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00:03Beneath the complexities of everyday life,
00:07the rules of our universe seem reassuringly simple.
00:13This solid bridge supports my weight.
00:17The water flowing underneath always goes downhill.
00:21And when I throw this stone,
00:24it always flies through the air following a predictable path.
00:29But as scientists peered deep into the tiny building blocks of matter,
00:36all such certainty vanished.
00:41They found the weird world of quantum mechanics.
00:46Deep down inside everything we see around us,
00:50we found a universe completely unlike our own.
00:54To paraphrase one of the founders of quantum mechanics,
00:56everything we call real is made up of things
01:00that cannot be themselves regarded as real.
01:07Around a hundred years ago,
01:09some of the world's greatest scientists
01:11began a journey down the rabbit hole
01:14into the strange and the bizarre.
01:18They found that in the realm of the very small,
01:21things could be in two places at once.
01:26That their fates are dictated by chance.
01:31And that reality itself defies all common sense.
01:37And at stake, that everything we thought we knew about the world
01:42might turn out to be completely wrong.
01:48The story of our descent into scientific madness
01:51begins with a most unlikely object.
01:59Berlin, 1890.
02:02Germany is a new country recently unified
02:06and hungry to industrialize.
02:09In this newly unified Germany,
02:11a number of new engineering companies were founded.
02:14They spent millions buying the European patent
02:17for Edison's new invention, the light bulb.
02:23The light bulb was the epitome of modern technology,
02:26a great optimistic symbol of progress.
02:32Engineering companies quickly realized
02:35there were fortunes to be made
02:37building street lights for the new German Empire.
02:42But what they didn't realize
02:44was that they would also unleash a scientific revolution.
02:50Strangely enough, this humble object
02:52is responsible for the birth of the most important theory
02:55in the whole of science, quantum mechanics.
02:58A theory that I've spent my life studying.
03:03And that's because back in 1900,
03:06the light bulb presented a rather strange problem.
03:10Engineers knew that if you heated the filament with electricity,
03:14it glowed.
03:16The physics that underpinned this, though,
03:19was completely unknown.
03:23But something as basic as the relationship
03:25between the temperature of the filament
03:27and the color of light it produces
03:30was still a complete mystery.
03:33A mystery they were obviously keen to solve.
03:37And with the help of the new German state,
03:39they saw how to steal a march on their competitors.
03:50In 1887, the German government invested millions
03:55in a new technical research institute here in Berlin,
03:58the Physikalische Technische Reichsstaat, or PTR.
04:01Then, in 1900, they enlisted a bright,
04:05if somewhat straight-laced scientist to help work here.
04:08His name was Max Planck.
04:17Planck took on a deceptively simple problem.
04:19Why the color of the light changes
04:22as the filament gets hotter.
04:27To get a sense of the puzzle facing Planck,
04:30I'm going to ride this bicycle
04:32with an old-fashioned lamp
04:34powered by an old-fashioned dynamo.
04:47Obviously, the faster I go, the brighter the light.
04:50The more I pedal, the more electricity the dynamo produces,
04:54the hotter the filament in the lamp,
04:56and the brighter the light.
04:58But the light the bulb makes isn't just getting brighter.
05:02It's changing color, too.
05:07As I speed up, the color shifts
05:09from red to orange to yellow.
05:13Right, now I'm going to really belt it.
05:18Now the bulb's filament is getting even hotter.
05:21But although it certainly gets brighter,
05:24the color seems to stay the same.
05:28Yellow-white.
05:33Why doesn't the light get any bluer?
05:40To investigate, Planck and his colleagues built this,
05:44a black-body radiator.
05:46It's a special tube they could heat
05:48to a very precise temperature
05:49and a way to measure the color
05:52or frequency of the light it produced.
06:00Nowadays, over a hundred years later,
06:03the PTR still do exactly this kind of measurement,
06:06just much more accurately.
06:10The temperature inside here is 841 degrees centigrade.
06:16I can feel the heat coming off
06:18and it's glowing with a lovely orangey-red color.
06:26It's about the same color as my bike light
06:29when I'm cycling slowly.
06:33But I want to see something hotter still.
