0:00:04 > 0:00:08Beneath the complexities of everyday life,
0:00:08 > 0:00:12the rules of our universe seem reassuringly simple.
0:00:14 > 0:00:17This solid bridge supports my weight.
0:00:17 > 0:00:21The water flowing underneath always goes downhill
0:00:21 > 0:00:23and when I throw this stone...
0:00:25 > 0:00:29..it always flies through the air following a predictable path.
0:00:30 > 0:00:32But as scientists peered deep
0:00:32 > 0:00:34into the tiny building blocks of matter...
0:00:37 > 0:00:39..all such certainty vanished.
0:00:42 > 0:00:46They found the weird world of quantum mechanics.
0:00:46 > 0:00:50Deep down inside everything we see around us,
0:00:50 > 0:00:54we found a universe completely unlike our own.
0:00:54 > 0:00:57To paraphrase one of the founders of quantum mechanics,
0:00:57 > 0:01:01everything we call real is made up of things
0:01:01 > 0:01:04that cannot be themselves regarded as real.
0:01:08 > 0:01:11Around 100 years ago, some of the world's greatest scientists
0:01:11 > 0:01:16began a journey down the rabbit hole into the strange and the bizarre.
0:01:18 > 0:01:21They found that in the realm of the very small,
0:01:21 > 0:01:23things could be in two places at once...
0:01:26 > 0:01:29..that their fates are dictated by chance...
0:01:31 > 0:01:35..and that reality itself defies all common sense.
0:01:38 > 0:01:40And at stake, that everything we thought
0:01:40 > 0:01:45we knew about the world might turn out to be completely wrong.
0:01:48 > 0:01:52The story of our descent into scientific madness
0:01:52 > 0:01:55begins with the most unlikely object.
0:01:58 > 0:02:01Berlin, 1890.
0:02:03 > 0:02:07Germany is a new country, recently unified and hungry to industrialise.
0:02:09 > 0:02:11In this newly-unified Germany,
0:02:11 > 0:02:14a number of new engineering companies were founded.
0:02:14 > 0:02:17They'd spent millions buying the European patent
0:02:17 > 0:02:20for Edison's new invention, the light bulb.
0:02:23 > 0:02:26The light bulb was the epitome of modern technology,
0:02:26 > 0:02:29a great optimistic symbol of progress.
0:02:33 > 0:02:36Engineering companies quickly realised there were fortunes
0:02:36 > 0:02:40to be made building streetlights for the new German Empire.
0:02:43 > 0:02:46But what they didn't realise was that they would also unleash
0:02:46 > 0:02:48a scientific revolution.
0:02:49 > 0:02:51Strangely enough,
0:02:51 > 0:02:54this humble object is responsible for the birth
0:02:54 > 0:02:57of the most important theory in the whole of science -
0:02:57 > 0:03:01quantum mechanics, a theory that I've spent my life studying.
0:03:03 > 0:03:06And that's because, back in 1900,
0:03:06 > 0:03:09the light bulb presented a rather strange problem.
0:03:10 > 0:03:14Engineers knew that if you heated the filament with electricity,
0:03:14 > 0:03:15it glowed.
0:03:17 > 0:03:21The physics that underpinned this, though, was completely unknown.
0:03:23 > 0:03:26But something as basic as the relationship
0:03:26 > 0:03:28between the temperature of the filament
0:03:28 > 0:03:31and the colour of light it produces was still a complete mystery.
0:03:33 > 0:03:36A mystery they were obviously keen to solve.
0:03:37 > 0:03:39And, with the help of the new German state,
0:03:39 > 0:03:42they saw how to steal a march on their competitors.
0:03:51 > 0:03:55In 1887, the German government invested millions
0:03:55 > 0:03:58in a new technical research institute here in Berlin,
0:03:58 > 0:04:02The Physikalisch-Technische Reichsanstalt, or PTR.
0:04:02 > 0:04:05Then, in 1900, they enlisted a bright
0:04:05 > 0:04:08if somewhat straight-laced scientist to help work here.
0:04:08 > 0:04:11His name was Max Planck.
0:04:17 > 0:04:20Planck took on a deceptively simple problem -
0:04:20 > 0:04:24why the colour of the light changes as the filament gets hotter.
0:04:28 > 0:04:30To get a sense of the puzzle facing Planck,
0:04:30 > 0:04:34I'm going to ride this bicycle with an old-fashioned lamp
0:04:34 > 0:04:36powered by an old-fashioned dynamo.
0:04:47 > 0:04:50Obviously the faster I go, the brighter the light.
0:04:50 > 0:04:54The more I pedal, the more electricity the dynamo produces,
0:04:54 > 0:04:57the hotter the filament in the lamp and the brighter the light.
0:04:59 > 0:05:03But the light the bulb makes isn't just getting brighter,
0:05:03 > 0:05:04it's changing colour, too.
0:05:07 > 0:05:12As I speed up, the colour shifts from red to orange to yellow.
0:05:13 > 0:05:15Right, now I'm going to really belt it.
0:05:19 > 0:05:21Now the bulb's filament is getting even hotter,
0:05:21 > 0:05:23but although it certainly gets brighter...
0:05:25 > 0:05:29..the colour seems to stay the same - yellow-white.
0:05:33 > 0:05:36Why doesn't the light get any bluer?
0:05:40 > 0:05:44To investigate, Planck and his colleagues built this,
0:05:44 > 0:05:46a black-body radiator.
0:05:46 > 0:05:50It's a special tube they could heat to a very precise temperature
0:05:50 > 0:05:53and a way to measure the colour or frequency
0:05:53 > 0:05:55of the light it produced.
0:06:00 > 0:06:05Nowadays, over 100 years later, the PTR still do exactly
0:06:05 > 0:06:09this kind of measurement, just much more accurately.
0:06:10 > 0:06:14The temperature inside here is 841 degrees centigrade.
0:06:17 > 0:06:20I can feel the heat coming off and it's glowing
0:06:20 > 0:06:22with a lovely orangey-red colour.
0:06:27 > 0:06:31It's about the same colour as my bike light when I'm cycling slowly.
0:06:33 > 0:06:37But I want to see something hotter still.
0:06:37 > 0:06:41The temperature inside here is about 2,000 degrees centigrade...
0:06:46 > 0:06:50..and it's glowing with a much brighter, whiter-coloured light.
0:06:50 > 0:06:52To produce light of this intensity and colour
0:06:52 > 0:06:55requires a power of about 40 kilowatts.
0:06:55 > 0:07:00Now, that's equivalent to about 400 mes on a bike cycling very fast,
0:07:00 > 0:07:04or the combined output of the entire Tour de France.
0:07:06 > 0:07:09Although the light is whiter, it's red-white -
0:07:09 > 0:07:12there's very little blue.
0:07:12 > 0:07:15Why is blue so much harder to make than red?
0:07:17 > 0:07:20And further up the spectrum, beyond blue,
0:07:20 > 0:07:24the so-called ultraviolet, is hardly produced at all -
0:07:24 > 0:07:27even when we look at things as hot as the sun.
0:07:31 > 0:07:36Even the sun, at a temperature 5,500 degrees centigrade,
0:07:36 > 0:07:39produces mostly white visible light
0:07:39 > 0:07:41and makes remarkably little ultraviolet light,
0:07:41 > 0:07:44given how hot it is. Why is this?
0:07:44 > 0:07:47Why is ultraviolet light so hard to make?
