What does it mean for an electron to spin? Why do we have such a hard time measuring it? And why doesn’t Paul Dirac get any attention anymore, anyway? I discuss these questions and more in today’s Ask a Spaceman!
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Hosted by Paul M. Sutter, astrophysicist at The Ohio State University, Chief Scientist at COSI Science Center, and the one and only Agent to the Stars (http://www.pmsutter.com).
EPISODE TRANSCRIPTION (AUTO-GENERATED)
You know what time it is. It's time for Ask a Spaceman. I'm your host, Paul Sutter. You've got questions and I've got answers. You know how the show works, but let's take it for another spin. You go online, Twitter, Facebook, use the hashtag AskASpaceman. Send me some questions and I will pick a small subset of them to answer on this show only because there's a lot of questions and not a lot of shows. I got to work on that. You can also email me directly at AskASpaceman at gmail.com. You can also go to AskASpaceman.com. or youtube.com slash paul m sutter to check out the videos all those places can accept questions how awesome is that we have one simple goal with this show complete knowledge of time and space and on the road to complete knowledge of time and space we have today's questions from so many people asking the same question this is fantastic we've got dean b via email what is fundamental spin we've got p e via email how does an electron have quote unquote spin we've got at n-i-r-b-n-z on twitter could you explain once and for all what does the spin property mean in quantum mechanics We've got Kerry Kael via email.
Why are so many things spinning? Where does that energy come from? We've got at Sojournal via Twitter, 10 bucks. If you can explain the spin of the electron. Now, I don't often take bets. But you know what? At Sojournal, as you're listening to me, prepare an envelope. I want you to put on that envelope the address Paul M. Sutter, P.O. Box 3322, Columbus, Ohio, 43210-3322. You can put that $10 in that envelope because I think... At the end of today, you're going to owe me 10 bucks. I think we're going to do it. We're going to do quantum spin to your satisfaction. I'm laying it out there. I'm laying it out there. I haven't even recorded. I don't know how well this is going to turn out, but I've got a good feeling about this. Now, since I don't often take bets, I don't want to just take your money and do something frivolous with it. I want you, this is going to be a game. I want you to choose a cheese. That's right, a cheese, and I will buy that cheese and eat it with your $10, and I'll put the picture on Twitter.
I know you're thinking this is a little out of left field. It's just never come up before. I love cheese. I like cheese. In fact, if you just want to mail me cheese directly instead of contributing to Patreon, that's P.O. Box 3322, Columbus, Ohio, 43210-3322. I'm a big fan of cheese, and I can't believe in the years that this podcast has gone on, it's never come up. But there it is. I believe 90% of my diet is cheese. But seriously, you know what? If I've successfully explained spin, I want to reward myself. You know, I want to congratulate myself. I don't often congratulate myself. And what better way? than with a good cheese. Now, first things again to everyone for all the amazing and thought provoking questions you've asked all these years. I can't thank you enough. This show drives on your questions and you are all definitely curious about more than I thought you would be like spin. good old spin, quantum spin. When I started this show, I never thought I'd get questions about spin, but here I am.
And you know what? I'm pretty happy for it. I love talking about astronomy, astrophysics, and quantum mechanics. But as you know, quantum mechanics is such a thick, rich, dense soup of a topic. It is, after all, an entirely field of physics. It's hard for me to find entry points, little dabbles here in quantum mechanics, but spin, in my opinion, is the classic, that's not the right word, the prototypical quantum mechanical subject. And it's a great way to start entering into this quantum world and working it into the show is through spin. And yes, there's all the sexy quantum mechanics stuff like superposition and probabilities and entanglement and teleportation and spooky action at a distance. And that's all great. That's all cool stuff. And historically speaking, if I wanted to follow a narrative of our development of quantum mechanics, we started figuring out all that stuff before we figured out spin. But you know what? That stuff and don't worry, I'm sure eventually I'll get to do shows about all that stuff.
A lot of that knowledge, a lot of that language, the perspective of looking at quantum mechanics is due to a particular physicist by the name of Erwin Schrodinger. And you may have heard of Schrodinger's equation or Schrodinger's cat. That's a long story on the cat thing. So feel free to ask. But all that stuff. All that good stuff, superposition, the uncertainty principle, things like that, that's just waves. The Schrodinger equation, the mathematics that's used to describe a lot of quantum mechanics, is a wave equation. It's an equation that describes the behavior of waves. So superposition, uncertainty principle, normal waves do that too. It's just in quantum mechanics, they acquire this new interpretation. But there's a lot of analogies that we can use in the macroscopic world where we can take things that we're already familiar with. And because the equations are exactly the same, they're just applied in a new scenario. We can build some mental models. We can make some analogies. And that's all great, except for spin.
There's nothing quite like spin. And as I'm about to tell you, I think spin is a purely quantum mechanical thing, despite its name. And I'll get to the whole name thing. There's nothing quite like it in the macroscopic world. And so what's the big deal? What's the big deal? Let's start with an electron. How many numbers do you need to totally describe an electron? Well, an electron has mass. There's one. It has charge. There's two. So at least two numbers you need to describe any electron in the universe. But some experiments in 1922 found something funny. These two physicists, Otto Stern and Walter Gerlach, shot some silver atoms through a magnetic field. And apparently, no, no, no, shot some silver atoms through a magnetic field. And apparently they had some motivation for doing this and they weren't just bored. They were intent on looking at this. And as the atoms passed through the magnetic field, they got deflected. Their paths got bent away from a straight line. Hmm, that's interesting.
Not totally unexpected, but that's pretty cool. Okay. Shooting these silver atoms through the magnetic field and their paths get curved. But they got curved in two directions. The beam of atoms was split. Some atoms would curve up and some atoms would curve down. down and it's at this point in the life of a scientist where you go from okay okay pretty cool result that's nice to uh stern and gerlach pretty much had no idea what was going on And remember, the magnetic field in their apparatus, in their experiment, isn't changing. It's just staying there. Nothing's changing about the beam of silver atoms being shot through the magnetic field. They just turn it on and go. But something, somewhere, somehow is making a decision. Each silver atom chooses, for lack of a better word, to go up or down as it gets shot through this magnetic field. How do we even begin to explain this? Presumably, that's what Stern and Gerlach said, but in German. And seriously, until 1922, nothing like this had been encountered before.
Nothing. This was a very unique thing. This was a very, very special thing. If you pretend... You know nothing about electrons and atoms or quantum mechanics. We do know about how charged objects operate in magnetic fields. We do know that if you have a metal ball, let's just say you got a metal ball and it has an electric charge. It can interact with a magnetic field. It can spin in the same direction as the magnetic field. So the magnetic field is pointing in one direction and then the axis of this spinning ball can point in the same direction or it can point in an opposite direction. It can point in any direction you want. And if it's not pointed in the same direction as the magnetic field, then the magnetic field can push on that charged metal ball to rotate it. This is very simple, James Clerk Maxwell, electrodynamics, seriously kindergarten level physics. I mean, learned this decades ago. And if you take these spinning charged metal balls and shoot them through a magnetic field, they'll get deflected.
So it's that easy, right? Like if we're performing this experiment with not silver atoms, but with giant metal balls, it would be very, very simple to interpret. We would get a very, very simple result, which would be, yes, the paths of these charged metal spinning balls would get deflected. They would end up on the opposite side. And so the natural intuitive, the first thing a physicist's mind goes to when you see this result of the silver atoms and then very, very quickly we realized it wasn't the silver atoms doing the work, but the electrons themselves. The first step, the first thing you think of is, oh, electrons have an extra property. They don't just have mass. They don't just have charge. They also have this property, and we're going to call it spin because it seems to act like an electron is a tiny spinning metal charged ball because that's what would happen if we were to run this experiment in 1850 or whatever. That's what would happen to the spinning charged metal balls. So this is, the electrons are doing pretty much the same thing.
And so they have an extra property. We'll call it spin because that seems to work well. So we can think of electrons as tiny little charged metallic balls and they're spinning really, really fast. And then when you shoot the electrons through a magnetic field, some will randomly be pointing up, some will randomly be pointing down, and that's how you get your split. Now, I want you to take that nice, neat, tidy, cozy little explanation, that understanding you have in your head of how an electron works like a spinning metal ball when we shoot it through a magnetic field. I want you to take that notion and put it in a box. I want you to lock that box. I want you to throw away the key. I want you to light that box on fire. I want you to take those ashes and put them in another box. And lock that box and throw away that key. And then I want you to attach that box to NASA's Parker Solar Probe as it crashes into the sun. I want you to never think of an electron like a spinning metal ball. Ever again.
