Daniel and Jorge Explain the UniverseIs the universe finite or infinite?
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Hey Daniel, do you think there are some questions out there that will never know the answer to?
M deep questions like when is the best time to eat a banana?
What do you mean is there a controversy about that?
Oh? Man, people argue about this stuff online, you know, slightly green or wait for the brown spots. There's a lot of debate.
Yeah.
Well, I don't discriminate. I think every banana is delicious. But I was thinking, you know, bigger questions, bigger questions about the.
Universe, like are we alone? Are there aliens out there?
You just went right for the alien button.
I have it right here on my desk for a reason.
But yeah, yeah, I'm talking about huge mind blowing questions, you know, like is like are we alone in the universe? You know, is humanity destined to wonder these about these questions forever or do you think we'll find answers eventually.
I think as long as we're around and we're putting some money in basic research, we'll keep making progress. So you know, it could take one hundred years, it could take a thousand years, but I think some days someone will come along with a clever way to give us an answer, even for the very biggest.
Questions, like when do aliens like to eat their bananas?
I think aliens like to eat you while you eat a banana.
I got dark. I got a bunch of dark spots all of a sudden. Hi am Jree. I'm a cartoonist and the creator of PhD comics.
I'm Daniel. I'm a particle physicist, and I love really big questions we might never know the answer to.
And where do you fall Daniel on the bananas, kille, green or spotty?
I think right in between. Green makes you just your mouth feel like you're eating cardboard, but too spotty, and you're basically eating a smoothie, which is not something you want.
That's why they invent the smoothies.
That's right. I actually like the red bananas. Have you had the red bananas?
I think I have. Yeah, they have them in Asia, right.
Yeah, and also in Central America. Slightly strawberry flavored, quite delicious.
It's got a bouquet of strawberry and oak carbon.
They are sassy but unpretentious.
We are Latino.
Here. We are shilling for big ban But welcome.
To our podcast Daniel and Jorge Explain the Universe and apparently bananas well.
In which we like to explore topics that are big, bigger even than questions about bananas, questions so big you can't even wrap your mind around them.
Yeah. It's our show produced by our Heart Radio, in which we think about some of the biggest questions that can be asked about this crazy and pretty large universe we live in, that's right.
And we like to take you to the forefront of knowledge, what scientists are thinking about, what the experts in the world have in their minds, but then translated so that you out there with your kids or listening on your commute or everybody who loves questions and wants to understand the universe can really dig into it and get something out of it more than just banana jokes.
Part of what would you like to do is not just talk about questions for which physicists know the answer and try to explain to you those answers. But we also kind of like to talk about the open questions out there, the questions that not even people with PhDs and universities have a good answ.
Are you saying PhDs don't give you all the answers?
I wish I hadn't ask for that, Danny Well.
I remember getting my PhD. It was a very unceremonious moment.
Did you get it by default? Is that what you're saying?
No? At Berkeley, when you turn in your PhD, you're in some sub sub basement and you give it to the margins lady and she measures the margins in your PhD, and if everything looks shipshape, she gives you a lollipop that says PhD on it. And that's really the moment you have a PhD?
Wow? Did they?
Did you?
Do you still have that lollipop?
You know, we recently cleaned out our house and I found a lollipop and my daughter Hazel, was like, why does this lollipop say PhD on it?
You found it, you still had it around, it around and did you let your daughter eat it? Because it's probably like twenty years old. Man.
I said, eat that and you will learn all the secrets of the universe.
Take the red lollipop and you will never go back.
No, but it's not very satisfying, you know that moment you get a PhD, you don't feel like you know that much more. It's like a slow accumulation of knowledge and understanding, and in some ways you learn how little you know about the universe. Right, You're like grappling with the infinity of our ignorance.
Yeah, I mean, at some point you basically become the world expert in a very particular topic, and you realize that if you don't know, nobody knows.
Like robots that run like cockroaches, for example.
Yeah, that's right, nobody knows how to make them practical. Nobody knows. If I don't know how to make them practical, nobody knows.
That's right. And one of the funnest questions, one of the really crazy things to think about, is just sort of the nature of the universe we find ourselves in.
Yeah, what is it like really? I mean, from we know what we can see from this little point in the universe in the milky way, from this little rock that we're all floating on. But and we can see some stuff out there, But what is the universe really like out there? You know, what's the shape of it? How far does it go? And what's out.
Thereascinating to me that this is a fascinating question, you.
Know, like it's fascinating that it's fascinating.
