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NARRATOR: Islands have edges.

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Planets have edges.

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Even galaxies have edges.

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But what about the universe?

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As explorers,

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as curious humans that we are,

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we're obsessed with boundaries
and limits,

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and we want to know -
does the whole thing,

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the universe have a limit?

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Does the universe have an edge?
Well, the answer is yes...and no.

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It depends on
what you mean by "edge".

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The edge of what we can see?

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The edge of where we can go?

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Or the edge of reality itself?

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Looking out to the edge
of the universe is
tremendously important

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to understand our place
in the universe itself.

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We're talking about our universe,
we're talking about the thing

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that we exist within,
the most fundamental thing there is.

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We're driven to understand it.

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There's always a desire to push
the knowledge to the edge.

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So, can we ever find
the edge of the universe?

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2016.
The Hubble Space Telescope turned

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toward a dark patch of sky
in the constellation Ursa Major.

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It captured an image
of an indistinct blob of light.

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The glow is from a distant galaxy
called GN-z11.

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It's the most distant galaxy
we've ever observed...

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..but is it the edge of the universe?

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The universe all around us
is filled with galaxies,

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so it's natural to say,
"Would there be a final galaxy?

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If you travelled far enough away,
would you finally be

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at the very last galaxy
in the universe,

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looking out into empty space?"

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That's a difficult question
to answer...

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..because there is a limit
to how far we can see.

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It all comes down
to the speed of light...

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..and the age of the universe.

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The key to understanding
the edge of the universe

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is that light travels
very, very fast,

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but not infinitely fast.

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It takes time for it to get
from one place in the universe
to the other.

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You open the curtains,
light fills the room,

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it doesn't seem to travel at all.

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But over the vast distances
of the universe

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you actually notice
this travel time.

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Even the sun,
93 million miles away,

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the light takes eight minutes
to get to us.

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When you look out at the stars
we start to think of distance
in terms of light-years,

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because it takes years for the light
to get from those stars to us.

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And then when you look at galaxies
you're talking about

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millions or billions
of light-years.

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When we look at the light
from galaxy GN-z11...

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..we're seeing light emitted
13.4 billion years ago.

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You can't really even find a galaxy
too much farther away than that,

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because the universe
is only 13.8 billion years old

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and it takes a certain amount of
time for galaxies to even form.

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So we're not gonna find
too many more galaxies
farther away than this.

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If things are far enough away,
there is no way

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that light can get to us
in the age of the universe.

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What this means is
there's a hard limit

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to the edge of the universe
that we can see,

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and this is set
by the age of the universe.

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GN-z11 sparked into life early
in the history of the universe...

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..just 400 million years
after the Big Bang.

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Before that, there were no stars
to send out light.

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If you look in any direction at all,

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you get all the way back
to when there were no stars,

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no galaxies,
nothing but very, very hot gas,

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and that sort of forms
a shell around us.

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That outer shell
is the cosmic microwave background.

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It's the oldest light
in the universe.

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The echo of the birth
of the universe,

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the Big Bang.

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The edge of our universe,
the very furthest thing
that we can see,

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is one of the earliest relics of
the formation of
the universe itself.

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That is
the cosmic microwave background.

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We call this the edge
of our observable universe.

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So we have an observable universe,
but beyond that, even if
there are things out there,

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there's no way we can see them

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because the light just could not
have gotten to us by now.

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As the name states,
the observable universe

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is simply the part of the universe
we can see.

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We can think of
the observable universe
sort of like a spotlight

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centred on wherever
you're standing right now,

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and you can see to the edge
of your spotlight and not beyond.

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But if you move a little bit to
the left, a little bit to the right,

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your observable universe
actually moves with you.

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For someone living in galaxy GN-z11,

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a totally different part
of the universe would be observable.

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So that distant galaxy

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is at the edge
of our observable universe...

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..and we are at the edge
of their observable universe.

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We have different spotlights.

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A wonderful thing to think about
is that there are other spheres
around other galaxies.

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There are other aliens
looking up into the sky tonight

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wondering what the true extent
of the universe is.

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The true extent of our universe
doesn't end with galaxy GN-z11.

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But when astronomers use
the Hubble Space Telescope

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to measure the distance to GN-z11...

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..they find something shocking.

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It's 32 billion light-years away...

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..three times further
than thought possible.

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So, if nothing can travel
faster than light,

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and we measure the distance
to this galaxy...

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how can it be
32 billion light-years away?

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There hasn't been enough time
in the history of the universe

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for light from GN-z11 to reach us.

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There must be
some mistake here, right?

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At this point, your brain
is probably thinking

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of leaping out of your skull
and running around screaming.

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Trust me, I know.

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I'm an astronomer,
I've been doing this my whole life,

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and this stuff twists
my imagination up.

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It's really hard to grasp this.

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How do we see a galaxy
that's 32 billion light-years away

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and only 13.4 billion years old?

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GN-z11 is further away
than it should be...

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..because something strange
is going on with our universe.

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It's expanding!

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If the universe is expanding,

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then where does its edge lie?

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And can we ever reach it?

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13.8 billion years ago,
a speck of energy burst into life.

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We call it the Big Bang.

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Space and time pushed out
in all directions.

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Ever since,
our universe has expanded.

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But the way it's expanding

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makes finding an edge
a major challenge.

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The universe is expanding,

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and expands according to
a very simple law

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that the farther away
a galaxy is from us

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the faster it appears
to be receding away from us.

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The furthest galaxies are moving
at very high speeds.

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The most distant galaxy
we've ever spotted, GN-z11,

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seems to have moved 32 billion
light-years away from us...

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..in just 13.4 billion years.

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That's faster
than the speed of light.

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We can measure the speeds

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with which galaxies
are moving away from us.

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And many, many galaxies
are moving away from us

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at speeds faster than
the speed of light.

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Sounds like it's breaking
the law, right?

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There's this idea you've been told
that relativity says

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nothing goes faster
than the speed of light.

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OK. You've been lied to.

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PHIL: Space itself
can do what it wants.

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It makes the rules,
it can break the rules.

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That rule applies to matter,
not to space itself.

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Space can expand
at whatever rate it wants.

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A simple way to think
of this expansion law

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is imagine standing
on an infinite rubber sheet

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that stretches all the way
out into the distance

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and you're standing
on the same place -

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you can mark it with a little X.

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Now, all the sheet expands
in every direction.

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So if it expands by a factor of two,

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another galaxy that was, say,
one foot away from you

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is now two feet away from you
as we stretch the sheet.

