Playing with Fire From Ice to Fire: The Incredible Science of Temperature

Everything around us exists somewhere on a vast scale,

from cold to hot.

The tiniest insects, all of us, the Earth, the stars,

even the universe itself, everything has a temperature.

I'm Dr Helen Czerski.

In this series, I'm going to unlock temperature's deepest mysteries.

Across three programmes,

I'm going to explore the extremes of the temperature scale...

..from some of the coldest temperatures to the very hottest,

and everything in between.

I'm a physicist, so my treasure map is woven from the fundamental

physical laws of the universe,

and temperature is an essential part of that.

It's the hidden energy contained within matter.

And the way that energy endlessly shifts and flows

is the architect that has shaped our planet.

And the universe.

It's not often that I get up at 5am to watch a pond,

but this one's worth watching.

In this programme, I'll be exploring the incredible science of heat.

What temperatures does it reach on the inside there?

-100 million degrees.

-That's just a ludicrous number!

I'll reveal how our ability to harness heat lies behind some of

humanity's greatest achievements,

from the molten metals that gave us tools...

..to the searing energy of plasmas

that offer the promise of almost unlimited power.

Temperature is in every single story that nature has to tell, and in this

series, I'll be exploring why, what temperature means,

how it works, and just how deep its influence

on our lives and our world really is.

I love steam engines because they're so raw.

You can see where the energy's coming from and where it's going to.

This one's called Braveheart. It was built in 1951

and still going strong.

The steam out there is amazing!

WHISTLE BLOWS

Steam locomotives like Braveheart are a symbol of an age

when it seemed that our ability to harness heat knew no bounds...

..allowing us to drive our trains, run our factories,

and propel our ships.

The age of steam was about building machines to get stuff done,

but to get the engineering right, people had to ask previously

unanswered questions about what heat really was.

And with the answers came an understanding of just how much heat

could do for us.

We're going past the modern world and the houses and computers and

technology that we take for granted,

all of which require a control of heat.

All of that is built on the foundation of the Industrial

Revolution, things like this engine.

Right at the heart of the engine is the rawest bit and the first form

of heat humans learned to control, and that is fire.

In all of human history, there can be few moments more

significant than the discovery of fire.

The spark is so brief,

such a tiny flash of light, and yet the start of such a huge story.

A long time ago, perhaps around a million years, our ancestors

could sit around a fire for the first time when they chose.

And I'm sure that fire was just as mesmerising for them as it is for

us, this flood of heat and light conjured up at will.

You don't need any understanding of physics to appreciate this,

or to be fascinated by it.

It must have seemed amazing that something as apparently dead

and inert as wood could suddenly change into flame,

releasing so much heat.

Our ancestors couldn't have known it,

but mastering that spark opened the door

to a whole new way of being human.

The ability to create fire provided our ancestors with warmth,

protection, and a means of cooking food.

But for all the usefulness of fire,

unlocking its full potential was still a long way off.

For almost all of human history, we had no idea what heat could do

for us because we just didn't know what it really was.

It wasn't that long ago that people thought heat was a substance

in its own right.

A weightless fluid called caloric

that could flow in and out of solids and liquids,

altering their temperature.

Not until the early 20th century did we discover that heat isn't

a substance, but something else entirely.

To show you,

I'm going to heat up my favourite snack.

What I've got here are popcorn kernels.

And each one is the seed of a plant.

But inside them, they've got a little bit of water.

What's happening is that energy is flowing into the kernels.

And the water molecules, as they heat up,

are moving faster and faster.

That water, the liquid water,

is being pulled apart and so the liquid is becoming a gas

and the popcorn kernels are filling up with steam.

Every single one of these kernels is now a very small pressure cooker.

And eventually...

POPPING

Ooh!

And the pressure bursts the kernel out of shell.

The whole kernel turns inside out, and then you get popcorn.

That is flying everywhere!

And the important point here

is that the heat energy is all about movement.

As atoms and molecules take energy on board, they start to speed up.

The faster the movement, the hotter the substance is.