0:07:50 > 0:07:55This remarkable failure of common sense so perplexed scientists
0:07:55 > 0:07:59of the late 19th century that they gave it a very dramatic name.
0:07:59 > 0:08:02They called it the ultraviolet catastrophe.
0:08:04 > 0:08:07Planck took a crucial first step to solving this.
0:08:07 > 0:08:10He found the precise mathematical link
0:08:10 > 0:08:14between the colour of light, its frequency and its energy.
0:08:14 > 0:08:17But he didn't understand the connection.
0:08:19 > 0:08:21However, it was another weird anomaly
0:08:21 > 0:08:24that would really put the cat amongst the pigeons.
0:08:26 > 0:08:29In the late 19th century, scientists were studying
0:08:29 > 0:08:33the then newly-discovered radio waves and how they were transmitted.
0:08:33 > 0:08:36And to do that, they were building experimental rigs
0:08:36 > 0:08:39very similar to this one. Basically, by spinning this disc,
0:08:39 > 0:08:43they could generate huge voltages that caused sparks
0:08:43 > 0:08:46to jump across the gap between the two metal spheres.
0:08:49 > 0:08:51But, in doing so,
0:08:51 > 0:08:55they discovered something very unexpected to do with light.
0:08:55 > 0:08:59They found that, by shining a powerful light source
0:08:59 > 0:09:04on the spheres, they could make the sparks jump across more easily.
0:09:04 > 0:09:08This suggested a mysterious and unexplained connection
0:09:08 > 0:09:10between light and electricity.
0:09:16 > 0:09:20To understand what was happening, scientists used this.
0:09:20 > 0:09:23It's called a gold leaf electroscope.
0:09:23 > 0:09:28It's basically a more sensitive version of the spark gap apparatus.
0:09:28 > 0:09:30Now, first of all, I have to charge it up.
0:09:34 > 0:09:37What I'm doing is adding an excess of electrons
0:09:37 > 0:09:40that are pushing the two gold leaves apart.
0:09:40 > 0:09:42Now, first I take red light
0:09:42 > 0:09:45and shine it on the metal surface
0:09:45 > 0:09:47and nothing happens.
0:09:47 > 0:09:49Even if I increased the brightness of the light,
0:09:49 > 0:09:51still the gold leaves aren't affected.
0:09:53 > 0:09:57Now I'll try this special blue light, rich in ultraviolet.
0:09:59 > 0:10:02Immediately, the gold leaves collapse.
0:10:12 > 0:10:16Light can clearly remove static electric charge from the leaves.
0:10:16 > 0:10:20It can somehow knock out the electrons I added to them.
0:10:22 > 0:10:26But why is ultraviolet light so much better at doing this than red light?
0:10:29 > 0:10:34This new puzzle became known as the photoelectric effect.
0:10:36 > 0:10:40The ultraviolet catastrophe and the photoelectric effect
0:10:40 > 0:10:43were big problems for physicists,
0:10:43 > 0:10:47because neither could be understood using the best science of the time.
0:10:48 > 0:10:52The science that said, quite unequivocally,
0:10:52 > 0:10:54that light was a wave.
0:10:58 > 0:10:59All around us,
0:10:59 > 0:11:03we see light behaving in a perfectly common-sense wavy way.
0:11:03 > 0:11:07Look at the shadow of my hand. It's fuzzy round the edges.
0:11:09 > 0:11:13We understand this as the light hitting the side of my hand
0:11:13 > 0:11:16and bending and smearing out slightly,
0:11:16 > 0:11:19just like water waves around an obstruction.
0:11:19 > 0:11:22Perfectly common-sense, wave-like behaviour.
0:11:25 > 0:11:29And here's something else, something rather beautiful.
0:11:29 > 0:11:31Look at these soap bubbles.
0:11:31 > 0:11:32Shine a light on them,
0:11:32 > 0:11:36and gorgeous coloured patterns emerge from nowhere.
0:11:36 > 0:11:41And this was easily explained if you accept that light was a wave,
0:11:41 > 0:11:46reflecting off the outer and inner layers of the thin soap film
0:11:46 > 0:11:48and breaking up into the colours of the rainbow.
0:11:49 > 0:11:53Rather like ripples on the surface of water,
0:11:53 > 0:11:58light was simply ripples of energy spreading through space
0:11:58 > 0:12:02and this was as firmly accepted as the fact that the earth was round.
0:12:03 > 0:12:07But although this wave theory works perfectly well for shadows
0:12:07 > 0:12:11and bubbles, when it came to the ultraviolet catastrophe
0:12:11 > 0:12:13and photoelectric effect...
0:12:15 > 0:12:17..the wheels started coming off.
0:12:17 > 0:12:21The problem was this - how could light do this?
0:12:21 > 0:12:25To truly grasp how absurd this phenomenon was,
0:12:25 > 0:12:29it might be useful to consider how waves in water behave.
0:12:30 > 0:12:32Hey!
0:12:36 > 0:12:41This is the wave tank at the RNLI's headquarters in Dorset.
0:12:41 > 0:12:45It's used to train lifeboat teams to deal with a range of different
0:12:45 > 0:12:51kinds of water waves. First, small waves, just 30 centimetres high.
0:12:51 > 0:12:53These waves don't have much energy,
0:12:53 > 0:12:57hardly enough energy to knock this top can off the other.
0:13:00 > 0:13:04But when the waves grow to over a metre and a half,
0:13:04 > 0:13:07it's a very different proposition.
0:13:07 > 0:13:10And they're really throwing me about.
0:13:10 > 0:13:14There's no way I can keep this can balanced on the top.
0:13:22 > 0:13:26It's clear what water waves are telling us -
0:13:26 > 0:13:29bigger, more intense waves have more power.
0:13:31 > 0:13:34They easily knocked me and the cans around.
0:13:39 > 0:13:41So if light was a wave,
0:13:41 > 0:13:46more intensity should knock out more electrons.
0:13:46 > 0:13:48But that's not what happened.
0:13:48 > 0:13:51Remember, no matter how intense the red light was,
0:13:51 > 0:13:55it still didn't budge electrons from the metal.
0:13:55 > 0:14:00But, weirdly, weak ultraviolet worked within seconds.
0:14:00 > 0:14:04So thinking of light as a wave just wasn't adding up.
0:14:08 > 0:14:11To resolve this, someone needed to think the unthinkable
0:14:11 > 0:14:15and, in 1905, someone did. You may well have heard of them.
0:14:15 > 0:14:17His name was Albert Einstein.
0:14:28 > 0:14:32This is the Archenhold-Sternwarte Observatory in Berlin.
0:14:34 > 0:14:39Perched on top is a strange, huge iron and steel construction,
0:14:39 > 0:14:43but it's not a gun, it's actually a telescope.
0:14:47 > 0:14:51Built in 1896, the telescope was one of the largest of its kind
0:14:51 > 0:14:55in the world and made the observatory the go-to place
0:14:55 > 0:14:59to engage and astound the public in new science.
0:15:00 > 0:15:04Albert Einstein gave a very famous public lecture here
0:15:04 > 0:15:08on his theory of relativity which is of course what he's most famous for.
0:15:08 > 0:15:11But it's not the work that won him the Nobel Prize.
0:15:18 > 0:15:23In 1905, he'd also come up with a new theory to explain
0:15:23 > 0:15:28the photoelectric effect and what he suggested was revolutionary
0:15:28 > 0:15:30and even heretical.