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But another galaxy
was ten feet away from you.

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Expand that by a factor of two
and now it's 20 feet away from you.

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So in the same amount of time,
one galaxy moved one foot

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where another galaxy moved ten feet.

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So the more stuff there is,
the more elastic

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between you and another galaxy,
the more it seems to expand
away from you.

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Expansion means
our observable universe

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stretches for a colossal 46 billion
light-years in all directions,

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92 billion light-years across...

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..and getting bigger by the second.

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This number
is so incomprehensibly large

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that it's difficult
to wrap your brain around.

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There are trillions of galaxies
within this volume.

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It's staggering. It's so much larger
than anything we're familiar with.

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If we were to travel to the edge
of the observable universe...

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..we would enter
even more unfamiliar territory.

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Imagine we're
in an ultra-fast spaceship.

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We leave the solar system,
then the Milky Way.

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As we travel deeper
into intergalactic space,

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things start to get really weird.

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For every million light-years we go
from the Milky Way...

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..the galaxies move away from us

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at around 13 miles per second faster.

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We have to accelerate
just to keep up.

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But the galaxies keep on moving,

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always beyond our reach.

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PHIL: Imagine you're a sprinter
on a race track.

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If you're running towards
the finish line, it may take you
a few seconds to cross it.

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But now imagine that
that finish line
is moving away from you.

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If it's moving away from you
at the same speed you're running,
you'll never reach it.

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And if it's moving faster
than the runner,

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then even faster runners
won't reach it.

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And that's sort of what we're
seeing here with the universe.

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Beyond a certain distance,
galaxies are racing away from us

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faster than the speed of light.

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It's a line called
the cosmic event horizon.

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And 97% of galaxies
we see in the observable universe

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are beyond this line and unreachable,

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including GN-z11.

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They're teasing us, "Look at me.
What a nice piece of real estate,"

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but we know even if
we started going there now,

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we could never reach them.

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Anything that has crossed
the cosmic event horizon

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is out of our reach for ever.

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But that's not the full picture,

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because the expansion rate
of the universe is changing.

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A little over 20 years ago,
astronomers discovered

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that the current rate
of the universe's expansion

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is accelerating, it's speeding up.

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Everyone thought that
the expansion of the universe

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should be slowing down
because of gravity.

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Galaxies have mass,
they have visible matter,

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dark matter,
light has energy as well.

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All known forms of energy slow down
the expansion of the universe

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because they act
in an attractive gravitational way.

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Imagine our surprise when we found

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the universe wasn't slowing down.

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It wasn't even going
at a constant speed.

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It's speeding up with time.

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That was a real jaw-dropper.

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Astronomers discovered
that distant galaxies

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were speeding away from us faster
than the laws of physics predicted.

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That is one of the most
bizarre things you can imagine.

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I know it doesn't sound like much,

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but imagine holding a rock
in front of you and letting it go,

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and instead of falling down,
it falls up.

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That is as big of a shock
as what the astronomers got.

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We did not expect the universe
to be accelerating its expansion.

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Astronomers suspect
a mysterious force is at work...

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..dark energy.

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Dark energy is what we think
is pushing the universe apart,

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causing this accelerating expansion.

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And the origin
and true physical nature

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of dark energy is a big mystery.

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Thanks to dark energy,
more and more galaxies

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are crossing the cosmic event horizon

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and leaving the observable universe.

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These galaxies
are lost to us for ever.

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There are galaxies
that we can see today

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that in a few million years, say,
we won't be able to see

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because the edge
of the observable universe

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has basically moved in closer
than that galaxy.

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That's going to happen all the time,

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and in a trillion years
or something like that

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all these galaxies
that we see in our sky

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00:15:42,960 --> 00:15:47,000
will be completely invisible,
because they'll be beyond the edge
of the universe.

244
00:15:50,840 --> 00:15:55,200
So eventually every last galaxy
will be so far away from us

245
00:15:55,280 --> 00:15:58,520
that light cannot reach us
through that expanding space.

246
00:15:58,600 --> 00:16:01,560
It's almost as if
you're driving through a dark desert

247
00:16:01,640 --> 00:16:05,080
in your car and the very,
very last town that ever exists

248
00:16:05,160 --> 00:16:08,360
has gone over the horizon, and
there'll never be any light again.

249
00:16:11,920 --> 00:16:14,240
We can see less and less
of the universe

250
00:16:14,320 --> 00:16:16,400
as we go into the future.

251
00:16:16,480 --> 00:16:18,640
What a strange thought.

252
00:16:18,720 --> 00:16:22,680
So that means we should build
all the telescopes we can now!

253
00:16:24,080 --> 00:16:27,280
There's a limit to the universe
we can see

254
00:16:27,360 --> 00:16:29,680
even with the most
advanced telescopes.

255
00:16:31,720 --> 00:16:35,960
But what lies beyond is one of
the biggest mysteries in astronomy.

256
00:16:38,720 --> 00:16:41,680
The greater universe
could be stranger

257
00:16:41,760 --> 00:16:43,960
than our wildest imagination.

258
00:16:46,120 --> 00:16:48,600
When you stand on the beach
and you look at the horizon

259
00:16:48,680 --> 00:16:52,680
and you kind of think,
"Oh, what beautiful lands are there
beyond the horizon,

260
00:16:52,760 --> 00:16:54,960
things I've never imagined before?"

261
00:16:55,960 --> 00:16:59,080
It's so natural,
it's so human to ask,

262
00:16:59,160 --> 00:17:02,720
"What lies beyond that? What is
the true extent of the universe?"

263
00:17:08,080 --> 00:17:12,040
The observable universe contains
trillions of galaxies.

264
00:17:14,360 --> 00:17:17,760
It's about
92 billion light-years across.

265
00:17:20,080 --> 00:17:24,400
But astronomers believe this isn't
the full extent of the universe.

266
00:17:27,720 --> 00:17:30,960
What we don't know is
how much of the universe

267
00:17:31,040 --> 00:17:33,040
is our observable universe.

268
00:17:33,120 --> 00:17:35,400
It could be
a tiny, microscopic speck

269
00:17:35,480 --> 00:17:38,360
of this much more vast universe.
We just don't know.

270
00:17:40,720 --> 00:17:44,160
We have no idea
how much stuff there is
outside the observable universe,

271
00:17:44,240 --> 00:17:47,960
but because, by definition,
it's outside the observable universe

272
00:17:48,040 --> 00:17:50,880
we really don't know right now.

273
00:17:50,960 --> 00:17:53,240
So what is out there?

