Is the Universe infinite?

Is the universe finite or infinite?

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Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, do you think there are some questions out
there that will never know the answer to? 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

(00:33):
I was thinking, you know, a 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 questions about

(00:55):
these questions forever? Or do you think we'll find answers eventually?
I think as long as we were around and we're
putting some money in basic research, we'll keep making progress.
So you know, it could take a 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

(01:18):
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 Jorge. I'm a cartoonist

(01:42):
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, Kale, 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

(02:02):
eating a smoothie, which is not something you want. That's
why they invented smoothies. That's right. I actually like the
red bananas. If 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

(02:23):
are sassy but unpretentious. Here we are La. Here we
are shilling for Big Banana. But welcome to our podcast.
Daniel and Jorge explain the universe and apparently bananas, in
which we like to explore topics that are big, bigger
even than questions about bananas, questions so big you can't

(02:44):
even wrap your mind around them. Yeah, it's our show
produced by our Hard Radio, in which we think about
some of the biggest questions that they're 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

(03:04):
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 our what you have 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

(03:26):
out there, the questions that not even people with PhD
s and universities have a good answer for. Are you
saying PhDs don't give you all the answers? I wish
I had an answer for that, Danny. I remember getting
my PhD. It was 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

(03:46):
sub basement and you give it to the margins lady
and she measures the margins and your PhD, and if
everything looks ship shape, she gives you a lollipop that
says PhD on it. And that's the moment you have
a PhD? Uh? Did they? Um? Did you do? You
still have that lollipop? You know, we recently cleaned out
our house and I found a lollipop and my daughter Hayes,

(04:07):
I was like, why does this lollipop say PhD on it?
You found it? You still head it around, lapping 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.
It's the red deal. Take the red lollipop and you
will never go back. No, but it's not very satisfied.

(04:28):
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 of our ignorance. Yeah,
I mean, at some point you basically become the world's
expert in a very particular topic, and you realize that

(04:49):
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 what

(05:12):
is it like really? I mean from we we know
what we can see from this little point in the universe,
in the 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 how far does it go? And and what's out there?
And it's fascinating to me that this is a fascinating question,

(05:34):
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 they're 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

(05:56):
we want to know the answers to tells me something
about who we are and how we think. Do you
think this question of um, 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 what is this
thing that we live in? Yeah, we want to understand ourselves.
We also really want to understand our context. You know,

(06:19):
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 wants 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

(06:41):
the only continent or are there more continent or 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 space and how far does it go? And it
really reveals something about the way they thought. To the
on the podcast, we'll be tackling one of these big

(07:02):
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 modeling lee enormous, right, but enormous

(07:24):
but with a limit like finite finite? Right? 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 believe the argument was infinite and

(07:45):
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 exactly. I like the low tech solution to some
of these questions. Wait, so you're saying the Greeks wondering
that the universe was infinite. So they knew kind of

(08:05):
generally what was going on, like we're a planet, we're
in a solar system, and then they were 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

(08:26):
and the question was how far do they go? So
how tall is the ceiling in this bio dome 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, like it has an

(08:46):
edge to 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 like hits an obstruction, in which case
that obstruction is past the edge. And so this is

(09:09):
sort of an argument that suggests that the universe must
be infinite. Oh, I see, it's kind of like um
testing how big your your your house is. You know,
if you walk and you hit a fence, throw a
javelin and if it hits something good, jump the fence
and see if there's another fence. Is that kind of
what you mean? Yeah, precisely. This argument is essentially trying
to say that it's nonsense for the universe to be finite.

(09:30):
There 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? Uh,
And the javelin is sort of a way to probe it. Now,
that argument of course, totally flawed. It's not approved to
the universe into Greek. Wrong, What do you can't prove
that the universe is infinite with the javelin? Turns out

(09:51):
we have learned one or two things since ancient times. 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 to the universe
or 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,

(10:13):
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. 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.

