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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, 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?
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...
it all started with an apple.
S. JAMES GATES, JR.
The triumph of Newton's equations
come from the quest
to understand the planets and the stars.
NARRATOR: And we've come a long way since.
BRIAN GREENE: Einstein gave the world
a new picture for what
the force of gravity actually is.
NARRATOR: Where he left off, string theorists now dare to go.
But how close are they to fulfilling Einstein's dream?
Watch The Elegant Universe right now.
THE ELEGANT UNIVERSE
Hosted By Brian Green
Einstein's Dream
A Theory of Everything?
BRIAN GREENE: Fifty years ago, this house was the scene of one of
the greatest mysteries of modern science,
a mystery so profound that today
thousands of scientists on the cutting edge of physics
are still trying to solve it.
Albert Einstein spent his last two decades
in this modest home in Princeton, New Jersey.
And in his second floor study
Einstein relentlessly sought a single theory so powerful
it would describe all the workings of the universe.
Even as he neared the end of his life
Einstein kept a notepad close at hand,
furiously trying to come up with the equations
for what would come to be known as the "Theory of Everything."
Convinced he was on the verge of
the most important discovery in the history of science,
Einstein ran out of time, his dream unfulfilled.
Now, almost a half century later,
Einstein's goal of unification --
combining all the laws of the universe
in one, all-encompassing theory --
has become the Holy Grail of modern physics.
And we think we may at last achieve Einstein's dream
with a new and radical set of ideas
called "string theory."
But if this revolutionary theory is right,
we're in for quite a shock.
String theory says
we may be living in a universe
where reality meets science fiction --
a universe of eleven dimensions
with parallel universes
right next door --
an elegant universe composed entirely
of the music of strings.
But for all its ambition,
the basic idea of string theory
is surprisingly simple.
It says that everything in the universe,
from the tiniest particle to the most distant star
is made from one kind of ingredient --
unimaginably small vibrating strands of energy
called strings.
Just as the strings of a cello
can give rise to a rich
variety of musical notes,
the tiny strings in string theory vibrate in a multitude of different ways
making up all the constituents of nature.
In other words, the universe is like
a grand cosmic symphony
resonating with all the various notes
these tiny vibrating strands of energy
can play.
String theory is still
in its infancy,
but it's already revealing
a radically new picture of the universe,
one that is both strange and beautiful.
But what makes us think we can understand
all the complexity of the universe,
let alone reduce it to a single "Theory of Everything?"
We have R mu nu, minus a half g mu nu R --
you remember how this goes --
equals eight Pi G T mu nu...
comes from varying the Einstein-Hilbert action,
and we get the field equations
and this term. You remember what this is called?
DOG BARKS: Vau, vau!
No that's the scalar curvature.
This is the ricci tensor.
Have you been studying this at all?
No matter how hard you try,
you can't teach physics to a dog.
Their brains just aren't wired
to grasp it.
But what about us?
How do we know that we're wired
to comprehend the deepest laws
of the universe?
Well, physicists today are confident that we are,
and we're picking up
where Einstein left off in his quest for unification.
Unification would be the formulation of a law
that describes, perhaps,
everything in the known universe from
one single idea, one master equation.
And we think that there might be this master equation,
because throughout the course of the last
200 years or so,
our understanding of the universe
has given us a variety of explanations
that are all pointing towards one spot.
They seem to all be converging
on one nugget of an idea
that we're still trying to find.
STEVEN WEINBERG
Unification is where it's at.
Unification is what
we're trying to accomplish.
The whole aim of fundamental physics
is to see more and more of the world's phenomena
in terms of fewer and fewer and simpler and simpler principles.
MICHAEL B. GREEN
We feel, as physicists, that if we can explain
a wide number of phenomena in a very simple manner,
that that's somehow progress.
There is almost an emotional aspect to the way
in which the great theories in physics
sort of encompass a wide variety
of apparently different physical phenomena.
