Frozen Solid 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 first programme,

I'm going to venture to the bottom of the temperature scale.

I'll explore how cold has fashioned the world around us

and why frozen doesn't mean what you might think.

The salt looks like that here.

It would look like that if I took it into a sauna

because it's a frozen solid.

And I'll descend to the very limits of cold

where the everyday laws of physics break down

and a new world of scientific possibility begins.

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.

This is Eldhraun, in the south of Iceland,

and it's a great place to start the story of temperature

because this weird landscape around me -

all these lumps and bumps -

this was sculpted by the interplay between hot and cold.

Just over 230 years ago,

a huge fissure in the ground opened up over there - 25km long -

and a huge amount of lava flooded out

in an event that lasted eight months.

It's thought that over 500 square kilometres

was covered in molten red rock.

When the lava came out of the ground,

it was at about 850 degrees Celsius,

but it met cool air, and heat flowed from hot to cold

because that's the way our universe works.

And, as the lava cooled, it froze, and this landscape is what you get

when the hot innards of the Earth meet cold

and are fixed in a form that will last for millennia.

But the mysterious ability of cold to create solid matter

is something we've only recently uncovered.

DOGS BARK

We've always been familiar with the experience of cold and heat,

but, until recently, we didn't understand what they actually were,

and, as the era of modern science dawned,

that lack of knowledge was becoming a barrier to progress.

I'm here at the Radcliffe Observatory in Oxford

and what it was built to observe is the cosmos.

Back in the 18th century,

this was one of the most foremost centres

of the new science of astronomy.

But, while looking up there,

they discovered they had a problem that started down here.

I'm meeting Amy Creese, who's a meteorological observer.

It's a role that was created here over 200 years ago

to solve a very specific problem caused by temperature.

Early observers made quite meticulous records

of the temperature

and that was because it was important to know

what the temperature was like in order to correct

for something called atmospheric refraction,

which is how much the light from a celestial object bends

as it comes into the Earth's atmosphere,

and that depends quite a lot on temperature.

So, in order to make very accurate measurements

of positions of stars, the observers found

that they needed to measure temperature, as well,

so they kept very good records of that.

So, even those people who were looking up at the cosmos

and thinking grand thoughts about the universe

needed to know about this quite mundane thing down here,

which was the temperature.

And you've got a book there with some of the early recordings on it.

I do. I have a book here from 1776.

It's some of the original recordings from Thomas Hornsby,

who founded this observatory.

And several times a day - he was much more keen than I am -

he came up here and took measurements

of pressure and temperature,

but he also made some quite funny notes in the margins.

For example, on the 26th of January 1776,

he wrote about how the wine in his study had started to freeze

because it had got very cold that day.

Which is a very important thing for a scientist to know about.

Yeah, and I'm glad that he wrote about it!

These are some of the earliest

regular measurements of temperature ever made,

and they were only possible thanks to one of the greatest

scientific innovations of the 18th century -

the modern thermometer.

The first thermometers were simple tubes filled with liquid.

If you put them in something warm, the liquid level would go up,

and if you put them in something cold,

the liquid level would go down.

That's not much use if you're trying to establish

a universal temperature scale that everyone can agree on.

Every inventor had their own idea of what that scale should be

and so no two thermometers were alike.

The solution that was arrived at was really clever.

It was to say that perhaps we can find fixed points.

So, perhaps there are situations

which are absolutely always the same temperature

and then everyone can agree on those points on the scale,

and then we can all calibrate our instruments.

The choices that stuck were those made by Daniel Fahrenheit,

who was a Polish physicist,

and he chose three fixed points that everyone else then followed.

So, the first one of his fixed points was this mixture here -

ammonium chloride and liquid water and water ice.

And that is a very interesting type of mixture

because when you mix those three things together,

they will find an equilibrium at a very specific temperature,

and Fahrenheit choice that as his starting point.

So, this is at zero degrees Fahrenheit.

Fahrenheit's second fixed point was a mixture of water and ice,

which will always settle at the same temperature -

32 degrees Fahrenheit,

more familiar to us these days as zero degrees Celsius.

And then there was one more fixed point

and Fahrenheit choice the temperature of the human body.

So, if you put a thermometer under your arm or under your tongue,

Fahrenheit said that was 96 on his scale,

and that was the beginning of the Fahrenheit scale.

