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.