Drawing of a magnetic field by French philosopher René Descartes, from his Principia Philosophiae, 1644.
What’s the connection between motion and magnetism? How can something like a black hole have a magnetic field? What’s the connection to quantum mechanics? How did Einstein use magnetism to develop special relativity? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
You may have noticed in this series that I talk about magnetic fields well, let's just say a lot. I talked about how planets get magnetic fields. I've talked about how galaxies get magnetic fields. I talked about how mag fields cause sunspots and aurora. And how could we possibly forget the wacky adventures of the magnetar?
But let's have a moment of honesty. Magnetic fields, as as painful as it is for me to say this, magnetic fields just aren't important in the universe. I know. I know. I know.
I'm I'm personally a big fan, but they're just they're just not there. They're just not there, man. On the very largest scales, like properly cosmological, the universe is neutral, and so magnetic fields just don't exist above a certain scale. And on small scales, they're almost always relegated to the background. They're almost always a supporting character.
They're there, but other physics are more important. They're never the star of the show. Oh, every once in a while, they'll get a big break, and we have to pay attention to them for a brief moment, But that fame is short lived and they don't get called back into the silver screen. It's back to just basic cable or or heaven forbid doing a podcast. But on the other hand, we we live with magnetic fields.
We rely on the Earth's magnetic field to tell us where to go when we're lost, and we rely on little handheld ones to stick things to our fridge, I guess. Okay. So maybe they're not even really important on Earth. I suppose there are a lot of important industrial uses for magnets out there too. But, I mean, come on.
If we were to take away all the magnetic fields in the universe, would we even notice? Would we even care? I would care. But that's just because I have a personal attachment to them, not because of their physical significance. So I think it's time.
I think it's time to put magnetic fields in the spotlight. Give them a moment to shine an episode dedicated just to them and them alone, not as in their connection to other physical processes, but in their own right. And then we can go back to ignoring them. Besides Magnetic fields might have some surprising hidden talents But first we got to define one a magnetic field is like any other field in physics. It's defined by how it makes you feel.
Do you feel sad or happy or grumpy or hungry or hangry? It's a field moves you. A force moves you. If you're in the vicinity of something that's generating a force and you respond to that force, you will start moving. And if you move around and map out the forces on you, like, okay, over here, I feel nudged a little bit to the left.
Okay. I'll I'll write that down. Now, over here, I feel, woah, I feel really big push, from the top. Wow. That's intense.
I'll I'll note that down. If you map that out in space, you are creating the field. You are mapping out the field. The field tells you how you feel in response to a force. So if something generates a magnetic force and you respond to the magnetic force, then mapping out that force in space gives you the field.
And magnetic fields make you feel tilted. Magnetic fields have the strange property that they never quite take you head on. They always move you sideways and they only ever affect you if you move. If you stand perfectly still, hold your breath, magnetic fields won't touch you. They'll ignore you.
They can't get you. But as soon as you start moving, bam. Well, given their typical strengths that we usually encounter, it's more like a a meh, not a bam, it's a nudge. But this nudge is always off to the side. A magnetic field's force is always perpendicular to the direction you're moving.
It always comes at you from the side. And so the motion in a magnetic field is all about curves. It's all about twisting. And this is why the magnetic field, if you ever if you just type in a search, you know, magnetic field, the pictures you come up aren't a bunch of arrows pointing around that you might see if you look for, say, an electric field or gravitational field. If you look up those kinds of fields, it'll be a whole bunch of arrows.
A field of arrows telling you where the force will push you. But because the magnetic field is always twisting, always curving, always curling you, it's easier to describe the magnetic field with lines, long threaded continuous lines. And, if you look at that magnetic field map, the lines tell you how they're gonna nudge you and how they're gonna twist you. And, the amount of lines tells you how strong that magnetic force is going to be. Where there's a lot of lines bunched together, that's a very strong magnetic force.
And where they're all spread out, that's a very weak magnetic force. This visualization is incredibly helpful because magnetic fields can be bent, they can be twisted, they can go into complicated shapes, they can fold over onto each other. In this language of lines that we use to describe magnetic fields, that is not an accident. That is not a quirk of history. That is a distinct purpose because the way magnetic fields behave is much easier to express through lines than a whole bunch of arrows.