And the crucial point about all this movement or energy

is its extraordinary ability to transform things.

Even matter itself.

I've come to Alderley Edge in Cheshire.

-This goes around your back...

-So this is just going round there.

-That's right.

-..to see some early evidence of how we learnt to take

advantage of this hidden ability of heat.

Dating back some 4,000 years,

the Alderley Edge mines are some of the oldest in Britain.

Mind the steps. It's a bit slippery in places.

Nigel Dibben from the Derbyshire Caving Club

has offered to take me inside.

Is your light on? There's mine on.

Follow me in.

Just mind your head here. It gets a bit low.

-So this is all man made?

-This is all man made.

You can see some of these pick marks on the wall along here.

I tell you what,

this is not the easiest commute for anyone coming to work down here.

Whatever's down the other end must have been pretty valuable.

After a few more minutes, we come to the heart of the mine.

This is what I really want you to have a look at.

That's spectacular!

That is a fabulous colour, isn't it?

It's such an unexpected colour to find in the gloom, isn't it?

And have a look down here as well.

There's a bit more down the bottom, this shaft here.

-Oh, yeah, a huge, great big stripe of it.

-Down the bottom there.

This mineral is chrysocolla, and it's dissolved into water

that's dripping through the rock here, and then it's been redeposited

in this beautiful sheet.

And the spectacular colour is a hint

as to why those early miners came down here.

The pure version of the mineral that the miners were after is this.

And this is malachite.

When I was a kid, I was fascinated by semi-precious gems,

and it was malachite that got me started on that.

I couldn't leave it alone.

It's a beautiful, deep, rich, green colour.

And it's not just me - it's been

used by humans for millennia as a green

pigment because of the way it looks.

But malachite doesn't just have style, it has substance.

Because when you take malachite and heat it up,

you start to transform it.

Malachite is a mineral,

which means it's made of lots of different types of atoms,

all bound up together, but you can't see what's in there.

But once you heat it up, you can drive off the smaller atoms,

the carbon and then the oxygen,

and then what you have left is the element that's at the heart of this,

which is this.

It's copper, the first metal to be smelted from its ore.

Copper is strong and malleable and shiny.

It's completely different from the mineral it came from.

And the clear implication is that heat can change things.

And so it wasn't far to the next step of the imagination.

Because if heat can change this into this, what else can it do?

The answer came when people realised that just as heat can turn rock into

metal, so with a little know-how

it could also be used to alter the metal itself.

In the year 793, Anglo-Saxon Britain came under attack...

..when Viking raiders first landed on the Northumbrian coast.

While the Vikings' reputation as fearsome warriors is

well-documented, what's less well-known is their skilful

craft work, especially with metals.

A skill calling not only for a sense of design, but also a sophisticated

understanding of temperature.

To find out more, I've come to meet historical blacksmith Jason Green.

-And how hot will it get in there?

-Around 1,300 degrees.

Under Jason's watchful eye, I'm going to attempt to make a Viking

dagger, a process that starts with heating up a small piece of steel,

before hammering it into shape.

-That's it.

-OK.

Not setting the grass on fire.

-The only way you're going to learn is to do it.

-By doing it.

Right, well, there is going to be a lot of doing, isn't there?

You can feel as it cools, it suddenly stops going anywhere!

-Yeah, it starts getting harder.

-Yeah.

Blow by blow, my dagger starts to take shape.

Both externally and, more importantly, deep inside the metal.

We don't tend to think of metals

as being crystals, but in fact they are.

That means their atoms are arranged

into a highly regular, repeating pattern.

What's happening as we heat is that the crystals are changing because

the heat makes them slightly more mobile,

it allows you to push atoms around.

As I hammer away, each impact rearranges the atoms inside...

..creating tiny knots within the crystalline structure.

As these knots accumulate,

it becomes harder for the atoms to move over each other.

And this helps make the metal stronger.

And so all of this raw action,

this hammering, thumping, and the heating,

is changing things at a very tiny scale inside the metal itself.

And that's what gives iron and steel

its strength and that's why it's so useful.