So this idea that we should be aiming
to unify our understanding is inherent, essentially,
to the whole way in which this kind of science progresses.
Newton's Embarrassing Secret
BRIAN GREENE: And long before Einstein, the quest for unification
began with the most famous accident
in the history of science.
As the story goes, one day in 1665,
a young man was sitting under a tree when,
all of a sudden, he saw an apple fall from above.
And with the fall of that apple, Isaac Newton
revolutionized our picture of the universe.
In an audacious proposal for his time,
Newton proclaimed that the force
pulling apples to the ground
and the force keeping the moon in orbit
around the earth were actually one and the same.
In one fell swoop, Newton unified the heavens and the earth
in a single theory he called gravity.
STEVEN WEINBERG:
The unification of the celestial with the terrestrial --
that the same laws that govern the planets in their motions
govern the tides and the falling of fruit here on earth --
it was a fantastic
unification of our picture of nature.
BRIAN GREENE: Gravity was the first force to be understood scientifically,
though three more would eventually follow.
And, although Newton discovered his law of gravity more than 300 years ago,
his equations describing this force make such
accurate predictions that we still make use of them today.
In fact scientists needed nothing more
than Newton's equations to plot the course of a rocket
that landed men on the moon.
Yet there was a problem.
While his laws described
the strength of gravity with great accuracy,
Newton was harboring an embarrassing secret:
he had no idea how gravity actually works.
For nearly 250 years,
scientists were content to look the other way
when confronted with this mystery.
But in the early 1900s,
an unknown clerk working in the Swiss patent office
would change all that.
While reviewing patent applications, Albert Einstein
was also pondering the behavior of light.
And little did Einstein know
that his musings on light
would lead him to solve Newton's mystery
of what gravity is.
At the age of 26, Einstein made a startling discovery:
that the velocity of light is a kind of
cosmic speed limit, a speed that nothing in the universe can exceed.
But no sooner
had the young Einstein published this idea
than he found himself squaring off
with the father of gravity.
The trouble was, the idea
that nothing can go faster than the speed of light
flew in the face of Newton's
picture of gravity.
To understand this conflict,
we have to run a few experiments.
And to begin with, let's create a cosmic catastrophe.
Imagine that all of a sudden, and without any warning,
the sun vaporizes and completely disappears.
Now, let's replay that catastrophe
and see what effect it would have on the planets
according to Newton.
Newton's theory predicts
that with the destruction of the sun,
the planets would immediately fly out of their orbits
careening off into space.
In other words, Newton thought that gravity was
a force that acts instantaneously
across any distance.
And so we would immediately feel
the effect of the sun's destruction.
But Einstein saw a big problem with Newton's theory,
a problem that arose from his work with light.
Einstein knew light doesn't travel instantaneously.
In fact, it takes eight minutes
for the sun's rays to travel the 93 million miles
to the earth.
And since he had shown that nothing, not even gravity,
can travel faster than light,
how could the earth be released from orbit
before the darkness resulting from the sun's disappearance
reached our eyes?
To the young upstart from the Swiss patent office
anything outrunning light was impossible,
and that meant the 250-year old Newtonian
picture of gravity was wrong.
S. JAMES GATES, JR.:
If Newton is wrong,
then why do the planets stay up?
Because remember, the triumph of Newton's equations
come from the quest to understand
the planets and the stars,
and particularly the problem of why the planets have the orbits that they do.
And with Newton's equations you could calculate the way
that the planets would move.
Einstein's got to resolve this dilemma.
BRIAN GREENE: In his late twenties, Einstein had to come up with
a new picture of the universe
in which gravity does not exceed the cosmic speed limit.
Still working his day job in the patent office, Einstein
embarked on a solitary quest to solve this mystery.
After nearly ten years of wracking his brain
he found the answer in a new kind of unification.
