The Bridge

Hello. My name is Stephen Hawking.

Physicist, cosmologist, and something of a dreamer.

Although I cannot move and I have to speak through a computer...

In my mind, I am free.

Free to explore the most profound mysteries of the cosmos.

Such as: Why is the universe the way it is?

Why does it follow rules and laws?

Why is there order instead of chaos?

Finding out leads us to the very deepest of secrets.

To the one principle that's at the heart of everything in the cosmos.

Check it out.

The recent excitement over the Higgs boson...

is just the latest step in a remarkable scientific voyage.

One that has taken us into orbit... and beyond.

I've devoted my life to this quest.

Because I believe it would lead us to the secrets of the universe itself.

This is the search for one I call the Grand Design:

The key to the cosmos.

The good news is: I think we found it.

Almost.

Getting here has been quite a journey.

It began one night 350 years ago...

in the small English town of Cambridge.

The year was 1665.

Death stalked the land as England fell under the plague's dark spell.

A student called Isaac Newton, fled town to escape the threat.

We fortunately got away, because Newton was a radical thinker...

who dare to see the universe in a completely new way.

Newton took the first steps in the search...

for the Grand Design by looking for the mysterious laws that govern nature.

He asked, "Why the things move?" "Why do they stop?"

and most famously: "Why do they fall to earth?"

You might not think answering such simple questions would change the world...

but it did.

Because Newton realized there was a force at work deep within the fabric of the universe...

that makes all objects attract one another: the force of gravity.

Gravity works not just on Earth but throughout the cosmos.

And its strength depends on just a couple of fundamental things:

The mass of the objects and their distance apart.

To find his answers, Newton invented a completely new mathematical language, called...

the calculus.

You don't need to know how it works, but it wasn't bad...

for a 23 year old.

Scientists all over the world still use it every day.

Newton's work, made it possible to predict everything...

from the orbits of planets around the stars...

and the precise timing of eclipses...

to the trajectories of raindrops.

Today, we theoretical physicists are still doing the same sorts of things as Newton.

And thankfully, we don't have to worry about the plague.

Although our work may seem complex, it's really quite simple.

We're trying to unravel the hidden mechanism...

that underlies everything.

We can only do it just because we stand on the shoulders of giants.

Scientists who — piece by piece — discovered what makes...

the universe tick.

In the mid-19th century, another of my scientific heroes

would make the next giant leap towards unraveling the key to the cosmos.

Enter James Clerk Maxwell.

A Victorian scientist who was fascinated by light.

In 1861, his deep-found fascination with light led to him inventing color photography.

He was so far ahead of his time, it would be another 30 years...

before the next full-color photograph was taken.

But that was just the beginning of what this remarkable man achieved.

It was when he turned his attention to a different realm of physics...

that everything changed.

This was the strange, almost magical connection...

between magnetism and electricity.

There is nothing complicated about it.

Move a magnet near a wire...

and you will cause electricity to flow through the wire.

Put electricity through a wire, and it would act like a magnet...

and deflect a compass.

So what connected them?

Maxwell's big idea was that magnetism and electricity...

are actually two facets of the same thing:

a wave of energy that was part electrical part magnetic.

He called them: electromagnetic waves.

But then came a surprise.

The mathematics told him these electromagnetic waves...

travel at extraordinary speed: 186,000 miles per second.

The exact same speed that had already been determined to be the speed of light.

This led to a stunning conclusion:

Light is an electromagnetic wave, too.

Maxwell connected electricity, magnetism, and light...

in a series of four equations...

that I consider to be one of the greatest discoveries...

in the history of science.

The equations called Maxwell's laws...

govern everything from the auroras that dance over the north and south poles...

to the modern electrical and communications technology...

that powers the planet.

Virtually every machine in the modern world...

from the computer to a power station to a washing machine...

works to the rules Maxwell reviewed.

Electromagnetism quite literally lights up our planet.

A fitting testament to a great mind.

But light is much more interesting than in Maxwell himself realized.

Although he didn't know it, he had actually uncovered one of the fundamental clues...

of the Grand Design.

That clue is the speed of light itself.

By the late 19th century, it looked like...

some of the great mysteries of the universe would be all wrapped up...

thanks to the groundbreaking discoveries of Newton and Maxwell.

