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Beneath the complexities
of everyday life,

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00:00:08,920 --> 00:00:12,920
the rules of our universe
seem reassuringly simple.

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00:00:14,840 --> 00:00:18,120
This solid bridge
supports my weight.

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00:00:18,120 --> 00:00:21,880
The water flowing underneath
always goes downhill

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and when I throw this stone...

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

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But as scientists peered deep

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into the tiny
building blocks of matter...

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..all such certainty vanished.

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They found the weird world
of quantum mechanics.

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Deep down inside everything
we see around us,

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we found a universe
completely unlike our own.

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To paraphrase one of the founders
of quantum mechanics,

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everything we call real
is made up of things

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that cannot be themselves
regarded as real.

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

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

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They found that in the realm
of the very small,

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things could be
in two places at once...

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

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..and that reality itself
defies all common sense.

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And at stake,
that everything we thought

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

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The story of our descent
into scientific madness

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begins with the most
unlikely object.

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Berlin, 1890.

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

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In this newly-unified Germany,

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a number of new engineering
companies were founded.

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They'd spent millions buying
the European patent

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for Edison's new invention,
the light bulb.

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The light bulb was
the epitome of modern technology,

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a great optimistic
symbol of progress.

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Engineering companies quickly
realised there were fortunes

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to be made building streetlights
for the new German Empire.

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

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a scientific revolution.

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Strangely enough,

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this humble object
is responsible for the birth

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of the most important theory
in the whole of science -

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

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And that's because, back in 1900,

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the light bulb presented
a rather strange problem.

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Engineers knew that if you heated
the filament with electricity,

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it glowed.

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

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But something as basic
as the relationship

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between the temperature
of the filament

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

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A mystery they were obviously
keen to solve.

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And, with the help
of the new German state,

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they saw how to steal a march
on their competitors.

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In 1887, the German government
invested millions

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in a new technical research
institute here in Berlin,

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The Physikalisch-Technische
Reichsanstalt, or PTR.

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Then, in 1900,
they enlisted a bright

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if somewhat straight-laced
scientist to help work here.

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His name was Max Planck.

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Planck took on a deceptively
simple problem -

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

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To get a sense of the puzzle
facing Planck,

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

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powered by an old-fashioned dynamo.

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Obviously the faster I go,
the brighter the light.

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

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

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

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it's changing colour, too.

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

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

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Now the bulb's filament
is getting even hotter,

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but although it certainly
gets brighter...

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

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Why doesn't the light
get any bluer?

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To investigate, Planck
and his colleagues built this,

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a black-body radiator.

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

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and a way to measure the colour
or frequency

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of the light it produced.

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

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this kind of measurement,
just much more accurately.

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The temperature inside here
is 841 degrees centigrade.

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

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with a lovely orangey-red colour.

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

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But I want to see something
hotter still.

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

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

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To produce light of this intensity
and colour

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requires a power
of about 40 kilowatts.

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

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or the combined output
of the entire Tour de France.

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

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there's very little blue.

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Why is blue so much harder
to make than red?

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And further up the spectrum,
beyond blue,

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

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

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

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produces mostly white visible light

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and makes remarkably little
ultraviolet light,

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given how hot it is. Why is this?

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Why is ultraviolet light
so hard to make?

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This remarkable failure of common
sense so perplexed scientists

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

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They called it
the ultraviolet catastrophe.

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Planck took a crucial first step
to solving this.

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He found the precise
mathematical link

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

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But he didn't understand
the connection.

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However, it was another
weird anomaly

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that would really put
the cat amongst the pigeons.

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In the late 19th century,
scientists were studying

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

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And to do that, they were building
experimental rigs

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

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they could generate huge voltages
that caused sparks

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to jump across the gap between
the two metal spheres.

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But, in doing so,

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they discovered something very
unexpected to do with light.

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They found that, by shining
a powerful light source

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

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This suggested a mysterious
and unexplained connection

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between light and electricity.

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To understand what was happening,
scientists used this.

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It's called a gold leaf
electroscope.

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

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

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What I'm doing is adding
an excess of electrons

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that are pushing
the two gold leaves apart.

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Now, first I take red light

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and shine it on the metal surface

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and nothing happens.

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Even if I increased
the brightness of the light,

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still the gold leaves
aren't affected.

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

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Immediately,
the gold leaves collapse.

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Light can clearly remove static
electric charge from the leaves.

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

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00:10:22,520 --> 00:10:27,000
But why is ultraviolet light so much
better at doing this than red light?

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00:10:30,080 --> 00:10:34,840
This new puzzle became known
as the photoelectric effect.

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The ultraviolet catastrophe
and the photoelectric effect

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were big problems for physicists,

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

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The science that said,
quite unequivocally,

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that light was a wave.

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All around us,

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

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

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

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and bending
and smearing out slightly,

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just like water waves
around an obstruction.

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Perfectly common-sense,
wave-like behaviour.

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And here's something else,
something rather beautiful.

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Look at these soap bubbles.

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Shine a light on them,

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and gorgeous coloured patterns
emerge from nowhere.

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

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

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and breaking up
into the colours of the rainbow.

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Rather like ripples
on the surface of water,

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light was simply ripples of energy
spreading through space

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

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But although this wave theory
works perfectly well for shadows

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and bubbles, when it came to
the ultraviolet catastrophe

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and photoelectric effect...

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..the wheels started coming off.

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00:12:17,800 --> 00:12:21,800
The problem was this -
how could light do this?

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To truly grasp how absurd
this phenomenon was,

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

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Hey!

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

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

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

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These waves don't have much energy,

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

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

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it's a very different proposition.

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And they're really
throwing me about.

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

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It's clear what water waves
are telling us -

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bigger, more intense waves
have more power.

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00:13:32,080 --> 00:13:34,560
They easily knocked me
and the cans around.

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So if light was a wave,

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more intensity
should knock out more electrons.

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But that's not what happened.

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Remember, no matter
how intense the red light was,

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it still didn't budge
electrons from the metal.

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But, weirdly, weak ultraviolet
worked within seconds.

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00:14:00,880 --> 00:14:04,360
So thinking of light as a wave
just wasn't adding up.

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00:14:08,640 --> 00:14:12,120
To resolve this, someone needed
to think the unthinkable

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00:14:12,120 --> 00:14:16,320
and, in 1905, someone did.
You may well have heard of them.

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00:14:16,320 --> 00:14:18,200
His name was Albert Einstein.

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00:14:28,640 --> 00:14:32,480
This is the Archenhold-Sternwarte
Observatory in Berlin.

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00:14:34,400 --> 00:14:40,120
Perched on top is a strange,
huge iron and steel construction,

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00:14:40,120 --> 00:14:43,800
but it's not a gun,
it's actually a telescope.

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00:14:47,760 --> 00:14:52,280
Built in 1896, the telescope was
one of the largest of its kind

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00:14:52,280 --> 00:14:56,360
in the world and made
the observatory the go-to place

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00:14:56,360 --> 00:15:00,000
to engage and astound
the public in new science.

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00:15:01,400 --> 00:15:04,560
Albert Einstein gave a very famous
public lecture here

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00:15:04,560 --> 00:15:08,960
on his theory of relativity which is
of course what he's most famous for.

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00:15:08,960 --> 00:15:12,200
But it's not the work
that won him the Nobel Prize.

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00:15:18,920 --> 00:15:23,520
In 1905, he'd also come up
with a new theory to explain

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00:15:23,520 --> 00:15:29,320
the photoelectric effect and
what he suggested was revolutionary

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00:15:29,320 --> 00:15:31,120
and even heretical.

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00:15:35,440 --> 00:15:39,840
He argued that we have to forget all
about the idea that light is a wave

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00:15:39,840 --> 00:15:45,360
and think of it instead as a stream
of tiny, bullet-like particles.

208
00:15:45,360 --> 00:15:49,720
The term he used to describe
a particle of light was a quantum.

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00:15:52,280 --> 00:15:56,360
To Einstein, a quantum
was a tiny lump of energy

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00:15:56,360 --> 00:15:59,800
and although in 1905
the word wasn't new,

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00:15:59,800 --> 00:16:03,480
the idea that light
could be a quantum seemed crazy.

