Are Magnets The Most Familiar Mystery On Earth?

0:00This episode is brought to you by Cancer Research UK. Dinosaurs walked the earth 180 million years ago. But, you know, cancer was part of their story too. Scientists have found tumors in ancient fossils. Well, that is part of the reason why cancer is a big, big part of our story, right? It's the other side of evolution. It's the most complex disease that we face. There are more than 200 types of cancer in total, each with distinct characteristics, challenges and mysteries. And that complexity demands scale. Cancer Research UK is the world's largest charitable funder of cancer research, with more than 4 ,000 scientists, doctors and nurses working across more than 20 countries in the search for answers, and then sharing their discoveries beyond borders.

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1:21Welcome to the Rest is Science. I am Hannah Fry. Hi. I'm a mathematician. If you cut me open, I would bleed equations. But I have been making science programs for about a decade or so. And I'm Michael Stevens. I am a barbecue pitmaster in my personal life. But you may know me from my YouTube channel, Vsauce, where I make science content, math content, psychology content, whatever I'm interested in. And today I'm interested in magnets. Why do we live on a magnet? Can animals feel magnetism? And I'm believe that humans can feel magnetism emotionally. But first of all, Hannah, how do magnets work?

2:03I mean, I can give you a science-y sounding answer about electrons and about spin and about spin cancelling each other out and the properties of particular atoms. But if you really want to know how they work, I mean, at some level, it's going to involve of acceptance, Michael. It does. At some level, we have to just say magnets work because we find ourselves in a universe where they do. It is a fundamental ingredient in our universe, just like gravity. The thing is, people don't question gravity in the same way because we experience it so often. It's normal that things fall. It's a little bit like saying, well, how did your aunt end up in the hospital?

2:44Oh, well, because she fell over on some ice. And it's like, okay, but why? Yeah. Feynman draws this great comparison to asking how magnets work and why they work to, well, what level of explanation do you want? She's in the hospital because she slipped on the ice. Why did she slip on the ice? Well, because ice is slippery. Yeah, but why is ice slippery? Well, because water expands when it freezes. And so a pressure that compresses it causes it to melt a little bit. So you're actually standing on a liquid layer on ice. Well, why does water expand when it freezes? Well, the molecular properties of water are quite unique.

3:21Hydrogen bonds. And this goes on and on. But why did she slip? Well, because she's old. Well, why does that affect it? Well, because your balance and your muscle control changes. And psychologically, maybe she was distracted. And suddenly you realize there is no end. Same with magnetism. Same with gravity. Same with everything. OK, the insane clown posse asked just a few months ago, Donald Trump, the president of the United States, said no one knows what a magnet is. And they got ridiculed for statements like that. I don't know who Insane Cloud Posse are. Are you not a Juggalette? Do I look like a Juggalette?

3:54Well, no, but I thought for the show you wash off the makeup. Is this not Faygo? I really am that boring, Michael. My goodness. All right, look. So years ago in a song called Miracles, they rapped about magnets. Yeah. How do they work? And a lot of people ridiculed them for that statement. Because this is the thing, right? You can answer it at different levels. You can start off and you can say you take a particular type of metal, something like iron, and you interact it with an existing magnetic field. There is a force of attraction or repulsion that appears. And then you could go a bit deeper and you could say there are field lines that appear around a magnetized object, which you can see with iron filings.

4:35And you go a bit deeper still and you could say down at the level of electrons, it's to do with the spin cancelling out or not in some materials. You kind of keep going and keep going and keep going. but ultimately even once you're taking quantum effects into account there there is always a level at which you just have to sort of say because it is yes there's a level where you have to say look ultimately it is an intrinsic property of our universe in the same way that there is gravity why well i don't know if there wasn't maybe we wouldn't have evolved to ask but that's a just so story even the the magnetic field lines you see when you sprinkle iron filings on a magnet, even those aren't showing you the field as it was before you sprinkled it.

5:22The iron affects that field. So you're seeing the iron's effect on the magnet. But what did it look like before? It just was. And so I think that, yeah, if you want an answer when people start making fun of those who are curious about magnets, you're right. It's ultimately the existence of charge, like the electromagnetic force as a thing in our universe is a property stuff has. And we know that when charges move, they create a magnetic field. They just do, okay? But here's the thing. Electrons are charged and they move in a sense around a nucleus, but they don't spin. And yet we say they have spin.