A New Picture of Gravity
PETER GALISON
Einstein came to think of the three dimensions of space
and the single dimension of time
as bound together in a single fabric of "space-time."
It was his hope
that by understanding the geometry of this
four-dimensional fabric of space-time,
that he could simply talk about
things moving along surfaces
in this space-time fabric.
BRIAN GREENE: Like the surface of a trampoline,
this unified fabric is warped and stretched
by heavy objects like planets and stars.
And it's this warping or curving of space-time
that creates what we feel as gravity.
A planet like the earth is kept in orbit,
not because the sun reaches out and instantaneously
grabs hold of it, as in Newton's theory,
but simply because it follows curves
in the spatial fabric caused by the sun's presence.
So, with this new understanding of gravity,
let's rerun the cosmic catastrophe.
Let's see what happens now if the sun disappears.
The gravitational disturbance that results
will form a wave that travels across the spatial fabric
in much the same way that a pebble
dropped into a pond makes ripples
that travel across the surface of the water.
So we wouldn't feel a change
in our orbit around the sun
until this wave reached the earth.
What's more, Einstein calculated that these ripples of gravity
travel at exactly the speed of light.
And so, with this new approach,
Einstein resolved the conflict with Newton
over how fast gravity travels.
And more than that, Einstein gave the world a new picture
for what the force of gravity actually is:
it's warps and curves in the fabric of space and time.
Einstein called this new picture of gravity "General Relativity,"
and within a few short years Albert Einstein
became a household name.
S. JAMES GATES, JR.: Einstein was like
a rock star in his day.
He was one of the most widely known
and recognizable figures alive.
He and perhaps Charlie Chaplin were
the reigning kings of the popular media.
MARCIA BARTUSIAK People followed his work.
And they were anticipating...because of this wonderful thing
he had done with general relativity,
this recasting the laws of gravity out of his head...
there was a thought he could do it again,
and they, you know, people want to be in on that.
BRIAN GREENE: Despite all that he had achieved
Einstein wasn't satisfied.
He immediately set his sights on an even grander goal,
the unification of his new picture of gravity
with the only other force known at the time,
electromagnetism.
Now electromagnetism is a force
that had itself been unified
only a few decades earlier.
In the mid-1800s,
electricity and magnetism
were sparking scientists' interest.
These two forces seemed to share a curious relationship
that inventors like Samuel Morse were taking
advantage of in newfangled devices, such as the telegraph.
An electrical pulse sent through a telegraph wire
to a magnet thousands of miles away
produced the familiar dots and dashes of Morse code
that allowed messages to be transmitted across the continent
in a fraction of a second.
Although the telegraph was a sensation,
the fundamental science driving it
remained something of a mystery.
But to a Scottish scientist named James Clark Maxwell,
the relationship between electricity and magnetism
was so obvious in nature that it demanded unification.
If you've ever been on top of a mountain
during a thunderstorm you'll get
the idea of how electricity and magnetism are closely related.
When a stream of electrically charged particles flows,
like in a bolt of lightning, it creates a magnetic field.
And you can see evidence of this on a compass.
Obsessed with this relationship,
the Scot was determined to explain
the connection between electricity and magnetism
in the language of mathematics.
Casting new light on the subject,
Maxwell devised a set
of four elegant mathematical equations
that unified electricity and magnetism
in a single force called "electromagnetism."
And like Isaac Newton's before him,
Maxwell's unification took science a step closer
to cracking the code of the universe.
JOSEPH POLCHINSKI
That was really the remarkable thing,
that these different phenomena
were really connected in this way.
And it's another example of diverse phenomena
coming from a single underlying building block or a single underlying principle.
WALTER H.G. LEWIN
Imagine that everything that you can think of
which has to do with electricity and magnetism
can all be written in four very simple equations.
Isn't that incredible? Isn't that amazing?
I call that elegant.
PETER GALISON: Einstein thought that this was
one of the triumphant moments of all of physics
and admired Maxwell hugely for what he had done.