And I mean it. Why? Because it's wrong. And this was realized right away. For one, we can calculate. given simple electromagnetism, how much the electron should respond to the magnetic field. This is a quantity called the magnetic moment of the electron for the curious. And if you have a spinning charged metal ball, you can calculate for how fast it's spinning, how much is charged, how massive it is, how much it ought to react to a magnetic field. And you apply those numbers to what we know, the mass and charge of an electron, and you get the wrong answer. And we know that electrons are really, really, really small. In fact, they may be infinitely small. That's another episode. And if they're just the size of our current detection limit, if they're just below that, if we think they're a certain size, that means to get that kind of a magnetic moment, the one we actually measure, that kind of reaction to a magnetic field that we actually measure, it means they're spinning faster than the speed of light, which is kind of wrong.
And if all the electrons being shot through this magnetic field had random spin directions, if they moved all over the place, if you imagine like these little arrows attached to the electrons, then some will already be pointing in the same direction as the magnetic field. Some will be a little bit off. Some will be way off. They should get tossed around randomly. If they have random spin directions, they should get spread out randomly and evenly on the opposite side. Because some will react strongly based on their spin and some will react weakly. Some will get deflected a little bit. Some will get deflected a lot. So it should be spread out evenly. Instead, they separate into two and only two camps. And this is the real kicker. Because no matter how we measure it, we always only ever get two answers for how much the electron is spinning. It's always either up or down. And the amount of upness and the amount of downness is always the same. We don't see any intermediate values. We don't see this nice continuous spread on the opposite side of the instrument.
We only see two clumps. And that is something that normal spinning metal balls simply wouldn't do. To sort this out, to really put us in the right perspective, we have to go in a fully quantum mechanical state of mind. We need to put blinders on either side of our eyes so we resist the temptation to apply our normal everyday macroscopic experiences and expectations of the world to the subatomic realm. Down there, things are simply different and it's not going to make intuitive sense. But nature doesn't care if nature makes intuitive sense. She doesn't care. Nature is nature. Electrons are going to do what they're going to do. And we have to, we absolutely have to discard. our classical worldview. Just like we do for relativity, we have to do it for quantum mechanics. The rules are simply different down there. So now we must go down that quantum road, full speed ahead, unwavering, unflinchingly, no stopping, except for one pit stop for Patreon. Patreon.com slash PM Sutter is how you can support this show and keep all the episodes going.
And the YouTube show, like every education outreach thing I do, it's supported all by contributions like yours at patreon.com. I can't thank you enough. And if you can't, I know I tease you that you ought to contribute even if you can't. Believe me, that is just a joke. If you can't contribute, I still appreciate you just listening to the show. If you can tell someone, if you arrive you on iTunes or Google or on the website or tell people on the internets, I really appreciate that too. Thank you so much. Now that we've had that appropriate stop, we can continue down the quantum road. In the early days of quantum mechanics, I mean, wait, like 1910s, 1920s, physicists really around the world were coming up with all sorts of rules to explain the subatomic world. There were all these experiments going on, you know, revealing what's happening to atoms and particles. And the physicists, the theorists were really trying to catch up, like piece it all together, put it into some coherent framework.
And they did. They had things pretty much sewn up tight in a compact set of mathematical descriptions, which is another episode, except for spin. So they were able to explain all these interesting, complicated phenomena, except for spin. And spin was just kind of tacked on. So they said, like, here's a self-consistent, elegant, beautiful description of the microscopic world. And then here's some stuff on spin. And of course, as the rules were developed, as we learn more about what quantum spin is really like, shorthand and notations were developed. And they seem, when you first encounter these shorthands and notations, they seem non-intuitive and awkward. And that's because they're non-intuitive and awkward. So what are some of the rules of quantum spin? Well, first it's, you guessed it, it's quantized. What does quantized mean? It means it comes in little packets or steps. The difference between the classical and the quantum world is the difference between a nice gentle slope in a staircase.
A nice gentle slope, I can choose to be anywhere I want on that slope. I can just be a little tiny bit. I can be a lot. I can step down just a little bit. I can go half a step, a quarter step, an eighth of a step. It doesn't matter. The slope lets me be anywhere. But on a staircase, I can only be on one step at a time. If I try to walk in half steps, I'm going to fall over. I can't do it. I can only go on this step, the number two step, number three step, number four step, number five step. And that's it. That's what the quantum world means. Things like angular momentum, things like energy only come in little packets, only come in steps. You can't have anything you want. And this is showcased really, really well with spin. When it comes to spin, an electron can only be measured to have, and here's a notation, here's a definition, either plus a half or negative a half spin. That's it. If you go to measure the spin of an electron, like, hey, how much is that electron spinning? You'll only measure one of two numbers, plus one half or minus one half.
And the one half, again, that's just a notation, a definition, a way to kind of untangle this mess. There are other kinds of particles than electrons, of course. Some kinds of particles can have three values of spin. They can have one. They can have zero. Or they can have minus one. Some other kinds of particles can have four measurements of spin. You can get plus three halves, plus one half, minus one half, or minus three halves. Even others can have five measurements of spin. Two, one, zero, minus one, or two. There are two kinds of spin. And all particles in the universe are either halfers, spin one half, or they're holers, spin one or spin two or spin three. And we've encountered this before. These kinds of particles have names. The fermions are the halfers in the universe. They are the ones with spin half or spin three halves or five halves. And the bosons are the holers. These are the spin zero, the spin one, the spin two. And I need to do a whole other episode on how this spinniness, that's my name, by the way, determines what kind of particle is and what are the implications.
That's a whole other episode, super fun, but not the subject of today. So an electron, let's go back to electrons. You can only see, you can only measure the spin of electron. You only have two values that will come out of your experiment, either plus a half or minus half. And I need to talk a moment about this concept of measurement. Because a lot of times you'll read or even I'll say like, oh, the electron has spin plus one half. I'm actually being a little bit lazy when I say that. And this is a very subtle point, but I think it needs to be made because this cuts right to the heart of quantum mechanics. Let's say I have a box of electrons, a big old box, lots of electrons. They can have any spin direction they want. Like the analogy I used before, imagine a bunch of little arrows, right? painted on all these electrons. And that arrow represents their spin direction. They all have the same magnitude of spin. So they're all spinning at the same rate, but they can be oriented any way they want.
All the electrons are jumbled up, so the arrows point all over the place. Then I start throwing them through the magnetic field. The magnetic field has a direction to it. It has an up and a down sense to it. For each individual electron, it has a little arrow being shot through this apparatus. That arrow will be maybe it's pointing up and a little bit sideways. Maybe it's pointing mostly sideways and just a little bit up. Maybe it's almost exactly split at a diagonal halfway between sideways and up. It doesn't matter though. It doesn't matter what that orientation of the spin is. I will only ever measure the arrows pointing straight up or straight down. I don't get to learn about how much the arrows are pointed left or right. This is a fundamental limit to measurements in the quantum world, and it's kind of a big deal. Imagine, imagine, imagine you bought your new smartphone. It has GPS, it has maps in it, turn-by-turn navigation. You're super excited. You plug it into your car and you say, hey, I want to go to that restaurant.
And instead of giving you accurate directions like, oh, you need to travel north-northeast for one mile, then turn left, you know, all the instructions. What if instead there's only two lights, right? And lights tell you north or south. And that's it. So let's say, well, if you say, oh, how far to that restaurant? Oh, it's two miles north. And then you travel two miles north. And then you ask again, hey, where's the restaurant? Oh, it's two miles south. So you travel two miles south. Okay, now where's the restaurant? It's two miles north. No, no, no, it's two miles south. No, no, no, it's two miles north. And that's the only ever answer you can get out of your GPS. You would immediately throw it in the trash. Unfortunately, we can't throw nature in the trash. We're stuck with it. This is a fundamental limit of measurement. The electrons can have whatever spin they want. If they're pointing just a little bit up, just a hair up, we will measure it as all the way up, spin plus half. If they're pointing just a little bit down, Just a tiny, tiny, tiny bit down as they're thrown through this magnetic field.