0:15:35 > 0:15:39He argued that we have to forget all about the idea that light is a wave
0:15:39 > 0:15:44and think of it instead as a stream of tiny, bullet-like particles.
0:15:44 > 0:15:49The term he used to describe a particle of light was a quantum.
0:15:51 > 0:15:55To Einstein, a quantum was a tiny lump of energy
0:15:55 > 0:15:59and although in 1905 the word wasn't new,
0:15:59 > 0:16:03the idea that light could be a quantum seemed crazy.
0:16:06 > 0:16:09And yet following Einstein's heretical line of thought
0:16:09 > 0:16:11to its logical conclusion
0:16:11 > 0:16:15solved all the problems with light at a single stroke.
0:16:22 > 0:16:26I'll try to explain how this helps using a rough analogy.
0:16:26 > 0:16:29Of course, like all analogies, it's far from perfect
0:16:29 > 0:16:32but hopefully it'll give you a sense of the physics
0:16:32 > 0:16:37to help you understand why thinking of light as a stream of particles
0:16:37 > 0:16:39solves the mystery of the photoelectric effect.
0:16:44 > 0:16:49In this analogy, these red balls represent Einstein's light quanta.
0:16:50 > 0:16:54'And those cans over there are the electricity held in the metal.'
0:16:54 > 0:16:56Now, in the original experiment,
0:16:56 > 0:16:59they made electricity flow from the surface of the metal
0:16:59 > 0:17:02by shining light on it. In my analogy, I'm going to try
0:17:02 > 0:17:06and knock those tin cans over using these red balls.
0:17:17 > 0:17:20'Absolutely no effect.
0:17:23 > 0:17:25'That's just like red light.'
0:17:27 > 0:17:29According to Einstein,
0:17:29 > 0:17:33each particle of red light carries very little energy
0:17:33 > 0:17:36because red light has a low frequency.
0:17:36 > 0:17:41'So even a very bright red light with many red light particles
0:17:41 > 0:17:45'can't dislodge any electrons from the metal plates,
0:17:45 > 0:17:48'just like the red balls.'
0:17:48 > 0:17:52Now I'm going to use heavier balls like these blue golf balls
0:17:52 > 0:17:55and I'm going to try and knock off the tin cans with these.
0:18:00 > 0:18:03'They're like the ultraviolet light in the experiment.
0:18:06 > 0:18:12'Now, each individual light particle carries more energy
0:18:12 > 0:18:15'because ultraviolet light is higher frequency.'
0:18:22 > 0:18:25Just a few of them, like a dim ultraviolet light,
0:18:25 > 0:18:28are enough to knock the electrons out of the metal plate
0:18:28 > 0:18:31and collapse the gold leaf.
0:18:33 > 0:18:38So Einstein's idea that light is made up of tiny particles or quanta
0:18:38 > 0:18:41is a wonderful explanation of the photoelectric effect.
0:18:41 > 0:18:43I remember when I first learnt about this,
0:18:43 > 0:18:47being blown away by its sheer elegance and simplicity.
0:18:50 > 0:18:54But what's more, Einstein's nifty idea also helped solve
0:18:54 > 0:18:57Planck's mystery of the light bulb.
0:18:57 > 0:18:59There was more red than ultraviolet
0:18:59 > 0:19:04because ultraviolet quanta took so much more energy to make,
0:19:04 > 0:19:07about 100 times more energy.
0:19:07 > 0:19:09No wonder there are so few of them.
0:19:11 > 0:19:14That moment at the beginning of the 20th century
0:19:14 > 0:19:17signalled a genuine revolution
0:19:17 > 0:19:21because it demonstrated that the kind of physical science
0:19:21 > 0:19:24that people were doing right back to Newton and Laplace,
0:19:24 > 0:19:27and people like that, that you needed a completely new approach.
0:19:30 > 0:19:32Physics has never recovered from that moment
0:19:32 > 0:19:35in the sense that it's built on that moment,
0:19:35 > 0:19:37that's where modern physics really began.
0:19:40 > 0:19:44But Einstein's theory also left physicists with a dizzying paradox
0:19:44 > 0:19:47defying all common sense.
0:19:47 > 0:19:51Light was definitely a wave which explained shadows and bubbles.
0:19:53 > 0:19:56And now it was definitely a particle too -
0:19:56 > 0:20:00Einstein's quanta explaining the photoelectric effect
0:20:00 > 0:20:02and the ultraviolet catastrophe.
0:20:03 > 0:20:09Then just a few years after Einstein's brilliant, crazy idea,
0:20:09 > 0:20:13the paradox got a lot deeper and a whole lot weirder.
0:20:15 > 0:20:19Because what seemed to be a curious mystery about light
0:20:19 > 0:20:24was about to become a battleground about the nature of reality itself.
0:20:34 > 0:20:361922.
0:20:36 > 0:20:40The Western world was in the grip of a revolution, a cultural revolution.
0:20:41 > 0:20:44James Joyce's Ulysses is published,
0:20:44 > 0:20:47Stravinsky is at the height of his powers
0:20:47 > 0:20:50and Chaplin has just released his first serious movie.
0:20:50 > 0:20:53The Ottoman Empire collapses.
0:20:53 > 0:20:57Europe is still recovering from the war to end all wars
0:20:57 > 0:20:59in which millions of men lost their lives.
0:20:59 > 0:21:02Russia is newly communist.
0:21:02 > 0:21:06Meanwhile, America is exporting jazz to the world.
0:21:06 > 0:21:08- Thank you. - MUSIC PLAYS
0:21:14 > 0:21:18'In arts, politics, literature, economics,
0:21:18 > 0:21:21'there was an insatiable appetite for change.
0:21:21 > 0:21:24'This was the birth of modernism.'
0:21:24 > 0:21:28# You've got a heart that there's no way of knowing
0:21:28 > 0:21:32# Can see where you are but can't see where you're going
0:21:32 > 0:21:35# And I'm stuck here still
0:21:35 > 0:21:39# I'm tangled up with you
0:21:42 > 0:21:47# This whole world can be so uncertain... #
0:21:47 > 0:21:51But, and I might get into trouble for saying this,
0:21:51 > 0:21:54I would argue that the upheaval that took place in physics
0:21:54 > 0:21:57at this time would eclipse them all
0:21:57 > 0:22:01and have far longer lasting consequences.
0:22:01 > 0:22:04It had begun with the discovery of the weird
0:22:04 > 0:22:07and contradictory wave/particle nature of light,
0:22:07 > 0:22:11it ended up as an epic battle fought between the greatest minds
0:22:11 > 0:22:15in science for the highest possible stakes -
0:22:15 > 0:22:17the nature of reality itself.
0:22:18 > 0:22:23# I know I deserve you, I know you're my saviour
0:22:23 > 0:22:27# But when I observe you, you change your behaviour... #
0:22:27 > 0:22:31'On one side, a new wave of modernist revolutionary scientists
0:22:31 > 0:22:36'and their leader, the brilliant Danish physicist, Niels Bohr.
0:22:37 > 0:22:41'On the other side, the voice of reason, Albert Einstein,
0:22:41 > 0:22:45'at the height of his powers and now world-famous,
0:22:45 > 0:22:47'a formidable adversary.'
0:22:47 > 0:22:50# Tangled up with you... #
0:22:52 > 0:22:54The battle raged for decades.