274
00:17:53,320 --> 00:17:56,920
One theory is that space outside
the observable universe

275
00:17:57,000 --> 00:18:00,680
is pretty much the same
as our own cosmic neighbourhood.

276
00:18:03,040 --> 00:18:05,160
It's just more universe,
it's just like here.

277
00:18:05,240 --> 00:18:07,800
It's just far enough away
that we can't see it.

278
00:18:07,880 --> 00:18:10,000
So it's not like
there's bizarre places

279
00:18:10,080 --> 00:18:12,640
where time runs backwards
or aliens have two heads...

280
00:18:12,720 --> 00:18:14,840
Well, yeah, maybe.

281
00:18:16,720 --> 00:18:20,080
But further out, in the deepest parts
of the greater universe...

282
00:18:20,160 --> 00:18:22,160
all bets are off.

283
00:18:24,680 --> 00:18:28,720
We expect that as you go,
sort of, twice or three times
beyond the observable universe,

284
00:18:28,800 --> 00:18:31,280
it's probably very similar
to the universe we inhabit.

285
00:18:31,360 --> 00:18:33,360
But if you go a thousand times,

286
00:18:33,440 --> 00:18:38,920
or a million times farther,
who knows what you might see?

287
00:18:39,000 --> 00:18:41,800
It might be very, very different
if we go far enough away.

288
00:18:44,600 --> 00:18:48,640
Strangely, it all comes back
to the expansion of the universe,

289
00:18:48,720 --> 00:18:51,240
and one crucial detail
in that process.

290
00:18:54,840 --> 00:18:58,040
There was a brief moment
in the very early history
of the universe

291
00:18:58,120 --> 00:19:02,080
where its expansion
accelerated hugely.

292
00:19:02,160 --> 00:19:04,160
This acceleration
is called inflation,

293
00:19:04,240 --> 00:19:07,600
and in a brief moment,
the universe itself expanded

294
00:19:07,680 --> 00:19:10,200
at multiple times
the speed of light.

295
00:19:12,400 --> 00:19:15,720
Inflation was a formative moment
for our universe.

296
00:19:17,520 --> 00:19:19,520
By the time it stopped,

297
00:19:19,600 --> 00:19:22,000
the universe's basic characteristics
were set.

298
00:19:24,720 --> 00:19:26,720
There are
these fundamental constants

299
00:19:26,800 --> 00:19:29,560
that describe
the phenomena in our universe.

300
00:19:29,640 --> 00:19:33,160
The fundamentals of matter,
and light, and space-time.

301
00:19:34,680 --> 00:19:36,680
But some scientists think

302
00:19:36,760 --> 00:19:39,240
there could be regions
of the greater galaxy

303
00:19:39,320 --> 00:19:41,480
where inflation has never stopped.

304
00:19:44,080 --> 00:19:47,640
The idea is the greater universe
is expanding

305
00:19:47,720 --> 00:19:50,520
at an insane speed,
but here and there, occasionally,

306
00:19:50,600 --> 00:19:53,960
a little region will stop inflating

307
00:19:54,040 --> 00:19:57,640
and just expand at the normal rate.

308
00:19:57,720 --> 00:19:59,560
Inflation can end somewhere

309
00:19:59,640 --> 00:20:01,920
and that gives rise
to the universe we live in,

310
00:20:02,000 --> 00:20:04,040
while inflation continues
somewhere else.

311
00:20:06,000 --> 00:20:09,400
Parts of the greater universe
that continued to inflate

312
00:20:09,480 --> 00:20:12,240
would be left
with different laws of physics.

313
00:20:14,040 --> 00:20:16,240
These incredibly violent
inflation processes

314
00:20:16,320 --> 00:20:20,240
actually monkeyed with
the very fabric of space itself,

315
00:20:20,320 --> 00:20:23,320
so that a lot of the things that
we were taught are laws of physics

316
00:20:23,400 --> 00:20:25,400
are different there.

317
00:20:26,720 --> 00:20:30,040
So in essence, inflation gives us
a very natural way

318
00:20:30,120 --> 00:20:33,120
to make this patchwork quilt
of different parts of the universe

319
00:20:33,200 --> 00:20:35,760
where things seem different.

320
00:20:35,840 --> 00:20:38,880
So what we can imagine
is a super-large-scale structure

321
00:20:38,960 --> 00:20:42,640
where there's different regions
of the universe, domains,

322
00:20:42,720 --> 00:20:45,200
then each domain has different
local laws of physics.

323
00:20:48,560 --> 00:20:50,560
These different parts of the universe

324
00:20:50,640 --> 00:20:54,120
are separated by frontiers
called "domain walls".

325
00:20:57,840 --> 00:21:00,640
We have similar boundaries on Earth.

326
00:21:00,720 --> 00:21:03,960
Whenever you have something that
can be in many different states,

327
00:21:04,040 --> 00:21:06,200
you can end up with domain walls.

328
00:21:06,280 --> 00:21:09,840
If I were a fish swimming around
in the Arctic

329
00:21:09,920 --> 00:21:12,240
near an iceberg,

330
00:21:12,320 --> 00:21:14,320
there would be a domain boundary

331
00:21:14,400 --> 00:21:17,520
between the water being
in the liquid state where I am

332
00:21:17,600 --> 00:21:20,120
and the solid state inside the ice.

333
00:21:20,200 --> 00:21:23,280
A domain wall is the wall
between two domains.

334
00:21:23,360 --> 00:21:26,440
If it's water, this could be ice,
this could be liquid.

335
00:21:26,520 --> 00:21:30,400
If we're talking about space,
this could be a kind of space,
maybe, you can live in,

336
00:21:30,480 --> 00:21:34,960
and this could be the kind of space
where you don't wanna be.

337
00:21:35,040 --> 00:21:38,000
Crossing a domain wall
would be very bad news

338
00:21:38,080 --> 00:21:40,080
for anyone who dared to try.

339
00:21:41,160 --> 00:21:43,880
Cross that domain wall
and the laws of physics

340
00:21:43,960 --> 00:21:45,960
could change dramatically.

341
00:21:46,040 --> 00:21:48,040
The number of dimensions
could change.

342
00:21:48,120 --> 00:21:50,880
If we were able to travel
to places in the universe

343
00:21:50,960 --> 00:21:53,560
where the laws of physics
are different, we would die...

344
00:21:54,560 --> 00:21:57,320
..because all of the chemistry
going on in our bodies

345
00:21:57,400 --> 00:22:00,360
depends very, very sensitively
on the laws of physics.