But a blade's strength doesn't come from hammering alone.

It also requires clever manipulation of its temperature.

The knife's now back in the forge, glowing cherry red,

and that means it's about 800 degrees C.

And that matters because the crystal structure at this temperature,

this is the one we want. It's very strong, it's really useful.

If I let it cool down slowly,

it will change back to the room temperature structure.

And so in order to keep this crystal structure, so it's a useful knife,

this is what we do...

..which is very satisfying!

As the hot metal is plunged into the water,

its temperature plummets in just a few seconds.

By cooling it so quickly,

the atoms haven't got time to shift into the shape that they want to

have, and so they're stuck, locked in with a very strong structure.

Finally, one last round of heating to remove any remaining brittleness.

There we go. One finished fighting blade.

I'm so impressed that with such simple tools

you can make something so useful.

That's brilliant. Thank you very much.

By turning wood into flames,

rock into metal,

and soft metal into hard,

our ancestors' growing understanding that heat could transform matter

altered the course of human civilisation.

But for thousands of years,

this knowledge was only applied to solids.

The next leap forward would see people using heat to exploit another

form of matter, one with astonishing potential.

Gas.

But to understand how gases respond to heat,

we first need to take a step back and look at what gases are

and how they behave.

Humans love a bit of spectacle,

anything with colour and music and fun.

But the stereotype of a scientific experiment is almost exactly the

opposite, a dusty basement with someone who hasn't seen daylight for

a week, writing down measurements that no one will ever read.

But there have been exceptions. There have been experiments

set up with the theatrical drama to match their scientific significance.

And one of my favourites happened in 1654, and it was all organised

by a man called Otto von Guericke.

The aim of the experiment was to demonstrate a very specific and

extraordinary property of gases.

And heading up the guest list was none other than the Holy

Roman Emperor, Ferdinand III.

When the Emperor and his guests were all seated,

it was time for the star of the show, and that was two metal

hemispheres like these, with flat inner surfaces.

Von Guericke placed the two hemispheres together and then

started to take out the air from the inside.

This created a vacuum which held the two halves of the sphere together.

And it was what Von Guericke

did next that made everyone pay attention.

He set up a team of horses on either side...

..put the sphere in between,

and gave the command for the horses to pull.

To show you what happened next,

we're going to attach our sphere to the modern equivalent

of Von Guericke's horses - a pair of 4X4s.

Stand by.

I'm actually quite nervous.

Three, two, one.

Go!

So the tension's out of the rope.

So now a little bit on the accelerator, just up to 1,000.

Feel it taking the strain.

OK. Keep going up to 1,300.

Can feel it in the car.

OK, up to 1,600.

The engine is not happy!

I think we might have established the sphere really works.

OK, let's pause there, so stop.

It's impressive. It really is impressive.

Just as our sphere stood up to a pair of 4X4s,

so Von Guericke's was also able

to resist the pull of two sets of horses.

To Von Guericke, it was the proof of something he'd long suspected,

that gases like air exert an incredibly strong force.

All the air around me looks completely calm, but it isn't.

It's a gigantic, three-dimensional game of molecular bumper cars.

Even in one cubic centimetre of air,

there are nearly 30 million trillion air molecules and they're bumping

into each other all the time.

Just one molecule will collide several billion times every second.

And every single collision gives a little bit of a push, and so if they

bump into us, they push, and that's what air pressure is.

And the question then is -

if I'm being pushed on by this pressure all the time,

why aren't I being squeezed?

And the answer is that every time I breathe in,

I'm taking air molecules into my lungs.

And they're pushing out on the walls of my lungs and because the

inward push and the outward push exactly balance, I don't notice.

And that was why Von Guericke needed to generate a vacuum.

You can only see how strong air pressure really is when you take

away the push from the other side. At the end of the demonstration,

all they needed to do was let a little bit of air back in and it was

almost as though the pressure hadn't been there.

The ability of molecules to exert pressure is one of the most

fundamental properties of not just air, but all gases.

But Von Guericke's discovery also raised an important question.

