# Nova (1974–…): Season 30, Episode 13 - The Elegant Universe: String's the Thing - full transcript

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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 best-selling 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've said that there

are 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...

We accidentally discovered

string theory.

The humble beginnings

of a revolutionary idea.

I was completely convinced

it was going to say,

"Susskind is the next Einstein."

This seemed crazy to people.

I was depressed, I was unhappy.

The result was

I went home and got drunk.

Obsession drives scientists

to pursue the holy grail

of physics,

but are they ready

for what they discover?

Step into the bizarre world

of "The Elegant Universe,"

right now.

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NATIONAL SCIENCE FOUNDATION,

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THE GEORGE D. SMITH FUND,

THE U.S. DEPARTMENT OF ENERGY,

THE CORPORATION

FOR PUBLIC BROADCASTING

and VIEWERS LIKE YOU

Corporate funding for NOVA

is provided by Sprint

and Microsoft.

Additional funding

is provided by:

Dedicated to education

and quality television.

Funding for "The Elegant

Universe" is provided by:

To enhance public understanding

of science and technology.

And by the National

Science Foundation,

where discoveries begin.

Additional funding is provided

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Major funding for NOVA

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It's a little-known secret,

but for more than half a century

a dark cloud has been looming

over modern science.

Here's the problem.

Our understanding

of the universe is based

on two separate theories.

One is Einstein's

general theory of relativity.

That's a way of understanding

the biggest things

in the universe...

Things like stars and galaxies.

But the littlest things

in the universe...

Atoms and subatomic particles...

Play by an entirely different

set of rules

called quantum mechanics.

These two sets of rules are each

incredibly accurate

in their own domain,

but whenever

we try to combine them

to solve some of the deepest

mysteries in the universe,

disaster strikes.

Take the beginning

of the universe, the Big Bang.

At that instant, a tiny nugget

erupted violently.

Over the next 14 billion years,

the universe expanded and cooled

into the stars, galaxies

and planets we see today.

But if we run the cosmic film

in reverse,

everything that's now rushing

apart comes back together.

So the universe gets

smaller, hotter and denser

as we head back

to the beginning of time.

As we reach the Big Bang...

When the universe was

both enormously heavy

and incredibly tiny...

Our projector jams.

Our two laws of physics,

when combined, break down.

But what if we could unite

quantum mechanics

and general relativity and see

the cosmic film in its entirety?

Well, a new set of ideas

called string theory

may be able to do that,

and if it's right,

it would be one of

the biggest blockbusters

in the history of science.

Someday, string theory may be

able to explain all of nature...

From the tiniest bits of matter

to the farthest reaches

of the cosmos...

Using just one

single ingredient:

tiny, vibrating strands

of energy, called "strings."

But why do we have to rewrite

the laws of physics

to accomplish this?

Why does it matter

if the two laws that we have

are incompatible?

Well, you can think of it

like this.

Imagine you lived

in a city ruled not

by one set of traffic laws,

but by two separate sets of laws

that conflicted with each other.

As you can see, it would be

pretty confusing.

What the heck are you doing?!

To understand this place,

you'd need to find a way

to put these two conflicting

sets of laws together

into one all-encompassing set

that makes sense.

We work on the assumption

that there is a theory out there

and it's our job...

If we're sufficiently smart

and sufficiently industrious...

To figure out what it is.

We don't have a guarantee,

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.

But before we can find

that theory,

we need to take

a fantastic journey

to see why

the two sets of laws we have

conflict with each other.

And the first stop

on this strange trip is

the realm of very large objects.

To describe the universe

on large scales,

we use one set of laws...

Einstein's general theory

of relativity,

and that's a theory

of how gravity works.

General relativity

pictures space

as sort of like a trampoline,

a smooth fabric that heavy

objects like stars and planets

can warp and stretch.

Now, according to the theory,

these warps and curves create

what we feel as gravity.

That is, the gravitational pull

that keeps the earth

in orbit around the sun

is really nothing more

than our planet following

the curves and contours

that the sun creates

in the spatial fabric.

But the smooth,

gently curving image of space

predicted by the

laws of general relativity

is not the whole story.

To understand the universe

on extremely small scales,

we have to use our other

set of laws... quantum mechanics.

And as we'll see,

quantum mechanics paints

a picture of space

so drastically different

from general relativity

that you'd think

they were describing

two completely

separate universes.

To see the conflict

between general relativity

and quantum mechanics,

we need to shrink

way, way, way down in size.

And as we leave the world

of large objects behind...

and approach

the microscopic realm,

the familiar picture of space

in which everything

behaves predictably

begins to be replaced

by a world with a structure

that is far less certain.

And if we keep shrinking,

getting billions and billions

of times smaller

than even the tiniest

bits of matter...

Atoms and the tiny particles

inside of them...

The laws of the very small,

quantum mechanics, say

that the fabric of space

becomes bumpy and chaotic.

Eventually, we reach

a world so turbulent

that it defies common sense.

Down here, space and time are

so twisted and distorted

that the conventional ideas

of left and right...

up and down...

even before and after,

break down.

There's no way to tell

for certain that I'm here...

or here...

or both places at once.

Or maybe I arrived here...

before I arrived here.

In the quantum world, you just

can't pin everything down.

It's an inherently

wild and frenetic place.

The laws in the quantum world

are very different

from the laws

that we are used to.

And is that surprising?

Why should the world of the very

small, at an atomic level...

Why should that world obey

the same kind of rules and laws

that we are used to in our world

with apples and oranges and

walking around on the street?

Why would that world

behave the same way?

The fluctuating, jittery picture

of space and time

predicted by quantum mechanics

is in direct conflict

with the smooth, orderly,

geometric model

of space and time

described by general relativity.

But we think that everything...

From the frantic dance

of subatomic particles

to the majestic swirl

of galaxies...

Should be explained by just

one grand physical principle,

one master equation.

If we can find that equation,

how the universe really works

at every time and place

will at last be revealed.

You see, what we need

is a theory

that can cope with the very tiny

and the very massive...

One that embraces both quantum

mechanics and general relativity

and never breaks down, ever.

For a physicist,

finding a theory

that unites general relativity

and quantum mechanics

is the holy grail,

because that framework

would give us a single

mathematical theory

that describes all the forces

that rule our universe.

General relativity describes the

most familiar of those forces...

Gravity.

But quantum mechanics

describes three other forces.

The strong nuclear force.

That's responsible

for gluing protons and neutrons

together inside of atoms.

Electromagnetism...

which produces

light, electricity

and magnetic attraction.

And the weak nuclear force.

That's the force responsible

for radioactive decay.

Every event in the universe,

from the splitting of an atom

to the birth of a star,

is nothing more than these four

forces interacting with matter.

Albert Einstein spent

the last 30 years of his life

searching for a way

to describe the forces of nature

in a single theory.

And now string theory

may fulfill

his dream of unification.

For centuries,

scientists have pictured

the fundamental ingredients

of nature...

Atoms and the smaller particles

inside of them...

As tiny balls or points.

But string theory proclaims

that at the heart

of every bit of matter

is a tiny, vibrating strand

of energy, called a string.

And a new breed

of scientist believes

these minuscule strings

are the key

to uniting the world of the

large and the world of the small

in a single theory.

The idea

that a scientific theory

that we already have

in our hands

could answer

the most basic questions

is extremely seductive.

For about 2,000 years,

all of our physics, essentially,

has been based on...

Essentially we were talking

about billiard balls.

The very idea of the string

is such a paradigm shift,

because instead

of billiard balls,

you have to use

little strands of spaghetti.

But not everyone is enamored

of this new theory.

So far no experiment

has been devised

that can prove

these tiny strings exist.

Let me put it bluntly:

There are physicists

and there are string theorists.

It is a new discipline,

a new, you may call it, tumor.

You can call it what you will.