All of those scientists and engineers

could calibrate their thermometers using those same three points.

They could divide up the temperature scale

in exactly the same way,

and, finally, the real science of temperature could begin.

The thermometer opened up a whole world of possibilities

for astronomy, meteorology, and, of course, medicine.

But it also brought with it a paradox.

While we now had a standard scale to record temperature,

we still didn't have any scientific explanation

of what temperature really was, of what made things hot or cold.

Some of the earliest scientific theories propose

that temperature was a physical substance.

One idea was that heat was a weightless liquid called caloric

that warmed things up.

Another theory suggested that

cold consisted of frigorific particles.

These ideas persisted until the late 18th century

when they were thrown into doubt by a discovery about heat

that would ultimately transform our understanding of cold.

In the 1790s, an American-born inventor

working in Germany called Count Rumford

applied his mind to the study of heat,

and this is the report that he wrote on his work.

And I love this document because it's written in a very human way.

Count Rumford was overseeing the manufacture of cannons

by German artillerymen

when he noticed something very curious

as they bored holes into the cold metal.

To show you, I've got a battery-powered drill

and an infrared camera that will reveal

what happens to the temperature of the metal as I drill through it.

And I'm just going to drill through this piece of metal here.

And have a look on the infrared camera.

You can see the spot around where I was drilling has warmed up

and I can feel the heat with my finger.

So, even a simple drilling experiment like this

can generate heat.

And this was exactly what Count Rumford observed

as he watched the cannon-makers at work.

As they bored through the metal, the cold iron got hotter.

The other important thing that Count Rumford noticed

was that the heat didn't run out.

You could keep drilling and the metal just got hotter.

And, in his opinion, that put a very big dent

in the theory of caloric

because if heat is a fluid flowing from a hot place to a cold place,

at some point, that fluid is going to run out.

Rumford had discovered something fundamental about temperature,

of what makes matter hot or cold,

yet it would be nearly a century

before it was fully recognised and explained.

And the first step towards an explanation would come

from a completely different branch of science altogether.

In 1827, Scottish botanist Robert Brown

was deep into his research on flowering plants.

It was an exciting time in biology because of the new realisation

that inside the very tiny plant cell,

there was even tinier machinery making everything work.

Brown was particularly interested in pollen...

..so he took pollen grains back to his laboratory,

suspended them in drops of water

and looked at them under his microscope.

And what he saw was the pollen grains sitting in the water,

but, from them, there were emerging even smaller particles,

and when he watched those particles,

they were moving, they were jiggling about.

So, the first thing that Brown did was check whether they were alive,

but they weren't, and he tried with lots of different materials,

and what he saw was that every time there was a particle that small,

just on the edge of what the microscope could see,

it would always be just jiggling about,

whatever it was made of, and he had no idea why that was.

The answer didn't come until nearly 100 years later

in a paper written in 1905 by Albert Einstein,

and it's a really elegant paper.

Einstein's paper drew together two crucial ideas.

First, that all matter was made of atoms,

and, second, that these atoms were constantly moving about.

This finally solved the mystery of Robert Brown's jiggling particles.

They were being bombarded by billions

of smaller, invisible atoms,

and Einstein's explanation depended on one fundamental point -

that the movement of atoms was directly linked

to their temperature.

The physical existence of our universe

is all about the relationship between matter and energy,

and this paper was where that story really started.

Einstein understood that heat is just the energy that atoms have

due to their movement,

and the measure of that movement energy is temperature.

The more energy, the faster the movement

and the higher the temperature.

More than a century after Rumford had puzzled over

what was heating up his cannons, Einstein had explained it.

The very act of boring through the metal

was adding energy to the atoms,

increasing their movement and, so, making the metal hotter.

This definition of heat also means something profound

for our understanding of cold

because if heat is the measure of energy,

of the movement of atoms,

then cold is simply an absence of energy,

a lack of motion.

And this is vital to understanding

how every single solid thing in our universe came into being.

To show you why, I'm back in Iceland -

the perfect place to explore the relationship

between cold and matter.

This is Breidamerkurjokull Glacier.

Here, matter exists side by side in three very different forms.

This is made of water molecules,

there's water molecules dripping off the roof here,

and the air I'm breathing out also contains some water molecules.

