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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.
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 have said that
There were 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...
It all started with an apple.
The triumph of Newton』s equations
Come from the quest to understand the planets and the stars.
And we've come a long way since.
Einstein gave the world a new picture for
What the force of gravity actually is.
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.
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?
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.
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.
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.
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,
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.
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.
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.
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 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.
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.
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
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.
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.
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.
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,
People want to be in on that.
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 new fangled 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 Newton』s before him,
Maxwell's unification took science a step closer
To cracking the code of the universe.
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.
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.
Einstein thought that this was
One of the triumphant moments of all of physics
And admired Maxwell hugely for what he had done.
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.
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.
Today, this is the goal of the 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.
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.
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's
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.
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.
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.
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.
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.
Led by Danish physicist noels boor,
These scientists
Were uncovering an entirely new realm of the universe.
Atoms,
Long thought to be the smallest constituents of nature,
a found it's consisted a even small parties
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.
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.
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 is 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 noels boor 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 cafe,
You might think you'd lost your mind.
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.
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 experience
The weirdness of quantum mechanics.
But here in the quantum cafe,
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.
I'll try.
"I』ll try," she says.
You see,
They're not used to people
Placing definite orders here in the quantum cafe,
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 seem as real to their inhabitants
As our universe seems to us.
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.
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.
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.
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.
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."
Yet, experiment after experiment showed
Einstein was wrong.
And that quantum mechanics really does describe
how the world works at the subatomic level.
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.
Quantum mechanics is fantastically accurate.
There has never been a prediction of quantum mechanics
that has contradicted an observation,
Never.
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.
You try to put those two pieces of mathematics together,
they do not coexist peacefully.
You get answers that the probabilities
of the event you're looking at are infinite.
Nonsense, it's not profound,
it's just nonsense.
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.
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 everywhere.
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.
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.
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.
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 machine guns of the other side.
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.
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.
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.
And another that uses quantum mechanics
to study the tiniest 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
living under the same roof.
There just seemed to be no way to combine quantum mechanics
and general relativity in a single theory
that could describe the universe on all scales.
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 World War I,
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 Schwarz child』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.
Schwarz child』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.
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.
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.
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.
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.
So it has incredible unification power,
It unifies our understanding
of all these different kinds of particles.
So unity of the different forces and particles is achieved
because they all come from different kinds of
vibrations of the same basic string.
It's a simple idea with far-reaching consequences.
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.
But this seductive new theory is also controversial.
Strings, if they exist, are so small,
there's little hope of ever seeing one.
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,