Relativistic beaming

00:00:03
Speaker 1: Hey, extraordinaries. Quick note for today's episode, I want to let you know about my new book, To Aliens Speak Physics. It's all about whether or not we can use physics as a universal language to communicate with aliens, or whether physics is more human than many people imagine. And the book features cute cartoons of aliens from my friend Andy Warner. If you've enjoyed my science outreach and wondered how you could support me, this is how The book is out in November fourth. Look for the link on the book website www dot alienspeakphysics dot com. Okay, on to today's episode. Whenever we look out into the universe, we see something new that astonishes us, something beyond our wildest imaginings of what the universe could do. It's a rich tapestry of extreme physics, and yet all of it is described by physics deep down. The mechanisms that create black holes, or collide galaxies or explode stars are rooted in the basic principles of physics, and so far we've been able to come up with explanations for how these incredible events occur and how the universe is vast and beautiful cosmos is shaped. Today, we're going to dig into the physics that underlies one of the most dramatic and literally brilliant phenomena in space, astrophysical jets. Welcome to Daniel and Kelly's extraordinary, brilliant universe.

00:01:44
Speaker 2: Hello.

00:01:45
Speaker 3: I'm Kelly Waidersmith.

00:01:46
Speaker 4: I study parasites and space, and before looking at today's outline, I didn't know that astrophysical jets was a phrase that's worth it.

00:01:54
Speaker 1: Hi, I'm Daniel. I'm a particle physicist, and if I had a baseball team, I might call them the astrophysical Jets.

00:02:01
Speaker 4: Oh, is that because your brain is fixating on the astros part?

00:02:06
Speaker 1: It just sounds like a team that would score a lot of runs.

00:02:08
Speaker 3: Yeah, yeah, no and throw their balls really fast. I think.

00:02:12
Speaker 4: So, when I was looking through the outline today, it occurred to me that this doesn't feel like it's in your main area of research, and so I was wondering if you could tell us, like, when you are researching something that you don't have knowledge of, like right at your fingertips, what is your process like for preparing our outline?

00:02:31
Speaker 1: Yeah, well, that's fascinating because this actually is sort of right on the edge of my area of research. I'm sort of card carrying particle physicists, which means that most of my career is like, smash particles together at the large had drunk collider, see what new kind of stuff comes out. But it's always been super interested in space, and like many people, I got into it because of astronomy. But then I kind of discovered like astronomy is mostly standing around in the cold looking at fuzzy things through a telescope, and that wasn't as exciting to me. Apologies to us. I mean, there's out there who love that and thank you for doing it, but it wasn't for me. But later in my career I got reinterested in astrophysics, which isn't looking through a telescope, but it's like trying to understand the physics behind what's going on out there, like how does the star work, et cetera. So the last few years I've actually written some papers on the centers of galaxies and neutron stars and supernova and stuff like this, but it is a little bit far from my core area of expertise. So it means reading a lot of.

00:03:28
Speaker 3: Papers, got it. How many are you reading.

00:03:34
Speaker 1: All of the papers all of them.

00:03:36
Speaker 3: Wow.

00:03:37
Speaker 1: I mean what I try to do is find a review in that area, like, find somebody who knows the fields, who's written like a broad perspective, read that really carefully, and then read a bunch of the papers it references to make sure I know what's being summarized and what's actually going on, and what's the sort of the lore all this kind of stuff. But it's a lot of work to try to really understand a new field well enough to try to contribute something to it. And you know, I think you were asking though about like if you want to talk about something on the podcast, not necessarily like write a bunch of papers about it. But I finally it's kind of similar to talk about something on the podcast. You got to understand it really well because my co host is really smart and asks hard questions, and if I want to explain things correctly in a way that actually clicks in people's minds, I got to have it all up in my brain. So yeah, it means reading a lot of papers when it's not my area. Is that your experience? Also?

00:04:28
Speaker 2: Yeah?

00:04:28
Speaker 4: Yeah, when I'm working on my outline, I'm always asking myself wwda, which is what will Daniel ask, trying to figure out, like what other things do I need to research that I am prepared for whatever?

00:04:40
Speaker 1: Daniel asks, Yeah, I feel like everybody in my life who I get to know, I have sort of like a mini version of them in my head, like a little model of them which constantly gets you know, improved and updated and of course the most interesting people, it's never actually correct, which is why they can wonderfully surprise me. But I feel like that's a big part of understanding who's somebody is. It's like comparing them to like the little model you have of who they are and what they might say and how they might react. It's super fun.

00:05:06
Speaker 4: Do the models in here, and then we should change subjects back to what we meant to talk about today.

00:05:11
Speaker 3: But do the models in your head?

00:05:12
Speaker 4: Like, are they like miniature people that you see or are you just imagining their personalities without their bodies?

00:05:19
Speaker 3: Like what do you imagine?

00:05:21
Speaker 1: Are they in the room with me right now?

00:05:23
Speaker 2: Right?

00:05:23
Speaker 5: So?

00:05:24
Speaker 1: I'm not schizophrenic, I'm pretty sure. No, it's just like you, I just asked myself, you know, what would my kids say in this situation? Or how would my wife react to having this for dinner? Or what would Kelly ask me if I said X y Z. You know, I think they're just useful for trying to understand who somebody is. But I also think it's helpful for understanding yourself because you end up building also a model of yourself and turning the inwards and anyway, I have my own Bonker's theory of consciousness, but that's not what today's episode is about.