BRIAN GREENE: About 50 years after Maxwell unified
electricity and magnetism,
Einstein was confident
that if he could unify his new theory of gravity with Maxwell's electromagnetism,
he'd be able to formulate a master equation
that could describe everything, the entire universe.
S. JAMES GATES, JR.:
Einstein clearly believes that the universe
has an overall grand and beautiful pattern to the way that it works.
So to answer your question,
why was he looking for the unification?
I think the answer is simply
that Einstein is one of those physicists
who really wants to know the mind of God, which means the entire picture.
A Strange New World
BRIAN GREENE: Today, this is the goal of string theory:
to unify our understanding of everything
from the birth of the universe
to the majestic swirl of galaxies
in just one set of principles,
one master equation.
Newton had unified the heavens and the earth
in a theory of gravity.
Maxwell had unified electricity and magnetism.
Einstein reasoned all that
remained to build a "Theory of Everything"--
a single theory that could encompass all the laws of the universe --
was to merge his new picture of gravity
with electromagnetism.
AMANDA PEET: He certainly had motivation.
Probably one of them might have been aesthetics,
or this quest to simplify.
Another one might have been just the physical fact
that it seems like the speed of gravity
is equal to the speed of light.
So if they both go at the same speed,
then maybe that's an indication of some underlying symmetry.
BRIAN GREENE: But as Einstein began trying to unite
gravity and electromagnetism
he would find that the difference in strength between these two forces
would outweigh their similarities.
Let me show you what I mean.
We tend to think that gravity is a powerful force.
After all, it's the force that, right now,
is anchoring me to this ledge.
But compared to electromagnetism,
it's actually terribly feeble.
In fact, there's a simple little test to show this.
Imagine that I was to leap from this rather tall building.
Actually, let's not just imagine it.
Let's do it.
You'll see what I mean.
Now, of course, I really should have been flattened.
But the important question is:
what kept me from crashing through the sidewalk
and hurtling right down to the center of the earth?
Well, strange as it sounds,
the answer is electromagnetism.
Everything we can see, from you and me
to the sidewalk, is made of tiny bits of matter
called atoms.
And the outer shell of every atom
contains a negative electrical charge.
So when my atoms collide
with the atoms in the cement
these electrical charges repel each other
with such strength that just a little piece of sidewalk
can resist the entire Earth's gravity and stop me from falling.
In fact the electromagnetic force
is billions and billions of times stronger
than gravity.
NIMA ARKANI-HAMED
That seems a little strange, because gravity keeps our feet to the ground,
it keeps the earth going around the sun.
But, in actual fact,
it manages to do that only because
it acts on huge enormous conglomerates of matter,
you know -- you, me, the earth, the sun --
but really at the level of individual atoms,
gravity is a really incredibly feeble tiny force.
BRIAN GREENE: It would be an uphill battle for Einstein to unify
these two forces of wildly different strengths.
And to make matters worse, barely had he begun
before sweeping changes
in the world of physics would leave him behind.
STEVEN WEINBERG: Einstein had achieved so much
in the years up to about 1920,
that he naturally expected that he could go on
by playing the same theoretical games
and go on achieving great things.
And he couldn't.
Nature revealed itself in other ways
in the 1920s and 1930s,
and the particular tricks and tools that Einstein had at his disposal
had been so fabulously successful,
just weren't applicable anymore.
The Quantum Cafe
BRIAN GREENE: You see, in the 1920s a group of young scientists
stole the spotlight from Einstein
when they came up with an outlandish
new way of thinking about physics.
Their vision of the universe was so strange,
it makes science fiction look tame,
and it turned Einstein's quest
for unification on its head.
Unification! Unification!
Led by Danish physicist Niels Bohr,
these scientists were uncovering an entirely
new realm of the universe.
Atoms,
long thought to be the smallest constituents
of nature, were found to
consist of even smaller particles:
the now-familiar nucleus
of protons and neutrons orbited by electrons.