0:22:54 > 0:22:57Actually, in some ways, it still does.
0:22:57 > 0:23:01It was fought across the world in universities, at conferences,
0:23:01 > 0:23:05in bars and cafes, it would reduce grown men to tears
0:23:05 > 0:23:10and it began with a deceptively simple experiment.
0:23:10 > 0:23:15# This whole world can be so uncertain... #
0:23:15 > 0:23:19'But weirdly, it was an experiment that wasn't even about light,
0:23:19 > 0:23:23'it was about the particles that make electricity.'
0:23:23 > 0:23:28# To somebody else... #
0:23:28 > 0:23:32In the mid-1920s, an experiment was carried out
0:23:32 > 0:23:34at Bell Laboratories in New Jersey in America
0:23:34 > 0:23:39which uncovered something entirely unexpected about electrons.
0:23:39 > 0:23:41Now, at the time it was accepted without question
0:23:41 > 0:23:44that electrons were these tiny lumps of matter,
0:23:44 > 0:23:49small but solid particles, like miniature billiard balls.
0:23:49 > 0:23:52In the experiment, they fired a beam of electrons at a crystal
0:23:52 > 0:23:54and watched how they scattered.
0:23:54 > 0:23:59Now, that's entirely equivalent to taking a beam of electrons,
0:23:59 > 0:24:01say from an electron gun,
0:24:01 > 0:24:06and firing it at a screen with two slits in it
0:24:06 > 0:24:10so that the electrons pass through the slits
0:24:10 > 0:24:12and hit another screen at the back.
0:24:15 > 0:24:17What the Bell scientists found
0:24:17 > 0:24:19shocked the physics world to the core.
0:24:22 > 0:24:26To understand why, consider a similar experiment with water waves.
0:24:27 > 0:24:30I've set up a simple experiment.
0:24:30 > 0:24:34I have a water ripple tank placed on top of an overhead projector,
0:24:34 > 0:24:39I have a generator producing waves that pass through two narrow gaps.
0:24:39 > 0:24:44The projector beams the image of the waves onto the back wall.
0:24:44 > 0:24:47You can see as the waves come in from the left
0:24:47 > 0:24:49and squeeze through the two gaps,
0:24:49 > 0:24:53they spread out on the other side and interfere with each other.
0:24:53 > 0:24:56What this means is that when you get the crest from one wave
0:24:56 > 0:25:00meeting the crest from another, they add up to make a higher wave.
0:25:00 > 0:25:04But when the crest from one meets a trough, they cancel out.
0:25:07 > 0:25:10This gives rise to these characteristic lines
0:25:10 > 0:25:13leading to the signature wave pattern.
0:25:15 > 0:25:17Bands of light and dark.
0:25:20 > 0:25:22Whenever you see these light and dark bands,
0:25:22 > 0:25:23the signature wave pattern,
0:25:23 > 0:25:27you know without doubt that you've got wave-like behaviour.
0:25:36 > 0:25:39So guess what they saw in New Jersey.
0:25:39 > 0:25:44Now it seemed that firing electrons, tiny solid particles,
0:25:44 > 0:25:48through the two gaps produced exactly the same kind of pattern,
0:25:48 > 0:25:51bands of light and dark.
0:25:53 > 0:25:56First, light, for a long time believed to be a wave,
0:25:56 > 0:25:59was found to sometimes behave like particles
0:25:59 > 0:26:02and now electrons, for a long time believed to be particles,
0:26:02 > 0:26:04were behaving like waves.
0:26:04 > 0:26:07But it was actually stranger than that.
0:26:07 > 0:26:10The wave pattern wasn't merely some result
0:26:10 > 0:26:13of the entire beam of electrons.
0:26:13 > 0:26:15More recently this experiment has been repeated
0:26:15 > 0:26:21in labs around the world by firing one electron at a time
0:26:21 > 0:26:23through the slits onto the screen.
0:26:26 > 0:26:31At first, each electron seems to land randomly on the screen.
0:26:33 > 0:26:39But gradually a pattern forms, the signature wave pattern.
0:26:39 > 0:26:43Let me be quite clear about just how weird this is.
0:26:43 > 0:26:45Remember from the wave tank experiment
0:26:45 > 0:26:49where the signature wave pattern only exists
0:26:49 > 0:26:52because each wave passes through both slits
0:26:52 > 0:26:56and then its two pieces interfere with each other.
0:26:56 > 0:26:59But here, every individual electron,
0:26:59 > 0:27:02each single particle is passing alone through the slits
0:27:02 > 0:27:05before it hits the screen.
0:27:05 > 0:27:08And yet, each single electron is still contributing
0:27:08 > 0:27:10to the signature wave pattern.
0:27:12 > 0:27:16Each electron has to be behaving like a wave.
0:27:20 > 0:27:23To explain this strange result,
0:27:23 > 0:27:27Niels Bohr and his colleagues created quantum mechanics,
0:27:27 > 0:27:32a crazy theory of light and matter that embraced contradiction
0:27:32 > 0:27:35and didn't care that it was almost impossible to understand.
0:27:35 > 0:27:37As Niels Bohr himself said,
0:27:37 > 0:27:41anyone who isn't shocked by quantum theory hasn't understood it.
0:27:41 > 0:27:44So, viewers, I'm going to take our tiny electron
0:27:44 > 0:27:47and use it to delve deep into the heart of reality.
0:27:48 > 0:27:51And, yes, prepared to be shocked
0:27:51 > 0:27:54because this is the only way to explain what we observe
0:27:54 > 0:27:57when a single electron travels through the slits
0:27:57 > 0:27:59and hits the screen.
0:27:59 > 0:28:01Quantum mechanics says this...
0:28:02 > 0:28:06..we can't describe what's travelling as a physical object.
0:28:08 > 0:28:09All we can talk about
0:28:09 > 0:28:12are the chances of where the electron might be.
0:28:14 > 0:28:19This wave of chance somehow travels through both slits
0:28:19 > 0:28:23producing interference just like the water wave.
0:28:24 > 0:28:26Then when it hits the screen,
0:28:26 > 0:28:30what was just the ghostly possibility of an electron
0:28:30 > 0:28:33mysteriously becomes real.
0:28:35 > 0:28:39Let me try and capture just how weird this is with an analogy.
0:28:39 > 0:28:41If I spin this coin...
0:28:46 > 0:28:48Then all the time it's spinning, it's a blur,
0:28:48 > 0:28:51I can't tell if it's heads or tails
0:28:51 > 0:28:56but if I stop it, I force it to decide and it's heads.
0:28:56 > 0:29:01So before it was sort of not heads or tails but a mixture of both
0:29:01 > 0:29:05but as soon as I've stopped it, I've forced it to make up its mind.
0:29:05 > 0:29:07This is what Bohr and his supporters
0:29:07 > 0:29:10claimed was happening with our electrons.
0:29:14 > 0:29:20In a sense, as it spins, the coin is both heads and tails.
0:29:20 > 0:29:24Similarly, the electrons' wave of chance
0:29:24 > 0:29:29passes through both slits, two paths at the same time.
0:29:30 > 0:29:33Our coin then stops at heads.
0:29:35 > 0:29:39The ethereal wave of probability hits the screen
0:29:39 > 0:29:42and only then becomes a particle.
0:29:42 > 0:29:47The quantum world was unlike anything ever seen before.