212
00:16:06,920 --> 00:16:09,800
And yet following Einstein's
heretical line of thought

213
00:16:09,800 --> 00:16:11,920
to its logical conclusion

214
00:16:11,920 --> 00:16:15,600
solved all the problems with light
at a single stroke.

215
00:16:22,880 --> 00:16:26,480
I'll try to explain how this helps
using a rough analogy.

216
00:16:26,480 --> 00:16:29,600
Of course, like all analogies,
it's far from perfect

217
00:16:29,600 --> 00:16:32,720
but hopefully it'll give you
a sense of the physics

218
00:16:32,720 --> 00:16:37,720
to help you understand why thinking
of light as a stream of particles

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00:16:37,720 --> 00:16:40,320
solves the mystery
of the photoelectric effect.

220
00:16:44,640 --> 00:16:49,640
In this analogy, these red balls
represent Einstein's light quanta.

221
00:16:50,720 --> 00:16:54,520
'And those cans over there are
the electricity held in the metal.'

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00:16:54,520 --> 00:16:56,400
Now, in the original experiment,

223
00:16:56,400 --> 00:16:59,560
they made electricity flow
from the surface of the metal

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00:16:59,560 --> 00:17:03,040
by shining light on it.
In my analogy, I'm going to try

225
00:17:03,040 --> 00:17:06,440
and knock those tin cans over
using these red balls.

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00:17:18,120 --> 00:17:20,720
'Absolutely no effect.

227
00:17:23,400 --> 00:17:25,320
'That's just like red light.'

228
00:17:28,280 --> 00:17:30,120
According to Einstein,

229
00:17:30,120 --> 00:17:34,000
each particle of red light
carries very little energy

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00:17:34,000 --> 00:17:37,280
because red light
has a low frequency.

231
00:17:37,280 --> 00:17:42,000
'So even a very bright red light
with many red light particles

232
00:17:42,000 --> 00:17:45,760
'can't dislodge any electrons
from the metal plates,

233
00:17:45,760 --> 00:17:48,440
'just like the red balls.'

234
00:17:48,440 --> 00:17:52,960
Now I'm going to use heavier balls
like these blue golf balls

235
00:17:52,960 --> 00:17:56,000
and I'm going to try and knock off
the tin cans with these.

236
00:18:00,480 --> 00:18:03,760
'They're like the ultraviolet light
in the experiment.

237
00:18:06,440 --> 00:18:12,480
'Now, each individual light particle
carries more energy

238
00:18:12,480 --> 00:18:16,160
'because ultraviolet light
is higher frequency.'

239
00:18:22,720 --> 00:18:26,000
Just a few of them,
like a dim ultraviolet light,

240
00:18:26,000 --> 00:18:29,360
are enough to knock the electrons
out of the metal plate

241
00:18:29,360 --> 00:18:31,320
and collapse the gold leaf.

242
00:18:33,720 --> 00:18:38,760
So Einstein's idea that light is
made up of tiny particles or quanta

243
00:18:38,760 --> 00:18:42,200
is a wonderful explanation
of the photoelectric effect.

244
00:18:42,200 --> 00:18:44,360
I remember when I first
learnt about this,

245
00:18:44,360 --> 00:18:47,680
being blown away by its sheer
elegance and simplicity.

246
00:18:50,560 --> 00:18:54,720
But what's more, Einstein's
nifty idea also helped solve

247
00:18:54,720 --> 00:18:57,600
Planck's mystery of the light bulb.

248
00:18:57,600 --> 00:19:00,080
There was more red than ultraviolet

249
00:19:00,080 --> 00:19:04,640
because ultraviolet quanta
took so much more energy to make,

250
00:19:04,640 --> 00:19:07,680
about 100 times more energy.

251
00:19:07,680 --> 00:19:09,600
No wonder there are so few of them.

252
00:19:12,240 --> 00:19:15,120
That moment at the beginning
of the 20th century

253
00:19:15,120 --> 00:19:17,760
signalled a genuine revolution

254
00:19:17,760 --> 00:19:21,960
because it demonstrated that
the kind of physical science

255
00:19:21,960 --> 00:19:24,880
that people were doing right back
to Newton and Laplace,

256
00:19:24,880 --> 00:19:28,320
and people like that, that you
needed a completely new approach.

257
00:19:30,560 --> 00:19:33,160
Physics has never recovered
from that moment

258
00:19:33,160 --> 00:19:35,520
in the sense that it's built
on that moment,

259
00:19:35,520 --> 00:19:37,840
that's where modern physics
really began.

260
00:19:40,400 --> 00:19:44,880
But Einstein's theory also left
physicists with a dizzying paradox

261
00:19:44,880 --> 00:19:48,040
defying all common sense.

262
00:19:48,040 --> 00:19:52,280
Light was definitely a wave
which explained shadows and bubbles.

263
00:19:54,360 --> 00:19:57,400
And now it was definitely
a particle too -

264
00:19:57,400 --> 00:20:00,520
Einstein's quanta explaining
the photoelectric effect

265
00:20:00,520 --> 00:20:02,440
and the ultraviolet catastrophe.

266
00:20:04,080 --> 00:20:09,520
Then just a few years after
Einstein's brilliant, crazy idea,

267
00:20:09,520 --> 00:20:13,600
the paradox got a lot deeper
and a whole lot weirder.

268
00:20:16,000 --> 00:20:20,240
Because what seemed to be a
curious mystery about light

269
00:20:20,240 --> 00:20:25,000
was about to become a battleground
about the nature of reality itself.

270
00:20:34,560 --> 00:20:36,600
1922.

271
00:20:36,600 --> 00:20:40,960
The Western world was in the grip of
a revolution, a cultural revolution.

272
00:20:42,280 --> 00:20:45,080
James Joyce's Ulysses is published,

273
00:20:45,080 --> 00:20:47,760
Stravinsky is at the height
of his powers

274
00:20:47,760 --> 00:20:50,960
and Chaplin has just released
his first serious movie.

275
00:20:50,960 --> 00:20:54,280
The Ottoman Empire collapses.

276
00:20:54,280 --> 00:20:57,720
Europe is still recovering from
the war to end all wars

277
00:20:57,720 --> 00:21:00,320
in which millions of men
lost their lives.

278
00:21:00,320 --> 00:21:02,400
Russia is newly communist.

279
00:21:02,400 --> 00:21:06,480
Meanwhile, America is exporting
jazz to the world.

280
00:21:06,480 --> 00:21:08,760
Thank you.
MUSIC PLAYS

281
00:21:14,840 --> 00:21:18,440
'In arts, politics,
literature, economics,

282
00:21:18,440 --> 00:21:21,880
'there was an insatiable appetite
for change.

283
00:21:21,880 --> 00:21:24,920
'This was the birth of modernism.'

284
00:21:24,920 --> 00:21:29,120

there's no way of knowing

285
00:21:29,120 --> 00:21:33,040

but can't see where you're going

286
00:21:33,040 --> 00:21:35,800


287
00:21:35,800 --> 00:21:39,600


288
00:21:43,360 --> 00:21:48,280

so uncertain...

289
00:21:48,280 --> 00:21:51,960
But, and I might get into trouble
for saying this,

290
00:21:51,960 --> 00:21:55,000
I would argue that the upheaval
that took place in physics

291
00:21:55,000 --> 00:21:58,280
at this time would eclipse them all

292
00:21:58,280 --> 00:22:01,440
and have far longer
lasting consequences.

293
00:22:01,440 --> 00:22:04,600
It had begun with
the discovery of the weird

294
00:22:04,600 --> 00:22:08,200
and contradictory wave/particle
nature of light,

295
00:22:08,200 --> 00:22:12,360
it ended up as an epic battle
fought between the greatest minds

296
00:22:12,360 --> 00:22:15,440
in science for the highest
possible stakes -

297
00:22:15,440 --> 00:22:17,600
the nature of reality itself.

298
00:22:19,400 --> 00:22:24,240

I know you're my saviour

299
00:22:24,240 --> 00:22:28,280

you change your behaviour...