Yeah, that it's one of the questions that people want answers to. That people want to know this about the universe. You know, people have all sorts of questions about how stars are formed, and how planets are made, and whether there are aliens out there, but people are also just curious about like how far does it go? How does that all work? And to me that says something about you know, what we want to know the answers to tells me something about who we are and how we think.
Do you think this question of what the universe is like out there is kind of primal to us as human beings, Like there's something innate in our nature that really kind of wants to know kind of what is this thing that we live in?
Yeah, we want to understand ourselves. We also really want to understand our context. You know, like if you're born in a little village, you wonder what else is out there beyond the edge of the village. And if you're an explorer, you wonder like what's deep in that jungle and what's out beyond the oceans? And it's just something in humanity that us to know, like what is the nature of their context? What defines our existence? What's beyond these four walls?
Yeah, you can imagine being like an early explorer and thinking, you know, is this the only continent or are there more continent? Are there three continents out there if I keep going? Or are there an infinite number of continents?
Yeah? And I love some of these sort of ancient arguments, you know, from like the Greeks and those folks, about like how big is the space and how far does it go? And it really reveals something about the way they thought.
So to the on the podcast, we'll be tackling one of these big questions about the universe, probably maybe the biggest question you can ask about the universe, So to the on the podcast, we'll be asking the question is the universe infinite.
Or is it just mind bogglingly enormous.
Right, but enormous but with a limit like finite finite, right, like does it have a size or does it go on literally forever and ever.
Yeah, it's totally a fascinating question. And I was reading about this and I came across the sort of ancient argument for the universe being infinite. Some of the ancient Greeks they believed the argument was infinite and they just needed a javelin to prove their point.
Really, wow, all this time, you guys have been building telescopes and space telescopes and particle colliders. All you needed was a stick.
Yeah, exactly. I like the low tech solution to some of these questions.
Wait, so you're saying the Greeks wondered if the universe was infinite, so they knew kind of generally what was going on, like we were a planet, we're in a solar system, and then where they're just wondering if the Earth was infinite.
No, they knew that, you know, there were planets. In fact, the Babylonians knew that there were other planets out there in the sky. We didn't know the nature of space or you know, that the Sun was just an example of other stars, but they had the sense that, you know, the sky wasn't a ceiling. The things went on for a while and the question was how far do they go?
And so how tall is the ceiling in this biodome we live in?
Yeah, and they had this fun sort of thought experiment because they couldn't venture out into space, and involved throwing a javelin. And the argument goes something like this, like, imagine the universe is finite.
Okay, we like it has an edge to it.
It has an edge to it. Now go to that edge and throw a javelin. What happens Either the javelin keep going, in which case you're not actually at the edge right and repeat forever, or the javelin bounces off something it like hits an obstruction, in which case that obstruction is past the edge. And so this is sort of an argument that suggests that the universe must be infinite.
Oh I see, it's kind of like testing how big your house is. You know, if you walk and you hit a fence, throw a javelin and if it hits something, go jump the fence and see there's another fence. Is that kind of what you mean?
Yeah, precisely. And this argument is essentially trying to say that it's nonsense. For the universe to be finite is no way in which an edge makes any sense at all, because if you get to an edge, then you know you have to think what's past it, and the javelin is sort of a way to probe it. Now, that argument, of course, totally flawed. It's not a proof that the universe is.
Into the Greeks for wrong. What you can't prove that the universe is infinite with a javelin.
Turns out we have learned one or two things since ancient times javelins. Yeah, and we'll get into it in much more detail, But the essential idea is that this is flawed because the universe could be closed. It could be that the universe sort of loops on itself, and you could keep throwing that javelin and picking it up and throwing it and picking it up and throwing it forever, even in a finite universe.
Interesting, it is possible, you're saying for the universe to have a wall at the end.
I actually do think that that's not nonsense, that it's possible for the universe to have a wall. But I think the simpler idea is for the universe to sort of just be closed on itself, like on the surface of a sphere. You know, if you're gonna throw javelin and then follow it and pick it up and throw it, you could do that on the surface of the Earth basically forever without hitting a wall. That doesn't mean the Earth is infinite, it's not.
All right, Well, let's get into the different scenarios for what the universe can look like, whether it's infinite or not infinite or finite. But it's kind of a big question, right, It's kind of a deep question whether or not the universe goes on forever or whether it has a limit. But we were wondering, you know, we kind of like to wonder here, how important is this question? And does it really matter to people?
Yeah, so I walked around campus and instead of asking them if they thought the universe was infinite, I asked them if it mattered to them, does it make a difference in your life if the universe goes on literally forever or is just really really big? And then I asked them if they wanted to know the answer. And I think their replies to that we're quite revealing.