00:05:52
Speaker 4: So when you sent me the outline for relativistic beaming, I was like, I just have zero clue what this means. We'll give us, like some background what we should expect in this episode.

00:06:03
Speaker 1: Yeah, this episode is about how the universe is constantly surprising us. How every time we look out into space we see something new and weird, something that doesn't quite make sense. And yet if we apply our knowledge of physics, it turns out we can crack it. We can make sense that we can explain why it's happening, and often it gives us an incredible view of what's going on in extreme situations, you know, the cores of galaxies, when things are really hot and dense and fast. But it requires us to put together a lot of little pieces of physics. Gravity electromagnetism, even special relativity, in order to explain what we see out there in the universe. And so today's episode is a story of like several decades, almost a century, of trying to understand some stuff we see out in space and finally putting it together. And the last piece of that is a process called relativistic beaming. And I had a bunch of listeners write to me and ask me about these astrophysical jets that are omitted from the centers of galaxies, and so I thought, let's do a deep dive into all the physics that makes those happen, especially that last bit, which I've never heard anybody cover with a popular treatment. So that was the motivation for today's episode.

00:07:16
Speaker 3: Awesome.

00:07:16
Speaker 4: I'm excited about today's episode because I love those moments where, like, you've studied a bunch of different topics that don't necessarily seem connected, but they turn out to be the building blocks that you need to understand something completely different that you probably wouldn't have been able to understand if you hadn't done all of that background sort of foundational work ahead of time.

00:07:34
Speaker 3: Exactly, So let's learn, all.

00:07:36
Speaker 1: Right, And so relativistic beaming is the last piece of it, but maybe the least well known. So I decided to go out there and ask our audience if they knew anything about relativistic beaming to help us calibrate. If you would like to participate for our future episodes, please don't be shy. Write to us questions at Danielankelly dot org. In the meantime, ask yourself, do you know what relativistic beaming is? Here's what our listeners had to say.

00:08:02
Speaker 5: Relativistic beaming.

00:08:03
Speaker 2: I think that's when Chapman from the Yankees beams you in the head by accident, and if that happens with one of his fastballs, and then when that happens, time will definitely go slower for you.

00:08:15
Speaker 5: So relativistic relativistic beaming, well.

00:08:19
Speaker 6: I can't remember off the top of my head. I think it was to do with synchotrons. When electrons are going near the speed of light and go around a corner. The radiation they release very is in a very tightly controlled spatial beam because of relativistic effects.

00:08:35
Speaker 5: Relativistic beaming is the ability of a thought to enter my mind and then instantly disappear as soon as I try to act on it. But in physics, maybe it's something to do with moving near massless particles near the speed of light and taking advantage of some of the relativistic changes that occur as a result.

00:08:55
Speaker 4: I think my favorite answer, I mean, they were all great, but was how a thought enters my mind and then it's disappears. Relativistic beaming in that sense happens to me like fifty times a day lately.

00:09:05
Speaker 1: You have that experience where you have an idea and then you try to write it down before it leaves your brain, and sometimes it's like no pencil or paper, you can't get to your phone, and you're like, oh, no, it's gonna go away.

00:09:15
Speaker 4: Yeah, it does, it go away, because it does for me a lot of the time. Or are you just scared?

00:09:19
Speaker 3: But it stays?

00:09:20
Speaker 1: We does, okay, No, And then sometimes I have like the remnants of the idea. I'm like, I remember feeling this way about it and it was something about that and what was it?

00:09:29
Speaker 4: And man, yep, no, I all think to myself, don't get distracted, don't get distracted while I'm looking for the pencil, and then I always think about, like, oh, did we make it as lunch this morning? And then it's gone and anyway, Okay, so the thought that we do not want to forget is what is relativistic beaming.

00:09:45
Speaker 1: That's right. And the story starts with the things shooting out of the centers of galaxies. These things called astrophysical jets. And if you have a mental image of a galaxy, you're probably imagining something like a disc. You've got a bunch of stars a swirl together, and that's what the Milky Way looks like, and that's what Andromeda looks like. But there's another really important feature of galaxies that's not always visible to the naked eye, and these are the jets that shoot up and down from the poles of the galaxy, out of the center. So instead of just imagining a disk, imagine a huge beam of light beaming up and down relative to the plane of the disk. These are astrophysical jets.

00:10:26
Speaker 3: Okay.

00:10:26
Speaker 4: And so the jet is made of light. And just so I make sure i'm picturing thing because you said photons, right.

00:10:32
Speaker 1: Well, the jet has light in it. They are bright, but they're actually not just made of light. They're mostly plasma so they're like high speed particles. There's electrons, there's protons, and there are photons as well.

00:10:43
Speaker 4: Wow, okay, so I just want to make sure that I've got my image of a galaxy correct. So at the center of our solar system there's the Sun, but at the center of a galaxy there isn't necessarily some big thing. It's just what is at the center of a galaxy. We talked about it maybe being a black hole, but we don't really know right well.

00:10:59
Speaker 1: There are lot of questions about what's at the center of the galaxy because it's hard to see it's so dense there. You know, I think a lot of people have the image of a galaxy as like just a bunch of stars sprinkled around, But there's a big density variation, Like the center of the galaxy is much denser than the outskirts. It's sort of like, you know, there's Manhattan and then there's the suburbs, and then there's the excerbs, and we live kind of in the suburbs. It's not very dense, but it's not as rural as it is further out in the galaxy. But near the center there's a lot of stars and there's a lot of gas and dust, so it is difficult to study the center of the galaxy. But we do know a lot about the centers of the galaxies also by looking at other galaxies, and so far, every galaxy we've studied has a super massive black hole at its center, with a couple of exceptions in cases where we're like pretty sure the super massive black hole has been ejected by like a recent collision or something.

