Original subtitles

Now, on NOVA,

take a thrill ride into a world

stranger than science fiction,

where you play the game by breaking some rules,

where a new view of the universe

pushes you beyond the limits

of your wildest imagination.

This is the world of "string theory,"

a way of describing every force and all matter

from an atom to earth, to the end of the galaxies --

from the birth of time to its final tick,

in a single theory, a "Theory of Everything."

Our guide to this brave new world

is Brian Greene, the bestselling author and physicist.

BRIAN GREENE

And no matter how many times I come here,

I never seem to get used to it.

NARRATOR: Can he help us solve

the greatest puzzle of modern physics --

that our understanding of the universe

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

NARRATOR: Resolving that contradiction eluded even Einstein,

who made it his final quest.

After decades,

we may finally be on the verge of a breakthrough.

The solution is strings,

tiny bits of energy vibrating

like the strings on a cello,

a cosmic symphony

at the heart of all reality.

But it comes at a price:

parallel universes and 11 dimensions,

most of which

you've never seen.

BRIAN GREENE: We really may live in a universe

with more dimensions than meet the eye.

AMANDA PEET People who have said that there

were extra dimensions of space

have been labeled crackpots, or people who are bananas.

NARRATOR: A mirage of science and mathematics

or the ultimate theory of everything?

S. JAMES GATES, JR.

If string theory fails to

provide a testable prediction,

then nobody should believe it.

SHELDON LEE GLASHOW:

Is that a theory of physics,

or a philosophy?

BRIAN GREENE: One thing that is certain is that string theory

is already showing us that

the universe may be a lot stranger

than any of us ever imagined.

NARRATOR: Coming up tonight...

GABRIELE VENEZIANO We accidentally discovered string theory.

NARRATOR:...the humble beginnings

of a revolutionary idea.

LEONARD SUSSKIND

I was completely convinced it was going to say,

"Susskind is the next Einstein."

JOSEPH LYKKEN This seemed crazy to people.

LEONARD SUSSKIND: I was depressed, I was unhappy.

The result was I went home and got drunk.

NARRATOR: 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.

THE ELEGANT UNIVERSE

Hosted By Brian Greene

String's the Thing

Two Conflicting Sets of Laws

BRIAN GREENE: 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.

To understand this place,

you'd need to find a way

to put those two conflicting sets of laws together

into one all-encompassing set that makes sense.

MICHAEL DUFF

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.

STEVEN WEINBERG

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.

BRIAN GREENE: 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 billion 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.

WALTER H.G. LEWIN

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?

BRIAN GREENE: 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.

One Master Equation

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 physicists,

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 it splitting an the atom

to the birth a the star

is nothing more then 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 miniscule strings

are the key to uniting the world of the large

and the world of the small

in a single theory.

JOSEPH LYKKEN: The idea that a scientific theory

that we already have in our hands

could answer the most basic questions

is extremely seductive.

S. JAMES GATES, JR.: 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.

BRIAN GREENE: But not everyone

is enamored of this new theory.

So far

no experiment has been devised

that can prove these tiny strings exist.

SHELDON LEE GLASHOW

And let me put it bluntly.

There are physicists

and there are string theorists.

It is a new discipline,

a new -- you may call it a tumor --

you can call it what you will,

but 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 can 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.

BRIAN GREENE: 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 it's birth

has been turned to something an the meet.

Which goes like this...

The Birth of String Theory

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.

GABRIELE VENEZIANO

I see occasionally, written in books, that,

uh,

that this model was invented

by chance or was, uh,

found in the 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.

BRIAN GREENE: 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.

LEONARD SUSSKIND:

To this day I remember the formula.

The formula was...

and I looked at it, and I said,

"This is so simple even I can figure out what this is."

BRIAN GREENE: 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.

LEONARD SUSSKIND:

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 that

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.

BRIAN GREENE: 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.

LEONARD SUSSKIND:

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,

"this paper's 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 the result was

I went home and got drunk.

BRIAN GREENE: As Susskind drowned his sorrows

over the rejection of his far out idea,

it appeared string theory

was dead.

The Standard Model

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.

SHELDON LEE GLASHOW:

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.