300
00:22:28,280 --> 00:22:32,360
'On one side, a new wave of
modernist revolutionary scientists

301
00:22:32,360 --> 00:22:36,600
'and their leader, the brilliant
Danish physicist, Niels Bohr.

302
00:22:38,120 --> 00:22:41,800
'On the other side, the voice
of reason, Albert Einstein,

303
00:22:41,800 --> 00:22:45,440
'at the height of his powers
and now world-famous,

304
00:22:45,440 --> 00:22:47,360
'a formidable adversary.'

305
00:22:47,360 --> 00:22:50,400


306
00:22:52,400 --> 00:22:54,800
The battle raged for decades.

307
00:22:54,800 --> 00:22:57,440
Actually, in some ways,
it still does.

308
00:22:57,440 --> 00:23:01,600
It was fought across the world in
universities, at conferences,

309
00:23:01,600 --> 00:23:05,840
in bars and cafes,
it would reduce grown men to tears

310
00:23:05,840 --> 00:23:11,280
and it began with
a deceptively simple experiment.

311
00:23:11,280 --> 00:23:16,280

so uncertain...

312
00:23:16,280 --> 00:23:20,320
'But weirdly, it was an experiment
that wasn't even about light,

313
00:23:20,320 --> 00:23:23,520
'it was about the particles
that make electricity.'

314
00:23:23,520 --> 00:23:29,160


315
00:23:29,160 --> 00:23:32,520
In the mid-1920s,
an experiment was carried out

316
00:23:32,520 --> 00:23:35,360
at Bell Laboratories
in New Jersey in America

317
00:23:35,360 --> 00:23:39,720
which uncovered something
entirely unexpected about electrons.

318
00:23:39,720 --> 00:23:42,240
Now, at the time it was accepted
without question

319
00:23:42,240 --> 00:23:45,280
that electrons were these
tiny lumps of matter,

320
00:23:45,280 --> 00:23:49,400
small but solid particles,
like miniature billiard balls.

321
00:23:49,400 --> 00:23:53,240
In the experiment, they fired
a beam of electrons at a crystal

322
00:23:53,240 --> 00:23:55,240
and watched how they scattered.

323
00:23:55,240 --> 00:23:59,440
Now, that's entirely equivalent
to taking a beam of electrons,

324
00:23:59,440 --> 00:24:01,640
say from an electron gun,

325
00:24:01,640 --> 00:24:06,400
and firing it at a screen
with two slits in it

326
00:24:06,400 --> 00:24:10,320
so that the electrons
pass through the slits

327
00:24:10,320 --> 00:24:13,080
and hit another screen at the back.

328
00:24:15,360 --> 00:24:17,440
What the Bell scientists found

329
00:24:17,440 --> 00:24:20,200
shocked the physics world
to the core.

330
00:24:22,480 --> 00:24:26,960
To understand why, consider a
similar experiment with water waves.

331
00:24:28,320 --> 00:24:30,600
I've set up a simple experiment.

332
00:24:30,600 --> 00:24:34,880
I have a water ripple tank placed
on top of an overhead projector,

333
00:24:34,880 --> 00:24:40,000
I have a generator producing waves
that pass through two narrow gaps.

334
00:24:40,000 --> 00:24:44,800
The projector beams the image
of the waves onto the back wall.

335
00:24:44,800 --> 00:24:47,440
You can see as the waves
come in from the left

336
00:24:47,440 --> 00:24:49,680
and squeeze through the two gaps,

337
00:24:49,680 --> 00:24:53,880
they spread out on the other side
and interfere with each other.

338
00:24:53,880 --> 00:24:57,000
What this means is that when
you get the crest from one wave

339
00:24:57,000 --> 00:25:01,160
meeting the crest from another,
they add up to make a higher wave.

340
00:25:01,160 --> 00:25:05,000
But when the crest from one meets
a trough, they cancel out.

341
00:25:07,560 --> 00:25:10,800
This gives rise to these
characteristic lines

342
00:25:10,800 --> 00:25:13,760
leading to the signature
wave pattern.

343
00:25:16,040 --> 00:25:17,760
Bands of light and dark.

344
00:25:20,440 --> 00:25:22,760
Whenever you see these
light and dark bands,

345
00:25:22,760 --> 00:25:24,360
the signature wave pattern,

346
00:25:24,360 --> 00:25:28,240
you know without doubt that
you've got wave-like behaviour.

347
00:25:36,480 --> 00:25:39,640
So guess what
they saw in New Jersey.

348
00:25:39,640 --> 00:25:44,720
Now it seemed that firing electrons,
tiny solid particles,

349
00:25:44,720 --> 00:25:49,040
through the two gaps produced
exactly the same kind of pattern,

350
00:25:49,040 --> 00:25:51,400
bands of light and dark.

351
00:25:53,920 --> 00:25:56,880
First, light, for a long time
believed to be a wave,

352
00:25:56,880 --> 00:25:59,520
was found to sometimes
behave like particles

353
00:25:59,520 --> 00:26:02,640
and now electrons, for a long time
believed to be particles,

354
00:26:02,640 --> 00:26:05,160
were behaving like waves.

355
00:26:05,160 --> 00:26:07,720
But it was actually
stranger than that.

356
00:26:07,720 --> 00:26:10,480
The wave pattern
wasn't merely some result

357
00:26:10,480 --> 00:26:13,560
of the entire beam of electrons.

358
00:26:13,560 --> 00:26:16,120
More recently this experiment
has been repeated

359
00:26:16,120 --> 00:26:21,480
in labs around the world by
firing one electron at a time

360
00:26:21,480 --> 00:26:24,040
through the slits onto the screen.

361
00:26:27,360 --> 00:26:31,680
At first, each electron seems to
land randomly on the screen.

362
00:26:33,920 --> 00:26:40,360
But gradually a pattern forms,
the signature wave pattern.

363
00:26:40,360 --> 00:26:44,160
Let me be quite clear about
just how weird this is.

364
00:26:44,160 --> 00:26:46,280
Remember from the
wave tank experiment

365
00:26:46,280 --> 00:26:49,560
where the signature wave pattern
only exists

366
00:26:49,560 --> 00:26:53,360
because each wave passes
through both slits

367
00:26:53,360 --> 00:26:56,840
and then its two pieces
interfere with each other.

368
00:26:56,840 --> 00:26:59,480
But here, every individual electron,

369
00:26:59,480 --> 00:27:03,160
each single particle is passing
alone through the slits

370
00:27:03,160 --> 00:27:05,600
before it hits the screen.

371
00:27:05,600 --> 00:27:09,280
And yet, each single electron
is still contributing

372
00:27:09,280 --> 00:27:11,040
to the signature wave pattern.

373
00:27:12,720 --> 00:27:16,400
Each electron has to be
behaving like a wave.

374
00:27:21,280 --> 00:27:24,120
To explain this strange result,

375
00:27:24,120 --> 00:27:27,960
Niels Bohr and his colleagues
created quantum mechanics,

376
00:27:27,960 --> 00:27:32,640
a crazy theory of light and matter
that embraced contradiction

377
00:27:32,640 --> 00:27:35,800
and didn't care that it was
almost impossible to understand.

378
00:27:35,800 --> 00:27:37,760
As Niels Bohr himself said,

379
00:27:37,760 --> 00:27:41,760
anyone who isn't shocked by
quantum theory hasn't understood it.

380
00:27:41,760 --> 00:27:44,800
So, viewers, I'm going
to take our tiny electron

381
00:27:44,800 --> 00:27:48,040
and use it to delve deep
into the heart of reality.

382
00:27:49,360 --> 00:27:51,440
And, yes, prepared to be shocked

383
00:27:51,440 --> 00:27:54,920
because this is the only way
to explain what we observe

384
00:27:54,920 --> 00:27:57,480
when a single electron
travels through the slits

385
00:27:57,480 --> 00:27:59,760
and hits the screen.

386
00:27:59,760 --> 00:28:01,840
Quantum mechanics says this...

