All language subtitles for NOVA.S30E12.The.Elegant.Universe.Einsteins.Dream.720p.WEB-DL.PBS.AAC2.0.H264-RTN

NARRATOR:
Now, on NOVA...

Take a thrill ride into a world
stranger than science fiction,

where you play the game
by breaking some rules;

where a new view of the universe

pushes you beyond the limits
of your wildest imagination.

This is the world
of string theory--

a way of describing every force
and all matter,

from an atom to Earth,
to the end of the galaxies;

from the birth of time
to its final tick--

in a single theory,
a theory of everything.

Our guide to this brave
new world is Brian Greene,

the best-selling author
and physicist.

GREENE:
And no matter how many times
I come here,

I never seem to get used to it.

NARRATOR:
Can he help us solve

the greatest puzzle
of modern physics?

(thunder crashing)

That our understanding
of the universe

is based on two sets of laws
that don't agree.

(crash)

NARRATOR:
Resolving that contradiction

eluded even Einstein,
who made it his final quest.

After decades, we may finally be
on the verge of a breakthrough.

The solution is... strings--

tiny bits of energy vibrating
like the strings on a cello,

a cosmic symphony
at the heart of all reality.

But it comes at a price--

parallel universes
and 11 dimensions,

most of which you've never seen.

We really may live in a universe

with more dimensions
than meet the eye.

WOMAN:
People who've said that there
are extra dimensions of space

have been labeled crackpots
or people who are bananas.

NARRATOR:
A mirage of science
and mathematics,

or the ultimate theory
of everything?

MAN:
If string theory
fails to provide

a testable prediction,
then nobody should believe it.

Is that a theory of physics
or a philosophy?

One thing that is certain...

GREENE:
Is that string theory
is already showing us

that the universe
may be a lot stranger

than any of us
ever imagined.

NARRATOR:
Coming up tonight...

It all started with an apple.

MAN:
The triumph
of Newton's equations come

from the quest to understand
the planets and the stars.

NARRATOR:
And we've come a long way since.

Einstein gave the world
a new picture

for what the force of gravity
actually is.

NARRATOR:
Where he left off,

string theorists now dare to go.

But how close are they
to fulfilling Einstein's dream?

Watch "The Elegant Universe"
right now.

Major funding for NOVA is
provided by the following:

(dog barking)

(thunder)

(door creaking)

50 years ago,
this house was the scene

of one of the greatest mysteries
of modern science--

a mystery so profound
that today

thousands of scientists
on the cutting edge of physics

are still trying to solve it.

Albert Einstein spent
his last two decades

in this modest home
in Princeton, New Jersey.

And in his second floor study,

Einstein relentlessly
sought a single theory

so powerful it would describe

all the workings
of the universe.

Even as he neared
the end of his life,

Einstein kept a notepad
close at hand,

furiously trying to come up
with the equations

for what would come to be known
as the "theory of everything."

Convinced he was on the verge

of the most important discovery
in the history of science,

Einstein ran out of time,
his dream unfulfilled.

Now, almost
a half-century later,

Einstein's goal of unification--

combining all the laws
of the universe

in one all-encompassing theory--

has become the holy grail
of modern physics,

and we think we may at last
achieve Einstein's dream

with a new and radical set
of ideas called string theory.

But if this revolutionary theory
is right,

we're in for quite a shock.

(explosion)

String theory says we may be
living in a universe

where reality
meets science fiction...

a universe of 11 dimensions

with parallel universes
right next door.

An elegant universe,

composed entirely
of the music of strings.

(playing slow, even notes)

But for all its ambition,

the basic idea of string theory
is surprisingly simple.

It says that everything
in the universe,

from the tiniest particle
to the most distant star,

is made from one kind
of ingredient:

unimaginably small,
vibrating strands of energy

called strings.

(cellists playing in harmony)

Just as the strings of a cello
can give rise

to a rich variety
of musical notes...

the tiny strings
in string theory vibrate

in a multitude
of different ways,

making up all the constituents
of nature.

In other words, the universe
is like a grand cosmic symphony,

resonating with all
the various notes

these tiny, vibrating strands
of energy can play.

