Is the whole Universe spinning?

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Speaker 1: Hey, Daniel, do you ever get frustrated by, you know, asking the biggest questions in the universe all the time? Never? I live for it. I mean, what could be more fun? How about finding the answers sometimes? Oh? There are always answers out there waiting for us to discover them. You know, really, you think all the big questions in the universe have answers to them? What if they don't. I think that even the deepest, hardest, craziest questions in the universe have answers, and one day some human will know them. Hopefully that human is you or one of our listeners. What if the big questions don't have answers? You mean, like, is white chocolate really chocolate? Can you just say? I don't know. That's what I told my kids all the time. That's an answer, right? How about ask your mother? Is that an answer to the question? Who ate all the chocolate? That one is also asked your mother? Hi? I'm poor Hammy car Tennis and the creator of PhD comics. Hi, I'm Daniel. I'm a particle of physicist and a professor at UC Irvine, and I will continue to eat chocolate until all the questions are answered. I thought you were going to say that you you ate all the chocolate, but I guess you still have answers to find. That's right. As long as they keep making chocolate, I'll keep asking questions and looking for answers. I'm not sure where this correlation between answers and chocolate came from, Daniel, Are you sure you're not just making it up? I'm not making that up. You know that there's a tight correlation between the amount of chocolate consumed per capita and the number of Nobel prizes one per capita for countries Switzerland is like knocking it out of the park. Isn't that weird? Though? Isn't the Nobel price based out of Switzerland? No, it's Sweden, man, Hello Sweden. Wrong as w country were. Clearly, Sweden just needs to be more chocolate. Clearly, everybody needs to eat more chocolate. I mean, it's good for everything. It's good for your mind, it's good for your soul. I'm not sure it's good for your waistline or your heart a little bit. That's a true correlation, right like that. If you make a plot of the countries that have Nobel Prizes and the amount of chocolate they eat per capita. Supposedly, I mean I haven't looked into the data, but supposedly there's a correlation, right, Yeah, I think we have a plot of that in the opening of one of our books. So the data is there, the correlation is true. The causality, I'm not exactly sure. It doesn't mean that you should force feed chocolate to your school children in order to get Nobel prizes in twenty years. You mean, like the causality could be the other way around, Like winning a lot of Noble prices could cause you to eat a lot of chocolate. Yeah. Maybe they're just celebrating their Noble prizes by eating chocolate, right, Or maybe there's a chemical and chocolate. It could be big chocolate, you know, just be a big conspiracy, Oh, conspiracy between big chocolate and big Nobele. Yeah, the big science and big chocolate. They're already trying to make us all bigger. I guess that's right exactly. We're asking bigger questions by making our scientists even bigger, especially around the waistline. I occupy a larger and larger fraction in the universe every year, that's right. If you occupy larger fractions of the universe. There's less of the universe to explore. Technically speaking, Yeah, well that must be what dark matter is. After all, it's just overweight physicists. I guess eating dark chocolate because white chocolate not chocolate. Maybe that's what happened out in space, like all these aliens that are more advanced, you know, sort of reach the chocolate singularity and just based out of the electronic neetic spectrum. See how much progress we've made already. I don't think anybody ever said the phrase chocolate singularity before. So that's progress right there, chocolate fuel progress. Yeah, just talking about chocolate makes even a noble price boom. But anyways, welcome to our podcast, Daniel and Horror Explain the Universe, a production of I Heart Radio in which we eat our way towards answers to some of the biggest, tastiest, most delicious question humans have ever asked. Questions about the size of the universe, the shape of the universe, the motion of the universe, the fundamental nature of the reality we find ourselves in as curious conscious beings trying desperately to understand what's going on and where is my next piece of chocolate? Coming from that's right, because the universe is full of amazing and incredible things like delicious snacks and dark matter and sometimes dark delicious snacks at all at the same time, and we're all just wondering what's going on, what's out there, and how does it work. And some of us get to do that for a living. I mean, ask questions, not each chocolate. Nobody pays me to do that yet, but it does fuel my curiosity about the fundamental nature of what things are like out there. Yeah, I guess you're a professional question asker, Daniel. Would you say that's that's how you can describe a researcher. Yeah. Everybody who have decided to spend their life doing science has done so because there's a question that drives them. Of course, I'm curious about lots of questions in science that I don't personally work on, But everybody who's a scientist has their own personal questions that drive them to spend their life asking it. You know. Maybe it's about how birds communicate and choose partners. Maybe it's about how the Earth was formed. Maybe it's about the fundamental nature of space and time. It's this personal curiosity that drives science forward. Yeah, and you forgot the very important question how do you make chocolate fat free? That would also move signs forward a lot. Apparently, you mean, how do you make chocolate chocolate free? How do you make chocolate still delicious but a little healthier? Maybe? Yeah, The answer that one is you can't. The unhealthiness is part of why it's so delicious. Oh, I say, it's the guilty pleasure of it that you like. Oh my goodness, it's just got a little extra dark. But I guess. Yeah, you're a professional question asker, which means you're also sort of a professional head spinner, right, because I mean your job is not just to answer questions that your job is to find answers to crazy questions and find crazy answers right to leave people's head spinning. That's right. Sometimes when you under these deep questions about the universe, you wonder, like, how can we even get started? How do we take that first step towards that future where some human knows the answer to those questions as like an actual fact. Sometimes it can be inconceivable to probe such big questions and know like how to get started. Yeah, and hopefully it's a future we're not just one human not the answer. Hopefully that person gets to tell other humans yes. But there's always a first human, right. That's the joy of discovery, to be the first person to know something about the universe that nobody else knows, just like there was a first person to taste every kind of fruit on Earth, or first person to see every single mountain or to walk along a beach. There's always a first and that's the joy of discovery. You want to be the one who figures it out, not reading about it in the science times of the newspaper, right right, Although the way science works, you're never quite the first human, right because you always need things to be pu reviewed, right, like somebody needs to check your answer. You can't just like come up with an idea and