0:29:49 > 0:29:53It's hard to overstate just how crazy this is.
0:29:53 > 0:29:56Bohr was effectively claiming that one can never know
0:29:56 > 0:30:00where the electron actually is at all until you measure it
0:30:00 > 0:30:04and it's not just that you don't know where the electron is,
0:30:04 > 0:30:08it's weirdly as though the electron itself is everywhere at once.
0:30:11 > 0:30:14Bear in mind that electrons are among the commonest
0:30:14 > 0:30:17and most basic building blocks of reality
0:30:17 > 0:30:20and yet here's Bohr saying that only by looking
0:30:20 > 0:30:24do we actually conjure their position into existence.
0:30:25 > 0:30:29It's like there's a curtain between us and the quantum world
0:30:29 > 0:30:33and behind it there is no solid reality...
0:30:35 > 0:30:38..just the potential for reality.
0:30:40 > 0:30:45Things only become real when we pull back the curtain and look.
0:30:45 > 0:30:47And this view, ladies and gentlemen,
0:30:47 > 0:30:51became known as the Copenhagen interpretation.
0:30:51 > 0:30:53APPLAUSE
0:30:58 > 0:31:00Persuasive as it might seem,
0:31:00 > 0:31:05many people couldn't stomach Niels Bohr's outlandish ideas.
0:31:05 > 0:31:09And they found a natural leader in the most powerful man in science.
0:31:11 > 0:31:13Albert Einstein hated this interpretation
0:31:13 > 0:31:16with every fibre of his being.
0:31:16 > 0:31:17He famously said,
0:31:17 > 0:31:21"Does the moon cease to exist when I don't look at it?"
0:31:22 > 0:31:26He was very unhappy because it gave limits to knowledge
0:31:26 > 0:31:28that he didn't think should be final.
0:31:28 > 0:31:32He thought there should be a better underlying theory.
0:31:36 > 0:31:40Over the next ten years, Einstein and Bohr would argue passionately
0:31:40 > 0:31:44about whether quantum mechanics meant giving up on reality or not.
0:31:48 > 0:31:53Then, with two other scientists, Nathan Rosen and Boris Podolsky,
0:31:53 > 0:31:57Einstein thought they'd found a way to win the argument.
0:31:57 > 0:32:00He was convinced he'd found a fatal flaw
0:32:00 > 0:32:04in the Copenhagen interpretation and it's claim that reality
0:32:04 > 0:32:08was summoned into existence by the act of looking at it.
0:32:08 > 0:32:10At the heart of Einstein's argument
0:32:10 > 0:32:13was an aspect of quantum mechanics called entanglement.
0:32:13 > 0:32:18Now, entanglement is this special, incredibly close relationship
0:32:18 > 0:32:22between a pair of quantum particles whose fates are intertwined.
0:32:22 > 0:32:25For example, if they were created in the same event.
0:32:30 > 0:32:32Let me try and explain this
0:32:32 > 0:32:35by imagining the two particles are spinning coins.
0:32:40 > 0:32:43Imagine these coins are two electrons
0:32:43 > 0:32:49created from the same event and then moved apart from each other.
0:32:49 > 0:32:52Quantum mechanics says that, because they're created together,
0:32:52 > 0:32:54they're entangled.
0:32:54 > 0:32:57And now many of their properties are for ever linked,
0:32:57 > 0:32:59wherever they are.
0:32:59 > 0:33:02Remember, the Copenhagen interpretation says that
0:33:02 > 0:33:06until you measure one of the coins, neither of them is heads or tails.
0:33:06 > 0:33:09In fact, heads and tails don't even exist.
0:33:09 > 0:33:13And here's where entanglement makes this weird situation even weirder.
0:33:15 > 0:33:18When we stop the first coin and it becomes heads...
0:33:20 > 0:33:23..because the coins are linked through entanglement,
0:33:23 > 0:33:27the second coin will simultaneously become tails.
0:33:29 > 0:33:30And here's the crucial thing.
0:33:30 > 0:33:34I can't predict what the outcome of my measurement will be,
0:33:34 > 0:33:37only that they will always be opposite.
0:33:37 > 0:33:38Einstein seized on this.
0:33:40 > 0:33:44Because it meant that something was happening between the two coins
0:33:44 > 0:33:46that was almost too crazy to imagine.
0:33:48 > 0:33:52It's as if the two coins are secretly communicating.
0:33:52 > 0:33:56Communicating instantaneously across space and time.
0:33:56 > 0:34:00Even if the first coin was on Earth and the other was on Pluto.
0:34:01 > 0:34:03Einstein refused to believe
0:34:03 > 0:34:07this instantaneous, faster-than-light communication.
0:34:07 > 0:34:11His theory of relativity said that nothing could travel that fast.
0:34:11 > 0:34:12Not even information.
0:34:13 > 0:34:17So, how could one coin instantaneously know
0:34:17 > 0:34:20how the other would land?
0:34:20 > 0:34:24He disparagingly called it "spooky action at a distance"
0:34:24 > 0:34:29and claimed it was a fatal flaw in the Copenhagen interpretation.
0:34:29 > 0:34:31What's more, he had a better idea.
0:34:33 > 0:34:37Einstein believed there was a simpler interpretation.
0:34:37 > 0:34:40That somehow the destiny of the two coins, whether or not they
0:34:40 > 0:34:45ended up heads or tails, was already fixed long before we observed them.
0:34:47 > 0:34:49He said that although it seemed the coin
0:34:49 > 0:34:54was deciding to be, say, heads, at the moment of observation,
0:34:54 > 0:34:57actually, that decision was taken long before.
0:34:58 > 0:35:00It was just hidden from us.
0:35:03 > 0:35:04In Einstein's mind,
0:35:04 > 0:35:08quantum particles were nothing like spinning coins.
0:35:08 > 0:35:12They were more like, say, a pair of gloves, left and right,
0:35:12 > 0:35:15separated into boxes.
0:35:15 > 0:35:20We don't know which box contains which glove until we open one,
0:35:20 > 0:35:24but when we do, and find, say, a right-handed glove,
0:35:24 > 0:35:28immediately, we know that the other box contains the left-handed glove.
0:35:28 > 0:35:33But, crucially, this requires no spooky action at a distance.
0:35:33 > 0:35:36Neither glove has been altered by the act of observation.
0:35:36 > 0:35:38Both of them were either
0:35:38 > 0:35:40left or right-handed glove from the beginning.
0:35:40 > 0:35:43And the only thing that has changed is our knowledge.
0:35:45 > 0:35:49So, which is the true description of reality?
0:35:49 > 0:35:53Bohr's coins, which only become real when we look at them...
0:35:56 > 0:35:59..and then magically communicate to each other,
0:35:59 > 0:36:02or Einstein's gloves, which are hidden from us,
0:36:02 > 0:36:06but are definitely left or right from the beginning?
0:36:06 > 0:36:09In other words, is there an objective reality,
0:36:09 > 0:36:12as Einstein believed, or not, as Bohr maintained?
0:36:13 > 0:36:17In the late 1930s, as the world plunged into war,
0:36:17 > 0:36:19there was no way to answer this question.
0:36:19 > 0:36:23The battle to understand the nature of reality was deadlocked.
0:36:30 > 0:36:32The war rolled across Europe
0:36:32 > 0:36:36and many of the leading scientists fled to the United States.
0:36:38 > 0:36:42Then, as the Second World War led inextricably to the Cold War...