String theory is still
in its infancy,

but it's already revealing

a radically new picture
of the universe,

one that is both strange
and beautiful.

But what makes us think
we can understand

all the complexity
of the universe,

let alone reduce it to
a single theory of everything?

We have R mu nu minus
a half g mu nu R--

you remember how this goes--

equals eight pi g T mu nu.

Comes from varying
the Einstein-Hilbert Action,

and we get the field equations
and this term.

You remember
what this is called.

(barks twice)

No, that's the scalar curvature.

This is the Ricci Tensor.

(sighs)

Have you been
studying this at all?

GREENE:
No matter how hard you try,

you can't teach physics
to a dog.

Their brains just aren't wired
to grasp it.

But what about us?

How do we know that we're wired

to comprehend the deepest laws
of the universe?

Well, physicists today
are confident that we are,

and we're picking up
where Einstein left off

in his quest for unification.

Unification would be
the formulation of a law

that describes perhaps
everything in the known universe

from one single idea,
one master equation.

And we think that there might
be this master equation

because throughout the course
of the last 200 years or so,

our understanding
of the universe

has given us a variety
of explanations

that are all pointing
towards one spot.

They seem to all be converging
on one nugget of an idea

that we're still trying to find.

MAN:
Unification is where it's at.

Unification is what we're trying
to accomplish.

The whole aim
of fundamental physics

is to see more and more
of the world's phenomena

in terms of fewer and fewer and
simpler and simpler principles.

MAN:
We feel, as physicists,
that if we can explain

a wide number of phenomena
in a very simple manner,

that that's somehow progress.

There is almost
an emotional aspect

to the way in which
the great theories in physics

sort of encompass a wide variety

of apparently different
physical phenomena.

So this idea that
we should be aiming

to unify our understanding
is inherent, essentially,

to the whole way in which
this kind of science progresses.

GREENE:
And long before Einstein,

the quest for unification began

with the most famous accident
in the history of science.

As the story goes,
one day in 1665,

a young man was sitting under
a tree when all of a sudden,

he saw an apple fall from above.

And with the fall of that apple,

Isaac Newton revolutionized
our picture of the universe.

In an audacious proposal
for his time,

Newton proclaimed that the force
pulling apples to the ground...

and the force keeping the moon
in orbit around the earth

were actually one and the same.

In one fell swoop,

Newton unified the heavens
and the earth

in a single theory
he called gravity.

WEINBERG:
The unification of the celestial
with the terrestrial--

that the same laws that govern
the planets in their motions

govern the tides and the falling
of fruit here on earth.

It was a fantastic unification
of our picture of nature.

Gravity was the first force
to be understood scientifically,

though three more
would eventually follow.

And although Newton discovered
his law of gravity

more than 300 years ago,

his equations
describing this force

make such accurate predictions

that we still make use
of them today.

In fact, scientists needed

nothing more
than Newton's equations

to plot the course of a rocket
that landed men on the moon.

MAN (over radio):
Eleven, this is Houston.

GREENE:
Yet, there was a problem.

While his laws described
the strength of gravity

with great accuracy,

Newton was harboring
an embarrassing secret:

He had no idea how gravity
actually works.

For nearly 250 years,

scientists were content
to look the other way

when confronted
with this mystery.

But in the early 1900s,

an unknown clerk working
in the Swiss patent office

would change all that.

While reviewing
patent applications,

Albert Einstein was also
pondering the behavior of light.

And little did Einstein know
that his musings on light

would lead him to solve Newton's
mystery of what gravity is.

At the age of 26, Einstein
made a startling discovery--

that the velocity of light
is a kind of cosmic speed limit,

a speed that nothing
in the universe can exceed.

But no sooner had the young
Einstein published this idea

than he found himself squaring
off with the father of gravity.

The trouble was

the idea that nothing can go
faster than the speed of light

flew in the face
of Newton's picture of gravity.

To understand this conflict, we
have to run a few experiments.

And to begin with, let's create
a cosmic catastrophe.

Imagine that all of a sudden
and without any warning,

the sun vaporizes
and completely disappears.

Now let's replay
that catastrophe

and see what effect
it would have on the planets

according to Newton.