have absolute certain that it's true, can you. You can't ever certain. But there are moments when you do an experiment and the results are just very conclusive. These moments of discovery where you've designed your experiment in a way to like corner the universe to revealing some truth to you. That a podcast about pulsar discovery last year in which we played audio of the astronomers watching the data come in in real time and having that moment of discovery. They almost shouted eureka live on the tape. So sometimes there really are those moments when you know something about the universe. Yeah, I guess you. Then you want to be the first human to be with like you know something. But it does leave your head spinning. This search for truth and the cosmos and searching for the idea of how things work in the explanation for why things are the way they are, including some very interesting questions about whether or not things are even spinning in the first place. That's right, because we see lots of things spinning in the universe. We know the Earth is spinning, and the Earth is spinning around the Sun, which is spinning around the center of the galaxy, which is itself spinning, which is moving around the center of the galaxy cluster. And so naturally, as a curious human being, you wonder how far does that spinning go? Yes, So to be on the program, we'll be asking the question, is the whole universe spinning? So, Daniel, how are we spinning? This question is the question it's self spinning? Is that what you're saying? This question definitely makes my head spin. And it's been one that's been debated sort of on the boundary of physics and philosophy for a few hundred years. You know, not just the question of is our universe spinning? But could it be spinning? What does it mean or the whole universe to spin spinning relative to what you know? Could you even tell if the whole universe was spinning? It's really fascinating question that tells us a lot about the nature of space itself and what is absolute and what is relative? Right, Because I guess you can, you know, ask if ace itself is spinning, right, because you know, we know sort of spaces kind of a thing that can bend and twist and ripple. What if the space itself is turning around? Well, you just blew my mind? Man? Did I make your head spin? Didn't make you want to eat more chocolate? That's always the case. That's a pretty low standard. But yeah, you can ask questions like is the stuff in space spinning? Is space itself spinning? What would space itself be spinning? Relative to? All sorts of really fun, big, mind blowing head spinning questions are involved. Well, I guess the question we're asking today is the universe spinning. So really we're we're asking about the whole shebanging like everything right. Yeah, now here's another one, Daniel, Is the multiverse spinning, It's definitely spinning money for the Marvel Universe. The Marvel Universe is spinning out cash. Yeah, they're shooting out of Spider Man's spinning web shooters. If the universe is spinning, and that spin is random, then you can imagine there might be a whole set of universes which each have their own different random spin. And then if we measured our universe to have a particular spin, we could ask like, well, why this spin not some other spin? You can add the multiverse that basically any questions, But then the multiverse would would be infinite and it would average out to nothing or not. I think this conversation is spinning out of control. Dinner, let's get back on track here. Yes, So, as usually, we were wondering how many people out there had thought about this spinning question, whether or not the whole universe is spinning or not, And so Daniel went out there into the wilds of the Internet to ask people the question, is the whole universe spinning? And thanks again to our cadre of volunteers who participate in these difficult to answer questions without any chance to do any googling. If that sounds fun to you and you'd like to hear your voice on the podcast, we would love to hear from you, So please don't be shy. It's free, it's easy, it's fun right to us. Two questions at Daniel and Joge dot com. Think about it for a second. Do you feel or think that the universe around you is spinning? Here's what people had to say. It wouldn't surprise me if the whole universe is spinning because so many things are spinning. You know, we're on a spinning Earth that, in turn is revolving around the Sun, which in turn is revolving around the galaxy, and the galaxy itself is spinning, and then our galaxy is probably spinning around other galaxies in the local group, which then uh itself probably is spinning within the bigger group. Well, I feel like we know it's expanding, but I have no idea if I've heard that it's spinning or not. Everything else in the universe is spinning, so why wouldn't the universe spin? Yes, without question, the universe is definitely spinning. There's a center of gravity, But I don't think that there is a center of gravity across the entire universe. I think there are local galactic clusters that rotate um, but yeah, because they share a center of gravity, I don't think that there's a center of gravity across the universe. So I'm going to say, no, it's not. Spin is a little relative, right, So in order for it to be spinning, it would need to be spinning relative to something outside the universe. So that's really a question. Is there anything outside the universe for which the universe could be spinning? When I think about the universe, I think of everything like it's just a word that is sort of all encompassing, and I I don't know if if we were, if the universe is all encompassing, what would it be spinning relative to? Is it that, like space itself spinning? Which doesn't make a lot of intuitive sense to me. I mean, you could probably figure out if it was spinning using I don't know, like the Michaelson Morley experiment with like the the the ether or whatever you can like test for that kind of a thing. I guess if the whole universe was spinning, then everything were be moving at the same time. So I don't know how we'd be able to tell, but maybe it is. All right, a wide range of answers. Some people said yes, no hesitation there, yes it's spinning. We got the whole range of answers here. We got from yes it's spinning to what does that even mean? To how could you even tell? It? Really pretty much represents the entire spectrum of possible answers, right, even a little bit of a maybe there, maybe it's spinning. So it's a fascinating question, is the whole universe spinning? And so let's get into the particulars of that question. I have questions about it, Daniel, could uh could the university spinning? It's a really interesting question just to ask, like what does that mean? First of all? Right, like could the university spinning? Spinning? Relative to what we've talked on this podcast for a while about real activity, you know, we talked to Carlo Rovelli, for example, about relative motion, and he made a really interesting point that we've talked about a few times that if you, for example, lived in the universe all by yourself, there's nothing else in the universe, then velocity would have no meaning. Because velocity is purely a relative quantity. You can only be moving relative to other stuff, and so in an empty universe, velocity has no meaning. Wait, what if the like the universe has a like an extent, you know what I mean, like a border or an edge, then you could be being relative to that even if you were alone in the universe. In our understanding of relativity, space is