387
00:28:03,200 --> 00:28:06,720
..we can't describe what's
travelling as a physical object.

388
00:28:08,520 --> 00:28:10,040
All we can talk about

389
00:28:10,040 --> 00:28:12,760
are the chances of where
the electron might be.

390
00:28:14,720 --> 00:28:19,960
This wave of chance somehow
travels through both slits

391
00:28:19,960 --> 00:28:23,560
producing interference
just like the water wave.

392
00:28:24,760 --> 00:28:27,040
Then when it hits the screen,

393
00:28:27,040 --> 00:28:30,560
what was just the ghostly
possibility of an electron

394
00:28:30,560 --> 00:28:33,680
mysteriously becomes real.

395
00:28:35,560 --> 00:28:39,800
Let me try and capture just how
weird this is with an analogy.

396
00:28:39,800 --> 00:28:41,920
If I spin this coin...

397
00:28:46,520 --> 00:28:49,160
Then all the time it's spinning,
it's a blur,

398
00:28:49,160 --> 00:28:51,520
I can't tell if it's heads or tails

399
00:28:51,520 --> 00:28:57,080
but if I stop it, I force it
to decide and it's heads.

400
00:28:57,080 --> 00:29:02,360
So before it was sort of not heads
or tails but a mixture of both

401
00:29:02,360 --> 00:29:06,000
but as soon as I've stopped it,
I've forced it to make up its mind.

402
00:29:06,000 --> 00:29:08,280
This is what Bohr and his supporters

403
00:29:08,280 --> 00:29:11,120
claimed was happening
with our electrons.

404
00:29:15,240 --> 00:29:20,880
In a sense, as it spins,
the coin is both heads and tails.

405
00:29:20,880 --> 00:29:24,640
Similarly,
the electrons' wave of chance

406
00:29:24,640 --> 00:29:29,480
passes through both slits,
two paths at the same time.

407
00:29:31,280 --> 00:29:34,040
Our coin then stops at heads.

408
00:29:35,680 --> 00:29:39,440
The ethereal wave of probability
hits the screen

409
00:29:39,440 --> 00:29:43,400
and only then becomes a particle.

410
00:29:43,400 --> 00:29:47,680
The quantum world was unlike
anything ever seen before.

411
00:29:49,720 --> 00:29:53,440
It's hard to overstate
just how crazy this is.

412
00:29:53,440 --> 00:29:57,040
Bohr was effectively claiming
that one can never know

413
00:29:57,040 --> 00:30:01,240
where the electron actually is
at all until you measure it

414
00:30:01,240 --> 00:30:04,640
and it's not just that you
don't know where the electron is,

415
00:30:04,640 --> 00:30:09,160
it's weirdly as though the electron
itself is everywhere at once.

416
00:30:11,480 --> 00:30:14,440
Bear in mind that electrons
are among the commonest

417
00:30:14,440 --> 00:30:17,720
and most basic building blocks
of reality

418
00:30:17,720 --> 00:30:21,160
and yet here's Bohr saying
that only by looking

419
00:30:21,160 --> 00:30:24,360
do we actually conjure
their position into existence.

420
00:30:25,480 --> 00:30:29,800
It's like there's a curtain
between us and the quantum world

421
00:30:29,800 --> 00:30:34,280
and behind it
there is no solid reality...

422
00:30:35,640 --> 00:30:38,360
..just the potential for reality.

423
00:30:40,800 --> 00:30:45,720
Things only become real when we
pull back the curtain and look.

424
00:30:45,720 --> 00:30:48,360
And this view, ladies and gentlemen,

425
00:30:48,360 --> 00:30:51,400
became known as the
Copenhagen interpretation.

426
00:30:51,400 --> 00:30:53,360
APPLAUSE

427
00:30:59,360 --> 00:31:01,160
Persuasive as it might seem,

428
00:31:01,160 --> 00:31:05,880
many people couldn't stomach
Niels Bohr's outlandish ideas.

429
00:31:05,880 --> 00:31:10,200
And they found a natural leader
in the most powerful man in science.

430
00:31:11,560 --> 00:31:14,400
Albert Einstein hated
this interpretation

431
00:31:14,400 --> 00:31:16,760
with every fibre of his being.

432
00:31:16,760 --> 00:31:18,120
He famously said,

433
00:31:18,120 --> 00:31:21,520
"Does the moon cease to exist
when I don't look at it?"

434
00:31:22,760 --> 00:31:26,760
He was very unhappy because
it gave limits to knowledge

435
00:31:26,760 --> 00:31:28,880
that he didn't think
should be final.

436
00:31:28,880 --> 00:31:32,600
He thought there should be
a better underlying theory.

437
00:31:36,560 --> 00:31:41,240
Over the next ten years, Einstein
and Bohr would argue passionately

438
00:31:41,240 --> 00:31:45,040
about whether quantum mechanics
meant giving up on reality or not.

439
00:31:49,160 --> 00:31:54,040
Then, with two other scientists,
Nathan Rosen and Boris Podolsky,

440
00:31:54,040 --> 00:31:58,240
Einstein thought they'd found a way
to win the argument.

441
00:31:58,240 --> 00:32:00,440
He was convinced he'd found
a fatal flaw

442
00:32:00,440 --> 00:32:04,440
in the Copenhagen interpretation
and it's claim that reality

443
00:32:04,440 --> 00:32:08,600
was summoned into existence
by the act of looking at it.

444
00:32:08,600 --> 00:32:10,480
At the heart of Einstein's argument

445
00:32:10,480 --> 00:32:14,240
was an aspect of quantum
mechanics called entanglement.

446
00:32:14,240 --> 00:32:18,600
Now, entanglement is this special,
incredibly close relationship

447
00:32:18,600 --> 00:32:22,960
between a pair of quantum particles
whose fates are intertwined.

448
00:32:22,960 --> 00:32:25,800
For example, if they were
created in the same event.

449
00:32:31,120 --> 00:32:32,400
Let me try and explain this

450
00:32:32,400 --> 00:32:35,840
by imagining the two particles
are spinning coins.

451
00:32:41,360 --> 00:32:43,880
Imagine these coins
are two electrons

452
00:32:43,880 --> 00:32:49,560
created from the same event and
then moved apart from each other.

453
00:32:49,560 --> 00:32:52,920
Quantum mechanics says that,
because they're created together,

454
00:32:52,920 --> 00:32:54,760
they're entangled.

455
00:32:54,760 --> 00:32:57,960
And now many of their properties
are for ever linked,

456
00:32:57,960 --> 00:33:00,240
wherever they are.

457
00:33:00,240 --> 00:33:02,880
Remember, the Copenhagen
interpretation says that

458
00:33:02,880 --> 00:33:07,240
until you measure one of the coins,
neither of them is heads or tails.

459
00:33:07,240 --> 00:33:09,880
In fact, heads and tails
don't even exist.

460
00:33:09,880 --> 00:33:13,800
And here's where entanglement makes
this weird situation even weirder.

461
00:33:15,560 --> 00:33:18,920
When we stop the first coin
and it becomes heads...

462
00:33:20,840 --> 00:33:23,680
..because the coins are linked
through entanglement,

463
00:33:23,680 --> 00:33:27,520
the second coin will
simultaneously become tails.

464
00:33:29,640 --> 00:33:31,240
And here's the crucial thing.

465
00:33:31,240 --> 00:33:35,000
I can't predict what the outcome
of my measurement will be,

466
00:33:35,000 --> 00:33:37,800
only that they
will always be opposite.

467
00:33:37,800 --> 00:33:39,280
Einstein seized on this.

468
00:33:40,880 --> 00:33:44,920
Because it meant that something was
happening between the two coins

469
00:33:44,920 --> 00:33:47,320
that was almost
too crazy to imagine.

470
00:33:49,000 --> 00:33:52,960
It's as if the two coins
are secretly communicating.

471
00:33:52,960 --> 00:33:57,120
Communicating instantaneously
across space and time.

472
00:33:57,120 --> 00:34:00,360
Even if the first coin was on Earth
and the other was on Pluto.