346
00:22:02,000 --> 00:22:05,800
So you could just dissipate
like Thanos' Snap and you're gone.

347
00:22:06,840 --> 00:22:09,720
Domain walls might be
the closest we get

348
00:22:09,800 --> 00:22:12,280
to locating an edge in the universe.

349
00:22:13,680 --> 00:22:16,040
It depends on how
you define the edge.

350
00:22:16,120 --> 00:22:19,440
If it is the realm where
the laws of our universe operate,

351
00:22:19,520 --> 00:22:22,920
then these domain walls are,
in essence, the edge
of the universe.

352
00:22:27,600 --> 00:22:29,680
But this is all just theory.

353
00:22:33,320 --> 00:22:35,320
If we ever really are to work out

354
00:22:35,400 --> 00:22:37,720
what the true size and shape
of the universe is,

355
00:22:37,800 --> 00:22:40,360
we're going to have to look
for clues that are close to us.

356
00:22:41,360 --> 00:22:44,920
Clues that could answer
the ultimate question.

357
00:22:45,000 --> 00:22:47,560
How big is the rest
of the greater universe...

358
00:22:48,920 --> 00:22:51,160
..and could it go on for ever?

359
00:22:58,640 --> 00:23:00,920
For tens of thousands of years,

360
00:23:01,000 --> 00:23:05,240
mankind has gazed in wonder
at the vastness of the cosmos.

361
00:23:06,360 --> 00:23:08,840
But just how extensive is it?

362
00:23:10,240 --> 00:23:12,760
If we could answer that question,

363
00:23:12,840 --> 00:23:16,160
it might help us to understand
our place in the universe.

364
00:23:17,560 --> 00:23:20,480
One of the fundamental questions
in science is,

365
00:23:20,560 --> 00:23:23,200
"How big is the universe?"

366
00:23:23,280 --> 00:23:26,000
To answer the question,
"How big is the universe?"

367
00:23:26,080 --> 00:23:29,200
we have to answer the question,
"What shape is the universe?"

368
00:23:29,280 --> 00:23:31,280
And by "shape" I mean geometry,

369
00:23:31,360 --> 00:23:34,640
I mean, "How is the universe curved
on its larger scales?"

370
00:23:35,960 --> 00:23:39,160
If we were to discover
that the universe does have

371
00:23:39,240 --> 00:23:42,360
some sort of geometric curvature,

372
00:23:42,440 --> 00:23:46,400
then this might imply
that it wraps around

373
00:23:46,480 --> 00:23:50,560
in on itself
over incredibly large distances,

374
00:23:50,640 --> 00:23:54,160
and that if you could travel
in one direction long enough,

375
00:23:54,240 --> 00:23:56,240
you would end up
at your starting point.

376
00:23:56,320 --> 00:23:58,760
Another version is that
the universe is more like

377
00:23:58,840 --> 00:24:02,400
an infinite flat plane, OK,
no curvature at all.

378
00:24:03,400 --> 00:24:05,800
The further you travel, well,
the further you get,

379
00:24:05,880 --> 00:24:08,360
and you never get back
to where you started.

380
00:24:10,920 --> 00:24:12,920
To work out the shape of something,

381
00:24:13,000 --> 00:24:16,120
we would normally just
step back and take a look,

382
00:24:16,200 --> 00:24:19,400
but clearly, moving outside
the universe is a non-starter.

383
00:24:20,960 --> 00:24:23,240
We can't jump in a rocket and fly

384
00:24:23,320 --> 00:24:25,600
a thousand times larger
than our cosmic horizon

385
00:24:25,680 --> 00:24:29,320
and see what the shape
of the universe is.
We just can't do that!

386
00:24:29,400 --> 00:24:33,000
Our human perspective on
the larger universe is so limited,

387
00:24:33,080 --> 00:24:37,720
so if we wanna figure out
what the larger shape and scale
of the universe is,

388
00:24:37,800 --> 00:24:39,840
we're going to have to be
very clever indeed.

389
00:24:42,560 --> 00:24:44,560
One way to be clever

390
00:24:44,640 --> 00:24:48,400
is to think of the geometry of
the universe in its simplest terms.

391
00:24:49,400 --> 00:24:52,320
JAMES: When we talk about
the geometry of the universe,

392
00:24:52,400 --> 00:24:54,720
we really are talking about
geometry.

393
00:24:54,800 --> 00:24:57,880
In order to do geometry,
you have to take measures.

394
00:24:57,960 --> 00:25:00,200
You need a cosmic ruler to do this.

395
00:25:00,280 --> 00:25:02,440
And it turns out
there's a great cosmic ruler

396
00:25:02,520 --> 00:25:04,800
known as
baryon acoustic oscillations.

397
00:25:07,680 --> 00:25:10,880
Baryon acoustic oscillations
are ripples

398
00:25:10,960 --> 00:25:13,120
in the cosmic microwave background...

399
00:25:14,480 --> 00:25:16,520
..the oldest light in the universe.

400
00:25:19,200 --> 00:25:23,520
As the universe expanded
these ripples were imprinted in space

401
00:25:23,600 --> 00:25:26,320
in a uniform way.

402
00:25:26,400 --> 00:25:31,320
They provide a cosmic ruler
to measure vast distances over time

403
00:25:31,400 --> 00:25:34,400
so we can gauge
if the universe expands

404
00:25:34,480 --> 00:25:37,800
in curved space or over a flat plane.

405
00:25:40,840 --> 00:25:42,840
When we use these cosmic rulers

406
00:25:42,920 --> 00:25:45,280
to try to back out
the shape of the universe,

407
00:25:45,360 --> 00:25:48,560
we're sure to a few per cent
accuracy that the universe is flat.

408
00:25:51,080 --> 00:25:55,640
If the universe is flat,
we could set off into the cosmos

409
00:25:55,720 --> 00:25:58,200
and continue travelling for ever.

410
00:26:00,920 --> 00:26:03,240
There may be no edge
to our universe...

411
00:26:04,680 --> 00:26:07,760
..because a flat universe
can be infinite.

412
00:26:09,160 --> 00:26:13,000
Now we're thinking of the universe
as something that does go on
for ever,

413
00:26:13,080 --> 00:26:16,960
that the stars and galaxies
never have an end.

414
00:26:17,040 --> 00:26:20,880
And how can something
truly infinite really exist?

415
00:26:20,960 --> 00:26:23,760
Infinity is weird because we...

416
00:26:23,840 --> 00:26:26,840
It's a concept of...
Cos it's endless.