isotropic and homogeneous, meaning there are no special locations in space. The kind of cosmology you're talking about where there are special locations in space would break a lot of the rules that we think we understand, you know, like conservation of momentum. And so for now, let's operate in a universe that's essentially either infinite in extent or where every location in space is the same. So you can't be moving just relative to space. You have to be moving relative to other stuff in space. So from that point of view, you can't ask questions like is the whole universe moving? Right now, imagine the universe filled with stars and galaxies just like ours. It doesn't make sense to ask is all of that stuff moving? Because it would have nothing to move relative to if you're talking about the whole universe. So from that point of view, things like you know, velocity are purely relative. And the question then is like, is the same thing true for spin? If you were alone in a universe, all by yourself, an empty space, could you tell if you were spinning? Right? Because velocity, like you're saying, is relative, but acceleration is not relative. Right, Like, you could also ask the question is the universe accelerating? And that one would have meaning right because moving could also apply to acceleration. Yes, velocity is purely relative, but acceleration is absolute. Some you can measure the acceleration of an object. If you were alone in the universe, you could build a little device that would tell you whether or not you were accelerating. And it's not a complicated device. You could just like have a ball in a box and hold it steady. If the ball moves towards one side, it means you are accelerating in the other direction. That's just how an accelerometer works. Or if you're inside a box, you can tell whether you're accelerating because you would feel an effective gravity against one side or the box. Right, So you can build an accelerometer. You don't have to be accelerating relative to any other thing. You can be accelerating relative to absolute space time. It's interesting the history of how we came to these realizations. The first person to think about this question whether spin is absolute or relative? You know, can you be spinning relative to space or do you have to spin relative to other stuff? Was actually Isaac Newton, Right, yeah, because where I guess the question is whether or not we're spinning or not, and that one can be relative, right, or not relative. So that was a question, if you're spinning, what are you spinning relative to Are you spinning relative to space or are you only spinning relative to other stuff out there in the universe? Right, So it's spin sort of like acceleration or is it more like velocity? And at the time, of course of Newton, Newton thought that velocity was absolute, that you were moving relative to space itself. But he proposed a really fun thought experiment as a way to sort of like measure whether or not you're spinning and basically, like you know, a spinometer in the same way you can build an accelerometer to see if you're accelerating. He thought, let's build an experiment that measures whether or not you're spinning. And it's pretty simple. You take a bucket of water, and you know the surface of the water is flat, and then you start spinning the bucket and you spin it sort of along its access You could like twist a rope on the handle or something like that. Now, what happens, of course, is that the bucket starts spinning, and then eventually the water gets dragged along by the buckets, and now the water is also spinning. And what happens is that the water gets pushed up against the sides of the buckets. So the surface of the water is no longer flat. It's now concave. And so this, he says, is a way to measure whether or not you're spinning, because even if you're like on the bucket, if you're like an ant on the bucket, you can still tell that it's concave. Right every it you will agree that the surface of the water is now concave. So it's like a way to measure the fact that you are spinning, right, It's sort of like it's trying to measure whether or not there are centripetal acceleration or forces on you because like, if you're spinning, something must be causing you to to spin to change directions all the time, and that's what the centripetal acceleration is. Yes, centripe tol acceleration is pulling you towards the center's changing your velocity vector so you're moving in a circle. For example, centrifugal acceleration is the fictional force that you feel because you're in a non inertial reference frame, so that the ant, for example, or the water feels that force pushing them outwards. You know, the same way. For example, if you are on a spaceship out in deep space and it was spinning, you would feel an effective gravity. You'd be able to walk along the inside of that spinning can as if there was gravity. This is one plan for having effective gravity and deep spaces. Make your spaceships spin, right. So Newton thought, well, obviously, then spin is absolute right that you're spinning relative to spa ace, because you can build this thing that measures it. And even if you were in the frame of the bucket, if you were spinning with the bucket, you could still tell that the bucket was spinning. So now imagine that bucket in deep and empty space. You should be able to tell whether you're spinning or not based on whether the water is concave or whether it's flat. So Newton was like, I'm very sure that spin is absolute, meaning that you could tell maybe if the universe, the whole universe, was spinning, right, like if we were just sitting here and we felt this in tripital exceloration um, even though nothing seemed to be moving, then maybe you could tell that the everything was spinning exactly. And if spin is absolute, then you could measure the spin of the universe, right, because the whole universe could be spinning in space, and if that was happening, according to Newton, you could measure it. But not everybody agreed with Newton. There was a guy named Ernst Mock who came along and said, no, no, no, that's not true at all. Spin is purely relative. Motion is relative, after all, right, like if you're moving through space, that's purely relative. So Mark said, maybe everything is relative, maybe all motion in the universe, acceleration and velocity, and this is before relativity, right before Einstein said maybe everything is relative. And then people said, well, what about the bucket, right, the bucket can tell if you're rotating, and Mark said, well, the bucket is rotating relative to the stars out there. That's why the surface of the bucket gets concave, because it's rotating not relative to space, but to the stars out there in the universe. That doesn't make a lot of sense. It doesn't make a lot of sense to me either. But remember, at the time, people didn't really understand the notion of space and time and relativity, and these questions were sort of up in the air, you know. People wanted to know, for example, if velocity is relative, as we all accept, then why isn't acceleration relative? Like you said it earlier that velocity is relative but acceleration is not. But why not? And at the time people didn't understand, and so this was sort of a beautiful idea to say, well, maybe everything is actually relative, and the reason that the surface of the bucket gets concave is somehow because of the distant stars. And on the other side, people say, well, that's ridiculous, Like, are you saying that if you spun a bucket in empty space that it would stay flat? Right? He