Newton's theory predicts that
with the destruction of the sun,

the planets would immediately
fly out of their orbits,

careening off into space.

In other words, Newton thought
that gravity was a force

that acts instantaneously
across any distance,

and so we would immediately
feel the effect

of the sun's destruction.

But Einstein saw a big problem
with Newton's theory--

a problem that arose
from his work with light.

Einstein knew light doesn't
travel instantaneously.

In fact, it takes eight minutes
for the sun's rays

to travel the 93 million miles
to the earth.

And since he had shown that
nothing, not even gravity,

can travel faster than light,

how could the earth
be released from orbit

before the darkness resulting
from the sun's disappearance

reached our eyes?

To the young upstart
from the Swiss patent office,

anything outrunning light
was impossible,

and that meant the 250-year-old
Newtonian picture of gravity

was wrong.

MAN:
If Newton is wrong,
then why do the planets stay up?

Because remember, the triumph
of Newton's equations

come from the quest
to understand

the planets and the stars,
and particularly the problem

of why do the planets
have the orbits that they do.

And with Newton's equations,

you can calculate the way
that the planets will move.

Einstein's got to resolve
this dilemma.

GREENE:
In his late 20s,
Einstein had to come up

with a new picture
of the universe

in which gravity does not exceed
the cosmic speed limit.

Still working his day job
in the patent office,

Einstein embarked on a solitary
quest to solve this mystery.

After nearly ten years
of wracking his brain,

he found the answer
in a new kind of unification.

MAN:
Einstein came to think
of the three dimensions of space

and the single dimension of time

as bound together in
a single fabric of space-time.

It was his hope that
by understanding

the geometry of this
four-dimensional fabric of
space-time

that he could simply talk about
things moving along surfaces

in this space-time fabric.

GREENE:
Like the surface
of a trampoline,

this unified fabric
is warped and stretched

by heavy objects
like planets and stars.

And it's this warping,
or curving, of space-time

that creates what we feel
as gravity.

A planet like the earth
is kept in orbit

not because the sun reaches out

and instantaneously grabs hold
of it as in Newton's theory,

but simply because it follows
curves in the spatial fabric

caused by the sun's presence.

So, with this new understanding
of gravity,

let's rerun
the cosmic catastrophe.

Let's see what happens now
if the sun disappears.

The gravitational disturbance
that results

will form a wave that travels
across the spatial fabric

in much the same way that
a pebble dropped into a pond

makes ripples that travel
across the surface of the water.

So we wouldn't feel a change
in our orbit around the sun

until this wave
reached the earth.

What's more, Einstein calculated
that these ripples of gravity

travel at exactly
the speed of light.

And so, with this new approach,

Einstein resolved
the conflict with Newton

over how fast gravity travels.

And more than that, Einstein
gave the world a new picture

for what the force of gravity
actually is:

it's warps and curves
in the fabric of space and time.

Einstein called this new picture
of gravity "general relativity,"

and within a few short years,

Albert Einstein became
a household name.

GATES:
Einstein was like a rock star
in his day.

He was one of
the most widely known

and recognizable figures alive.

He and perhaps Charlie Chaplin
were the reigning kings

of the popular media.

People followed his work.

And they were anticipating,
because of this wonderful thing

he had done
with general relativity--

this recasting the laws
of gravity out of his head--

there was the thought
he could do it again

and they... you know,
people want to be in on that.

GREENE:
Despite all that
he had achieved,

Einstein wasn't satisfied.

He immediately set his sights
on an even grander goal:

the unification of
his new picture of gravity

with the only other force known
at the time, electromagnetism.

Now, electromagnetism is a force
that had itself been unified

only a few decades earlier.

In the mid-1800s,
electricity and magnetism

were sparking
scientists' interest.

These two forces seemed to share
a curious relationship

that inventors like Samuel Morse
were taking advantage of

in newfangled devices
such as the telegraph.

(telegraph clicking)

An electrical pulse, sent
through a telegraph wire

to a magnet thousands
of miles away,

produced the familiar dots
and dashes of Morse code

that allowed messages to be
transmitted across the continent

in a fraction of a second.

