All language subtitles for BBC.The.Secrets.of.Quantum.Physics.1of2.Einsteins.Nightmare.1080p.HDTV.x264.AAC

Beneath the complexities
of everyday life,

the rules of our universe
seem reassuringly simple.

This solid bridge
supports my weight.

The water flowing underneath
always goes downhill

and when I throw this stone...

..it always flies through the air
following a predictable path.

But as scientists peered deep

into the tiny
building blocks of matter...

..all such certainty vanished.

They found the weird world
of quantum mechanics.

Deep down inside everything
we see around us,

we found a universe
completely unlike our own.

To paraphrase one of the founders
of quantum mechanics,

everything we call real
is made up of things

that cannot be themselves
regarded as real.

Around 100 years ago, some
of the world's greatest scientists

began a journey down the rabbit hole
into the strange and the bizarre.

They found that in the realm
of the very small,

things could be
in two places at once...

..that their fates
are dictated by chance...

..and that reality itself
defies all common sense.

And at stake,
that everything we thought

we knew about the world might
turn out to be completely wrong.

The story of our descent
into scientific madness

begins with the most
unlikely object.

Berlin, 1890.

Germany is a new country, recently
unified and hungry to industrialise.

In this newly-unified Germany,

a number of new engineering
companies were founded.

They'd spent millions buying
the European patent

for Edison's new invention,
the light bulb.

The light bulb was
the epitome of modern technology,

a great optimistic
symbol of progress.

Engineering companies quickly
realised there were fortunes

to be made building streetlights
for the new German Empire.

But what they didn't realise
was that they would also unleash

a scientific revolution.

Strangely enough,

this humble object
is responsible for the birth

of the most important theory
in the whole of science -

quantum mechanics, a theory
that I've spent my life studying.

And that's because, back in 1900,

the light bulb presented
a rather strange problem.

Engineers knew that if you heated
the filament with electricity,

it glowed.

The physics that underpinned this,
though, was completely unknown.

But something as basic
as the relationship

between the temperature
of the filament

and the colour of light it produces
was still a complete mystery.

A mystery they were obviously
keen to solve.

And, with the help
of the new German state,

they saw how to steal a march
on their competitors.

In 1887, the German government
invested millions

in a new technical research
institute here in Berlin,

The Physikalisch-Technische
Reichsanstalt, or PTR.

Then, in 1900,
they enlisted a bright

if somewhat straight-laced
scientist to help work here.

His name was Max Planck.

Planck took on a deceptively
simple problem -

why the colour of the light
changes as the filament gets hotter.

To get a sense of the puzzle
facing Planck,

I'm going to ride this bicycle
with an old-fashioned lamp

powered by an old-fashioned dynamo.

Obviously the faster I go,
the brighter the light.

The more I pedal, the more
electricity the dynamo produces,

the hotter the filament in the lamp
and the brighter the light.

But the light the bulb makes
isn't just getting brighter,

it's changing colour, too.

As I speed up, the colour
shifts from red to orange to yellow.

Right,
now I'm going to really belt it.

Now the bulb's filament
is getting even hotter,

but although it certainly
gets brighter...

..the colour seems to stay
the same - yellow-white.

Why doesn't the light
get any bluer?

To investigate, Planck
and his colleagues built this,

a black-body radiator.

It's a special tube they could heat
to a very precise temperature

and a way to measure the colour
or frequency

of the light it produced.

Nowadays, over 100 years later,
the PTR still do exactly

this kind of measurement,
just much more accurately.

The temperature inside here
is 841 degrees centigrade.

I can feel the heat coming off
and it's glowing

with a lovely orangey-red colour.

It's about the same colour as my
bike light when I'm cycling slowly.

But I want to see something
hotter still.

The temperature inside here
is about 2,000 degrees centigrade...

..and it's glowing with a much
brighter, whiter-coloured light.

To produce light of this intensity
and colour

requires a power
of about 40 kilowatts.

Now, that's equivalent to about
400 mes on a bike cycling very fast,

or the combined output
of the entire Tour de France.

Although the light is whiter,
it's red-white -

there's very little blue.

Why is blue so much harder
to make than red?

And further up the spectrum,
beyond blue,

the so-called ultraviolet,
is hardly produced at all -

even when we look at things
as hot as the sun.