00:11:50
Speaker 3: You can eject a super massive black hole.

00:11:53
Speaker 4: We've probably talked about this on a prior episode and my bad memory is why life is so endlessly surprising to me?

00:11:59
Speaker 1: But whoa yeah, yeah, Well, when galaxies merge, what happens is the center's merge. It takes a long long time and then the black holes merge, but not always. And if you have like three galaxies merging at the same time, two of them can work together and eject the third one. And so yeah, gravitational kicks can eject the super massive black hole from the core. It's not something we understand super well. We do think that there are super massive black holes at the cores of these galaxies. So imagine a very very dense center of the galaxy and then fewer stars as you move away from the center, and then from the center shooting up and down. Are these massive astrophysical jets. They can go for like hundreds of thousands of light years.

00:12:39
Speaker 3: Wow.

00:12:39
Speaker 4: But they're not necessarily coming from the super massive black hole, but just from like the general center.

00:12:45
Speaker 1: We're going to dig into that later in the episode. But we think they are connected. Yes, And this was one of the central puzzles of astrophysical jets and continues to be like what exactly is powering them? It's a really fun question.

00:12:56
Speaker 4: Okay, so stuff is shooting out, is it just kind of like trickling out or is it moving really fast?

00:13:02
Speaker 1: These are some of the fastest things in the universe. Like, these jets are shooting particles out, often very close to the speed of light. Like the energy of particles in these jets is often much higher than energy of particles in our experiments here on Earth, like the Large Hadron Collider, we accelerate particles to have an energy of like around five to seven tarra electron volts. That's trillions of electron volts. But these galactic centers can accelerate particles to much higher energies, which connects to lots of fascinating mysteries, like we see super high energy particles arriving on Earth and we don't understand where they come from. And one theory is that they're being kicked to super high energy by these galactic accelerators. Essentially that these centers of galaxies are like enormous guns shooting out particles at super high energies.

00:13:53
Speaker 3: But then you so you shoot the particles out, and then where do they go? What happens to them?

00:14:00
Speaker 1: Well, if you believe that it's natural, right, and there's really fun theories out there about how like maybe aliens have megastructures and they're engineering the centers of galaxies to do particle physics experiments. That would be awesome two galaxies pointing at each other. But in general, they just shoot out into the universe. And you can see these astrophysical jets if you look at it in the right spectrum, really beautiful. You should google these images. They're spectacular. Often these things are bigger than the galaxies themselves. They just shoot out into the universe and then you know there are magnetic fields out there in the universe, and these are mostly charged particles and so they bend and they fly around. And one reason why we like to study the cosmic rays, the super high energy cosmic raise is that they are less bent than the other particles. So you want to know where something came from. If it's like gotten bent and zipped around and changed direction one hundred times, it's hard to tell. But if it's come mostly straight at you, and high energy particles get less bent by magnetic fields, then it's easier to sort of point back in the sky and say where it came from. Reason why we look for the super high energy particles because they tend to point back to their source more than lower energy particles. Yeah, very cool.

00:15:07
Speaker 3: Okay, so they're going really fast. It's a lot of different stuff.

00:15:11
Speaker 4: Is this like a narrow beam or is this like pretty wide and spread out.

00:15:15
Speaker 1: It's pretty narrow like when they were first discovered, And we'll dig into that. These are little sources like in the sky. They're pretty small, which is one reason why it was such a puzzle. And it's sort of amazing. And you know, you have these galaxies and they're emitting these things up and down the north and south pole from the center and it's fascinating because it's not shot out by the super massive black hole itself. Obviously, black holes do not emit photons. It's not like, you know, black holes are shooting particles out into space or anything like that. But the environment the black hole creates, and in general, the environment of the galaxy might be the thing that's powering these particles and creating these beams.

00:15:56
Speaker 3: Awesome.

00:15:57
Speaker 4: So we've gotten to the what, and now we're going to take a break, and then we'll get to the why. Why do you get astrophysical jets? Not the baseball team? All right, we're back. We've described astrophysical jets, and now Daniel's going to help us understand why you get these bursts of loads of different kinds of things coming out of the center of galaxies.

00:16:36
Speaker 1: Yeah, and I think a fun way to attack this is to take us through the history in the last century of people trying to understand them and putting together lots of different things simultaneously. Because astrophysical jets are connected to something else we've probably heard about, which are quasars. Let's put astrophysical jets in our pocket for a minute and talk about the history of quasars, why they were confusing, how we understand them, and then how they come back together to help us understand astrophysical jets.

00:17:04
Speaker 3: Let's get into the quasar question.

00:17:07
Speaker 1: So quasars are fun because, like the word itself is like sounds super cool and science fiction. And they're name because we didn't understand what they were, right. So quasars are short for quasi stellar objects. And they were first found back in the nineteen fifties when we didn't really understand a lot about the galaxy. Oh it's only like twenty or thirty years since we understood that there were other galaxies out there in the universe, and we started studying these things they called nebula that turned out to be other distant galaxies. And then in parallel, we had the birth of radio astronomy. You know, astronomy used to be just in the optical like you look through a telescope and you see, what do I see out there? And let's make a map? But radio and World War two led to the advent of radio telescopes. People listening to the sky in the radio and what they found were these radio sources in the nineteen fifties where they couldn't find any optical objects as well. They were like hmm, there's something out there that's emitting in the radio, but the telescopes at the time couldn't see anything there in the visible so they're like, what are these things?