(10:37):
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 would like to wonder, hear,

(10:57):
how important is this 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 they replies to that we're

(11:19):
quite revealing. You asked them a two part question. First,
you asked him the technical one, and then you asked
them if they wanted to know the answer. All right, well,
think about it. Does if you listening, think about it
for a second, do you think the universe is infinite
or just really big? And would you want to know
the answer either way? Here's what people have to say.
I think it does matter because it changes the scale

(11:42):
of how it's just 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 no,
not really. It's interesting, but not really, not really. But

(12:03):
do you want to know? Yes, 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 the first time 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

(12:24):
whole reason they joined physics 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 a day to day no, probably not. But

(12:45):
do you want to know that? The answer I think
I would like to know the answer. 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 unfathomedly lige alright, No, do you want
to know the answer? Though? Yeah? So what 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

(13:07):
interested in knowing the answer to that. Yeah, alright, So
why do you want to know? I think I'm someone
who just likes to know things, and it's it's hard
to actually articulate the reasons for that, but because it
doesn't matter to me personally, but I'm intellectually curious. Al right. Well,
it seems like people answered both of your questions. The
answered whether they thought that the universe was infinite or

(13:30):
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
it in is when they're green or when they have
spots in them. Right, there's no practical difference. Um. But
then when I asked people if they wanted to know
the answer, they perked up and maybe they thought like

(13:52):
I had the answer and I could totally he teased
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 would be
like to learn that truth that could change your relationship
with the cosmos? Right, And I think a lot of

(14:12):
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, acknowledge that knowing whether the universe is infinite
or not won't make a difference in their lives. But
everyone still wants to know. Everybody wants to know, and

(14:35):
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 that were they disappointed

(14:55):
that you didn't know the answer to this question, that
you wouldn't give it to the I said, hey, you
got to tune into the podcast. So yeah, I did.
Let at a time, You're 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

(15:16):
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 there's sort of no third option,
is there, I guess? 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 bologny. But now you're right,

(15:40):
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 radoculously big,
it could be like mind blowing the unfathomably big. It

(16:03):
could be like tend to that that big, or could
be actually infinite about tend to the ridiculous lead duntlessly 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

(16:26):
it's finite or it's infinite, and one of those is true,
And one day some human might actually know that truth
and have that experience that all these listeners, all those
interviewees were 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. Give Daniel more money.

(16:52):
All right, Well, 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 day, and what
would happen if we found out. But first let's take
a quick break, all right, Daniel we're asking the question

(17:19):
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, and we

(17:39):
can go through the various scenarios. But you know, we're
talking about big questions here, and the questions we don't
have a lot of history in getting answers to. It's
not like we've been a 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 universe. So we've

(18:00):
got to be open minded. Things that seem crazy to
us are going to be based on our experience here, right.
So like if you think, oh, nobody has a PhD
yet on answering this question exactly, we all got to
be open minded. So anything that works mathematically, even if
it's like revolting to our physical intuition, we've got to

(18:21):
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's 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,

(18:42):
it 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 we finite or could
be infinite? Both of those things are possible. Well, let's
cycle each one at a time and and see what
each one would need. So let's say the let's let's

(19:05):
assume the universe is finite, meaning that there is a
limited amount of space in space um and and that
the universe doesn't go on forever. At some point, uh
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 the on the other

(19:26):
side of the wall. You're gonna take your javel and
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. It is the background on which

(19:49):
all of this physics happens. But it's not nothing. It's dynamical.
It can twist and bend and ripple and do all
sorts of crazy stuff. It's got like substance to that
to it, right, the space of 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's there
is nothing, like literally nothing, and then there's space, which

(20:10):
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.

(20:32):
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 like an 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,

(20:53):
and then space could be something. Yeah, but then you've
sort of just redefined space to be the nothingness, and
then you could ask how far does that nothing go on?
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 by that's a confusing sentence.

