Nova (1974–…): Season 38, Episode 17 - The Fabric of the Cosmos: The Illusion of Time - full transcript

Lying just beneath
everyday reality

is a breathtaking world,

where much of what we perceive
about the universe is wrong.

Physicist and best-selling
author Brian Greene takes you

on a journey that bends the
rules of human experience.

BRIAN GREENE:
Why don't we ever see events
unfold in reverse order?

According to the laws
of physics, this can happen.

It's a world
that comes to light

as we probe the most extreme
realms of the cosmos,

from black holes
to the Big Bang

to the very heart
of matter itself.



I'm going to have
what he's having.

Here, empty space teems
with ferocious activity.

Our universe may be
one of many,

and the three-dimensional world
merely a mirage.

GREENE:
But how could this be?

How could we be so wrong
about something so familiar?

Does it bother us?

Absolutely.

?here's no principle

built into the laws of nature

that say that theoretical
physicists have to be happy.

It's a game-changing
perspective

that opens up a whole new world
of possibilities.

Coming up...



GREENE:
Look around any train station,

and you can see how time
rules our lives.

But time is not what it seems.

There may be no distinction

between past, present
and future.

GREENE:
If time isn't what
we all think it is,

then what is it?

Did it have a beginning?

Will it have an end?

Where did it come from?

The "Illusion of Time"

on "The Fabric of the Cosmos,"
right now on NOVA.

Major funding for NOVA is
provided by the following:

And...

And by the Corporation
for Public Broadcasting

and by contributions
to your PBS station from:

Major funding
for "The Fabric of the Cosmos"

is provided by
the National Science Foundation.

And...

Supporting original research
and public understanding

of science, technology,
engineering and mathematics.

Additional funding
is provided by...

And the George D. Smith Fund.

BRIAN GREENE:
"Once upon a time."

That magical phrase at the
beginning of every good story.

But what is the story of time?

People say that time flies,
that time is money,

we waste time, we kill time,
we try to save time.

But what do we really know
about time?

Well, like this river,
time seems to flow endlessly

from one moment to the next.

And the flow of time seems
to always be in one direction:

toward the future.

But that may not be right.

Discoveries over
the last century

have shown that much
of what we think about time

may be nothing more
than an illusion.

Contrary to everyday experience,
time may not flow at all.

Our past may not be gone.

Our future may already exist.

It turns out time itself
can speed up or slow down.

And events that we think can
unfold in only one direction

can also unfold in reverse.

But how could this be?

How could we be so wrong about
something so familiar?

And if time isn't what we all
think it is, then what is it?

Did it have a beginning?

Will it have an end?

Where did it come from?

JANNA LEVIN:
We'd like to corner time
as a thing,

but it defies that completely
by being momentary,

by only having definitions
that hearken back to the notion

of time itself.

Time is the thing that everyone
knows intimately

until you ask them
to tell you about it.

ALAN GUTH:
"What is time?" is really

the $64,000 question to physics.

There's basically
no aspect of time

which I feel we really
fully understand.

GREENE:
So how do you begin
to unlock a mystery

as deep and elusive as time?

Well, one way is to measure it.

And using clocks of all
different shapes, sizes

and kinds,
we've been measuring time

with ever-greater accuracy
for thousands of years.

The first clock was one that you
could say ticks just once a day:

the rotating Earth.

From the repetition of our
planet's daily rotation

on its axis

to its yearly orbit
around the sun,

we have always used
the predictable,

consistent motion of the Earth
to measure time.

We're always looking for things
that repeat over and over again,

and that repetition, that cycle
of things forms a clock.

That's all time becomes
is some repetitive process.

GREENE:
Measuring the Earth's motion
with a sundial,

we divided the day into hours.

WILLIAM PHILLIPS:
The Earth rotates once a day,

and we tick off the days

by looking at the rising
and the setting of the sun.

GREENE:
With the swing of a pendulum,

we divided hours
into minutes and seconds.

With the vibration
of a quartz crystal,

we improved accuracy
to the thousandths of a second.

But the National Institute
of Standards and Technology

in Colorado is the place to go

if you really want to know
what time it is.