417
00:26:26,920 --> 00:26:31,320
What does that mean? Who knows?
I don't know. (LAUGHS)

418
00:26:33,760 --> 00:26:37,280
PHIL: Infinity is a concept
more than anything else.

419
00:26:37,360 --> 00:26:41,400
Our brains aren't evolved for that.
We evolved living in the plains,

420
00:26:41,480 --> 00:26:43,920
we were apes looking for food.

421
00:26:44,000 --> 00:26:47,480
We weren't evolved
to think about the universe

422
00:26:47,560 --> 00:26:50,280
and all of this stuff.

423
00:26:50,360 --> 00:26:52,880
HAKEEM: I just can't stop
contemplating this stuff.

424
00:26:52,960 --> 00:26:55,680
The idea of infinity
and these large numbers

425
00:26:55,760 --> 00:26:58,760
and even the tininess
of everything, it's nuts.

426
00:26:58,840 --> 00:27:02,280
Yeah, thinking about infinity makes
my head hurt a little bit.

427
00:27:06,240 --> 00:27:10,320
An infinite universe
has profound implications

428
00:27:10,400 --> 00:27:13,160
for understanding
our place in the cosmos.

429
00:27:14,360 --> 00:27:17,200
It guarantees we're not alone.

430
00:27:18,880 --> 00:27:20,880
If the universe is infinite,

431
00:27:20,960 --> 00:27:23,400
then there could be
an infinite number of galaxies

432
00:27:23,480 --> 00:27:25,480
that have planets with life,

433
00:27:25,560 --> 00:27:27,560
an infinite number without life,

434
00:27:27,640 --> 00:27:29,800
then because life
did appear here on Earth,

435
00:27:29,880 --> 00:27:32,880
it's physically possible,
therefore it will definitely happen

436
00:27:32,960 --> 00:27:34,960
elsewhere in the universe.

437
00:27:35,040 --> 00:27:40,240
In a flat universe, alien life could
come in an infinite number of forms.

438
00:27:41,720 --> 00:27:44,400
But there is an altogether
stranger guarantee.

439
00:27:45,760 --> 00:27:48,720
If the universe has no edge,
this means

440
00:27:48,800 --> 00:27:52,480
that things that seem like
they are impossible become possible.

441
00:27:55,520 --> 00:27:58,120
Every possible arrangement
of matter,

442
00:27:58,200 --> 00:28:02,200
every possible history of a galaxy,

443
00:28:02,280 --> 00:28:05,800
of a solar system,
of a planet like Earth

444
00:28:05,880 --> 00:28:09,360
is possible and is happening
right now

445
00:28:09,440 --> 00:28:12,800
in parallel to us
somewhere over there.

446
00:28:14,760 --> 00:28:17,720
So that means that
there has to be another place

447
00:28:17,800 --> 00:28:20,200
that has a galaxy just like ours...

448
00:28:21,200 --> 00:28:23,200
..and it would have
an Earth just like ours.

449
00:28:23,280 --> 00:28:25,320
It would have people,

450
00:28:25,400 --> 00:28:28,960
it would have another version
of you, another version of me.

451
00:28:29,040 --> 00:28:32,320
It's 100% guaranteed that there's
another Max Tegmark out there...

452
00:28:33,640 --> 00:28:36,760
..having exactly this conversation,
and, in fact, many of them.

453
00:28:37,760 --> 00:28:41,200
An infinite universe
full of Max Tegmarks

454
00:28:41,280 --> 00:28:43,720
may be a strange concept,

455
00:28:43,800 --> 00:28:47,680
but what's truly mind-bending
is understanding the physics

456
00:28:47,760 --> 00:28:49,760
of a flat universe.

457
00:28:49,840 --> 00:28:53,560
If the universe is infinite
and it's expanding,

458
00:28:53,640 --> 00:28:55,640
what is it expanding into,

459
00:28:55,720 --> 00:28:58,000
and what did it expand from?

460
00:28:58,080 --> 00:29:01,800
And was there ever an edge
to the universe?

461
00:29:02,840 --> 00:29:05,600
Fortunately the answer is that

462
00:29:05,680 --> 00:29:08,040
it doesn't make sense
to ask that question.

463
00:29:09,920 --> 00:29:13,360
Everything is expanding, including
the universe that we exist within.

464
00:29:14,400 --> 00:29:17,120
So, in fact, it's not
expanding into anything

465
00:29:17,200 --> 00:29:19,200
because it is everything.

466
00:29:23,640 --> 00:29:27,320
To help understand what's going on
in an infinite universe,

467
00:29:27,400 --> 00:29:29,520
we need to go back to the Big Bang.

468
00:29:32,480 --> 00:29:35,400
We wanna think of the Big Bang
as an explosion in space,

469
00:29:35,480 --> 00:29:37,800
like it happened someplace,

470
00:29:37,880 --> 00:29:40,440
but there wasn't any place
before the Big Bang.

471
00:29:40,520 --> 00:29:43,680
Space existed inside
of the Big Bang itself,

472
00:29:43,760 --> 00:29:48,000
so it's not an explosion in space,
it's an explosion OF space.

473
00:29:48,080 --> 00:29:50,480
We're sometimes told that
at the Big Bang

474
00:29:50,560 --> 00:29:53,680
the universe started out
very, very small and then got big,

475
00:29:53,760 --> 00:29:57,680
but how can a finite point
become infinite?

476
00:29:57,760 --> 00:30:00,000
Well, if the universe is infinite,

477
00:30:00,080 --> 00:30:03,760
then it was also infinite
at the Big Bang.

478
00:30:03,840 --> 00:30:05,920
And this is a tough thing
to think about.

479
00:30:06,000 --> 00:30:09,760
Think about it this way.
In an infinite universe
the galaxies go on for ever,

480
00:30:09,840 --> 00:30:13,320
and now there's a great distance
between every galaxy.

481
00:30:13,400 --> 00:30:16,800
But once upon a time, the galaxies
were closer together, say,

482
00:30:16,880 --> 00:30:18,880
half their current distance apart.

483
00:30:18,960 --> 00:30:22,000
But they still went on for ever -
the universe was still infinite.

484
00:30:23,440 --> 00:30:25,560
In a flat universe,

485
00:30:25,640 --> 00:30:29,240
space was infinite
from the beginning.

486
00:30:29,320 --> 00:30:32,320
There was never
a single point in space

487
00:30:32,400 --> 00:30:34,400
where the Big Bang happened.