We'll measure them as negative one half. And that's it. That's all we will ever measure. Ever. We don't get to learn how much left or right. We don't get to learn how much up or down. Just if it's any bit up, it gets counted as up. And any bit down, it gets counted as down. Now, if I repeated my... Magnetic field Stern-Gerlach experiment apparatus. Instead of having an up-down magnetic field, I rotated it so it was a left-right magnetic field and repeated the experiment. Then I would get to learn about the electrons left-right. Are they pointing left? Are they pointing right? But I don't get to do it at the same time as the up-down. In fact, I destroy my knowledge of the up-down. And I can't do it at the same time. I never get to learn both simultaneously. So there's a limitation in the direction I can measure an electron spin. I can only measure its uppiness or its downiness. And I never get to learn how much of uppiness or downiness. Just are you in the up bucket or are you in the down bucket? And that's it.
So that's what spin is. Electrons and all particles have this weird property that we call spin. Different particles have different allowed values of spin, but we are fundamentally limited in how well we can measure it. And this is why this experiment, this 1922 experiment by Stern and Gerlach is so striking because usually the effects of quantum mechanics are really subtle. Like you have to do these really detailed measurements and you tease out the quantum nature and it's all spooky and weird, but very deep down, this is like a tabletop experiment that you can do at home and blasting right in your face is the true nature of quantum mechanics. right there, and you can't get around it. How are we gonna make sense of this? Where does this property of spin come from? What does it mean? Well, like I said, in regular quantum mechanics, all this stuff, quote unquote, regular quantum mechanics, by the way. All this stuff is just tacked on with the justification, experiment said so. Like we have this grand, beautiful, theoretical infrastructure to explain quantum mechanics.
And then here's some rules of spin that we can't really predict. And by regular quantum mechanics, I mean the Schrodinger equation. The Schrodinger equation does not include spin in its mathematics. That equation is a whole other episode, but I need to bring it up because Schrodinger wasn't the only person to work on quantum mechanics. He wasn't alone. There was lots of people. And now it's time. There's someone else I need to introduce. One of my all-time favorite scientists. I mean, James Clerk Maxwell, don't worry, still king of the mountain. But putting up a good fight for that. Oh, my gosh. Paul Adrian Maurice Dirac. Paul Dirac, just a beast of a scientist. I am not joking around. By the way, he signed his papers P.A.M. Dirac. So people would wonder why I signed my own papers P.M. Sutter. There's the inspiration for that. Paul Dirac just made things happen. He wouldn't just solve problems. He would develop new mathematical techniques on the fly to solve those problems. And then decades later, actual mathematicians will go back and prove that what he did by pure intuition was actually legit.
This is the kind of guy that could and did go toe-to-toe with Einstein and win. He shared his Nobel Prize with Schrodinger in 1933 for developing quantum mechanics, but he doesn't get as much love today. You know, the Schrodinger equation is everywhere, if you pay attention to that kind of stuff. But outside of physics, he's not really in popular culture. But inside of physics, his name is on like half the stuff. Like the Dirac this, the Dirac that, the Dirac operator, Dirac function, the Dirac equation. He gets plain name recognition inside the world of physics. And here's why I'm bringing up Dirac. Here's the deal. Quantum mechanics was developed in roughly the 1920s. By the 1920s, special relativity was already old news. Einstein developed that in 1905. And everyone knew that special relativity was a thing. And if you're going to make a theory of nature, if you're going to describe some aspect of nature, it has to incorporate special relativity, the rules of special relativity. Schrodinger attempted to make an equation that described what the experiments of quantum mechanics were revealing, but also obeyed the rules of special relativity, what we knew to be true from a special relativity.
That equation didn't make any sense at all. So he abandoned it. He said, let's just let's just figure out all that special relativity stuff later. And instead, he went casting about found the existing language of wave equations, was able to apply that successfully to quantum mechanics. And that's our Schrodinger equation, using that to like interpret and understand our results from those experiments. So you can use the Schrodinger equation to make predictions and knowing that it's not going to be correct in pure special relativity. So it's an incomplete picture of nature. Then comes Dirac, who at about the same time is doing the same similar work. He wants to develop some equations to describe what the experiments in quantum mechanics are saying. But he really, really wants to bring in special relativity. He really wants to make this happen. He arrives at the same place Shrody did, and he figured it out. The trick, because initially these equations make no sense at all. The trick was to, you know, completely restructure the mathematical description.
So instead of casting about for some existing language, he invented a whole new language. He said, no, no, no, no. Here's how you interpret it. You're going to speak a new language. That equation, the Dirac equation, describes everything the Schrodinger equation does and more. It includes special relativity. So it's like having two languages to describe the same phenomena. The Schrodinger equation is based on one language, the language of waves, and we can use it to make progress and make understanding The Dirac equation includes the Schrodinger equation and so much more in a completely new language. That's another episode to really dig into that, what that language is and how we interpret quantum mechanics via the Dirac equation. It includes the Schrodinger equation, includes special relativity, and it includes spin. The concept of spin. naturally appeared in Dirac's solution for quantum mechanics. If spin wasn't already discovered, he would have been credited with predicting it. He ended up getting vindicated, though.
His equations not only included the Schrodinger equation and special relativity and spin, also naturally included something we had known before, something we now call antimatter. Antimatter was predicted by Paul Dirac in this exact same equation. So what is spin? Well, it's a horrible word for one. Spin, the concept of spin is based on our classical understanding of electrodynamics that simply do not apply to the quantum world. Better term might be, I don't know, magnetic response of a particle. I don't know. I guess there's no word. We should have just made up a word. We should have made up a word and we'd be so much better off. If you're lazy, you can think of it like an actual spinning metal ball. like pretend these little particles are charged metal balls. And, but let's not, let's not. And you know what? We're never going to speak like that again. Spin is built into the universe. Asking where it comes from is like asking where mass or charge comes from. It seems to be fundamentally connected to the relationship.
between quantum mechanics and special relativity. So at one level, you can think of it as just a set of rules. Like, hey, nature says there's this quantity called spin, and it's different kinds of particles that have different amounts of spin, just like different particles can have different amounts of charge or mass. But because of the rules of quantum mechanics, we're limited in how much of that spin, of that quantity, we can actually measure. But spin appears to come. instead from this unification between quantum mechanics and special relativity. It appears to be something much, much deeper than a set of rules. It's at this point, and it's so cool, it's so cool that you can do a tabletop experiment, See this quantum mechanical effect, which is usually super subtle. Quantum mechanics is not easy to tease out, but there it is right in your face, making itself manifest, making itself known. And not only is it this manifestation of quantum mechanics, it's at this intersection between quantum mechanics and special relativity.
Is that confusing? Too bad, Mother Nature doesn't care. I'd like to thank my top Patreon contributors this month, Justin G, Matthew K, Kevin O, Justin R, Chris C, and Helga B. Thanks so much to you and all my Patreon contributors, patreon.com slash pmsutter, to help make this show possible. Be this show. And thanks again. The other part of making this show, this show is the questions. I'd like to thank Dean B, Pete E, at Nurbans, Carrie Kale, and at Sojournal, who is going to send me 10 bucks. P.O. Box 3322, Columbus, Ohio, 4320-3322. Please. keep those questions coming. Go to ask a spaceman.com. You can follow me directly on Twitter at Paul, Matt Sutter, also on Facebook, also on youtube.com slash Paul M Sutter. Uh, go to iTunes, go, go forth and spread the word of spin. And I'll see you next time for more complete knowledge of time and space.
What’s the balancing act happening inside stars? Why is iron the fusion limit? How long does it take for a star to blow? I discuss these questions and more in today’s Ask a Spaceman!
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Keep those questions about space, science, astronomy, astrophysics, physics, and cosmology coming to #AskASpaceman for COMPLETE KNOWLEDGE OF TIME AND SPACE!
Big thanks to my top Patreon supporters this month: Justin G., Michael Z., Kevin O., Justin R., Chris C., Helge B., Tim R., Michael C., Lars H., Ray S., John F., James L., Mark R., Alan M., David B., Silvan W., Edward H., and Alex P.!
Music by Jason Grady and Nick Bain. Thanks to WCBE Radio for hosting the recording session, Greg Mobius for producing, and Cathy Rinella for editing.
Hosted by Paul M. Sutter, astrophysicist at The Ohio State University, Chief Scientist at COSI Science Center, and the one and only Agent to the Stars (http://www.pmsutter.com).