STEVE JEFFERTS:
This is U.S. official time.

It doesn't get any more accurate
than this.

GREENE:
Here they measure time
with mind-boggling accuracy

using one of the smallest
objects in the universe:

an atom of a rare metal
called cesium.

PHILLIPS:
Atoms have a natural frequency.

And anything that vibrates,

that is giving you repetitive
motion, can be a clock.

The frequency at which
the cesium atom ticks

is the official timekeeper
for the world.

GREENE:
When a cesium atom
is bombarded with energy,

it vibrates, or ticks,
giving off pulses of light

over nine billion times
a second.

JEFFERTS:
We count the ticks
of the cesium atom.

And the cesium atom ticks

at this 9,1 92,631 ,770
ticks in a second.

And so every time you count up
to that number,

one second has gone by.

And you get one second
after one second,

after one second
after one second.

PHILLIPS:
This is just astounding.

My watch gains or loses a second
every couple of months.

We're talking about clocks that
would only gain or lose a second

in 1 00 million years.

And that kind of story, where
we take one measure of time

and replace it with something
that we decide is more accurate,

has been the constant
reform process of physics

over hundreds of years.

GREENE:
But no matter how accurate
our clocks have become,

time remains a mystery.

Clocks can tell us
what time it is,

but they haven't been able to
tell us what time itself is.

What is it
we're actually measuring?

We may not know what time is,

but the experience
of the passage of time

is a fundamental part
of our lives.

We're always thinking about
time, remembering the past,

making plans for the future,

living our lives within time's
constant tick, tick, tick.

I mean, look around
any train station

and you can see
how time rules our lives.

What may not be so obvious

is that the rise of train travel
played a key role

in one of the most startling
discoveries about time.

(train horn blowing)

Tickets, please, sir.

Train running on time?

Yes, sir.

Thank you.

GREENE:
In the early days
of train travel,

time posed a unique problem.

Back then, each town set
their own particular time.

Noon was when the sun
was directly overhead,

you know, more or less.

And what time it was
in another city,

well, you know,
that hardly mattered.

And to complicate things
even further,

trains would carry
the time of the city

where they began their journey.

So, if I was going
from Paris to Geneva,

I would be on Paris time
the whole way,

since that's where I started.

But were I going the other
direction, from Geneva to Paris,

I'd be on Geneva time.

PETER GALISON:
And as you began to have more
and more train lines crossing,

and more and more
different times

located at that interchange,

it became a nightmare
of confusion.

GREENE:
The need to coordinate clocks
over great distances

became a huge issue,

especially when the cities were
connected by a single track.

And here's where the modern
story of time begins.

As the need
for synchronized clocks

became ever more critical,

a young physicist
named Albert Einstein

took a job at the patent office
in Bern, Switzerland.

GALISON:
It was a ringside seat

to all of the great inventions
of the time.

The patents showed how
new and exciting ways

to synchronize clocks with the
exchange of telegraph signals,

clocks that were synchronized
by radio waves,

all made the synchronization
of time, and what time was,

and how it was measured,

something immediately important
and exciting for Einstein.

GREENE:
Einstein would soon shake up
the world

with a radical insight
into the nature of time.

And these mechanical devices
provided unexpected inspiration.

Einstein realized that these
attempts to synchronize clocks--

they were much more than merely
creative inventions.

Instead, he realized that they
were revealing a deep crack

in our understanding
of time itself.

Most people view time

in a pretty simple,
straightforward way.

Time ticks the same
for everyone everywhere.

It's a common-sense picture

established by the father
of modern science, Isaac Newton.

JIM GATES:
Time for Isaac Newton
is something that is

an immutable property
of the universe.

Time always changes
at the same rate.

Time just goes along,

and there's really nothing
we can do about it.

GREENE:
Sensible as Newton's picture
of time may seem,

Einstein realized
it wasn't right.

He discovered that time could
run at different rates.

As strange as it sounds,

this means that time for me
may not be the same

as time for you.

Einstein's discovery smashed
Newton's conception of reality.