So that's how you feel around a magnetic field. That's how it moves you and how we can depict it moving. But how do you make a field in the first place? How do you generate that magnetic force? Originally, we thought that magnetic fields were produced by magnetic charges.
You know, like an electron or an electric charge makes an electric field. Maybe there are magnetic charges that make magnetic fields, and let's just call them, say, North and South, you know, just for the sake of convenience. Instead of just having positive and negative electric charge, we're gonna have a North and South magnetic charge. And let's not really talk about the fact that you can never pull a magnet apart, and if you take a magnet and split it, and by the way, a magnet is now what we're calling the thing that makes magnetic fields. If you take a magnet and split it in half, you don't get an isolated north and south charge.
You always just get a teensy tiny magnets that individually have smaller north and south charges that act like a bigger like, let's just not talk about that. Wait. You know what? Let's talk about that. Let's talk about exactly that.
Because that seems like kind of a big deal. This is not how electric charges behave. This is not how gravitational charges behave. This is something different. What is going on inside a magnet?
And to understand what's happening inside of a magnet, we have to understand what's happening inside of a magnet. Very, very zen statement there. But but let's dig in. We need to understand what makes the magnetic field. What makes the magnetic field in a magnet, a magnet that you can hold in your hands, is the exact same thing that makes a magnetic field anywhere else, like in the sun or the core of the Earth or a magnetar.
What makes a magnetic field is moving charges. A charge sitting by itself has an electric field. If you're another charge, you'll either be pulled towards it or pushed away from it. Pretty straightforward, right? But if you take that exact same charge, don't change anything about it, but just flick it, kick it, give it a little nudge, get it moving, all of a sudden you have magnetic fields.
Charges in motion generate magnetic fields. And the field that it generates is never in the same direction of motion. The field that a moving charge makes is always perpendicular to the motion of the charge itself, and the way the force impacts you to what you feel is always perpendicular to your motion. So there's this intimate connection between magnetic fields and motion. In order to make a magnetic force or magnetic field, you need motion.
And in order to feel a magnetic field, in order to feel a magnetic force, you also need motion. So you can imagine a bunch of charges running down a straight line. The magnetic field they generate is circular. It wraps around it almost like a helix. You can imagine that that's this circular twisting tube of magnetic fields of magnetic force around this line of charges.
And if you were to take that straight wire with these loops of magnetic field around it and twist the wire itself to make a hoop or a circle, then all those spirals, all those hoops of magnetic fields will connect together, and you'll get the familiar pattern of a North South magnet. Now, isn't that interesting? If you compare the magnetic field of a bar magnet, you get a certain picture. And if you look at the magnetic field of a hoop of wire with charges running around in a circle, you get a very similar pattern. I wonder what's going on.
Perhaps. Perhaps. Run just just go with me here. Maybe the magnetic field of a bar magnet isn't made by moving charges, but a bar magnet isn't exactly moving, is it? Unless you, you know, throw it, but if you just put it there on the table and don't touch it, it's not moving.
So how can something that's not moving have moving charges? Bar magnets don't move, but it's made of atoms. And atoms have electrons, and the electrons have spin. You know that wonderfully fundamental quantum property that we dug into in glorious and delicious detail a few episodes back. Spin is something fundamental to, well, fundamental particles.
If you have a tiny particle like an electron or proton or quark or whatever, it has this strange property known as spin. It has mass. It has charge. It has spin. So just like an electron has this fundamental property of charge that it can use to generate an electric field, and it has this fundamental property called mass that it can use to generate a gravitational field.
It has this fundamental property called spin that it can use to generate a magnetic field. Now it's like we explored in the episode about spin. It's very, very tempting to think of an electron as a tiny little metal ball that's spinning really fast, and this is a metallic charged thing that is spinning and that will generate an electric field. It'll generate a magnetic field because of its motion. Even though it's really tempting to think of it that way, that's not how they actually are.