said, absolutely, If you spin your bucket and then you start removing the stars out there in the universe that the surface of the bucket would go from concave to flat, that somehow those distant, distant stars are telling the bucket surface what to do. I feel like MOK should have stuck to like sound research, maybe not trying to do astronomy here. But I guess you're saying that's kind of the thinking at the time, like maybe if you sort of like removed a reference frame, and maybe that's kind of what he was getting at. It's like, if you remove the reference frame of the stars, then somehow, um, maybe you lose all relativity. Yeah. Max said that spinning is relative. If you're not spinning relative to something else, then you're not spinning. Spinning in an empty universe has no meaning that you could never tell. You know that if you were in an empty universe and I started to spin you, you wouldn't feel your arms get pushed out towards the side. If you build a spaceship and spun it in an empty universe, you wouldn't feel effective gravity that comes somehow from the distant stars that are defining a reference frame. And this was a beautiful idea, and even Einstein liked that. Einstein thought Oh, this sounds really nice, and he tried really hard to work it into his theory of relativity, because I think the idea was that motion is relative, right, like everything is relative. So Einstein basically came up later and said, actually, turns out Newton is wrong and also Mock is wrong. They're both wrong in slightly different ways. All right, Well, let's get into how Einstein proved them wrong and whether or not the whole universe is spinning, because my head is spinning a little bit right now, and I kind of made a little break, so we'll be right back. All right, we're asking the question is the whole universe spinning? I am getting a little dizzy just thinking about and talking about this question. And where we left off was that Einstein looked at Newton and then he looked at another researcher back in the day, and he said, um, you're both wrong. Actually, maybe spinning is also relative. So Einstein said, definitely, Newton is wrong, because Newton said that all motion is absolute, that space is like its own absolute frame, and velocity is absolute, that in an empty universe and object moving in his train line, you could somehow measure its velocity relative to absolute space. So Einstein said, no, that's ridiculous. There is no preferred location in space, There is no preferred velocity in space. That doesn't make any sense. And he built that into his theory of relativity. But he also said that Mack was wrong because Mak wanted to go even further and say, well, even acceleration is relative, and Einstein liked that idea, but he found that it didn't actually work. And so what Einstein says is that space is relative. And we've talked up on the podcast a few times about how time is relative. Also different observers in the universe feel timed differently. But when you put the two together, space time, you actually do get something absolute. Einstein didn't actually like calling his theory relativity theory because he thought it oversawled the relativity aspect of it. He wanted to call it invariance theory. But anyway, what he found was that space is relative and time is relative, but that space time together had some absolutes in it that lets you measure whether something is accelerating, and spinning counts as accelerating. So acceleration and spin are absolute, but velocity is relative. I guess it sort of maybe depends on your definition, right, Like is spinning motion also motion, or is spinning motion like something that's not motioned, you know what I mean? Like maybe you're saying, you know, spinning involves acceleration, and therefore it's not sort of the same as moving with a constant velocity exactly. Spinning is definitely a kind of acceleration. So then the question really is is acceleration absolute. If acceleration is absolut if you can measure acceleration out in deep space, that means that spinning is absolute, that you could also measure spin in deep space. And so Einstein's theory of relativity shows us that velocity is relative, but that acceleration is absolute. That you can tell if an object is accelerating, which also means you can tell if it's spinning, right, which means that if you even if you were alone in the universe, you can still get dizzy. You can get dizzy in an empty universe. And we've seen this other times when we've talked about Einstein's theory of relativity and spin. Because Einstein's theory, for example, predicts a difference between the gravity of an object that's spinning a gravity of an object that's not spinning. A spinning object has this weird effect frame dragging, where like drags space along behind it a little bit. And Newton doesn't predict that because Newton says there's no difference between the gravity of a spinning object and a non spinning object, and that wouldn't make any sense if spinning was purely relative. Right, So spinning objects have a different gravitational effect, and therefore you must be able to tell the difference between spinning and non spinning objects, right, But I wonder if that is that something. I guess that's just kind of built into the laws of the universe that we that we can tell, Like why does the universe draw the line between velocity and acceleration, because you know, mathematically, it's just like one derivative over. It is just one derivative like it could have been acceleration is also invariant, but then the derivative of acceleration the next one over, is not relative. It does seem sort of arbitrary to like draw the line at the first derivative and not the second derivative. And there are a lot of really deep ideas here. One of them has to do with symmetry and conservation laws. We'll talk about that in an episode coming up soon on Nuther's Theorem. I think the best way to think about it is to think about what is invariant, what is preserved in the universe. The reason, for example, that you can tell that something is accelerating versus something is not is because of their path through space time. You know, this has to do with things like free fall. Right, if you are just moving through space without accelerating, you don't feel any forces, which you're effectively doing is making a straight line through space time. Like your path through space time is just a straight line. If you're accelerating, you like burn your rocket. If you're being pushed by something, then you make a curve in spacetime. So the reason that you can tell whether something is accelerating or not is because everybody agrees. All observers agree whether or not you're making a straight line in space time or a curve in spacetime. That's what it means to say that spacetime itself is absolute, that the universe can tell the difference between straight paths and spacetime and curved paths in spacetime. Well, I guess that's kind of what I mean, like, you know, you said, it's all sort of depends or it's all because of some symmetry in the universe. But then like, what if the universe they didn't have those symmetries, or what if we lived in another universe without those symmetries, would those RULs be different? Or is that invariant about the velocity and acceleration kind of baked into math itself. It's not baked in the math. You could construct cosmologies where things were different. This is the universe that we find ourselves in. These are the rule that we discover. You know. It's sort of like asking the question and why is momentum conserved? And the answer is