And the theories of Einstein and Maxwell
were useless at explaining
the bizarre way these tiny bits of matter
interact with each other inside the atom.
PETER GALISON: There was a tremendous mystery
about how to account for all this,
how to account for what was happening to the nucleus
as the atom began to be pried
apart in different ways.
And the old theories were totally inadequate to the task of explaining them.
Gravity was irrelevant. It was far too weak.
And electricity and magnetism was not sufficient.
BRIAN GREENE: Without a theory to explain this strange new world,
these scientists were lost
in an unfamiliar atomic territory
looking for any recognizable landmarks.
Then, in the late 1920s, all that changed.
During those years, physicists developed
a new theory called "quantum mechanics,"
and it was able to describe the microscopic
realm with great success.
But here's the thing:
quantum mechanics was so radical a theory
that it completely shattered all previous ways
of looking at the universe.
Einstein's theories demand
that the universe is orderly and predictable,
but Niels Bohr disagreed.
He and his colleagues proclaimed
that at the scale of atoms and particles,
the world is a game of chance.
At the atomic or quantum level, uncertainty rules.
The best you can do,
according to quantum mechanics,
is predict the chance or probability
of one outcome or another.
And this strange idea
opened the door to an unsettling new picture of reality.
It was so unsettling
that if the bizarre features of quantum mechanics
were noticeable in our everyday world,
like they are here in the Quantum Café,
you might think you'd lost your mind.
WALTER H.G. LEWIN: The laws in the quantum world
are very different from the laws
that we are used to.
Our daily experiences are totally different
from anything that you would see in the quantum world.
The quantum world is crazy.
It's probably the best way to put it:
it's a crazy world.
BRIAN GREENE: For nearly 80 years,
quantum mechanics has successfully claimed
that the strange and bizarre are typical
of how our universe actually behaves
on extremely small scales.
At the scale of everyday life,
we don't directly experiencethe
weirdness of quantum mechanics.
But here in the Quantum Café,
big, everyday things sometimes
behave as if they were microscopically tiny.
And no matter how many times I come here,
I never seem to get used to it.
I'll have an orange juice, please.
BARTENDER: I'll try.
BRIAN GREENE: "I'll try," she says.
You see, they're not used to people placing
definite orders here in the Quantum Café,
because here everything is ruled by chance.
While I'd like an orange juice,
there is only a particular probability
that I'll actually get one.
And there's no reason to be disappointed
with one particular outcome or another,
because quantum mechanics suggests
that each of the possibilities
like getting a yellow juice or a red juice
may actually happen.
They just happen to happen
in universes that are parallel to ours,
universes that seemas real to their inhabitants
as our universe seems to us.
WALTER H.G. LEWIN: If there are a thousand possibilities,
and quantum mechanics cannot, with certainty,
say which of the thousand it will be,
then all thousand will happen.
Yeah, you can laugh at it and say,
"Well, that has to be wrong."
But there are so many other things in physics
which -- at the time that people came up with --
had to be wrong, but it wasn't.
Have to be a little careful, I think,
before you say this is clearly wrong.
BRIAN GREENE: And even in our own universe,
quantum mechanics says there's a chance
that things we'd ordinarily think of as impossible
can actually happen.
For example there's a chance
that particles can pass right through walls or barriers
that seem impenetrable to you or me.
There's even a chance
that I could pass through something solid, like a wall.
Now, quantum calculations do show
that the probability for this to happen in the everyday world
is so small that I'd need
to continue walking into the wall
for nearly an eternity before having a reasonable chance of succeeding.
But here, these kinds of things happen all the time.
EDWARD FARHI
You have to learn to abandon those assumptions
that you have about the world
in order to understand quantum mechanics.
In my gut, in my belly,
do I feel like I have a deep intuitive
understanding of quantum mechanics?
No.
BRIAN GREENE: And neither did Einstein.