Einstein says that time
is not just a label

on the whole universe;

time is experienced
individually.

What Einstein gave us is a much,
much richer picture

where everybody
has their own private time,

which runs
at their own private rates.

There isn't time in a sense
of a universal tick-tock;

there were times.

GREENE:
Einstein came to this
shocking revelation

by uncovering
a hidden connection

between space and time.

What Einstein figured out is
that there's a profound link

between motion through space
and the passage of time.

Roughly speaking,

the more you have of one,
the less you have of the other.

To see how this works,
let's take a little ride.

Right now, I'm heading due north
at 60 miles an hour.

And that means all my motion
is in the northward direction.

But let's now turn
onto a different road

and head northwest.

I'm still going
60 miles an hour,

but I'm not making as much
progress toward the north

as I was a minute ago.

And that's because some
of my northward motion

has been diverted, or shared
with, my westward motion.

Einstein realized that time
and space are linked

in much the same way
that north and west are.

And with this
surprising insight,

Einstein would overthrow
the common-sense idea

that time ticks the same
for everyone.

Here's what I mean.

That guy over there would say
that I'm not moving at all.

But I am.

I may not be moving
through space,

but I am moving through time.

I mean, after all,
my watch just keeps on ticking

and ticking.

And as long
as I'm standing still--

that is, not moving
through space--

Einstein said that all
of my motion is through time.

But look what happens
if I walk toward that guy.

We've exaggerated it,
but because I'm now in motion,

he'll perceive my watch
ticking slower.

That's because
from his perspective,

some of my previous motion
through time is being diverted

into my motion through space.

And it's not just my watch.

If we really exaggerate
the effect,

he'd perceive all my movement,
my voice,

everything about me
slowing down.

And now that I've stopped
moving,

the passage of time on our
watches once again agrees.

This was Einstein's
key insight:

that motion through space
affects the passage of time.

DAVID KAISER:
It's mind-blowing
that you and I will not agree

on measurements of time.

Isn't time separate
from us, right?

Why should my measurement of
time depend on how I am moving,

or how you're moving?

That doesn't make any sense.

Time itself is running
more slowly

for the person who's moving.

That's amazing.

No one before Einstein
ever imagined

that that sort of thing
would happen.

That was uniquely Einstein.

GREENE:
So why don't we ever see this
in everyday life?

Well, at the slow speeds
we move here on Earth,

motion's impact on time is so
tiny we don't experience it.

But the effect is real
and can be measured.

To do this, all you need
are a couple of atomic clocks

and a jet airplane.

And this experiment
was carried out in 1 971

when scientists flew an atomic
clock around the world

and then compared it
to one on the ground.

As Einstein predicted,
the two clocks no longer agreed.

They differed by only a few
hundred billionths of a second,

but that was very real proof

of motion's effect
on the passage of time.

PHILLIPS:
Einstein's theory
has been tested again

and again and again.

And it all hangs together.

It really forms the basis

for the way we understand
much of the way nature works.

These effects,
which used to be considered

sort of obscure and very small,

are very in-your-face
with today's technology.

GREENE:
With the discovery
of this unexpected link

between space and time,
Einstein realized that the two

could no longer be thought of
as separate things.

Instead, space and time
are fused together

in what came to be called
"spacetime."

Einstein unified the idea
of space with the idea of time

into this four-dimensional
structure called "spacetime."

GREENE:
And this fusion of space
and time would lead Einstein

to perhaps the most mind-bending
realization of all:

The sharp difference we see
between past,

present and future
may only be an illusion.

In our day-to-day lives,

we experience time
as a continuous flow.

But it can also be useful
to think of time

as a series of snapshots
or moments,

and everything that happens

can be thought of
as the unfolding of moment

after moment

after moment.

And if we picture all moments,
or snapshots, lined up--

every moment here on Earth,

every moment of Earth
orbiting the sun,

and every moment
throughout the entire universe--

we would see every event
that has ever happened

or will ever happen.