00:18:10
Speaker 4: So then are they quasi objects because they're like, maybe that's from space. I don't know, maybe we messed something up, like why, yeah, why the quasi part?

00:18:18
Speaker 1: Yeah exactly. They didn't understand what they were, so they're like, maybe there's something like stars because they're sort of localized, but there's no visible object there initially, so it was a real mystery at first, which is why I think they went for like, let's give this kind of a fuzzy name so we don't know, we don't paint ourselves in the corner, which hey, maybe that was wise.

00:18:37
Speaker 4: Oh yeah, so maybe after they figured out what it was, they should have called it like straight up stellar objects, like we're totally sure about this.

00:18:43
Speaker 1: But they're not stars, right, they're not stars, So it was good that they left.

00:18:47
Speaker 3: That fuzz Oh so stellar means star, not just like spacey.

00:18:50
Speaker 1: Yes, exactly, like a star.

00:18:53
Speaker 3: Yeah, yeah, Kelly is really good with words.

00:18:56
Speaker 1: All right, Moving on, and they knew it was that they had a very small angular size, right, like they could tune the radio telescopes in a certain direction and see it and then turn it a little further away and not see it right, and so you could tell that this thing was coming from a localized spot in the sky. And by the sixties there were like hundreds of these things had been cataloged, and so people started a dedicated search for like, let's look to see if we can find something that corresponds to them. And then in the early sixties they finally found one. They found like a what looked like a faint star right at the location of the radio source, but the spectrum of it didn't really make sense. It was very confusing to understand, like what this object was.

00:19:38
Speaker 3: What was it? It was a quasar? Then what is a quasar?

00:19:42
Speaker 1: And what is a quasar? That was the question, right, So first thing they just look at the spectrum and they noticed, like the spectrum implies that it's really really really red shifted, meaning that it's really really far away. Because remember, as we look out into space, we're not just looking back in time. We're looking at things moving away from us. Hubble's big discovery was that the further away something is the faster it seems to be moving away from us. And so if you find something which is super red shifted, meaning it's moving away from us very fast, that also means it's super distant. And that was one of the early puzzles. It's like, Okay, we're seeing this star. It's not super bright, but it's crazy far away, which means that at its source it's got to be like insanely bright for us to see it. These things were apparently like most of the way across the universe, and yet somehow we were still seeing them. So people were like, wow, what this doesn't make sense if our calculations are correct. There's an incredible source of energy being shot at us from across the universe. How can that be right?

00:20:44
Speaker 3: Yeah? How can that be right? It's like a murder mystery. I'm waiting for the end.

00:20:49
Speaker 1: Yeah, exactly. And so you have this combination of like extreme velocity and distance, yet we're able to see it. And this implies some intense source of power, right, you need something in order to generate something to make these things brighter. Essentially, it would have to be like a thousand times brighter than the entire Milky Way for us to see it across the universe, so we're talking like three billion light years away, and so the early explanation was that you have an active galactic nuclei. So the centers of the galaxies are not just like here's a bunch of stars all swirling around and they're just sort of denser than they are out here, but that there's something else going on. And this is essentially at the same time as we're starting to understand, hey, are black holes are real thing. They're not just like a calculation that Einstein and his friends did, are like an actual thing out there in the universe. And so this all came together into this cohesive explanation that the center of the galaxy is very dense gravitationally, and you have these super massive black holes at their cores, and the gravity of that super massive black hole combined with its magnetic field, is generating these astrophysical jets.

00:22:02
Speaker 4: Okay, so a quasar is the center of a galaxy where you have a black hole and all of that denseness and stuff is shooting out.

00:22:08
Speaker 1: Yes, exactly as seen from Earth. So like you could describe it in several ways. You could say you have an active galactic nucleus, which is generating these jets from Earth. If you see it, you call it a quasar, and so it's sort of the union of these things as seen from different angles.

00:22:23
Speaker 3: So does every galaxy have a quasar?

00:22:26
Speaker 1: No, every galaxy does not necessarily have a quasar. It's fascinating. Not every galaxy has an active nucleus, right, It's really interesting.

00:22:35
Speaker 4: And is that because not every galaxy has a black hole at its center? Maybe, or this is a different reason.

00:22:40
Speaker 1: We don't know. It's a real mystery. It seems like the universe made a lot of quasars about ten billion years ago, and since then it hasn't been making very many of them. So like that's why most of them are far away, Like there aren't quasars that are like quasaring right now nearby, Like the Andromeda doesn't have a quake, so Heart doesn't have massive bright pulls of plasma shooting out from both sides of it. You have to look further away, which implies it was in the past. So there was something in the conditions of the universe ten billion years ago which was really quasari, and it's no longer. Is the universe very quasari.

00:23:17
Speaker 4: It's like the fashion of the universe, like neon leggings were big in the eighties and quasars were big at someway, but both went out of style. Yeah exactly, but the leggings are coming back. Maybe quasars come.