They have focused

on questions which

experiment cannot address.

They will deny that...

These string theorists...

But it's a kind of physics

which is not yet testable.

It does not make predictions

that have anything to do

with experiments that

could be done in the laboratory

or with observations

that could be made

in space or from telescopes.

And I was brought up to believe,

and I still believe,

that physics is

an experimental science.

It deals with the results

to experiments

or, in the case

of astronomy, observations.

From the start,

many scientists thought

string theory was

simply too far out.

And, frankly, the strange way

the theory evolved

in a series of twists,

turns and accidents

only made it seem more unlikely.

In fact even its birth has been

turned into something of a myth,

which goes like this.

In the late 1960s,

a young Italian physicist

named Gabriele Veneziano

was searching

for a set of equations

that would explain

the strong nuclear force...

The extremely powerful glue

that holds the nucleus

of every atom together,

binding protons to neutrons.

As the story goes,

he happened on a dusty book

on the history of mathematics,

and in it he found

a 200-year-old equation,

first written down by a Swiss

mathematician, Leonhard Euler.

Veneziano was amazed to discover

that Euler's equations,

long thought to be nothing more

than a mathematical curiosity,

seemed to describe

the strong force.

He quickly published a paper

and was famous ever after

for this accidental discovery.

I see occasionally

written in books

that, uh... that this model

was invented by chance

or was, uh, found

in a math book,

and, uh, this makes me

feel pretty bad.

What is true

is that the function

was the outcome

of a long year of work,

and we accidentally

discovered string theory.

However it was discovered,

Euler's equation,

which miraculously explained

the strong force,

took on a life of its own.

This was the birth

of string theory.

Passed from colleague

to colleague,

Euler's equation ended up

on the chalkboard

in front of a young American

physicist, Leonard Susskind.

To this day

I remember the formula.

The formula was...

And I looked at it, and I said,

"You know, this is so simple

even I could figure out

what this is."

Susskind retreated

to his attic to investigate.

He understood that

this ancient formula described

the strong force mathematically.

But beneath

the abstract symbols,

he had caught a glimpse

of something new.

And I fiddled with it.

I monkeyed with it.

I sat in my attic, I think,

for two months, on and off,

but the first thing

I could see in it,

it was describing

some kind of particles

which had internal structure,

which could vibrate,

which could do things, which

wasn't just a point particle.

And I began to realize

what was being described here

was a string...

An elastic string,

like a rubber band or like

a rubber band cut in half.

And this rubber band could

not only stretch and contract

but wiggle.

And marvel of marvels,

it exactly agreed

with this formula.

I was pretty sure at that time

that I was the only one

in the world who knew this.

Susskind wrote up his discovery,

introducing the revolutionary

idea of strings.

But before his paper

could be published,

it had to be reviewed

by a panel of experts.

I was completely convinced

that when it came back,

it was going to say,

"Susskind is the next Einstein,

or maybe even the next Newton."

And it came back saying, "Nah,

this paper is not very good.

Probably shouldn't

be published."

I was truly

knocked off my chair.

I was depressed, I was unhappy.

I was saddened by it.

It made me a nervous wreck,

and, uh, the result was

I went home and got drunk.

As Susskind drowned his sorrows

over the rejection

of his far-out idea,

it appeared

string theory was dead.

Meanwhile, mainstream science

was embracing particles

as points, not strings.

For decades, physicists

had been exploring

the behavior

of microscopic particles

by smashing them together

at high speeds,

and studying those collisions.

In the showers

of particles produced,

they were discovering

that nature is far richer

than they thought.

Once a month there'd be

a discovery of a new particle...

The rho meson, the

omega particle, the B particle,

the B1 particle,

the B2 particle, phi, omega.

More letters were used

than exist in most alphabets.

It was a population explosion

of particles.

It was a time

when graduate students

would run through the halls

of a physics building

and say, "They discovered

another particle,

and it fit the theories,"

and it was all so exciting.

And in this zoo

of new particles,

scientists weren't

just discovering

building blocks of matter.

Leaving string theory

in the dust,

physicists made a startling

and strange prediction...

That the forces of nature can

also be explained by particles.

Now, this is

a really weird idea,

but it's kind of like

a game of catch

in which the players like me...

And me are particles of matter.

And the ball

we're throwing back and forth

is a particle of force.

It's called

a messenger particle.

For example,

in the case of magnetism...

The electromagnetic force...

This ball would be a photon.

The more of these

messenger particles, or photons,

that are exchanged between us,

the stronger

the magnetic attraction.

And scientists predicted

that it's this exchange

of messenger particles

that creates

what we feel as force.

Experiments confirmed

these predictions

with the discovery

of the messenger particles

for electromagnetism, the

strong force and the weak force.

And using these newly

discovered particles,

scientists were closing in

on Einstein's dream

of unifying the forces.

Particle physicists reasoned

that if we rewind

the cosmic film

to the moments

just after the Big Bang...

Some 14 billion years ago

when the universe was

trillions of degrees hotter...

The messenger particles

for electromagnetism

and the weak force

would have been

indistinguishable.

Just as cubes of ice melt

into water in the hot sun,

experiments show

that as we rewind

to the extremely hot conditions

of the Big Bang,

the weak and electromagnetic

forces meld together

and unite into a single force,

called the electroweak.

And physicists believe

that if you roll the cosmic film

back even further,

the electroweak would unite

with the strong force

in one grand superforce.

Although that has yet

to be proven,

quantum mechanics

was able to explain

how three of the forces operate

on the subatomic level.

And all of a sudden

we had a consistent theory

of elementary particle physics,

which allows us to describe

all of the interactions...

Weak, strong

and electromagnetic...

In the same language.

It all made sense, and, uh,

it's all in the textbooks.

Everything was converging

toward a simple picture

of the known particles

and forces...

A picture

which eventually became known

as the standard model.

I think I gave it that name.

Professors Sheldon Glashow,

Abdus Salam and Steven Weinberg.

The inventors

of the standard model...

Both the name and the theory...

Were the toasts

of the scientific community,

receiving Nobel Prize

after Nobel Prize.

But behind the fanfare

was a glaring omission.

Although the standard model

explained three of the forces

that rule the world

of the very small,

it did not include

the most familiar force.

Gravity.

Overshadowed

by the standard model,

string theory became

a backwater of physics.

Most people in our community

lost complete interest

in string theory.

They said, "Okay, that was

a very nice, elegant thing

but had nothing

to do with nature."

It's not taken seriously

by much of the community,

but the early pioneers

of string theory are convinced

that they can smell reality

and continue to pursue the idea.

But the more these diehards

delved into string theory,

the more problems they found.

Early string theory had

a number of problems.

One was that it predicted

a particle which we know

is unphysical.

It's what's called a tachyon...

A particle that travels

faster than light.

There was this discovery

that the theory requires

ten dimensions,

which is very disturbing,

of course,

since it's obvious that

that's more than there are.

It had this massless particle

which was not seen

in experiments.

So these theories

didn't seem to make sense.

This seemed crazy to people.

Basically, string theory

was not getting off the ground.

People threw up their hands

and said, "This can't be right."

By 1973, only a few young

physicists were still wrestling

with the obscure equations

of string theory.

One was John Schwarz,

who was busy tackling

string theory's

numerous problems,

among them, a mysterious

massless particle...

Predicted by the theory

but never seen in nature...

And an assortment of anomalies,

or mathematical inconsistencies.

We spent a long time trying

to fiddle with the theory.

We tried all sorts of ways

of making the dimension be four,

getting rid of

these massless particles

and tachyons and so on,

but it was always ugly

and unconvincing.

For four years,

Schwarz tried to tame

the unruly equations

of string theory,

changing, adjusting,

combining and recombining them

in different ways.

But nothing worked.

On the verge

of abandoning string theory,

Schwarz had a brainstorm:

Perhaps his equations

were describing gravity.