Although the telegraph
was a sensation,

the fundamental science
driving it

remained something of a mystery.

But to a Scottish scientist
named James Clerk Maxwell,

the relationship between
electricity and magnetism

was so obvious in nature
that it demanded unification.

(thunder)

If you've ever been on top of a
mountain during a thunderstorm,

you'll get the idea of how
electricity and magnetism

are closely related.

When a stream of electrically
charged particles flows,

like in a bolt of lightning,
it creates a magnetic field,

and you can see evidence of this
on a compass.

(thunder)

(thunder)

Obsessed with this relationship,

the Scot was determined
to explain the connection

between electricity
and magnetism

in the language of mathematics.

Casting new light
on the subject,

Maxwell devised a set

of four elegant
mathematical equations...

that unified electricity
and magnetism

in a single force
called electromagnetism.

And like Isaac Newton
before him,

Maxwell's unification
took science a step closer

to cracking the code
of the universe.

MAN:
That was really
the remarkable thing

that these different phenomena

were really connected
in this way.

And it's another example
of diverse phenomena

coming from a single
underlying building block

or a single
underlying principle.

Imagine that everything
that you can think of

which has to do
with electricity and magnetism

can all be written
in four very simple equations.

Isn't that incredible?

Isn't that amazing?

I call that elegant.

Einstein thought that this

was one of the triumphant
moments of all of physics

and admired Maxwell hugely
for what he had done.

About 50 years

after Maxwell unified
electricity and magnetism,

Einstein was confident
that if he could unify

his new theory of gravity
with Maxwell's electromagnetism,

he'd be able to formulate
a master equation

that could describe everything,
the entire universe.

Einstein clearly believes
that the universe has

an overall grand
and beautiful pattern

to the way that it works.

And so to answer the question,

why was he looking
for the unification?

I think the answer is simply

that Einstein is one
of those physicists

who really wants to know
the mind of God,

which means the entire picture.

GREENE:
Today this is the goal
of string theory--

to unify our understanding
of everything

from the birth
of the universe

to the majestic swirl
of galaxies

in just one set of principles--
one master equation.

Newton had unified the heavens
and the earth

in the theory of gravity.

Maxwell had unified electricity
and magnetism.

Einstein reasoned
all that remained

to build a theory
of everything--

a single theory
that could encompass

all the laws of the universe--

was to merge his new picture of
gravity with electromagnetism.

WOMAN:
He certainly had motivation.

Probably one of them might
have been aesthetics,

or this quest to simplify.

Another one might have been
just the physical fact

that it seems like
the speed of gravity

is equal to the speed of light.

So if they both go
at the same speed,

then maybe that's an indication
of some underlying symmetry.

GREENE:
But as Einstein began trying

to unite gravity
and electromagnetism,

he would find that
the difference in strength

between these two forces would
outweigh their similarities.

Let me show you what I mean.

We tend to think that gravity
is a powerful force.

After all, it's the force

that right now is anchoring me
to this ledge.

But compared
to electromagnetism,

it's actually terribly feeble.

In fact, there's a simple
little test to show this.

Imagine that I was to leap
from this rather tall building.

Actually, let's not
just imagine it.

Let's do it.

You'll see what I mean.

Now, of course, I really
should have been flattened.

But the important question is:

what kept me from crashing
through the sidewalk

and hurtling right down
to the center of the earth?

Well, strange as it sounds,
the answer is electromagnetism.

Everything we can see
from you and me to the sidewalk

is made of tiny bits of matter
called atoms.

And the outer shell
of every atom

contains a negative
electrical charge.

So when my atoms collide
with the atoms in the cement,

these electrical charges repel
each other with such strength

that just a little piece
of sidewalk

can resist the entire
earth's gravity

and stop me from falling.

In fact,
the electromagnetic force

is billions and billions
of times stronger than gravity.

MAN:
That seems a little strange

because gravity keeps our feet
to the ground,

it keeps the earth
going around the sun.

But in actual fact,
it manages to do that

only because it acts

on huge, enormous
conglomerates of matter--

you know, you, me,
the earth, the sun.

But really at the level
of individual atoms,

gravity is a really
incredibly feeble, tiny force.