If cold air molecules could have such a powerful effect,

what might be achieved if those same molecules were heated up?

I've travelled to the north of England to meet a bunch

of enthusiasts with a head for heights.

Harry Stringer is from the Pennine Region Balloon Association.

He's been flying hot air balloons for over 25 years.

-So, where are we going today?

-Well, we'll clear the tree tops here...

-That sounds like a good start!

-Yeah, and then we'll go up

-to about 1,000 feet.

-OK.

-Hands on.

Hands on.

The very first hot-air balloon, launched in 1783, was the brainchild

of two brothers called Joseph and Etienne Montgolfier.

Oh, we're free.

OK, we're way.

One story goes that Joseph had been staring into his fireplace one

evening, when he had the idea of filling a paper bag with hot-air.

On letting the bag go, he observed that it began to rise.

And this encouraged the brothers to repeat the experiment, but this time

with a much larger, purpose-built balloon.

Until the 1780s,

the sky was just a place for clouds and birds, and humans certainly

didn't go up there. But when the first balloons came along,

people could look up and wonder, what's it like up there?

And the problem, if you were curious about the sky,

was that gravity was holding you down to the ground.

But the really ingenious thing about hot-air balloons is how they use

heat, together with the force of gravity itself, to get around this.

The mechanism of these is beautifully simple.

There's a bag above me, filled with hot-air.

What the burner does is it allows the balloonist to play around with

the density of the air by controlling its temperature.

And as the air inside there is heated up,

and it could get up to 100 Celsius, it expands.

As the air expands, its individual molecules push outwards,

making the air inside the balloon less dense.

And that's where gravity comes into play.

Gravity is pulling everything,

everything I can see, down to the ground.

But because the air inside the balloon is less dense than the air

around it, everything around us is being pulled down more,

so it's squeezing the less dense balloon upwards and so balloonists

are floating on top of the denser air around them.

But temperature doesn't just enable a balloon to rise,

it also controls how it falls.

So, how do you make us come down?

We'll have a parachute vent. It's massive. You can see it.

I could pull this red line...

-Yeah.

-..and it will open the valve and then I just close it and the

gulp of hot air loss will cause the balloon to descend.

-We are safe.

-Can we stand up now?

-We can. We can.

The discovery that heating up air could make it expand enough to lift

people into the skies was a milestone in human innovation.

And it wasn't long before we began to put that very same heat energy

to a much more practical purpose.

It was something that emerged from a very 18th-century problem.

300 years ago, mine owners in Britain

were facing a serious crisis.

Since many ore deposits sat well below the water table,

they were finding that their mines could go only as deep

as the drainage technology at the time allowed,

resulting in many mines going out of business.

What was needed was a way to haul all that water up to the surface,

so that the miners could get to the ore below.

And in 1712, an ironmonger called Thomas Newcomen hit upon the answer,

with the world's first commercial steam engine.

And it worked by harnessing

the immense energy contained within hot steam.

The principle behind Newcomen's engine is exactly the same one that

Otto von Guericke had demonstrated.

And that is how hard air pressure can push,

especially when there's a vacuum on the other side.

I've got a plastic bottle here

with some water in the bottom, and I'm going to put it in the microwave

to heat the water up.

What's happening inside the microwave is that the water

molecules are being given energy and they're not just heating up,

but some of them are turning into a gas, into steam.

And that steam is starting to fill up the bottle.

And it's what happens next that's important.

Tip it into this water here.

SHE LAUGHS

And you can see that what happens is that the bottle has been crushed

and it's now full of water.

And the reason for that is that as it filled up with steam,

the air was pushed out.

And then when I cooled the steam down,

it condensed from a gas back into a liquid,

which takes up much less space.

So there's a partial vacuum left in bottle and so there was all the air

pressure pushing in, nothing pushing back, and the bottle was crushed.

This is the principle that Newcomen used to drive his engine.

At the heart of Newcomen's engine lay a large metal cylinder,

housing a piston and filled with hot steam.