But that meant reconsidering

the size of these tiny strands

of energy.

We weren't thinking about

gravity up till that point,

but as soon as we suggested

that maybe we should be dealing

with a theory of gravity,

we had to radically change

our view

of how big these strings were.

By supposing that strings

were a hundred billion billion

times smaller than an atom,

one of the theory's vices

became a virtue.

The mysterious particle

John Schwarz had been

trying to get rid of

now appeared to be a graviton,

the long-sought-after particle

believed to transmit gravity

at the quantum level.

String theory had produced

the piece of the puzzle

missing from the standard model.

Schwarz submitted

for publication

his groundbreaking new theory

describing how gravity works

in the subatomic world.

It seemed very obvious to us

that it was right,

but there was really no reaction

in the community whatsoever.

Once again, string theory

fell on deaf ears.

But Schwarz

would not be deterred.

He had glimpsed the holy grail.

If strings described gravity

at the quantum level,

they must be the key

to unifying the four forces.

He was joined in this quest by

one of the only other scientists

willing to risk his career

on strings, Michael Green.

In a sense, I think,

we had a quiet confidence

that the string theory

was obviously correct,

and it didn't matter much

if people didn't see it

at that point.

They would see it down the line.

But for Green's confidence

to pay off,

he and Schwarz

would have to confront

the fact that

in the early 1980s,

string theory still had

fatal flaws in the math

known as "anomalies."

An anomaly is just

what it sounds like:

It's something that's strange

or out of place,

something that doesn't belong.

Now this kind of anomaly

is just weird.

But mathematical anomalies

can spell doom

for a theory of physics.

They're a little complicated,

so here's a simple example:

Let's say we have a theory

in which these two equations...

describe one physical property

of our universe.

Now, if I solve this equation

over here and I find x = 1,

and I solve this equation

over here and find x = 2,

I know my theory has anomalies

because there should

only be one value for x.

Unless I can revise my equations

to get the same value for x

on both sides,

the theory is dead.

In the early 1980s,

string theory was riddled

with mathematical anomalies

kind of like these,

although the equations

were much more complex.

The future of the

theory depended

on ridding the equations of

these fatal inconsistencies.

After Schwarz and Green battled

the anomalies in string theory

for five years, their work

culminated late one night

in the summer of 1984.

It was widely believed

that these theories

must be inconsistent

because of anomalies.

Well, for no really good reason,

I just felt

that had to be wrong,

because I... I felt string

theory has got to be right,

therefore

there can't be anomalies.

So we decided, "We got

to calculate these things."

Amazingly, it all boiled down

to a single calculation.

On one side of the blackboard,

they got 496.

And if they got the matching

number on the other side,

it would prove string theory

was free of anomalies.

I do remember, um...

a particular moment

when John Schwarz and I

were talking at the blackboard

and working out these numbers

which had to fit,

and they just had

to match exactly.

I remember joking with

John Schwarz at that moment,

because there was

thunder and lightning.

There was a big mountain storm

in Aspen at that moment,

and I remember saying

something like, you know,

"We must be getting pretty close

"because the gods

are trying to prevent us

completing this calculation."

And indeed they did match.

The matching numbers meant the

theory was free of anomalies

and it had

the mathematical depth

to encompass all four forces.

So we recognized

not only that the strings

could describe gravity

but they could also describe

the other forces,

so we spoke in terms

of unification.

And we saw this

as a possibility of realizing

the dream that Einstein had

expressed in his later years

of unifying the different forces

in some deeper framework.

We felt great.

That was

an extraordinary moment,

because we realized

that no other theory

had ever succeeded

in doing that.

But by now,

it's like crying wolf,

each time we had done something

I figured everyone's going to be

excited, and they weren't.

So I figured... well, by now

I didn't expect

much of a reaction.

But this time,

the reaction was explosive.

In less than a year, the number

of string theorists leapt

from just a handful to hundreds.

Up to that moment,

the longest talk I'd ever given

on the subject was five minutes

at some minor conference,

and then suddenly

I was invited all over the world

to give talks

and lectures and so forth.

String theory was christened

the "theory of everything."

In early fall of 1984, I came

here, to Oxford University,

to begin my graduate studies

in physics.

Some weeks after, I saw a poster

for a lecture by Michael Green.

I didn't know who he was,

but then again, I really

didn't know who anybody was.

But the title of the lecture

was something like

"The Theory of Everything."

So how could I resist?

This elegant new version

of string theory seemed capable

of describing all

the building blocks of nature.

Here's how.

Inside every grain of sand...

are billions of tiny atoms.

Every atom is made

of smaller bits of matter...

Electrons orbiting a nucleus

made of protons and neutrons...

Which are made of even smaller

bits of matter, called quarks.

But string theory says this

is not the end of the line.

It makes the astounding claim

that the particles making up

everything in the universe

are made

of even smaller ingredients...

Tiny, wiggling strands of energy

that look like strings.

Each of these strings

is unimaginably small.

In fact,

if an atom were enlarged

to the size

of the solar system...

a string would only be

as large as a tree.

And here's the key idea:

Just as different vibrational

patterns, or frequencies,

of a single cello string

create what we hear

as different musical notes,

the different ways

that strings vibrate

give particles

their unique properties,

such as mass and charge.

For example, the only difference

between the particles

making up you and me

and the particles that transmit

gravity and the other forces

is the way

these tiny strings vibrate.

Composed of an enormous number

of these oscillating strings,

the universe can be thought of

as a grand cosmic symphony.

And this elegant idea

resolves the conflict

between our jittery,

unpredictable picture of space

on the subatomic scale

and our smooth picture of space

on the large scale.

It's the jitteriness

of quantum mechanics

versus the gentleness

of Einstein's general

theory of relativity

that makes it so hard

to bridge the two,

to stitch them together.

Now, what string theory does,

it comes along

and basically calms the jitters

of quantum mechanics.

It spreads them out

by virtue of taking

the old idea of a point particle

and spreading it out

into a string.

So the jittery behavior is there

but it's just

sufficiently less violent

that quantum theory

and general relativity

stitch together perfectly

within this framework.

It's a triumph of mathematics.

With nothing but these tiny

vibrating strands of energy,

string theorists claim to be

fulfilling Einstein's dream

of uniting all forces

and all matter.

But this radical new theory

contains a chink in its armor.

No experiment can ever check up

what's going on

at the distances

that are being studied.

No observation can relate

to these tiny distances

or high energies.

That is to say, there ain't no

experiment that could be done,

nor is there any observation

that could be made

that would say

"You guys are wrong."

The theory is safe,

permanently safe.

Is that a theory of physics

or a philosophy?

I ask you.

People often criticize

string theory

for saying that

it's very far removed

from any direct

experimental test,

and it's... surely

it's not really, uh...

a branch of physics

for that reason.

And I... my response

to that is simply

that they're going

to be proved wrong.

Making string theory

even harder to prove

is that in order to work,

the complex equations require

something that sounds

like it's straight out

of science fiction:

extra dimensions of space.

We've always thought,

for centuries, that there

was only what we can see,

you know, this dimension,

that one and another one.

There was only three dimensions

of space and one of time.

And people who've said

that there were

extra dimensions of space

have been labeled

as, you know, crackpots

or people who were bananas.

Well, string theory

really predicts it.

To be taken seriously,

string theorists had to explain

how this bizarre prediction

could be true,

and they claim that the far-out

idea of extra dimensions

may be more down to earth

than you'd think.

Let me show you what I mean.

I'm off to see a guy who was

one of the first people

to think

about this strange idea.

I'm supposed to meet him

at 4:00 at his apartment

on Fifth Avenue and 93rd Street

on the second floor.

Now, in order to get

to this meeting,

I need four pieces

of information,

one for each of the three

dimensions of space...