STEVEN WEINBERG: It was a time

when graduate students

would run through the halls

of a physics building saying

they discovered another particle,

and it fit the theories.

And it was all so exciting.

BRIAN GREENE: 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 "super-force."

Although that has yet to be proven,

quantum mechanics was able to explain

how three of the forces operate

on the subatomic level.

SHELDON LEE GLASHOW:

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 it's all in the textbooks.

STEVEN WEINBERG:

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.

BRIAN GREENE: 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.

GABRIELE VENEZIANO: Most people

in our community lost, completely,

interest in string theory. They said,

"Okay, that was a very nice elegant thing

but had nothing to do with nature."

S. JAMES GATES, JR.: 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.

BRIAN GREENE: But the more these

diehards delved into

string theory

the more problems they found.

JOSEPH LYKKEN:

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.

JOHN H. SCHWARZ

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.

CUMRUN VAFA

It had this massless particle

which was not seen in experiments.

MICHAEL B. GREEN: So these theories didn't seem to make sense.

JOSEPH LYKKEN: This seemed crazy to people.

CUMRUN VAFA: Basically,

string theory was not getting off the ground.

JOSEPH LYKKEN: People threw up their hands and said,

"This can't be right."

Wrestling with String Theory

BRIAN GREENE: 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.

JOHN H. SCHWARZ:

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 the tachyons and so on,

but it was always ugly and unconvincing.

BRIAN GREENE: 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.

JOHN H. SCHWARZ:

We weren't thinking about gravity up 'til 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.

BRIAN GREENE: 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.

JOHN H. SCHWARZ:

It seemed very obvious to us that it was right.

But there was really no reaction

in the community whatsoever.

BRIAN GREENE: 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.

MICHAEL B. 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.

BRIAN GREENE: 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 if 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.

JOHN H. SCHWARZ:

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've got to calculate these things."

BRIAN GREENE: 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.

MICHAEL B. GREEN:

I do remember 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.

BRIAN GREENE: The matching numbers

meant the theory was free of anomalies.

And it had the mathematical depth

to encompass all four forces.

JOHN H. SCHWARZ:

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.

MICHAEL B. GREEN:

We felt great.

That was an extraordinary moment,

because we realized

that no other theory had ever succeeded in doing that.

JOHN H. SCHWARZ:

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, I figured...

by now I didn't expect

much of a reaction.

BRIAN GREENE: But this time the reaction was explosive.

In less than a year,

the number of string theorists

leapt from just a handful to hundreds.

MICHAEL B. GREEN:

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.

BRIAN GREENE: String theory was christened

"The Theory of Everything."

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 theory

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.

SHELDON LEE GLASHOW:

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.

MICHAEL B. GREEN:

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, um, um,

a branch of physics, for that reason.

And I, my response to that is simply

that they're going to be proved wrong.

BRIAN GREENE: 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.

AMANDA PEET:

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.

BRIAN GREENE: 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.

Multiple Dimensions

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 four o'clock at his apartment

at 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.

THEODOR KALUZA

Look. He says here,

"I like your idea."

So why does he delay?

BRIAN GREENE: 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.

MICHAEL DUFF:

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.

EDWARD WITTEN

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.

JOSEPH LYKKEN:

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.

BRIAN GREENE: 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

sixdimensional 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.

Five Flavors of String Theory

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 other 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 miniscule extradimensional 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 openended 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.

MICHAEL B GREEN:

One attitude that people

who didn't like string theory could take was,

"Well, you have five theories, so it's not unique."

JOHN H. SCHWARZ:

This was a peculiar state of affairs,

because we were looking just to describe

one theory of nature and not five.

JOSEPH LYKKEN:

If there's five of them, well maybe there's

smart enough people would find twenty of them.

Or maybe there's an infinite number of them,

and you're back to just searching

around at random for theories of the world.

CUMRUN VAFA: 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?

EDWARD WITTEN: 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?

BRIAN GREENE: String theory seemed

to be losing steam once again.

And frustrated by a lack of progress,

many physicists abandoned the field.

NARRATOR: Will string theory prove to be a "Theory of Everything"

or will it unravel into a "Theory of Nothing?"

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