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
You know what time it is. It's time for Ask a Spaceman. I'm your host, Paul Sutter. You've got questions and I've got answers. You know how this show works, but let's blow it up one more time. You go online to Twitter, Facebook, use the hashtag Ask a Spaceman. I'll pick those questions out and I'll blast an answer right back to you or six months later or two years later, but eventually an answer will get back to you. You can also follow me directly on Twitter and Facebook. My name is at Paul Matt Sutter. You can also go to the website at AskASpaceman.com. Leave some comments on the episodes. You can have a conversation there. It's all good. You can also go to YouTube.com slash Paul M. Sutter. We have one simple goal with this show. Complete knowledge of time and space. And on the road to complete knowledge of time and space, we have today's questions from at Ianut Branea. asking, why do supernova happen? I don't understand what triggers the explosion. And also, why is iron the limit when fusion stops inside a star? And also, at thomaspadden1, also on Twitter, supernova, coolest or hottest thing ever? Great questions.
And today, I'm going to give you a simple recipe that you can follow at home to make your own supernova. You need a load of hydrogen. Around 10 to the 31 kilograms or two times 10 to the 31 pounds. Don't worry if you're a little bit off. This measurement doesn't have to be exact. You need a dash of carbon, nitrogen, and oxygen. Set your oven to 10 million Kelvin and set your timer to around 10 million years. And all you need to do is wait and eventually you'll get... The big boom. A supernova explosion. It is just that easy. It's crazy to think how simultaneously simple and complex the natural world can be. It's a very simple ingredients. It's a very simple process. And yet it leads to this incredibly rich, dynamic situation and one of the most powerful explosions in the universe. That simple process. that takes all these ingredients the hydrogen the carbon the nitrogen the oxygen and turns it into a supernova is gravity it's all driven by gravity good old-fashioned newton and einstein down at the bottom of your feet gravity gravity plus time gives you supernova chew on this The same force.
The same force that holds your feet to the ground is the exact same force that provides the energy required for a star to go supernova. That is one of the most beautiful things. One of the things I really love about physics is the universality of physics, how the same physics here on Earth apply across the universe. And the understanding that we develop to figure out why our feet stick to the ground help us understand some of these crazy exotic phenomena happening on the other side of the universe. And I need to say, so much of astrophysics is driven by simple gravitational collapse. You have a cloud of stuff. It's unstable. It starts collapsing in on itself. You get galaxies. You get galaxies. Stars, you get supernova. This is an example of the power of gravity. Sure, gravity may be the weakest of the four forces, but it can play some tricks that the others can't. Just give it, you know, a few million years. In the case of stars, it's a balancing act. You have a giant blob of hydrogen, and gravity is pulling it together.
Gravity is what pulls it into a spherical shape. Gravity would love for the star to collapse into an infinitely dense point. That's all gravity desires, is to take stuff and squish it down even tighter. But it gets stopped. It gets prevented from squishing that star down into an infinitely dense point because... The deeper you go into the core, as all this material is compressing in on itself, the deeper you go, the higher the pressures get, just like diving into the ocean. The gravity of the Earth would love to pull the ocean down into the core. And so as you go lower and lower in the ocean, the pressures get higher and higher. Higher pressures when it comes to gases like the sun or a star, higher pressures mean higher temperatures. eventually. The pressures are so high that protons, who normally hate each other and want nothing to do with each other, they are like charges, so they repel. They are forced to live together inside the same nucleus, and that's a fusion party. The pressures, the gravity delivers a enough overwhelming pressure.
In fact, this is the definition of a star. The dividing line between a planet or a brown dwarf and a star is, is there enough gravitational pressure at the core to ignite a fusion reaction? There's two main chain reactions that happen to convert hydrogen into helium. And since I'm talking about supernova, I'm going to talk about more massive stars. Stars at least around eight or so times more massive than the sun. And in this reaction, you start off with four hydrogen atoms. And you sprinkle in some carbon, nitrogen, and oxygen. And these act like a catalyst to... shall we say, enhance the reaction. They make the nuclear chain reaction go faster than it normally would and be a little bit more energetic than it normally would. At the end of the day, you start off with four bits of hydrogen and some sprinkling of carbon, nitrogen, and oxygen. They return. You still keep the carbon, nitrogen, and oxygen. They get recycled to be used in another reaction. But instead of four hydrogen atoms, you get a helium atom.
And the helium atom is slightly less massive than the original four hydrogen atoms. And we live in a universe where mass is equal to energy. And so this process gives off energy. You lose mass, boom, you get some energy out of it. This process, this nuclear reaction has a few other byproducts. It spits out some positrons, which are the evil twin of the electron. I need to get into antiparticles someday. Just ask the question. Spits off a couple neutrinos, and neutrinos, a very ghostly particle, usually don't play a role, but they'll become important later. And of course, photons, light, radiation. Ladies and gentlemen, this is a nuclear bomb. A fusion bomb, to be precise, a hydrogen bomb. And what do bombs do? They explode. So gravity wants to pull the star ever smaller, but it's resisted by this perpetual nuclear explosion going on in the core. These stay in balance. If gravity were to win a little bit, let's say you were to temporarily stop The nuclear reaction is happening in a star.
Then gravity would start to wind, pull it even tighter. But the core of the star would get scrunched up even tighter with even higher pressures, with even higher temperatures, which would increase the reaction rate, which would make the star want to blow up a little bit more. So it reinflates. And if the fusion were to win a little bit, say you're able to temporarily ramp it up where you can get a lot of fusion going, well, that would tend to inflate the star, expand the star, which would cool it off in the core, which would lower the reaction rate, which would mean gravity would temporarily have the upper hand. And so we have a balance between the crushing gravity of the star's own weight and the energetic explosion happening because of these nuclear chain reactions. This balance is called hydrostatic equilibrium. Equilibrium because it's in balance. Hydrostatic because, well, it's not hydro. It's not water. It's gas. Static is not moving. It's just a historical term, all right? We're going to live with it.
In this case, in the case of stars, this happens all over the universe, by the way. Anytime another force resists gravitational collapse, you get what's called hydrostatic equilibrium. Why doesn't our atmosphere collapse? Hydrostatic equilibrium. How do giant clusters of galaxies maintain their shape for billions of years? Hydrostatic equilibrium. How does a star keep from collapsing into an infinitely dense point? Hydrostatic equilibrium provided by nuclear fusion. Anytime another force resists gravitational collapse, and this is usually a byproduct of the gravitational collapse itself, gravity is kind of its own worst enemy. Gets a little bit too eager, wants to collapse things into an infinitely dense point, but that leads to a condition that resists the collapse itself. So... Sorry, gravity. I mean, maybe if you're a little bit nicer, you'd get what you wanted, but you had to go all in single-mindedly and you end up with all this fusion. So stars stay in balance for a long time. It's all hunky-dory for millions or even billions of years until you run out of hydrogen in the core.
Now, you can have tons of hydrogen left over in the star everywhere else, but if it's not in the core, no fusion for you. Just like Patreon. Hey, there can be all these monies floating around the world, but unless they're put into patreon.com slash pmsutter, they're not doing anything useful or important. or vital for the perpetuation of our civilization. You know, they have to be in a very specific place. They have to be submitted to patreon.com slash pmsutter to help this show keep going. Very apt analogy. One of my best analogies ever. So you can have tons of hydrogen in the star, but if it's not in the core, it's not going to get any work done. You're not going to have any fusion. But what happens in the core? As this fusion party is happening, you're creating helium. Where does the helium go? It sinks down into the core, deeper into the core. And you can fuse helium, but in the normal everyday life of a star, it's not hot enough. The pressures aren't high enough. So it gets shoved into a little ball.
And you end up eventually, and eventually depends on the size of the star, how quickly it fuses, blah, blah, blah. The fusion, the actual hydrogen fusion, the furnace gets moved into a shell around the helium core. And how intense is nature? What's happening, it's starting to happen inside of our own sun right now. It'll take a few more billion years for it to really play out. But it's building up a core of inert, dead helium. And surrounding that, beginning to surround that, is a shell of hydrogen fusion. And it's surrounded by the ferocity of the rest of the star. Eventually, eventually, eventually... The burning of hydrogen in the shell turns off. And yes, I know it's not technically burning. It's not combustion. It's not fire. But it's called burning because it's fun to call it hydrogen burning. So it's fun. So shut up. Don't worry about that. Just call it hydrogen burning. It's the same as hydrogen fusion. It's just a name. Another historical term. Eventually, that shell runs out of gas.