You asked them a two part question. First, you asked them the technical one, and then you ask them if they wanted to know the answer. All right, well, think about it. Does of you listening? Think about it for a second. Do you think the universe is infinite or is really big? And would you want to know the answer either way. Here's what people had to say.
I think it does matter because it changes the scale of how it's just weird.
It's just weird to think that it's infinite. You can't really see that it's infinite, but it's just endless to me.
No, No, it's interesting, but it doesn't urge me to care, which the answer is.
Okay, no, not really.
It's interesting, but not really, not really. But do you want to know?
Yes?
I do, So why do you want to know if it doesn't make a difference. I don't know.
It's a question nobody can really answer right now, and I just think that's interesting. I definitely want to know, but it doesn't influence my everyday to life. So then why do you want to know out of curiosity? I don't like not knowing things. I think that's the whole reason joined physics, So it's to gain more knowledge about the universe that we live in.
I think it'd be cool to know, but it doesn't really affect me.
So why is it cool to know there's.
A higher or lower chance of like finding like a new planet, or maybe like there's a higher chance of finding aliens? I'd say on day to day No, probably not.
But do you want to know that the answered?
I think I would like to know the answer? Yeah?
Why do you want to know? Because it would be nice to.
Be able to say that I know whether or not it's infinite or just unfathomly large.
All right, No, do.
You want to know the answer though, Yeah? So why do you want to know the answer? Just because you always want to know the answers to everything? I think if it is infinite, that would blow my mind. I would be interested in knowing the answer to that yet. All right, So why do you want to know?
Like I think, I'm someone who just likes to know things, and it's hard to actually articulate the reasons for that, but because.
It doesn't matter to me personally, but I'm intellectually curious.
All right. Well, it seems like people answered both of your questions. The answered whether they thought the universe was infinite or just really big, and whether or not they wanted to know the answer, and they were both kind of different.
Yeah, most people felt like, no, of course, it doesn't make a difference. You know, it doesn't change how you're going to live your life. It doesn't mean you should eat bn in is when they're green or when they have spots in them. Right, there's no practical difference. But then when I asked people if they wanted to know the answer, they perked up, and maybe they thought like I had the answer and I couldn't that or something.
Totally. You tease them and then you let them down.
But the thing is you should have seen the curiosity in their eyes, like, imagine knowing for a moment. They imagine what to be like to learn that truth that could change your relationship with the cosmos. Right, And I think a lot of people put that really well. One of my favorite responses was the guy who said that I'm someone who likes to know things, and it's hard to articulate the reason why, but it is something really deep, you know that you'd like to know.
Interesting. Yeah, I guess it. So most people. It seems acknowledged that knowing whether the universe is infinite or not one way get difference in their lives. But everyone still wants to know.
Everybody wants to know, and I want to know.
Yeah, there is sort of an inherent curiosity in people.
Yeah, and I think this being one of the biggest of questions really touches on that because, as we were saying before, I think it connects to our curiosity not just about ourselves, but about this place we live in. And it's sort of the biggest question about the biggest topic.
And so were they disappointed that you didn't know the answer to this question or that you wouldn't give it to them?
I said, hey, you got to tune into the podcast. So no, yeah, I did let.
One listener at a time. You're teasing one listener at a time.
I did leave these people disappointed unfortunately, you know, but hey, if they listen to podcast, maybe they can understand something about what we do know about the nature of this question.
Well, it seems like there are only two options. Either the universe is really big or the universe is infinite. There's sort of no third option, is there?
Yes, So yeah, the third option is you know, the simulation when you're actually trapped in a room somewhere in the universe is about ten feet across and everything you're experiencing is just blowney. But no, you're right. If what we're experiencing is real and it's not a simulation, then at the very minimum, the universe is ginormous.
But it could be really really big, or it could be extremely infinitely big.
Yeah, And there's a lot of gradations there. It could be like ridiculously big. It could be ridoculously big. It could be like mind blowingly unfathomably big. It could be like ten to that big, or it could be actually.
Infinite about ten to the ridiculously ridiculously big.
Yeah. I mean, any of these things. These are real numbers, you know. And the crazy thing for me is that one of these is true. Right, There is a truth out there. The universe actually does have a size. Either it's finite or it's infinite, and one of those is true, And one day some human I actually know that truth and have that experience that all these listeners, all those interviewees we're hoping to have.
Well, I feel really teased now. I hope you have an answer for me at the end here, Daniel, Otherwise I'm going to be disappointed as well.
The answer is fund more physics research.