488
00:30:35,680 --> 00:30:37,800
It happened everywhere.

489
00:30:39,560 --> 00:30:43,320
An infinite universe
offers infinite possibilities,

490
00:30:43,400 --> 00:30:45,720
but no edge to space.

491
00:30:49,600 --> 00:30:52,040
But there may be
another kind of edge,

492
00:30:52,120 --> 00:30:56,720
one that will only reveal itself
if the universe dies.

493
00:31:06,280 --> 00:31:09,480
We live in an infinite
and expanding universe.

494
00:31:11,200 --> 00:31:15,240
Space has no edge -
it goes on for ever.

495
00:31:20,160 --> 00:31:24,400
But there could be a different kind
of edge to our universe -

496
00:31:24,480 --> 00:31:26,680
an edge of time.

497
00:31:28,200 --> 00:31:31,920
The universe seems to have begun
13.8 billion years ago,

498
00:31:32,000 --> 00:31:34,640
in the past,
so there's some inclination,

499
00:31:34,720 --> 00:31:37,040
some impression
that it's finite in time.

500
00:31:37,120 --> 00:31:41,120
What we call the Big Bang is,
as far as we understand it,

501
00:31:41,200 --> 00:31:43,720
a beginning,
a start of the universe.

502
00:31:43,800 --> 00:31:45,920
The universe has a finite edge.

503
00:31:46,000 --> 00:31:48,240
Now, does it have an edge
in the future?

504
00:31:49,640 --> 00:31:52,000
We used to think
that time would someday

505
00:31:52,080 --> 00:31:54,080
come to a catastrophic end...

506
00:31:55,120 --> 00:31:59,400
..along with the planets, galaxies,
and all life in the universe.

507
00:32:01,840 --> 00:32:04,880
If we know there's a Big Bang,
if we know the universe started,

508
00:32:04,960 --> 00:32:07,200
expanded and cooled,
it's very natural

509
00:32:07,280 --> 00:32:09,840
to wonder whether someday
the expansion will stop,

510
00:32:09,920 --> 00:32:13,320
reverse and come back.
And that's a Big Crunch.

511
00:32:17,760 --> 00:32:20,760
In a Big Crunch,
our expanding universe

512
00:32:20,840 --> 00:32:22,920
will begin to contract.

513
00:32:23,000 --> 00:32:25,840
Stars and planets will
smash into each other.

514
00:32:27,960 --> 00:32:29,960
Galaxies will collide.

515
00:32:31,480 --> 00:32:34,440
And all of the life left in space
will be compressed

516
00:32:34,520 --> 00:32:37,600
with all other matter
into a singularity.

517
00:32:46,640 --> 00:32:48,640
If this theory is true,

518
00:32:48,720 --> 00:32:52,400
then the universe will have
a beginning...and an end of time.

519
00:32:54,880 --> 00:32:57,080
If we live in a universe
that will expand,

520
00:32:57,160 --> 00:32:59,760
stop expanding,
and then go back into a crunch,

521
00:32:59,840 --> 00:33:01,840
then it has, in effect, two edges.

522
00:33:03,400 --> 00:33:06,200
But there's
a much stranger possibility.

523
00:33:07,880 --> 00:33:10,440
Perhaps the end is but a beginning

524
00:33:10,520 --> 00:33:12,800
where the universe
is an oscillating universe.

525
00:33:12,880 --> 00:33:16,560
It has a Big Bang-like beginning,
it expands to a maximum size

526
00:33:16,640 --> 00:33:18,640
and then goes back
into a Big Crunch,

527
00:33:18,720 --> 00:33:20,720
and does that over and over.

528
00:33:22,600 --> 00:33:25,320
We could be residents of a universe

529
00:33:25,400 --> 00:33:28,320
created from the ashes of another.

530
00:33:28,400 --> 00:33:32,480
A single universe in a stream
of bouncing universes...

531
00:33:35,480 --> 00:33:39,960
..each full of galaxies,
planets and life.

532
00:33:40,040 --> 00:33:43,000
But our most recent observations
of the universe

533
00:33:43,080 --> 00:33:46,320
suggest a Big Crunch
isn't in the cards.

534
00:33:49,200 --> 00:33:52,120
Once again, dark energy is key.

535
00:33:55,960 --> 00:33:58,840
It's pushing our universe apart.

536
00:33:58,920 --> 00:34:01,880
The universe is being
dominated right now by dark energy,

537
00:34:01,960 --> 00:34:05,000
and because the dark energy
doesn't seem to be fading away,

538
00:34:05,080 --> 00:34:08,240
it's easy to imagine
eternal expansion toward the future.

539
00:34:09,720 --> 00:34:14,720
Dark energy seems to be overpowering
the large-scale effects of gravity.

540
00:34:17,680 --> 00:34:19,680
But some theories suggest

541
00:34:19,760 --> 00:34:23,080
that this strange force
could switch sides.

542
00:34:24,320 --> 00:34:28,360
It's possible that
the currently repulsive dark energy

543
00:34:28,440 --> 00:34:33,080
will someday change sign and
become gravitationally attractive.

544
00:34:33,160 --> 00:34:36,520
If that's true, and if there's
enough of this energy

545
00:34:36,600 --> 00:34:38,840
or if it grows in magnitude,

546
00:34:38,920 --> 00:34:42,640
then the universe would slow down,

547
00:34:42,720 --> 00:34:44,720
eventually come to a stop,

548
00:34:44,800 --> 00:34:49,720
and then reverse its motion
and collapse in on a Big Crunch.

549
00:34:49,800 --> 00:34:53,360
So in that case,
despite the current acceleration,

550
00:34:53,440 --> 00:34:56,040
the universe could ultimately
reverse its motion

551
00:34:56,120 --> 00:34:58,880
and collapse in on itself.

552
00:34:58,960 --> 00:35:03,680
Dark energy really is little more
than a word for our ignorance,

553
00:35:03,760 --> 00:35:07,880
and until we can figure out more
about what this stuff is,

554
00:35:07,960 --> 00:35:10,280
we will not know
our ultimate destiny

555
00:35:10,360 --> 00:35:12,640
and whether things are gonna
keep flying apart

556
00:35:12,720 --> 00:35:15,320
or come crashing together or what.

557
00:35:19,240 --> 00:35:22,880
But most scientists think
a Big Crunch is unlikely.

