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Kurzgesagt – In a Nutshell
The Paradox of an Infinite Universe
The Paradox of an Infinite Universe
Kurzgesagt – In a Nutshell
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11:20 · Feb 27, 2024
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Is
the
universe
infinite?
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0:04
Is the universe infinite?
0:06
Does it have an edge?
0:08
And if so, what would you see if you went there?
0:12
Today we know that the universe had a beginning 14 billion years ago
0:16
and that it's been expanding ever since.
0:18
But something that's expanding should also have an edge, right?
0:22
Well, it sort of does.
0:24
Information can’t travel faster than light.
0:27
That means that we can only see parts of the universe whose light has
0:30
had time to reach us in the last 14 billion years.
0:34
When we look outwards, what we see is a sphere centered on us,
0:37
the observable universe.
0:40
But it gets a tad more complicated – because the universe has been expanding,
0:44
we know that the most far away things whose light we can see are
0:48
actually 45 billion lightyears from us right now.
0:51
So the observable universe is a sphere with a radius of 45 billion light
0:56
years.
0:57
It contains around 200 billion galaxies, each with hundreds of billions of stars.
1:03
So for us, there is an edge:
1:06
We are looking at the past until there is just no past left.
1:10
This edge is really more like an edge in time
1:13
and in a sense meaningless.
1:16
The real universe is for sure bigger than what we can see.
1:19
But how much?
1:21
There are two options – either the universe is really big but finite,
1:25
or it’s truly infinite.
1:28
A finite universe means that, if you want to fill it with ice cream,
1:31
you can do it.
1:33
You need a lot of ice cream, but it is a finite amount.
1:37
But that leads to a weird problem.
1:39
Such a universe should have an edge, a cosmic wall where space ends.
1:45
And if there is an edge there should be something outside that edge.
1:49
But the universe by definition is “all there is”,
1:52
so how could there be stuff outside all there is?
1:56
Does the idea of something outside of everything even make sense?
2:01
Except of course there is a physics hack:
2:03
You can have a universe that has no border,
2:06
but that still could be filled with a finite amount of ice cream.
2:10
What if the universe is truly finite?
2:13
Imagine an orange and a really tiny ant.
2:17
The ant can only see a small piece of the skin – just
2:20
as you can only see a small chunk of the universe.
2:24
But if the ant starts walking,
2:26
it will eventually walk around the whole orange
2:28
and be back to its starting point.
2:30
So the skin of an orange is not infinite –
2:33
but it doesn’t have a border.
2:35
The universe is not like the skin of an orange,
2:37
but it could be very similar – instead of a sphere,
2:40
it could be a hypersphere, where 3D-space is curled on itself.
2:44
Which is impossible for your brain to visualize unfortunately.
2:47
But the point is: no borders,
2:49
no outside of the hyper sphere – from our human ant perspective,
2:53
our whole 3D space is like the peel of the orange.
2:56
If you were aboard a spaceship flying in a straight line,
2:59
you would eventually come back to Earth.
3:01
How does any of this make sense?
3:03
The actual physics is hard,
3:05
so we have to simplify and lie a bit here.
3:08
But in a nutshell it all boils down to gravity:
3:11
The way it works is that mass creates gravity by bending spacetime.
3:16
This bending is the strongest where the mass is,
3:18
but sort of stretches on forever,
3:20
like a very mild tension in the fabric of spacetime itself.
3:25
This could bend the whole universe in a way where it bends back on
3:28
itself,
3:29
which then makes the hypersphere.
3:30
If you find this confusing, we are with you.
3:34
If the universe happens to be a hypersphere,
3:36
how could we find out how big it is?
3:38
On Earth we can see things disappear below the horizon
3:41
and that helps us calculate how big the Earth is.
3:44
Scientists tried to find some sort of “universe horizon”
3:48
that would reveal the scale of the cosmic sphere –
3:50
but didn’t see anything.
3:52
Which means that if the universe is a hypersphere,
3:55
it needs to be
3:55
so big that from our perspective it looks like we are living on a
3:58
flat surface.
4:00
For this to make sense,
4:01
a hyperspherical universe should be at least 1,000 times bigger than our observable part.
4:07
It could be a trillion times bigger for all we know,
4:09
but not smaller than that.
4:11
Some scientists thought all of this is way too straight forward
4:14
and came up with a wilder option:
4:17
The universe could be like the frosting of a donut.
4:19
A hyperdonut – also impossible to visualize for your brain.
4:23
This, too, means that if you travel in a straight line,
4:26
you'd get back to where you started.
4:28
But with fun complications.
4:30
In a hyperdonut universe there is not the same amount of stuff in every
4:34
direction.
4:35
If two spaceships fly in different directions,
4:37
one could get back to the start way earlier.
4:41
This also means that light from faraway galaxies would do fun and confusing stuff,
4:46
in a sort of cosmic hall-of-mirrors effect.
4:49
We could see far away things in two places – but not just that,
4:52
but we would see it in different moments in time!
4:55
Because its light would have taken much longer to travel in one direction than
4:59
the other!
5:00
You could see a star being born in front of you
5:03
and see that same star die on the opposite side of the sky.
5:07
How big would such a hyper donut universe be?
5:10
Well, because of its strange geometry,
5:12
actually this is kind of the smallest possible universe – potentially just a few
5:16
times bigger than the observable universe.
5:19
But it could also be way, way bigger.
5:21
We don’t know.