I see, give Daniel more money. All right, Well, so let's get into this question a little bit more, and let's get into how we would know if the universe is infinite or fine and what would happen if we found out. But first, let's take a quick break.
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All right, Daniel, we're asking the question is the universe infinite or is it finite? And I guess my first question is, you know, is it possible? Which of these is possible? Are they? Are these both possible possibilities for the universe? Can the universe be really infinite? Or does it make sense for the universe to have a wall at the end and be closed off.
I think unfortunately, all of these things are possible. We can go through the various scenarios. But you know, we're talking about big questions here, and the questions that we don't have a lot of history in getting answers to. It's not like we've been to lots of universes. We've seen what happens, and we have a lot of experience where grizzled veterans in measuring universes. Now this is going to be the first time if we ever do get an answer, that we would learn this about a universe. So we got to be open minded. Things that seem crazy to us are going to be based on our experience here, right, And so like if you think.
Oh, nobody has a PhD yet on answering this question.
What you're saying exactly, we all got to be open minded. So anything that works mathematically, even if it's like revolting to our physical intuition, we got to be open to it.
Because well, we've been making progress. Right before, we thought the universe was just us, and then we find out that it's the size of the galaxy, and then we find out there is more galaxy. So I feel like our understanding of how big the universe is has been getting bigger progressively.
Yeah, we've been biggering and biggering our imagination of the universe. But still all that is doesn't really address the question of whether it's finite or whether it's actually infinite. That just sort of increases the sort of minimum distance of the universe as we look further and further out. So, you know, so to answer your question, the universe could be finite or it could be infinite. Both of those things are possible.
Well, let's tackle each one at a time and see what each one would mean. So let's say, let's assume the universe is finite, meaning that there is a limited amount of space in space, and the the universe doesn't go on forever. At some point it ends. It has like a volume, like a set number that's the volume of the universe. Is that possible. Wouldn't that mean that there's like a wall at the end. And if there's a wall, what's on the other side of the wall.
You're gonna take your javelin, You're gonna go to that wall and you're gonna poke it. Right.
Yeah, I'm gonna spend forty billion years traveling just to throw that javelin.
Well, you have to remember what we're talking about here is the shape of space, and space is not nothing. It's not emptiness.
Right.
Space is a thing that we are existing, is the background on which all of this physics happens. But it's not nothing. It's dynamical. It can twist and bend and rip them do all sorts of crazy stuff.
It's got like substance to it to it. Right, the space of the universe.
I don't know if substance is the right word, but it has a nature to it. It's not nothing.
I see, there is nothing, like literally nothing, and then there's space, which has something.
We don't know if nothing is a physical possibility. Right, So when we're talking about like the edge of space, yes, it's possible for space to be finite in different ways. In one way, it could have an edge, like you could just like you get to the edge of space and there's and that's it. Now you might ask, well, what's beyond it? Well, the answer could be that there's nothing. There is not a thing beyond it. It's like when you get to the north pole and you want to go more north, there's just no more northiness to go.
Couldn't it be like a like an island and you get to the edge of an island and beyond that there's nothingness.
It could be an island. The edge is defined by the water, right, So what's the water in this analogy.
Right, Well, the water could be nothing, yeah, the space could be something.
Yeah. But then you've sort of just redefined space to be the nothingness. And then you could ask, well, how far does that nothing go on? It doesn't really answer the question, but there's a real possibility that space is all there is, that there is that nothing isn't an option that there's space and then there's space and there's not there's no place where there's no space.
Nothing is not an option. Boy, that's a confusing sentence.
And you just you get to that edge and you just can't go any further.
Like what happens if you try to keep going, you just would you would not go anywhere.
Maybe it's helpful to think about other examples where space is not simply connected, like in a black hole. If you're in a black hole, you can only go in one direction. You can only go closer to the center of the black hole, because that's the only path. The shape of space will allow It's not just that the gravity is really strong. It's the bending of space that makes that means that every path points towards the center of the black hole, right, and so in the same way, if you got to like the edge of the universe, it just means that there's nowhere to go but back or sideways there as.
I see, like the universe might redirect you.
Yeah, Like what happens when you get to the edge of the subway system, right, Well, you either ride it back or you transfer to another line.
Like you would throw a javelin at the edge of the universe and it would just get deflected to the side maybe, yeah, or come back at you inspere you.
Yeah, And that doesn't mean that there's a wall there holding it in. It just means space ends. That's a total possibility.
Space gets spacey or wonky at the edges.