Even the sun, at a temperature
5,500 degrees centigrade,

produces mostly white visible light

and makes remarkably little
ultraviolet light,

given how hot it is. Why is this?

Why is ultraviolet light
so hard to make?

This remarkable failure of common
sense so perplexed scientists

of the late 19th century that they
gave it a very dramatic name.

They called it
the ultraviolet catastrophe.

Planck took a crucial first step
to solving this.

He found the precise
mathematical link

between the colour of light,
its frequency and its energy.

But he didn't understand
the connection.

However, it was another
weird anomaly

that would really put
the cat amongst the pigeons.

In the late 19th century,
scientists were studying

the then newly-discovered radio
waves and how they were transmitted.

And to do that, they were building
experimental rigs

very similar to this one.
Basically, by spinning this disc,

they could generate huge voltages
that caused sparks

to jump across the gap between
the two metal spheres.

But, in doing so,

they discovered something very
unexpected to do with light.

They found that, by shining
a powerful light source

on the spheres, they could make the
sparks jump across more easily.

This suggested a mysterious
and unexplained connection

between light and electricity.

To understand what was happening,
scientists used this.

It's called a gold leaf
electroscope.

It's basically a more sensitive
version of the spark gap apparatus.

Now, first of all,
I have to charge it up.

What I'm doing is adding
an excess of electrons

that are pushing
the two gold leaves apart.

Now, first I take red light

and shine it on the metal surface

and nothing happens.

Even if I increased
the brightness of the light,

still the gold leaves
aren't affected.

Now I'll try this special
blue light, rich in ultraviolet.

Immediately,
the gold leaves collapse.

Light can clearly remove static
electric charge from the leaves.

It can somehow knock out
the electrons I added to them.

But why is ultraviolet light so much
better at doing this than red light?

This new puzzle became known
as the photoelectric effect.

The ultraviolet catastrophe
and the photoelectric effect

were big problems for physicists,

because neither could be understood
using the best science of the time.

The science that said,
quite unequivocally,

that light was a wave.

All around us,

we see light behaving in a perfectly
common-sense wavy way.

Look at the shadow of my hand.
It's fuzzy round the edges.

We understand this as the light
hitting the side of my hand

and bending
and smearing out slightly,

just like water waves
around an obstruction.

Perfectly common-sense,
wave-like behaviour.

And here's something else,
something rather beautiful.

Look at these soap bubbles.

Shine a light on them,

and gorgeous coloured patterns
emerge from nowhere.

And this was easily explained
if you accept that light was a wave,

reflecting off the outer and inner
layers of the thin soap film

and breaking up
into the colours of the rainbow.

Rather like ripples
on the surface of water,

light was simply ripples of energy
spreading through space

and this was as firmly accepted as
the fact that the earth was round.

But although this wave theory
works perfectly well for shadows

and bubbles, when it came to
the ultraviolet catastrophe

and photoelectric effect...

..the wheels started coming off.

The problem was this -
how could light do this?

To truly grasp how absurd
this phenomenon was,

it might be useful to consider
how waves in water behave.

Hey!

This is the wave tank at the RNLI's
headquarters in Dorset.

It's used to train lifeboat teams
to deal with a range of different

kinds of water waves. First, small
waves, just 30 centimetres high.

These waves don't have much energy,

hardly enough energy to knock
this top can off the other.

But when the waves grow
to over a metre and a half,

it's a very different proposition.

And they're really
throwing me about.

There's no way I can keep this can
balanced on the top.

It's clear what water waves
are telling us -

bigger, more intense waves
have more power.

They easily knocked me
and the cans around.

So if light was a wave,

more intensity
should knock out more electrons.

But that's not what happened.

Remember, no matter
how intense the red light was,

it still didn't budge
electrons from the metal.

But, weirdly, weak ultraviolet
worked within seconds.

So thinking of light as a wave
just wasn't adding up.

To resolve this, someone needed
to think the unthinkable

and, in 1905, someone did.
You may well have heard of them.

His name was Albert Einstein.

This is the Archenhold-Sternwarte
Observatory in Berlin.

Perched on top is a strange,
huge iron and steel construction,

but it's not a gun,
it's actually a telescope.

Built in 1896, the telescope was
one of the largest of its kind

in the world and made
the observatory the go-to place

to engage and astound
the public in new science.