space is the same everywhere? All right, well, why is this space the same everywhere? It's not required by mathematics. You could have a universe where the rules of physics vary from place to place. We just don't seem to have that universe. So this is sort of like a discovery of ours. The universe seems to obey this principle, and these are the consequences of that principle. Why does it obey this principle? We don't know, But we've discovered it. It's sort of like saying, okay, the speed of light is invariant for everybody. Everybody measures the speed of light to be the same. Why, we don't know, but here are the consequences of it. This is the invariance that Einstein discovered that space is not invariant, but space time is. You know. Another way to think about these paths is to think about like, you know, imagine how you get from your house to your work. Somebody could draw like a set of axes X, y Z and say Joge gets from his house to his work and from this coordinate to that coordinate. And somebody else could come along and say, well, I have a totally different set of coordinates for Jorges path from him to work, and those are totally different coordinates, but doesn't matter. And it doesn't matter because the path itself is the same. The two observers would look at the path and say, oh, it's a straight line or no, it's a wiggle because he stops at the fridge on the way from his bed to his cartooning studio. Right, So people agree about the shape of the path even if they don't agree about the actual coordinates. That's what it means that space time, your path through space time is absolute, even if your coordinates are arbitrary. Well sometimes I work from bed. Even everyone would agree that it's a short community. Yeah, exactly. If your path is a point, then everybody would agree. Yeah, that's the whole point of being a cartoonist is to live in a point. But I think what you're saying is that you know, it's sort of baked into the laws of at least this universe that we live in, that you can't tell if something is spinning, Like it's not something that you can hid, or it's not something that you can like maybe never tell. But the question we're asking today is whether the whole universe is spinning, which kind of makes me wonder, like feel like, now it could go either way, because yes, you can tell if something is spinning in the universe, but what if the universe itself is spinning? Could you tell? Here? I think we have to be careful about what we mean by the universe. And I'm talking now about the matter in the universe. Like I take all the stuff, the galaxies, the dust, the gas, the dark matter, is all of that stuff spinning and that here it's spinning relative to space time. Spacetime itself can that spin? Like, well, what would that be spinning relative too? There's no external metric for spacetime itself to spin. So we're talking about, like, you know, the Earth is spinning, is it possible that everything else is also spinning in that same way that the Earth is spinning? Oh? I see You're you've been spinning this question the whole time, Daniel. You're really asking is the whole stuff in the universe spinning? You're not asking if the universe is spinning. Well, it depends on whether you're including space time in that spin. And I would say it doesn't make sense for space time itself to spin, but stuff in space time can spin relative to space time. Well, I guess maybe let's explore that a little bit. Like we've talked about space being a thing, How do you know it's not spinning? It could be spinning even if it's not relative to anything. Right, the same way that we talked about expansion of spacetime, we don't say that spacetime is expanding into something else. We only have intrinsic measurements. We are in spacetime. There's nothing outside of space time to measure, so we can only measure internally. Right, So we say spaces expanding, people ask what is it expanding into? Well, it's not expanding into anything. It's just increasing the relative distances between stuff in space. So in the same way, you can't really ask what is spacetime rotating in It's not in anything. It just sort of is. And so there's no sense in which space time could spin. But I thought you were going the other way, because you know, you can say that space is expanding, but it's not expanding relative to anything. Couldn't I just say spacetime is spinning, but it's just not spinning relative to anything. Then what does that mean for it to be spinning? What does it mean for its expanding nothingness? It means that the distances between stuff are getting larger, right, But what does it mean for space time itself as a whole to spin? You can tell whether's something is spinning relative to space time same way you can tell whether you know distances are growing, but you can't tell whether spacetime itself is spinning. You could maybe, like maybe if I was alone in the universe and I got dizzy, you know what I mean? Like I guess what I'm wondering is, could there be like an inherent centripital acceleration to the universe. I think it's possible. Mathematically, there is always an ambiguity there, even in the case of expansion, because general relativity has this weird difficulty by talking about the velocities of things that are very far away, you always define either relative velocities or velocities relative to space. And so probably there's a way you could construct a universe that had a rotating space time that had an inherent built in centrifugal acceleration. You know, for example, a curvature of space would give you that effect if space was curved in such a way that you effectively felt a negative gravity. And there are some folks out there argue that dark energy may partially be due to some like inherent and trifugal force due to the rotation of space time itself, rather than some expansion of space. WHOA wait, so like the space and could be spinning and it might be the answer to dark energy. That's one thing people were probably and we'll talk about how people use type one a supernova to try to answer that question. You a little bit when we talk about how to measure whether space is spinning. All right, we'll leave that for another episode, but I guess in this episode we are now shifting the question too. Is everything in the universe in spacetime it's self spinning, because you know, it could be spinning, right Like, like you said earlier, the Earth is spinning, and we're spinning around the Sun, and the Sun is spinning around the gaxy and the GALSSI. It's probably spinning around some larger cluster of galaxies, and that could also be spinning relative to other clusters. You know, is everything may be spinning. It's a really fun question, and what we expect, what physics predicts, is sort of counterintuitive because on one hand, we see that everything is spinning, as you say, the Earth is spinning, of the galaxy is spinning, but we don't actually expect, on very very large scales for all the stuff in the universe to be spinning. That if you added up all the stuff in the universe, we actually predict that it should have zero spin, right because it seems kind of implausible almost that everything would not to be spinning, that it would be sort of standing still. It doesn't. It's actually a very specific prediction of inflation. Remember, inflation is this prediction that the universe started out much much denser, much much more compact, and then spread out and expanded and blew out, and that all the structure we see in the universe comes from like the little quantum fluctuations in density from the early universe. And it's that