It runs out of hydrogen. So gravity wins! Yay! Finally, its resistance is futile. The fusion has finally shut off once and for all, and gravity can get back to work of collapsing that star into a single, infinitely dense point. But now you have a bunch of helium in the core, and initially the helium isn't doing anything at all, just sitting there being hot. But as gravitational collapse continues and continues and continues, once the core reaches about 100 million degrees, the pressures are high enough, the temperatures are high enough, helium can start to fuse. Helium fuses? turns into carbon, oxygen, a little bit of nitrogen sometimes, and releases energy. Hydrostatic equilibrium rules the day again. But helium burning is hotter and furiouser. Why? Well, helium fusion isn't quite as efficient as hydrogen fusion. It doesn't release as much energy. So it has to really struggle to overcome gravity. This is operating at 100 billion degrees with an incredibly compressed core. To maintain that balance, the helium has to burn a lot faster than the hydrogen did.
So this phase, the helium burning phase of a star is much shorter. And the cycle is repeating again inside of massive stars. It's building up a core of unburnt, of inert carbon and oxygen. With the helium fusion happening in a shell around it, and sometimes hydrogen fusion can reignite in a shell around that, but eventually you run out of helium in the shell and gravity wins. Then carbon and oxygen can start fusing. That happens at the billion Kelvin mark. So 10 times hotter than where helium fuses, you get carbon and oxygen fusion. With a shell of helium fusion around that, a shell of hydrogen fusion, it's crazy. But then that stops because you run out of carbon and oxygen. Well, what do carbon and oxygen fuse into? They fuse into silicon. And silicon fusion ignites at around 3 billion Kelvin. And at this stage, you end up with the hottest, nastiest, seven-layer bean dip you've ever shoved your tortilla chip into. You have on the surface a skin of hydrogen. that's not doing anything. Below that, you have a skin of hydrogen that's fusing.
Below that, you have a skin of helium that's fusing. Below that, carbon that's fusing, neon that's fusing, oxygen that's fusing, silicon that's fusing. And at the very, very core, a ball of iron. That's what silicon fuses into, nickel and then iron. And as we move up the fusion chain to more and more massive elements, the energy production gets less efficient. You get less bang for your buck. fusing carbon or silicon than you do fusing helium or hydrogen. And it's at iron where the nightmare begins because it costs energy to fuse iron. When you fuse lighter elements, you gain energy. When you fuse iron and heavier elements, you lose energy. So gravity, the incredible pressure of this massive star weighing down on this core of iron, squeezes two iron bits together, but it loses air. There's nothing to fight it. There's even less resistance to gravity. So gravity pushes the attack, drives up the temperatures even more. And the fusion rate goes up even more and almost instantaneously. The star is thrown out of balance.
Some iron gets fused in this process, but most of the iron remains because this happens very quickly. That first hydrogen burning stage in a star lasts, you know, a few million years. And for small stars can last tens of billions of years. But for these big massive stars, at least eight times more massive than the sun, a couple dozen times more massive than our sun. This is over in a few million years. The helium lasts a million years. Tops. Carbon fusion goes on for 600 years. Neon burning lasts a single year. Oxygen lasts six months. Silicon fusing into iron, that takes a day. A single day. And you've run out of silicon. Then you make that ball of iron in the core. It tries to fuse, but there's nothing to stop the overwhelming gravitational collapse. Spoiler alert, gravity is going to win. But in this battle between collapse and explosion, there's going to be one big epic showdown. You have a core of iron. This core of iron is only a few miles across or kilometers across, surrounded by enough gas to fill a dozen suns, and there is nothing holding that gas back.
The iron core itself is made of protons and neutrons. And there's a bunch of electrons just hanging out, not bothering anybody. But the pressures are intense enough to shove, to literally shove those electrons into the protons. This process turns the protons into neutrons and also spits out some neutrinos. The iron, you start off with a ball of iron, it gets converted into a giant ball of neutrons, right? raw neutrons, like a gigantic citywide atomic nucleus. And the neutrons can hold themselves up from degeneracy pressure. Just like electrons, you can only fit so many electrons in a box before the box starts spilling over. You can only fit so many neutrons in a box before the neutrons start spilling over. Once this ball of neutrons is formed, this neutron core, You have the entire weight of the star, dozens of suns worth of material, that the intense gravitational pressure converted that iron core into a ball of neutrons, but it can't press it any further. It is supported. It does resist gravity.
So the entire material slams into that neutron ball and ricochets. And boom. That process, going from formation of the Iron Core to triggering the launch of a supernova, about 15 minutes. 15 minutes. This is a process that was set up millions of years in the past. And the final moments happen before this show is over. That's the initial explosion. That's the triggering of a supernova. But here's where things get tricky. We don't fully understand supernova. Welcome to, you know, science. But we've been running simulations of supernova. What's going on inside these explosions? It's complicated stuff. You know, you've got all these fusion chain reactions going on, the multiple layers, the multiple elements. You've got radiation. It's spinning. You bet there's magnetic fields. And I swear there should be a special tone every time I say magnetic field in a show. I should get a different voice like magnetic fields. Like it's the secret word of this show. There are magnetic fields happening. There are neutrinos.
Which usually no one cares about neutrinos. But what we found when we started making our initial simulations of supernovas, you know, 10, 20, 30 years ago, the shock front, the rebounding, you get this rebound off the core and then you get this shock front, this wave of explosive energy that starts propagating away to start filling out the rest of the star and make it go boom. It kind of stalls, peters out, slows down. It was thought that neutrinos might come to the rescue. Even though they don't usually interact with matter, there's enough neutrinos produced And this kind of event qualifies as enough that enough of them blow. It's like a subtle wind, a gentle wind. So there's this stalled shock front in the middle of the star. It wants to explode, but it doesn't quite have enough oomph to do it. But then there's this flood of neutrinos and kind of reinvigorates it. We thought neutrinos would do the trick, but complex simulation so that... That again, the shockwave that wants to blow up the star, the final revenge of nuclear fusion against gravity, but that peters out too.
What happens after that? We don't know. Because it's in this stage that things get really complicated really fast. In our simulations of supernova, we have a lot of trouble getting supernova to, you know, supernova. We're not exactly sure. We're pretty confident on the triggering mechanism, that this is the physics, that the iron core forms, it turns into a neutron ball, and there's a bounce and a rebound. And we know this because we've seen the neutrinos themselves. This was a major revelation back in 1987. There was a supernova that we saw. With our eyeballs, our telescopes. And we also detected this supernova using our neutrino detectors. And we actually saw the neutrinos first because the neutrinos were able to burst out of there, out of the scene, and stream straight to our eyeballs while the explosion hadn't quite happened yet. So the neutrinos actually popped out first. There was a flash of neutrinos. There's almost a supernova neutrinos first before there was a supernova of light.
And yes, I'd love to do an episode on neutrinos. Just feel free to ask. We saw the flash of neutrinos in our neutrino detector, and then we saw the flash of the explosion in our telescopes. So we knew, we know this general picture is right, but the details, what happens to actually... motivate that explosion to actually get it going, to get it through, get its second wind or its third wind to actually produce the brilliant display that we see. We think it might have something to do with unstable modes where these explosions aren't perfectly spherically symmetrical. It's not like a ball. or a shell, maybe this shock front happening inside of a star slows down, but then it starts to get unstable and starts to slosh back and forth. And then eventually one of these sloshing modes, like a resonating bridge or something just goes nuts. And that's what goes to the explosion. We're not exactly sure. And sometimes we see hints of failed supernova. Stars that get turned into neutron stars, the leftover raw cores, without having the big explosion.
We call these core collapse or type two supernova. It's the end of the line for massive stars in our universe. Just like our sun will end up as a planetary nebula and eventually a white dwarf, massive stars almost all the time end up as supernova. The core, what happens to that? It either continues on as a core in which we call it a neutron star. Sometimes it gets ripped apart and there's nothing left. It gets vaporized. And sometimes gravity wins. And in that final moment, that unrush of collapse, there is enough pressure to even overwhelm the degeneracy of neutrons. And it does tip the scale and it does get to collapse all that matter into an infinitely dense point, which is what we call a black hole. This is how black holes are formed in our universe, are through the deaths of massive stars. In these final moments, in that 10 or 15 minutes of catastrophe at the end of a life of a massive star, the amount of energy released is around, in a typical supernova like this, 10 to the 44 joules.