Albert Einstein gave a very famous
public lecture here

on his theory of relativity which is
of course what he's most famous for.

But it's not the work
that won him the Nobel Prize.

In 1905, he'd also come up
with a new theory to explain

the photoelectric effect and
what he suggested was revolutionary

and even heretical.

He argued that we have to forget all
about the idea that light is a wave

and think of it instead as a stream
of tiny, bullet-like particles.

The term he used to describe
a particle of light was a quantum.

To Einstein, a quantum
was a tiny lump of energy

and although in 1905
the word wasn't new,

the idea that light
could be a quantum seemed crazy.

And yet following Einstein's
heretical line of thought

to its logical conclusion

solved all the problems with light
at a single stroke.

I'll try to explain how this helps
using a rough analogy.

Of course, like all analogies,
it's far from perfect

but hopefully it'll give you
a sense of the physics

to help you understand why thinking
of light as a stream of particles

solves the mystery
of the photoelectric effect.

In this analogy, these red balls
represent Einstein's light quanta.

'And those cans over there are
the electricity held in the metal.'

Now, in the original experiment,

they made electricity flow
from the surface of the metal

by shining light on it.
In my analogy, I'm going to try

and knock those tin cans over
using these red balls.

'Absolutely no effect.

'That's just like red light.'

According to Einstein,

each particle of red light
carries very little energy

because red light
has a low frequency.

'So even a very bright red light
with many red light particles

'can't dislodge any electrons
from the metal plates,

'just like the red balls.'

Now I'm going to use heavier balls
like these blue golf balls

and I'm going to try and knock off
the tin cans with these.

'They're like the ultraviolet light
in the experiment.

'Now, each individual light particle
carries more energy

'because ultraviolet light
is higher frequency.'

Just a few of them,
like a dim ultraviolet light,

are enough to knock the electrons
out of the metal plate

and collapse the gold leaf.

So Einstein's idea that light is
made up of tiny particles or quanta

is a wonderful explanation
of the photoelectric effect.

I remember when I first
learnt about this,

being blown away by its sheer
elegance and simplicity.

But what's more, Einstein's
nifty idea also helped solve

Planck's mystery of the light bulb.

There was more red than ultraviolet

because ultraviolet quanta
took so much more energy to make,

about 100 times more energy.

No wonder there are so few of them.

That moment at the beginning
of the 20th century

signalled a genuine revolution

because it demonstrated that
the kind of physical science

that people were doing right back
to Newton and Laplace,

and people like that, that you
needed a completely new approach.

Physics has never recovered
from that moment

in the sense that it's built
on that moment,

that's where modern physics
really began.

But Einstein's theory also left
physicists with a dizzying paradox

defying all common sense.

Light was definitely a wave
which explained shadows and bubbles.

And now it was definitely
a particle too -

Einstein's quanta explaining
the photoelectric effect

and the ultraviolet catastrophe.

Then just a few years after
Einstein's brilliant, crazy idea,

the paradox got a lot deeper
and a whole lot weirder.

Because what seemed to be a
curious mystery about light

was about to become a battleground
about the nature of reality itself.

1922.

The Western world was in the grip of
a revolution, a cultural revolution.

James Joyce's Ulysses is published,

Stravinsky is at the height
of his powers

and Chaplin has just released
his first serious movie.

The Ottoman Empire collapses.

Europe is still recovering from
the war to end all wars

in which millions of men
lost their lives.

Russia is newly communist.

Meanwhile, America is exporting
jazz to the world.

Thank you.
MUSIC PLAYS

'In arts, politics,
literature, economics,

'there was an insatiable appetite
for change.

'This was the birth of modernism.'

# You've got a heart that
there's no way of knowing

# Can see where you are
but can't see where you're going

# And I'm stuck here still

# I'm tangled up with you

# This whole world can be
so uncertain... #

But, and I might get into trouble
for saying this,

I would argue that the upheaval
that took place in physics

at this time would eclipse them all

and have far longer
lasting consequences.

It had begun with
the discovery of the weird

and contradictory wave/particle
nature of light,

it ended up as an epic battle
fought between the greatest minds

in science for the highest
possible stakes -

the nature of reality itself.

# I know I deserve you,
I know you're my saviour

# But when I observe you,
you change your behaviour... #

'On one side, a new wave of
modernist revolutionary scientists

'and their leader, the brilliant
Danish physicist, Niels Bohr.

