Original subtitles

Dr. Helen Czerski: Everything around us exists somewhere

on a vast scale from cold...to hot.

Whether living or dead, solid or liquid, visible or

invisible, everything has a temperature.

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.

Across three programs, we're going to explore

the extremes of the temperature scale,

from some of the coldest temperatures...

to the very hottest

and everything in-between.

In this program, we're going to venture to the bottom

of the temperature scale.

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

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

And we'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 we'll show you why.

[Dogs barking]

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 foremost

centers of the new science of astronomy.

But while looking up there, they discovered they had

a problem that started down here.

Amy Creese is a Meteorological Observer.

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

to solve a very specific problem caused by temperature.

Creese: 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 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.

Czerski: So even those people who are 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 earlier

recordings in 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 in 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.

Creese: And I'm glad that he wrote about it.

Ha ha ha ha!

Czerski: 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, and 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.

A solution that was arrived that 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 3 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 3 things together, they will

find an equilibrium at a very specific temperature.

And Fahrenheit chose that as his starting point, so this is

at 0 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 0 degrees Celsius.

And then there was one more fixed point, and Fahrenheit

chose 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 3 points. They could

divide up the temperature scale in exactly the same way,

and finally the really 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 proposed

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.

And you can see just what that was using a simple hand drill

and an infrared camera.

And I'm just gonna drill through this piece

of metal here.

[Drilling]

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 fingers.

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.

[Drilling]

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

recognized 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

realization that inside the very tiny plant cell, there

was an even tinier mechanism 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 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 no idea why that was.

The answer didn't come until 1905 in a paper written by

Albert Einstein that 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 entire universe came into being.

To show you why, we're in Iceland, the perfect place to

explore the relationship between cold and matter.

This is Breidamerkurjokull glacier.

Here, matter exists side-by-side in 3 very

different forms.

[Plop]

Nearly everything in this cave is made of water molecules,

from the ice itself to the water flowing through it

and even in the air.

Billions upon billions of the same type of molecule, all

in the same place but behaving in 3 different ways:

as a solid, a liquid and a gas.

Each of these 3 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

and 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 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 800 degrees Celsius.

Liquid iron freezes to become a solid metal

at around 1500 degrees Celsius.

Liquid tungsten turns into a solid at nearly 3500 degrees.

It's exactly the same process that transforms liquid water

into solid ice at 0 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 one of the world's leading experts

in how they were formed.

Thordarson: So here we have these beautiful regular

columns, and, these extend at 10, 15 meters up

into the cliff face.

Columns like this are fairly unusual.

Czerski: These columns tell a story of how the intricacies

of cooling and freezing have shaped

the fabric of our planet.

Thordarson: So this column here which is about 80 centimeters

in width here, this width is actually a function

of the cooling.

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

the surface, and it also cool fastest where 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

buildup 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.

Czerski: 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?

Thordarson: Not exactly, but I would guess between

10 and 20 years.

Czerski: 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 this potential

of cooling and freezing 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 we're not here because of those materials.

We're 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 1,000 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, we're going to meet someone who works

with glass day in, day out.

This is Walter Pinches, a glassmaker carrying on

a tradition that's changed little in 800 years.

How hot is it in there?

Pinches: 1250, 1300.

Czerski: 1300 degrees C.

It's only 2 meters away. Ha ha ha!

Standing next to the fiery glow of the furnace, it's easy

to think that the key to glassmaking 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 color's just mixing into the liquid as you go along.

Color's already twisted in, you've already got your pattern.

Czerski: 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 that 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 favorite 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.

The modern world is built of solids,

like glass, that we have 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 look

outwardly 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've failed to understand these

changes, our pursuit of progress has ended

in catastrophe.

On the 15th of April 1912, "Titanic," that unsinkable

symbol of luxury, struck an iceberg and sank.

There were 2,200 people onboard, and more than

1,500 of them died.

Titanic was built of 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,

we've come to the Cammell Laird shipyard in Merseyside,

where marine engineers are working on their

latest project.

This is the Royal Research ship "Sir David Attenborough."

When complete, she will be one of the most modern

and advanced polar research ships in the world.

And Captain Ralph Stevens, will be responsible

for navigating 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?

Stevens: Well, for us, it's quite

a revolutionary material, and that allows us to take in ...

