as they did that, they expanded the hunting ground dramatically. And so now the community had to come up with, well, how are we going to search these other mass ranges? And so I've kind of become, you know, I went from, it's imminent early in my career, to resign that this is a lifelong and potentially, you know, beyond, you know, my time scale, search because there's just so many possibilities.

As a dark matter physicist, I sure hope we detect it, but I am resigned to it, possibly not happening in my lifetime. Boy, you can really hear the emotion in his voice. Yeah, but as physicists like to say, the universe is under no obligation to be easy for us humans to figure out. and these things often take time. I like to point out that the ancient Greeks had the idea of the atom nearly 2,000 years before anyone had any experimental evidence that atoms really existed.

Well, let's hope we don't have to wait 2,000 years to solve the dark matter mystery. Fingers crossed. Thanks a lot, Dan Falk, is a science journalist in Toronto. And now it's time for our Quarks and Quarks listener question. Hello, I'm John Smith, and I'm calling in from Valdeemont, Quebec. My question is, why doesn't flowing water freeze at the same temperature as still water? For the answer, we're going to Dr. Josh Culpepper, who is a postdoctoral researcher in hydrology at York University in Toronto. Hi, welcome to Quarks and Quarks.

Thank you for having me. Now, doesn't all fresh water freeze at the same temperature zero degrees Celsius? is? Yes, it does. What they seem to be getting at is that there's a combination of cold temperatures that are required, and there are also physical dynamics at play when a river is flowing. Okay, well, let's get to the river. I mean, I think we've all seen that even a really cold days, they're still flowing when everything else is flowing around them. So what's going on? Yeah, so what's happening is that the water mass as a whole is constantly flowing, where it can bring up warmer water from underneath and the cold water on top can kind of plunge down. So what happens is as opposed to a lake

where you have a nicely stratified and calm surface where the ice can freeze quite easily. When you have a river system, that system has to get to zero degrees all the way through, which can be quite challenging, especially if there's a lot of water or the water's flowing really rapidly. And you have additional water coming in, warmer water coming in and constantly kind of supplying a source of heat. So the whole river has to cool down to zero degrees before you can start to get that freezing. Oh, I see. So a lake, the cold water's on top, it's warmer down below and they don't mix. Exactly. So, all right, so it can freeze faster. But how does the movement then of the water in a river affect how it eventually does freeze? Well, you'll notice, especially on rivers,

where you might have some slower areas near the shores, that often what happens is the ice forms at the shore first and then kind of moves in. Because that water is moving more slowly, the water is more shallow near the shore, it's easier for that water to freeze. What's happening in the faster part, the center part, is that sometimes that flow can interrupt the ice crystals

from forming like a solid lattice like you would on a lake where it's very calm. So it can take really cold temperatures to actually freeze a fast-flowing river. And can they freeze completely? Rivers can freeze completely. I mean, it sort of depends a little bit on the type of river. A very shallow creek or river could freeze completely. It probably freeze also before a deeper system, though a very deep system,

probably it won't freeze all the way through unless it's incredibly cold. Well, I'm thinking about something really big like Niagara Falls. Can it freeze over? You'll get little bits of freezing here and there, like from the mist that sprayed onto the rocks and onto the areas where you might look onto the whole river. But that whole river is not going to freeze over completely. Why not? It's not cold enough. There's too much water.

Oh, too much of the warm water still mixing around in it. Yeah, it would take an incredible amount of heat loss to freeze that over. What about smaller waterfalls? Yeah, smaller waterfalls can freeze over. You'll see that, you know, on like smaller hikes, you might see like a smaller waterfall that'll freeze over. Often what happens is while that water is moving over the cliff face or hill that it's causing the waterfall, that spray of mist will start to freeze along the side where the main waterfall is. And it can, of course, build up and build up over the course of the winter.

And smaller waterfalls can freeze over completely. Again, it just takes a sufficient amount of heat loss. Well, are there any other conditions that impact how long it takes for flowing water? to freeze? You know, primarily the big governing factor is how cold it is. That's really what's going to determine when and how things freeze. If it's very, very cold, all the heat from that water can go into the atmosphere and it can start to freeze. Otherwise, the rate of flow can also impact when that river starts to freeze over. But the water is still freezing at zero degrees. It just might take longer if it's turbulent. Exactly. Yep, still freezing at zero degrees. Dr. Culpepper, thank you so

much. Thank you. Dr. Josh Culpepper is a postdoctoral researcher in hydrology at York University in Toronto. And that's it for Quirks and Quarks this week. If you'd like to get in touch with us, our email is Quirx at cbc.ca.ca. You can find our webpage at cbc.ca.ca slash quirks, where you can read my latest blog or listen to our audio archives. You can also follow our podcast, get us on SiriusXM, or download the CBC Listen app. It's free from the App Store or Google Play. Quarks and Quarks is produced by Rosie Fernandez, Amanda Bukowitz, Livia Diring, and Dan Falk.

Our acting senior producer is Sonia Biting. I'm Bob McDonald. Thanks for listening. For more CBC podcasts, go to cbc.ca.ca slash podcasts.