And if 10 to the 44 seems too abstract, that's about the same amount of energy released by the sun over its entire 10 billion year lifetime. The formation of the iron core, the fusion of the iron core takes about a day. The intense pressures converting that into a ball of neutrons takes about 15 minutes and the explosion itself takes about a couple seconds. The source of all that energy, 10 billion years of our sun's output released in those moments is gravity. Gravity drove this entire crazy train. Yeah, there was nuclear fusion and neutrinos and blah, blah, blah. But those processes were set in motion by simple gravity. The processes were driven by gravitational class, by the conversion of gravitational energy into blowing up energy. So are supernova the coolest or hottest things? I would say neither and kind of both. Supernovae obviously are not the coldest thing, taking it literally, the coldest thing in the universe, but they're not the most energetic events. The active galactic nuclei swamp them out in terms of raw energy output.
They are the most intense explosions, but they're also kind of both. You know what? It's not often that a star dies in such spectacular fashion. And when they do, 10 to 44 joules released as photons and neutrinos, you can literally see these stars blow up from the other side of the universe. Which is pretty cool. There's not a lot of things you can see from the other side of the universe. They will outshine galaxies for a couple weeks. Kind of bright. Thank you so much for listening, and especially thanks to all my Patreon contributors and the top ones, Helgi B., Justin G., Justin R., Kevin O., Michael Z., and Chrissy. For this month, who knows what the list will be next month. Go to patreon.com slash pmsutter for more info. Thank you so much for iTunes reviews. Thanks so much for asking me follow-up questions on Twitter and Facebook and on the website. I love the discussions. You can find out more about following me directly on Twitter and Facebook. My name is at Paul Matsar. Or go to YouTube.com slash Paul Matsar.
And, oh, how could I not thank the people who asked the questions at Ian Uprenea and at ThomasPatton1 on Twitter for asking these wonderful questions that led to today's episodes. And I will see you next time for more Complete Knowledge of Time and Space.
How does the universe generate giant magnetic fields? Is it astrophysical or primordial? How do we find them anyway? Why am I not talking about Maxwell? I discuss these questions and more in today’s Ask a Spaceman!
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Hosted by Paul M. Sutter, astrophysicist at The Ohio State University, Chief Scientist at COSI Science Center, and the one and only Agent to the Stars (http://www.pmsutter.com).
EPISODE TRANSCRIPTION (AUTO-GENERATED)
You know what time it is. It's time for Ask a Spaceman. I'm your host, Paul Sutter. You've got questions and I've got answers. You know how this show works. But let me wrap around it one more time. You go online to Twitter or Facebook. Use the hashtag Ask a Spaceman. Send questions to me and I will send answers to you. Fair trade, I would say. You could also go to the website AskASpaceman.com. Go to YouTube.com slash PaulMSutter. Note, that is a slightly different address than it used to be because YouTube... I don't want to talk about YouTube right now, but it's just a different address. YouTube.com slash Paul M. Sutter for all your Ask a Spaceman visual needs. You can also follow me directly on Twitter and Facebook. My name is at Paul Matt Sutter. We have one simple goal with this show. Complete knowledge of time and space. And on the road to complete knowledge of time and space, we have today's questions. We have Chris N. via email. How do galaxies get giant magnetic fields? And we have PE via email.
How are magnetic fields detected in the universe? By the way, before I really dig into the meat of this episode, if you want to be cool, you can't call them magnetic fields. You have to call them B-fields. That's right, B. The letter B or a bumblebee, B fields. We have E fields is short for electric fields, and M was already taken, and B kind of rhymes, so that's nice. So B fields it is. I'm doing this to save you from your own embarrassment. You know I always have your best interests at heart. I don't want you schmoozing at some fancy pants astronomy party and you make a major faux pas by saying magnetic fields. That's way too many syllables. Be cool. Say B fields. Now, for the sake of clarity, I will break my own rule temporarily. And I'm only doing this because I like you. I will become less cool. And I won't use the term bee field throughout the episode unless I feel like it. I will try to stick to magnetic field throughout the episode so that you know what I'm talking about. But just know that that's an educational thing I'm doing so that you know what I'm talking about.
But when you're out there in the real world at your fancy astronomy parties, bee fields. Be cool, bee field. Now, if anyone ever tells you that magnetic fields are boring... then I want you to cut them out of your life forever. I'm not even joking. I don't care if they're a family member. I don't care if they're your best friend. If they say to you, magnetic fields are boring, you're done. You're done. And I know you're a true believer in magnetic fields because you've actually downloaded this episode and you're listening to it right now. And you will be handsomely rewarded for knowing that magnetic fields are not boring. Magnetic fields are always there for you. In physics, you have a complicated process with no clear mechanism? Magnetic fields. Strange observations with no immediate explanation? No. Magnetic fields. Need a source of pressure? Magnetic fields. Need to transfer energy? Magnetic fields. Magnetic fields are the trusty workhorse of astrophysics, and they are everywhere. And maybe that's why they don't get so much love and attention.
Everyone's all about quantum gravity and black holes. No, no, no, no. Bring it back to good old-fashioned magnetic fields. They are so useful. They're the friend that you can always count on without you realizing. You know you have someone in your life If you think back like, wow, that person has always been there for me when I needed it the most. That person is the living embodiment of an astrophysical magnetic field. And if you have such a friend, I want you to go and thank them right now. It's easy to say the words magnetic field. What the heck does that mean? What is a field of magnetism? A field of magnetism? You know, these words make sense alone, but together it's a little bit weird. A field of magnetism. You can visualize a magnetic field. If you take a bar magnet, put it on a table... and then sprinkle some iron filings around it, the bits of iron will squirm around, and then eventually, and you shake them off, you do this whole grade school level science experiment, you see the iron filings will trace out this classic dumbbell shape around the north and south poles of that bar magnet.
So you can visualize what the magnetic field looks like, and here's the trick. To bend your mind this morning, this afternoon, evening, middle of the night, whenever you're listening to this, your mind's about to get bent. The visualization of the field is the field. The visualization, when you sprinkle those iron filings around and you see these lines traced out, that visualization, that picture you made of the field is the magnetic field. Any field, a physical field, tells other objects how to move. In this case, a bar magnet sets up a magnetic field, or more accurately, there's a magnetic field associated with the bar magnet, but whatever. Bar magnet sets up a magnetic field. This field permeates all of space and other bits of metal react to the presence of the magnetic field and do what the magnetic field tell it to do. Like move around or twist or wiggle or whatever. And just because I'm feeling particularly saucy today, I need to tell you that the magnetic field isn't just a mathematical contrivance.
It's not just like a picture. It's not just a way for a bar magnet to communicate its feelings and intentions to bits of iron floating around it. It's a real physical object. The magnetic field works. is a real physical object. And I know maybe that doesn't sound cool, but it sounds pretty awesome to me. And how we know that, how the magnetic field isn't just an artifact of the presence of a magnetized object, it's its own thing, that's another episode. So feel free to ask the question I'd love to dig in. Now, I have a confession to make. Before I go further into astrophysical magnetic fields, I was typing up my notes for this episode. I was putting the picture together and I kind of went on a little tangent about magnetic fields and electric fields and James Clerk Maxwell. And you know how much I adore Maxwell. And I kind of sort of totally wrote an entire episode about the development of electromagnetism. And then by the time I came back to the main topic and wrapped that up, I realized it would be, well, not the longest episode I've ever made, but a pretty long one and one where I didn't do either topic enough justice that they deserved.
So I've taken out all the development of electromagnetism, all the Maxwell stuff out, saved it for another episode, just waiting for you to ask. I'm not going to tell you what to ask, but if you were to email me or on my Twitter feed or Facebook or whatever, if the phrase, say, Paul, could you do an episode on the history of electromagnetism? If that would just happen to pop up, I'm on it. I could have it out in a day. So just keep that in the back of your mind. And if the following presentation, what I'm about to talk about with astrophysical magnetic field seems a little short-winded, I apologize, but I'm going to blame Maxwell. To apologize, I won't do a corny Patreon pitch, except for right now, go to patreon.com slash pmsutter to help support the show. This episode is not about Maxwell as much as I'd like it to be, but I wanted to emphasize the whole point of digging into Maxwell and electromagnetism and all that was to show how magnetic fields show up everywhere. They are a fundamental feature of our universe.