6:07Okay, we're getting really into the weeds here because there is no quick answer. but if we accept that electrons have a charge and that they move in certain ways, some of which don't involve movement at all, spin, I'm looking at you, they create a magnetic field and those magnetic fields tend to be not lined up in most things but in certain materials they can line up and have a macroscopic manifestation and that's what a magnet exhibits. In iron, in nickel, I mean very few things that actually have magnetic properties. That's right. And we just so happen to be living on a planet full of the stuff.

6:45But how did the Earth get magnetized? I remember being a kid doing the magnet thing in like fourth grade and you would take a paperclip and swipe it over a magnet over and over again. And now the paperclip is a magnet. Did someone do that to Earth? Well, the Earth is sort of doing it to itself inside. inside and within within the earth's uh you know if you're going to go down into the center of the air you have this this solid iron core and then you have this uh this sort of magma right this sort of swirling uh metallic substance and because the earth is spinning um but also because it's very very hot in the core and cooler at the outside you have these currents so this this magnetized metallic liquid is continually moving around that's one of the things that supports and continues, the fact that the Earth has this magnet within it.

7:35So what we don't know is where it started. We know, or at least there's the theory, the geodynamo theory that says that moving conductive liquids can maintain and even grow a magnetic field. Do they produce one by themselves? I don't think so. But if there's another magnetic field around, like, say, from the early sun, the sun may have been much more magnetic when it was young. And the Earth could have been in that stronger magnetic field. It's sloshing around this molten iron and that magnetic field gets kind of ripped and stretched and it can persist because of this rolling motion. And it still persists today.

8:17Because I think this is it, right? OK, we started off by joking about how no one knows how magnets work. But why is the Earth itself a magnet? I mean, this is a very strange thing, if you think about it, that North and South, magnetic North, magnetic South, which, by the way, ever so slightly different from geographical North and South, that they are there and can be sensed by aligning magnets floating them on water. Like, why is that? Where did the Earth's magnetic field come from? Yeah, we don't totally know. It's not like the earth has a giant horseshoe shaped Wile E. Coyote magnet inside of it.

8:55What the earth does have inside of it, though, is a lot of metal. It's got a solid metallic core because pressure is so high. A little further out, it's got molten metal, mainly iron. There's some nickel. There's some sulfur. There's other stuff there, but most of it's iron. And iron has a property where its atoms can get lined up. But the iron in the outer core is too hot to be a magnet. OK, like a magnet couldn't even survive down there, and yet it's magnetic. So there's this whole theory called geodynamo theory, which says somehow a spinning planet that's cooling, that has molten ferromagnetic metals in it, will have swirls of this fluid because of its own rotation.

9:35and a moving electrically conductive fluid can grab onto a magnetic field and hold it so that it persists and can even grow. And so at some point in Earth's history, it grabbed onto a magnetic field, maybe from our young son, which may have been more magnetic in the past. And it's just been holding on to that. I mean, it's not like there's a battery down there that's sending current through wires. It's just that deep inside the planet, there's molten metal that's moving. And somehow that motion amplifies or holds on to magnetic fields, like a dynamo that keeps this magnetic field alive. So the solar wind can stretch it and shape it, but the source is within.

10:19We're carrying it with us. And that means the entire Earth is just this big magnet covered in life. Because the fact that the Earth is spinning is key here, right? And the fact that the Earth is cooling down is also key. because you have this temperature difference between that really, really hot center, the central core, and then it's much cooler on the outside as you get towards the Earth's crust. That temperature difference, it's sort of like imagine a pot of water. As it's boiling, you have this circulation current that happen from the very hot bottom to the much cooler top, which moves the water around in the same way that it's moving all the liquid inside the Earth around.

10:54But then also the spin on the Earth is helping as well. You have this Coriolis effect. So you get all of these little eddies, these little currents, these little swirls inside of the Earth. And that is what is perpetuating this magnetism that somehow or other initially arrived on Earth that the Earth has grabbed onto and kept going for all of these billions of years. You know what's exciting is that just recently we managed to create a geodynamo in a lab. Some researchers took a two cubic meter vessel of molten sodium, which is really electrically conductive, and they spun it up like in a giant blender so that parts of it were moving at like 15 meters a second.