Every location in space, and
each and every moment in time,

from the birth of our universe
at the Big Bang

some 1 4 billion years ago

to the formation of stars
in the Milky Way galaxy,

to the creation of Earth
4 1 /2 billion years ago,

to the time of the dinosaurs,

to events happening
on Earth today,

like me working in my office.

Thinking about spacetime
like this

led Einstein to overturn
our everyday picture

of past, present and future.

To get a feel for this,
you have to think

about the seemingly simple
concept of "now."

For me, a list of things

that I consider to be happening
right now might include

the tick of noon
on my office clock,

my cat just now jumping
from the windowsill,

things happening far away

like a pigeon in Venice taking
flight at this very moment,

a meteor just now hitting
the moon...

and the explosion of a star at
the far reaches of the universe.

These and all other events
that I think are happening

at the same moment in time,

but in different regions
of our universe,

make up what I intuitively
think of as "now."

You can picture them as lying
on a single slice of spacetime.

Let's call it a "now slice."

Common sense would say
that you and I and everyone else

will agree on what's happening,
or what exists, right now,

moment after moment
after moment.

That is, we would all agree

on what lies on a given
"now slice."

But Einstein showed that,

strangely, when you take
motion into account,

this common-sense picture
of time goes out the window.

To see what I mean, think of
spacetime as a loaf of bread.

Einstein realized that just
as there are different ways

to cut a loaf of bread
into individual slices,

there are different ways
to cut spacetime

into individual now slices.

That is, because motion affects
the passage of time,

someone who is moving will have
a different conception

of what's happening right now,

and so they'll cut the loaf
into different now slices.

Their slices will be
at a different angle.

That person who's moving will...
will tilt the knife,

will be carving out these slices
at a different angle.

They won't be parallel
to my slices of time.

To get a feel for the bizarre
effect this can have,

imagine an alien, here,

in a galaxy ten billion
light-years from Earth.

And way over there on Earth,
the guy at the gas station.

Now, if the two
are sitting still-

not moving in relation
to one other-

their clocks tick off time
at the same rate

and so they share
the same now slices,

which cut straight
across the loaf.

But watch what happens
if the alien hops on his bike

and rides directly away
from Earth.

Since motion slows
the passage of time,

their clocks will no longer tick
off time at the same rate.

And if their clocks
no longer agree,

their now slices
will no longer agree either.

The alien's now slice cuts
through the loaf differently.

It's angled towards the past.

GREENE:
Since the alien is biking
at a leisurely pace,

his slice is angled to the past
by only a miniscule amount.

But across ten billion
light years,

that tiny angle results
in a huge difference in time.

So what the alien would find
on his angled now slice--

what he considers as happening
right now on Earth--

no longer includes our friend
at the gas station

or even 40 years earlier,
when our friend was a baby.

Amazingly, the alien's
now slice has swept back

through 200 years
of Earth history

and now includes events

that we consider part
of the distant past, like...

(classical music)

Beethoven finishing
the fifth symphony.

KAISER:
Even at a relatively slow speed,

we can have actually
tremendous disagreements

on our labeling of now,
what happens at the same time,

if we're spread out
far enough in space.

And if that's
not strange enough,

the direction you move
makes a difference, too.

Watch what happens when
the alien turns around

and bikes toward Earth.

The alien's new now slice is
angled to toward the future,

and so it includes events
that won't happen on Earth

for 200 years,

perhaps our friend's
great-great-great granddaughter

teleporting from Paris
to New York.

Once we know that your now can
be what I consider the past,

or your now can be
what I consider the future,

and your now is every bit
as valid as my now,

then we learn that the past
must be real.

The future must be real.

They could be your now.

That means past, present,
future: all equally real.

They all exist.

SEAN CARROLL:
If you believe
the laws of physics,

there's just as much reality
to the future and the past

as there is
to the present moment.

The past is not gone,

and the future
isn't non-existent.

The past, the future,
and the present are all existing

in exactly the same way.

Just as we think of all of space
as being "out there,"

we should think of all of time
as being "out there," too.

Everything that has ever
happened, or will happen.

It all exists.