GREENE:
It would be an uphill battle
for Einstein

to unify these two forces
of wildly different strengths.

And to make matters worse,
barely had he begun

before sweeping changes
in the world of physics

would leave him behind.

WEINBERG:
Einstein had achieved so much
in the years up to about 1920

that he naturally expected
that he could go on

by playing the same
theoretical games

and go on achieving
great things.

And he couldn't.

Nature revealed itself in other
ways in the 1920s and 1930s,

and the particular
tricks and tools

that Einstein
had at his disposal

that had been
so fabulously successful

just weren't applicable anymore.

GREENE:
You see, in the 1920s,
a group of young scientists

stole the spotlight
from Einstein

when they came up
with an outlandish new way

of thinking about physics.

Their vision of the universe
was so strange

it makes science fiction
look tame,

and it turned Einstein's quest
for unification on its head.

MECHANICAL VOICE:
Unification, unification.

GREENE:
Led by Danish physicist
Niels Bohr,

these scientists were uncovering

an entirely new realm
of the universe.

Atoms-- long thought to be

the smallest constituents
of nature--

were found to consist
of even smaller particles,

the now familiar nucleus
of protons and neutrons

orbited by electrons.

And the theories
of Einstein and Maxwell

were useless at explaining

the bizarre way
these tiny bits of matter

interact with each other
inside the atom.

GALISON:
There was a tremendous mystery

about how to account
for all this--

how to account for what was
happening to the nucleus

as the atom began to be
pried apart in different ways.

And the old theories
were totally inadequate

to the task of explaining them.

Gravity was irrelevant--
it was far too weak--

and electricity and magnetism
was not sufficient.

GREENE:
Without a theory to explain
this strange new world,

these scientists were lost in
an unfamiliar atomic territory

looking for
any recognizable landmarks.

Then in the late 1920s,
all that changed.

During those years,

physicists developed a new
theory called quantum mechanics,

and it was able to describe

the microscopic realm
with great success.

But here's the thing:

quantum mechanics
was so radical a theory

that it completely shattered

all previous ways
of looking at the universe.

Einstein's theories demand

that the universe
is orderly and predictable.

But Niels Bohr disagreed.

He and his colleagues proclaimed

that at the scale
of atoms and particles,

the world is a game of chance.

At the atomic or quantum level,
uncertainty rules.

The best you can do
according to quantum mechanics

is predict the chance,
or probability,

of one outcome or another.

And this strange idea...

Thanks.

opened the door to an unsettling
new picture of reality.

It was so unsettling

that if the bizarre features
of quantum mechanics

were noticeable
in our everyday world,

like they are here
in the Quantum Café,

you might think
you'd lost your mind.

LEWIN:
The laws in the quantum world

are very different from
the laws that we are used to.

Our daily experiences
are totally different

from anything that you would see
in the quantum world.

The quantum world is crazy.

It's probably
the best way to put it.

It's a crazy world.

For nearly 80 years,

quantum mechanics
has successfully claimed

that the strange and bizarre
are typical

of how our universe
actually behaves

on extremely small scales.

At the scale of everyday life,
we don't directly experience

the weirdness
of quantum mechanics.

But here in the Quantum Café,

big, everyday things
sometimes behave

as if they were
microscopically tiny.

And no matter
how many times I come here,

I never seem to get used to it.

I'll have
an orange juice, please.

I'll try.

"I'll try," she says.

You see, they're not
used to people

placing definite orders
here in the Quantum Café

because here,
everything is ruled by chance.

While I'd like an orange juice,

there's only
a particular probability

that I'll actually get one.

And there's no reason
to be disappointed

with one particular
outcome or another

because quantum
mechanics suggests

that each of
the possibilities--

like getting a yellow juice
or a red juice--

may actually happen.

They just happen to happen

in universes that
are parallel to ours--

universes that seem as
real to their inhabitants

as our universe seems to us.

LEWIN:
If there are a thousand
possibilities

and quantum mechanics
cannot with certainty say

which of the thousand
it will be,

then all thousand will happen.

Yeah, you can laugh at it

and say,
"Well, that has to be wrong."