expansion itself that we think would have killed effectively any rotation. Remember what happens when you're spinning. If you're like on ice in your figure skater, if you pull your arms towards yourself, then you start to spin faster. And that's why, for example, the Earth is spinning because it started out from a cloud of stuff that was very gently spinning and then collapsed, and the is now spinning faster. But expansion has the opposite effect. So as the universe expands, we expected to spin less and less. Because inflation expanded the universe by like ten to the thirty than any tiny random rotation of that initial blob of stuff, we would expect that to be effectively zeroed out by inflation. Right, I guess maybe I'm getting a little confused here because of what we're actually asking, Like, I guess, are we asking whether all this stuff in the universe has a zero or non zero average spin? Do you know what I mean? Like, for sure I can spin in my chair here, and the Earth is for sure spinning, and the Sun is definitely spinning, and the galaxy and the galaxy clusters are definitely moving relative to other things. I guess is the question, like, if you average out everything, all the stuff in the universe, does it have a spin to it or does it all cancel out to exactly zero? Is that kind of what we're really asking here? Yeah, draw line through space and measure the spin relative to that line, and then ask is that overall spinning all the stuff in the universe? Is that zero or not? Right? Sort of like if you have a snow globe, maybe I'm thinking, you know, you can have all the snowflakes inside moving and twirling and looking chaotic, but overall you can sort of tell whether or not this stuff in it is spinning relative to the globe. What you're saying is that it maybe has to do with the beginning of the unse like if this stuff at the beginning of the universe had a little bit of a spin, that's the only way it could still have spin today, Yeah, because angular momentum is conserved. Right, If you are spinning, you will always be spinning. There's no way to stop something from spinning unless you're coming in from the outside with some other spin to cancel it out. If you're talking about the whole universe, then there is nothing outside and so if the whole universe somehow started out spinning, it should still be spinning today, although that spin would get dampened out and go to very very small values because of the expansion of the universe, like a figure skater shooting her arms out for like millions and millions of miles or spin, but then go to the to the zero. So that's what we're talking about, is is there spin to this stuff in the universe and it should persist if it's there, right, Because I think what you're saying is that the universe had no spin at the Big Bang when it was super super tiny small, and thence if it had no spin, there's no way for it to gain spin since then like it can't like push off against anything right to get spin. There's no way to spin the entire universe. There's nothing to like push against. Right. If you're talking about the whole universe is nothing that you can push against, because that would mean something outside the universe and the universe is everything, right, unless maybe, like spacetime itself has a like a lean, like a lean to it, right, or something, you know, like it has a preference for a matter and not antimatter. It has a preference for certain things, you know, four wards some time and not backwards some time. Could it also have like a little bit of a spin preference. Yeah, we usually ask this question under the assumption that spacetime is homogeneous, meaning it's the same everywhere and in every direction. But it's certainly possible if we discovered an overall spin to the universe to wonder that comes from and you know, could be generated by like some features in spacetime that aren't the same everywhere I see. But your your basic answer is that the stuff in the universe has any spin. Now, it must be due to any kind of spin it has at the beginning of the universe. But even if it had a little bit of a rotation at the beginning of the universe, that would have all sort of gone to almost zero by now. Yes, So we would be very surprised if we look out into the universe, measure its overall spin, and find it to be spinning at any significant rate. That would be a big shock. It would make all your head spin for sure, relative to all the chocolate you're eating. It would be a fantastic and delicious discovery because it would be a clue that's something we think is true, is not that there's something else going on that we don't understand, and like the biggest, most delicious scales. All right, well, let's get into how we could tell if the universe, or at least the stuff in the universe, is spinning at any significant rate, because I think Daniel is hungry, so let's get that answer asap. But first let's stick another quick break. Alright, we're talking about whether or not all this stuff in the universe is spinning, Dan, and you said it's unlikely for it to have any kind of spin, because expanding the universe would have killed any sort of momentum. But what if I'm thinking, what if the universe was spinning a lot at the beginning, couldn't it also have some sort of remnant spin twit at this point? Yeah, it would, and we might be able to measure that if we devise tests that are very very sensitive, and so you're right that the current theories of physics suggests that spin is dampened by expansion. But we don't have an idea for why the universe would or would not be spinning originally. So if the universe was somehow born spinning super duper fast, we could still measure that today, and that would be interesting because our cosmology allows for the universe to spin. It's possible for the whole universe to have been born spinning, for it to have been a spinning baby shot out of the universe womb and doing a triple lets. And remember we don't really know much about that birth. We talked about possible theories of like infloton fields decaying into normal matter. There's are really displaceholders for like where to have future ideas the shape of those ideas, but we definitely don't have ideas firm enough to where we can say, like, the universe should not have been born with any spin. For sure, there's a huge opening there for ways the universe could have been born and allowing for it to spin, And definitely Einstein's ther relativity says it is possible. It's meaningful for the stuff in the universe to be spinning, right, So I guess that means that if you do discover that all the stuff in the universe is spinning right now, it maybe shouldn't be that surprising. You'll just tell you if it was spinning at the beginning or not. It would tell you that it was spinning a lot at the beginning, and then you'd have to go into your theories and say, like, well, how do I make that happen? You know? Is it a diverse where the spin is somehow random and we just got a big serving of it is necessary because of some property of the universe beforehand, the universe is inherently always spinning. It would be a lot of really fun questions. Yeah, or maybe the universe mom just hit a lot of chocolate before the baby universe was born and made the baby universe spin out of control. All right, So then what are some of the ways that we could maybe test today, like where we are now in this spinning or not spinning universe, whether or not things are spinning everything, whether or not everything is spinning. So people have been thinking about this for decades, and remember Einstein's theory