Billions upon billions of the same type of molecule

all in the same place,

but behaving in three different ways -

as a solid, a liquid, and a gas.

Each of these three states is a consequence of temperature,

of how fast the molecules of water are moving...

..and when the water reaches its freezing point

and changes from a liquid to a solid,

something extraordinary is happening

in the hidden world of its molecules,

something we can't see by looking at ice at this massive scale.

To understand it, we need to look at something very much smaller,

something that's also frozen, even if it might not look like it.

This is table salt - sodium chloride.

About as common as you can get.

And, even here, you can see that the salt's a little bit sparkly.

If I put it under the microscope, now you can see what's going on.

Those tiny little grains of salt here have flat faces -

they're little cubes - and every single grain is the same.

Not a perfect cube, but they've all got a cubic shape,

and it's those flat faces that are reflecting the light

and making the salt sparkle.

And that's an indication of something deeper down

in the structure of the salt.

Salt is made of equal numbers of sodium and chloride ions.

The chloride ions are assembled in rows and columns,

so that they sit on a square grid.

The smaller sodium ions fit into the spaces in between.

A salt crystal is just a giant grid, like this -

a cube that's a million or so atoms long on each side.

This is the hidden structure of a crystal.

Its atoms are no longer free to move around each other.

Each one is locked in its own place on the grid.

So, the salt looks like that here.

It would look like that if I took it into a sauna because it's frozen.

It's a frozen solid.

Freezing is simply what happens when the molecules of a substance

no longer have enough energy to move past each other,

and so they become fixed in position.

And this doesn't always happen at a temperature

that we would consider cold.

For salt, it happens at about 800 degrees Celsius.

We associate freezing with water ice,

but that's just because water is important to us.

The concept of freezing is far bigger,

and the transition from liquid to solid

can happen at a huge range of temperatures,

depending on the substance.

Liquid iron freezes to become a solid metal

at around 1,500 degrees Celsius.

Liquid tungsten turns into a solid

at nearly 3,500 degrees Celsius.

It's exactly the same process

that transforms liquid water into solid ice

at zero degrees Celsius.

As with other liquids, the molecules in liquid water

have enough energy to keep moving past each other.

But, as they cool, the molecules slow down.

As water reaches its freezing point,

they arrange themselves in tightly fixed positions,

forming a hexagonal lattice - a crystalline structure.

The beautiful symmetry of snowflakes comes, in part,

from this microscopic hexagonal form.

Here, deep in this cave of ice, it exists on a massive scale,

and, in fact, the very process of cooling and freezing is key

to how the entire planet formed.

Some 4 billion years ago,

the Earth was covered in molten rock.

As we've seen in the striking landscapes of Iceland,

that lava eventually cooled and froze into solid rock,

and, sometimes, the way it cooled

created something truly extraordinary.

The hexagonal columns of basalt at Reynisfjara

are one of Earth's natural wonders

and Professor Thor Thordarson,

a volcanologist from the University of Iceland,

is going to help me understand how they formed.

There's lots of basalt in the world,

but not all of it has this amazing structure here.

So, here, we have these beautiful, regular columns,

and these extend 10m, 15m up into the cliff edge.

Columns like this are fairly unusual.

These columns tell a story

of how the intricacies of cooling and freezing

have shaped the fabric of our planet.

So, this column here, which is about 80cm in width here,

this width is actually the function of the cooling.

So, if you think of a lava flow, it starts cooling from the surface,

and it also cools fastest when it is close...

in contact with the atmosphere.

As the lava cools and freezes, it also shrinks,

as its molecules arrange themselves into a solid structure.

This happens more quickly at the surface,

where the lava meets the air,

and more slowly underneath, where it stays warmer.

And if the rate of shrinking is great enough,

the cooling lava at the surface is under so much stress that it cracks,

and often the most efficient way

to dissipate this huge build-up of stress

is to crack at an angle of 120 degrees -

the angle that gives us a hexagon.

As the rock beneath the surface also continues to cool,

these cracks extend downwards,

creating the colossal pillars we see today.

Can you tell from the size of these how quickly these cooled?

I mean, did these take a day to form, or a week, or a year?

Can you tell?

Not exactly, but I would guess between ten and 20 years.

This landscape was formed because lava began to cool and freeze

at just the right speed for the laws of physics to create a masterpiece.