But there are so many
other things in physics

which at the time
that people came up with

had to be wrong, but it wasn't.

You have to be
a little careful, I think,

before you say
this is clearly wrong.

GREENE:
And even in our own universe,

quantum mechanics says
there's a chance

that things we'd ordinarily
think of as impossible

can actually happen.

For example, there's a chance
that particles

can pass right through
walls or barriers

that seem impenetrable
to you or me.

There's even a chance

that I could pass through
something solid, like a wall.

Now, quantum calculations
do show

that the probability for this
to happen in the everyday world

is so small
that I'd need to continue

walking into the wall
for nearly an eternity

before having a reasonable
chance of succeeding.

But here, these kind of things
happen all the time.

MAN:
You have to learn

to abandon those assumptions
that you have about the world

in order to understand
quantum mechanics.

In my gut, in my belly,
do I feel like I have

a deep, intuitive understanding
of quantum mechanics?

No.

GREENE:
And neither did Einstein.

He never lost faith
that the universe behaves

in a certain
and predictable way.

The idea that all we can do
is calculate the odds

that things will turn out
one way or another

was something Einstein
deeply resisted.

MAN:
Quantum mechanics says
that you can't know for certain

the outcome of any experiment.

You can only assign
a certain probability

to the outcome
of any experiment.

And this Einstein
disliked intensely.

He used to say,
"God does not throw dice."

GREENE:
Yet experiment after experiment
showed Einstein was wrong

and that quantum mechanics
really does describe

how the world works
at the subatomic level.

LEWIN:
So quantum mechanics
is not a luxury--

something that
you can do without.

I mean,
why is water the way it is?

Why does light
go straight through water?

Why is it transparent?

Why are other things
not transparent?

How do molecules form?

Why are they reacting
the way they react?

The moment that
you want to understand anything

at an atomic level--
as nonintuitive as it is--

at that moment you can only make
progress with quantum mechanics.

FARHI:
Quantum mechanics
is fantastically accurate.

There has never been a
prediction of quantum mechanics

that has contradicted
an observation-- never.

GREENE:
By the 1930s,

Einstein's quest for unification
was floundering

while quantum mechanics
was unlocking

the secrets of the atom.

Scientists found that
gravity and electromagnetism

are not the only forces
ruling the universe.

Probing the structure
of the atom,

they discovered two more forces.

One, dubbed
the strong nuclear force,

acts like a super-glue,

holding the nucleus
of every atom together,

binding protons to neutrons.

And the other,
called the weak nuclear force,

allows neutrons
to turn into protons,

giving off radiation
in the process.

At the quantum level,

the force we're most
familiar with-- gravity--

was completely overshadowed
by electromagnetism

and these two new forces.

Now, the strong and weak forces
may seem obscure,

but in one sense at least,

we're all very much aware
of their power.

At 5:29 on the morning
of July 16, 1945,

that power was revealed
by an act

that would change
the course of history.

In the middle of the desert
in New Mexico,

at the top of a steel tower

about a hundred feet above
the top of this monument,

the first atomic bomb
was detonated.

It was only about
five feet across,

but that bomb packed a punch

equivalent to about
20,000 tons of TNT.

(huge explosion)

With that powerful explosion,

scientists unleashed
the strong nuclear force--

the force that keeps
neutrons and protons

tightly glued together
inside the nucleus of an atom.

By breaking the bonds
of that glue

and splitting the atom apart...

vast, truly unbelievable amounts

of destructive energy
were released.

(Geiger counter clicking)

We can still detect remnants
of that explosion

through the other nuclear
force-- the weak nuclear force--

because it's responsible
for radioactivity.

And today,
more than 50 years later,

the radiation levels
around here are still

about ten times higher
than normal.

So although in comparison to
electromagnetism and gravity

the nuclear forces act
over very small scales,

their impact on everyday life
is every bit as profound.

But what about gravity--
Einstein's general relativity?

Where does that fit in
at the quantum level?

Quantum mechanics tells us

how all of nature's forces
work in the microscopic realm

except for the force of gravity.

Absolutely no one could
figure out how gravity operates

when you get down to the size of
atoms and subatomic particles.