00:23:28
Speaker 1: There nostalgia for the early universe. I don't know. These things are dangerous, right, so it's crazy. The oldest quasar we've seen comes from a galaxy we visualized six hundred and ninety million years after the Big Bang, so that's less than a billion years after you know, the first atoms are formed, and then finally stars are formed, and then galaxies come together, and already you have the conditions necessary to create a quasar. And this is one of the bigger puzzles in the early universe recently, is like how things got so big so fast? You know, how did you get super massive black holes at the centers of galaxies so quickly after the beginning of the universe. Our simulations can't explain that. James Webspace Telescope, which looks in their infrared, can see super distant, super old stuff and it's visualized galactic formation that nobody understands either. Like very early in the universe, you have galaxies that are much bigger than anything we can understand. So this is a core question, and it's like, how does stuff come together and form structure in the early universe so quickly. There's definitely an element there that we don't understand in Quasars are a big part of that. Why did they come up in the early universe? Why aren't they making them anymore?

00:24:41
Speaker 4: Why did they go out of style like the fashion in the eighties, which I would say was a really good era. So okay, so the jets are different than the quasars. The jets come out of the quasars, and so why are the quasars making the jets?

00:24:53
Speaker 1: Yeah, I would say quasars are the thing we see from Earth, right, Okay, the jets are the sort of underlying physical process says that generates our observation of the quasar. But you're right, it's a good question. What is making these jets. They're super bright, they're super intense. We think they're powered by this active galactic nuclei and fundamentally the black holes at their cores. Right, these are super massive black holes. And remember black holes come in two categories. There's like there's a star that burned up all of its fuel, and the fusion is no longer providing pressure to keep that star puffed up, and so the gravity eventually wins and it collapses and you get a black hole that's going to give you a black hole up to like fifty eighty maybe one hundred times the mass of our Sun. But the black holes are the centers of galaxies. These things are like millions or billions of times the mass of the Sun. So definitely not the collapse of an individual star, and again a big mystery as to how they form. But they're enormously massive, and they're at the centers of these galaxies, and we think that the gravitational energy of these black holes, as well as their spin, is what's powering these astrophysical jets.

00:26:00
Speaker 4: All right, So as a biologist, yeah, it's counterintuitive to me because I feel like the main thing I know about black holes is that they like suck things in if it gets close enough.

00:26:09
Speaker 3: Yeah, and so now we're talking.

00:26:10
Speaker 4: About stuff getting seemingly spit out of black holes.

00:26:14
Speaker 3: So what bridge the gap there for me?

00:26:16
Speaker 1: Yeah, it's a good question. And remember that black holes are super gravitational and powerful, but they're not magical right there, just like suck everything in. You can, for example, orbit a black hole the way you can orbit the Sun, because a gravity from a black hole is just gravity. So if you're at the right velocity and at the right radius, you can orbit a black hole forever. It's not going to magically suck you in. So let's think about how black holes are the centers of galaxies work. Well, you have the black hole, then you have stuff swirling around it, right, so that stuff could in principle stay in orbit forever. So just because it's near the black hole doesn't mean it's going to get sucked in necessarily. And that's why you have like this accretion disc. If you think about your image of a black hole, it's not just like a black sphere. It's like a disc of stuff that's orbiting around it. That's the stuff that's like on deck for going into the black hole. Hasn't fallen in yet, all right, So why does it actually fall in. It falls in because there's friction. Like if you're just orbiting a black hole or a star, you could do that forever. But if you and ten trillion of your friends are all orbiting a star, you're gonna bump into each other, and occasionally somebody's gonna get nudged into the center right out of orbit. That's what an accretion disk is. It's like a huge cloud of gas and dust and little bits, and there's friction between them. They bump into each other, they pull on each other gravitationally, so some of the stuff falls in. All right, So particles are now falling in towards the black hole. But black holes also have magnetic fields. They're not just gravitational objects. And what do magnetic fields do? They bend the path of charge particles. Think about particles coming from the Sun towards the Earth. What happens to them? They don't just hit us down here on the Earth. They get deflected by the magnetic field, and they get deflected towards the North pole and towards the South pole, and some of them, because the magnetic field is different at the north pole, go in at the atmosphere there, which is what causes the Northern lights or the Southern lights. Those are super high energy particles from space hitting the atmosphere and then glowing gorgeous. Yeah, and so just like the Earth has gravity but also has a magnetic field. Particles falling into the blank hole will get deflected up towards the north pole or the south pole because of the super intense magnetic field, and so they get sped up by the gravitational field and then they get bent by the magnetic field and then shot up or down the poles.

00:28:37
Speaker 3: Ah.

00:28:37
Speaker 4: Okay, so everything that's getting shot out then has some charge.

00:28:41
Speaker 1: Yes, almost everything here is going to be charged, because it's very hard to stay neutral. Like if you have protons and electrons and they're in hydrogen atom, the energy of that bond is tiny compared to the energy of these particles, and so they're just going to blow apart. Right, So basically everything is plasma here. Everything is charged, and so you get plasma. They gets sho up and down the north and south poles, and plasma itself glows. So where do the photons come from? Right? Photons come from these particles getting bent by the magnetic fields. Because how does a charge particle change direction? Every time a charge particle change direction, it emits a photon. That's the only way you can do it. Can't just be like I'm going this way, now I'm going that way. And the way it does it is by emitting a photon. It's like I'm going straight, I'm gonna go right. I'm going to admit a photon for the left. Therefore I'm going to recoil against it to the right. Just like if you're flying through space and you want to change direction, what do you do? You fire your rockets and you shoot some stuff out in the opposite direction that you want to go. Charge particles have to do that also, and they shoot out photons. They have like an infinite supply of photons inside of them. You can imagine. They don't literally have all those photons. They just you know, dump some of their energy into the electromagnetic field with momentum the opposite direction that they need to go, and so then they go that way. And there's nobody like driving these particles. I'm making them sound like they're making these decisions, but this is just the process anyway. So stuff falls in towards the center gets routed towards the poles using a magnetic field, and that's how you get these jets. And the power comes from the gravitational energy of the black hole.

