(Xive TV theme)
- [Voiceover] 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
- [Voiceover] 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.
- [Voiceover] 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.
(coin rattling)
- [Voiceover] The story of our descent
into scientific madness begins with a most
unlikely object.
Berlin, 1890.
Germany is a new country, recently unified
and hungry to industrialize.
- In this newly unified Germany
a number of new engineering companies were founded.
They had spent millions buying the European patent
for Edison's new invention, the light bulb.
- [Voiceover] The light bulb was the epitome
of modern technology.
A great optimistic symbol of progress.
Engineering companies quickly realized
there were fortunes to be made
building streetlights for the new German empire.
But what they didn't realize
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.
- [Voiceover] 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 color of light it produces
was still a complete mystery.
- [Voiceover] 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.
- [Voiceover] Planck took on a deceptively simple problem,
why the color 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.
(classical music)
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.
- [Voiceover] But the light the bulb makes
isn't just getting brighter,
it's changing color too.
As I speed up, the color shifts
from red, to orange, to yellow.
- Right, now I'm going to really belt it.
- [Voiceover] Now the bulb's filament
is getting even hotter.
But although it certainly gets brighter,
the color seems to stay the same, yellow-white.
Why doesn't the light get any bluer?
(suspenseful music)
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 color, or frequency
of the light it produced.
Nowadays, over a hundred 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 color.
- [Voiceover] It's about the same color 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 color of light.
To produce light of this intensity and color
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.
- [Voiceover] 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 of
five and a half thousand 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.
- [Voiceover] Planck took a crucial first step
to solving this.
He found the precise mathematical link
between the color of light, its frequency,
and its energy.
But he didn't understand the connection.
However, it was another weird anomoly
that would really put the cat amongst the pigeons.
- In the late 19th century, scientists were studying
that their 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 cause 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 increase the brightness of the light,
still the gold leaves aren't affected.
- [Voiceover] Now I'll try this special blue light,
rich in ultraviolet.
- Immediately, the gold leaves collapse.
(electricity humming and cracking)
- [Voiceover] 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 photo-electric effect.
The ultraviolet catastrophe,
and the photo-electric 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 around the edges.
- [Voiceover] We understand this
as the light hitting the sides of my hand
and bending and smearing out slightly.
Just like water waves around an obstruction.
- Perfectly common sense, wave-like behavior.
- [Voiceover] And here's something else,
something rather beautiful.
Look at these soap bubbles.
Shine a light on them, and gorgeous colored 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 colors 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 worked perfectly well
for shadows and bubbles,
when it came to the ultraviolet catastrophe
and the photo-electric 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!
- [Voiceover] 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 centimeters high.
- These waves don't have much energy,
hardly enough energy to knock this top can
off the other.
- [Voiceover] But when the waves grow
to over a meter and a half,
it's a very different proposition.
- And they're nearly throwing me about.
There's now way I can keep this can balanced on the top.
(chuckles)
- [Voiceover] It's clear what water waves are telling us.
Bigger, more intense waves have more power.
They easily knock 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 him,
his name was Albert Einstein.
- [Voiceover] 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.
(dramatic instrumental music)
- [Voiceover] In 1905, he'd also come up with a new theory
to explain the photo-electric 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 photo-electric effect.
In this analogy, these red balls represent
Einstein's light quanta.
- [Voiceover] 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.
(upbeat instrumental music)
- [Voiceover] 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 plate.
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.
- [Voiceover] They're like the ultraviolet light
in the experiment.
Now, each individual light particle
carries more energy.
Because ultraviolet light is higher frequency.
(upbeat instrumental music)
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 photo-electric effect.
I remember when I first learnt about this,
being blown away by its sheer elegance and simplicity.
(electricity humming)
- [Voiceover] 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 a hundred times more energy.
No wonder there are so few of them.
- [Graham] That moment, at the beginning of the 20th century
signaled 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.
- [Voiceover] 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 photo-electric 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.
- [Voiceover] Because what seemed to be
a curious mystery about light
was about to become a battleground
about the nature of reality itself.
(jazz music)
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.
(jazz music)
- [Voiceover] In art, politics, literature, economics,
there was an insatiable appetite for change.
This was the birth of modernism.
♫ You've got a heart but there's no way of knowin'
♫ You 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.
♫ You know I deserve you
♫ I know you're my savior
♫ But when I observe you you change your behavior ♫
- [Voiceover] 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.
- 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 ♫
- [Voiceover] But weirdly it was an experiment
that wasn't even about light.
It was about the particles that make electricity.
- 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.
- [Voiceover] 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.
- [Voiceover] 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 behavior.
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.
- [Voiceover] 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.
- [Voiceover] 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.
- [Voiceover] And yet, each single electron
is still contributing to the signature wave pattern.
- Each electron has to be behaving like a wave.
(jazz music)
- [Voiceover] 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, prepare 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.