A little faster or slower, and these columns wouldn't exist.

They stand as evidence that solid rock,

the fabric of our world, is frozen,

and the architect that sculpted it is temperature.

And as we humans have built architectural wonders of our own,

so we've learned to harness the potential of cooling,

to change the very nature of matter.

This is Ely Cathedral.

It's been here for nearly 1,000 years,

and, over the centuries,

countless craftsmen have taken local raw materials,

limestone and oak,

and transformed them into this vast and intricate structure.

But I'm not here because of those materials.

I'm here to see something else.

The stained glass windows here are breathtaking,

and they only exist thanks to the unique properties of glass

that emerge as it cools.

It's only when you're right in close like this

that you can really appreciate these fabulous windows.

Each one of these panels

is illuminating the cathedral with a story.

But the story that you can see from down there

is built of a thousand smaller stories

that you can only see up here

because every single one of these pieces of glass

is carrying its own distinctive history

of how cooling shaped it and locked in its properties.

To understand why,

I've come to meet someone who works with glass day in, day out.

This is Walter Pinches,

a glass-maker carrying on a tradition

that's changed little in 800 years.

-How hot is it in there?

-1,250-1,300.

1,300 degrees C?

It's only 2m away!

SHE LAUGHS

Standing next to the fiery glow of the furnace,

it's easy to think that the key to glass-making is heat.

But the real key to this process is what happens

when the glass comes out of the furnace and begins to cool.

And the colour's just mixing into the liquid as you go along.

The colour's already twisted in. You've already got your pattern.

Cooling is a process that craftsmen like Walter

learn to control precisely.

When the hot glass first emerges, it's molten,

so, like all liquids,

its molecules are still free to move and slide over each other,

and this gives Walter a brief window of time to manipulate its shape.

But, with every passing second, the glass is cooling,

especially at the surface, where it's in contact with the air.

What's amazing about this is that the inside and the outside

are different temperatures, and right in at a molecular level,

everything in there is different.

Everywhere is behaving differently because of its temperature.

Starting at the surface, the glass begins to freeze.

Its atoms slow down and come to rest in fixed positions,

and they do so in a way that's unlike many other solids.

This is my favourite bit - when it just blows up like a balloon.

As we've seen, when other substances freeze,

like water or salt, their atoms become fixed

in the ordered structure of a crystal.

But glass is different.

It cools more quickly and so its atoms don't have time

to arrange themselves in a regular pattern.

Instead, they freeze in the disordered,

chaotic arrangement of a liquid,

and this gives glass one of its most valuable properties.

Unconstrained by a rigid, crystalline structure,

it can be worked and manipulated into an infinite number of forms.

This is the clever bit -

hot molecules at the bottom flowing quickly,

cooler ones at the top flowing more slowly.

By precisely controlling the heating and cooling of glass,

craftsmen like Walter can create shapes and forms

that are truly unique.

Liquids are at their most beautiful when they're flowing freely,

but they change so quickly

that we almost never get to appreciate the details.

But glass-blowing is this fabulous process

of sculpting a moment in time

and then catching it by cooling it for us all to admire.

The modern world is built of solids like glass

that we created by controlling the process of cooling and freezing.

But that change from liquid to solid isn't the end of the story.

As a solid becomes colder, it may outwardly look the same,

but, in the hidden world of atoms and molecules,

it can still be changing

in ways that utterly transform how it behaves.

And, occasionally, when we fail to understand these changes,

our pursuit of progress has ended in catastrophe.

Some events in history are so unexpected, so shocking

that the mentality of an entire society is divided

into before and after.

And, for our nation's maritime history,

that cusp came on the 15th of April 1912,

when news filtered out from London and New York

that the gigantic Titanic, that unsinkable symbol of luxury,

had struck an iceberg and had sunk.

There were 2,200 people on that ship,

and 70% of them died that day.

Titanic was built from state-of-the-art steel.

As with glass, we'd learned over centuries

to make steel incredibly strong

through precisely honed processes of heating and cooling.

Nobody doubted she was strong enough

to stand up to the extreme cold of the Arctic.

To understand what went wrong,

I've come to the Cammell Laird shipyard in Merseyside

where marine engineers are working on their latest project.