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?

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

Czerski: 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 behavior of steel can fundamentally change.

And the key to this change was cold.

Steel, like many metals, is ductile.

That means that 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 gonna 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,

-80 Celsius, very, very cold.

The difference will be very obvious.

So here we go.

First... the steel at room temperature.

[Banging]

So, here's the cold one.

Down at -80 Celsius.

[Banging]

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 the 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.

[Bang]

With this in mind, modern shipbuilders are able to avoid

the mistakes of their predecessors.

Stevens: We did some calculations. We went through

the last 10 years of temperatures our ships have

been exposed to, and that we came to 25 degrees and then

reduced it down to -35.

So the game is that you want the steel to give

a little bit, but--and not snap.

Stevens: That's it. 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.

Czerski: 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, we're reducing the temperature

of the disc to -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 a little track here

for quite a while.

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

The behavior 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 disk close to the magnetic track,

a strong electric current begins to flow in the disk.

This in turn, generates its own magnetic field.

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

and so the disk 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 super-cooled

materials have given us a powerful new tool

in engineering and medicine.

Some countries already use a supersized 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 super-cools

massive coils of copper wire to a temperature of

-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?

And how do we get there?

[Liquid bubbling]

We know that as you cool materials down, they tend to

turn into liquids and then solids, but actually

the question of how cold you could make something started

with gasses, and this was the kind of experiment

that was used.

What I've got here are 4 beakers, each of which is

at a different temperature.

They range from -5 to 50 degrees Celsius.

Into each, we're placing a syringe containing

15 milliliters 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 that's interesting

here, it's something else.

After 5 minutes, the air that's heated to 50 degrees

has expanded from 15 to 16 milliliters, while

the air that's cooled to -5 has reduced to

14 milliliters.

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 is

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 -273.15--

a fantastically low temperature, but below that

there's nowhere to go. That's the coldest you can get.

[Wind howling]

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 -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 ultra-cold,

within fractions of a degree of Absolute Zero.

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

could revolutionize 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

4 degrees of Absolute Zero,

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

Hinds: There are ways to make helium a bit colder,

but to get to the millionth 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.

Czerski: 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 colors 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 0 or a 1,

but these quantum materials can be 0 and 1

at the same time.

Link these multi-tasking bits together, and they can do vast

numbers of calculations simultaneously,

far faster than any conventional computer chip.

Hinds: 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.

Czerski: The world of Absolute Zero

is a strange new realm of physics

and one we're only just beginning to get to

grips with.

But there's something ironic about the vast efforts

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.

In 1964, in a small laboratory in New Jersey,

astrophysicists Robert Wilson and Arno Penzias

stumbled upon a discovery that changed our understanding

of the universe forever...

revealing something profound about its temperature.

And helping us decipher exactly what they found is Tim O'Brien,

an astrophysicist at The University of Manchester

and the Director of the Jodrell Bank Observatory.

So, at some point during every undergraduate physicists

degree, they hear the names Penzias and Wilson.

Tell me what they did.

O'Brien: So these were these two great characters

that, were working in the USA in the 1960s.

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.

[Faint hissing]

It was a faint hiss, and that faint hiss came from

everywhere in 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 a big telescope that

the pigeons were sitting in. Washed it all out--

No, the stuff was still there.

Czerski: There remained only one possible explanation for this

noise, and it had enormous implications for our view

of the universe.

This strange hissing was coming from beyond

our own galaxy.

O'Brien: It's what we now know, and they didn't know

at the time, is what we call the Cosmic Microwave

Background, the fading glow of the Big Bang.

Where was this coming from?

O'Brien: Yeah, 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 is 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 it radiates.

And this means there's a precise relationship between

temperature and color.

So, when an astronomer sees a star of a certain color,

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 this

star has a temperature of around 3200 degrees Celsius.

And this is Spica, a star that glows a brilliant

bluish-white.

This shorter wavelength is indicative of a young,

hot star that's burning at a temperature of around

22000 degrees Celsius.

Travel back the other way towards longer wavelengths,

and things get cooler.

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 -270 degrees Celsius,

Only 2.7 degrees warmer than Absolute Zero.

Away from our nice warm bubble on planet Earth, the universe

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 tantalizing

possibility, that maybe, just maybe,

we might not be alone in this vast universe.