GREENE:
From Leonardo da Vinci

laying the final brushstroke
on the Mona Lisa

to the signing of the
Declaration of Independence,

to your first day at school,

to events that from our
perspective are yet to happen,

like the first humans
landing on Mars.

With this bold insight,

Einstein shattered
one of the most basic concepts

of how we experience time.

"The distinction between past,
present, and future,"

he once said, "is only an
illusion, however persistent."

But if every moment in time
already exists,

then how do we explain the very
real feeling that time,

like this river, seems
to endlessly rush forward?

Well, maybe we've been deceived,
and time does not flow.

Perhaps the river of time
is more like a frozen river

with every moment
forever locked in place.

ALBERT:
The most vivid example
about the way the world is

has to do with
this flow of time.

Physics does radical violence

to this everyday experience
of time.

LEVIN:
Our entire experience of time
is constantly in the present.

And all we ever grasp
is that instant moment.

TEGMARK:
There is nothing
in the laws of physics

that picks out one now
over any other now.

And it's just from our
subjective viewpoints

that it feels like
things are changing.

GREENE:
Just the way an entire movie
exists on celluloid,

all of time may already exist.

The difference is that
in the movies,

a projector lights up or selects
each frame as it goes by.

But in the laws of physics,

there is no evidence of
something like a projector light

that selects one moment
over another.

Our brains may create this
impression, but in reality,

what we all experience
as the flow of time

may be nothing more
than an illusion.

But if time, like this frozen
river, does not flow,

and all of time is "out there,"

is it possible to travel
to the future or the paste?

BOARDING ANNOUNCEMENT:
Now departing for year 2060,
Flight 24.

GREENE:
And if we could time travel,

would it be anything
like what we all imagine?

"Catapult you through time into
a world that has yet to be&"

"The Time Travelers!"

"Suppose something goes wrong
with the time machine again?"

"Throw the switch, Jed!"

"Could we go anywhere we want
at any time?"

"We're going to attempt
time travel."

GREENE:
No one outside Hollywood

has made a working time machine
just yet.

But surprisingly, time travel
might be possible.

BOARDING ANNOUNCEMENT:
Now boarding, flight 24
to Black Hole A Star.

One way to travel through time

is to make use of a strange
feature of gravity.

The familiar force that keeps
our feet planted to the ground

can have a profound impact
on time.

Hi.
Hello.

See you later, sir.

Right, much later.

GREENE:
So how can gravity be used
to make a time machine?

Well Einstein's theories show
that gravity, like motion,

can affect time.

It's as if gravity can pull
on time, slowing its passage.

And the stronger
the gravitational pull,

the more time slows.

Here on Earth, the effect
is too small to notice,

but still very real.

Compared to someone living on
the top floor of a skyscraper,

someone living on the bottom

experiences time elapsing
a little slower

because gravity is just
a tiny bit stronger

closer to the ground.

But if you could travel
to a black hole,

the effect of gravity on time
would be huge.

Formed when large stars
collapse in on themselves,

black holes have immense
gravitational pull,

millions and even billions of
times stronger than the Earth's.

And if someone watched you
travel close to a black hole,

they'd see time for you
slow down dramatically.

LEVIN:
You near that black hole will
appear to your friend far away

to be moving slowly,
talking slowly,

biologically aging slowly.

To them years are passing, while
for you it might be minutes.

GREENE:
So depending on the black hole's
size and how close I get,

if I spend an hour
or two in orbit...

something like 50 years will
have passed back on Earth.

I will have traveled
to Earth's future.

Hello, sir.

Hi.

Long time, no see.

Time travel becomes you.

Thank you.

Kind of like
a fountain of youth.

So when I return, I'll find
myself in the future.

Everyone else
will have aged 50 years,

but me, I'll have aged
only a couple of hours.

Now, time travel to the future
is one thing.

But what about time travel
to the past?

Well, that might
be possible too,

using something predicted
by Einstein's equations

known as a wormhole.

If wormholes exist,

they would be kind of like
shortcuts through spacetime,

tunnels that would link not
just one place with another,

but also one moment
with another.

A wormhole would connect

one part in spacetime
to another part in spacetime

which is at an earlier time,

like a sort of subway system
through time.