I've been on a lot of ships,

but I haven't ever been quite this excited

to be on the back deck of a ship

because this is the Royal Research Ship

Sir David Attenborough in the process of being built.

We're surrounded by the innards of a ship,

all these individual pieces that will build the final structure.

And what's brilliant about it is that the oceans are raw

and the structures you need to sail on them are raw,

and this is what's going on -

steel being welded to build one of the most modern

polar research ships in the world.

Joining me on board the Sir David Attenborough

is Captain Ralph Stevens.

It will be his responsibility to navigate this huge vessel

through icy polar waters.

It's astonishing to me that we're still building ships of steel.

You know, we associate steel with the Industrial Revolution

150 years ago, and, yet, we are still building ships from steel.

Why is it so good?

Well, for us, it's quite a revolutionary material

in that it allows us to take impacts.

It's quite common for us to say some of the ice is as hard as steel.

And some of the glacial ice, it's rock-hard

and it's noticeably different.

When you hit a piece, you'll hear a big clang throughout the ship.

LOUD CLANG

And so we want the hull to be able to take all of these forces

that it's exposed to without cracking.

And steel can do that job?

Steel can do that. The right steel can do that.

But, ironically, steel may actually have been Titanic's Achilles heel...

..because what the engineers of the day didn't fully understand

is that, under certain conditions,

the behaviour of steel can fundamentally change.

And the key to this change was cold.

Steel, like many metals, is ductile.

That means it can stretch when put under stress -

a property that's useful in a huge structure like a ship.

Few had imagined that, in the cold, this crucial property might change.

Got a sample of shipbuilding steel here

with a little notch in the bottom,

and I'm going to do this experiment twice -

once with this one, which is at room temperature,

and once with an identical sample,

which has been in the dry ice here at minus 80 Celsius.

Very, very cold. The difference will be very obvious.

So, here we go.

First, the steel at room temperature.

So, here's the cold one, down at minus 80 Celsius.

This is the sample at room temperature,

and you can see that it bent, absorbed the energy,

absorbed the energy, but it didn't snap,

whereas this one - this is the cold temperature one -

and the surface looks really different.

There's all this speckled pattern and that's a snap.

This was brittle fracture. You don't want your ship doing this.

Cold has changed the nature of the steel,

making it more brittle.

And it's this that some experts now think

could have played a significant role in the Titanic disaster.

Analysis of metal taken from the wreckage

suggests that, rather than flexing on collision with the iceberg,

the hull and rivets had become brittle and they fractured.

With this in mind, modern shipbuilders are able

to avoid the mistakes of their predecessors.

We did some calculations.

We went through the last ten years of temperatures

our ships have been exposed to.

We came to 25 degrees and then reduced it down to minus 35.

So, the game is that you want the steel to give a little bit,

-but not snap.

-That's it.

We don't... We can't afford to have it fracture,

and if the worst came to the worst,

you want that steel to deform rather than crack.

The tragic irony of Titanic is that she was constructed from metals

that we've been using for centuries.

We thought we understood them...

..but cold altered them in ways that no-one expected.

Since then, we've been much more aware

of the hidden changes that can occur within materials

when they're cooled far below their freezing point.

And, by pushing temperatures lower and lower,

we're beginning to unlock some strange and exciting

new properties of matter.

This is a material with a very long name.

It's yttrium barium copper oxide, and it doesn't look like very much.

There's very strong magnets here and it's not responding to them.

It doesn't conduct electricity.

Doesn't seem very interesting,

but, when you cool it down, it changes completely.

Using liquid nitrogen, I'm reducing the temperature of the disc

to minus 196 degrees Celsius.

And, now, when I bring it close to the magnets,

something unexpected happens.

It's levitating.

And it will scoot around on the little track here for quite a while.

So, something's changed. We've cooled it down.

The behaviour changed completely.

And that's because cold has altered the material at the atomic scale.

Materials conduct electricity when electrons travel through them,

but the atoms in a conductor are an obstacle to the flow of electrons

because, as electrons bump into them, they lose energy.

At extremely low temperatures,

the electrons can team up into pairs

and then the attraction between the electron pairs

helps them navigate through the atoms far more easily.

So, when I bring the disc close to the magnetic track,

a strong electric current begins to flow in the disc.

This, in turn, generates its own magnetic field.

The magnets in the track and the disc repel each other,

and so the disc levitates.

