Original subtitles

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, bestselling author and physicist.

BRIAN 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 --

that our understanding of the universe

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

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.

BRIAN GREENE: We really may live in a universe

with more dimensions than meet the eye.

AMANDA PEET People who have said that there were 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?

S. JAMES GATES, JR.

If string theory fails to provide

a testable prediction,

then nobody should believe it.

SHELDON LEE GLASHOW

Is that a theory of physics,

or a philosophy?

BRIAN GREENE: One thing that is certain

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.

S. JAMES GATES, JR.

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.

BRIAN GREENE: 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.

THE ELEGANT UNIVERSE

Hosted By Brian Green

Einstein's Dream

A Theory of Everything?

BRIAN GREENE: Fifty 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.

String theory says

we may be living in a universe

where reality meets science fiction --

a universe of eleven dimensions

with parallel universes

right next door --

an elegant universe composed entirely

of the music of strings.

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.

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?

DOG BARKS: Vau, vau!

No that's the scalar curvature.

This is the ricci tensor.

Have you been studying this at all?

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.

STEVEN WEINBERG

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.

MICHAEL B. GREEN

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.

Newton's Embarrassing Secret

BRIAN 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.

STEVEN 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.

BRIAN GREENE: 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.

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.

S. JAMES GATES, JR.:

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 the planets have the orbits that they do.

And with Newton's equations you could calculate the way

that the planets would move.

Einstein's got to resolve this dilemma.

BRIAN GREENE: In his late twenties, 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.

A New Picture of Gravity

PETER GALISON

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.

BRIAN 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.

S. JAMES GATES, JR.: 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.

MARCIA BARTUSIAK 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 a thought he could do it again,

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

BRIAN 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.

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 Clark Maxwell,

the relationship between electricity and magnetism

was so obvious in nature that it demanded unification.

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.

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's before him,

Maxwell's unification took science a step closer

to cracking the code of the universe.

JOSEPH POLCHINSKI

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.

WALTER H.G. LEWIN

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.

PETER GALISON: Einstein thought that this was

one of the triumphant moments of all of physics

and admired Maxwell hugely for what he had done.

BRIAN GREENE: 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.

S. JAMES GATES, JR.:

Einstein clearly believes that the universe

has an overall grand and beautiful pattern to the way that it works.

So to answer your 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.

A Strange New World

BRIAN 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 a 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.

AMANDA PEET: 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.

BRIAN 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.

NIMA ARKANI-HAMED

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.

BRIAN 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.

STEVEN 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

had been so fabulously successful,

just weren't applicable anymore.

The Quantum Cafe

BRIAN 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.

Unification! Unification!

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.

PETER 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.

BRIAN 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

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 Caf�,

you might think you'd lost your mind.

WALTER H.G. 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.

BRIAN GREENE: 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 experiencethe

weirdness of quantum mechanics.

But here in the Quantum Caf�,

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.

BARTENDER: I'll try.

BRIAN GREENE: "I'll try," she says.

You see, they're not used to people placing

definite orders here in the Quantum Caf�,

because here everything is ruled by chance.

While I'd like an orange juice,

there is 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 seemas real to their inhabitants

as our universe seems to us.

WALTER H.G. 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.

Have to be a little careful, I think,

before you say this is clearly wrong.

BRIAN 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 kinds of things happen all the time.

EDWARD FARHI

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.

BRIAN 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.

MICHAEL DUFF

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."

BRIAN GREENE: Yet, experiment after experiment

showed Einstein was wrong

and that quantum mechanics really does describe

how the world works at the subatomic level.

WALTER H.G. 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 non-intuitive as it is,

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

EDWARD FARHI: Quantum mechanics

is fantastically accurate.

There has never been

a prediction of quantum mechanics

that has contradicted an observation,

never.

Gravity - The Odd Man Out

BRIAN 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 16th, 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 twenty thousand tons of TNT.

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.

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 10 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.

S. JAMES GATES, JR.: You try to put those two pieces

of mathematics together,

they do not coexist peacefully.

STEVEN WEINBERG: You get answers

that the probabilities of the event

you're looking at are infinite.

Nonsense, it's not profound,

it's just nonsense.

NIMA ARKANI-HAMED: It's very ironic because it was the first force

to actually be understood in some decent

quantitative way, but, but,

but it still remains split

off and very different from, from the other ones.

S. JAMES GATES, JR.: 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.

Strings to the Rescue

BRIAN 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.

STEVEN WEINBERG: Einstein, in his later years,

got rather detached from the work of physics

in general and, and stopped reading people's papers.

I didn'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.

S JAMES GATES, JR.: When the community of theoretical physicists

begins 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.

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.

STEVEN WEINBERG: 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 barbwire

trenches and machines guns of the other side.

BRIAN 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.

S. JAMES GATES, JR.:

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.

RIGHT SIDE BRIAN GREENE: 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...

LEFT SIDE BRIAN GREENE: ...and another that uses quantum mechanics

to study the tiniest of objects,

like atoms and particles.

This has been kind of like having two families

that just cannot get along

and never talk to each other...

RIGHT SIDE BRIAN GREENE: ...living under the same roof.

LEFT SIDE BRIAN GREENE: There just seemed to be no way to combine

quantum mechanics...

RIGHT SIDE BRIAN GREENE: ...and general relativity in a single theory

that could describe the universe on all scales.

BRIAN 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.

While stationed on the front lines

in WWI,

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

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.

EDWARD WITTEN

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.

BRIAN 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.

MICHAEL B. 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.

MICHAEL DUFF: 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.

MICHAEL B. GREEN: So it has incredible

unification power,

it unifies our understanding

of all these different kinds

of particles.

EDWARD WITTEN: So unity

of the different forces and particles

is achieved because they all

come from different kinds of vibrations

of the same basic string.

BRIAN GREENE: It's a simple idea

with far-reaching consequences.

JOSEPH LYKKEN

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.

Science of Philosophy?

BRIAN GREENE: But this seductive new theory

is also controversial.

Strings, if they exist,

are so small,

there's little hope of ever seeing one.

JOSEPH 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.

S. JAMES GATES, JR.: If string theory fails

to provide

a testable prediction,

then nobody should believe it.

On the other hand,

there is 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.

STEVEN WEINBERG:

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.

But ask me a hundred years from now,

then I can tell you.

BRIAN 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

sea shore, 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,

two hundred and fifty 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?

SHELDON LEE 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?

STEVEN 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.

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