So let's say I wanted
to go back in time

and meet myself at the beginning
of this program.

If a wormhole connected
here and there,

all I'd need to do
is step through.

Hey, good to see you again.

Thanks, good to be back.

Well, that would be
kind of weird,

but the real problem
with time travel to the past

is that things would get pretty
confusing pretty quickly.

I mean, imagine I were to change
something about my past,

like preventing my parents
from meeting.

Would that mean
I'd never be born?

If you do travel to the past,
you can't change things

that we know are true
about the past

because they already happened.

So if you go back

and kill who you thought
was your grandpa,

that must have been
some other guy

you thought
was your grandfather,

and everything
must somehow become

beautifully self-consistent,

even if it's in a twisted way.

GREENE:
And if you can travel
to the past,

why haven't we been overrun
by tourists from the future?

I mean, think about it.

We haven't seen any intrepid
time travelers

popping into
and out of our world--

at least, most people
don't think we have--

so it's probably safe to assume
that time travel to the past

just isn't possible,
at least not yet.

But since the math
hasn't yet ruled it out,

we can't dismiss time travel
to the past entirely.

PHILLIPS:
So it's not at all clear

that it could ever be
a practical reality,

but at least in principle,
it doesn't seem to be forbidden.

My guess is that
it's impossible,

but it's striking that we still
haven't been able

to rigorously prove that.

GREENE:
While it seems likely
that traveling to the past

is out of reach,

what about the fact, so common
to our everyday experience,

that time itself seems to move
in only one direction...

toward the future?

We call this the arrow of time.

CARROLL:
The arrow of time

is probably
the most blatant fact

about the universe we live in

that we don't completely
understand.

Why we live in a universe that
has a directionality to time

is a mystery.

JOSEPH LYKKEN:
This is not true of space.

In space, I can go
from New York to Chicago

and then I can change my mind
and go from Chicago to New York.

So there is a one-way
aspect to time

that we don't understand
at a fundamental level.

PHILLIPS:
Why doesn't it go backwards?

What does it even mean

that time goes forward
from the past into the future?

GREENE:
So what can we say about where
the arrow of time comes from?

Why do we only see events
unfold in one direction?

Why don't we ever see them
happen in reverse order?

Well, it must be
the laws of physics.

I mean, surely they don't allow
something like this to happen.

Well, actually they do.

The laws of physics

are the mathematical equations
we use to describe everything

from the behavior of atoms
to the swirl of galaxies.

They've been devised
and confirmed

through centuries of observation
and experiment.

But surprisingly, there's
nothing in the laws of physics

that says events have to unfold
through the familiar sequence

we call "forward in time."

According to these equations,

events could just as well
unfold in reverse order.

GATES:
Most of the equations we use

to describe what we see
in the universe around us

don't have an arrow of time
attached to them.

They're equations that work
equally well

moving forward in time
or moving backwards in time.

There's this contradiction
between the physics,

which seems fundamentally
reversible,

and so much of our life
that seems irreversible.

GREENE:
Though it flies in the face
of everyday experience,

the laws of physics actually say

bizarre things like these
are possible.

But how could this be?

Well, the answer is not as
far-fetched as you might think.

Here's why.

We all know what will happen
if I drop this glass of wine.

Now, the idea that this mess
could somehow reverse itself

and form back into a solid glass
filled with wine seems absurd.

But according to the laws
of physics, this can happen.

All I need to do is reverse
the velocities of everything.

Every piece of glass,
every drop of wine,

every molecule and atom in the
liquid, glass, table and air.

Just reverse
all their velocities

and... voil?!

So if the laws of physics

don't care about whether glasses
shatter or unshatter,

why don't we ever
see them unshatter?

How can we square
the laws of physics

with our everyday experience?

Something must be missing
in our understanding.

But what?

What's responsible
for the arrow of time?

(wolf howling)

Like many good mysteries,

this one leads us to a graveyard
in our search for clues.

In Vienna, near the final
resting places

of Beethoven, Brahms,
Schubert and Strauss,

is 1 9th-century
Austrian physicist

Ludwig Boltzmann's tombstone.