0:36:44 > 0:36:47..American science, backed by dollar bills
0:36:47 > 0:36:49and a new vision of the future, boomed.
0:36:51 > 0:36:55Remember, after the war, physicists came back raring to go
0:36:55 > 0:37:00and tried to apply the ideas of quantum theory to atoms,
0:37:00 > 0:37:03the interaction between electrons and light and what have you,
0:37:03 > 0:37:06you didn't need to worry about the philosophical side of things
0:37:06 > 0:37:08to make progress with that.
0:37:08 > 0:37:11So, as you say, it really took a back seat.
0:37:13 > 0:37:18Quantum mechanics led to a profound understanding of semiconductors,
0:37:18 > 0:37:21which helped create the modern electronic age.
0:37:23 > 0:37:26It produced lasers, revolutionising communications,
0:37:26 > 0:37:28breathtaking new medical advances.
0:37:32 > 0:37:35And breakthroughs in nuclear power.
0:37:39 > 0:37:43Quantum mechanics was so successful that most working physicists
0:37:43 > 0:37:48deliberately chose to ignore Einstein's objections.
0:37:48 > 0:37:52It simply didn't matter to them because it worked.
0:37:52 > 0:37:56They even coined a phrase for it, "Shut up and calculate."
0:38:00 > 0:38:04And the price for this success was that Bohr and Einstein's debate
0:38:04 > 0:38:08on the reality of the quantum world was simply brushed under the carpet.
0:38:12 > 0:38:15And amidst all this success and pragmatism,
0:38:15 > 0:38:18there were few who still worried what it all meant.
0:38:19 > 0:38:23But as the '50s rolled headlong into the '60s, one lone dissenter
0:38:23 > 0:38:27worked out how to settle the argument once and for all.
0:38:39 > 0:38:41John Bell, I think it's fair to say,
0:38:41 > 0:38:44isn't well known to the general public.
0:38:44 > 0:38:48But to physicists like me, he's, well, an hero.
0:38:48 > 0:38:52He was an original thinker with real courage in his convictions.
0:38:52 > 0:38:57And the story of his rise to become one of the greats of physics
0:38:57 > 0:39:01is made even more remarkable when you consider how he started.
0:39:01 > 0:39:06He was born in Belfast in the 1920s into a poor, working-class family.
0:39:06 > 0:39:08His father was a horse dealer.
0:39:08 > 0:39:10And they really struggled to get him
0:39:10 > 0:39:13into Queen's University Belfast to study physics.
0:39:13 > 0:39:17He was the only one in his family to even finish school.
0:39:17 > 0:39:22This, I believe, made him insatiably curious, fiery and stubborn.
0:39:28 > 0:39:32I remember meeting John Bell in 1989, a year before he died.
0:39:32 > 0:39:35We were both at a conference in America
0:39:35 > 0:39:38and we happened to be sharing a lift just after both attending
0:39:38 > 0:39:41a talk on quantum mechanics.
0:39:41 > 0:39:45Keen to say something to the great John Bell, I said I thought
0:39:45 > 0:39:49that the speaker's conclusions were completely crazy.
0:39:49 > 0:39:53He stared at me with his piercing blue eyes and, for a moment,
0:39:53 > 0:39:56I thought my fledgling physics career was going down the drain.
0:39:56 > 0:40:00But as the lift doors opened and he was about to leave, he said,
0:40:00 > 0:40:02"Yes, I completely agree with you.
0:40:02 > 0:40:05"Haven't they heard of the helium problem?"
0:40:05 > 0:40:08To this day, I'm not quite sure what the helium problem is,
0:40:08 > 0:40:11but I was just so relieved that John Bell and I agreed.
0:40:23 > 0:40:25For many years, he worked here,
0:40:25 > 0:40:28at Britain's atomic energy research centre, Harwell,
0:40:28 > 0:40:32who built this early experimental nuclear reactor called DIDO.
0:40:36 > 0:40:39It was here that he started pondering the deep
0:40:39 > 0:40:43and worrying questions quantum mechanics raised.
0:40:43 > 0:40:47Did the quantum world only exist when it was observed?
0:40:47 > 0:40:51Or was there a deeper truth out there, waiting to be discovered?
0:40:52 > 0:40:55In fact, he was so troubled, he began to wonder
0:40:55 > 0:40:59if there was a problem at the heart of quantum mechanics.
0:41:01 > 0:41:05He famously said, "I hesitate to think it might be wrong,
0:41:05 > 0:41:08"but I know it is rotten."
0:41:08 > 0:41:11And so, in the early 1960s, Bell decided to try
0:41:11 > 0:41:14and resolve the crisis at the heart of quantum physics.
0:41:14 > 0:41:16It was an epic challenge.
0:41:16 > 0:41:19After all, how do you check if something is real,
0:41:19 > 0:41:24if something is or isn't there, all without looking?
0:41:24 > 0:41:27How do you look behind the curtain without pulling it open?
0:41:28 > 0:41:34But John Bell came up with a brilliant way of doing exactly that.
0:41:36 > 0:41:39I think this is one of THE most ingenious ideas
0:41:39 > 0:41:41in the whole of physics.
0:41:41 > 0:41:44It's certainly one of the most difficult to understand and explain.
0:41:44 > 0:41:47But I'm going to try and have a go and, yes,
0:41:47 > 0:41:49I'm afraid I'm going to use another analogy.
0:41:49 > 0:41:53This time, I'm going to play a game of cards.
0:41:53 > 0:41:56But it's one for the highest possible stakes,
0:41:56 > 0:41:58the nature of reality itself.
0:42:00 > 0:42:05The card game is against a mysterious quantum dealer.
0:42:05 > 0:42:09The cards he deals represent any subatomic particles,
0:42:09 > 0:42:11or even quanta of light, photons.
0:42:13 > 0:42:16And the game we'll play will ultimately tell us
0:42:16 > 0:42:19whether Einstein or Bohr was right.
0:42:20 > 0:42:23Now, the rules of the game are deceptively simple.
0:42:23 > 0:42:27The dealer's going to deal two cards face down.
0:42:27 > 0:42:30If they're the same colour, I win.
0:42:30 > 0:42:33If they're different colours, I lose.
0:42:39 > 0:42:44So I have a red, so I need another red to win.
0:42:44 > 0:42:47That's black. I lose.
0:42:50 > 0:42:53Again, opposite colours. I've lost both those.
0:43:01 > 0:43:03That's four in a row.
0:43:08 > 0:43:13That's six pairs in a row that I've lost. OK.
0:43:13 > 0:43:15I think I know what the dealer's doing here.
0:43:15 > 0:43:18Clearly, the deck has been rigged in advance
0:43:18 > 0:43:22so that every pair came out as opposite colours.
0:43:22 > 0:43:26But there's a simple way to catch the dealer out.
0:43:26 > 0:43:30So what we can do now is change the rules of the game.
0:43:30 > 0:43:35This time, if they are the opposite colour, I win.
0:43:40 > 0:43:45But once again, every time, my evil quantum opponent beats me.
0:43:54 > 0:43:57But again, I can see what the crafty dealer could have done.
0:43:57 > 0:44:00Maybe while I wasn't looking, he's switched the pack
0:44:00 > 0:44:04and rigged it so that it always lands in his favour.
0:44:04 > 0:44:06Now every pair is the same colour.