A street, an avenue

and a floor number...

And one more for time,

the fourth dimension.

You can think about these

as the four dimensions

of common experience...

Left-right, back-forth,

up-down and time.

As it turns out,

the strange idea that there

are additional dimensions

stretches back almost a century.

Our sense that we live

in a universe

of three spatial dimensions

really seems beyond question.

But in 1919, Theodor Kaluza,

a virtually unknown

German mathematician,

had the courage

to challenge the obvious.

He suggested

that maybe, just maybe,

our universe has

one more dimension

that for some reason

we just can't see.

No, he says here,

"I like your idea."

So, why does he delay?

You see, Kaluza

had sent his idea

about an additional spatial

dimension to Albert Einstein.

And although Einstein was

initially enthusiastic,

he then seemed to waver

and for two years held up

publication of Kaluza's paper.

Eventually, Kaluza's paper

was published,

after Einstein decided extra

dimensions were his cup of tea.

Here's the idea.

In 1916, Einstein showed

that gravity is nothing

but warps and ripples

in the four familiar dimensions

of space and time.

Just three years later,

Kaluza proposed

that electromagnetism

might also be ripples.

But for that to be true,

Kaluza needed a place

for those ripples to occur.

So Kaluza proposed an additional

hidden dimension of space.

But if Kaluza was right,

where is this extra dimension?

And what would

extra dimensions look like?

Can we even begin

to imagine them?

Well, building

upon Kaluza's work,

the Swedish physicist

Oskar Klein suggested

an unusual answer.

Take a look at the cables

supporting that traffic light.

From this far away, I can't see

that they have any thickness.

Each one looks like a line,

something with only

a single dimension.

But suppose we could explore one

of these cables way up close,

like from the point of view

of an ant.

Now, a second dimension,

which wraps around the cable,

becomes visible.

From its point of view, the ant

can move forwards and backwards,

and it can also move clockwise

and counterclockwise.

So, dimensions can come

in two varieties.

They can be long and unfurled,

like the length of the cable,

but they can also be

tiny and curled up,

like the circular direction

that wraps around it.

Kaluza and Klein made

the wild suggestion

that the fabric of our universe

might be

kind of like

the surface of the cable,

having both big,

extended dimensions...

The three that we know about...

But also tiny, curled-up

dimensions, curled up so tiny,

billions of times smaller

than even a single atom,

that we just can't see them.

And so our perception

that we live in a universe

with three spatial dimensions

may not be correct after all.

We really may live in a universe

with more dimensions

than meet the eye.

So, what would these

extra dimensions look like?

Kaluza and Klein proposed

that if we could shrink down

billions of times,

we'd find one extra

tiny, curled-up dimension

located at every point in space.

And just the way an ant can

explore the circular dimension

that wraps around

a traffic-light cable,

in theory, an ant that is

billions of times smaller

could also explore this tiny,

curled-up circular dimension.

This idea that extra dimensions

exist all around us

lies at the heart

of string theory.

In fact, the mathematics

of string theory demand

not one but six

extra dimensions,

twisted and curled

into complex little shapes

that might look

something like this.

If string theory is right,

we would have to admit

that there are really

more dimensions out there,

and I find that

completely mind-blowing.

If I take the theory

as we have it now literally,

I would conclude that the

extra dimensions really exist.

They're part of nature.

When we talk

about extra dimensions,

we literally mean

extra dimensions of space

that are the same

as the dimensions of space

that we see around us.

And the only difference between

them has to do with their shape.

But how could these

tiny extra dimensions,

curled up into

such peculiar shapes,

have any effect

on our everyday world?

Well, according to string

theory, shape is everything.

Because of its shape,

a French horn can produce

dozens of different notes.

When you press one of the keys,

you change the note

because you change the shape

of the space inside the horn

where the air resonates.

And we think the curled-up

spatial dimensions

in string theory work

in a similar way.

If we could shrink down

small enough

to fly into one of these

tiny six-dimensional shapes

predicted by string theory,

we would see how

the extra dimensions

are twisted and curled back

on each other,

influencing how strings,

the fundamental ingredients

of our universe,

move and vibrate.

And this could be the key

to solving one of nature's

most profound mysteries.

You see, our universe is kind

of like a finely tuned machine.

Scientists have found that

there are about 20 numbers...

20 fundamental constants

of nature...

That give the universe the

characteristics we see today.

These are numbers like

how much an electron weighs,

the strength of gravity,

the electromagnetic force,

and the strong and weak forces.

Now, as long as we set the dials

on our universe machine

to precisely the right values

for each of these 20 numbers,

the machine produces

the universe we know and love.

But if we change the numbers

by adjusting the settings

on this machine

even a little bit,

the consequences are dramatic.

For example, if I increase

the strength of

the electromagnetic force,

atoms repel one another

more strongly,

so the nuclear furnaces that

make stars shine break down.

The stars, including our sun,

fizzle out...

and the universe as we know it

disappears.

So, what exactly in nature

sets the values of these

20 constants so precisely?

Well, the answer could be

the extra dimensions

in string theory.

That is, the tiny, curled-up

six-dimensional shapes

predicted by the theory

cause one string to vibrate

in precisely the right way

to produce what we see

as a photon

and another string to vibrate

in a different way,

producing an electron.

So according to string theory,

these minuscule

extra-dimensional shapes

really may determine

all the constants of nature,

keeping the cosmic symphony

of strings in tune.

By the mid-1980s, string theory

looked unstoppable.

But behind the scenes,

the theory was in tangles.

Over the years, string theorists

had been so successful

that they had constructed

not one but five different

versions of the theory.

Each was built on strings

and extra dimensions,

but in detail, the five theories

were not in harmony.

In some versions, strings

were open-ended strands;

in others,

they were closed loops.

At first glance,

a couple of versions even

required 26 dimensions.

All five versions

appeared equally valid,

but which one was describing

our universe?

This was kind

of an embarrassment

for string theorists,

because on the one hand,

we wanted to say

that this might be it...

The final description

of the universe.

But then in the next breath

we had to say,

"And it comes in five flavors,

five variations."

Now, there's one universe,

you expect there to be

one theory and not five.

So this is an example

where more is definitely less.

One attitude that people who

didn't like string theory

could take was

"Well, you have five theories,

so it's not unique."

This was a peculiar

state of affairs,

because we were looking just

to describe one theory of nature

and not five.

If there's five of them,

well, maybe there's...

smart enough people

would find 20 of them,

or maybe there's

an infinite number of them,

and you're back to just

searching around at random

for... for theories

of the world.

Maybe one of these

five string theories

is describing our universe.

On the other hand, which one?

And why... what are

the other ones good for?

But having five string theories,

even though it's big progress,

raises the obvious question:

If one of those theories

describes our universe,

then who lives

in the other four worlds?

String theory seemed

to be losing steam once again,

and frustrated

by a lack of progress,

many physicists

abandoned the field.

Will string theory prove

to be a theory of everything,

or will it unravel

into a theory of nothing?

On NOVA's Web site, go behind

the scenes with Brian Greene,

journey into the subatomic

world, play with strings,

picture other dimensions,

and much more.

To order this program

on VHS or DVD

or the book

The Elegant Universe,

please call WGBH

Boston Video at

1-800-255-9424.

Next time on NOVA:

Can string theory be saved?

Do we live in a world

of extra dimensions

with parallel universes

just beyond our reach?

The thrill ride continues

as Brian Greene pushes

physics to the limit

on the next episode of

"The Elegant Universe."

NOVA is a production

of WGBH Boston.

Corporate funding for NOVA

is provided by Sprint

and Microsoft.

Additional funding

is provided by:

Dedicated to education

and quality television.

Funding for "The Elegant

Universe" is provided by:

To enhance public understanding

of science and technology.

And by the National

Science Foundation,

where discoveries begin.