00:30:15
Speaker 4: So the centers of galaxies that have astrophysical jets are only the centers of galaxies that have a black hole in the middle. Is that fair to say because you need the black hole's magnetic field.

00:30:27
Speaker 1: Yeah, exactly, And that's because almost every single galaxy has a super massive black hole at its heart. OK, So that's pretty safe to say anyway.

00:30:36
Speaker 3: And do all black holes make these jets?

00:30:38
Speaker 1: Yeah, that's a great question. And so some of these active galactic nuclei have all the conditions we think for making a jet, but they don't have jets. Right, there's a big, powerful black hole there, there's a swirling mass of gas and dust and particles, but there's no jet. And so, as you said earlier, this is not something we currently understand, and there's a lot of current research on like understanding the magnetic environment around a black hole, because it's fun to even think about, Like, well, why do black holes have magnetic fields? Anyway? Aren't they gravitational objects?

00:31:10
Speaker 3: Yeah?

00:31:11
Speaker 2: Why?

00:31:13
Speaker 1: Black holes are really weird things. And there's only three things that they can have. They can have a mass, right, you can put stuff into a black hole and it grows, so it's mass increases. But they can also have an electric charge, right, Like what happens if you have a black hole and it's neutral. And you drop an electron into a black hole, well, now it has a charge. Because the universe serves electric charge. It can't just eat the electric charge and then boom, it's gone. You've deleted it from the universe. The universe conserves electric charge. So if you drop an electron into a black hole, we don't know what happens to the electron. Is it's still an electron, whatever, is it something else? But we do know that that charge is now added to the event horizon. So we don't know what's going on inside the black hole, but you can think of the event horizon itself now as charged.

00:32:00
Speaker 4: So do most black holes have a negative or a positive charge? Or is it like split fifty to fifty? What is the predominant charge of black holes?

00:32:07
Speaker 1: Yeah, great question. Most black holes have either a positive or a negative charge. It's about split fifty to fifty. We think we haven't measured this in detail for any black holes. But you know, imagine that you randomly throw in positive and negative particles and you do it a billion times. In order for the black hole to be neutral, those would have to be exactly equal. It's like flipping a coin a trillion times and getting exactly half of them to be heads and exactly half of them to be tailed very very unlikely. So we think there probably are no electrically neutral black holes in the universe. Probably every black hole out there has some charge because it's eaten positives and negatives, and the chances that they add up exactly to zero basically zero. So now you have a black hole has gravity, has mass, and it has electric charge. There's one more thing black holes can have, which is spin, because the universe also conserve angular momentum. Right, momentum is just like if you're moving through space. Newton tells us that you can't just move without something pushing you or this conservation of momentum. Well, spin is also conserved. If you like set something spinning in space, it'll spin forever. The only way to stop is to come with some external torque. Same thing is true if you drop something into a black hole instead of just dropping an object straight in imagine if you drop an object so it hits the black hole like sort of near the edge, sort of like spinning a bicycle wheel right, pushing on the edge to make it spin rather than poking it in the center. Same thing. If you drop an object into a black hole, you can make it spin, and it conserves that spin. The universe can't just get rid of angular momentum. So now you have an object which has an electric charge and it has to spin. What happens with that?

00:33:48
Speaker 3: You've got a magnetic field.

00:33:50
Speaker 1: Boom, Oh my gosh, congratulations exactly. And because magnetic fields are not generated by magnetic charges, we don't know if monopoles exist in the universe. Right, the only way to make a magnetic field in our universe is to combine electric charges and motion. So currents generated magnetic fields. Spinning objects generated magnetic fields. So black holes have magnetic fields. And this is fun to think about. The way I visualize it is that I put the charge and spin on the event horizon. We don't know what's going on inside, so you don't have to worry about like, how is the information getting from inside the black hole to the outside. Just imagine a spinning sphere of charge and that generates a magnetic field. The event horizon is conceptually similar to that.

00:34:37
Speaker 4: Okay, so all black holes. It sounds like all black holes should have magnetic fields, because probably none of them are neutral. Exactly, but they don't all make the jets. Is that because some of them have stronger magnetic fields than others or we really don't know.

00:34:52
Speaker 1: We really don't know. Yeah, it's a mystery active galactic nuclei hot area of research and definitely not something we understand. You know, when they do this, it's really dramatic. One other way to study these things is not to look for the active galactic nuclei, but to try to study the black holes and their magnetic fields in more detail. And so, for example, we have image black holes, a couple of them, right, Remember these pictures that look like a Crispy Kreme donut of the accretion disk around a black hole. Super awesome from the event Horizon telescope. That's the stuff that's swirling around the black hole, waiting on deck maybe to go in. Well, a few years after they released that image, they released a follow up image where they studied the magnetic field lines near the black hole. By looking at the polarization of the photons that come from different parts of the accretion disc, they can understand the magnetic fields and polarization of photons is kind of weird. It's because photons are vector objects. They're not just like a location in space. They also have like a direction, and so they can essentially spin as they move. And so photons have like this little vector extra vector that you can measure. We don't have to dick into the details now, but we are studying the polarization of these photons that come from the vicinity of black holes and trying to understand which models of magnetic fields in the vicinity of black holes make the most sense best agree with the data with what we see out there in the universe.