473
00:34:02,040 --> 00:34:03,880
Einstein refused to believe

474
00:34:03,880 --> 00:34:07,840
this instantaneous,
faster-than-light communication.

475
00:34:07,840 --> 00:34:12,040
His theory of relativity said that
nothing could travel that fast.

476
00:34:12,040 --> 00:34:13,320
Not even information.

477
00:34:14,400 --> 00:34:18,000
So, how could one coin
instantaneously know

478
00:34:18,000 --> 00:34:20,480
how the other would land?

479
00:34:20,480 --> 00:34:24,920
He disparagingly called it
"spooky action at a distance"

480
00:34:24,920 --> 00:34:29,440
and claimed it was a fatal flaw
in the Copenhagen interpretation.

481
00:34:29,440 --> 00:34:31,920
What's more, he had a better idea.

482
00:34:34,200 --> 00:34:37,480
Einstein believed there was
a simpler interpretation.

483
00:34:37,480 --> 00:34:41,240
That somehow the destiny of
the two coins, whether or not they

484
00:34:41,240 --> 00:34:45,520
ended up heads or tails, was already
fixed long before we observed them.

485
00:34:47,920 --> 00:34:50,160
He said that although
it seemed the coin

486
00:34:50,160 --> 00:34:54,600
was deciding to be, say, heads,
at the moment of observation,

487
00:34:54,600 --> 00:34:58,080
actually, that decision
was taken long before.

488
00:34:59,160 --> 00:35:00,840
It was just hidden from us.

489
00:35:03,920 --> 00:35:05,280
In Einstein's mind,

490
00:35:05,280 --> 00:35:09,200
quantum particles were
nothing like spinning coins.

491
00:35:09,200 --> 00:35:13,320
They were more like, say,
a pair of gloves, left and right,

492
00:35:13,320 --> 00:35:16,200
separated into boxes.

493
00:35:16,200 --> 00:35:21,400
We don't know which box contains
which glove until we open one,

494
00:35:21,400 --> 00:35:25,160
but when we do, and find,
say, a right-handed glove,

495
00:35:25,160 --> 00:35:29,240
immediately, we know that the other
box contains the left-handed glove.

496
00:35:29,240 --> 00:35:33,440
But, crucially, this requires
no spooky action at a distance.

497
00:35:33,440 --> 00:35:37,080
Neither glove has been altered
by the act of observation.

498
00:35:37,080 --> 00:35:38,480
Both of them were either

499
00:35:38,480 --> 00:35:41,320
left or right-handed glove
from the beginning.

500
00:35:41,320 --> 00:35:44,080
And the only thing that has changed
is our knowledge.

501
00:35:45,880 --> 00:35:49,640
So, which is
the true description of reality?

502
00:35:49,640 --> 00:35:53,520
Bohr's coins, which only become
real when we look at them...

503
00:35:56,520 --> 00:35:59,680
..and then magically communicate
to each other,

504
00:35:59,680 --> 00:36:02,720
or Einstein's gloves,
which are hidden from us,

505
00:36:02,720 --> 00:36:06,520
but are definitely left or right
from the beginning?

506
00:36:06,520 --> 00:36:09,600
In other words,
is there an objective reality,

507
00:36:09,600 --> 00:36:12,880
as Einstein believed,
or not, as Bohr maintained?

508
00:36:14,400 --> 00:36:17,480
In the late 1930s,
as the world plunged into war,

509
00:36:17,480 --> 00:36:20,320
there was no way to answer
this question.

510
00:36:20,320 --> 00:36:24,080
The battle to understand the nature
of reality was deadlocked.

511
00:36:30,680 --> 00:36:33,000
The war rolled across Europe

512
00:36:33,000 --> 00:36:36,520
and many of the leading scientists
fled to the United States.

513
00:36:39,000 --> 00:36:42,520
Then, as the Second World War
led inextricably to the Cold War...

514
00:36:45,040 --> 00:36:47,680
..American science,
backed by dollar bills

515
00:36:47,680 --> 00:36:50,120
and a new vision of the future,
boomed.

516
00:36:52,080 --> 00:36:55,760
Remember, after the war,
physicists came back raring to go

517
00:36:55,760 --> 00:37:00,640
and tried to apply the ideas
of quantum theory to atoms,

518
00:37:00,640 --> 00:37:04,320
the interaction between electrons
and light and what have you,

519
00:37:04,320 --> 00:37:07,280
you didn't need to worry about
the philosophical side of things

520
00:37:07,280 --> 00:37:09,080
to make progress with that.

521
00:37:09,080 --> 00:37:11,680
So, as you say,
it really took a back seat.

522
00:37:14,160 --> 00:37:18,600
Quantum mechanics led to a profound
understanding of semiconductors,

523
00:37:18,600 --> 00:37:21,360
which helped create
the modern electronic age.

524
00:37:23,520 --> 00:37:26,800
It produced lasers,
revolutionising communications,

525
00:37:26,800 --> 00:37:29,120
breathtaking new medical advances.

526
00:37:33,280 --> 00:37:35,400
And breakthroughs in nuclear power.

527
00:37:39,760 --> 00:37:43,920
Quantum mechanics was so successful
that most working physicists

528
00:37:43,920 --> 00:37:48,520
deliberately chose to ignore
Einstein's objections.

529
00:37:48,520 --> 00:37:52,560
It simply didn't matter to them
because it worked.

530
00:37:52,560 --> 00:37:56,560
They even coined a phrase for it,
"Shut up and calculate."

531
00:38:00,640 --> 00:38:04,560
And the price for this success
was that Bohr and Einstein's debate

532
00:38:04,560 --> 00:38:08,800
on the reality of the quantum world
was simply brushed under the carpet.

533
00:38:12,760 --> 00:38:15,640
And amidst all this success
and pragmatism,

534
00:38:15,640 --> 00:38:18,440
there were few who still worried
what it all meant.

535
00:38:19,600 --> 00:38:24,040
But as the '50s rolled headlong
into the '60s, one lone dissenter

536
00:38:24,040 --> 00:38:27,840
worked out how to settle
the argument once and for all.

537
00:38:39,640 --> 00:38:41,960
John Bell, I think it's fair to say,

538
00:38:41,960 --> 00:38:45,000
isn't well known
to the general public.

539
00:38:45,000 --> 00:38:49,040
But to physicists like me,
he's, well, an hero.

540
00:38:49,040 --> 00:38:53,160
He was an original thinker with
real courage in his convictions.

541
00:38:53,160 --> 00:38:57,720
And the story of his rise to become
one of the greats of physics

542
00:38:57,720 --> 00:39:01,960
is made even more remarkable
when you consider how he started.

543
00:39:01,960 --> 00:39:06,720
He was born in Belfast in the 1920s
into a poor, working-class family.

544
00:39:06,720 --> 00:39:08,800
His father was a horse dealer.

545
00:39:08,800 --> 00:39:10,480
And they really struggled to get him

546
00:39:10,480 --> 00:39:14,280
into Queen's University Belfast
to study physics.

547
00:39:14,280 --> 00:39:17,720
He was the only one in his family
to even finish school.

548
00:39:17,720 --> 00:39:23,160
This, I believe, made him insatiably
curious, fiery and stubborn.

549
00:39:28,920 --> 00:39:33,360
I remember meeting John Bell
in 1989, a year before he died.

550
00:39:33,360 --> 00:39:35,880
We were both at a conference
in America

551
00:39:35,880 --> 00:39:39,280
and we happened to be sharing a lift
just after both attending

552
00:39:39,280 --> 00:39:41,600
a talk on quantum mechanics.

553
00:39:41,600 --> 00:39:46,280
Keen to say something to the great
John Bell, I said I thought

554
00:39:46,280 --> 00:39:50,360
that the speaker's conclusions
were completely crazy.

555
00:39:50,360 --> 00:39:53,600
He stared at me with his piercing
blue eyes and, for a moment,

556
00:39:53,600 --> 00:39:57,360
I thought my fledgling physics
career was going down the drain.

557
00:39:57,360 --> 00:40:00,840
But as the lift doors opened
and he was about to leave, he said,

558
00:40:00,840 --> 00:40:02,560
"Yes, I completely agree with you.