Cooling this steam with water simultaneously created a vacuum

and caused the weight of the atmosphere to push down on the

piston, driving the engine.

The cylinder was then refilled with hot steam and the cycle repeated.

Soon, Newcomen's steam engines were popping up all over Britain.

Each one a symbol of heat's ability to perform useful work.

But Newcomen's design had one major weakness.

The brilliant thing about steam engines is that they convert heat

energy, this type of energy you can't really see, into mechanical

work, the sort of thing that can push pistons

and turn wheels and do practical things.

And Newcomen's engine worked, but it was spectacularly inefficient -

of all the energy and the coal that went in,

only one or 2% was converted into useful, mechanical work.

The mystery was why.

Where was all that heat energy going?

And what could be done to retrieve it?

To discover the answer, I've come to Coldharbour Mill in Devon.

Originally built in 1797,

it's one of the oldest steam-powered woollen mills left in Britain.

OK, so we won't kill anybody with the other end...

John Jasper runs the mill's giant steam engine.

-Like this.

-Like that?

-You are a natural. Right.

-OK.

And the first thing any steam engine needs, of course, is steam.

And today, John's invited me to help him make some.

-So, tell me about these boilers.

-This is a Lancashire boiler.

It holds 20,000 gallons of water.

Above that water level, you have steam.

-And you get a bit of steam...

-Great.

So it's basically a sort of steam kettle.

So these bits are the heating elements.

Effectively you're shovelling fire into the heating elements...

-That's it.

-And then all of this is the kettle, which is full of water.

-That's right.

-But instead of coming out of the spout...

-Yes.

-..it goes to a steam engine.

It just takes a little longer to get to the boil.

-I'd better do some more shovelling then!

-Yeah.

The engine here is a descendant of a type that was built to address the

problem of Newcomen's lost energy.

It was designed by a Scottish instrument maker called James Watt.

Watt had recently become familiar with a new theory of heat.

Creating steam is all about putting heat energy into water,

but there's this strange observation,

which is that as you start to heat water up,

you see the thermometer rise and it goes up and up and up,

and then it gets to 100 degrees and it won't go any further.

So you can be pumping in huge amounts of heat energy and yet

the thermometer isn't moving.

And that's because once water reaches its boiling point,

all that heat energy is being used up to turn the water into steam.

And that led to the idea that there are two forms of heat.

There's the sort which causes the thermometer to rise,

but there's another type and that is the energy needed just to turn the

water from a liquid into a gas at the same temperature.

And that heat is called latent heat.

The amount of latent heat needed to turn water from a liquid

into a gas is enormous.

And the reason that all this matters for steam engines is that steam

is expensive in terms of energy.

And when you've got it, you certainly don't want to waste it.

It was this revelation that creating steam requires huge amounts

of latent heat that was one of the main reasons why Newcomen's engine

was so wasteful.

At the heart of every steam engine, there's a piston.

That's where the hot gas molecules

are pushing to create mechanical work.

The problem with Newcomen's engine was that in order to reset,

the water needed to be condensed, cooled down.

And that happened inside the piston,

so the metal itself had to be cooled down as well.

And then you needed to use more steam energy to heat it up again

to create the next stroke.

In order to conserve all that valuable steam,

Watt came up with an ingenious invention.

Watt's solution was a condenser and this is it.

So instead of having the condensation happening inside the

piston, the steam was vented out to a separate chamber and that was

where the condensation occurred.

And the reason it was a brilliant solution was that the hot parts of

the engine stayed hot and the cool parts of the engine stayed cool.

And much less heat was wasted.

Watt's great insight

that the more an engine can conserve heat, the more efficient it will be,

was a watershed moment in the history of steam power.

Other improvements followed,

such as the introduction of steam at high pressure

to generate even greater force.

These innovations ushered in a mechanical revolution...

..founded upon the energy of hot gas molecules.

But as our population grew and our coal supplies dwindled,

so we began to turn elsewhere for our energy.

And in some places,

that's involved tapping into a different source of heat...

..one that's responsible for some of the most violent natural phenomena

on the planet.