Yeah, it gets because space is about it's about the relative connections of these pieces, right. It's not an emptiness. It's a thing where it's this fluid we're moving through. And what we're talking about is the curvature of space, the connectedness of space, the topology of it. If you want to get technical how it's connected to other bits, and and that changes, right, that changes when you put mass in something, it changes how that stuff is connected.
All right. So that's one possibility is that the universe is finite with an edge that is kind of wonky and doesn't let you poke through it.
Yeah, that would be really weird and awesome to discover. But it could also be finite in the way the surface of the Earth is finite but doesn't have an edge. And then it could be we call that closed. It could be essentially be curved. And we know that that space could be curved, right, gravity and energy bends it, and so it could just be that you keep going and you just sort of loop back to where you started.
Right. It's like asking what is the length of a circle? Like if you take a length of string and you tie it to itself into a circle, Like, what's the length of it. It's finite, but it's not like a set number because it just goes around and around.
Yeah, and it doesn't have an edge, right, there's no wonky bit there. You just keep going. And so that's totally possible, and that is consistent. If space curves in a certain way, then it can loop back on itself. And so we're talking about now, is this curvature of space is really important in understanding the possible shapes of the universe, which determine the possible sizes, right, because if, for example, space is curved in that way it's like the surface of a sphere, then it can't be infinite.
It cannot If space is curved.
Right, space is curve like the surface of a sphere, then it cannot be infinite. Yeah, then it would have to be closed and therefore finite.
Really couldn't it be like a spiral, Like it's curved, but it just keeps spiraling outwards forever.
Like facility pasta or something.
Yeah, like just take a string and keep spiraling it up.
It's interesting. I mean, you can have topologies that are like a doughnut, you know, or a bagel or something that have positive curvature, but that's.
Still like a seashell, you know, like it just keeps spiraling out, folding outwards. Isn't it possible to.
Yeah, I wonder about that, you know, Or like the surface of a cylinder. The surface of a cylinder is also curved, but could go on forever, but then the curvature would only be in one dimension. So I think if the curvature is sort of even in every direction, then then the sphere is the only A sphere or a toroid is the only shape that's consistent with that curvature.
What if it it's like a slinky, you know, like a spring, then it could be finite, infinite and curved.
Well, but I think that a slinky is essentially the same topologically as a straight cylinder. It's just sort of bent in another space, and that doesn't have curvature in every direction. But when we're talking about measuring the curvature of space, it's something innate, and so it would be the same in every direction.
Oh, I see. If it's curved inwards in all directions, then you sort of have to fold in on itself.
And so that's another possibility.
Right.
The universe could be finite and have a wonky edge, or it could be finite and be closed on itself and like the surface of a sphere without any edges.
So that's how a finite universe can make sense without imagining like a giant brick wall at the end, which doesn't make sense.
Yeah.
Yeah, But and you say you always like to say that this possibility that the universe is finite is sort of maybe comforting to human brains, but it is kind of a crazy idea, the idea that the universe ends at some point, or that there's it doesn't go on forever.
Yeah. Do you think it's comforting to think that the universe is finite because because infinity is hard to imagine.
Yeah, I mean, infinity is kind of scary, right, It's like it's like living in an infinite house. It's like a nightmare.
The clean electricity, the cleaning.
Bill, yeah, the cleaning bill, and the how many beds you have to buy? Keys, how many ikea furniture the rest of us you have to build. I mean, it's a it's the stuff nightmares are made up.
It could literally be building furniture forever. Yeah. And once you lose your keys, they're just gone, man, They're gone.
Yeah, that's true.
I'm not sure which is more comforting. But I think the idea of a finite universe is really odd because then there's a number, right, the universe would have a size, And then you have to ask, like why that number? Why not seven times that number or half that number? What is important? About that number, and you wonder like is it random or is it fixed? Is it determined by something?
Right? It would there would be like a fixed number of uh know, electrons in it, you know, like there would be a seven point three trillion point one.
And not one electron model or less?
Right, is there? Yeah, that's what makes a physicist worry.
It's just hard to grapple with. But that's why we're doing physics, right. We're asking these really basic questions about the universe so that then we can do the philosophy. Is so that then we can like do the therapy be like, all right, turns out this is the universe we live in. What does that mean? How do I live my life in that kind of universe?
And does that mean that when you go to a restaurant and you look at the menu, like the menu makes you uncomfortable? Like why are there only thirteen dishes here? Why aren't there fourteen dishes or twelve?
I know that that doesn't arise from some defundamental principle of the universe. That arises from some like argument you know, between the managers when they were founding the restaurants. So not every number you spot reveals something deep about the universe.