559
00:40:02,560 --> 00:40:05,360
"Haven't they heard of
the helium problem?"

560
00:40:05,360 --> 00:40:08,640
To this day, I'm not quite sure
what the helium problem is,

561
00:40:08,640 --> 00:40:11,880
but I was just so relieved
that John Bell and I agreed.

562
00:40:23,680 --> 00:40:25,480
For many years, he worked here,

563
00:40:25,480 --> 00:40:28,720
at Britain's atomic energy
research centre, Harwell,

564
00:40:28,720 --> 00:40:32,520
who built this early experimental
nuclear reactor called DIDO.

565
00:40:37,280 --> 00:40:40,040
It was here that he started
pondering the deep

566
00:40:40,040 --> 00:40:43,880
and worrying questions
quantum mechanics raised.

567
00:40:43,880 --> 00:40:47,560
Did the quantum world only exist
when it was observed?

568
00:40:47,560 --> 00:40:51,280
Or was there a deeper truth
out there, waiting to be discovered?

569
00:40:53,360 --> 00:40:56,080
In fact, he was so troubled,
he began to wonder

570
00:40:56,080 --> 00:40:59,400
if there was a problem at the heart
of quantum mechanics.

571
00:41:01,720 --> 00:41:06,080
He famously said, "I hesitate
to think it might be wrong,

572
00:41:06,080 --> 00:41:08,920
"but I know it is rotten."

573
00:41:08,920 --> 00:41:11,600
And so, in the early 1960s,
Bell decided to try

574
00:41:11,600 --> 00:41:14,800
and resolve the crisis at the heart
of quantum physics.

575
00:41:14,800 --> 00:41:16,720
It was an epic challenge.

576
00:41:16,720 --> 00:41:20,360
After all, how do you check
if something is real,

577
00:41:20,360 --> 00:41:24,680
if something is or isn't there,
all without looking?

578
00:41:24,680 --> 00:41:28,040
How do you look behind the curtain
without pulling it open?

579
00:41:29,280 --> 00:41:34,320
But John Bell came up with a
brilliant way of doing exactly that.

580
00:41:37,200 --> 00:41:39,560
I think this is
one of THE most ingenious ideas

581
00:41:39,560 --> 00:41:41,960
in the whole of physics.

582
00:41:41,960 --> 00:41:45,200
It's certainly one of the most
difficult to understand and explain.

583
00:41:45,200 --> 00:41:47,440
But I'm going to try
and have a go and, yes,

584
00:41:47,440 --> 00:41:50,200
I'm afraid I'm going to use
another analogy.

585
00:41:50,200 --> 00:41:53,440
This time, I'm going
to play a game of cards.

586
00:41:53,440 --> 00:41:57,000
But it's one
for the highest possible stakes,

587
00:41:57,000 --> 00:41:59,240
the nature of reality itself.

588
00:42:01,160 --> 00:42:05,440
The card game is against
a mysterious quantum dealer.

589
00:42:05,440 --> 00:42:09,640
The cards he deals
represent any subatomic particles,

590
00:42:09,640 --> 00:42:12,120
or even quanta of light, photons.

591
00:42:13,440 --> 00:42:16,800
And the game we'll play
will ultimately tell us

592
00:42:16,800 --> 00:42:19,480
whether Einstein or Bohr was right.

593
00:42:21,200 --> 00:42:24,360
Now, the rules of the game
are deceptively simple.

594
00:42:24,360 --> 00:42:28,160
The dealer's going to deal
two cards face down.

595
00:42:28,160 --> 00:42:31,360
If they're the same colour, I win.

596
00:42:31,360 --> 00:42:33,640
If they're different colours,
I lose.

597
00:42:40,200 --> 00:42:45,040
So I have a red,
so I need another red to win.

598
00:42:45,040 --> 00:42:47,680
That's black. I lose.

599
00:42:50,760 --> 00:42:54,080
Again, opposite colours.
I've lost both those.

600
00:43:01,400 --> 00:43:03,680
That's four in a row.

601
00:43:09,160 --> 00:43:13,680
That's six pairs in a row
that I've lost. OK.

602
00:43:13,680 --> 00:43:16,240
I think I know
what the dealer's doing here.

603
00:43:16,240 --> 00:43:18,840
Clearly,
the deck has been rigged in advance

604
00:43:18,840 --> 00:43:23,080
so that every pair
came out as opposite colours.

605
00:43:23,080 --> 00:43:26,800
But there's a simple way
to catch the dealer out.

606
00:43:26,800 --> 00:43:30,760
So what we can do now
is change the rules of the game.

607
00:43:30,760 --> 00:43:35,320
This time, if they are
the opposite colour, I win.

608
00:43:41,120 --> 00:43:45,960
But once again, every time,
my evil quantum opponent beats me.

609
00:43:54,400 --> 00:43:57,760
But again, I can see what the crafty
dealer could have done.

610
00:43:57,760 --> 00:44:00,680
Maybe while I wasn't looking,
he's switched the pack

611
00:44:00,680 --> 00:44:04,400
and rigged it so that
it always lands in his favour.

612
00:44:04,400 --> 00:44:06,480
Now every pair is the same colour.

613
00:44:11,200 --> 00:44:14,200
Rigged decks, remember, were
what Einstein thought

614
00:44:14,200 --> 00:44:17,640
was really happening
in the entanglement experiment.

615
00:44:17,640 --> 00:44:22,240
He said that, just like the gloves
were already placed in the box,

616
00:44:22,240 --> 00:44:26,400
so the evil dealer stacked the cards
before we played.

617
00:44:27,600 --> 00:44:31,080
But Niels Bohr's idea
was very different.

618
00:44:31,080 --> 00:44:37,360
He said red and black don't even
exist until you turn them over.

619
00:44:37,360 --> 00:44:41,000
Bell's genius was that he came up
with a way of deciding once

620
00:44:41,000 --> 00:44:45,440
and for all who was right -
Einstein or Bohr.

621
00:44:45,440 --> 00:44:47,080
This is how he did it.

622
00:44:47,080 --> 00:44:51,200
I'm now not going to tell the dealer
which game I want to play,

623
00:44:51,200 --> 00:44:54,240
same colours wins,
or different colour wins,

624
00:44:54,240 --> 00:44:56,840
until after he's dealt the cards.

625
00:45:00,680 --> 00:45:06,240
Now, because he can never predict
which rules I'm going to play by,

626
00:45:06,240 --> 00:45:08,640
he can never
stack the deck correctly.

627
00:45:10,280 --> 00:45:13,880
Now he can't win...or can he?

628
00:45:15,120 --> 00:45:19,080
So now the rules are,
different wins.

629
00:45:22,240 --> 00:45:24,000
They're the same. OK.

630
00:45:25,080 --> 00:45:26,720
Same colour wins.

631
00:45:30,040 --> 00:45:33,080
This gets to the very
heart of Bell's idea.

632
00:45:33,080 --> 00:45:36,600
If we now start playing
and I win as many as I lose,

633
00:45:36,600 --> 00:45:38,520
then Einstein was right.

634
00:45:38,520 --> 00:45:42,320
The dealer is just a trickster
with a gift for slight of hand.

635
00:45:44,200 --> 00:45:48,600
Reality may be tricky, but it
does have an objective existence.

636
00:45:52,320 --> 00:45:53,960
But what if I lose?

637
00:45:53,960 --> 00:45:58,800
Well, then I'm forced to admit that
there is no sensible explanation.

638
00:46:01,800 --> 00:46:05,920
Each card must be sending
secret signals to the other

639
00:46:05,920 --> 00:46:11,160
across space and time,
in defiance of everything we know.

640
00:46:11,160 --> 00:46:15,280
I'm forced to accept that,
at the fundamental quantum level,

641
00:46:15,280 --> 00:46:18,080
reality is truly unknowable.

642
00:46:23,200 --> 00:46:28,280
Bell reduced this idea into
a single mathematical equation

643
00:46:28,280 --> 00:46:33,680
that tells us once and for all
what seemed unanswerable.