of relativity is only about a hundred years old. To this concept that the universe can spin absolutely relative to space time. He's only about a hundred years old, and it took a while for people to understand what it means. You know, Einstein actually believed Mark for decades until later in his life when he was like, no, beautiful, but I think he's wrong. And it's really only like fifty or so years that people have been are thinking about this in a way that they test it. And one of the simplest ideas is just like, well, let's just look and see if the universe appears to be spinning relative to some fixed point. You mean looking at all the stuff in the universe, not the universe itself. Yeah, all the stuff in the universe. And so, for example, are the galaxies, the distant galaxies? Are they spinning relative to the Milky Way or relative to the Solar System? And the cool thing is that you can treat the Solar System, because itself is spinning, as a sort of gyroscope. That the Solar System is spinning, and it's going to continue to spin, and because of conservation of Anglo momentum, it has to keep spinning and basically the same direction, so it like defines an access and then you can ask, well, is the rest of the universe sort of spinning around us relative to the Milky Way? What do you mean? I would that tell you if the universe was spinning, I mean the stuff in the universe was spinning. Well, it would tell you whether the universe was spinning relative to the Milky Way. At least we can measure the Milky Way spin, right, because we can tell whether an individual object needs spinning, and so that gives us like a reference point, and then we can ask whether the rest of the universe is spinning relative to us. So we can measure our spin relative to spacetime, and then we can measure the rest of the universe is spin relative to the Milky Way, just by looking at the galaxies and saying, like, are they moving relative to us? Oh? I see, But isn't that sort of a given, Like don't we expect the Milky Way to be, you know, spinning in some weird direction relative to everything else, all the stuff in the universe. We can measure the Milky Way spin relative to spacetime because it's absolute, and then we can measure the motion of the distance galaxies relative to the Milky Way, and if that's exactly the opposite, then that suggests that the whole rest of the universe is essentially not spinning. Oh, I see, Because you're saying that you can measure the spinning of stuff relatives to spacetime. There is sort of like a reference frame in the universe, and that's called space time. And you're saying, because the Milky Way is close to us, we can measure that pretty accurately. It gives us sort of a compass of how we're spinning relative to spacetime. And then you're saying, let's look at everything else and see if it's rotating relative to that exactly. So, say you're spinning around inside of a room, you can have a way to measure your own spin. You have like a gyroscope that tells you your own spin, and you can also measure your spin relative to the walls. So those two numbers agree, that tells you the walls are not spinning. Those two numbers disagree. That tells you, Oh, well, I guess the walls are also spinning. So that's what we can do. We can measure our spin of the Milky Way and then we can measure the spin of the Milky Way relative to the distant galaxies, and then we can ask are the distant galaxies spinning in absolute sense? But I guess my question is why do even need to go that far? Like, couldn't I just use the gyroscope here on Earth and then measure the galaxies relative to my little gyroscope. Yeah, but the Milky Way is a better gyroscope than your little gyroscope. The Milky Way and the Solar System provide pretty nice gyroscope because it's a huge amount of mass and a very long lever arm, and so they're very precise. But there are lots of ways you can construct this experiment. The real limitation, though, is that it's very hard to observe the motion of those distant galaxies because they're really really far away, and so they hardly move at all. And we've only been looking at them for like a hundred years. It's only Hubble a hundred years ago they even discover that there were other galaxies out there. So if they are moving, they're moving very very slow, not in a way that we can tell relative to spacetime, right, They're moving super slow. They're moving they're expanding, right, they're moving away from us. But now we're talking about not radial motion away from us, the rotational motion right perpendicular to the line between us and them, and we haven't seen that kind of rotation. Like I think you're saying, like, if you correct for the motion of the Earth and correct for the motion of the Earth around the Solar System and the milk away and all that, it would be hard to tell because they're so far away, Like for us to see them spinning around and around the cosmos, it would have to be moving like ridiculously fast, right, Because we can measure their velocity in a radial direction pretty well because of Doppler effect. Measuring their velocity in the other direction, you know, requires seeing their change in their own relative to the Milky Way, which requires like you know, seeing them actually move, and that's really hard to do for stars and even harder to do for galaxies. They need to be either moving really really fast, or you have to observe for like a billion years and we're only a hundred years in. Yeah, that's the real limitation. Like it's possible, and you just need a lot of time, or they need to be moving really really fast in an obvious way that we can see it, and that's not happening. Can you just to ask him? Call him all right? So that that one has its limitations. You're not gonna gonna get a Nobel prize anytime soon with that method. What are some other ways we can measure? Other things we can do is to look at the expansion of the universe. So we know that things that are far away from us are moving away from us, and we know that that expansion is accelerating. So this is the discovery of dark energy. That something is stretching out. The universe seems to be creating new space between points all of the time. And the interesting thing is that cosmologists say that if the whole universe was rotating, if everything was spinning, then that would look a little different. That expansion would look different than it would if the universe was not spinning. And so the way that we measure that the universe is expanding is a couple of ways. One is that we look at type one A supernova, these standard candles that we know exactly how bright they should be, and so when we see one, we can tell how far away it is because of how bright we see it. We have these like tracer points in the universe, so we can see their expansion over time, and we can see the history of the expansion of the universe. And if there was a swirl in that expansion, if the universe was rotating at the same time as expanding, then that would look a little different than if it was just expanding in a purely radial way. What do you mean, because I would mess with the I guess the basic motion of things, right, because it suddenly has an extra component of velocity and it's feeling an extra component of exploration. Yeah. Imagine you're like on a Merry Go Round and you have a bunch of friends and you all throw ping pong balls out. If you're spinning, or if you're not spinning, then those ping pong balls have a different path, right, They would curve if you're spinning. That's one way to measure or rotation. And so we're trying to