5:23
Ok.
5:24
So much for finite universes.
5:26
But… What if it's truly infinite and space goes on forever?
5:31
What would that be like?
5:33
What if the universe is truly infinite?
5:37
Actually, the cosmological model used by most scientists, describes an infinite universe.
5:43
We mostly use it to calculate what happens inside our observable chunk,
5:47
but if taken literally, it predicts an infinite universe.
5:51
An infinite universe goes on, well, forever,
5:55
with no border anywhere – also impossible to visualize.
5:59
Wherever you look you'll find more and more stuff in every possible direction.
6:04
This kind of breaks our brain a bit for a few reasons:
6:07
First of all, if the universe is infinite,
6:10
is it also eternal and has been there forever?
6:14
Was there a time where there was nothing everywhere
6:16
and then there was something everywhere?
6:19
Well we don’t know –
6:20
but we have a lot of evidence for the big bang,
6:22
so it really seems like the universe started at some point in the past.
6:27
But wait – since the big bang the universe has been expanding.
6:31
How can an infinite thing that is everywhere become even bigger?
6:35
Cosmic expansion just means that the distance between galaxies is growing with time.
6:40
Even an infinite space can become bigger.
6:43
Welcome to the paradoxes of infinity.
6:46
Infinity gets much weirder though.
6:49
As you travel with your spaceship in a straight line, you find new galaxies,
6:53
stars and planets, new wonders, new weird stuff,
6:56
probably new aliens and new lifeforms stranger than you could ever imagine.
7:00
But after a long time,
7:02
you might find the most special thing in the universe: Yourself.
7:06
An exact copy of you watching this video right now.
7:10
How can that be?
7:11
Well, everything in existence is made of a finite amount of different particles.
7:16
And a finite number of different particles can only be combined in a finite
7:19
number of ways.
7:21
That number may be
7:21
so large that it feels like infinity to our brains –
7:24
but it's not really.
7:26
If you have finite options to build things,
7:28
but infinite space that is full of things in all directions forever,
7:31
then it makes sense that by pure chance, there will likely be repetition.
7:36
Although just because something is possible, doesn't mean it will actually happen.
7:40
Maybe the universe goes on forever but only the boring stuff repeats itself endlessly.
7:44
Maybe there is really only one you.
7:47
But if the laws of physics are the same everywhere, then far,
7:50
far away gas could have given birth to stars and planets,
7:53
where primitive genetic information could emerge from chemistry.
7:56
Which might have ended up in cells
7:58
and animals that evolved in really unlikely ways.
8:01
And eventually apes that learn how to create online videos.
8:05
It happened at least once, so the chance is not 0.
8:09
Even if the chance of there being an exact copy of you watching this
8:12
video right now is almost zero – almost zero is still a huge number
8:17
in an infinite universe.
8:19
Unfortunately you'll never meet.
8:21
Because almost zero still means the chance is incredibly small.
8:25
Earth as it exists right now is so unlikely,
8:28
you’d have to travel incredibly far to find a second identical Earth:
8:31
Some 10 to the 10 to the 29 – a 1 followed by 100
8:35
octillion zeros – times the size of the observable universe.
8:39
So far that it kind of means forever far away.
8:42
Still, in a truly infinite universe,
8:45
every combination of particles could reasonably not just (sighs) It's time to stop now.
8:47
While these scenarios are possible on paper – we've entered science philosophy here.
8:47
Currently none of these ideas are testable or proveable.
8:47
If the universe is infinite we will never know.
8:47
The reality is, for us the universe is finite and has an edge.
8:47
And we can see the edge, the edge in time.
8:47
Everything we can interact with is within the observable universe and most likely,
8:47
this will stay true forever.
8:47
Even if this finite universe feels small to you,
8:47
it's more than big enough to fulfill all the dreams
8:47
that we and our descendants can ever come up with.
8:47
Infinitely enough for all of us.
8:47
While we can’t be sure whether the universe is infinite,
8:47
many fascinating questions do have definitive answers in science, math,
8:47
and technology - and you can learn all about them on Brilliant.org.
8:47
Brilliant will make you a better thinker and problem solver,
8:47
with thousands of hands-on lessons on just about anything you may be curious about.
8:47
They recently launched a whole new suite of data courses
8:47
that use real data to tell powerful stories
8:47
while training your brain to see trends through the chaos of truly massive data
8:47
sets.
8:47
You’ll answer practical questions like:
8:47
how to maximize revenue hosting on airbnb using the same analysis techniques
8:47
that scientists use to explore the distribution of galaxies
8:47
and unravel the mysteries of the cosmos,
8:47
a foundation that can take your understanding to astronomical heights.
8:47
We’ve also partnered with Brilliant to create a series of lessons to take your
8:47
scientific knowledge to the next level.
8:47
These lessons let you further explore the topics in our most popular videos,
8:47
from rabies and mammalian metabolism to climate science and supernovae.
8:47
Each lesson on Brilliant is interactive,
8:47
like a one-on-one version of a kurzgesagt video.
8:47
And you can get started whenever, wherever - right from whatever device you’d like.
8:47
To get hands-on with kurzgesagt lessons and explore everything Brilliant has to offer,
8:47
you can start your free,
8:47
30-day trial by signing up at Brilliant.org/nutshell.
8:47
There’s even an extra perk for kurzgesagt viewers:
8:47
the first 200 people to use the link get 20% off an annual membership
8:48
once their trial ends.
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