He never lost faith that the universe
behaves in a certain
and predictable way.
The idea that all we can do is calculate the odds
that things will turn out one way or another
was something Einstein deeply resisted.
MICHAEL DUFF
Quantum mechanics says that you
can't know for certain
the outcome of any experiment;
you can only assign a certain probability
to the outcome of any experiment.
And this, Einstein disliked intensely.
He used to say "God does not throw dice."
BRIAN GREENE: Yet, experiment after experiment
showed Einstein was wrong
and that quantum mechanics really does describe
how the world works at the subatomic level.
WALTER H.G. LEWIN:
So quantum mechanics is not a luxury, something
that you can do without.
I mean why is water the way it is?
Why does light go straight through water? Why is it transparent?
Why are other things not transparent?
How do molecules form?
Why are they reacting the way they react?
The moment that you want to understand
anything at an atomic level,
as non-intuitive as it is,
at that moment, you can only make progress with quantum mechanics.
EDWARD FARHI: Quantum mechanics
is fantastically accurate.
There has never been
a prediction of quantum mechanics
that has contradicted an observation,
never.
Gravity - The Odd Man Out
BRIAN GREENE: By the 1930s, Einstein's quest
for unification was floundering,
while quantum mechanics
was unlocking the secrets of the atom.
Scientists found that gravity
and electromagnetism
are not the only forces ruling the universe.
Probing the structure of the atom,
they discovered two more forces.
One, dubbed the "strong nuclear force,"
acts like a super-glue,
holding the nucleus of every atom together,
binding protons to neutrons.
And the other, called the "weak nuclear force,"
allows neutrons to turn into protons,
giving off radiation in the process.
At the quantum level,
the force we're most familiar with,
gravity, was completely overshadowed
by electromagnetism and these two new forces.
Now, the strong and weak forces
may seem obscure,
but in one sense at least,
we're all very much aware of their power.
At 5:29 on the morning of July 16th, 1945,
that power was revealed by an act
that would change the course of history.
In the middle of the desert, in New Mexico,
at the top of a steel tower about
a hundred feet above the top of this monument,
the first atomic bomb was detonated.
It was only about five feet across,
but that bomb packed a punch
equivalent to about twenty thousand tons of TNT.
With that powerful explosion, scientists
unleashed the strong nuclear force,
the force that keeps neutrons and protons
tightly glued together inside the nucleus of an atom.
By breaking the bonds of that glue
and splitting the atom apart,
vast, truly unbelievable amounts
of destructive energy were released.
We can still detect remnants of
that explosion through
the other nuclear force,
the weak nuclear force,
because it's responsible for radioactivity.
And today, more than 50 years later,
the radiation levels around here are still
about 10 times higher than normal.
So,
although in comparison to electromagnetism and gravity
the nuclear forces act over very small scales,
their impact on everyday life is every bit as profound.
But what about gravity?
Einstein's general relativity?
Where does that fit in at the quantum level?
Quantum mechanics tells us
how all of nature's forces work in the microscopic realm
except for the force of gravity.
Absolutely no one could
figure out how gravity operates
when you get down to the size of atoms
and subatomic particles.
That is, no one could figure out
how to put general relativity and quantum mechanics together into one package.
For decades,
every attempt to describe the force of gravity
in the same language as the other forces --
the language of quantum mechanics --
has met with disaster.
S. JAMES GATES, JR.: You try to put those two pieces
of mathematics together,
they do not coexist peacefully.
STEVEN WEINBERG: You get answers
that the probabilities of the event
you're looking at are infinite.
Nonsense, it's not profound,
it's just nonsense.
NIMA ARKANI-HAMED: It's very ironic because it was the first force
to actually be understood in some decent
quantitative way, but, but,
but it still remains split
off and very different from, from the other ones.
S. JAMES GATES, JR.: The laws of nature
are supposed to apply everywhere.