It's it's just a fundamental number that we must associate with electrons. The electrons actually have no spatial extent. They're not balls spinning around. They're just electrons. Quit trying to describe them using terms because they are not classical objects.
They are quantum objects. Sorry. Brant over. The end result is that electrons have have a magnetic field, and their magnetic field looks a lot like the magnetic field of a small spinning metal ball. And the electrons aren't really orbiting around a nucleus in an atom.
You know, it's very tempting to have that very classical picture of these electrons spinning around and orbiting their nucleus like the planets spin and orbit around the sun. It's very tempting to have that picture. That's not an appropriate picture. And I don't know how. Someone please explain to me how this outdated image of an atom with a nucleus with electrons spinning around became the universal symbol for all of physics and sometimes all of science.
Even though it is a hundred years out of date, I just don't, Someone explain that to me, please. But even though we it's improper to think of electrons as spinning metal balls orbiting around a nucleus, the magnetic field they generate is exactly as if they were. The electron has an electric charge and it's moving and makes a magnetic field. Each electron, each atom, each molecule has a teensy tiny magnetic field associated with it. But in most materials, if you just pick up a random object like a banana or a pillow or just whatever, the electrons will pair up together where there'll be one electron spinning one direction and another electron spinning the opposite direction, and their magnetic fields cancel each other out.
So on the outside, the macroscopic view, we see no big magnetic field because all the teensy tiny magnetic fields have canceled out. But if there are unpaired electrons, if there are bachelor electrons or bachelorette electrons floating around in that material that don't have a pair. They have their own teensy tiny magnetic field, but they'll tend to have all sorts of random orientations, and they can't get their act together. They can't coordinate. And so once again, there's no magnetic field on the outside, not because they're paired up, but because you have trillions, gajillions of electrons all jiggling around all with random orientations and directions, and there's nothing nothing produced out of that mess.
But in some materials, in some circumstances, you can have extra electrons floating around and they all line up. This is a permanent magnet. Because magnetic fields can add and subtract from each other, we can take a bunch of teensy tiny magnetic fields from each individual microscopic subatomic electron and add them together to make a magnetic field that you can feel. The electrons and atoms and molecules in something like a permanent magnet create magnetic fields because they're just moving charges. And because they're moving charges, they can also be influenced by magnetic fields.
They can feel the magnetic force, and this is exactly how a fridge magnet works. A fridge magnet is made of a material that's chosen. So it's a strong magnet, which means there's a bunch of extra electrons and all the electrons line up, but they're arranged in alternating strips so that the magnetic fields from each strip reinforce each other on one side, the the side you're gonna put on the fridge, and they cancel out on the other, the the souvenir picture side. And your fridge is also made of metal, usually stainless steel, and it has a bunch of electrons, and it would love to be a magnet, but it can't get its act together because all of its electrons are doing all sorts of random uncoordinated things. But when the permanent magnet comes close, the magnetic field that it has is like a strict Catholic school teacher nun, And with a few detentions and some slaps on the wrist with a ruler, it gets those electrons in line on the fridge and a bond is formed.
Isn't this somewhat interesting? Contrast that with electric fields, which are comparatively just just boring. I mean, yeah, electric fields can shock you, which I guess is pretty cool, but do they levitate objects off the ground? Yes. Well, well, can they do it sideways?
Yes. Okay. Just never mind. Alright. Magnetic fields are cool.
But if magnetic fields always come from moving charges, that is the only way in our universe to make a magnetic field. How could something like a black hole have a magnetic field? Because black holes can have magnetic fields. How? If the most important thing to know about black holes is that nothing gets out, not even light, How could it be possibly possible for a black hole to generate a magnetic field, which is obviously pointing out of the surface of the black hole?
But there's you can't get out, so so what's going on? The magnetic field of a black hole is not caused by the black hole. Just like the gravity of the black hole is not caused by the black hole. The magnetic field, the charge, the gravitational field, the whole deal of a black hole is caused by the remnant of the dead star that formed the black hole. You remember I did this episode very very early on on what it would be like to fall into a black hole?