measure the rotation of all the stuff in the universe by looking at the path of these tracers, these type one a supernova to see is it consistent which is flying straight out or is it consistent with curving. We can't observe them for very long. But we have these momentary tracers, then you can do some sort of like back calculation to say, like are they consistent with rotation or are they consistent with pure expansion? Oh? I see. I think what you're saying is like, like if everything in the universe was in a Merry Go round, we would see the stars, the supernova or at least the stuff around like the equator. That stuff would be maybe expanding faster, maybe or moving differently than the stuff like above and below us on the Merry Go round. Yeah, if there's an overall rotation, that implies that there's a rotation in some plane, right, some access around which you are rotating, and that essentially defines a north and a south just the same way like the Earth's rotation defines a north pole and a south pole, and the Solar system has a north and the south defined by its rotation. So that would create different directions in the sky where things look different, basically a huge anisotropy. We actually talked about on the podcast once about this access of evil, this idea that maybe things in the universe do look a little bit different north to south if depending on how you define it. That's exactly the kind of thing you would expect to see if the universe was rotating. You'd expect to see some difference in one half of the sky versus the other half of the sky. Right, you would expect it to be a little wider in the middle around the waistline. The universe is either rotating or been eating too much chocolate, you know, one of those two. There's only two posibilities. You can use similar tests with another very sensitive probe of the universe's expansion. That's the cosmic microwave background radiation. That's this light from the very very early universe. And we were talking about how if this rotation exists, it should have existed a long time ago. It's actually much more powerful to see it earlier on, before the universe expanded so much, because the rotation would be more dramatic and so in the same way that if the universe is rotating, it would affect the way things are expanding, it would also affect that plasma that generates did that CNB light The plasma is very sensitive to like how much dark matter there is and how it's moving, and how the normal matter is sloshing around into and out of those dark matter gravitational wells, and if there was an overall spin, it would affect those patterns in very subtle ways, and people have studied those and not seeing any evidence for universal rotation. M M. It's kind of like you we talked about it before, how the cosmic microwave background is like a picture of the baby universe, like an early picture of the universe, and so you you don't see any Basically, what you're saying is you don't see any spin in that picture, nor do you see it being uh, like the baby's not extra wide around the middle. Yeah, And it's a little subtle because it's not like you're looking at the actual universe and seeing it's spin. You're looking at like the patterns that spin would cause in the clumpiness of the universe. It would make very distinctive patterns. And this is actually the most sensitive test we have for the universe's rotation, is this cosmic microwave background light. In their early eighties, people saw some weird stuff in the sky from radio signals that led them to believe maybe the universe was rotating. There's a paper in two I read from a guy in Manchester in Birch who claimed to be measuring the rotation of the universe that attended the minus thirteen radiants per year. But this measurement in the CNB is more powerful and is not consistent with any rotation, So people think Burchess measurement was probably wrong. Well, I'm getting this sort of the picture that if you're asking the question, is the stuff in the universe spinning? If it was spinning, then you would see some sort of like you said, an isotriptropy, meaning like it would the universe would look differently whether you're looking at the spinning direction or whether you're looking at the its waistline or the equator of the spinning stuff in the universe. And it seems like for for all accounts looking at galaxies and looking at the baby picture of the universe, there is no different or like preferred direction that you can see as far as we can tell. Here's a study we talked about on the Access of Evil episode where people looked at the spin of Alexis and they found that galaxies in one direction of sky tend to be spinning left more than any other direction of the sky. And they claim this is maybe evidence for the universe to be rotating. But a lot of people criticize that paper and think that it probably underestimates systematic effects that could cause that, you know, local gravitational effects or or other uncertainties that they didn't take into account. So overall, there's no evidence for universal rotation, although our cosmology allows it. Right, it is possible our universe to rotate, it just doesn't seem to be doing that right, or at least not to tend to the negative nine radiance per year, which sounds like a small number. It's like point zero zero zero zero nine zeros and then a one. But if you, like you said before, like you know, the universe expanded tend to the what like thirties sixty two times since the Big Bang, And so it could just be that are these instruments this way of knowing, it's not accurate enough yet, Like there could be a little bit of spin down there still underneath the what we can measure. You will never have infinite precision, and so either the universe is not spinning or spinning very very gently. Yeah, it's a kitty merry go around. You know, nobody wants us to throw up all that chocolate. Yeah, nobody wants the six flags crazy roller coaster eversion of the universe. So I guess the answer is, is the stuff in the universe spinning? Not that we can tell, but it could still be spinning a little bit. We just don't know for sure yet. And fascinatingly, we could tell if the stuff in the universe was spinning. Spin itself is absolute, and that tells you something really deep about the nature of space and time and motion. That velocity is relative, but acceleration is not. Acceleration is absolute relative to space time itself. Yeah, and it would also sort of tell you that the universe kind of has a direction, right, It kind of would have a north and a south, and a tropical zone. It prefers dark chocolate to white chocolate. All right, Well, um, we hope your head is not spinning from all that discussion about spinning. Well, we hope that it does sort of make you think about whether or not the things around you are moving the way you think they are, and whether or not the universe was maybe born a pretty chill baby or a crazy spinning baby. And I hope you come away a little bit in awe of the fact that we can ask such amazing enormous questions about the nature of the cosmos, and that we have ways just from this tiny spinning rock finding initial answers to these questions. Yeah, and that we can talk about it in an hour and only make forty seven chocolate references. That's right, only make the forty seven bad chocolate plants. All right, Well, we hope you enjoyed that. Thanks for joining us, see you next time. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of I heart Radio. For more podcast from my heart Radio, visit the i heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows. H