So if Einstein's laws
are supposed
to apply everywhere,
and the laws of quantum mechanics
are supposed to apply everywhere,
well you can't have two separate everywheres.
Strings to the Rescue
BRIAN GREENE: In 1933, after fleeing Nazi Germany,
Einstein settled in Princeton, New Jersey.
Working in solitude, he stubbornly continued
the quest he had begun more than a decade earlier,
to unite gravity and electromagnetism.
Every few years,
headlines appeared,
proclaiming Einstein was on the verge of success.
But most of his colleagues
believed his quest was misguided
and that his best days were already behind him.
STEVEN WEINBERG: Einstein, in his later years,
got rather detached from the work of physics
in general and, and stopped reading people's papers.
I didn't even think he knew
there was such a thing as the weak nuclear force.
He didn't pay attention to those things.
He kept working on the same problem
that he had started working on as a younger man.
S JAMES GATES, JR.: When the community of theoretical physicists
begins to probe the atom,
Einstein very definitely gets left out of the picture.
He, in some sense, chooses not
to look at the physics coming from these experiments.
That means that the laws of quantum mechanics
play no role in his sort of further investigations.
He's thought to be this doddering,
sympathetic old figure
who led an earlier revolution but somehow fell out of it.
STEVEN WEINBERG: It is as if a general
who was a master of horse cavalry,
who has achieved great things
as a commander at the beginning of the First World War,
would try to bring mounted cavalry
into play against the barbwire
trenches and machines guns of the other side.
BRIAN GREENE: Albert Einstein died on April 18, 1955.
And for many years it seemed that Einstein's dream
of unifying the forces in a single theory
died with him.
S. JAMES GATES, JR.:
So the quest for unification
becomes a backwater of physics.
By the time of Einstein's death
in the '50s,
almost no serious physicists
are engaged in this quest for unification.
RIGHT SIDE BRIAN GREENE: In the years since,
physics split into two separate camps:
one that uses general relativity
to study big and heavy objects,
things like stars, galaxies and the universe as a whole...
LEFT SIDE BRIAN GREENE: ...and another that uses quantum mechanics
to study the tiniest of objects,
like atoms and particles.
This has been kind of like having two families
that just cannot get along
and never talk to each other...
RIGHT SIDE BRIAN GREENE: ...living under the same roof.
LEFT SIDE BRIAN GREENE: There just seemed to be no way to combine
quantum mechanics...
RIGHT SIDE BRIAN GREENE: ...and general relativity in a single theory
that could describe the universe on all scales.
BRIAN GREENE: Now, in spite of this,
we've made tremendous progress
in understanding the universe.
But there's a catch:
there are strange realms of the cosmos
that will never be fully understood
until we find a unified theory.
And nowhere is this more evident
than in the
depths of a black hole.
A German astronomer named
Karl Schwarzschild
first proposed
what we now call black holes
in 1916.
While stationed on the front lines
in WWI,
he solved the equations
of Einstein's general relativity
in a new and puzzling way.
Between calculations of artillery trajectories,
Schwarzschild figured out
that an enormous amount of mass,
like that of a very dense star,
concentrated in a small area,
would warp the fabric of space-time
so severely
that nothing, not even light,
could escape its gravitational pull.
For decades,
physicists were skeptical
that Schwarzschild's calculations
were anything more than theory.
But today
satellite telescopes probing deep
into space
are discovering regions
with enormous gravitational pull
that most scientists believe
are black holes.
Schwarzschild's theory
now seems to be reality.
So here's the question:
if you're trying to figure out
what happens in the depths of a black hole,
where an entire star is crushed
to a tiny speck,
do you use general relativity
because the star is incredibly heavy
or quantum mechanics
because it's incredibly tiny?
Well, that's the problem.
Since the center of a black hole
is both tiny and heavy,
you can't avoid using
both theories at the same time.
And when we try to put the two theories together
in the realm of black holes,
they conflict. It breaks down.