This is an example of superconductivity.

Once it's cooled down below the critical temperature,

the properties of the material change.

It becomes able to conduct electrical currents

without any resistance.

And it also changes how it responds to magnets.

The peculiar electromagnetic properties

of supercooled materials

have given us a powerful new tool in engineering and medicine.

Some countries already use a super-sized version

of this magnetic levitation effect in their high-speed rail systems.

Having no contact with the track,

trains run faster and more smoothly and efficiently.

And, inside MRI scanners,

liquid helium supercools massive coils of copper wire

to a temperature of minus 269 degrees Celsius.

At this extreme cold,

an electric current can flow with almost zero resistance,

which helps generate the powerful and stable magnetic field

that the MRI machine needs.

The extraordinary discoveries we've made

at extremely low temperatures are now driving

one of the biggest scientific quests of the modern age.

How cold is it possible to go?

How do we get there?

And what new properties of matter might we uncover?

The first step on that journey is to understand how things cool down.

Take this humble cup of tea.

I always drink my tea far too quickly

because the experience of a lifetime tells me that, if I don't,

it will cool down.

Heat will flow out of the tea, which is warm,

into its surroundings, which are cooler.

If I look at this glass of iced water here,

this is cooler than the surroundings,

and, if I leave that alone, it will heat up

until it matches the temperature of everything around it.

This is a demonstration of a fundamental principle of physics -

the second law of thermodynamics.

Heat flows from hot to cold until equilibrium is reached.

We can see this in action through the thermal imaging camera.

The hot tea is cooling and the chilled water is warming

until both are the same temperature as their surroundings.

It's a law that can't be broken, but it also raises a question.

How can you ever make something colder than its surroundings,

like an ice cube?

Here's the problem.

At some point, this ice was liquid water,

and, to cool it down, to freeze it,

heat had to flow out of it to make it colder,

but that seems to go against this fundamental law.

So, how is this possible?

The answer to that question can be found here.

This 33,000-square-metre

food distribution centre in Warwickshire

handles almost 200,000 home grocery deliveries every day,

and much of it is chilled well below ambient temperature.

The invention of refrigeration

made an enormous difference to our society

because it allowed us to control our food supply.

So, something like this - frozen carrots -

was probably frozen just after it left the field, and, since then,

it's passed through an unbroken chain of cold -

refrigerated lorries, refrigerated warehouses,

all the way to us,

and places like this gigantic freezer are part of that.

And, after being in here,

I will never take frozen food for granted ever again

because it's so cold. It's minus 22!

And when you stop to think about that, it's strange.

How DO you make this building so much colder

than the ambient temperature here in balmy Warwickshire?

The secret to this place is the same as the hidden workings

of the fridge-freezer in your kitchen...

..and it begins with something counterintuitive.

The odd thing about the process of making something cold

is that it starts with a huge input of energy,

and that happens here.

These are compressors.

They're taking ammonia gas, and all the energy is being used

to squeeze the gas to a high pressure.

And, at the same time, that heats it up,

so what leaves here is both at high pressure and high temperature.

Next, this pressurised ammonia needs to be cooled down.

Ammonia gas comes out of the plant room downstairs

at 100 degrees C, and this is where it's cooled down.

It's flowing through all these pipes in the inside of here.

And the water falling down is cooling it down

much closer to room temperature.

This is where the energy is lost

from the refrigerant fluid - the ammonia.

By the time it leaves here, it's much cooler and it's a liquid.

Crucially, even though the ammonia is now cooler,

it's still under pressure.

Releasing this pressure is the secret

of how this vast warehouse space is cooled,

and I can show you how using something very familiar.

What happens is the same as when you have an aerosol spray

and you spray it and you can see it. I've got a thermometer here.

If I spray the bottom of the thermometer,

the temperature goes right down.

This process is called adiabatic cooling.

As the high-pressure gas leaves the can,

it pushes outwards on the air around it and expands.

But that push uses energy that can only come

from the movement of the atoms,

and so the expanding gas cools down.

The exact same process happens

as liquid ammonia is pumped into the warehouse.

The high-pressure liquid passes through a valve,

and, still contained within the pipes,

it rapidly expands into a gas

causing its temperature to instantly drop.