11:39And sure enough, it like took the existing magnetic fields in the room, one of which was from Earth, and held on to it and magnified it. and even after they stopped spinning it, it maintained that magnetic field. That's probably what Earth does. But this is what's so strange because it's like, okay, well, where did that original magnetic field come from? Because it's not there anymore. I mean, you used to get a little bit of magnetism from the sun, especially when you have solar flares and so on. But it's not enough to have – there must have been some seed somewhere at some point that originally – this kind of intergalactic bar magnet.

12:19that sort of floated past our planet that gave it the original magnetism. I know. Some aliens came by on their bar magnet ship four and a half billion years ago, and they were like, oh, crap, we just made a mess of that planet. They tried to clean it up, and they were like, it's not going away. We'll just get out of here and don't tell anyone. But thank goodness our planet is a magnet because that magnetic field interacts with charged particles that fly at us from the sun and protect us. They redirect those particles so that our atmosphere isn't just obliterated by them. This is where you get the northern lights, right?

12:56Exactly. But it's where the Earth's magnetic field is concentrated, sort of exiting from the north and south. And then that is where you see these northern lights as interacting with the sun's magnetic field. Yeah, the charged particles get funneled into the atmosphere there. They create these beautiful lights, but that also protects our atmosphere as a whole from just being peeled away by this powerful radiation. Mars, not a magnet. And guess what? Tell me about its atmosphere. Does it protect the Martian surface from ultraviolet light? Absolutely not. Think again. Serious sunburn on the surface of Mars.

13:30Serious sunburn. They call it the red planet for a reason. That's right. Well, one other interesting thing about the fact that it is this geodynamo, right? the fact that it's the swirl of the inside of the earth that is creating this magnet or that is allowing this magnet to exist, means that it's nowhere near as stable as you think. And the really nice thing is you can see in rock, especially when you have volcanoes, for example, as lava ends up solidifying, if there's iron inside of that lava, the iron will line up with the earth's magnetic field, then cool and then get frozen in place. which means that you can go back really, really ancient lava flows and you can see the direction that the Earth's magnetic field was pointing in.

14:15And it's not always that north is north and south is south. This thing has flipped. It's flipped many, many, many, many, many times over the history of the Earth. Do you think it's bad when it flips? It would be pretty confusing, wouldn't it, if you were out on a sailboat? That would be a hilarious public announcement. Hey, everyone, compasses still work, but opposite the way they say they do. Exactly. Like the end that's painted red is now going to point south for maybe only 12 years, maybe for another few million. We think Earth's magnetic poles flip on average about every 450 ,000 years. But we're in a long stretch of stability.

14:54Yeah, it's been about 800 ,000 years since they flipped. So some people say we're due for a flip and it's going to be catastrophic. But it probably takes about a thousand years for the flip to actually happen. Oh, it's not. It's not like an instant. It's not like you wake up one day and north is south and south is north. Oh, bummer. That's not how it works. I think that sometimes these reversals might be just another consequence of something else terrible that happened. Because we can find that extinction events and pole reversals might line up. It could be that an impact creates a lot of volcanoes and change in Earth's interior that can lead to a switch.

15:34But really, the animals all died, not because of the magnetic switch, but because of the ash in the air, because of the impact. Also, Earth's magnetic north pole is actually Earth's south magnetic pole. Oh, really? Is it upside down? Yeah. Oh! It's upside down because we made up a rule of how to name the poles of a magnet. And we use the right-hand rule. Sure. If electrical current flows this way from my palm to my fingertips using my right hand, my thumb points in the north direction. That's up. Yeah, so we make magnets because we had to arbitrarily decide which one's north, which one's south.

16:08And this is the field that you can see if you put in iron filings. Exactly. Which way round is it wrapping? That's right. And so, yeah, you turn some current on through a coil, it goes this way, that way's north. Oh, shoot. When you make a compass using this mechanism, the north points to the north. But they should repel since they're the same, which means that actually as a magnet, Earth's north pole is its south pole. tell you what i don't understand i've actually until literally this conversation i have never thought to be curious about it when it flips right on the occasions when it flips it does it is that north and south swap it's not like suddenly north is at the equator and and presumably that must be because of the spin that must be well it can it can there's a thing called an excursion where the north pole doesn't just like flip but it like goes to the equator and back it wobbles it can yeah it can wobble quickly but it's stable along that axis right of like the it's most stable that way yes because of the earth spin because of the earth but why doesn't it exactly align with the with the geographic north because because geographic north south is is if you put a skewer through the earth on the axis that it spins around that would be the geographic north south and then then the magnetic north south is ever so slightly off yeah why i I don't know.