644
00:46:33,680 --> 00:46:36,120
How reality really is.

645
00:46:37,360 --> 00:46:42,320
John Bell published his idea in 1964
and the extraordinary thing is,

646
00:46:42,320 --> 00:46:46,360
at the time, the entire
physics community ignored him.

647
00:46:46,360 --> 00:46:50,840
Total silence. It seems
the world simply wasn't ready.

648
00:46:53,280 --> 00:46:57,040
Perhaps it was because his equation
seemed untestable,

649
00:46:57,040 --> 00:47:00,720
or just because nobody thought
it was worth investigating.

650
00:47:00,720 --> 00:47:04,120
But that was about to change.

651
00:47:04,120 --> 00:47:07,440
And the change would
come from a very unexpected place.

652
00:47:10,520 --> 00:47:15,560

of the age of Aquarius

653
00:47:15,560 --> 00:47:20,040


654
00:47:20,040 --> 00:47:25,560


655
00:47:25,560 --> 00:47:30,440


656
00:47:31,560 --> 00:47:34,560
America was in crisis over Vietnam,

657
00:47:34,560 --> 00:47:37,840
Watergate, feminism,
the Black Panthers.

658
00:47:37,840 --> 00:47:41,280
And while all this was going on,
a small group of hippy physicists

659
00:47:41,280 --> 00:47:44,280
were working at the University
of Berkeley in California.

660
00:47:44,280 --> 00:47:46,040
They did all the hippy things -

661
00:47:46,040 --> 00:47:48,000
they smoked dope, they popped LSD,

662
00:47:48,000 --> 00:47:51,400
they debated things like Buddhism
and telepathy.

663
00:47:51,400 --> 00:47:53,720


664
00:47:53,720 --> 00:47:57,440


665
00:47:57,440 --> 00:47:59,880
And they loved quantum mechanics.

666
00:47:59,880 --> 00:48:02,240
In its weird version of reality,

667
00:48:02,240 --> 00:48:05,200
they saw parallels
with their own esoteric beliefs.

668
00:48:08,200 --> 00:48:11,760


669
00:48:11,760 --> 00:48:14,280


670
00:48:16,360 --> 00:48:18,800
Their hippy, New Age-style physics

671
00:48:18,800 --> 00:48:21,320
also caught
the attention of the public,

672
00:48:21,320 --> 00:48:23,440
who read their crazy hippy books

673
00:48:23,440 --> 00:48:27,080
that mixed quantum mechanics
with Eastern mysticism.

674
00:48:27,080 --> 00:48:30,080
Books like The Tao Of Physics,

675
00:48:30,080 --> 00:48:32,640
The Dancing Wu Li Masters

676
00:48:32,640 --> 00:48:36,480
and my personal favourite,
Space-Time And Beyond -

677
00:48:36,480 --> 00:48:38,920
Towards An Explanation
Of The Unexplainable.

678
00:48:40,560 --> 00:48:42,640
But more importantly for our story,

679
00:48:42,640 --> 00:48:44,560
the story of quantum mechanics,

680
00:48:44,560 --> 00:48:47,640
these hippy physicists
also turned their attention

681
00:48:47,640 --> 00:48:50,480
to Einstein's
now-famous thought experiment

682
00:48:50,480 --> 00:48:53,680
and what it told us
about the nature of reality.

683
00:48:55,560 --> 00:48:58,560
They saw Niels Bohr's
secret signalling

684
00:48:58,560 --> 00:49:02,360
as proof that
physics supported their own ideas.

685
00:49:02,360 --> 00:49:06,520
Because if two particles could
spookily communicate across space,

686
00:49:06,520 --> 00:49:12,520
then ESP, telepathy and clairvoyance
were probably true as well.

687
00:49:12,520 --> 00:49:16,120
If only they could prove
it really existed.

688
00:49:16,120 --> 00:49:18,280
Then, in 1972, they realised that,

689
00:49:18,280 --> 00:49:21,960
with a bit of mathematical
slight of hand,

690
00:49:21,960 --> 00:49:27,000
they could take Bell's equation
and experimentally test it.

691
00:49:27,000 --> 00:49:29,480
One of their group, John Clauser,

692
00:49:29,480 --> 00:49:32,200
borrowed some equipment
from the lab he was working in

693
00:49:32,200 --> 00:49:37,120
and set up the first genuine and
ultimate test of quantum mechanics.

694
00:49:40,240 --> 00:49:43,080
This is a picture
of that first experiment,

695
00:49:43,080 --> 00:49:46,200
built of leftovers
and stolen equipment.

696
00:49:46,200 --> 00:49:49,880
Over the next few years,
it was improved by a team

697
00:49:49,880 --> 00:49:54,040
led by Alain Aspect in Paris,
making its results more reliable.

698
00:49:55,440 --> 00:49:59,520
Over ten years after Bell
first proposed his equation,

699
00:49:59,520 --> 00:50:03,000
finally, it could be
put to the test.

700
00:50:03,000 --> 00:50:05,320
This is a modern version
of the experiment

701
00:50:05,320 --> 00:50:08,600
first carried out by John Clauser
and then Alain Aspect.

702
00:50:12,880 --> 00:50:15,520
Here, a crystal converts laser light

703
00:50:15,520 --> 00:50:19,200
into pairs of entangled
light quanta, photons,

704
00:50:19,200 --> 00:50:21,560
making two very precise beams.

705
00:50:27,560 --> 00:50:30,960
These photons are passed around
and bent back again

706
00:50:30,960 --> 00:50:33,640
until they pass through
these detectors.

707
00:50:33,640 --> 00:50:36,040
The two photons
are like the two cards

708
00:50:36,040 --> 00:50:38,160
the evil dealer
places in front of me.

709
00:50:40,440 --> 00:50:44,320
We'll measure a property
of the photons called polarisation,

710
00:50:44,320 --> 00:50:48,400
which is equivalent to the colour
of the playing cards in my game.

711
00:50:48,400 --> 00:50:53,000
So, for instance, winning with two
matching red cards might be the same

712
00:50:53,000 --> 00:50:56,480
as two photons with
matching polarisation.

713
00:50:56,480 --> 00:50:58,440
But because this is
quantum mechanics,

714
00:50:58,440 --> 00:51:01,360
it's more complicated
than my simple card game.

715
00:51:01,360 --> 00:51:03,520
And these dials here allow me

716
00:51:03,520 --> 00:51:06,520
to measure a second property
of the photons as well.

717
00:51:06,520 --> 00:51:08,000
Now that's equivalent to me

718
00:51:08,000 --> 00:51:11,240
not only trying to guess
the colour of the face of the cards,

719
00:51:11,240 --> 00:51:14,880
but also trying to guess
the colour of the back of the cards.

720
00:51:14,880 --> 00:51:18,360
OK, so we're now going to switch on
the laser and start the experiment.

721
00:51:24,360 --> 00:51:26,560
So this number here gives me

722
00:51:26,560 --> 00:51:29,720
the number of photon pairs
coming through the experiment.

723
00:51:29,720 --> 00:51:32,920
That's equivalent to the
pairs of cards in my game.

724
00:51:32,920 --> 00:51:34,440
The graph here, dropping down,

725
00:51:34,440 --> 00:51:38,680
gives me the probability that
I can win, that I'm guessing right.

726
00:51:38,680 --> 00:51:41,600
The more photons,
the more accurate it becomes.

727
00:51:41,600 --> 00:51:44,520
I'll stop at an uncertainty
of about 1%.

728
00:51:46,200 --> 00:51:50,760
And the final answer is 0.56,
so if I...

729
00:51:52,040 --> 00:51:53,880
..put that into my equation,

730
00:51:53,880 --> 00:51:56,920
I now need to run the experiment
three more times,

731
00:51:56,920 --> 00:52:01,760
corresponding to the four different
settings of these dials.

732
00:52:01,760 --> 00:52:07,280
Each run is now like a different
set of rules for the quantum dealer.

733
00:52:07,280 --> 00:52:09,480
And when I add them up
and get the answer,

734
00:52:09,480 --> 00:52:12,600
if it's less than two,
then Einstein was right.