0:44:10 > 0:44:13Rigged decks, remember, were what Einstein thought
0:44:13 > 0:44:17was really happening in the entanglement experiment.
0:44:17 > 0:44:21He said that, just like the gloves were already placed in the box,
0:44:21 > 0:44:26so the evil dealer stacked the cards before we played.
0:44:27 > 0:44:30But Niels Bohr's idea was very different.
0:44:30 > 0:44:36He said red and black don't even exist until you turn them over.
0:44:36 > 0:44:40Bell's genius was that he came up with a way of deciding once
0:44:40 > 0:44:45and for all who was right - Einstein or Bohr.
0:44:45 > 0:44:46This is how he did it.
0:44:46 > 0:44:50I'm now not going to tell the dealer which game I want to play,
0:44:50 > 0:44:53same colours wins, or different colour wins,
0:44:53 > 0:44:56until after he's dealt the cards.
0:45:00 > 0:45:05Now, because he can never predict which rules I'm going to play by,
0:45:05 > 0:45:08he can never stack the deck correctly.
0:45:09 > 0:45:13Now he can't win...or can he?
0:45:14 > 0:45:18So now the rules are, different wins.
0:45:21 > 0:45:23They're the same. OK.
0:45:24 > 0:45:26Same colour wins.
0:45:29 > 0:45:32This gets to the very heart of Bell's idea.
0:45:32 > 0:45:36If we now start playing and I win as many as I lose,
0:45:36 > 0:45:38then Einstein was right.
0:45:38 > 0:45:42The dealer is just a trickster with a gift for slight of hand.
0:45:43 > 0:45:48Reality may be tricky, but it does have an objective existence.
0:45:51 > 0:45:53But what if I lose?
0:45:53 > 0:45:58Well, then I'm forced to admit that there is no sensible explanation.
0:46:01 > 0:46:05Each card must be sending secret signals to the other
0:46:05 > 0:46:10across space and time, in defiance of everything we know.
0:46:10 > 0:46:14I'm forced to accept that, at the fundamental quantum level,
0:46:14 > 0:46:17reality is truly unknowable.
0:46:22 > 0:46:27Bell reduced this idea into a single mathematical equation
0:46:27 > 0:46:33that tells us once and for all what seemed unanswerable.
0:46:33 > 0:46:35How reality really is.
0:46:36 > 0:46:41John Bell published his idea in 1964 and the extraordinary thing is,
0:46:41 > 0:46:45at the time, the entire physics community ignored him.
0:46:45 > 0:46:50Total silence. It seems the world simply wasn't ready.
0:46:52 > 0:46:56Perhaps it was because his equation seemed untestable,
0:46:56 > 0:47:00or just because nobody thought it was worth investigating.
0:47:00 > 0:47:03But that was about to change.
0:47:03 > 0:47:07And the change would come from a very unexpected place.
0:47:10 > 0:47:15# This is the dawning of the age of Aquarius
0:47:15 > 0:47:19# Age of Aquarius
0:47:19 > 0:47:25# Aquarius
0:47:25 > 0:47:30# Aquarius. #
0:47:31 > 0:47:34America was in crisis over Vietnam,
0:47:34 > 0:47:37Watergate, feminism, the Black Panthers.
0:47:37 > 0:47:40And while all this was going on, a small group of hippy physicists
0:47:40 > 0:47:43were working at the University of Berkeley in California.
0:47:43 > 0:47:45They did all the hippy things -
0:47:45 > 0:47:47they smoked dope, they popped LSD,
0:47:47 > 0:47:51they debated things like Buddhism and telepathy.
0:47:51 > 0:47:53# When the moon
0:47:53 > 0:47:57# Is in the Seventh House... #
0:47:57 > 0:47:59And they loved quantum mechanics.
0:47:59 > 0:48:01In its weird version of reality,
0:48:01 > 0:48:04they saw parallels with their own esoteric beliefs.
0:48:07 > 0:48:11# And love will steer the stars
0:48:11 > 0:48:13# This is the dawning of...#
0:48:16 > 0:48:18Their hippy, New Age-style physics
0:48:18 > 0:48:20also caught the attention of the public,
0:48:20 > 0:48:23who read their crazy hippy books
0:48:23 > 0:48:26that mixed quantum mechanics with Eastern mysticism.
0:48:26 > 0:48:29Books like The Tao Of Physics,
0:48:29 > 0:48:32The Dancing Wu Li Masters
0:48:32 > 0:48:36and my personal favourite, Space-Time And Beyond -
0:48:36 > 0:48:38Towards An Explanation Of The Unexplainable.
0:48:40 > 0:48:42But more importantly for our story,
0:48:42 > 0:48:44the story of quantum mechanics,
0:48:44 > 0:48:47these hippy physicists also turned their attention
0:48:47 > 0:48:50to Einstein's now-famous thought experiment
0:48:50 > 0:48:53and what it told us about the nature of reality.
0:48:55 > 0:48:58They saw Niels Bohr's secret signalling
0:48:58 > 0:49:01as proof that physics supported their own ideas.
0:49:01 > 0:49:06Because if two particles could spookily communicate across space,
0:49:06 > 0:49:12then ESP, telepathy and clairvoyance were probably true as well.
0:49:12 > 0:49:15If only they could prove it really existed.
0:49:15 > 0:49:17Then, in 1972, they realised that,
0:49:17 > 0:49:21with a bit of mathematical slight of hand,
0:49:21 > 0:49:26they could take Bell's equation and experimentally test it.
0:49:26 > 0:49:29One of their group, John Clauser,
0:49:29 > 0:49:31borrowed some equipment from the lab he was working in
0:49:31 > 0:49:36and set up the first genuine and ultimate test of quantum mechanics.
0:49:39 > 0:49:42This is a picture of that first experiment,
0:49:42 > 0:49:45built of leftovers and stolen equipment.
0:49:45 > 0:49:49Over the next few years, it was improved by a team
0:49:49 > 0:49:53led by Alain Aspect in Paris, making its results more reliable.
0:49:55 > 0:49:59Over ten years after Bell first proposed his equation,
0:49:59 > 0:50:02finally, it could be put to the test.
0:50:02 > 0:50:04This is a modern version of the experiment
0:50:04 > 0:50:08first carried out by John Clauser and then Alain Aspect.
0:50:12 > 0:50:15Here, a crystal converts laser light
0:50:15 > 0:50:18into pairs of entangled light quanta, photons,
0:50:18 > 0:50:21making two very precise beams.
0:50:27 > 0:50:30These photons are passed around and bent back again
0:50:30 > 0:50:33until they pass through these detectors.
0:50:33 > 0:50:35The two photons are like the two cards
0:50:35 > 0:50:37the evil dealer places in front of me.
0:50:40 > 0:50:43We'll measure a property of the photons called polarisation,
0:50:43 > 0:50:47which is equivalent to the colour of the playing cards in my game.
0:50:47 > 0:50:52So, for instance, winning with two matching red cards might be the same
0:50:52 > 0:50:56as two photons with matching polarisation.
0:50:56 > 0:50:57But because this is quantum mechanics,
0:50:57 > 0:51:00it's more complicated than my simple card game.
0:51:00 > 0:51:03And these dials here allow me
0:51:03 > 0:51:06to measure a second property of the photons as well.
0:51:06 > 0:51:07Now that's equivalent to me
0:51:07 > 0:51:10not only trying to guess the colour of the face of the cards,
0:51:10 > 0:51:14but also trying to guess the colour of the back of the cards.