17:26I don't know either. They're different by about 11 degrees. And the magnetic pole keeps moving. It's moving from Canada to Siberia right now. It's actually quite close to the geographic North Pole, the actual pole that the Earth spins around. But it used to be much further south down into Canada. It moves at about 40 kilometers a year, about 25 miles a year at tops from what we've observed. I've always wanted to go there and watch a compass needle stop pointing in some direction, like on the surface, but just point down. Like that right there. That's it. Here you go. OK, because you basically have this kind of boiling blob of liquid metal that is wobbling around all over the place.

18:16And it just it just wobbles roughly in the same place as this kind of skewer through the earth and the way that it spins. I don't know what causes it to drift around and not keep lining up with our rotation axis. But there are a lot of factors that affect Earth's magnetic field. Even the tides and the motion of the ocean changes it because saltwater is electrically conductive. And so it's a mess. If you want to be really precise by using a compass, you're going to notice that it's just not – it's all over the place. Also, the Earth's spin is not absolutely consistent. No, it isn't. It isn't. But why 11 degrees?

18:58But then I think maybe we're just thinking about it as though at this moment in time, it's 11 degrees. But we shouldn't think of it as static in time. It's something that's right. You sped it up and watched it over the last four billion years. It's kind of all over the shop. Yes. And it and reversals seem very random. They don't seem to correlate with any known other process. Okay, I'll tell you what else would be different. If it did flip, not only would sailors end up going in the wrong direction, but there is one paper that came out about 10 years ago that talked about dogs pooping. Have you heard about this?

19:34I have. Is it true? I don't know. First of all, tell our listeners what you're talking about. All right, so this is a paper. It was called Dogs are Sensitive to Small Variations of the Earth's Magnetic Field, published in 2013. Peer reviews, right? Sure, yeah. Frontiers in Zoology was the journal. And I mean, credible researchers over a two-year period, they followed 70 dogs from 37 different breeds. They recorded the directions of the dog's spines during their defecations and their urinations. And they demonstrated statistically that they appeared to align with the Earth's magnetic field direction.

20:12When they pooped. Yeah. One thing I will say is that there have been replication studies on this. People have tried to repeat the experiment and have not found a statistical correlation. I'm not surprised. I believe a dog could maybe sense Earth's magnetic field. However, why they would want to poop facing north, I don't know. I can't think of a helpful evolutionary reason. No, I agree with you. I agree with you. But that's interesting, though, that you wouldn't be surprised if they could sense it. Because, I mean, you and me sitting here, how's your sensation on the magnetic field of the earth?

20:46I can't feel it. No, me neither. I'm not aware of it. It's happening. It's going through my body. I am in it, and yet I don't feel it. It seems as though other creatures can, and that is what we're going to be talking about once we come back after this break. If you happen to be listening to this podcast while out on a dog walk, just whip out a compass and take note of which direction. Maybe we should turn this into a citizen science experiment. I love that. An experiment. Take photos of your dog pooping. send them directly to Hannah, right? Yeah. We need close-ups, guys. It's for science. My email address is vsource at therestofscience.com.

21:35Welcome back. Good to see you. Hannah, we were talking about dogs. We were talking about defecation. Let's change the subject. But not totally. I want to keep talking about animals because there's evidence that many animals, many living things on Earth can actually sense and feel Earth's magnetic field. Of course there is. I mean, if you take migratory birds, for instance, how on Earth, this has been a question, a longstanding question for hundreds and hundreds of years, how do robins, for example, know where they're going? How do they move around to different climates, fly south for the winter?

22:08and there was always this question of how on earth do they know where they're going and it looks like and looks like actually is the operative word here it looks like they are seeing the magnetic field of the earth it looks like they're seeing it looks like they are seeing it tell me more okay so back in the 1960s they had some experiments where they put robins in in special cages which had sort of like a type of paper on the outside so if the bird wanted to fly its wings would just hit the paper leave ink on the paper and you could tell which direction they were aiming for right okay now these particular cages were also sort of cone shaped so the bird was sort of sitting in the middle and as it would fly up you could tell um and then what they started doing was they started messing with the magnetic field around this particular cage measure with the intensity of it, measure with the direction of it, measure with the inclination of it, which direction the field lines are going in.

