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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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

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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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