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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 bestselling author and physicist.

And no matter how|many times I come here,

I never seem to get used to it.

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.

We really may live in a universe with|more dimensions than meet the eye.

People who have said that there|were extra dimensions of space

have been labeled crackpots,or|people who are bananas.

A mirage of science and mathematics|or the ultimate theory of everything?

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 is that|string theory is already showing us

that the universe may be a lot stranger

than any of us ever imagined.

Coming up tonight...

it all started with an apple.

The triumph of Newton's|equations come from the quest

to understand the planets and the stars.

And we've come a long way since.

Einstein gave the world a new picture

for what the force of|gravity actually is.

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.

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.

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.

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.

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.

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.

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.

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.

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 morewould|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.

Eleven, this is Houston.

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 replaythat 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.

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.

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.

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.

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.

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.

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 Clerk 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 before him,

Maxwell's unification|took science a step closer

to cracking the code of the universe.

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.

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.

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.

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.

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.

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.

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.

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

stole the spotlight from Einstein

when they came upwith|an outlandish new way

of thinking about physics.

Their vision of the|universewas 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 constituentsof nature-

were found to consist of|even smaller particles,

the now familiar nucleus|of protons and neutrons

orbited by electrons.

And the theoriesof Einstein and Maxwell

were useless at explaining

the bizarre way these|tiny bits of matter

interact with each|other inside the atom.

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.

Without a theory to explain|this strange new world,

these scientists were lost in|an unfamiliar atomic territory

looking forany 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 noticeablein our everyday world,

like they are here in the Quantum Caf�,

you might think you'd lost your mind.

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 universeactually 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 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.

I'll try.

"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'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.

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.

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.

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.

And neither did Einstein.

He never lost faith that|the universe behaves

in a certainand predictable way.

The idea that all we can|do is calculate the odds

that things will turn|outone way or another

was something Einstein deeply resisted.

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. "

Yet experiment after experiment

showed Einstein was wrong

and that quantum mechanics|really does describe

how the world works at|the subatomic level.

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.

Quantum mechanics is|fantastically accurate.

There has never been a|prediction of quantum mechanics

that has contradicted|an observation- never.

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.

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 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 languageas|the other forces-

the language of quantum mechanics-

has met with disaster.

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 atare 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.

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.

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.

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.

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.

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.

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 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 fabricof|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 staris|crushed to a tiny speck,

do you use general relativity-

because the staris 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.

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.

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.

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.

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.

But this seductive new|theory is also controversial.

Strings- if they exist-are so small

there's little hope of ever seeing one.

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.

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 pebbleor 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?

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?

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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