Additional funding is provided

by Volkswagen

and by:

and:

Major funding for NOVA

is also provided

by the Corporation

for Public Broadcasting

and by contributions

to your PBS stations from:

We are PBS.

---

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.

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've said that there

are 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...

We accidentally discovered

string theory.

The humble beginnings

of a revolutionary idea.

I was completely convinced

it was going to say,

"Susskind is the next Einstein."

This seemed crazy to people.

I was depressed, I was unhappy.

The result was

I went home and got drunk.

Obsession drives scientists

to pursue the holy grail

of physics,

but are they ready

for what they discover?

Step into the bizarre world

of "The Elegant Universe,"

right now.

Captioning sponsored by SPRINT,

MICROSOFT, the PARK FOUNDATION,

ALFRED P. SLOAN FOUNDATION,

NATIONAL SCIENCE FOUNDATION,

VOLKSWAGEN,

THE GEORGE D. SMITH FUND,

THE U.S. DEPARTMENT OF ENERGY,

THE CORPORATION

FOR PUBLIC BROADCASTING

and VIEWERS LIKE YOU

Corporate funding for NOVA

is provided by Sprint

and Microsoft.

Additional funding

is provided by:

Dedicated to education

and quality television.

Funding for "The Elegant

Universe" is provided by:

To enhance public understanding

of science and technology.

And by the National

Science Foundation,

where discoveries begin.

Additional funding is provided

by Volkswagen

and by:

and:

Major funding for NOVA

is also provided

by the Corporation

for Public Broadcasting

and by contributions

to your PBS stations from:

It's a little-known secret,

but for more than half a century

a dark cloud has been looming

over modern science.

Here's the problem.

Our understanding

of the universe is based

on two separate theories.

One is Einstein's

general theory of relativity.

That's a way of understanding

the biggest things

in the universe...

Things like stars and galaxies.

But the littlest things

in the universe...

Atoms and subatomic particles...

Play by an entirely different

set of rules

called quantum mechanics.

These two sets of rules are each

incredibly accurate

in their own domain,

but whenever

we try to combine them

to solve some of the deepest

mysteries in the universe,

disaster strikes.

Take the beginning

of the universe, the Big Bang.

At that instant, a tiny nugget

erupted violently.

Over the next 14 billion years,

the universe expanded and cooled

into the stars, galaxies

and planets we see today.

But if we run the cosmic film

in reverse,

everything that's now rushing

apart comes back together.

So the universe gets

smaller, hotter and denser

as we head back

to the beginning of time.

As we reach the Big Bang...

When the universe was

both enormously heavy

and incredibly tiny...

Our projector jams.

Our two laws of physics,

when combined, break down.

But what if we could unite

quantum mechanics

and general relativity and see

the cosmic film in its entirety?

Well, a new set of ideas

called string theory

may be able to do that,

and if it's right,

it would be one of

the biggest blockbusters

in the history of science.

Someday, string theory may be

able to explain all of nature...

From the tiniest bits of matter

to the farthest reaches

of the cosmos...

Using just one

single ingredient:

tiny, vibrating strands

of energy, called "strings."

But why do we have to rewrite

the laws of physics

to accomplish this?

Why does it matter

if the two laws that we have

are incompatible?

Well, you can think of it

like this.

Imagine you lived

in a city ruled not

by one set of traffic laws,

but by two separate sets of laws

that conflicted with each other.

As you can see, it would be

pretty confusing.

What the heck are you doing?!

To understand this place,

you'd need to find a way

to put these two conflicting

sets of laws together

into one all-encompassing set

that makes sense.

We work on the assumption

that there is a theory out there

and it's our job...

If we're sufficiently smart

and sufficiently industrious...

To figure out what it is.

We don't have a guarantee,

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.

But before we can find

that theory,

we need to take

a fantastic journey

to see why

the two sets of laws we have

conflict with each other.

And the first stop

on this strange trip is

the realm of very large objects.

To describe the universe

on large scales,

we use one set of laws...

Einstein's general theory

of relativity,

and that's a theory

of how gravity works.

General relativity

pictures space

as sort of like a trampoline,

a smooth fabric that heavy

objects like stars and planets

can warp and stretch.

Now, according to the theory,

these warps and curves create

what we feel as gravity.

That is, the gravitational pull

that keeps the earth

in orbit around the sun

is really nothing more

than our planet following

the curves and contours

that the sun creates

in the spatial fabric.

But the smooth,

gently curving image of space

predicted by the

laws of general relativity

is not the whole story.

To understand the universe

on extremely small scales,

we have to use our other

set of laws... quantum mechanics.

And as we'll see,

quantum mechanics paints

a picture of space

so drastically different

from general relativity

that you'd think

they were describing

two completely

separate universes.

To see the conflict

between general relativity

and quantum mechanics,

we need to shrink

way, way, way down in size.

And as we leave the world

of large objects behind...

and approach

the microscopic realm,

the familiar picture of space

in which everything

behaves predictably

begins to be replaced

by a world with a structure

that is far less certain.

And if we keep shrinking,

getting billions and billions

of times smaller

than even the tiniest

bits of matter...

Atoms and the tiny particles

inside of them...

The laws of the very small,

quantum mechanics, say

that the fabric of space

becomes bumpy and chaotic.

Eventually, we reach

a world so turbulent

that it defies common sense.

Down here, space and time are

so twisted and distorted

that the conventional ideas

of left and right...

up and down...

even before and after,

break down.

There's no way to tell

for certain that I'm here...

or here...

or both places at once.

Or maybe I arrived here...

before I arrived here.

In the quantum world, you just

can't pin everything down.

It's an inherently

wild and frenetic place.

The laws in the quantum world

are very different

from the laws

that we are used to.

And is that surprising?

Why should the world of the very

small, at an atomic level...

Why should that world obey

the same kind of rules and laws

that we are used to in our world

with apples and oranges and

walking around on the street?

Why would that world

behave the same way?

The fluctuating, jittery picture

of space and time

predicted by quantum mechanics

is in direct conflict

with the smooth, orderly,

geometric model

of space and time

described by general relativity.

But we think that everything...

From the frantic dance

of subatomic particles

to the majestic swirl

of galaxies...

Should be explained by just

one grand physical principle,

one master equation.

If we can find that equation,

how the universe really works

at every time and place

will at last be revealed.

You see, what we need

is a theory

that can cope with the very tiny

and the very massive...

One that embraces both quantum

mechanics and general relativity

and never breaks down, ever.

For a physicist,

finding a theory

that unites general relativity

and quantum mechanics

is the holy grail,

because that framework

would give us a single

mathematical theory

that describes all the forces

that rule our universe.

General relativity describes the

most familiar of those forces...

Gravity.

But quantum mechanics

describes three other forces.

The strong nuclear force.

That's responsible

for gluing protons and neutrons

together inside of atoms.

Electromagnetism...

which produces

light, electricity

and magnetic attraction.

And the weak nuclear force.

That's the force responsible

for radioactive decay.

Every event in the universe,

from the splitting of an atom

to the birth of a star,

is nothing more than these four

forces interacting with matter.

Albert Einstein spent

the last 30 years of his life

searching for a way

to describe the forces of nature

in a single theory.

And now string theory

may fulfill

his dream of unification.

For centuries,

scientists have pictured

the fundamental ingredients

of nature...

Atoms and the smaller particles

inside of them...

As tiny balls or points.

But string theory proclaims

that at the heart

of every bit of matter

is a tiny, vibrating strand

of energy, called a string.

And a new breed

of scientist believes

these minuscule strings

are the key

to uniting the world of the

large and the world of the small

in a single theory.

The idea

that a scientific theory

that we already have

in our hands

could answer

the most basic questions

is extremely seductive.

For about 2,000 years,

all of our physics, essentially,

has been based on...

Essentially we were talking

about billiard balls.