I see, all right, all right, all right, so then that's the finite possibility. So step me through the infinite possibility, because to me, this one's the one that seems impossible, like, how can something be infinite? But you're saying the universe could be infinite.
The universe could totally be infinite. Yeah, I mean if space is very simple and flat, you know, like the surface of a flat plane, then it could just keep going on. It could just go on and on and on. And in some ways that's sort of the simplest idea because it doesn't require an edge or complicated topology. I mean it requires you to somehow invent an infinite number of electrons, which is, you know, a whole other problem you have to tackle it.
Something, yeah, like an infinite amount of energy, right, and matter and possibilities like there's It means that there are versions of this Earth somewhere out there in the same universe as us.
And if you had a lot of experience measuring universes and finding them to be finite, I would understand your skepticism, right, But you have exactly the same experience that everybody else has, which is you have no idea how big a universe should be or whether it's weird to have an infinite universe, And so it could totally be and it could be very natural.
You're saying that I don't have a PhD in universe sizing.
You have an incredible array of talents, but that is not one of them.
Well, all right, so I guess as a physicist you kind of have to treat all possibilities as possible. You know, if it's possible, then it's a possibility.
Yeah, And so all you can do is say what's possible? What would make sense from the physics point of view? What can we make work, you know, mathematically and theoretically, And then you go out and you confront those theories with the data and you say, well, do they predict something we can measure? Is there something in the universe out there that can give us a clue as to which of these scenarios we are actually living in?
Right, because there might be clues out there that tell you whether the universe is finite or infinite without having to go to the edge to check that's right.
We certainly can learn something about this question by doing actual measurements that don't involve javelins.
Well, what do we know right now about the universe, Like, how big do we think it is as we know it right now? How big do we think it is?
Well, we've been looking out into space for a while and we have these really clever tools now for measuring how far away stuff is. You know, we talked about this on the podcast, measuring the distance to nearby stars by basically looking at them in two ways like binocular vision, and then using supernovas and stuff to measure the distance to even further stuff. So basically what we can do is we can ask, like how far out can we see? That sets a minimum size for the universe, just like you know, what's the observable universe, right.
Like what's the furthest thing that we can see with our eyes or telescopes, And that tells us at least the universe has to be that big.
That's right. And your naive calculation might be, well, it must be the speed of light times the age of the universe, which is almost fourteen billion years, and so you might think, oh, it's a sphere fourteen billion light years in radius, but that's actually an underestimate the universe that we can see is much much bigger than that.
Right, that is so naive, Daniel, That's like what a third year graduated in physics might think.
But no, I don't mean to be kind of saying at all. I mean like that would make a lot of sense that that's all we can see because light has to travel here from those places. And we talk a lot on the podcast about how you know, the further away you look, the further back in time you're looking at, eventually you run out of time. The universe is not infinitely old, so stuff that's really far away just hasn't had a chance to get here.
Yeah, well that's how we would think it would be. But you're saying that we can tell the size by how far the furthest thing we can see is.
Yeah, And it turns out that stuff is further away than the age of the universe times the speed of light because the universe is not static, it's expanding. Stuff is moving away from us. So light has gotten to us from stuff that is now further away than the speed of light times the age of the universe.
Because we know that, right, whether it's finite or infinite, we know it's getting bigger.
We know it's stretching, it's expanding, it's getting more spread out. So the furthest we can see is about forty three billion light years away.
Wow, or I guess twenty one billion light years each way, No.
It's forty three billion light years in radius. So it's a diameter of more than ninety billion light years across, which is I mean, it's a huge number. Like, why do anybody even need more universe than that?
Well, depends on how many Daniel and jorgez you want.
We've got big plans here, folks.
Were more room we need, we need more of us obviously. All right, So our current estimate of how big the universe is is ninety six billion light years. What meaning it would take you right now ninety six billion years to go from one end to the other or more I guess since it's growing.
It's growing. Yeah, so you could to actually go from one edge of the observable universe to the other because it's growing and space is expanding faster than light can move through it. So a photon on one edge of the universe will never reach the other side of the observable universe.
So we know at least it right now as far as we know is eighty six billion light years wid.
Yeah, and that's the minimum, right, So that's an at least and that's already a really big number. But we know something else about the universe, which is really important. We know not just how far we can see. We know something about the shape of space. We know like how things are curving.
All right, let's get into the shape of space and how shapely it is and whether or not it is actually infinite or just crazy big. But first, let's take another quick break.