(21:17):
And then 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 what 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

(21:40):
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've got to like the
edge of the universe, it just means that there's nowhere
to go but back or sideways. I see, like the
universe might redirect you. Yeah, like what happens when you
get to the edge of the subway system right where

(22:02):
you either you know, 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 or come back at you
and spirit. Yeah, And that doesn't mean that there's a
wall they're 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

(22:24):
about the relative connections of these pieces, right, It's not
an emptiness. It's a thing where it's like this fluid
we're moving through. And what we're talking about is the
curvature 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

(22:46):
is connected. All right. So that's one possibility is that
the universe is finite with an edge that is kind
of wonking 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

(23:08):
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 leg the string string and
you tie it to itself into a circle, like, what's
the length of it. It's finite, but it's not um
like a set number because it just goes around and around. Yeah,

(23:29):
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,

(23:53):
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 curve, right, If space is curved like
the surface of the 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 fucilit pasta

(24:13):
or something. Yeah, Like just take a string and keep
spiraling it up. It's interesting. Um, you can have topologies
that are like a donut, you know, a bagel or
something that have positive curvature, but that's still like a
like a like a like a seashell, you know, like
it just keep sparling out folding outwards. Isn't it possible

(24:36):
to Yeah, I wonder about that, you know, or like
the surface of a cylinder, surface of a cylinder is
also um 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

(24:57):
a toroid is the only shape that's consistent with that curvature.
What if it looks like a slinky, you know, like
a like a spring, then it could be finite, infinite
and curved. Well, but I think the slinky is essentially
the same topologically as a straight cylinder. It's just sort
of bent in another space, and that doesn't have curature
in every direction. But we're when we're talking about measuring

(25:18):
the curvature of space, it's something innate and it would
be the same in every direction. Oh, I see. If
it's 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. Um and like the surface of
a sphere without any edges. So that that's how a

(25:38):
finite universe can make sense without imagining like a giant
brick wall at the end, which doesn't make sense. Yeah. Yeah,
And but and you say you always like to say
that this possibility that the universe is finite, it's 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 there's it doesn't go

(26:00):
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 electricity bill, yeah, the cleaning bill,
and the how many beds you have to buy, how

(26:21):
many I kea furniture a dress as you have to build.
I mean, it's a it's the stuff nightmares are made up.
You could literally be building furniture forever. Yeah, and once
you lose your keys, they're just gone. Man, They're gone.
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

(26:43):
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, you know, electrons
in it. You know, like there would be uh seven

(27:04):
point three trillion point one and not one electron more
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

(27:25):
the therapy. Were 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 many
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 de fundamental principle of the universe.

(27:49):
That arises from some argument you know, between the managers
um when they were founding the restaurants, and not every
number you spot reveal something deep about the universe. Alright, alright, alright,
so then that's the finite possibility. So step me through
the infinite possibility, because to me, this one is the
one that seems impossible, like how can something be infinite?
But you're saying the universe could be infinite. The universe

(28:12):
could totally be infinite. Yeah, I mean if space is
very simple and flat, you know, like the surface of
a 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,

(28:34):
which is you know, a whole other problem you have
to tackle at something 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

(28:58):
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 that I don't have a PhD in universizing.
You have an incredible array of talents, but that is
not one of them. Well, um, all right, so I

(29:21):
guess as a physicist you kind of have to treat
all possibilities. Is 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 it is there's

(29:44):
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 think about this question
by doing actual measurements that don't involve javelins. Well, what

(30:04):
do we know right now about the universe? Like, how
how big do we think it is? As as we
know it right now? How big do you 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 can 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

(30:28):
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.

(30:48):
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 for 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

(31:13):
year gradu stude 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 in
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

(31:35):
chance to get here. Yeah, well that's how how how
many we would think it would be. But you're saying
that we can tell the size by how far the
then 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

(31:56):
away from us. So light has gotten to us from
stuff that is how 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 or or I guess twenty one

(32:19):
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 anybody even need more
universe than that? Well, depends on how many Daniel and
jorgees you want? You want. We've got big plans here.