00:36:18
Speaker 4: Cool, all right, So let's take a break. And we've talked about astrophysical jets, and I realized that in my head, I've decided that that is what relativistic beaming means.

00:36:28
Speaker 3: But maybe that's not actually true. So let Daniel shake in his head.

00:36:31
Speaker 4: No, so let's clear up Kelly's misconceptions after the break. All right, So, astro physical jets are those things that get shot out of quasars that have black holes, but not all of them.

00:37:02
Speaker 3: And I thought that.

00:37:03
Speaker 4: This resulted in relativistic beaming, like the beam that comes out of the center is But no, you.

00:37:09
Speaker 3: Shook your head. No, So what is relativistic beaming Daniel.

00:37:13
Speaker 1: Yes, so we have these astrophysical jets. We understand something about how they're made. They're extraordinarily powerful. We used to call them quasars when we saw them in the sky, and we see them all over the universe. There's lots of them that we spotted. We've I think identified seven hundred and fifty thousand different quasars in the umbers, which is a lot. Most of them are not pointed at us right, so we can see them even if they're not pointed at us. And the most dramatic pictures you'll see online are ones we see sort of from the side. We can see them from the side because they emit photons. Also from the side, they hit each other and they glow, et cetera, et cetera. But the brightest ones are the ones pointed right at us. Like if there's a galaxy out there that's oriented perfectly, so we're looking exactly at the plane of the galaxy and the core of the nuclei is pointed like directly at the Earth, then those particles are shooting exactly towards us when they're emitted from the galaxy, and then they benefit from a super awesome extra special boost that makes that galactic core even brighter than it otherwise would be. And that's relativistic beaming.

00:38:21
Speaker 4: Ah, okay, So it's not brighter just because it's pointing at us, making it easier to see.

00:38:25
Speaker 3: It's brighter for some other reason.

00:38:27
Speaker 1: That's right. It is brighter because it's pointing at us, and that makes it easier to see. But plus it gets souped up because of this relativistic effect, which you could also consider to just be like the relativistic version of the Doppler effect. Right, anything that's moving towards you is going to get blue shifted. Anything moving away from you is going to get red shifted. And it's easy to understand that. It's something we experience every day. If you hear a police car drive by you, you hear the sound that its siren makes changes as it passes you. Right, when it's approaching, it's a higher sound. When it's moving away, it's a lower sound. Why is that. It's because the wavelength gets shifted to longer wavelengths when it's moving away from you. Right. If you just imagine, like a source moving away from you, it's going to draw out longer wavelengths than a source moving towards you, and so each wavelength is a little bit shorter. That's a generic Doppler effect. Things moving away from us are red shifted, which is also how we can infer distance because there's a relationship between red shift and distance. In the universe, things moving towards us are blue shifted, and we see this in the sky like not everything in the sky is moving away from us, Andrameda, for example, is overcoming the expansion of the universe and local gravity is pulling it towards us. So Andromeda in the sky is blue shifted, not red shifted. Okay, so that's the Doppler effect, which is something fairly well known. But special relativity changes everything. Right, normal Doppler effect is what happens when things are pretty slow and not moving super fast, like the way I move here on Earth. Right, especially now that I'm fifty years old.

00:39:57
Speaker 3: You're a spry fifty Daniel.

00:40:00
Speaker 1: Thank you. Astro Physically speaking, I'm quite yes, these quasars make me feel like a spring chicken. But when relativity comes into play, things change, and astrophysical jets are moving near the speed of light relative to us, and so they benefit from the relativistic Doppler effect, which super enhances the brightness in the direction of motion the energy of these things in the direction of motion. For two reasons.

00:40:29
Speaker 7: Reason one and reason number one is our old friend, length contraction, and the rule of thumb for length contraction is moving objects seem shorter.

00:40:41
Speaker 1: So if you're just like looking at a ruler and it's sitting next to you, you measure it's a meter long. If instead it's zooming towards you at nine tenths the speed of light, and then you measure it, you're not going to measure it to be a meter long. You're going to measure it to be less than a meter. Moving objects are shorter, and that's a super fun and mind bending consequence special relativity which I love thinking about. And people often ask me like, well, why is it shorter? And you know, I think that's a really revealing question because the answer is it only actually makes sense for it to be shorter in the universe, where the speed of light is fixed for all observers, it has to be shorter. It wouldn't actually make sense to measure that meter stick as a meter if it's moving. But our intuition is that speed shouldn't change the length of things. Kelly thinks, Oh, and my daughter's running across the yard. She's the same person, in the same size as she was when she was standing still, and mostly she is almost she is you can't tell, which is what gives us this intuitive feeling that length shouldn't depend on velocity. But we're wrong. It actually does. There's no good reason why length shouldn't depend on velocity. So this question, like why does length depend on velocity, reveals again just our bias towards things we find intuitive. If I told you, oh, the length doesn't depend on velocity, you wouldn't ask me why. You should just be like, yeah, yeah, cool. Anyway, that's a digression on the special relativity. But in this case, what's happening is the thing is shooting right at us, moving very very high speeds. Right. So from the point of view of that object, the distance between it and the Earth is contracted, right because it sees the Earth moving towards it at really high speeds, right, So we're seeing it as if it was closer, right. So relativity is like shrinking the distance between us and the center of this galaxy. This furnace where the black hole is shooting bullets at us at super high speed.