That is, no one could figure out

how to put general relativity
and quantum mechanics

together into one package.

For decades, every attempt
to describe the force of gravity

in the same language
as the other forces--

the language
of quantum mechanics--

has met with disaster.

GATES:
You try to put those two
pieces of mathematics together,

they do not coexist peacefully.

You get answers that
the probabilities

of the event you're looking at
are infinite.

Nonsense.

It's not profound,
it's just nonsense.

It's very ironic,

because it was the first force
to actually be understood

in some decent,
quantitative way,

but it still remains split off

and very different
from the other ones.

The laws of nature are
supposed to apply everywhere.

So if Einstein's laws are
supposed to apply everywhere

and the laws
of quantum mechanics

are supposed
to apply everywhere,

well, you can't have
two separate everywheres.

GREENE:
In 1933, after fleeing
Nazi Germany,

Einstein settled
in Princeton, New Jersey.

Working in solitude,

he stubbornly
continued the quest

he had begun
more than a decade earlier

to unite gravity
and electromagnetism.

Every few years,
headlines appeared

proclaiming Einstein
was on the verge of success.

But most of his colleagues
believed his quest was misguided

and that his best days
were already behind him.

WEINBERG:
Einstein in his later years

got rather detached from
the work of physics in general

and stopped reading
people's papers.

I don't even think he knew

there was such a thing
as the weak nuclear force.

He didn't pay attention
to those things.

He kept working
on the same problem

that he had started working on
as a younger man.

GATES:
When the community
of theoretical physicists

begin to probe the atom,

Einstein very definitely
gets left out of the picture.

He in some sense chooses
not to look at the physics

coming from these experiments.

Uh, that means that
the laws of quantum mechanics

play no role in his sort of
further investigations.

He's thought to be

this doddering,
sympathetic, old figure

who led an earlier revolution
but somehow fell out of it.

It is as if a general who was
a master of horse cavalry,

who has achieved great things
as a commander

at the beginning
of the First World War,

would try to bring
mounted cavalry into play

against the barbed wire,
trenches and machine guns

of the other side.

GREENE:
Albert Einstein died
on April 18, 1955.

And for many years
it seemed that Einstein's dream

of unifying the forces
in a single theory

died with him.

GATES:
So the quest for unification

becomes a backwater of physics.

By the time of Einstein's
death in the '50s,

almost no serious physicists

are engaged in this quest
for unification.

In the years since, physics
split into two separate camps:

one that uses general relativity
to study big and heavy objects,

things like stars, galaxies
and the universe as a whole...

and another that uses
quantum mechanics

to study the tiniest
of objects

like atoms
and particles.

This has kind of been like
having two families

that just cannot get along
and never talk to each other...

living under the same roof.

There just seemed
to be no way

to combine
quantum mechanics...

and general relativity

in a single theory
that could describe

the universe
on all scales.

GREENE:
Now, in spite of this,

we've made tremendous progress
in understanding the universe.

But there's a catch.

There are strange realms
of the cosmos

that will never be
fully understood

until we find a unified theory.

And nowhere is this
more evident...

than in the depths
of a black hole.

A German astronomer
named Karl Schwarzschild

first proposed what we now call
"black holes" in 1916...

(bombs whistle)

while stationed on the front
lines in World War I.

He solved the equations
of Einstein's general relativity

in a new and puzzling way.

Between calculations
of artillery trajectories,

Schwarzschild figured out
that an enormous amount of mass,

like that of a very dense star
concentrated in a small area,

would warp the fabric
of space-time so severely...

(explosion)

that nothing, not even light,

could escape
its gravitational pull.

For decades,
physicists were skeptical

that Schwarzschild's
calculations

were anything more than theory.

But today satellite telescopes
probing deep into space

are discovering regions
with enormous gravitational pull

that most scientists believe
are black holes.

Schwarzschild's theory
now seems to be reality.

So here's the question.

If you're trying to figure out

what happens
in the depths of a black hole,

where an entire star
is crushed to a tiny speck,

do you use general relativity--

because the star
is incredibly heavy--

or quantum mechanics,
because it's incredibly tiny?