The very idea of the string

is such a paradigm shift,

because instead

of billiard balls,

you have to use

little strands of spaghetti.

But not everyone is enamored

of this new theory.

So far no experiment

has been devised

that can prove

these tiny strings exist.

Let me put it bluntly:

There are physicists

and there are string theorists.

It is a new discipline,

a new, you may call it, tumor.

You can call it what you will.

They have focused

on questions which

experiment cannot address.

They will deny that...

These string theorists...

But it's a kind of physics

which is not yet testable.

It does not make predictions

that have anything to do

with experiments that

could be done in the laboratory

or with observations

that could be made

in space or from telescopes.

And I was brought up to believe,

and I still believe,

that physics is

an experimental science.

It deals with the results

to experiments

or, in the case

of astronomy, observations.

From the start,

many scientists thought

string theory was

simply too far out.

And, frankly, the strange way

the theory evolved

in a series of twists,

turns and accidents

only made it seem more unlikely.

In fact even its birth has been

turned into something of a myth,

which goes like this.

In the late 1960s,

a young Italian physicist

named Gabriele Veneziano

was searching

for a set of equations

that would explain

the strong nuclear force...

The extremely powerful glue

that holds the nucleus

of every atom together,

binding protons to neutrons.

As the story goes,

he happened on a dusty book

on the history of mathematics,

and in it he found

a 200-year-old equation,

first written down by a Swiss

mathematician, Leonhard Euler.

Veneziano was amazed to discover

that Euler's equations,

long thought to be nothing more

than a mathematical curiosity,

seemed to describe

the strong force.

He quickly published a paper

and was famous ever after

for this accidental discovery.

I see occasionally

written in books

that, uh... that this model

was invented by chance

or was, uh, found

in a math book,

and, uh, this makes me

feel pretty bad.

What is true

is that the function

was the outcome

of a long year of work,

and we accidentally

discovered string theory.

However it was discovered,

Euler's equation,

which miraculously explained

the strong force,

took on a life of its own.

This was the birth

of string theory.

Passed from colleague

to colleague,

Euler's equation ended up

on the chalkboard

in front of a young American

physicist, Leonard Susskind.

To this day

I remember the formula.

The formula was...

And I looked at it, and I said,

"You know, this is so simple

even I could figure out

what this is."

Susskind retreated

to his attic to investigate.

He understood that

this ancient formula described

the strong force mathematically.

But beneath

the abstract symbols,

he had caught a glimpse

of something new.

And I fiddled with it.

I monkeyed with it.

I sat in my attic, I think,

for two months, on and off,

but the first thing

I could see in it,

it was describing

some kind of particles

which had internal structure,

which could vibrate,

which could do things, which

wasn't just a point particle.

And I began to realize

what was being described here

was a string...

An elastic string,

like a rubber band or like

a rubber band cut in half.

And this rubber band could

not only stretch and contract

but wiggle.

And marvel of marvels,

it exactly agreed

with this formula.

I was pretty sure at that time

that I was the only one

in the world who knew this.

Susskind wrote up his discovery,

introducing the revolutionary

idea of strings.

But before his paper

could be published,

it had to be reviewed

by a panel of experts.

I was completely convinced

that when it came back,

it was going to say,

"Susskind is the next Einstein,

or maybe even the next Newton."

And it came back saying, "Nah,

this paper is not very good.

Probably shouldn't

be published."

I was truly

knocked off my chair.

I was depressed, I was unhappy.

I was saddened by it.

It made me a nervous wreck,

and, uh, the result was

I went home and got drunk.

As Susskind drowned his sorrows

over the rejection

of his far-out idea,

it appeared

string theory was dead.

Meanwhile, mainstream science

was embracing particles

as points, not strings.

For decades, physicists

had been exploring

the behavior

of microscopic particles

by smashing them together

at high speeds,

and studying those collisions.

In the showers

of particles produced,

they were discovering

that nature is far richer

than they thought.

Once a month there'd be

a discovery of a new particle...

The rho meson, the

omega particle, the B particle,

the B1 particle,

the B2 particle, phi, omega.

More letters were used

than exist in most alphabets.

It was a population explosion

of particles.

It was a time

when graduate students

would run through the halls

of a physics building

and say, "They discovered

another particle,

and it fit the theories,"

and it was all so exciting.

And in this zoo

of new particles,

scientists weren't

just discovering

building blocks of matter.

Leaving string theory

in the dust,

physicists made a startling

and strange prediction...

That the forces of nature can

also be explained by particles.

Now, this is

a really weird idea,

but it's kind of like

a game of catch

in which the players like me...

And me are particles of matter.

And the ball

we're throwing back and forth

is a particle of force.

It's called

a messenger particle.

For example,

in the case of magnetism...

The electromagnetic force...

This ball would be a photon.

The more of these

messenger particles, or photons,

that are exchanged between us,

the stronger

the magnetic attraction.

And scientists predicted

that it's this exchange

of messenger particles

that creates

what we feel as force.

Experiments confirmed

these predictions

with the discovery

of the messenger particles

for electromagnetism, the

strong force and the weak force.

And using these newly

discovered particles,

scientists were closing in

on Einstein's dream

of unifying the forces.

Particle physicists reasoned

that if we rewind

the cosmic film

to the moments

just after the Big Bang...

Some 14 billion years ago

when the universe was

trillions of degrees hotter...

The messenger particles

for electromagnetism

and the weak force

would have been

indistinguishable.

Just as cubes of ice melt

into water in the hot sun,

experiments show

that as we rewind

to the extremely hot conditions

of the Big Bang,

the weak and electromagnetic

forces meld together

and unite into a single force,

called the electroweak.

And physicists believe

that if you roll the cosmic film

back even further,

the electroweak would unite

with the strong force

in one grand superforce.

Although that has yet

to be proven,

quantum mechanics

was able to explain

how three of the forces operate

on the subatomic level.

And all of a sudden

we had a consistent theory

of elementary particle physics,

which allows us to describe

all of the interactions...

Weak, strong

and electromagnetic...

In the same language.

It all made sense, and, uh,

it's all in the textbooks.

Everything was converging

toward a simple picture

of the known particles

and forces...

A picture

which eventually became known

as the standard model.

I think I gave it that name.

Professors Sheldon Glashow,

Abdus Salam and Steven Weinberg.

The inventors

of the standard model...

Both the name and the theory...

Were the toasts

of the scientific community,

receiving Nobel Prize

after Nobel Prize.

But behind the fanfare

was a glaring omission.

Although the standard model

explained three of the forces

that rule the world

of the very small,

it did not include

the most familiar force.

Gravity.

Overshadowed

by the standard model,

string theory became

a backwater of physics.

Most people in our community

lost complete interest

in string theory.

They said, "Okay, that was

a very nice, elegant thing

but had nothing

to do with nature."

It's not taken seriously

by much of the community,

but the early pioneers

of string theory are convinced

that they can smell reality

and continue to pursue the idea.

But the more these diehards

delved into string theory,

the more problems they found.

Early string theory had

a number of problems.

One was that it predicted

a particle which we know

is unphysical.

It's what's called a tachyon...

A particle that travels

faster than light.

There was this discovery

that the theory requires

ten dimensions,

which is very disturbing,

of course,

since it's obvious that

that's more than there are.

It had this massless particle

which was not seen

in experiments.

So these theories

didn't seem to make sense.

This seemed crazy to people.

Basically, string theory

was not getting off the ground.

People threw up their hands

and said, "This can't be right."

By 1973, only a few young

physicists were still wrestling

with the obscure equations

of string theory.

One was John Schwarz,

who was busy tackling

string theory's

numerous problems,

among them, a mysterious

massless particle...

Predicted by the theory

but never seen in nature...

And an assortment of anomalies,

or mathematical inconsistencies.

We spent a long time trying

to fiddle with the theory.