Heat then flows from the warm air inside the warehouse

to the much colder ammonia until they reach equilibrium,

just as the second law of thermodynamics

dictate they should.

And, since the equilibrium temperature

is far colder than that at which the air began,

this vast space cools down.

Refrigeration works because you keep moving the goalposts,

so that heat only ever flows from hot to cold.

No laws of physics are broken.

As long as you keep pumping a bit of energy in at the compressors,

the refrigerators will keep working.

As we saw, the process of lowering temperature requires energy...

..and this raises an interesting question.

How much colder is it possible to go?

We know that, as you cool materials down,

they tend to turn to liquid and then solids, but, actually,

the question of how cold you could make something

started with gases,

and this was the kind of experiment that was used.

What I've got here are four beakers,

each of which is at a different temperature.

They range from minus five to 50 degrees Celsius.

Into each, I'm placing a syringe

containing 15ml of air at room temperature.

This air will heat up or cool down

until it's at the same temperature as what's in the beaker.

So much science is about waiting,

and this is one of those experiments.

But it's not the change in temperature I'm interested in here.

It's something else.

After five minutes, the air that's heated to 50 degrees

has expanded from 15ml to 16ml,

while the air that's cooled to minus five

has reduced to 14ml.

In other words, there's a direct relationship

between the temperature of a gas and its volume.

So, the first scientists who saw this kind of relationship

did something very straightforward -

they plotted a graph that showed temperature against volume.

And, at the higher temperatures, the volume was higher,

and, as you go down to the lower and lower and lower temperatures,

the volume decreases.

And then there's a question because, at some point,

even though they couldn't see it, if that line kept going,

it was going to pass through zero volume,

and at that point and past that point,

what happens to the temperature? What does it mean?

And that was the first hint that there might be a limit

on just how cold you can go.

This observation led to a concept known as absolute zero -

the theoretical limit of cold.

And now we know exactly what it is.

On the Celsius scale, it's minus 273.15 -

a fantastically low temperature.

But, below that, there's nowhere to go.

That's the coldest you can get.

And it remains a theoretical point on the temperature scale.

The Boomerang Nebula 5,000 light years away from Earth

is the coldest place we know of in nature.

It's a star in the late stages of its life

that's shedding huge plumes of gas.

As this gas expands rapidly into the void of interstellar space,

it loses energy quickly,

resulting in its unusually low temperature

of minus 272 degrees Celsius.

But even this is one whole degree warmer

than absolute zero.

Though we've yet to find absolute zero

in the far reaches of the universe,

we're trying to create it ourselves much closer to home.

At Imperial College London, Professor Ed Hinds and his team

are working at the very limits of the ultracold,

within fractions of a degree of absolute zero.

It promises to open up a whole new world of physics,

which could revolutionise our future.

The stuff they're cooling here is tiny clouds of molecules.

Chilling them to absolute zero requires two phases of cooling.

First, using liquid helium,

they take them down to within four degrees of absolute zero.

But it's these last few degrees that pose the problem.

There are ways to make helium a bit colder,

but to get to the millionths of a degree,

there is no fluid that you can use,

so, instead, we use light.

By scattering the light, the molecules will get colder.

Even at this temperature,

the molecules still have some movement.

Photons in the laser light collide with the slowly moving molecules,

and, in that instant,

what little momentum they have is transferred to the photons.

The photons are scattered,

but the molecules slow down and so get even colder.

By using an array of different colours of laser light

in just the right order,

Ed and his team can reach temperatures

within a few millionths of a degree of absolute zero.

At these incredibly low temperatures,

materials begin to behave differently

at the subatomic or quantum level.

In this quantum state, they exhibit strange properties,

which might lead to a new type of computer.

A normal computer bit can only represent a zero or a one,

but these quantum materials can be zero and one at the same time.

Link these multitasking bits together,

and they can do vast numbers of calculations simultaneously

far faster than any conventional computer chip.

This opens up the possibility of quantum computing,

quantum sensing, quantum cryptography.

These are all ways of doing useful things,

but much better than can be done with conventional techniques.

The world of absolute zero is a strange new realm of physics

and one that we're only just beginning to get to grips with.

But there is something ironic about the vast effort required

to push things extremely close to absolute zero.

Because wait long enough - billions of years -

and everything will get there.

The universe itself is cold and it's getting colder.