Well, that's the problem.

Since the center of a black hole
is both tiny and heavy,

you can't avoid using both
theories at the same time.

And when we try to put
the two theories together

in the realm of black holes...

they conflict, it breaks down.

They give
nonsensical predictions,

and the universe
is not nonsensical.

It's got to make sense.

Quantum mechanics works
really well for small things,

and general relativity
works really well

for stars and galaxies.

But the atoms-- the small
things-- and the galaxies,

they're part
of the same universe,

so there has to be
some description

that applies to everything.

So we can't have one description
for atoms and one for stars.

GREENE:
Now, with string theory,
we think we may have found a way

to unite our theory of the large
and our theory of the small

and make sense of the universe
at all scales and all places.

Instead of a multitude
of tiny particles,

string theory proclaims that
everything in the universe--

all forces and all matter--

is made of one
single ingredient:

tiny vibrating strands of energy
known as strings.

GREEN:
A string can wiggle
in many different ways,

whereas, of course,
a point can't.

And the different ways
in which the string wiggles

represent the different kinds
of elementary particles.

It's like a violin string

and it can vibrate just like
violin strings can vibrate.

Each note, if you like,
describes a different particle.

GREEN:
So it has incredible
unification power.

It unifies our understanding

of all these different
kinds of particles.

So unity of the different forces
and particles is achieved

because they all come from
different kinds of vibrations

of the same basic string.

GREENE:
It's a simple idea
with far-reaching consequences.

What string theory does is
it holds out the promise

that, look, we can really
understand questions

that you might not
even have thought

were scientific questions,

questions about
how the universe began,

why the universe is
the way it is

at the most fundamental level.

The idea
that a scientific theory

that we already have
in our hands

could answer
the most basic questions

is extremely seductive.

GREENE:
But this seductive new theory
is also controversial.

Strings-- if they exist--
are so small

there's little hope
of ever seeing one.

LYKKEN:
String theory
and string theorists

do have a real problem.

How do you actually test
string theory?

If you can't test it in the way
that we test normal theories,

it's not science,
it's philosophy,

and that's a real problem.

If string theory fails to
provide a testable prediction,

then nobody should believe it.

On the other hand,

there's a kind of elegance
to these things,

and given the history

of how theoretical physics
has evolved thus far,

it is totally conceivable
that some,

if not all, of these ideas
will turn out to be correct.

I think a hundred years from
now, this particular period,

when most of the brightest
young theoretical physicists

worked on string theory will be
remembered as a heroic age,

when theorists tried
and succeeded

to develop a unified theory
of all the phenomena of nature.

On the other hand, it may be
remembered as a tragic failure.

My guess is that it will be
something like the former

rather than the latter.

Uh... but ask me a hundred years
from now, then I can tell you.

GREENE:
Our understanding
of the universe has come

an enormously long way
during the last three centuries.

Just consider this.

Isaac Newton, who was perhaps

the greatest scientist
of all time,

once said, "I have been like
a boy playing on the seashore,

"diverting myself
in, now and then, finding

"a smoother pebble
or a prettier shell than usual,

while the great ocean of truth
lay before me all undiscovered."

And yet, 250 years later,

Albert Einstein, who was
Newton's true successor,

was able to seriously suggest
that this vast ocean,

all the laws of nature,
might be reduced

to a few fundamental ideas

expressed by a handful
of mathematical symbols.

And today, a half-century
after Einstein's death,

we may at last be on the verge

of fulfilling his dream of
unification with string theory.

But where did this daring and
strange new theory come from?

How does string theory achieve
the ultimate unification

of the laws of the large
and the laws of the small?

And how will we know
if it's right or wrong?

GLASHOW:
No experiment can ever check up
what's going on

at the distances
that are being studied.

The theory is permanently safe.

Is that a theory of physics
or a philosophy?

WEINBERG:
It isn't written in the stars
that we're going to succeed,

but in the end,
we hope we will have

a single theory
that governs everything.

This NOVA program is available
on DVD.

The companion book
is also available.

To order, visit shopPBS.org,
or call 1-800-play-PBS.

Captioned by
Media Access Group at WGBH
access.wgbh.org