Etched on top is an elegant
equation: S=klogW.

It's the mathematical
formulation

of a powerful concept
known as entropy.

Entropy is a measure
of something

that we're all familiar with:
disorder, or randomness.

And it's an important idea
because there's a tendency

of everything in the universe
to move from order to disorder.

Here's a way to get
a feel for the idea.

Take my book.

All 569 pages of it.

It's very ordered,

with the first page
followed by the second,

followed by the third and so on.

But now let's tear the pages out
and let entropy go to work.

As you can see, the pages become
very disordered.

And the reason is simple:

There is only one way
for them to land in order,

but a huge number of ways
for them to land out of order,

and so it's much more likely

that they'll land
in a total mess.

And this is what we experience
in our daily lives:

things move from order
to disorder.

In this case,
from a neat, ordered book

to pages that are randomly
scattered.

Everywhere we look,

we see examples of entropy,
or disorder,

increasing
with the passage of time.

An egg breaks and splatters.

Ice cubes lose their orderly
shape as they melt into water.

Billowing smoke becomes
increasingly disordered.

GATES:
Ordered states become
disordered states,

and that appears to be, perhaps,

the direction
of an arrow of time.

We see sort of degrees
of messiness.

A measure of disorder
tends to increase

in one direction of time.

And so that, for Boltzmann,
begins to create an arc of time.

GREENE:
So maybe this is the answer.

Maybe the arrow of time comes
from the tendency of nature

to evolve toward
ever greater disorder.

This sure seems like progress,

but there's just one small
problem with this reasoning:

because the laws of physics

don't distinguish between
the future and the past,

entropy should increase
not only toward the future

but also toward the past.

And that makes no sense.

KAISER:
That's like saying
that entropy should increase

in either direction
that we look.

We could look backwards in time
and it should increase,

we could look forwards in time
and it should increase.

GREENE:
That would mean the pages
of my book in the past

would be disordered
and then come together

to form the neat,
ordered book in my hands.

And when's the last time you saw
something like that happen?

How could our everyday
experience be so at odds

with the laws of physics?

There must be a piece
of the puzzle that's missing.

If we're sure the past
had to be more ordered

and that everything tends
toward disorder

as the equations
of entropy tell us,

is there something else
besides the laws of physics

that might explain this?

Well, think of hitting
a baseball.

The laws of physics can help you
predict where it will land.

But those laws are not
the only things you need.

Run the film backward

and you can see that you also
need the initial conditions,

like how hard the ball was hit.

Similarly, if the laws
of physics

can't give us an explanation
for the arrow of time,

maybe we need to look further

to the initial conditions
of the universe.

That brings our attention
back to the Big Bang.

If the history of the universe
were like a movie

and you ran it backwards,

you'd see an increase in order
the further back in time you go.

Gradually, today's universe,

with billions of galaxies
clumped here and there,

would turn back into clouds
of gas and dust

as everything contracts.

CARROLL:
So these clouds of gas and dust

move closer and closer
to each other

so that if you get
far enough into the past,

they're squeezed into
a smaller and smaller volume.

We have now come to the place
where the buck finally stops.

If this represents all of space
at each moment of time,

then we can see there simply
isn't any more space and time

before this single moment.

So the ultimate source
of order, of low entropy,

must be the very beginning
of the universe: the Big Bang.

GATES:
The Big Bang
is a highly ordered state.

It's probably
the most ordered event

in all of physics.

And so, everything that
has come after that

has been an increase
in disorder.

KAISER:
What the Big Bang gives us

is a reason why the universe
might look different

when we look backwards
in time versus forward.

Moreover, when we go
back to early times,

the universe should have looked
not just different from today

but highly ordered.

CARROLL:
Why was the entropy low?

We don't know.

But at least we know
that there was a point

that the universe began in
when the entropy was low.

GREENE:
So our best understanding
is that the Big Bang

is what set the arrow of time
on its path.

You can picture this as
something like a wind-up clock.

Just as the stored energy
of a tightly wound clock

is released as it unwinds,

the universe has been unwinding
since the Big Bang,

becoming ever more disordered.

