Looking at the earth at night

reveals to us just how successful we've been

in harnessing and manipulating energy

and how important it is to our existence.

Energy is vital to us all.

We use it to build the structures that surround and protect us.

We use it to power our transport and light our homes.

And even more crucially, energy is essential for life itself.

Without the energy we get from the food we eat, we'd die.

But what exactly is energy?

And what makes it so useful to us?

In attempting to answer these questions,

scientists would come up with a strange set of laws

that would link together everything, from engines, to humans, to stars.

It turns out that energy, so crucial to our daily lives

also helps us make sense of the entire universe.

This film is the intriguing story

of how we discovered the rules that drive the universe.

It is the story of how we realised

that all forms of energy are destined to degrade and fall apart.

To move from order to disorder.

It's the story of how this amazing process

has been harnessed by the universe

to create everything that we see around us.

Over the course of human history,

we've come up with all sorts of different ways

of extracting energy from our environment.

Everything from picking fruit,

to burning wood, to sailing boats, to waterwheels.

But around 300 years ago, something incredible happened.

Humans developed machines

that were capable of processing extraordinary amounts of energy

to carry out previously unimaginable tasks.

This happened thanks to many people and for many different reasons,

but I'd like to begin this story

with one of the most intriguing characters

in the history of science.

One of the first to attempt to understand energy.

Gottfried Leibniz was a diplomat, scientist, philosopher and genius.

He was forever trying to understand the mechanisms

that made the universe work.

Leibniz like several of his great contemporaries

was absolutely convinced that the world we see around us

is a vast machine designed by a powerful and wise person.

And if you could understand how machines worked,

you could therefore understand how the universe

and the principles that had been used to make the universe worked as well.

So there was an extremely close relationship for Leibniz

between theology and philosophy on the one hand

and engineering and mechanics on the other.

It was this relationship between philosophy and engineering

that in 1676 would lead him to investigate

what at first sight seemed to be a very simple question.

What happens when objects collide?

This is was what Leibniz

and many of his contemporaries were grappling with.

So when these two balls bump into each other,

the movement of one gets transferred to the other.

It's as though something's been passed between them

and this that Leibniz called the living force.

He thought of it as a stuff,

as a real physical substance that gets exchanged during collisions.

Leibniz argued that the world is a living machine

and that inside the machine,

there is a quantity of living force put there by God at the Creation

that will stay the same forever.

So the amount of living force in the world will be conserved.

The puzzle was to define it.

Leibnitz would soon find a simple mathematical way

to describe the living force.

But he would also see something else.

EXPLOSION

He realised that in gunpowder, fire and steam,

his living force was being released in violent and powerful ways.

EXPLOSION

If this could be harnessed,

it could give humankind unimaginable power.

Leibniz would soon become fascinated

with ways of capturing the living force.

A prolific letter writer, Leibniz struck up correspondence

with a young French scientist called Denis Papin.

As they corresponded, Leibniz and Papin realised

the living force released in certain situations

could indeed be harnessed.

Heat could be converted in to some form of useful action.

But how far could this idea be taken?

Papin was in no doubt.

This is an extract from his letter to Leibniz...

"I can assure you that the more I go forward,

"the more I find reason to think highly of this invention,

"which in theory, may augment the powers of man to infinity.

"But in practice, I believe I can say without exaggeration,

"that one man by this means

"will be able to do as much as 100 others can do without it."

Now, you might expect me at this point to tell you

that Leibniz and Papin changed the world forever.

Well, they hadn't.

Their ideas had been profound and far reaching, yes,

but they hadn't really moved things forward.

For that, you need something much more tangible.

You need innovation, industry.

You need countless skilled workers and craftsmen

who are going to apply these ideas,

to experiment with them in novel and new ways.

Well, in the century that followed Leibniz and Papin,

this would take place in the most dramatic way imaginable.

150 years after Leibniz and Papin's discussions,

the living force had been harnessed in spectacular ways.

The machines they dreamed of had become a reality.

Steam engines were now the cutting edge of 19th century technology.

If you look at steps in civilisation,

then one great step was the steam engine, because it replaced muscle,

animal muscle, including our muscle, by steam power.

And the steam power was effectively limitless

and hugely important to doing almost unimaginable things.

But steam technology would do more than just transform human society.

It would uncover the truth about what Leibniz had called

the living force and reveal new insights

about the workings of our universe.

This is Crossness in south-east London.

It's an incredible industrial cathedral,

home to some of the most impressive Victorian steam engines ever built.