00:42:35
Speaker 3: It's bringing us closer to that, and that's what makes it brighter.

00:42:38
Speaker 1: That's one of the things that makes it brighter. It's reason number one. And that's why it's called relativistic beaming because it's like, the relativistic Doppler effect is making this much brighter if it's pointed at you.

00:42:49
Speaker 4: That's amazing that we figured that out, because you'd look out at the sky and you'd be like, some quasers are brighter than the other.

00:42:54
Speaker 3: But like to account for that anyway, go.

00:42:56
Speaker 1: Humans, And that's why I started this episode with, like, to understand this, you got to understand gravity of black holes, you got to understand magnetism of the bending, and then you've got to bring in the relativity to show why these things are so bright.

00:43:08
Speaker 3: So many blocks, all right? Reason two.

00:43:12
Speaker 1: Reason number two is the other fun bit of special relativity, which is time dilation. Right, So special relativity tells us that moving objects look shorter, but also that moving clocks run more slowly. Right, And so what's happening when you look at Equasar is relativity changes the frequency of these things. Right, We talked about how you go from red shift or blue shift depending on the velocity. Well, changing the color, changing the frequency also changes the energy, right, And so if these things are blue shifted, that makes them more energetic. So the particles are not just pointed at us, the moving at us at very high speed, and relativity boosts that to make them have more energy in our frame. And that's a confusing thing to think about, like how does relativity give something more energy? Remember that energy is conserved in a static universe. It's not actually conserved in our expanding universe, but it's not invariant, meaning like I can measure the energy of something and you can measure the energy to be different. If your daughter is running past you on the lawn, you measured her to have a certain velocity a certain kinetic energy. If your husband is running next to her, he says, no, she's not moving at all. She has no energy. So you two can disagree on how much energy she has. Because energy is frame dependent, we think it's conserved in a static universe, but it's not invariant, which often leads to confusion. So you and I can disagree about how much energy something has, and the energy of these astrophysical jest depends on the observer because energy is frame dependent. It's relative, it's not an absolute quantity.

00:44:45
Speaker 4: Okay, Okay, So while you were describing this, I realize that. So we're talking about charged particles moving super fast towards us. Is this s galactic cosmic radiation? Is this what the astronauts have to worry about?

00:44:57
Speaker 1: This is one source of that. Absolutely. Yeah. And when you're out there in space near the ISS, for example, this is one of those elements. You're absolutely right. It's a dangerous environment. And that partially comes from the Sun, and partially comes from inside our galaxy, and partially comes from other galaxies. We think the highest energy ones come from the centers of other galaxies.

00:45:18
Speaker 4: WHOA, Okay, so I should be saying that galactic cosmic radiation comes from quasars.

00:45:23
Speaker 3: No from astrophysical.

00:45:24
Speaker 1: Jets or quasars. Yeah, either one works, okay.

00:45:27
Speaker 3: And they're super bright because of relativistic beaming.

00:45:30
Speaker 1: Yes, absolutely, they're super bright even without the relativistic beaming. But then they're super double extra bright because of the relativistic beaming. The ones that are pointed right at us gets super enhanced because of these relativistic effects. So it's this incredible dance of all these pieces of physics, and it took us decades to put this all together, and so many different branches of physics and so many different historical traditions came together for us to like start to understand a coherent picture of what's going on inside galaxies and such an important thread in science, you know, is understanding things from different perspectives and like making sure the story you're telling is coherent when you come at it from different angles, and that's often how we unravel mysteries. Right, We're like, well, this seems to work, but wait, what about this piece? If I measure differently or if I come out from this angle, it's not making sense. Because we think, we hope the universe does make sense and that there is a story out there that we can unravel no matter how you look at it.

00:46:26
Speaker 4: And I really love human story, So let me tell an astronaut story really quick. So when I was reading astronaut memoirs, there's a lot of times where they'll talk about like being in space and then like a flash of light, it's like it passes through their eyeballs and they were kind of not sure what it was. It's probably kind of a scary experience. And I think that the main hypothesis to explain what's happening is that galactic cosmic radiation is passing through your eyeballs and it like lights up, you know, the receptors in your eye, and that's what you see. It's kind of scary and yeah, one super scary too. Kind of amazing though, to think that.

00:47:03
Speaker 3: Your vision is being impacted by something happening in a quasar and a distant galaxy.

00:47:10
Speaker 4: Anyway, What a crazy universe we live in. Also, I like it down here on Earth.

00:47:17
Speaker 1: I know it is nice to live here beneath the shelter of our magnetic field and our atmosphere, yeah, where our eyeballs are not getting pelted by bullets shot out by black holes from other very distinct galaxies.

00:47:27
Speaker 3: Thank you magnetic fields protecting our Earth.

00:47:32
Speaker 1: Thank you our fragile environment. And thanks to everybody out there for being curious about how the universe works and listening to this explanation for how the centers of distant galaxies combined gravity, electromagnetism, and relativity to shoot particles at you.

00:47:47
Speaker 3: See, y'all, next time.

00:47:55
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