'On the other side, the voice
of reason, Albert Einstein,

'at the height of his powers
and now world-famous,

'a formidable adversary.'

# Tangled up with you... #

The battle raged for decades.

Actually, in some ways,
it still does.

It was fought across the world in
universities, at conferences,

in bars and cafes,
it would reduce grown men to tears

and it began with
a deceptively simple experiment.

# This whole world can be
so uncertain... #

'But weirdly, it was an experiment
that wasn't even about light,

'it was about the particles
that make electricity.'

# To somebody else... #

In the mid-1920s,
an experiment was carried out

at Bell Laboratories
in New Jersey in America

which uncovered something
entirely unexpected about electrons.

Now, at the time it was accepted
without question

that electrons were these
tiny lumps of matter,

small but solid particles,
like miniature billiard balls.

In the experiment, they fired
a beam of electrons at a crystal

and watched how they scattered.

Now, that's entirely equivalent
to taking a beam of electrons,

say from an electron gun,

and firing it at a screen
with two slits in it

so that the electrons
pass through the slits

and hit another screen at the back.

What the Bell scientists found

shocked the physics world
to the core.

To understand why, consider a
similar experiment with water waves.

I've set up a simple experiment.

I have a water ripple tank placed
on top of an overhead projector,

I have a generator producing waves
that pass through two narrow gaps.

The projector beams the image
of the waves onto the back wall.

You can see as the waves
come in from the left

and squeeze through the two gaps,

they spread out on the other side
and interfere with each other.

What this means is that when
you get the crest from one wave

meeting the crest from another,
they add up to make a higher wave.

But when the crest from one meets
a trough, they cancel out.

This gives rise to these
characteristic lines

leading to the signature
wave pattern.

Bands of light and dark.

Whenever you see these
light and dark bands,

the signature wave pattern,

you know without doubt that
you've got wave-like behaviour.

So guess what
they saw in New Jersey.

Now it seemed that firing electrons,
tiny solid particles,

through the two gaps produced
exactly the same kind of pattern,

bands of light and dark.

First, light, for a long time
believed to be a wave,

was found to sometimes
behave like particles

and now electrons, for a long time
believed to be particles,

were behaving like waves.

But it was actually
stranger than that.

The wave pattern
wasn't merely some result

of the entire beam of electrons.

More recently this experiment
has been repeated

in labs around the world by
firing one electron at a time

through the slits onto the screen.

At first, each electron seems to
land randomly on the screen.

But gradually a pattern forms,
the signature wave pattern.

Let me be quite clear about
just how weird this is.

Remember from the
wave tank experiment

where the signature wave pattern
only exists

because each wave passes
through both slits

and then its two pieces
interfere with each other.

But here, every individual electron,

each single particle is passing
alone through the slits

before it hits the screen.

And yet, each single electron
is still contributing

to the signature wave pattern.

Each electron has to be
behaving like a wave.

To explain this strange result,

Niels Bohr and his colleagues
created quantum mechanics,

a crazy theory of light and matter
that embraced contradiction

and didn't care that it was
almost impossible to understand.

As Niels Bohr himself said,

anyone who isn't shocked by
quantum theory hasn't understood it.

So, viewers, I'm going
to take our tiny electron

and use it to delve deep
into the heart of reality.

And, yes, prepared to be shocked

because this is the only way
to explain what we observe

when a single electron
travels through the slits

and hits the screen.

Quantum mechanics says this...

..we can't describe what's
travelling as a physical object.

All we can talk about

are the chances of where
the electron might be.

This wave of chance somehow
travels through both slits

producing interference
just like the water wave.

Then when it hits the screen,

what was just the ghostly
possibility of an electron

mysteriously becomes real.

Let me try and capture just how
weird this is with an analogy.

If I spin this coin...

Then all the time it's spinning,
it's a blur,

I can't tell if it's heads or tails

but if I stop it, I force it
to decide and it's heads.

So before it was sort of not heads
or tails but a mixture of both

but as soon as I've stopped it,
I've forced it to make up its mind.

This is what Bohr and his supporters

claimed was happening
with our electrons.

In a sense, as it spins,
the coin is both heads and tails.

Similarly,
the electrons' wave of chance

passes through both slits,
two paths at the same time.

Our coin then stops at heads.