Just a short distance from Reykjavik

lies one of Iceland's top tourist attractions...

..an outdoor health spa known as the Blue Lagoon.

This is the real attraction around here.

Lovely warm water at 38 Celsius.

And full of minerals which are apparently very good for you.

So on a day like today and in a country with a reputation for being

chilly, this is clearly the perfect place to relax.

But despite appearances, this is no natural beauty spot.

In fact, the Blue Lagoon is entirely man-made...

..fed by hot water from the nearby Svartsengi geothermal power station.

Every day, Svartsengi produces enough electricity

for around 130,000 homes.

And the source of all that power is the same heat energy that created

Iceland in the first place.

Down below my feet, the Earth is far hotter than it is up here.

There's a huge amount of heat energy available, and anything that takes

advantage of it is known as a geothermal power source,

literally heat from the Earth.

The deeper you go, the hotter it gets.

To tap into that heat, Svartsengi sits above 13 boreholes,

stretching two kilometres into the rock below.

The basic premise here is that a mixture of hot water and steam is

pumped up from deep down and the steam is separated out and sent

through a turbine that generates 75 megawatts of electricity.

That goes into the grid.

And then the same steam comes back around and reheats the water and

that supplies domestic hot water

for about 20,000 homes on this peninsula.

For the engineers around here,

the hot water beneath their feet is just one massive treasure trove.

Utilising the heat of the planet itself has allowed us to take steam

power to a new level.

But today, scientists are attempting

to harness another even hotter form of energy...

..derived from a strange type of matter that here on Earth makes the

occasional spectacular appearance.

Inside the University of Manchester's high-voltage lab,

a team of researchers is getting ready to recreate one of the most

awesome natural phenomena on the planet.

Lightning.

This beast of a device is an impulse generator,

and this one is capable of generating two million volts

between the bottom and the top.

And here's how it works.

Normally, when you get a voltage, electric charge will flow.

But here, each of these red things is a capacitor, and so the electric

charge can't go anywhere. It's stored on the plates.

And that means that energy is building up.

And it's this point here that's the important bit

because when the switch over there is pressed,

all of that charge is going to get dumped through that point in around

a millionth of a second.

Here to show me what that means in practice is Dr Vidy Peesapati.

So, what we're going to do right now is make sure that no one else can

walk in, so if you want to press the black button...

-That one?

-Yes. That's the one.

-SIREN

Now it's ready. Now we basically have to set our voltage...

Under Vidy's supervision, I'm going to trigger a lightning strike...

-You want to press F4 on the keyboard.

-That one?

-Yeah.

..which we'll also capture using a high-speed camera.

-Now it's charging.

-So we can see the voltage going up here.

Absolutely. So, it takes around 60 seconds for the entire kit to be

-charged up.

-When this gets to the end, we'll be ready to go.

We'll let the siren go, telling us there's going to be a flashover.

And it automatically triggers the first stage.

60 seconds later, and the generator is ready to fire.

So, when I hear the siren... SIREN

CRACK

-That is an echo and a half, isn't it? Wow!

-It is very loud.

And that is basically a sonic boom.

-It's like a giant electric whip crack.

-It is, absolutely.

But it's only when you play back the slow motion video that you begin to

see exactly what lightning really is...

CRACK

..a superheated channel of air, with so much energy

that it's become an entirely different form of matter.

We're used to the idea of three states of matter - solid, liquid and

gas, but what we've got here is a fourth,

because the source of all of that light is a plasma.

From the sun's fiery surface

to the clouds of interstellar gas known as nebulae,

plasmas are found across our solar system and beyond.

And it's this superheated form of matter that scientists are hoping

will enable them to unlock a brand-new type of energy...

..by manipulating one of its strangest properties.

This is a Crookes tube, named after the British physicist William

Crookes, who was one of the people to design and use it in the 1870s.

This was the piece of equipment that opened the door to plasma physics.

It's a sealed glass vessel and it's got two electrodes -

the negative one here, and a positive one here.

And on the inside, there's just a little bit of gas

at very low pressure.