Constructed in 1854, Crossness houses four huge engines

that once required 5,000 tonnes of coal each year

to drive their 47-tonne beams.

Everything about this place seems to have been built to impress.

From the lavish ironwork -

the grand pillars like something out of a Greek or Roman temple.

It's the kind of effort you'd think would be lavished

on a luxury ocean liner for the rich and famous.

And yet this place was built to process sewage.

Although only a few workers and engineers would see inside it,

steam had become

such a vital part of Britain's power and economic prosperity

that it was afforded almost religious respect.

But for all the great success and immense power

that engines were bestowing on their creators

there was still a great deal of confusion and mystery

surrounding exactly how and why they worked.

In particular questions like, "How efficient could they be made?"

"Were there limits to their power?"

Ultimately, people wanted to know

just what might it be possible to achieve with steam.

The reason these questions persisted was simple almost no-one

had understood the fundamental nature of the steam engine.

Very few were aware of the cosmic principle which underpinned it.

These great lumbering machines we think of as the early steam engines

actually were the seed of understanding

of everything that goes on in the universe.

As unlikely as it sounds,

steam engines held within them the secrets of the cosmos.

This is the Chateau de Vincennes in Paris.

Events here would motivate one man's journey to uncover the cosmic truth

about the steam engine, and help to create a new science.

The science of heat and motion. Thermo-dynamics.

In March 1814, during the Napoleonic wars,

when Napoleon and his armies where fighting elsewhere,

Paris itself came under sustained attack

from the combined forces of Russia, Prussia and Austria.

Citizens were deployed around key locations to protect them.

This chateau was being defended by a group of inexperienced students

who were forced to retreat under sustained artillery fire.

One of them was a brilliant young scientist and soldier.

His name was Nicolas Leonard Sadi Carnot

and the humiliation he felt personally

would drive him and motivate him

to uncover a profound insight into how all engines work.

Carnot came from a highly-respected military family.

After the French defeat here and elsewhere around Europe,

he became determined to reclaim French pride.

What really bothered Carnot was the technological superiority

that France's enemies seemed to possess.

And Britain, in particular, had this huge advantage

both militarily and economically

because of its mastery of steam power.

So Carnot vowed to really understand how steam engines work

and use that knowledge for the benefit of France.

He says absolutely explicitly that if you could take away

steam engines from Britain

then the British Empire would collapse.

And he's writing in the wake of French military defeat

and he proposes to analyse,

literally, the source of British power

by analysing the way in which fire and heat engines work.

Living on half-pay with his brother Hippolyte

in a small apartment in Paris,

in 1824 Carnot wrote the now legendary

Reflections On The Motive Power Of Fire.

In just under 60 pages,

he developed and abstracted the fundamental way

in which all heat engines work.

Carnot saw that all heat engines

comprised of a hot source in cooler surroundings.

Now, Carnot believed heat was some kind of substance

that would flow like water from the hot to the cool.

And just like water falling from a height

the flow of heat could be tapped to do useful work.

Carnot's crucial insight

was to show that to make any heat engine more efficient

all you had to do was to increase the difference in temperature

between the heat source and cooler surroundings.

This idea has guided engineers for 200 years.

Ultimately, a car engine is more efficient than a steam engine

because it runs at a much hotter temperature.

Jet engines are more efficient still

thanks to the incredible temperatures they can run at.

Carnot had revealed

that heat engines weren't just a clever invention.

They were tapping into a deeper property of nature.

They were exploiting the flow of energy

between hot and cold.

Carnot had glimpsed the true nature of heat engines and, in the process,

begun a new branch of science.

But he would never see the impact his idea would have on the world.

In 1832, a cholera epidemic spread through Paris.

It was so severe, it would kill almost 19,000 people.

Now, back then, there was no real scientific understanding

of how the disease spread, so it must have been terrifying.

Carnot undaunted by the risks,

decided to study and document the spread of the disease.

But, unfortunately, he contracted it himself and was dead a day later.

He was just 36 years old.

A lot of his precious scientific papers were burned

to stop the spread of the contagion

and his ideas fell into temporary obscurity.

It seems the world wasn't quite ready for Carnot.

Carnot had made the first great contribution

to the science of thermodynamics.

But as the 19th century progressed the study of heat, motion and energy

began to grip the wider scientific community.

Soon, it was realised these ideas could do much more

than simply explain how heat engines worked.

Just as Leibniz had suspected with his notion of living force,

these ideas were applicable on a much grander scale.

By the mid 19th century,

scientists and engineers had worked out very precisely

how different forms of energy relate to each other.