The ethereal wave of probability
hits the screen

and only then becomes a particle.

The quantum world was unlike
anything ever seen before.

It's hard to overstate
just how crazy this is.

Bohr was effectively claiming
that one can never know

where the electron actually is
at all until you measure it

and it's not just that you
don't know where the electron is,

it's weirdly as though the electron
itself is everywhere at once.

Bear in mind that electrons
are among the commonest

and most basic building blocks
of reality

and yet here's Bohr saying
that only by looking

do we actually conjure
their position into existence.

It's like there's a curtain
between us and the quantum world

and behind it
there is no solid reality...

..just the potential for reality.

Things only become real when we
pull back the curtain and look.

And this view, ladies and gentlemen,

became known as the
Copenhagen interpretation.

APPLAUSE

Persuasive as it might seem,

many people couldn't stomach
Niels Bohr's outlandish ideas.

And they found a natural leader
in the most powerful man in science.

Albert Einstein hated
this interpretation

with every fibre of his being.

He famously said,

"Does the moon cease to exist
when I don't look at it?"

He was very unhappy because
it gave limits to knowledge

that he didn't think
should be final.

He thought there should be
a better underlying theory.

Over the next ten years, Einstein
and Bohr would argue passionately

about whether quantum mechanics
meant giving up on reality or not.

Then, with two other scientists,
Nathan Rosen and Boris Podolsky,

Einstein thought they'd found a way
to win the argument.

He was convinced he'd found
a fatal flaw

in the Copenhagen interpretation
and it's claim that reality

was summoned into existence
by the act of looking at it.

At the heart of Einstein's argument

was an aspect of quantum
mechanics called entanglement.

Now, entanglement is this special,
incredibly close relationship

between a pair of quantum particles
whose fates are intertwined.

For example, if they were
created in the same event.

Let me try and explain this

by imagining the two particles
are spinning coins.

Imagine these coins
are two electrons

created from the same event and
then moved apart from each other.

Quantum mechanics says that,
because they're created together,

they're entangled.

And now many of their properties
are for ever linked,

wherever they are.

Remember, the Copenhagen
interpretation says that

until you measure one of the coins,
neither of them is heads or tails.

In fact, heads and tails
don't even exist.

And here's where entanglement makes
this weird situation even weirder.

When we stop the first coin
and it becomes heads...

..because the coins are linked
through entanglement,

the second coin will
simultaneously become tails.

And here's the crucial thing.

I can't predict what the outcome
of my measurement will be,

only that they
will always be opposite.

Einstein seized on this.

Because it meant that something was
happening between the two coins

that was almost
too crazy to imagine.

It's as if the two coins
are secretly communicating.

Communicating instantaneously
across space and time.

Even if the first coin was on Earth
and the other was on Pluto.

Einstein refused to believe

this instantaneous,
faster-than-light communication.

His theory of relativity said that
nothing could travel that fast.

Not even information.

So, how could one coin
instantaneously know

how the other would land?

He disparagingly called it
"spooky action at a distance"

and claimed it was a fatal flaw
in the Copenhagen interpretation.

What's more, he had a better idea.

Einstein believed there was
a simpler interpretation.

That somehow the destiny of
the two coins, whether or not they

ended up heads or tails, was already
fixed long before we observed them.

He said that although
it seemed the coin

was deciding to be, say, heads,
at the moment of observation,

actually, that decision
was taken long before.

It was just hidden from us.

In Einstein's mind,

quantum particles were
nothing like spinning coins.

They were more like, say,
a pair of gloves, left and right,

separated into boxes.

We don't know which box contains
which glove until we open one,

but when we do, and find,
say, a right-handed glove,

immediately, we know that the other
box contains the left-handed glove.

But, crucially, this requires
no spooky action at a distance.

Neither glove has been altered
by the act of observation.

Both of them were either

left or right-handed glove
from the beginning.

And the only thing that has changed
is our knowledge.

So, which is
the true description of reality?

Bohr's coins, which only become
real when we look at them...

..and then magically communicate
to each other,

or Einstein's gloves,
which are hidden from us,

but are definitely left or right
from the beginning?

In other words,
is there an objective reality,

as Einstein believed,
or not, as Bohr maintained?

In the late 1930s,
as the world plunged into war,

there was no way to answer
this question.

