優雅的宇宙 - 102 - 字符串的東西。 (The Elegant Universe - 102 - String's the Thing)

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Now, on NOVA,
take a thrill ride into a world
stranger than science fiction,
where you play the game by breaking some rules,
where a new view of the universe
pushes you beyond the limits
of your wildest imagination.
This is the world of "string theory,"
a way of describing every force and all matter
from an atom to earth, to the end of the galaxies --
from the birth of time to its final tick,
in a single theory, a "Theory of Everything."
Our guide to this brave new world
is Brian Greene, the bestselling author and physicist.
BRIAN GREENE (Columbia University):
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 (University of Toronto): 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. (University of Maryland):
If string theory fails to
provide a testable prediction,
then nobody should believe it.
SHELDON LEE GLASHOW: (University of Boston)
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 (CERN): We accidentally discovered string theory.
NARRATOR: ...the humble beginnings
of a revolutionary idea.
LEONARD SUSSKIND (Stanford University):
I was completely convinced it was going to say,
"Susskind is the next Einstein."
JOSEPH LYKKEN (Fermilab): 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 (University of Michigan):
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 (University of Texas at Austin):
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 (Massachusetts Institute of Technology):
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 (Boston University):
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 (CERN):
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 (California Institute of Technology):
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 (Harvard University):
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 (University of Cambridge):
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 (ACTOR):
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 (Institute for Advanced Study):
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?"