Humans have always looked up at the sky

and asked questions about the stars and the structure of galaxies -

everything they could see.

But in the 1940s and '50s, a new type of question emerged

about what was between the stars and about what dark really was,

and this question opened the door on the nature of the universe.

And, then, in 1964, it was answered by accident.

In a small laboratory in New Jersey,

astrophysicists Robert Wilson and Arno Penzias

stumbled on a discovery

that changed our understanding of the universe forever,

revealing something profound about its temperature.

I'm meeting Professor Tim O'Brien,

an astrophysicist at the University of Manchester

and director of the Jodrell Bank Observatory.

So, at some point during every undergraduate physicist's degree,

they hear the names Penzias and Wilson.

Tell me what they did.

So, these were these two great characters

that were working in the USA in the 1960s,

and they built themselves a remarkable telescope.

It was incredibly well-built

to try and study the outer regions of the Milky Way,

and they were measuring very weak signals coming from space.

But there was this last bit of noise

that they had no idea where it came from.

They could not get rid of it.

It was a faint hiss,

and that faint hiss came from everywhere on the sky.

It had the same sort of strength, the same brightness

of the radio signal everywhere on the sky.

And they tried everything.

They tried all kinds of things, didn't they?

They did try everything. At one point,

they thought it might be coming from pigeon droppings in the telescope.

So, big telescope that the pigeons were sitting in.

Washed it all out - no, this stuff was still there.

There remained only one possible explanation for this noise,

and it had enormous implications for our view of the universe.

The strange hissing was coming from beyond our own galaxy.

It's what we now know and they didn't know at the time...

It's what we call the cosmic microwave background -

the fading glow of the Big Bang.

-Where was this coming from?

-It's coming from the whole sky. So, it's coming from everywhere.

And it's actually the light that was emitted by the universe

about 380,000 years after the Big Bang.

The cosmic microwave background radiation

is invisible to the naked eye, but it fills the universe.

If we could see it, the entire sky would glow

with a brightness that's astonishingly uniform

in every direction.

What's remarkable is that these microwaves carry information.

They allow us to take an accurate temperature

of the entire universe without the use of a thermometer.

A thermometer has a fundamental limitation,

which is that it has to be touching the thing that it's measuring.

And that's not much use if you're looking at the rest of the world

or even the rest of the universe.

But the laws of physics themselves offer another route

because every single object in the universe with a temperature

is radiating some of that energy away as light,

and every single object has a temperature.

The reason you can see me now on the infrared camera

is that I have a temperature, and so I'm glowing in the infrared -

effectively, a human infrared light bulb.

The temperature of an object determines the exact wavelengths

of the light that it radiates,

and this means that there's a precise relationship

between temperature and colour.

So, when an astronomer sees the star of a certain colour,

they know it has a certain temperature.

The reddest star visible to the naked eye is Mu Cephei.

The wavelength of red light that it radiates

tells us that this star has a temperature

of around 3,200 degrees Celsius.

And this is Spica,

a star that glows a brilliant blueish-white.

The shorter wavelength light is indicative of a young, hot star

that's burning at a temperature of around 22,000 degrees Celsius.

And then you can go back the other way -

cooling down through all those very hot temperatures,

down through our everyday temperatures,

and keep going and keep going.

The wavelengths get longer and longer.

Eventually, you reach the very long wavelengths

of the cosmic microwave background.

They're not part of the visible spectrum,

but the wavelengths of these microwaves

reveal its temperature,

and that temperature is cold.

Today, the cosmic microwave

background radiation glows

at a temperature of minus 270 degrees Celsius -

only 2.7 degrees warmer than absolute zero.

We live in a nice, warm bubble on planet Earth,

but out there in the universe, it isn't just very empty.

It's very, very cold.

But that's not the end of our story of temperature...

..because amidst the vast swathes of cold and nothingness,

we're starting to find other bubbles of warmth

out there in the universe -

planets with a temperature similar to our own,

which means they may have the right conditions

for liquid water and complex chemistry.

These discoveries are causing huge excitement among scientists

because they offer up the tantalising possibility

that maybe - just maybe -

we might not be alone in this vast universe.

Next time, the temperatures of life.

This is an awesome force of nature.

I'll discover how the slenderest knife-edge of temperature

here on Earth provided the catalyst for life to flourish.