And when Crookes turned up the voltage, this is what he saw.

So you can see that this is quite noisy, but there's

a green glow down this end of the tube.

Crookes called this eerie light radiant matter.

Crookes didn't understand what was going on, but we do.

And it's this.

When high voltage is applied across the two electrodes,

it frees up negatively charged electrons from the gas inside

that are then accelerated towards the flat end of the tube.

As they strike the glass, they excite the molecules on the surface,

causing them to give off light.

And it's the free movement of electrons like this that is the

defining characteristic of a plasma.

And which gives it one of its most distinctive properties.

I've got a magnet here, just a small one.

When I bring the magnet in here, you can see that beam of electrons is

being pushed to one side or the other.

It's being deflected by the magnet.

So I can actually control what's going on inside a plasma,

using electric and magnetic fields,

and that is what makes a plasma really interesting.

It's this in-built electromagnetism that's opened up the possibility of

one day channelling the enormous energy inside super hot plasma

and putting it to use...

..by exploiting here on Earth a different source of energy,

the very same type of energy that powers our sun.

Inside a vast hangar at the Culham Science Centre near Oxford

sits a machine so complex

it contains well over 100,000 separate parts.

This is a fusion reactor.

Its job is to channel streams of extremely hot plasma and use them

to manipulate matter at the atomic scale.

The aim is to unleash the power of the atom itself and reach

the holy grail of physics.

Nuclear fusion.

There's no way anyone would be this close to a fusion reactor if it was

running because it throws off enormous numbers of neutrons

which can do a lot of damage and that's why everything

around me here is surrounded in concrete, three metres thick.

Just at the moment, they're in a maintenance phase,

so we can get a little bit closer.

Here to give me a tour of the reactor is Dr Joanne Flanagan.

What exactly is it that all of this kit is trying to do?

We are essentially trying to create an artificial star.

Actually, we do, we create artificial stars.

We take hydrogen gas and heat it up to very high temperatures, where it

becomes ionised, it becomes a plasma.

What sort of temperatures does it reach on the inside there?

We routinely reach temperatures of about 100 million degrees,

which is about ten times hotter than the centre of the sun.

That's just a ludicrous number!

It's a number you can't even get your head around.

It's a crazy hot temperature. We need such high temperatures

because hydrogen nuclei repel each other.

To get them to stick, we need them to collide at high speed.

And that's fundamentally what temperature is.

High-speed particles.

Right, how do you make any thing that hot?

The first step is to run a current through the plasma,

like an old-style electrical light bulb.

And that gets us to a few tens of millions of degrees.

But then we need to pull additional heating systems online to boost us

-the rest of the way.

-So you're just throwing everything at it

to get energy into it.

Since there's no material on Earth that can withstand temperatures

of 100 million degrees, the scientists instead

contain the plasma by using its electromagnetism.

At the heart of the reactor lies a giant metal doughnut called a

tokamak that uses a powerful magnetic field to keep the plasma

confined long enough for the collisions

that cause fusion to happen.

To show me how it works, Jo takes me inside a full-sized mock-up.

The plasma would be in the space that we're in here and the magnetic

fields, where do they go?

The magnetic fields curve around in the shape of the vessel.

They have a sort of onion-like structure and they hold the plasma

to the shape of this vessel,

about five centimetres away from the edges.

And the plasma is then here in the middle, is it?

Right where you are.

As all this plasma is heated up,

so the hydrogen nuclei inside accelerate,

getting faster and faster until they reach a speed where they can get

close enough to fuse.

So, once you've had a successful collision, what happens next?

Then you have a very fast neutron that comes out of that reaction.

So it's the neutrons that are carrying the energy out is their

-speed.

-Yes.

-That will go flying off and it will heat something up.

Yeah.

The idea is that you would have a lithium blanket surrounding the

entire device which would capture

those neutrons and heat up, and you'd have heat

exchanger pipes that run through that blanket that would then heat

water to drive the steam turbines.

But if we're ever to master the searing temperatures of fusion,

then there's one major obstacle that still has to be overcome.