558
00:35:24,400 --> 00:35:27,880
For a while we didn't know
if the expansion of the universe

559
00:35:27,960 --> 00:35:31,120
was going to slow, stop,
and reverse itself

560
00:35:31,200 --> 00:35:33,520
because of gravity.
There are all these galaxies

561
00:35:33,600 --> 00:35:36,840
in the universe and they're pulling
on each other by their gravity.

562
00:35:36,920 --> 00:35:38,920
And if the expansion
isn't fast enough,

563
00:35:39,000 --> 00:35:43,200
that gravity might be strong enough
to stop the expansion
and re-collapse the universe.

564
00:35:43,280 --> 00:35:46,640
Now with dark energy, we know
that there's no way that can happen.

565
00:35:46,720 --> 00:35:49,240
The universe is
going to expand for ever,

566
00:35:49,320 --> 00:35:52,200
because dark energy is pumping it
full of acceleration.

567
00:35:53,200 --> 00:35:55,520
In order for there to be
a Big Crunch,

568
00:35:55,600 --> 00:35:59,000
our understanding of dark energy
would have to change a lot.

569
00:35:59,080 --> 00:36:01,960
That is, dark energy would have to
be extremely weird

570
00:36:02,040 --> 00:36:05,280
and turn off in some very funny way

571
00:36:05,360 --> 00:36:08,400
for the universe to suddenly
stop expanding and re-collapse.

572
00:36:11,040 --> 00:36:15,160
Without a Big Crunch, there is
no future edge to time.

573
00:36:17,240 --> 00:36:20,800
SEAN:
The universe is not only expanding,
but it's being driven by dark energy

574
00:36:20,880 --> 00:36:23,400
to expand faster and faster.

575
00:36:23,480 --> 00:36:26,600
And dark energy doesn't dilute away
as far as we can tell,

576
00:36:26,680 --> 00:36:29,760
so the simplest idea
is that the universe will simply

577
00:36:29,840 --> 00:36:32,560
continue to expand eternally
toward the future.

578
00:36:35,160 --> 00:36:38,440
Just like space,
time would go on for ever.

579
00:36:40,280 --> 00:36:43,920
It may sound like a better fate
for life in the universe,

580
00:36:44,000 --> 00:36:46,000
but it's not.

581
00:36:48,080 --> 00:36:51,120
One of the consequences
of this dark energy

582
00:36:51,200 --> 00:36:53,800
that's causing the acceleration
of the universe

583
00:36:53,880 --> 00:36:58,320
is that we eventually are headed
towards the Big Chill.

584
00:36:59,680 --> 00:37:03,200
I should say we're eventually headed
towards (DEEP VOICE) the Big Chill.

585
00:37:03,280 --> 00:37:06,440
So the universe
is getting colder and colder

586
00:37:06,520 --> 00:37:09,480
and things are getting
more and more spread out.

587
00:37:09,560 --> 00:37:14,040
So the accelerated, and continual,
and forever expansion
of our universe

588
00:37:14,120 --> 00:37:17,040
might make for a frankly
depressing end to time itself,

589
00:37:17,120 --> 00:37:21,200
the ultimate entropy-based
heat death of the universe

590
00:37:21,280 --> 00:37:25,120
where you would walk out
and see no stars in the sky...

591
00:37:26,120 --> 00:37:28,840
..see absolutely nothing.
There will come one day

592
00:37:28,920 --> 00:37:30,920
when the very last star
in the universe

593
00:37:31,000 --> 00:37:33,600
just fizzles out, and that is it.

594
00:37:36,440 --> 00:37:42,280
In the future, space will be
a cold, dark, infinite void

595
00:37:42,360 --> 00:37:44,360
where time goes on for ever.

596
00:37:45,800 --> 00:37:50,000
There will be nothing to do
but suffer in the eternal expanse.

597
00:37:51,480 --> 00:37:53,480
It's our inevitable fate

598
00:37:53,560 --> 00:37:56,760
if there's no future edge of time
in the universe.

599
00:37:58,640 --> 00:38:02,640
But even if there isn't
an edge to the universe,

600
00:38:02,720 --> 00:38:06,400
could there be edges
within the universe?

601
00:38:07,840 --> 00:38:09,840
(APPLAUSE)

602
00:38:09,920 --> 00:38:12,120
April 2019.

603
00:38:12,200 --> 00:38:16,840
An international team of astronomers
makes a special announcement.

604
00:38:16,920 --> 00:38:20,440
And we are delighted to be able
to report to you today

605
00:38:20,520 --> 00:38:24,760
that we have seen
and taken a picture of a black hole.

606
00:38:24,840 --> 00:38:27,560
Here it is.

607
00:38:27,640 --> 00:38:29,640
(APPLAUSE)

608
00:38:33,320 --> 00:38:36,080
It's a picture
of a supermassive black hole

609
00:38:36,160 --> 00:38:39,360
at the centre of the M87 galaxy,

610
00:38:39,440 --> 00:38:41,800
54 million light-years away.

611
00:38:42,920 --> 00:38:45,840
It may also be the first image

612
00:38:45,920 --> 00:38:48,080
of an edge in the universe.

613
00:38:49,360 --> 00:38:52,200
Black holes create
a really interesting scenario

614
00:38:52,280 --> 00:38:55,400
when we think about space,
and the universe having edges.

615
00:38:57,240 --> 00:39:01,120
This edge between space
outside and inside a black hole

616
00:39:01,200 --> 00:39:03,320
is called the "event horizon".

617
00:39:05,160 --> 00:39:07,400
The event horizon of a black hole
is a region

618
00:39:07,480 --> 00:39:10,120
within which once you cross inside,

619
00:39:10,200 --> 00:39:12,840
the gravitational tug is so strong

620
00:39:12,920 --> 00:39:14,920
that even light cannot escape,

621
00:39:15,000 --> 00:39:17,440
which means nothing can escape

622
00:39:17,520 --> 00:39:20,360
once you cross
inside the event horizon.

623
00:39:20,440 --> 00:39:22,760
So that really is sort of an edge,

624
00:39:22,840 --> 00:39:25,320
because it really does create
a boundary.

625
00:39:30,160 --> 00:39:34,200
The event horizon isn't
a physical barrier in space.

626
00:39:35,640 --> 00:39:39,120
The event horizon is an edge of
the part of the universe
we can visit,

627
00:39:39,200 --> 00:39:41,840
but it's not an edge in the sense
there's anything there.

628
00:39:43,280 --> 00:39:46,560
You would just pass right through it
if you got right up to that place.

629
00:39:46,640 --> 00:39:49,280
So it's sort of
a conceptual boundary

630
00:39:49,360 --> 00:39:52,280
between two different parts
of the universe.