The battle to understand the nature
of reality was deadlocked.

The war rolled across Europe

and many of the leading scientists
fled to the United States.

Then, as the Second World War
led inextricably to the Cold War...

..American science,
backed by dollar bills

and a new vision of the future,
boomed.

Remember, after the war,
physicists came back raring to go

and tried to apply the ideas
of quantum theory to atoms,

the interaction between electrons
and light and what have you,

you didn't need to worry about
the philosophical side of things

to make progress with that.

So, as you say,
it really took a back seat.

Quantum mechanics led to a profound
understanding of semiconductors,

which helped create
the modern electronic age.

It produced lasers,
revolutionising communications,

breathtaking new medical advances.

And breakthroughs in nuclear power.

Quantum mechanics was so successful
that most working physicists

deliberately chose to ignore
Einstein's objections.

It simply didn't matter to them
because it worked.

They even coined a phrase for it,
"Shut up and calculate."

And the price for this success
was that Bohr and Einstein's debate

on the reality of the quantum world
was simply brushed under the carpet.

And amidst all this success
and pragmatism,

there were few who still worried
what it all meant.

But as the '50s rolled headlong
into the '60s, one lone dissenter

worked out how to settle
the argument once and for all.

John Bell, I think it's fair to say,

isn't well known
to the general public.

But to physicists like me,
he's, well, an hero.

He was an original thinker with
real courage in his convictions.

And the story of his rise to become
one of the greats of physics

is made even more remarkable
when you consider how he started.

He was born in Belfast in the 1920s
into a poor, working-class family.

His father was a horse dealer.

And they really struggled to get him

into Queen's University Belfast
to study physics.

He was the only one in his family
to even finish school.

This, I believe, made him insatiably
curious, fiery and stubborn.

I remember meeting John Bell
in 1989, a year before he died.

We were both at a conference
in America

and we happened to be sharing a lift
just after both attending

a talk on quantum mechanics.

Keen to say something to the great
John Bell, I said I thought

that the speaker's conclusions
were completely crazy.

He stared at me with his piercing
blue eyes and, for a moment,

I thought my fledgling physics
career was going down the drain.

But as the lift doors opened
and he was about to leave, he said,

"Yes, I completely agree with you.

"Haven't they heard of
the helium problem?"

To this day, I'm not quite sure
what the helium problem is,

but I was just so relieved
that John Bell and I agreed.

For many years, he worked here,

at Britain's atomic energy
research centre, Harwell,

who built this early experimental
nuclear reactor called DIDO.

It was here that he started
pondering the deep

and worrying questions
quantum mechanics raised.

Did the quantum world only exist
when it was observed?

Or was there a deeper truth
out there, waiting to be discovered?

In fact, he was so troubled,
he began to wonder

if there was a problem at the heart
of quantum mechanics.

He famously said, "I hesitate
to think it might be wrong,

"but I know it is rotten."

And so, in the early 1960s,
Bell decided to try

and resolve the crisis at the heart
of quantum physics.

It was an epic challenge.

After all, how do you check
if something is real,

if something is or isn't there,
all without looking?

How do you look behind the curtain
without pulling it open?

But John Bell came up with a
brilliant way of doing exactly that.

I think this is
one of THE most ingenious ideas

in the whole of physics.

It's certainly one of the most
difficult to understand and explain.

But I'm going to try
and have a go and, yes,

I'm afraid I'm going to use
another analogy.

This time, I'm going
to play a game of cards.

But it's one
for the highest possible stakes,

the nature of reality itself.

The card game is against
a mysterious quantum dealer.

The cards he deals
represent any subatomic particles,

or even quanta of light, photons.

And the game we'll play
will ultimately tell us

whether Einstein or Bohr was right.

Now, the rules of the game
are deceptively simple.

The dealer's going to deal
two cards face down.

If they're the same colour, I win.

If they're different colours,
I lose.

So I have a red,
so I need another red to win.

That's black. I lose.

Again, opposite colours.
I've lost both those.

That's four in a row.

That's six pairs in a row
that I've lost. OK.

I think I know
what the dealer's doing here.

Clearly,
the deck has been rigged in advance

so that every pair
came out as opposite colours.

But there's a simple way
to catch the dealer out.

So what we can do now
is change the rules of the game.

This time, if they are
the opposite colour, I win.