We tried all sorts of ways

of making the dimension be four,

getting rid of

these massless particles

and tachyons and so on,

but it was always ugly

and unconvincing.

For four years,

Schwarz tried to tame

the unruly equations

of string theory,

changing, adjusting,

combining and recombining them

in different ways.

But nothing worked.

On the verge

of abandoning string theory,

Schwarz had a brainstorm:

Perhaps his equations

were describing gravity.

But that meant reconsidering

the size of these tiny strands

of energy.

We weren't thinking about

gravity up till that point,

but as soon as we suggested

that maybe we should be dealing

with a theory of gravity,

we had to radically change

our view

of how big these strings were.

By supposing that strings

were a hundred billion billion

times smaller than an atom,

one of the theory's vices

became a virtue.

The mysterious particle

John Schwarz had been

trying to get rid of

now appeared to be a graviton,

the long-sought-after particle

believed to transmit gravity

at the quantum level.

String theory had produced

the piece of the puzzle

missing from the standard model.

Schwarz submitted

for publication

his groundbreaking new theory

describing how gravity works

in the subatomic world.

It seemed very obvious to us

that it was right,

but there was really no reaction

in the community whatsoever.

Once again, string theory

fell on deaf ears.

But Schwarz

would not be deterred.

He had glimpsed the holy grail.

If strings described gravity

at the quantum level,

they must be the key

to unifying the four forces.

He was joined in this quest by

one of the only other scientists

willing to risk his career

on strings, Michael Green.

In a sense, I think,

we had a quiet confidence

that the string theory

was obviously correct,

and it didn't matter much

if people didn't see it

at that point.

They would see it down the line.

But for Green's confidence

to pay off,

he and Schwarz

would have to confront

the fact that

in the early 1980s,

string theory still had

fatal flaws in the math

known as "anomalies."

An anomaly is just

what it sounds like:

It's something that's strange

or out of place,

something that doesn't belong.

Now this kind of anomaly

is just weird.

But mathematical anomalies

can spell doom

for a theory of physics.

They're a little complicated,

so here's a simple example:

Let's say we have a theory

in which these two equations...

describe one physical property

of our universe.

Now, if I solve this equation

over here and I find x = 1,

and I solve this equation

over here and find x = 2,

I know my theory has anomalies

because there should

only be one value for x.

Unless I can revise my equations

to get the same value for x

on both sides,

the theory is dead.

In the early 1980s,

string theory was riddled

with mathematical anomalies

kind of like these,

although the equations

were much more complex.

The future of the

theory depended

on ridding the equations of

these fatal inconsistencies.

After Schwarz and Green battled

the anomalies in string theory

for five years, their work

culminated late one night

in the summer of 1984.

It was widely believed

that these theories

must be inconsistent

because of anomalies.

Well, for no really good reason,

I just felt

that had to be wrong,

because I... I felt string

theory has got to be right,

therefore

there can't be anomalies.

So we decided, "We got

to calculate these things."

Amazingly, it all boiled down

to a single calculation.

On one side of the blackboard,

they got 496.

And if they got the matching

number on the other side,

it would prove string theory

was free of anomalies.

I do remember, um...

a particular moment

when John Schwarz and I

were talking at the blackboard

and working out these numbers

which had to fit,

and they just had

to match exactly.

I remember joking with

John Schwarz at that moment,

because there was

thunder and lightning.

There was a big mountain storm

in Aspen at that moment,

and I remember saying

something like, you know,

"We must be getting pretty close

"because the gods

are trying to prevent us

completing this calculation."

And indeed they did match.

The matching numbers meant the

theory was free of anomalies

and it had

the mathematical depth

to encompass all four forces.

So we recognized

not only that the strings

could describe gravity

but they could also describe

the other forces,

so we spoke in terms

of unification.

And we saw this

as a possibility of realizing

the dream that Einstein had

expressed in his later years

of unifying the different forces

in some deeper framework.

We felt great.

That was

an extraordinary moment,

because we realized

that no other theory

had ever succeeded

in doing that.

But by now,

it's like crying wolf,

each time we had done something

I figured everyone's going to be

excited, and they weren't.

So I figured... well, by now

I didn't expect

much of a reaction.

But this time,

the reaction was explosive.

In less than a year, the number

of string theorists leapt

from just a handful to hundreds.

Up to that moment,

the longest talk I'd ever given

on the subject was five minutes

at some minor conference,

and then suddenly

I was invited all over the world

to give talks

and lectures and so forth.

String theory was christened

the "theory of everything."

In early fall of 1984, I came

here, to Oxford University,

to begin my graduate studies

in physics.

Some weeks after, I saw a poster

for a lecture by Michael Green.

I didn't know who he was,

but then again, I really

didn't know who anybody was.

But the title of the lecture

was something like

"The Theory of Everything."

So how could I resist?

This elegant new version

of string theory seemed capable

of describing all

the building blocks of nature.

Here's how.

Inside every grain of sand...

are billions of tiny atoms.

Every atom is made

of smaller bits of matter...

Electrons orbiting a nucleus

made of protons and neutrons...

Which are made of even smaller

bits of matter, called quarks.

But string theory says this

is not the end of the line.

It makes the astounding claim

that the particles making up

everything in the universe

are made

of even smaller ingredients...

Tiny, wiggling strands of energy

that look like strings.

Each of these strings

is unimaginably small.

In fact,

if an atom were enlarged

to the size

of the solar system...

a string would only be

as large as a tree.

And here's the key idea:

Just as different vibrational

patterns, or frequencies,

of a single cello string

create what we hear

as different musical notes,

the different ways

that strings vibrate

give particles

their unique properties,

such as mass and charge.

For example, the only difference

between the particles

making up you and me

and the particles that transmit

gravity and the other forces

is the way

these tiny strings vibrate.

Composed of an enormous number

of these oscillating strings,

the universe can be thought of

as a grand cosmic symphony.

And this elegant idea

resolves the conflict

between our jittery,

unpredictable picture of space

on the subatomic scale

and our smooth picture of space

on the large scale.

It's the jitteriness

of quantum mechanics

versus the gentleness

of Einstein's general

theory of relativity

that makes it so hard

to bridge the two,

to stitch them together.

Now, what string theory does,

it comes along

and basically calms the jitters

of quantum mechanics.

It spreads them out

by virtue of taking

the old idea of a point particle

and spreading it out

into a string.

So the jittery behavior is there

but it's just

sufficiently less violent

that quantum theory

and general relativity

stitch together perfectly

within this framework.

It's a triumph of mathematics.

With nothing but these tiny

vibrating strands of energy,

string theorists claim to be

fulfilling Einstein's dream

of uniting all forces

and all matter.

But this radical new theory

contains a chink in its armor.

No experiment can ever check up

what's going on

at the distances

that are being studied.

No observation can relate

to these tiny distances

or high energies.

That is to say, there ain't no

experiment that could be done,

nor is there any observation

that could be made

that would say

"You guys are wrong."

The theory is safe,

permanently safe.

Is that a theory of physics

or a philosophy?

I ask you.

People often criticize

string theory

for saying that

it's very far removed

from any direct

experimental test,

and it's... surely

it's not really, uh...

a branch of physics

for that reason.

And I... my response

to that is simply

that they're going

to be proved wrong.

Making string theory

even harder to prove

is that in order to work,

the complex equations require

something that sounds

like it's straight out

of science fiction:

extra dimensions of space.

We've always thought,

for centuries, that there

was only what we can see,

you know, this dimension,

that one and another one.

There was only three dimensions

of space and one of time.

And people who've said

that there were

extra dimensions of space

have been labeled

as, you know, crackpots

or people who were bananas.

Well, string theory

really predicts it.

To be taken seriously,

string theorists had to explain

how this bizarre prediction

could be true,

and they claim that the far-out

idea of extra dimensions

may be more down to earth

than you'd think.