735
00:52:12,600 --> 00:52:15,960
If it's greater than two,
then Bohr was right.

736
00:52:15,960 --> 00:52:18,480
OK, so now for the second setting.

737
00:52:18,480 --> 00:52:20,920
Just remember what
the experiment will show.

738
00:52:22,520 --> 00:52:25,640
If the numbers
come out less than two,

739
00:52:25,640 --> 00:52:29,000
then it's proof the dealer
has been stacking the deck.

740
00:52:29,000 --> 00:52:31,040
This was Einstein's view.

741
00:52:31,040 --> 00:52:34,920
OK, so the number I get this time
is 0.82.

742
00:52:40,080 --> 00:52:42,880
Now, reset for run three.

743
00:52:45,880 --> 00:52:48,360
But if the result
is greater than two,

744
00:52:48,360 --> 00:52:53,000
then the deck cannot be stacked
and something else is at work.

745
00:52:53,000 --> 00:52:56,920
OK, so the run three result
is -0.59.

746
00:52:56,920 --> 00:52:59,440
And finally, run four.

747
00:53:02,880 --> 00:53:05,720
This last number will finally reveal

748
00:53:05,720 --> 00:53:10,760
if the world follows common sense,
or something much more bizarre.

749
00:53:10,760 --> 00:53:14,800
OK, so our final result is in
and it's 0.56.

750
00:53:14,800 --> 00:53:16,640
So if we turn the laser off...

751
00:53:19,000 --> 00:53:21,880
Right, I'd better just work out
the answer.

752
00:53:27,520 --> 00:53:30,400
And there we have it, 2.53.

753
00:53:31,800 --> 00:53:33,840
It's a number greater than two.

754
00:53:33,840 --> 00:53:37,080
Absolute proof
that Albert Einstein was wrong

755
00:53:37,080 --> 00:53:39,520
and Niels Bohr was right.

756
00:53:48,840 --> 00:53:52,560
The significance of this result
is simply enormous.

757
00:53:52,560 --> 00:53:54,160
Just remember what it means.

758
00:53:54,160 --> 00:53:57,760
Einstein's version of reality
cannot be true.

759
00:53:57,760 --> 00:54:01,320
No amount of clever jiggery-pokery
with our experiment

760
00:54:01,320 --> 00:54:03,080
can cheat nature.

761
00:54:03,080 --> 00:54:05,360
The two entangled
photons' properties

762
00:54:05,360 --> 00:54:08,080
couldn't have been set
from the beginning,

763
00:54:08,080 --> 00:54:12,160
but are summoned into existence
only when we measure them.

764
00:54:14,880 --> 00:54:17,920
Something strange
is linking them across space.

765
00:54:17,920 --> 00:54:21,240
Something we can't explain
or even imagine

766
00:54:21,240 --> 00:54:23,760
other than by using mathematics.

767
00:54:23,760 --> 00:54:28,720
And weirder, photons do only
become real when we observe them.

768
00:54:30,480 --> 00:54:33,360
In some strange sense,
it really does suggest

769
00:54:33,360 --> 00:54:36,280
the moon doesn't exist
when we're not looking.

770
00:54:37,800 --> 00:54:40,000
It truly defies common sense.

771
00:54:41,800 --> 00:54:45,320
No wonder towards the end
of his life, Einstein wrote...

772
00:54:59,320 --> 00:55:01,840
The experiment only confirms this.

773
00:55:01,840 --> 00:55:05,360
Whatever is happening,
we just don't understand it.

774
00:55:07,920 --> 00:55:10,320
But it doesn't mean
we should stop looking.

775
00:55:12,160 --> 00:55:15,480
While it's true
that Einstein's dream of finding

776
00:55:15,480 --> 00:55:19,840
a reasonable, common-sense
explanation was shattered for good,

777
00:55:19,840 --> 00:55:22,160
my own personal view is that this

778
00:55:22,160 --> 00:55:25,360
doesn't necessarily banish
physical reality.

779
00:55:25,360 --> 00:55:29,440
Like Einstein, I still believe
there might be a more palatable

780
00:55:29,440 --> 00:55:33,880
explanation underlying the weird
results of quantum mechanics.

781
00:55:33,880 --> 00:55:38,800
One thing is clear, whether there
are physical, spooky connections,

782
00:55:38,800 --> 00:55:40,760
whether there are
parallel universes,

783
00:55:40,760 --> 00:55:44,840
whether we bring reality
into existence by looking,

784
00:55:44,840 --> 00:55:46,720
whatever the truth is,

785
00:55:46,720 --> 00:55:50,920
the weirdness of the quantum world
won't go away.

786
00:55:50,920 --> 00:55:53,360
It'll rear its ugly head somewhere.

787
00:55:56,920 --> 00:56:01,320
120 years ago, the greatest
scientific revolution ever

788
00:56:01,320 --> 00:56:03,360
was brought about by a light bulb.

789
00:56:07,040 --> 00:56:10,240
And scientists are still using
powerful light sources

790
00:56:10,240 --> 00:56:13,680
like x-rays to unlock
nature's mysteries.

791
00:56:18,480 --> 00:56:21,160
This is the Diamond Light Source.

792
00:56:21,160 --> 00:56:24,760
It's Britain's single largest
science facility.

793
00:56:24,760 --> 00:56:29,080
The x-rays produced here are
ten billion times more powerful

794
00:56:29,080 --> 00:56:30,800
than a hospital x-ray.

795
00:56:30,800 --> 00:56:34,880
With that's sort of power,
scientists can slice into matter

796
00:56:34,880 --> 00:56:37,240
and glimpse
those quantum secrets inside.

797
00:56:45,800 --> 00:56:49,440
Researchers here
are using this powerful light beam

798
00:56:49,440 --> 00:56:53,320
to investigate new materials
which may have the potential

799
00:56:53,320 --> 00:56:57,320
to bring about an electronics
breakthrough as great as any before.

800
00:57:02,400 --> 00:57:05,600
Just as the quantum pioneers
of the '20s and '30s

801
00:57:05,600 --> 00:57:09,800
ended up bringing about a scientific
and technological revolution,

802
00:57:09,800 --> 00:57:14,560
so this generation of physicists are
set to usher in a new quantum era.

803
00:57:16,480 --> 00:57:20,320
An era where Einstein's hated
quantum entanglement

804
00:57:20,320 --> 00:57:24,240
now produces unbreakable
computer security.

805
00:57:24,240 --> 00:57:28,080
New kinds of communication systems,
superfast computers

806
00:57:28,080 --> 00:57:31,080
and other advances
we can't yet even imagine.

807
00:57:40,560 --> 00:57:45,520
And this is why quantum mechanics
thrills and frustrates me.

808
00:57:45,520 --> 00:57:48,240
It's capricious,
it's counterintuitive,

809
00:57:48,240 --> 00:57:51,240
it even sometimes feels
just plain wrong.

810
00:57:51,240 --> 00:57:55,720
And yet it still surprises us
every day.

811
00:57:55,720 --> 00:57:59,280
And I, for one, believe that our
knowledge of the quantum world

812
00:57:59,280 --> 00:58:01,480
is still far from complete.

813
00:58:01,480 --> 00:58:06,040
That there are greater truths
about nature yet to be discovered.

814
00:58:06,040 --> 00:58:09,600
And that's still
what keeps me awake at night.

815
00:58:13,440 --> 00:58:17,480
Next week, join me as my journey
into the quantum world

816
00:58:17,480 --> 00:58:19,640
gets even more surprising.

817
00:58:19,640 --> 00:58:23,760
I investigate how its weird rules
are crucial for life

818
00:58:23,760 --> 00:58:26,960
and how the bizarre behaviour
of subatomic particles

819
00:58:26,960 --> 00:58:30,320
might even influence
evolution itself.

820
00:58:39,320 --> 00:58:43,960

I know you're my saviour

821
00:58:43,960 --> 00:58:46,000


822
00:58:46,000 --> 00:58:48,160


823
00:58:48,160 --> 00:58:50,920


824
00:58:50,920 --> 00:58:55,680