They give nonsensical predictions. And the universe is not nonsensical;
it's got to make sense.
EDWARD WITTEN
Quantum mechanics works really well
for small things, and general relativity
works really well for stars and galaxies,
but the atoms, the small things,
and the galaxies, they're part of the
same universe.
So there has to be some description
that applies to everything.
So we can't have one description for atoms
and one for stars.
BRIAN GREENE: Now, with string theory,
we think we may have found
a way to unite our theory of the large
and our theory of the small
and make sense of the universe
at all scales and all places.
Instead of a multitude of tiny particles,
string theory proclaims
that everything in the universe,
all forces and all matter
is made of one single ingredient,
tiny vibrating strands of energy
known as strings.
MICHAEL B. GREEN: A string
can wiggle in many different ways,
whereas, of course, a point can't.
And the different ways in which the string wiggles
represent the different kinds
of elementary particles.
MICHAEL DUFF: It's like a violin string,
and it can vibrate just like violin
strings can vibrate.
Each note if, you like,
describes a different particle.
MICHAEL B. GREEN: So it has incredible
unification power,
it unifies our understanding
of all these different kinds
of particles.
EDWARD WITTEN: So unity
of the different forces and particles
is achieved because they all
come from different kinds of vibrations
of the same basic string.
BRIAN GREENE: It's a simple idea
with far-reaching consequences.
JOSEPH LYKKEN
What string theory does is it
holds out the promise that,
"Look, we can really
understand questions that
you might not even have thought were scientific questions:
questions about how the universe began,
why the universe is the way it is
at the most fundamental level."
The idea that a scientific theory
that we already have in our hands
could answer the most basic questions
is extremely seductive.
Science of Philosophy?
BRIAN GREENE: But this seductive new theory
is also controversial.
Strings, if they exist,
are so small,
there's little hope of ever seeing one.
JOSEPH LYKKEN: String theory
and string theorists do have a real problem.
How do you actually test string theory?
If you can't test it in the way
that we test normal theories,
it's not science, it's philosophy,
and that's a real problem.
S. JAMES GATES, JR.: If string theory fails
to provide
a testable prediction,
then nobody should believe it.
On the other hand,
there is a kind of elegance to these things,
and given the history of how theoretical
physics has evolved thus far,
it is totally conceivable
that some if not all
of these ideas will turn out to be correct.
STEVEN WEINBERG:
I think, a hundred years from now,
this particular period,
when most of the brightest young theoretical physicists
worked on string theory,
will be remembered as a heroic age
when theorists tried and succeeded
to develop a unified
theory of all the phenomena of nature.
On the other hand, it may be remembered as a tragic failure.
My guess is
that it will be something like the former rather than the latter.
But ask me a hundred years from now,
then I can tell you.
BRIAN GREENE: Our understanding of the universe
has come an enormously long way
during the last three centuries.
Just consider this.
Isaac Newton,
who was perhaps the greatest scientist
of all time, once said,
"I have been like a boy playing on the
sea shore, diverting myself in now
and then finding a smoother pebble or a prettier shell than usual,
while the great ocean of truth
lay before me, all undiscovered."
And yet,
two hundred and fifty years later,
Albert Einstein,
who was Newton's true successor,
was able to seriously suggest
that this vast ocean,
all the laws of nature,
might be reduced to a few fundamental ideas
expressed by a handful
of mathematical symbols.
And today,
a half century after Einstein's death,
we may at last be on
the verge of fulfilling his dream of unification
with string theory.
But where did this daring and strange new theory come from?
How does string theory achieve
the ultimate unification of the laws of the large
and the laws of the small?
And how will we know if it's right or wrong?
SHELDON LEE GLASHOW: No experiment
can ever check up what's going on
at the distances that are being studied.
The theory is permanently safe.
Is that a theory of physics
or a philosophy?
STEVEN WEINBERG: 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.