Let me show you what I mean.

I'm off to see a guy who was

one of the first people

to think

about this strange idea.

I'm supposed to meet him

at 4:00 at his apartment

on Fifth Avenue and 93rd Street

on the second floor.

Now, in order to get

to this meeting,

I need four pieces

of information,

one for each of the three

dimensions of space...

A street, an avenue

and a floor number...

And one more for time,

the fourth dimension.

You can think about these

as the four dimensions

of common experience...

Left-right, back-forth,

up-down and time.

As it turns out,

the strange idea that there

are additional dimensions

stretches back almost a century.

Our sense that we live

in a universe

of three spatial dimensions

really seems beyond question.

But in 1919, Theodor Kaluza,

a virtually unknown

German mathematician,

had the courage

to challenge the obvious.

He suggested

that maybe, just maybe,

our universe has

one more dimension

that for some reason

we just can't see.

No, he says here,

"I like your idea."

So, why does he delay?

You see, Kaluza

had sent his idea

about an additional spatial

dimension to Albert Einstein.

And although Einstein was

initially enthusiastic,

he then seemed to waver

and for two years held up

publication of Kaluza's paper.

Eventually, Kaluza's paper

was published,

after Einstein decided extra

dimensions were his cup of tea.

Here's the idea.

In 1916, Einstein showed

that gravity is nothing

but warps and ripples

in the four familiar dimensions

of space and time.

Just three years later,

Kaluza proposed

that electromagnetism

might also be ripples.

But for that to be true,

Kaluza needed a place

for those ripples to occur.

So Kaluza proposed an additional

hidden dimension of space.

But if Kaluza was right,

where is this extra dimension?

And what would

extra dimensions look like?

Can we even begin

to imagine them?

Well, building

upon Kaluza's work,

the Swedish physicist

Oskar Klein suggested

an unusual answer.

Take a look at the cables

supporting that traffic light.

From this far away, I can't see

that they have any thickness.

Each one looks like a line,

something with only

a single dimension.

But suppose we could explore one

of these cables way up close,

like from the point of view

of an ant.

Now, a second dimension,

which wraps around the cable,

becomes visible.

From its point of view, the ant

can move forwards and backwards,

and it can also move clockwise

and counterclockwise.

So, dimensions can come

in two varieties.

They can be long and unfurled,

like the length of the cable,

but they can also be

tiny and curled up,

like the circular direction

that wraps around it.

Kaluza and Klein made

the wild suggestion

that the fabric of our universe

might be

kind of like

the surface of the cable,

having both big,

extended dimensions...

The three that we know about...

But also tiny, curled-up

dimensions, curled up so tiny,

billions of times smaller

than even a single atom,

that we just can't see them.

And so our perception

that we live in a universe

with three spatial dimensions

may not be correct after all.

We really may live in a universe

with more dimensions

than meet the eye.

So, what would these

extra dimensions look like?

Kaluza and Klein proposed

that if we could shrink down

billions of times,

we'd find one extra

tiny, curled-up dimension

located at every point in space.

And just the way an ant can

explore the circular dimension

that wraps around

a traffic-light cable,

in theory, an ant that is

billions of times smaller

could also explore this tiny,

curled-up circular dimension.

This idea that extra dimensions

exist all around us

lies at the heart

of string theory.

In fact, the mathematics

of string theory demand

not one but six

extra dimensions,

twisted and curled

into complex little shapes

that might look

something like this.

If string theory is right,

we would have to admit

that there are really

more dimensions out there,

and I find that

completely mind-blowing.

If I take the theory

as we have it now literally,

I would conclude that the

extra dimensions really exist.

They're part of nature.

When we talk

about extra dimensions,

we literally mean

extra dimensions of space

that are the same

as the dimensions of space

that we see around us.

And the only difference between

them has to do with their shape.

But how could these

tiny extra dimensions,

curled up into

such peculiar shapes,

have any effect

on our everyday world?

Well, according to string

theory, shape is everything.

Because of its shape,

a French horn can produce

dozens of different notes.

When you press one of the keys,

you change the note

because you change the shape

of the space inside the horn

where the air resonates.

And we think the curled-up

spatial dimensions

in string theory work

in a similar way.

If we could shrink down

small enough

to fly into one of these

tiny six-dimensional shapes

predicted by string theory,

we would see how

the extra dimensions

are twisted and curled back

on each other,

influencing how strings,

the fundamental ingredients

of our universe,

move and vibrate.

And this could be the key

to solving one of nature's

most profound mysteries.

You see, our universe is kind

of like a finely tuned machine.

Scientists have found that

there are about 20 numbers...

20 fundamental constants

of nature...

That give the universe the

characteristics we see today.

These are numbers like

how much an electron weighs,

the strength of gravity,

the electromagnetic force,

and the strong and weak forces.

Now, as long as we set the dials

on our universe machine

to precisely the right values

for each of these 20 numbers,

the machine produces

the universe we know and love.

But if we change the numbers

by adjusting the settings

on this machine

even a little bit,

the consequences are dramatic.

For example, if I increase

the strength of

the electromagnetic force,

atoms repel one another

more strongly,

so the nuclear furnaces that

make stars shine break down.

The stars, including our sun,

fizzle out...

and the universe as we know it

disappears.

So, what exactly in nature

sets the values of these

20 constants so precisely?

Well, the answer could be

the extra dimensions

in string theory.

That is, the tiny, curled-up

six-dimensional shapes

predicted by the theory

cause one string to vibrate

in precisely the right way

to produce what we see

as a photon

and another string to vibrate

in a different way,

producing an electron.

So according to string theory,

these minuscule

extra-dimensional shapes

really may determine

all the constants of nature,

keeping the cosmic symphony

of strings in tune.

By the mid-1980s, string theory

looked unstoppable.

But behind the scenes,

the theory was in tangles.

Over the years, string theorists

had been so successful

that they had constructed

not one but five different

versions of the theory.

Each was built on strings

and extra dimensions,

but in detail, the five theories

were not in harmony.

In some versions, strings

were open-ended strands;

in others,

they were closed loops.

At first glance,

a couple of versions even

required 26 dimensions.

All five versions

appeared equally valid,

but which one was describing

our universe?

This was kind

of an embarrassment

for string theorists,

because on the one hand,

we wanted to say

that this might be it...

The final description

of the universe.

But then in the next breath

we had to say,

"And it comes in five flavors,

five variations."

Now, there's one universe,

you expect there to be

one theory and not five.

So this is an example

where more is definitely less.

One attitude that people who

didn't like string theory

could take was

"Well, you have five theories,

so it's not unique."

This was a peculiar

state of affairs,

because we were looking just

to describe one theory of nature

and not five.

If there's five of them,

well, maybe there's...

smart enough people

would find 20 of them,

or maybe there's

an infinite number of them,

and you're back to just

searching around at random

for... for theories

of the world.

Maybe one of these

five string theories

is describing our universe.

On the other hand, which one?

And why... what are

the other ones good for?

But having five string theories,

even though it's big progress,

raises the obvious question:

If one of those theories

describes our universe,

then who lives

in the other four worlds?

String theory seemed

to be losing steam once again,

and frustrated

by a lack of progress,

many physicists

abandoned the field.

Will string theory prove

to be a theory of everything,

or will it unravel

into a theory of nothing?

On NOVA's Web site, go behind

the scenes with Brian Greene,

journey into the subatomic

world, play with strings,

picture other dimensions,

and much more.

To order this program

on VHS or DVD

or the book

The Elegant Universe,

please call WGBH

Boston Video at

1-800-255-9424.

Next time on NOVA:

Can string theory be saved?

Do we live in a world

of extra dimensions

with parallel universes

just beyond our reach?

The thrill ride continues

as Brian Greene pushes

physics to the limit

on the next episode of

"The Elegant Universe."

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