Our universe is made of stuff that stuff is positive and negative charges. They're moving around. If you have positive and negative charges moving around, boom, done. You have a magnetic field. But how do we know that magnetic fields are out there? It's hard to spot. since we don't exactly have iron filings spread around everywhere, which is a real shame. We're working on that. But in the meantime, we have to look for the influence of magnetic fields. And there's a few cool ways that we dig out or infer the existence of magnetic fields out in interstellar space and intergalactic space. One is we look for dust. Sorry, we look for dust. There's dust grains floating around, minding their own business. And some dust grains, if they're spinning, if they're rotating, if they have a little bit of energy, they will tend to align with magnetic field lines. Just like little bits of iron would, it's just little tiny bits of dust. And as they're vibrating, as they're spinning, as they're doing their little dusty thing, they emit radiation.
They can emit microwave radiation. And we can look for that microwave radiation. We can map out the direction that the dust is spinning. And then we can use that to map out radiation. the magnetic field lines. There's another way. If you send an electron shooting around, if you have an electron gun and pew pew, and you shoot out some electrons in a magnetized environment, the electrons will follow a winding corkscrew path around those magnetic field lines. And as it's following, as the electrons are following this winding corkscrew path, it will emit radiation. It will glow. This light is called synchrotron radiation. It has very unique characteristics. We can pick out synchrotron emission from distant objects in the universe. And we see the light, so we recognize it as synchrotron radiation. We can use that to measure the strength and the direction of the magnetic field in that patch of the universe. And we can also use background light. If light passes through a magnetized gas, its polarization starts to twist around.
I need to do a whole episode on light polarization and its uses. That's another great question. But for now, just know that light is electromagnetic radiation. It's waves of electricity and magnetism that kind of wave against each other. And the electricity and the magnetic radiation Fields that are doing the waving to make light are perpendicular to each other. So one's along one line and one's at a perpendicular line and they're both waving. They're both wiggling. And both of these lines are perpendicular to the direction of travel of the light itself. So if you if you take out your hand. And you make one of those finger guns where you're pointing your index finger straight forward and then you got your thumb straight up. So there's two fingers. And if you take your middle finger and poke it straight out of your hand, now you have three fingers poking out and they should all be at right angles to each other. And they should all be perpendicular to each other. So if you take your modified finger gun...
and point it at something, point your index finger at something, then you can say your thumb, if you wiggle your thumb right now, and go ahead and do this in a public place, if you're on the bus or whatever, it's cool. If you wiggle your thumb, that's, say, the electric field wiggling up and down, and then you wiggle your middle finger, careful how you do that, depending on how public a setting you're doing this in, If you wiggle your middle finger, that's the perpendicular magnetic field. And then your pointer finger is the direction the radiation is actually traveling, the direction the light is actually traveling. So you can do this. I'm doing it right now. You can't really hear me doing it, but my finger is traveling forward while my thumb and my middle finger are wiggling back and forth. And that's how I like to imagine electromagnetic radiation. And by convention... By definition, we had to pick some sort of definition for this. Where the angle of the electric field is, whether it's straight up and down or straight left and right or at some jaunty angle, that is what we call the polarization of the radiation.
And if this light passes through a magnetized medium, the polarization actually starts to turn. So if a beam of light is coming through a magnetized environment and say the electric field is pointing straight up and down, it will slowly start to turn. And it might end up after it goes far enough, instead of up and down, be left and right. And then maybe back to up and down and then back to left and right or at some weird angle. That is called Faraday rotation. And this allows us to pick out magnetic fields even when there's no electrons, there's no dust. We can look at the background light. We can look at, say, a galaxy. We can measure the properties of light. We can measure the polarization of light coming off that galaxy. And then look for a very similar galaxy that's, say, sitting behind a galaxy cluster, a much more massive object. Look at the light as it passes through the gas of that galaxy cluster. Galaxy clusters, by the way, are the largest gravitationally bound structures in the universe, host to galaxies, dark matter, and a very hot, thin gas structure.
threading between those galaxies and the light passing through that hot thing gas if that hot thing gas is magnetized the polarization will change and we can measure that and we can figure out the strength in the direction of the magnetic field in that gas in the cluster it's not the easiest method So it's not like we have magnetic maps of the whole universe. The dust maps that we use to map the magnetic field, that's mostly useful in our own galaxy. And then when we're looking out at other galaxies or galaxy clusters, that's when we use synchrotron emission. That's when we use this Faraday rotation of the polarization. So we just have bits and pieces. We do not have a complete census. of magnetic fields in our universe but we have enough data to paint a general picture and that general picture is their magnetic fields all over the place they're incredibly weak a million or a billion times weaker than the earth's magnetic field but they are there and they're measurable Where do we see these magnetic fields? Well, we know some strong ones.
The Earth has a strong magnetic field. Jupiter has a strong magnetic field. The Sun has a strong magnetic field. So we're used to the concept of small and astronomically small here. Objects having magnetic fields. But our galaxy has a large magnetic field. Clusters of galaxies have magnetic fields. This hot thing, gas... that makes up the bulk of the volume of galaxy cluster that the galaxies themselves are just kind of swimming in, itself is magnetized? I guess it's not so surprising to find magnetic fields somewhere, but it's interesting to find large ones, big ones. And I don't mean large as in strong, these are very weak fields, but large as in, you know... Large galaxy clusters of galaxies are a few million light years across and they are filled. With these magnetic fields, if you had a sensitive enough compass, you could wander out. Imagine doing this and how weird it is and how surprising it is. Taking a compass, wandering out into the spaces between galaxies. Your compass would pick out a magnetic field line.
It would wiggle. It would find a north for you. And you could follow those lines for tens of thousands of light years without changing direction. And after tens of thousands of light years, it would pick out a new magnetic field line, and you'd start following that. And you could follow those lines and make your way out to the edge of a cluster of galaxies a million light years away. How in the world... Or should I say, how in the universe did magnetic fields get there and how did they get so big? It's not about the strength, but the coherency. We see magnetic fields threading entire galaxies and we see magnetic fields that are tangled up at this scale of tens of thousands of light years, but are able to maintain coherency and consistency for tens of thousands of light years. We'd expect all pockets of magnetism, but... How do they all agree to point in the same direction? How do they work together to be so consistent? Something fishy is going on. And I should mention, before I dig into the mystery, that the galactic magnetic fields are absolutely gorgeous.
Look up, I'm sure you're able, capable of searching for this. Look up, you know, pictures of the magnetic field of the Milky Way galaxy. There are swoops and whirls. There's flows and curves. It almost looks like a river. It traces out the disk of our galaxy and the spiral structure of our galaxy. But there's knots everywhere. And worlds, there's flows coming off the center. It almost looks like a fingerprint. It's absolutely beautiful. The larger cluster magnetic fields that sit between galaxies are more tangled and messy. They don't seem to follow any particular pattern. And they're not tied to any specific galaxy inside the cluster. And it appears that... We think every galaxy cluster in the universe has a magnetic field, but we haven't seen one in every galaxy cluster because maybe they don't all light up because we can't see the rotation of the Faraday rotation. We don't get to see synchrotron emission. These seem to be rather chance observational events or chance energetic events.
So we think every galaxy cluster in every galaxy has a magnetic field. How does the universe produce such large magnetic fields? When you're faced with a large scale conundrum in the universe, you have two choices, the bottom up approach or the top down approach. The top down approach is maybe magnetic fields have soaked the universe since forever, since the early days of the Big Bang, and they've gradually weakened. They just carry along as galaxies form, as clusters form. They just carry along this primordial magnetic field. Somehow. Or maybe the universe started out completely unmagnetized and magnetic fields started to grow and fill up these gigantic structures. Somehow, you know, since the universe appears lousy with these large magnetic fields, it's positively infested, the natural thing, if you're an astrophysicist, is to assume they're primordial, that they've been there since the early universe. Then you don't have to worry about how they get inside a galaxy, how they grow, how they get strong.
It's just, nope, they've just been there since... Day one. Maybe it's some exotic process. Inflation is pretty crazy. Some weird things can happen in the first second of the universe. Maybe there's cosmic strings vibrating around. Maybe there's exotic symmetry breaking of forces. There's all sorts of contrived, I might say. theoretical avenues for in the crazy, convoluted, complex, messy place that the universe was in the first few hundred thousand years that you can just flood the place with magnetic fields and then boom, you're done. There's magnetic fields seeded in the early universe and we still have fossil magnetic fields today. But we have pictures of the early universe, the cosmic microwave background. And you can ask, hey, if the universe was highly magnetized, what would the cosmic microwave background look like? Would it change anything? The answer is yes. And then we can use observations of the cosmic microwave background, the afterglow pattern of the Big Bang, 300,000 years into the history of the universe.