- [Voiceover] Quantum mechanics says this,
we can't describe what's traveling 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 is 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 forced it to make up its mind.
This is what Bohr and his supporters claimed
was happening with our electrons.
- [Voiceover] In a sense,
as it spins the coin is both heads and tails.
Similarly, the electron's 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 ...
- [Voiceover] 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)
- [Voiceover] Persuasive as it may 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 fiber 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.
- [Voiceover] Over the next 10 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 its 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
that two particles are spinning coins.
- [Voiceover] 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 forever 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.
- [Voiceover] 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.
- [Voiceover] 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.
- [Voiceover] 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 observe them.
- [Voiceover] He said that although it semed
that 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,
then 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.
- [Voiceover] 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.
- [Voiceover] The war rolled across Europe,
and many of the leading scientists
fled to the United States.
Then, as the second World War led inexorably
to the Cold War,
American science, backed by dollar bills
and a new vision of the future,
boomed.
- [Graham] Remember, after the war,
physicists came back, raring to go,
and try 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.
- [Voiceover] Quantum mechanics
led to a profound understanding of semiconductors,
which helped create the modern electronic age.
It produced lasers, revolutionizing communications,
breath-taking 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."
- [Voiceover] 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, a 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 Queens University, Belfast
to study physics.
In fact, he was the only one in his family
to even finish school.
This, I believe, made him insatiably curious,
firey, 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 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 Center at Harwell,
who built this early experimental nuclear reactor
called DIDO.
- [Voiceover] It was here that he started pondering
the deep and worrying questions
that 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.
- [Voiceover] 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.
- [Voiceover] The card game is against
the 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 is going to deal two cards, face down.
If they're the same color, I win,
if they're different colors, I lose.
So, I have a red, so I need another red to win.
That's black, I lose.
Again, opposite colors.
I've lost both those.
That's four in a row.
That's six pairs in a row that I've lost.
Okay, I think I know what the dealer's doing here,
clearly deck has been rigged in advance
so that every pair come out as opposite colors.
- [Voiceover] 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 color, I win.
- [Voiceover] 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 it always lands in his favor.
Now every pair is the same color.
- [Voiceover] 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 color wins, or different color wins,
until after he's dealt the cards.
- [Voiceover] 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.
The same, okay.
Same color 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.
- [Voiceover] 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.
- [Voiceover] 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 radio silence.
It seems the world simply wasn't ready.
- [Voiceover] 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.
(psychedelic rock music)
- 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.
- [Voiceover] And they loved quantum mechanics.
In its weird version of reality
they saw parallels with their own esoteric beliefs.
- 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 favorite,
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.
- [Voiceover] They saw Niels Bohr's secret signaling
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 realized 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.
- [Voiceover] This is a picture of that first experiment,
built of leftovers and stolen equpiment.
Over the next few years, it was improved
by a team lead 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.
- [Voiceover] 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.
- [Voiceover] 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 polarization.
- Which is equivalent to the color of the playing cards
in my game.
- [Voiceover] So, for instance, winning
with two matching red cards
might be the same as two photons
with matching polarization.
- But because this is quantum mechanics
it's more complicated than my simple card game.
- [Voiceover] 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 color of the face of the cards,
but also trying to guess the color
of the back of the cards.
Okay, so we now are 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 the 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.
- [Voiceover] The more photons,
the more accurate it becomes.
I'll stop at an uncertainty of about 1%.
- And the final answer is .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.
- [Voiceover] 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, the Bohr was right.
Okay, so now for the second setting.
- [Voiceover] 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.
- Okay, so the number I get this time is 0.82.
Now, reset for run three.
- [Voiceover] But if the result is greater than two,
then the deck cannot be stacked
and something else is at work.
- Okay, so the run three result is -0.59.
And finally, run four.
- [Voiceover] This last number will finally reveal
if the world follows common sense
or something much more bizarre.
- Okay, so our final result is in,
and it's 0.56.
So if we turn the laser off.
Right, I better just work out the answer.
And there were 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.
- [Voiceover] 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, "All these 50 years
"of conscious brooding have brought me no nearer
"to the question, what are light quanta.
"Every Tom, Dick, and Harry thinks he knows it
"but he is mistaken."
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.
- [Voiceover] Like Einstein, I still believe there might be
a more palatable explanation underlying
the weird results of quantum mechanics.
But 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.
- [Voiceover] 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 sort of power, scientists can slice into matter
and glimpse those quantum secrets inside.
- [Voiceover] 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,
super-fast 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 counter-intuitive,
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.
- [Voiceover] 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 behavior of subatomic particles
might even influence evolution itself.
(jazz music)
♫ I know I deserve you
♫ I know you're my savior
♫ But when I observe you, you change your behavior
♫ So I'm stuck here still
♫ I'm tangled up with you ♫
(Xive TV theme)