0:51:14 > 0:51:18OK, so we're now going to switch on the laser and start the experiment.
0:51:23 > 0:51:26So this number here gives me
0:51:26 > 0:51:29the number of photon pairs coming through the experiment.
0:51:29 > 0:51:32That's equivalent to the pairs of cards in my game.
0:51:32 > 0:51:34The graph here, dropping down,
0:51:34 > 0:51:38gives me the probability that I can win, that I'm guessing right.
0:51:38 > 0:51:41The more photons, the more accurate it becomes.
0:51:41 > 0:51:44I'll stop at an uncertainty of about 1%.
0:51:45 > 0:51:50And the final answer is 0.56, so if I...
0:51:51 > 0:51:53..put that into my equation,
0:51:53 > 0:51:56I now need to run the experiment three more times,
0:51:56 > 0:52:01corresponding to the four different settings of these dials.
0:52:01 > 0:52:06Each run is now like a different set of rules for the quantum dealer.
0:52:06 > 0:52:09And when I add them up and get the answer,
0:52:09 > 0:52:12if it's less than two, then Einstein was right.
0:52:12 > 0:52:15If it's greater than two, then Bohr was right.
0:52:15 > 0:52:18OK, so now for the second setting.
0:52:18 > 0:52:20Just remember what the experiment will show.
0:52:22 > 0:52:25If the numbers come out less than two,
0:52:25 > 0:52:28then it's proof the dealer has been stacking the deck.
0:52:28 > 0:52:30This was Einstein's view.
0:52:30 > 0:52:34OK, so the number I get this time is 0.82.
0:52:39 > 0:52:42Now, reset for run three.
0:52:45 > 0:52:47But if the result is greater than two,
0:52:47 > 0:52:52then the deck cannot be stacked and something else is at work.
0:52:52 > 0:52:56OK, so the run three result is -0.59.
0:52:56 > 0:52:59And finally, run four.
0:53:02 > 0:53:05This last number will finally reveal
0:53:05 > 0:53:10if the world follows common sense, or something much more bizarre.
0:53:10 > 0:53:14OK, so our final result is in and it's 0.56.
0:53:14 > 0:53:16So if we turn the laser off...
0:53:18 > 0:53:21Right, I'd better just work out the answer.
0:53:27 > 0:53:30And there we have it, 2.53.
0:53:31 > 0:53:33It's a number greater than two.
0:53:33 > 0:53:36Absolute proof that Albert Einstein was wrong
0:53:36 > 0:53:39and Niels Bohr was right.
0:53:48 > 0:53:52The significance of this result is simply enormous.
0:53:52 > 0:53:53Just remember what it means.
0:53:53 > 0:53:57Einstein's version of reality cannot be true.
0:53:57 > 0:54:00No amount of clever jiggery-pokery with our experiment
0:54:00 > 0:54:02can cheat nature.
0:54:02 > 0:54:04The two entangled photons' properties
0:54:04 > 0:54:07couldn't have been set from the beginning,
0:54:07 > 0:54:11but are summoned into existence only when we measure them.
0:54:14 > 0:54:17Something strange is linking them across space.
0:54:17 > 0:54:20Something we can't explain or even imagine
0:54:20 > 0:54:23other than by using mathematics.
0:54:23 > 0:54:28And weirder, photons do only become real when we observe them.
0:54:30 > 0:54:32In some strange sense, it really does suggest
0:54:32 > 0:54:35the moon doesn't exist when we're not looking.
0:54:37 > 0:54:39It truly defies common sense.
0:54:41 > 0:54:45No wonder towards the end of his life, Einstein wrote...
0:54:58 > 0:55:01The experiment only confirms this.
0:55:01 > 0:55:05Whatever is happening, we just don't understand it.
0:55:07 > 0:55:10But it doesn't mean we should stop looking.
0:55:11 > 0:55:15While it's true that Einstein's dream of finding
0:55:15 > 0:55:19a reasonable, common-sense explanation was shattered for good,
0:55:19 > 0:55:21my own personal view is that this
0:55:21 > 0:55:24doesn't necessarily banish physical reality.
0:55:24 > 0:55:29Like Einstein, I still believe there might be a more palatable
0:55:29 > 0:55:33explanation underlying the weird results of quantum mechanics.
0:55:33 > 0:55:38One thing is clear, whether there are physical, spooky connections,
0:55:38 > 0:55:40whether there are parallel universes,
0:55:40 > 0:55:44whether we bring reality into existence by looking,
0:55:44 > 0:55:46whatever the truth is,
0:55:46 > 0:55:50the weirdness of the quantum world won't go away.
0:55:50 > 0:55:53It'll rear its ugly head somewhere.
0:55:56 > 0:56:00120 years ago, the greatest scientific revolution ever
0:56:00 > 0:56:03was brought about by a light bulb.
0:56:06 > 0:56:09And scientists are still using powerful light sources
0:56:09 > 0:56:13like x-rays to unlock nature's mysteries.
0:56:18 > 0:56:20This is the Diamond Light Source.
0:56:20 > 0:56:24It's Britain's single largest science facility.
0:56:24 > 0:56:28The x-rays produced here are ten billion times more powerful
0:56:28 > 0:56:30than a hospital x-ray.
0:56:30 > 0:56:34With that's sort of power, scientists can slice into matter
0:56:34 > 0:56:36and glimpse those quantum secrets inside.
0:56:45 > 0:56:49Researchers here are using this powerful light beam
0:56:49 > 0:56:52to investigate new materials which may have the potential
0:56:52 > 0:56:57to bring about an electronics breakthrough as great as any before.
0:57:01 > 0:57:05Just as the quantum pioneers of the '20s and '30s
0:57:05 > 0:57:09ended up bringing about a scientific and technological revolution,
0:57:09 > 0:57:14so this generation of physicists are set to usher in a new quantum era.
0:57:16 > 0:57:19An era where Einstein's hated quantum entanglement
0:57:19 > 0:57:23now produces unbreakable computer security.
0:57:23 > 0:57:27New kinds of communication systems, superfast computers
0:57:27 > 0:57:30and other advances we can't yet even imagine.
0:57:40 > 0:57:45And this is why quantum mechanics thrills and frustrates me.
0:57:45 > 0:57:47It's capricious, it's counterintuitive,
0:57:47 > 0:57:50it even sometimes feels just plain wrong.
0:57:50 > 0:57:55And yet it still surprises us every day.
0:57:55 > 0:57:58And I, for one, believe that our knowledge of the quantum world
0:57:58 > 0:58:01is still far from complete.
0:58:01 > 0:58:05That there are greater truths about nature yet to be discovered.
0:58:05 > 0:58:09And that's still what keeps me awake at night.
0:58:13 > 0:58:17Next week, join me as my journey into the quantum world
0:58:17 > 0:58:19gets even more surprising.
0:58:19 > 0:58:23I investigate how its weird rules are crucial for life
0:58:23 > 0:58:26and how the bizarre behaviour of subatomic particles
0:58:26 > 0:58:30might even influence evolution itself.
0:58:38 > 0:58:43# I know I deserve you I know you're my saviour
0:58:43 > 0:58:45# But when I observe you
0:58:45 > 0:58:47# You change your behaviour
0:58:47 > 0:58:50# So I'm stuck here still
0:58:50 > 0:58:55# I'm tangled up with you. #