23:05And they demonstrated that the birds would change the direction they wanted to fly in based on what was going on with the magnetic field. So this is like back in the 1960s and people are like, okay, cool, right? So robins are using the magnetic field of the earth or magnets somehow. They have some form of magnetoception. Exactly. Exactly. Since not just light and sound and touch, but magnetism. I mean, honestly, magnetoception makes it sound like it deserves its own superhero series. Because we use it to describe superheroes, because it's just unbelievable that you could sense this thing that we did not evolve to sense.

23:43Totally. But here is where it gets even wilder, because it looks like the sensing is all going on inside the eye of the robin. So there's a very particular protein that appears in a robin's eye. It's called a cryptochrome. And when blue light hits this protein, it forms this pair of electrons whose spin becomes quantum entangled. OK, so essentially what happens to one happens to the other. I mean, that's the most ridiculously oversimplified explanation of quantum ever. But just go with me. Yeah, I'm going. OK, so here's the thing. The Earth's magnetic field, which is incredibly weak when you're talking at this scale, But it's enough to really subtly change the probability that that pair of electrons will either recombine or separate out.

24:31So, I mean, effectively what's happening is it has this protein which is so sensitive to magnetic fields that where the magnetic field is present or stronger or which direction it's going in, it changes the chemistry in the retina, essentially. And that pattern, that chemical pattern, is interpreted by the robin's brain. So what we think it's actually seeing is this sort of faint ghostly pattern that aligns with magnetic north. So it's not like a metal detector. It's not got like a compass inside it. we think that it sees the magnetic field of the earth as a kind of directional glow within its field of vision wow isn't that amazing so it's probably a visual phenomenon probably a visual either things are glowing a bit more north or south it's not like its head sort of like to a magnet it's not like the eye is being like pulled a little bit all the time so north and they're going, oh, once it gets cold, I need to start following that pull.

25:39But maybe it's when it gets cold, I need to follow the glow. And I want to say brightness or color because it could be a third thing. Absolutely. That's also visual that we've never seen. Wow. Isn't that great? And here's the other thing about it. I mean, well, OK, there's many, many great things about this. For starters, that the idea of kind of quantum chemistry wizardry, like normally it doesn't really work when you interact it with sort of squishy bits of flesh you know like biology and quantum is is it's very very difficult to get these effects to work and this is something that's happening at you know bird's eye temperature you know within a biological being which is really extraordinary a being that literally has a bird brain literally literally has a bird brain it brings a whole new meaning to bird's eye view too doesn't it because suddenly birds i this bird particular bird's eye has got a quite interesting view um we also now know because of the experiments that they've done that it's not the intensity of the magnetic field that it is that it's um sort of seeing or being drawn to but the thing that that that makes it know where it is north or south is almost certainly the inclination of the magnetic field let me explain that ever so So if you imagine the Earth as though it's like a bar magnet with iron filings around it, those iron filings would go in this sort of a direction.

27:05So they kind of come out, they spurt out from the top where the North Pole is. From now on, I'm going to use inverted commas. Sure, fine. They spurt out from the North and then they go around the Earth and then re-enter. In like a hoop, like ears on the Earth. Exactly. And so if you think about it, at the equator, those lines are effectively going parallel to the line of the Earth itself. Whereas when you're up at the pole, they're coming straight out. They're coming perpendicularly out of the ground. And so you will be able to tell how far north or south you are by looking at the angle that those field lines make with the ground that you see beneath you.

27:46so what we think that they are doing is that is what is they are sensing the inclination right of those field lines um and the reason why we know this is because uh once again you can put this put the robin very safely and you know ethically in this particular cone-shaped cage with the with the paper around the outside and you can rotate the inclination angle using something called a Helmholtz coil, essentially a big electromagnetic device. Let's say the bird wants to be in South London, okay, this particular time of year, that's where it wants to be. If you change the angle of inclination of the magnetic field so that suddenly it believes that it is in Scotland, for instance, then the bird, the robin, will demonstrate something called Züggenruh, which is German for migration anxiety.

28:41Which is like, okay, get me the hell out of here. I do not want to be here. So you can trick the bird into believing that it is at the wrong spot on the earth, which is something that it is seeing with its eyes. Sees with its eyes. Isn't that amazing? We've got to teach birds to paint. And get them to go, oh yeah, you know, here's what an apple looks like. But then they draw all of these lines on it and we're like, what's that? And they're like, well, that's the magnetic inclination. You do not see that? Come on, you dumbass. Yeah, right? How do we do that? Okay. Isn't that good? That's really cool.