And if I were to watch you fall into a black hole, and yes, you're gonna be the guinea pig in this experiment, As you fall into the black hole from my perspective, you appear to get stuck on the surface. You become slower and slower and slower and more and more red shifted. Where I never quite see you cross the line into the black hole itself. You never quite fall into that event horizon, the point of no return. You just get slower and slower and slower, infinitely slow, but you never make the crossing from my perspective.
From your perspective, oh, from your perspective, oh, you're dead. But from my perspective, you get stuck on that surface. This happens in the formation of the black hole itself. As that star, as a massive star, is dying, as it's collapsing, the image of it, the picture of it, the surface of that star gets stuck to the event horizon from our perspective. The actual star does fall in.
The black hole is formed. But from our perspective, we just get this stuff glued to the surface to the event horizon. And so all the gravitational effects, the charge effects, the magnetic field effects that we might observe of the black hole are not coming from inside the black hole. They're coming from the surface of the black hole, and they are leftover remnants of all the stuff that fell into the black hole, including the original star itself. So if you're near a black hole, which I strongly recommend you never do this, but if you were to be near a black hole, you would experience that magnetic field not from the black hole itself, but from the ghost of the dead star.
And that dead star had moving charges, was perfectly capable of generating a magnetic field, and that magnetic field will persist. I've leaned really hard in this episode on the fact that the only way to make magnetic fields is through moving charges. This is how we understand permanent magnets. This is how we understand the fossil magnetic field generated around a black hole. But is that true?
I mean, this is the game of science as you question everything, even your assumptions. Way back in the day, we assumed that magnetic fields were caused by little tiny magnetic charges, a little north, a little south here, but then it turns out it could all be explained by moving electric charges. In fact, every magnet that we can see or generate or hypothesize about is caused by tiny little charges wiggling around, and there's no such thing as an isolated north or south pole. This is what we call the monopoles. But there doesn't have to be.
There's no law of the universe that says, Thou shalt not have monopoles. We just don't see them. And so because we never see them, we don't ever put them into the equations, especially Maxwell's equations that unified electricity and magnetism. He just looked around, didn't see a monopole, so he said, okay, line number two, there aren't any monopoles. But why not?
Why not? Why does our universe not have monopoles? Well, you remember our best friend, Paul Adrian Maurice Dirac? You know, one of the founders of quantum mechanics played a big role in this whole spin thing. Well, he was puntsing around with math because that's basically all he did for his entire life, And he found something surprising even to him.
He found that he can support this show through Patreon. Yes. Even a hundred years ago, he could've gone to patreon.com/pmsutter and kept this show going. All my education outreach activities are supported through your contributions and Paul Adrian Maurice Dirac's contributions if he were alive. What he really saw was that if you were to just pretend, like, just let's make a pretend universe where there happens to be a magnetic monopole floating around the North Pole, the South Pole, all by its lonesome, a magnetic charge, if you will.
If you were to take an electric charge, yeah, here's a here's a positive electric charge. I'm gonna put it over there and take a magnetic monopole, say a north a North pole by itself. I'm gonna put it next to it. Put them next to each other. What happens?
You can play around with the physics and see what happens they start spinning around each other ever so slightly and this spinning Isn't affected by how far away they are If they're very far away, they'll spin just the same, which is very strange. But spinning angular momentum momentum going in a circle if you take something and start moving in a circle is quantized. I haven't done a lot of episodes on quantum mechanics, so I haven't really dug into this topic. But one of the main takeaways, the big revelation that led to unlocking the quantum world is that angular momentum, momentum going in a circle is quantized in our universe. You cannot have any amount of angular momentum you want.
It comes in chunks. You have units. You can have one unit of angular momentum, two units of angular momentum, three eight units of angular momentum. This unit of angular momentum, known as Planck's constant, is very, very small. It's a very tiny number, which is why angular momentum to us here in macroscopic dimensions appears totally 100% continuous because we're at angular momentum value 3,500,400,365,271.