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All right, Daniel, So we know that the universe right now, or at least at some point, is at least eighty six billion light years wide. But we don't know if it keeps going on much more than that, or a little bit after that, or maybe that it's actually infinite and it goes on forever beyond eighty six billion light years that we can see. And so I guess the question is, how could we even tell you know we're sitting here, in this little rock, in this tiny galaxy, How could we possibly tell whether the universe goes on forever or not because we can't really get travel as far you know, forever to check.
Yeah, you're right, in some sense, we know literally nothing about what's passed there. Like there could be crazy blue dragons the size of galaxies out there and we would have no information about it, as photons from those dragons haven't reached our eyeballs, and so almost anything could be happening outside the observable universe. So any argument we're going to make is going to be sort of frustratingly indirect because we can't go out there, and even if you wait another billion years, the observable universe grows, we can see more of it, will never literally never see all of it. Right, It's not like we will could ever prove directly that the universe is infinite. So it's going to rely on some sort of more indirect argument.
Right, because you could send a probe out there to see if you hit a wall or if it curves around, but you would have to check forever to make sure that the universe doesn't go on forever.
So you could prove that the universe is finite, but you can't prove directly, at least that it's infinite. Like you could send a probe out there and it does hit a wall, or it loops back around itself and comes back to Earth and it's very surprised. Right. You could make that kind of demonstration, and people have looked, you know, they've looked out of the space to see like our stars repeating, do see the same pattern over and over again, which would suggest that the universe is sort of small and finite, and that light has zoomed through it a few times in the history of the universe. So you could prove that it's finite, but direct proof that it's infinite is actually impossible.
Wow, that's a little disturbing.
But we can make some interesting arguments, right, you know you said before you were making arguments before about how unusual or unnatural infinity is. We can make sort of the opposite set of arguments. We could argue that infinity is actually the most natural explanation.
You could find some mathematical proof that the universe is infinite. You mean, like, from what we can tell about the nature of the universe, it says our formulas say that the universe should be infinite or finite.
Yeah, you can make that kind of argument. And while that argument will never be like one hundred percent satisfactory because you can't ever prove that something is infinite, it could still be pretty satisfactory. And a lot of other things in physics or in science at least we accept with the same level proof. For example, we think that momentum is conserved everywhere in the universe. You know, we think that every time things bump into each other, the momentum is the same before and after. We haven't checked every single time rocks bump into each other, the momentum didn't leak out of the universe somewhere, right, we have a fundamental principle. There's a symmetry about translational variants that we invoke that we believe, and that momentum conservation momentum is a consequence of it. So in a similar way, we make some argument that implies the universe has to be infinite, then maybe we could believe it.
I see, like, maybe if a conservation of momentum necessitates, like it needs for the universe to be infinite for it to be true, then we might as well say that, yes, the universe is infinite, because if you believe conservation of momentum, then you have to believe that the universe is infinite.
Yes, and I have to necessitate that, right, So you need some theory of how the universe works that requires the universe to be infinite. That so the theory only works if the universe is infinite. And then you need to rule out all other possible theories and show that the data are consistent with your theory that requires an infinite universe. And in that case you could convince yourself that the universe is infinite.
If you had the right PhD thesis, you could make the you could convince yourself of anything.
Yeah, And so I went around and I actually asked cosmologists this question, you know, said, can you imagine a theory of inflation or of the early universe or of cosmology that only works if the universe is infinite? And that's stumped all of them.
Really, they couldn't imagine it.
Well, we have a theory of the universe. It's inflation that's most natural if the universe is infinite, but it could probably work in a finite universe.
Also, it can work either way. It doesn't assume an edge.
It doesn't assume an edge or a curvature. And you know, we've measured the curvature of the universe and we've measured to be really flat, like very very very very flat, and that's consistent with what we think happened to the very ginning of the universe. We think the universe expanded really really quickly and that stretched everything out. But to cosmologists, the universe is either infinite or like ridiculously big, like we're on the surface of a huge sphere. And to them it doesn't really make that much difference for their calculations because the chrismologist for the cosmologists, oh.
Man, so everyday people care more about whether the universe is fine out or infinite than cosmologists.
Well that's funny because I asked them, like, does it make a difference to your theory whether the universe is actually infinite or just really vast, like, you know, like it seems flat because we're on the surface of a huge sphere, but it's actually not. And the said, no, it doesn't make any difference. And then I said, well, would you like to know the answer? And then they were like, oh, of course, I'm definitely curious, and so in that way they share this wonderous curiosity, right.
Even though it doesn't affect their job either, even as cosmologists.
That's right.
But I did get an estimate, an estimate of how crazy the idea is or what this theory would look.