Because for now at least, we've yet to find a way of getting

more energy out from a fusion reactor...

..than we put in.

Fusion is such an enticing idea - there's no shortage of fuel,

there's almost no pollution, it would solve so many problems.

But impressive as all of this is,

it might not be the technology that crosses the line first.

Several years ago,

an idea came along that it might be possible to generate fusion in a

tiny bubble of gas inside a liquid.

Theory had predicted that by collapsing a bubble of gas

incredibly quickly, it might be possible to get the molecules inside

to heat up enough for fusion to occur.

But it couldn't be made to work in practice.

And the idea was discredited. It was basically thrown away.

But some new science has been done

and bubbles are back in the world of fusion.

Just up the road from the reactor

is one of the companies behind this technique.

And I've come to meet its co-founder, Dr Nick Hawker.

So, Nick, what's your solution to the problem of fusion?

The idea is instead of trying to hold the plasma together with

magnetic fields, you use an implosion of some kind

to both compress and heat the plasma.

And how do you set that up? How does that work in practice?

This is a plastic target.

In the middle is a little spherical cavity.

And then what we have is a high velocity projectile.

That comes in and it hits this side here.

That creates an enormously high pressure on this surface.

So the idea is that when you compress the gas in there,

because you do it so quickly, it heats up,

and that's where the energy comes from?

That's right, yeah.

The plasma exists for a few hundred nanoseconds.

To heat the pocket of gas inside the target,

Nick and his team hit it with a projectile,

travelling at almost 30,000 kilometres per hour.

SIREN

-Is everything armed?

-Everything is now armed.

Three, two, one, fire.

By filming the moment of impact with high-speed cameras set to record

at a billion frames per second,

the team have been able to capture the precise moment the plasma forms.

On the left of the screen is the view of the gas pocket from side on.

And on the right, the view from behind.

So you can see the shock -

this dark stuff here is the shock coming through.

That's the first shock which goes into the gas and even that is enough

to heat it a lot and it starts to turn...

Well, it turns into a plasma and starts to glow.

As the projectile strikes the target,

the gas collapses in on itself,

causing the molecules inside to heat up so violently that they emit

a light, briefly turning into a plasma.

It's beautiful, isn't it? You get this bright light

and it's a circle and then it becomes a ring

and the centre of it goes dark.

It's a very pretty way of doing it, isn't it?

This beautiful circle that appears out of nowhere.

And what sort of temperatures are reached in here?

Average temperature is something like

in the tens of thousands of Kelvin.

So this isn't hot enough for fusion, but you can...

If you hit it faster, can you reach the temperatures you need?

Yes, it's all about the velocity.

We think we need to go two or three times faster than this gun.

So we're looking at electromagnetically launching

a projectile to try to get to higher and higher velocities

and then to the temperatures we need for fusion.

But potentially you can get a huge amount of energy out of it,

-if it works.

-Well, a cavity this size,

if you burned all the fuel in there,

that would release about the same amount of energy as a barrel of oil,

so it's an enormous amount of energy.

I think it's very likely that fusion energy,

this technology made possible by fantastically high temperatures,

will form a significant power source in the future of our civilisation,

but the exciting thing about it is

that we don't know which path it's going to take.

This is the adventure of science and engineering.

Even though there's not yet one clear solution,

when it comes to fusion, the game is afoot.

In this series,

we've learned how nothing would exist without temperature.

From the searing heat of the early Earth

to the cooling that transformed it and allowed life to flourish,

temperature has been fundamental to the story of our planet.

But it has also driven our story.

As our understanding of temperature has grown,

so we've learned how to use it...

..to create new materials,

drive our machines...

..and to advance our technology.

Temperature is such a big idea, encapsulated in just one number.

As a physicist, it's the first thing I measure.

And as a human, it's the first thing I feel.

And yet our direct experience

of temperature is limited to a really narrow range.

But once you learn about what's beyond that, the extreme

heat, the extreme cold, and all the subtleties in between,

it's clear that the possibilities that temperature offers are endless.