TEGMARK:
Our universe started out in
a very unusually orderly state,

and that's
ultimately responsible

for the fact that time
seems to have a direction.

GREENE:
We don't yet know
why our universe began

in a highly ordered state,

but the fact that it did

means that every time
a glass shatters,

it's actually carrying forward
something set in motion

billions of years ago.

The glass breaks
but doesn't unbreak

because it's following
the natural drive

from order to disorder
that began with the Big Bang.

CARROLL:
We only ever move
from the past to the future.

And everything we see around us,
all the changes,

from the formation of stars
to our lives,

is all little epiphenomena,
surfers riding the wave

of increasing disorganization
in the universe

that defines the difference
between the past and the future.

So the Big Bang
may have stamped

the arrow of time
on our universe,

and everything that has happened
since may simply be the drive

toward ever greater disorder
that began with that event

1 3.7 billion years ago.

But if time had a beginning

and disorder is
always increasing,

does that mean
that time will have an end?

What will the universe be like
in the far, far future?

Recent discoveries are shedding
new light on this question.

The explosive force
of the Big Bang

sent space hurtling outward.

And as a result, the universe
is still expanding today.

Until recently,

most people thought that
expansion must be slowing down.

That is, we thought of space,
filled with galaxies,

as kind of like a car
traveling down a highway.

RADIO ANNOUNCER:
You're listening to WUNI, the
stellar sounds of the cosmos.

GREENE:
If the driver takes
his foot off the gas,

the car gradually slows down.

Similarly, we thought
the universe was expanding,

but at a slower and slower rate.

But surprisingly,
astronomers found

the expansion of the universe
is not slowing down.

It's accelerating.

It's as if someone's not taking
their foot off the gas pedal,

but stepping on it, causing
a turbo booster to kick in.

And that's making
the expansion of the universe

speed up more and more.

KAISER:
Our expansion will keep
accelerating in the future,

not slow down.

It goes against everything

we had kind of gotten
used to thinking about.

GREENE:
This has some very strange
implications for the future.

Because the expansion of our
universe is accelerating,

in the far future,
after 1 00 billion years or so,

all of the other
distant galaxies

will have hurtled
out of sight from us.

It will appear as if our galaxy
were in the middle of nothing.

A surprising outcome
is that our descendants

will be at a terrible loss.

Light from distant galaxies
has to travel so far to reach us

that when we look out at them,

we're actually looking
back in time.

So in the far future,

when those distant galaxies
are no longer visible,

astronomers will find that
the past, in cosmic terms,

is out of reach.

And as for the end of time,

one theory suggests
that eventually,

black holes will dominate
the cosmos.

Then, they too will evaporate,

leaving nothing
but random particles

drifting through space.

LEVIN:
In a far distant future
where everything has decayed

and everything's just
sort of smoothed out,

there's no change.

And without change, we don't
really have a clear notion

of the passage of time.

If you don't have
events happening,

then it's hard to see
how you would even imagine

that there was time.

You can't even tell which
direction of time is forward

and which is backward.

In a very real sense,

time itself will one day
lose its meaning.

GREENE:
About 350 years ago,

Isaac Newton, who was
one of the first

to think about time
scientifically,

wrote that he did not need
to define time

because it is something
"well-known to all."

But in trying to square

our experience of time
with the true nature of time,

we've been forced to challenge

some of our most
deeply held beliefs.

We now know that in every event

that goes from order
to disorder,

there's a link
to the Big Bang itself,

giving us the arrow of time.

The common-sense notion that one
true time governs the universe

has given way to a picture
in which time is different

for each and every one of us.

And the flow of time,

which seems to us as real
as the flow of a river,

may be nothing more
than an illusion.

Past, present and future
may all exist on equal footing.

Our everyday experience of time

will always exert
a powerful influence.

We will continue to imagine
that time is universal,

that the past is gone,
that the future is yet to be.

But because of our
scientific discoveries,

we can also look
beyond experience

and recognize that we are
part of a far richer

and far stranger reality.

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is provided by
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