And then you add one to that, you're not really gonna notice that discreteness. But if you get down to the subatomic world, you notice that discreteness. Angular momentum in our universe is quantized. It comes in discrete chunks. So you have this electric charge and you have this magnetic monopole and they're rotating around each other, and that doesn't matter how far away they are, and that angular momentum must be quantized.
There is a fundamental limit to the least amount of angular momentum that they can have. But if angular momentum is quantized and at least one magnetic monopole exists in our universe, Then, this says, Dirac's math says, that electric charge must also be quantized. And electric charge is indeed quantized in our universe. You have an electron. It has one value of the charge.
It doesn't have half it or a third of it or four fifths or a fraction, just one. You can have one unit of electric charge. You can have two units of electric charge. You can have three units of electric charge. You cannot have one and a half.
You can't have five and a quarter worth of electric charge. You can't. Electric charge is quantized. Angular momentum is quantized. And the connection is through magnetic monopoles.
If only one magnetic monopole I'll say it again because this is just such a far out ridiculous statement. If only one magnetic monopole exists in our universe, then this explains why electric charge is quantized. If you don't have that, then you just have to assume, you just have to state, yes, electric charge is quantized in our universe, but we don't know why. If there's one magnetic monopole in our universe, then you know why electric charge is quantized because it's connected to the fundamental quantization of angular momentum. Magnetic fields are all about motion and the motion of charges.
We'll leave aside that whole monopole business for now. But imagine you're just looking at a charge, good old electron, just sitting there being boring with its boring electric field. What if you start moving? Yeah. You start running past it.
Well, from your perspective, it looks like the charge is the one that's doing the moving. Right? From your perspective, you're staying still and everything around you is moving. But moving charges make magnetic fields. So if you're moving past an electron, are you looking at an electric field or a magnetic field?
Yes. You're looking at both. Electric fields and magnetic fields are the same thing. They're two sides of the same coin, exactly the same way that space and time are connected into this unified framework that we call space time, electric and magnetic fields cannot be thought of in isolation. You can only think you can only think of the electromagnetic field, the unified picture, and whether it looks more like an electric field or more like a magnetic field depends on your point of view.
You could say it's relative. It's exactly this line of thinking that led Einstein down the path of developing special relativity. Before space time, before the equivalence of mass and energy, there was the unification of electricity and magnetism, developed by James Clerk Maxwell in the mid eighteen hundreds, '50 years ahead of his time, he didn't know that he was writing down the very first relativistic equation. And we didn't realize it for fifty years after. This connection between electricity and magnetism lays the groundwork for all of modern physics.
Because all modern physics is built on special relativity, it's a cornerstone of how we look at the universe. And if we didn't understand this connection between magnetism and electricity, we wouldn't have that intuition. We wouldn't know what to do. So from the humble magnetic field that never gets to be a star player, you get special relativity, and you get possibly the quantization of electric charge. I'd say that's not too shabby for a supporting actor.
I'd like to thank Dan h on Facebook, David h on YouTube, and at Brenda Hannesberg over on Twitter for the questions that motivated today's episode about magnetism. Sorry to see you go magnets until next time. Don't forget, I wrote a book. Go to pmcenter.com/book. You can preorder and or order depending on when exactly you're listening to this episode.
It's called Your Place in the Universe Understanding Our Big Messy Existence, and, yes, magnetic fields are mentioned. They don't get their own chapter, though. And, of course, you can come on an Astro tour with me or one of our other fantastic storytellers. Trips that are signing up right now include Iceland, Costa Rica, Ireland, and Colorado. Go to astrotours.co.
And check out my radio show, spaceradioshow.com. And I'd like to thank my top Patreon contributors this month, Justin g, Kevin o, Chris c, and Helgeb b. Go to patreon.com/pmsudder to learn how you can support the show. And of course, keep asking those questions. Send those to ask a spaceman dot com or hashtag ask a spaceman on Twitter and Facebook.
You can also email askaspaceman@gmail.com. And you know what? If you don't wanna contribute, if you don't wanna go on a trip, you don't wanna buy the book, you please do me a favor and go to iTunes and leave a positive review. If you love magnetic fields, Five stars on iTunes. I'll see you next time for more complete knowledge of time and space.