(32:40):
Folks need a 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,

(33:00):
so you couldn't actually go from one edge of the
observed universe to the other because it's growing in space
is expanding faster than like can move through it. So
a photon on one edge of the 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 eight six billion light years y. Yeah,

(33:20):
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

(33:43):
first let's take another quick break, all right, Daniel, So
we know that the universe right now, or at least
at some point is at least eighty six billion light

(34:04):
years wide, but we don't know if it 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 and this tiny galaxy, how could

(34:27):
we possibly tell whether the universe goes on forever or not,
because we can't really get travel is 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 because photons

(34:48):
from those dragons haven't reached our eyeballs, and so almost
anything could be happening outside the observable universe. So any
argument we're gonna make, it's gonna be sort of frustratingly
in direc act 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

(35:09):
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,

(35:31):
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 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 you see the same pattern over and
over again, which would suggest that the universe is sort

(35:51):
of small and finite and that light is zoomed through
it a few times in the history of the universe.
So you could prove that it's finite, but direct 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

(36:13):
the opposite set of arguments. We could argue that infinity
is actually the most natural explanation. You can 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.

(36:34):
And while that argument will never be like a 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 of 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,

(36:56):
that the momentum is the same before and after. We
haven't checked every and all 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
invariance that we invoke that we believe, and that momentum conservation,
momentum is a consequence of it. So in a similar way,

(37:16):
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 uh 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,

(37:37):
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 um and show that
the data are consistent with your theory that requires an
infinite universe. And in that case you could convince yourself

(38:00):
that the universe is infinite. If you had the right
pH D 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 cosmology that only works if the universe is infinite,
and that stumped all of them. Really, they couldn't imagine it. Well,

(38:24):
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 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

(38:46):
what we think happened the very beginning of the universe.
We think the universe expanded really really quickly and that
stretched everything out um. 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 theologists.
For the cosmologists, oh man, so everyday people care more

(39:10):
about whether the universe is fine it are infinite than cosmologist.
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 it is. 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 curious, you know, And

(39:32):
so in that way they share this wondering curiosity, right,
even though it doesn't affect their job either, even as cosmologist.
That's right. But I did get an estimate, an estimate
of of how crazy the idea his or or what
this theory would look like. No, an estimate for sort
of the minimum size of the universe, given that what
we've measured is that the universe seems flat, than either

(39:55):
the universe is infinite or it's at least ten to
the ten to the ten to the 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 did those blow up to give

(40:17):
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 its cosmologies,
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 numbers,
just like an estimate some of them, I see. But

(40:39):
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 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,

(41:02):
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
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

(41:22):
this because those becausemologies and they they they believe the
universe is infinite. But when you pro them to give
you a minimum number, they just gave you a ridiculous,
made up, fake number. I mean, what kind of numbers
like ten to the tent to the tent to the tent.
That's that sounds like they just pull that out of
their pocket. Well, you know, you can read the paper
will link to it from this episode, and there actually

(41:46):
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
than either it's infinite or you're on the surface of
a doncillous the big sphere of radius. I don't even
know what you name that. Oh, I see what you're saying.
You're saying that if the universe is finite, then we

(42:07):
would see more curvature in it, because that's the only
way to explain the finite universes is kind of the
sphere or tourist type of thing. Oh I see, nice,
but the only 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

(42:28):
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 of of a sound equipment or something. It sounds
like sneakers that Bill and Teddible were in their next

(42:49):
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

(43:10):
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 we should all be preparing our
brains for the fact that this is an infinite universe
and there there's an infinite number of mees out there
in other earths, repeating itself after a while. I hope,

(43:33):
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 we're 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,

(43:54):
I've done them all, kind of like this episode, which
we have to wrap up soon because it's not infinite
amount of time we have and too bad, right, wouldn't
you just like to talk forever? Well? Are you sure?
But what people like to listen to us forever? A
couple of dudes talking friend infinitude. Daniel and Jorgey Talk Forever,

(44:18):
the longest podcast ever. It's just a continuous stream that
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 after a while
we just keep repeating the same jokes, although I don't
think you need an infinite universe for that thing, or
a podcast. We've already we already closed the curvature of that.

(44:41):
I think we've measured the curvature of this podcast. Yeah,
it's fairly round. It's about fifty podcasts episodes before we
start repeating the jokes. 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?

(45:04):
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 to 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.

(45:26):
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 Daniel and
Jorge dot com. 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 Apple Podcasts, or wherever you listen

(45:49):
to your favorite shows. Yeah.