But once again, every time,
my evil quantum opponent beats me.

But again, I can see what the crafty
dealer could have done.

Maybe while I wasn't looking,
he's switched the pack

and rigged it so that
it always lands in his favour.

Now every pair is the same colour.

Rigged decks, remember, were
what Einstein thought

was really happening
in the entanglement experiment.

He said that, just like the gloves
were already placed in the box,

so the evil dealer stacked the cards
before we played.

But Niels Bohr's idea
was very different.

He said red and black don't even
exist until you turn them over.

Bell's genius was that he came up
with a way of deciding once

and for all who was right -
Einstein or Bohr.

This is how he did it.

I'm now not going to tell the dealer
which game I want to play,

same colours wins,
or different colour wins,

until after he's dealt the cards.

Now, because he can never predict
which rules I'm going to play by,

he can never
stack the deck correctly.

Now he can't win...or can he?

So now the rules are,
different wins.

They're the same. OK.

Same colour wins.

This gets to the very
heart of Bell's idea.

If we now start playing
and I win as many as I lose,

then Einstein was right.

The dealer is just a trickster
with a gift for slight of hand.

Reality may be tricky, but it
does have an objective existence.

But what if I lose?

Well, then I'm forced to admit that
there is no sensible explanation.

Each card must be sending
secret signals to the other

across space and time,
in defiance of everything we know.

I'm forced to accept that,
at the fundamental quantum level,

reality is truly unknowable.

Bell reduced this idea into
a single mathematical equation

that tells us once and for all
what seemed unanswerable.

How reality really is.

John Bell published his idea in 1964
and the extraordinary thing is,

at the time, the entire
physics community ignored him.

Total silence. It seems
the world simply wasn't ready.

Perhaps it was because his equation
seemed untestable,

or just because nobody thought
it was worth investigating.

But that was about to change.

And the change would
come from a very unexpected place.

# This is the dawning
of the age of Aquarius

# Age of Aquarius

# Aquarius

# Aquarius. #

America was in crisis over Vietnam,

Watergate, feminism,
the Black Panthers.

And while all this was going on,
a small group of hippy physicists

were working at the University
of Berkeley in California.

They did all the hippy things -

they smoked dope, they popped LSD,

they debated things like Buddhism
and telepathy.

# When the moon

# Is in the Seventh House... #

And they loved quantum mechanics.

In its weird version of reality,

they saw parallels
with their own esoteric beliefs.

# And love will steer the stars

# This is the dawning of...#

Their hippy, New Age-style physics

also caught
the attention of the public,

who read their crazy hippy books

that mixed quantum mechanics
with Eastern mysticism.

Books like The Tao Of Physics,

The Dancing Wu Li Masters

and my personal favourite,
Space-Time And Beyond -

Towards An Explanation
Of The Unexplainable.

But more importantly for our story,

the story of quantum mechanics,

these hippy physicists
also turned their attention

to Einstein's
now-famous thought experiment

and what it told us
about the nature of reality.

They saw Niels Bohr's
secret signalling

as proof that
physics supported their own ideas.

Because if two particles could
spookily communicate across space,

then ESP, telepathy and clairvoyance
were probably true as well.

If only they could prove
it really existed.

Then, in 1972, they realised that,

with a bit of mathematical
slight of hand,

they could take Bell's equation
and experimentally test it.

One of their group, John Clauser,

borrowed some equipment
from the lab he was working in

and set up the first genuine and
ultimate test of quantum mechanics.

This is a picture
of that first experiment,

built of leftovers
and stolen equipment.

Over the next few years,
it was improved by a team

led by Alain Aspect in Paris,
making its results more reliable.

Over ten years after Bell
first proposed his equation,

finally, it could be
put to the test.

This is a modern version
of the experiment

first carried out by John Clauser
and then Alain Aspect.

Here, a crystal converts laser light

into pairs of entangled
light quanta, photons,

making two very precise beams.

These photons are passed around
and bent back again

until they pass through
these detectors.

The two photons
are like the two cards

the evil dealer
places in front of me.

We'll measure a property
of the photons called polarisation,

which is equivalent to the colour
of the playing cards in my game.

So, for instance, winning with two
matching red cards might be the same

as two photons with
matching polarisation.

But because this is
quantum mechanics,

it's more complicated
than my simple card game.

And these dials here allow me

to measure a second property
of the photons as well.

