But for more than half a century

A dark cloud has been looming over modern science.

Here is 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?

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 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.

We work on the assumption that there is a theory out there,

And it's our job,

If we're sufficiently smart and sufficiently industrious,

To figure out what it is.

We don't have a guarantee—

It isn't written in the stars that we're going to succeed

But in the end

We hope we will have a single theory

That governs everything.

But before we can find that theory,

We need to take a fantastic journey

To see why the two sets of laws We have

conflict with each other.

And the first stop on this strange trip

Is the realm of very large objects.

To describe the universe on large scales

We use one set of laws，Einstein's general theory of relativity,

And that's a theory of how gravity works.

General relativity pictures space as sort of like a trampoline

A smooth fabric

that heavy objects like stars and planets can warp and stretch.

Now, according to the theory,

These warps and curves create what we feel as gravity.

That is，The gravitational pull that keeps the earth In orbit around the sun

Is really nothing more than our planet Following the curves and contours

that the sun creates in the spatial fabric.

But the smooth, Gently curving image of space

predicted By the laws of general relativity

is not the whole story.

To understand the universe on extremely small scales,

We have to use our other set of laws, quantum mechanics.

And as we'll see,

Quantum mechanics paints a picture of space

So drastically different from general relativity

That you'd think they were describing

two completely separate universes.

To see the conflict between general relativity

And quantum mechanics

We need to shrink way, way, way down in size.

And as we leave the world of large objects behind

And approach the microscopic realm,

The familiar picture of space In which everything behaves predictably

Begins to be replaced by a world

With a structure that is far less certain.

And if we keep shrinking,

Getting billions and 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.

The laws in the quantum world

Are very different from the laws that we are used to.

And is that surprising?

Why should the world of the very small, at an atomic level,

Why should that world obey the same kind of rules and laws

that we are used to in our world, With apples and oranges

And walking around on the street?

Why would that world behave the same way?

The fluctuating jittery picture of space and time predicted

By quantum mechanics

is in direct conflict With the smooth, orderly,

Geometric model of space and time

described by general relativity.

But we think that everything,

From the frantic dance of subatomic particles

To the majestic swirl of galaxies,

Should be explained by just one grand physical principle,

One master equation.

If we can find that equation,

How the universe really works at every time and place

Will at last be revealed.

You see,

What we need is a theory

That can cope with the very tiny and the very massive,

One that embraces both quantum mechanics and general relativity,

And never breaks down, ever.

For 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 gravity.

The 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 nuclear forces.

That's the force responsible for radioactive decay.

Every event in the universe

From the splitting of a atom, to the birth of a star

Is nothing more than these four forces into acting with manner

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.

The idea that a scientific theory That we already have in our hands

could answer the most basic questions

Is extremely seductive.

For about 2,000 years,

All of our physics essentially has been based on...

Essentially we were talking about billiard balls.

The very idea of the string is such a paradigm shift,

Because instead of billiard balls,

You have to use little strands of spaghetti.

But not everyone is enamored of this new theory.

So far no experiment has been devised that

Can prove these tiny strings exist.

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.

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 turned something . which goes like this

In the late 1960s

a young Italian physicist,

Named Gabriele venetian,

Was searching for a set of equations

That would explain the strong nuclear force,

The extremely powerful glue that

Holds the nucleus of every atom together

Binding protons to neutrons.

As the story goes,

He happened on a dusty book on the history of mathematics,

And in it he found a 200-year old equation,

First written down by a Swiss mathematician, Leonhard Euler.

Veneziano was amazed to discover

that Euler』s equations, Long thought to be nothing More than a mathematical curiosity,

Seemed to describe the strong force.

He quickly published a paper and was famous

Ever after for this "accidental" discovery.

I see occasionally, written in books,

That, uh, that this model was invented by chance or was, ,

Found in the math book

,,this makes me feel pretty bad.

What is true

Is that the function was the outcome of a long year of work,

And we accidentally discovered string theory.

However it was discovered, Euler』s equation,

Which miraculously explained the strong force,

Took on a life of its own.

This was the birth of string theory.

Passed from colleague to colleague,

Euler's equation ended up on the chalkboard

In front of a young American physicist, Leonard Susskind.

To this day I remember the formula.

The formula was...

And I looked at it, and I said,

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

Susskind retreated to his attic to investigate.

He understood that this ancient formula described

The strong force mathematically,

But beneath the abstract symbols

He had caught a glimpse of something new.

And I fiddled with it, I moneyed 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