Like, estimate for sort of the minimum size of the universe. Given that what we've measured is that the universe seems flat, then either the universe is infinite or it's at least ten to the ten to the ten to the one hundred and twenty two megaparsex wide. What And that's an that's an argument from like looking at the quantum fluctuations in the early universe and asking how do those blow up to give us the universe that we have now, and if so, how much must it have blown up? And that's the number they come up with.
So if you ask as cosmologist, people who think about the universe and the beginning of it, they have a minimum size of the universe.
Some of them do. This is not like a widely believed number, it's just like an estimate.
Some of them, I see, Yeah, But so some of them, based on what they know about the universe, they think that the universe has to be at least that humongous number big.
That's right, But I think another interesting thing to understand is that to cosmologists, the universe being infinite is sort of the default. It's the idea that makes the most sense to them. They have no trouble with that at all. And the reason is that they invoke a very basic argument. They say, look, the universe should be the same everywhere, and if the universe is finite, then you ask questions like why is it finite? Why is it here? Why is it not over there? And so this sort of like universal principle that the universe should look the same everywhere makes it very natural for them to think of the universe as being infinite.
Right. I'm a little suspicious, Daniel. I feel like you asked those cosmologies and they believe the universe is infinite. But when you probe them to give you a minimum number, they just gave you a ridiculous, made up, fake number. I mean, what kind of number is like ten to the ten to the ten to the ten. That sounds like they just pulled that out of their pocket.
Well, you know, you can read the paper we'll link to it from this episode, and there actually is a calculation there, you know, and you start from small quantum fluctuations and what we know about the expansion of the universe. And if you want to get a universe that's this flat, where space really does seem flat, then either it's infinite or you're on the surface of a ridonculously big sphere of radius. I don't even know what you name that.
Well, oh, I see what you're saying. You're saying that if the universe is finite, then we would see more curvature in it, because that's the only way to explain the A finite universe is kind of the sphere or taurus type of thing. Mm hm, oh, I see nice. But the so the only way to explain the flatness that we see is by either assuming the universe is infinite or it's a ridiculous number.
Yes, and I hold that hope that you know, someday these cosmologists will think of a reason and will think of a way to actually test the infinitude of the universe.
Infinitude that sounds like like a I don't know, like a brand of a sound equipment or something.
It sounds like sneaker said, Bill and Ted will wear in their next great adventure. But you know, we're in the early days of understanding this question. You know, we don't understand how the universe came to be. There as a lot of possible constraints there. If we understood the mechanism that created the universe and started inflation, then we can answer questions like is it reasonable to have an infinite number of electrons or just ten to the seventy jillion? And so there's a lot more information coming down the pike, and so I'm optimistic that one day we will know the answer to this question.
All right, Well, it sounds kind of like you guys are leaning towards infinite, you know, it definitely sounds like you should all be preparing our brains for the fact that this is an infinite universe and there's an infinite number of means out there in other earths repeating itself after what.
I hope So, you know, I hope the universe goes on forever. It'd be sort of claustrophobic mentally to imagine that there's a limited amount of stuff. You know, that if we somehow were able to travel the stars, that there's like a full list of all the stars you could visit, and of course it'd be a big number. But it's not that comfortable to imagine that there's an end to that list, that at some point you'd be like, well, I've done them all.
Kind of liked this episode, which we have to wrap up soon because it's not infinite the amount of time we have.
And too bad, right, wouldn't you just like to talk forever?
Well? Sure, but when people like to listen to us forever a couple of dudes talking for an infinitude.
Daniel and Jorge talk forever.
Hey, yeah, the longest podcast ever. It's just a continuous stream.
And you'll never get to the end of it because we keep talking. If you started after we already began talking, you'll never catch.
Up, right, right, And then after a while, we just keep repeating the same jokes. Although I don't think you need an infinite universe for that though, or an infinite podcast. We've already we've already closed the curvature of that.
I think we've measured the curvature of this podcast. Yeah, yeah, it's fairly round.
It's about fifty podcasts episodes before we start repeating the checks.
But I think this is one of my funnest questions because it really does touch on our innate curiosity, our desire to understand the way the world works and our place in it, you know, And are we totally insignificant or are we just very insignificant? And that's an important question?
All right? Well, but in this universe we want to thank you for joining us and we hope you enjoyed that.
Thanks everyone for tuning in and for lending us a non infinite slice of your day. Before you still have a question after listening to all these explanations, please drop us a line. We'd love to hear from you. You can find us on Facebook, Twitter, and Instagram at Daniel and Jorge That's one word, or email us at Feedback at Danielandhorge dot com. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.
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