We can say, was the baby born with magnetic fields? That will affect how it looks in its first day's picture. And we can put limits on it, and it doesn't look like there were strong magnetic fields in the early universe. So... If magnetic fields haven't been there from the start, which would have been great because we could have put a lid on this problem, how did they get there? I think we have a mystery, and the short answer is we don't know, so feel free to just stop the episode now, I guess. But we do have a decent hypothesis. And here's the story, our best guess story, of how the universe got its magnetic fields. Let's assume the early universe was not magnetized at all. Fair enough. We have baby pictures of the universe. It doesn't appear to have strong magnetic fields. Let's just assume it's all clean and clear. And it can become magnetized via, of all things, Ohm's Law. Yeah, Ohm's law. You remember Ohm's law, electricity in a wire, voltage, current, resistance, V equals IR, all that.
That's really only an approximation. There's a bunch of parts in the derivation of the Ohm's law that get left out to generate that approximation. And if you put those back in, you can do some interesting things. There's some interesting consequences. Say you've got a bunch of electrons hanging out. And a bunch of protons hanging out. You've got plasma. And that plasma gets slammed with a shockwave from, I don't know, the first generation of stars blowing up. If that shockwave hits the mix of protons and electrons at an angle, some will move faster than others. They'll separate in a slightly weird way, and they'll produce a current of electricity. You've got charges moving. You've got a magnetic field. This mechanism is called the Biermann battery, and it's a good thing the discoverer was named Biermann. Otherwise, we wouldn't have this cute little alliterative phrase to describe it. Biermann battery battery. where you can get a magnetic field out of nothing at all. The good news is, easy peasy, instant, presto magnetic field.
All you need is some stars blowing up, boom, you get a magnetic field. Bad news, it's incredibly weak. Less than a billionth of a billionth of a gauss. That's less than a billionth of a billionth of the strength of the Earth's magnetic field, which is way weaker. That's a billion times weaker than what we see in galaxies and clusters of galaxies. So we've kind of solved one problem. We were able to generate a magnetic field, which is good, but they're too weak, which is bad. So we need to amplify and spread them. And the answer to making strong magnetic fields is dynamos. We've done episodes on dynamos before. You're all total experts on how dynamos work. Why does the Earth have a strong magnetic field? Dynamos. Why does the Sun have a strong magnetic field? Dynamos. Why does the galaxy have a strong magnetic field? Dynamo? Dynamo. Galaxies are really big, but they spin around every few hundred million years, so they've had plenty of rotation since they formed, enough to get a dynamo action going.
And if you would love to hear the details of how a dynamo works, refer back to the episode on sunspots. So there you go. The same physical process that gives us sunspots is the same physical process that gives galaxies their magnetic fields. Okay, so we've made a little bit of progress. You can go from unmagnetized universe to very slightly kind of sort of magnetized universe. Then you can have galaxies being magnetized. But what about the big boys? What about the clusters of galaxies? Those dudes aren't rotating. So how can we get a magnetic field? The dynamos that are operating... in galaxies can create strong magnetic fields, but not large ones, nothing bigger than a galaxy itself. So it needs to get blown out. How do you get, this is a fundamental question, how do you get a magnetic field that's residing inside of a galaxy blown out to fill out a cluster? Maybe it's from the galaxies themselves, you know, supernovas, there's galactic winds, there's all sorts of stuff leaking out of galaxies.
So maybe they pollute the clusters that they live in. Maybe it's from active galactic nuclei. And I need to do a whole episode on active galactic nuclei, the supermassive black holes in the centers of galaxies that are feeding. They have these massive jets. There's dynamos operating there. There's strong magnetic fields there. Definitely powerful enough. But the question is, can you have one tiny little source, a single black hole that can push enough magnetic field out to fill out a volume a million light years across? seems a little you know that seems stretching it a bit it's hard to tell Because we don't have enough data, we don't have enough maps of the magnetic fields, the large ones at least, in our universe, and we also don't have a lot of theoretical understanding. This is very complicated physics, requiring a lot of computer simulation, and we don't really have a very accurate picture of what happens to these magnetic fields as they're being blown around galaxies and maybe blown out and interacting with turbulence and cosmic rays and supernova.
How do galaxy clusters get these giant magnetic fields? We don't know. We have this very interesting picture where initially the universe was unmagnetized. It got this weak seed field, you know, a few hundred million years in with the first generation of stars. And then this seed field gets collected into small places in small, I mean, galaxy. And the dynamo action spins it up and amplifies it. And then somehow this amplified field gets pushed back out, blown back out into the cluster. That's a... kind of compelling picture, but we're not exactly sure. How do we test it? We need more observations, honestly. We need to get more magnetic field measurements between the clusters. We need to look at the filaments. We need to look at the cosmic voids. Do magnetic fields permeate these parts of the structures? Is it really, you know, is there something happening in the early universe that still partially magnetizes the universe? And so clusters, as they evolve, they just carry along magnetic field.
Or is it from this bottom-up approach where magnetic fields are seeded and amplified in these small, small little pockets and then get blown out to fill out the rest of the clusters? We honestly don't know. There are some preliminary hints that this picture might be correct. But there are so many intricacies and unknowns. I've presented to you, I've talked about one particular thread of thinking. to go from an unmagnetized to a magnetized universe, but there are many, many alternatives. And we're going to have some work to do to tangle it all out, but that's great because that is science. Thank you so much to my Patreon contributors this month, Helgi B., Justin G., Justin R., Kevin O., Michael Z., and Chris Z. Those are the top ones, but thank you to all the Patreon contributors. Go to patreon.com slash pmsutter for more info. And big thanks to Chris N and P.E. for the questions that led to this episode and accidentally... though they had no intention of doing so to another completely different episode about the history of electromagnetism and my best friend, James Clerk Maxwell.
Thank you again for the iTunes reviews, for the donations, for following me on Twitter and Facebook at Paul Matt Sutter. You can also go to the website, askaspaceman.com. Thank you for being you and for being there for me. like a magnetic field is there for an astrophysicist. See you next time for more Complete Knowledge of Time and Space.
What’s the deal with time crystals? Do they break physics as we know it? Or are they merely…neat? What do symmetries and the conservation of energy have to do with it? I discuss these questions and more in today’s Ask a Spaceman!
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Big thanks to my top Patreon supporters this month: Justin G., Kevin O., Justin R., Helge B., Tim R., Michael C., Lars H., Ray S., John F., James L., Mark R., Alan M., David B., and Silvan W.!
Music by Jason Grady and Nick Bain. Thanks to WCBE Radio for hosting the recording session, Greg Mobius for producing, and Cathy Rinella for editing.
Hosted by Paul M. Sutter, astrophysicist at The Ohio State University, Chief Scientist at COSI Science Center, and the one and only Agent to the Stars (http://www.pmsutter.com).
Do black holes exist, or are they merely theoretical constructs? What’s the evidence? And what about the singularity, surely that doesn’t happen, right? I discuss these questions and more in today’s Ask a Spaceman!
Support the show: http://www.patreon.com/pmsutter
All episodes: http://www.AskASpaceman.com
Follow on Twitter: http://www.twitter.com/PaulMattSutter
Like on Facebook: http://www.facebook.com/PaulMattSutter
Watch on YouTube: http://www.youtube.com/c/PaulMattSutter
Keep those questions about space, science, astronomy, astrophysics, physics, and cosmology coming to #AskASpaceman for COMPLETE KNOWLEDGE OF TIME AND SPACE!
Big thanks to my top Patreon supporters this month: Justin G., Kevin O., Justin R., Helge B., Tim R., Michael C., Lars H., Ray S., John F., James L., Mark R., Alan M., David B., and Silvan W.!
Music by Jason Grady and Nick Bain. Thanks to WCBE Radio for hosting the recording session, Greg Mobius for producing, and Cathy Rinella for editing.
Hosted by Paul M. Sutter, astrophysicist at The Ohio State University, Chief Scientist at COSI Science Center, and the one and only Agent to the Stars (http://www.pmsutter.com).