Now that's equivalent to me

not only trying to guess
the colour of the face of the cards,

but also trying to guess
the colour of the back of the cards.

OK, so we're now going to switch on
the laser and start the experiment.

So this number here gives me

the number of photon pairs
coming through the experiment.

That's equivalent to the
pairs of cards in my game.

The graph here, dropping down,

gives me the probability that
I can win, that I'm guessing right.

The more photons,
the more accurate it becomes.

I'll stop at an uncertainty
of about 1%.

And the final answer is 0.56,
so if I...

..put that into my equation,

I now need to run the experiment
three more times,

corresponding to the four different
settings of these dials.

Each run is now like a different
set of rules for the quantum dealer.

And when I add them up
and get the answer,

if it's less than two,
then Einstein was right.

If it's greater than two,
then Bohr was right.

OK, so now for the second setting.

Just remember what
the experiment will show.

If the numbers
come out less than two,

then it's proof the dealer
has been stacking the deck.

This was Einstein's view.

OK, so the number I get this time
is 0.82.

Now, reset for run three.

But if the result
is greater than two,

then the deck cannot be stacked
and something else is at work.

OK, so the run three result
is -0.59.

And finally, run four.

This last number will finally reveal

if the world follows common sense,
or something much more bizarre.

OK, so our final result is in
and it's 0.56.

So if we turn the laser off...

Right, I'd better just work out
the answer.

And there we have it, 2.53.

It's a number greater than two.

Absolute proof
that Albert Einstein was wrong

and Niels Bohr was right.

The significance of this result
is simply enormous.

Just remember what it means.

Einstein's version of reality
cannot be true.

No amount of clever jiggery-pokery
with our experiment

can cheat nature.

The two entangled
photons' properties

couldn't have been set
from the beginning,

but are summoned into existence
only when we measure them.

Something strange
is linking them across space.

Something we can't explain
or even imagine

other than by using mathematics.

And weirder, photons do only
become real when we observe them.

In some strange sense,
it really does suggest

the moon doesn't exist
when we're not looking.

It truly defies common sense.

No wonder towards the end
of his life, Einstein wrote...

The experiment only confirms this.

Whatever is happening,
we just don't understand it.

But it doesn't mean
we should stop looking.

While it's true
that Einstein's dream of finding

a reasonable, common-sense
explanation was shattered for good,

my own personal view is that this

doesn't necessarily banish
physical reality.

Like Einstein, I still believe
there might be a more palatable

explanation underlying the weird
results of quantum mechanics.

One thing is clear, whether there
are physical, spooky connections,

whether there are
parallel universes,

whether we bring reality
into existence by looking,

whatever the truth is,

the weirdness of the quantum world
won't go away.

It'll rear its ugly head somewhere.

120 years ago, the greatest
scientific revolution ever

was brought about by a light bulb.

And scientists are still using
powerful light sources

like x-rays to unlock
nature's mysteries.

This is the Diamond Light Source.

It's Britain's single largest
science facility.

The x-rays produced here are
ten billion times more powerful

than a hospital x-ray.

With that's sort of power,
scientists can slice into matter

and glimpse
those quantum secrets inside.

Researchers here
are using this powerful light beam

to investigate new materials
which may have the potential

to bring about an electronics
breakthrough as great as any before.

Just as the quantum pioneers
of the '20s and '30s

ended up bringing about a scientific
and technological revolution,

so this generation of physicists are
set to usher in a new quantum era.

An era where Einstein's hated
quantum entanglement

now produces unbreakable
computer security.

New kinds of communication systems,
superfast computers

and other advances
we can't yet even imagine.

And this is why quantum mechanics
thrills and frustrates me.

It's capricious,
it's counterintuitive,

it even sometimes feels
just plain wrong.

And yet it still surprises us
every day.

And I, for one, believe that our
knowledge of the quantum world

is still far from complete.

That there are greater truths
about nature yet to be discovered.

And that's still
what keeps me awake at night.

Next week, join me as my journey
into the quantum world

gets even more surprising.

I investigate how its weird rules
are crucial for life

and how the bizarre behaviour
of subatomic particles

might even influence
evolution itself.

# I know I deserve you
I know you're my saviour

# But when I observe you

# You change your behaviour

# So I'm stuck here still

# I'm tangled up with you. #