631
00:39:52,360 --> 00:39:55,560
If we did send a manned probe
to a black hole,

632
00:39:55,640 --> 00:39:58,360
it would be a one-way trip.

633
00:40:00,840 --> 00:40:05,440
The event horizons of black holes
are a sort of edge

634
00:40:05,520 --> 00:40:08,680
because once you pass through
an event horizon,

635
00:40:08,760 --> 00:40:12,200
you are cut off
from the rest of the universe.

636
00:40:12,280 --> 00:40:14,440
You can never go back out.

637
00:40:14,520 --> 00:40:19,120
You are outside of our universe.

638
00:40:19,200 --> 00:40:21,200
Once you've crossed
inside that region,

639
00:40:21,280 --> 00:40:24,480
you are never coming back out,
you know, and that's an edge.

640
00:40:26,160 --> 00:40:28,240
Once inside the black hole,

641
00:40:28,320 --> 00:40:31,000
the probe would be
in a separate part of space...

642
00:40:32,440 --> 00:40:34,840
..cut off from the rest
of the universe.

643
00:40:36,000 --> 00:40:38,360
Falling through the event horizon
of a black hole

644
00:40:38,440 --> 00:40:41,640
is like jumping over
the edge of a cliff.

645
00:40:41,720 --> 00:40:44,640
You can see the edge,
and you can see the edge go by,

646
00:40:44,720 --> 00:40:47,160
and then when you're at the bottom,
you can look up

647
00:40:47,240 --> 00:40:49,600
and see what's happening
at the top of the cliff,

648
00:40:49,680 --> 00:40:51,680
but you can never go back.

649
00:40:53,600 --> 00:40:56,680
At the bottom
of this black hole cliff

650
00:40:56,760 --> 00:40:58,760
sits a singularity,

651
00:40:58,840 --> 00:41:02,680
a region of space where
the laws of physics go off the rails.

652
00:41:04,560 --> 00:41:07,520
Deep toward that singularity
could be as surprising

653
00:41:07,600 --> 00:41:10,000
as you might imagine,
and yet still a possibility.

654
00:41:10,080 --> 00:41:12,800
If you map the space-time
around a black hole

655
00:41:12,880 --> 00:41:14,880
in a very particular way,

656
00:41:14,960 --> 00:41:17,360
there emerges
a sort of mirror universe,

657
00:41:17,440 --> 00:41:20,080
a parallel universe
on the other side of the black hole

658
00:41:20,160 --> 00:41:24,640
identical to our own and traversable
via the black hole.

659
00:41:30,200 --> 00:41:33,520
So black holes are not just edges
to our universe...

660
00:41:35,240 --> 00:41:38,720
..they may also be gateways
to other universes.

661
00:41:40,600 --> 00:41:44,360
It's highly conjecture,
but if there's ever gonna be

662
00:41:44,440 --> 00:41:47,600
a space or a region
where you're making connections

663
00:41:47,680 --> 00:41:49,680
with, say, some other universe,

664
00:41:49,760 --> 00:41:52,360
a black hole, in principle,
could be a portal to that.

665
00:41:56,280 --> 00:42:00,040
But it's highly unlikely
anyone will ever want to venture

666
00:42:00,120 --> 00:42:02,120
beyond an event horizon.

667
00:42:03,760 --> 00:42:07,120
And our pursuit of the other edges
in the cosmos

668
00:42:07,200 --> 00:42:09,600
offers little hope.

669
00:42:09,680 --> 00:42:13,600
We can never travel beyond
the cosmic event horizon...

670
00:42:15,360 --> 00:42:18,080
..and we will never be able
to see beyond the edge

671
00:42:18,160 --> 00:42:20,160
of our observable universe.

672
00:42:21,840 --> 00:42:25,120
So can we ever hope
to discover the true edge

673
00:42:25,200 --> 00:42:30,800
of the greater universe,
or find out if it even has one?

674
00:42:32,160 --> 00:42:34,600
My feeling is that probably
we should not think

675
00:42:34,680 --> 00:42:37,160
about edges for the universe.

676
00:42:37,240 --> 00:42:40,040
Everything you've ever seen
in your life is finite.

677
00:42:40,120 --> 00:42:42,560
It has an inside and an outside,
it has an edge.

678
00:42:44,160 --> 00:42:47,480
The universe might not be like that.
It's probably not like that.

679
00:42:47,560 --> 00:42:50,800
There's probably no sense
in which the universe has an edge.

680
00:42:53,680 --> 00:42:56,120
We used to think
that the ultimate limits

681
00:42:56,200 --> 00:42:58,600
on the future life
were set by nature.

682
00:42:58,680 --> 00:43:03,440
We couldn't get off the planet,
or there was nothing beyond
our solar system.

683
00:43:03,520 --> 00:43:07,640
Now we realise we have
this vast, vast cosmos out there

684
00:43:07,720 --> 00:43:12,640
and the ultimate limits are actually
simply our own imagination,

685
00:43:12,720 --> 00:43:15,320
and our ability to do
great things with it

686
00:43:15,400 --> 00:43:17,400
rather than self-destruct.

687
00:43:17,480 --> 00:43:20,120
Our future destiny
is in our own hands,

688
00:43:20,200 --> 00:43:22,480
and I find that very empowering.

689
00:43:24,040 --> 00:43:27,680
It is beautifully frustrating
to realise how limited we are,

690
00:43:27,760 --> 00:43:30,680
to realise that we're probably
never going to get a true view

691
00:43:30,760 --> 00:43:32,600
of the real extent of the universe.

692
00:43:32,680 --> 00:43:34,680
We should keep an open mind,
be humble,

693
00:43:34,760 --> 00:43:36,760
but I think we should
give up on the idea

694
00:43:36,840 --> 00:43:40,040
that things should have edges
cos that's what we're familiar with.

695
00:43:40,120 --> 00:43:42,200
The universe is something special.

696
00:43:42,280 --> 00:43:44,480
What matters to us,

697
00:43:44,560 --> 00:43:46,960
and will only ever matter to us,

698
00:43:47,040 --> 00:43:49,360
is the observable universe

699
00:43:49,440 --> 00:43:51,560
because that's the limit
of what we can see,

700
00:43:51,640 --> 00:43:54,120
and that is the limit
of what we can know.

701
00:43:54,200 --> 00:43:56,280
So there is
an edge to the universe -

702
00:43:56,360 --> 00:43:58,360
there's an edge to what we can know.

703
00:43:58,440 --> 00:44:00,440
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