But then, two American physicists stumbled into a completely unexpected discovery.

Albert Michelson and Edward Morley were investigating...

the implications of Maxwell's revelation that light is a form of wave...

traveling at 186,000 miles per second.

They figured that just as water waves, of waves of energy traveling in water...

and sound waves, of wave of energy traveling through air...

so light waves must also travel through... something.

They called this "something": the luminiferous ether.

Michelson and Morley proposed that space...

including the space between the sun and the earth...

Was filled with this mysterious ether.

They believed that sunlight must travel through the ether...

to reach earth.

Since the earth also move around the sun...

They must be traveling through this ether, too.

If so, this movement would cause what they called an 'ether wind'...

to blow over the surface of our planet.

Michelson and Morley believed this ether wind should be detectable here on Earth...

and designed an experiment to measure its effects.

They ran their experiment in a cramped basement.

But to understand the principle behind it...

I thought we'd get them out of the lab and stage it out here on the beach.

Since the earth is constantly orbiting the sun...

then the ether wind would be ever-present...

and would effect the speed of light here, on earth.

If a beam of light was traveling with the ether wind behind it...

the light should move faster.

But if light was traveling in the exact opposite direction,

and so was fighting oncoming ether wind, the speed of light should slow down.

And the difference between those two speeds should be measurable.

But they couldn't detect any difference.

Whichever way the light waves were pointing...

they couldn't find any slowing down or speeding up.

This result was deeply disturbing.

The luminiferous ether could not exist.

What's more, it meant the speed of light must be constant in all directions.

Modern experiments have repeatedly confirmed this discovery.

The speed of light remains fixed regardless of the direction in which it is traveling.

Michelson and Morley were deeply embarrassed...

about having to report they'd been wrong. The ether simply wasn't there.

But the experiment wasn't a total disaster.

It's one of the most important mistakes in the history of science.

Light is a wave that travels through nothing. What a strange idea.

And the discovery that the speed can not be varied, is also very odd.

Luckily, we physicists like strange things...

because they often lead us to breakthroughs.

It would take another of my heroes, Albert Einstein...

to discover the deep truth about the nature of light.

To explain how he did it, let me take you back to my childhood.

My old toy train is a perfect way of illustrating Einstein's profound question...

of what the fixed speed of light meant for the universe.

Einstein started with a simple question:

How fast is something like this little train moving?

Well, this is the 1950s and toy trains weren't all that good.

So let's say it's traveling at about one mile per hour.

But change where you look at it from, you get a different answer.

The earth spins on its axis at around 1,000 miles per hour.

So, seen from up here, my train is actually traveling much faster.

Of course, the earth as a whole is orbiting the sun at 67,000 miles per hour.

And even the sun isn't stationary. It is orbiting the center of the milky way.

And the Milky Way is moving through space, too.

So how fast is the train really moving?

Well, it all depends on where you see it from.

It could be 1 mile per hour, 1,000 miles per hour, 67,000 miles per hour...

Or many times faster.

So what's the problem?

Anyone can understand that speed is relative to your perspective.

But when you remember that unlike the speed of the toy train...

the speed of light is fixed no matter what your perspective...

then that common-sense view of things starts to break down.

To explain why, Einstein imagined a train, too.

Only his was full-sized.

He devised a thought experiment,

asking how something as simple as the lighting of a cigarette lighter...

would look to people with different points of view.

He imagined a man playing with his lighter in the center of a railway car...

being watched by two people at either end.

Both at exactly the same distance from the lighter.

Einstein said the interesting question is not "what" do these two people see,

but "when" do they see it?

Because they're in the same car, or moving together at the same speed,

the two observers see the light at exactly the same time.

But what about someone outside? What does she see?

From her perspective, with the train moving forward...

the beam of light needs to travel a little bit further before it reaches the man at the front.

On the other hand, the light moving towards the man at the back...

has a slightly shorter distance to go, since he is moving towards it.

So, she sees the light reach the man at the back before it hits the chap at the front.

What that means is that an event on a moving train...

takes place simultaneously if you're on the train...

but at different times, when you're standing on the platform.

Stop a moment...

and think about the implication of that.

It means that we can't say if the events really happen at the same time or not.

Reality itself depends on "where" you are.

Scale this up to the universe as a whole, and it gets even weirder.