06:36The temperature inside here is about 2,000 degrees centigrade.
06:45And it's glowing with a much brighter, whiter colored light.
06:49To produce light of this intensity in color
06:52requires a power of about 40 kilowatts.
06:55Now, that's equivalent to about 400 Mies on a bike cycling very fast,
07:00or the combined output of the entire Tour de France.
07:05Although the light is whiter, it's red-white.
07:09There's very little blue.
07:11Why is blue so much harder to make them red?
07:17And further up the spectrum, beyond blue,
07:20the so-called ultraviolet is hardly produced at all,
07:24even when we look at things as hot as the sun.
07:31Even the sun, at a temperature of 5,500 degrees centigrade,
07:36produces mostly white visible light
07:38and makes remarkably little ultraviolet light,
07:41given how hot it is.
07:43Why is this?
07:44Why is ultraviolet light so hard to make?
07:50This remarkable failure of common sense,
07:53so perplexed scientists of the late 19th century
07:56that they gave it a very dramatic name.
07:58They called it the ultraviolet catastrophe.
08:04Planck took a crucial first step to solving this.
08:07He found the precise mathematical link
08:10between the color of light, its frequency, and its energy.
08:14But he didn't understand the connection.
08:18However, it was another weird anomaly
08:21that would really put the cat amongst the pigeons.
08:25In the late 19th century,
08:27scientists were studying the then newly discovered radio waves
08:31and how they were transmitted.
08:33And to do that,
08:33they were building experimental rigs very similar to this one.
08:37Basically, by spinning this disc,
08:39they could generate huge voltages
08:41that cause sparks to jump across the gap
08:44between the two metal spheres.
08:49But in doing so,
08:50they discovered something very unexpected to do with light.
08:55They found that by shining a powerful light source on the spheres,
09:00they could make the sparks jump across more easily.
09:04This suggested a mysterious and unexplained connection
09:07between light and electricity.
09:16To understand what was happening, scientists use this.
09:20It's called a gold leaf electroscope.
09:23It's basically a more sensitive version of the spark gap apparatus.
09:27Now, first of all, I have to charge it up.
09:34What I'm doing is adding an excess of electrons
09:37that are pushing the two gold leaves apart.
09:40Now, first I take red light and shine it on the metal surface.
09:45And nothing happens.
09:47Even if I increase the brightness of the light,
09:49still the gold leaves aren't affected.
09:53Now, I'll try this special blue light, rich in ultraviolet.
09:59Immediately, the gold leaves collapse.
10:11Light can clearly remove static electric charge from the leaves.
10:16It can somehow knock out the electrons I added to them.
10:21But why is ultraviolet light so much better at doing this than red light?
10:29This new puzzle became known as the photoelectric effect.
10:35The ultraviolet catastrophe and the photoelectric effect
10:39were big problems for physicists.
10:43Because neither could be understood using the best science of the time.
10:47The science that said, quite unequivocally, that light was a wave.
10:58All around us, we see light behaving in a perfectly common sense wavy way.
11:03Look at the shadow of my hand. It's fuzzy around the edges.
11:08We understand this as the light hitting the sides of my hand
11:13and bending and smearing out slightly.
11:15Just like water waves around an obstruction.
11:19Perfectly common sense wave-like behaviour.
11:25And here's something else.
11:27Something rather beautiful.
11:28Look at these soap bubbles.
11:31Shine a light on them and gorgeous coloured patterns emerge from nowhere.
11:36And this was easily explained if you accept that light was a wave.
11:41reflecting off the outer and inner layers of the thin soap film.
11:45And breaking up into the colours of the rainbow.
11:49Rather like ripples on the surface of water,
11:53light was simply ripples of energy spreading through space.
11:57And this was as firmly accepted as the fact that the Earth was round.
12:03But although this wave theory worked perfectly well for shadows and bubbles,
12:08when it came to the ultraviolet catastrophe and the photoelectric effect,
12:14the wheels started coming off.
12:16The problem was this.
12:18How could light do this?
12:21To truly grasp how absurd this phenomenon was,
12:25it might be useful to consider how waves in water behave.
12:36This is the wave tank at the RNLI's headquarters in Dorset.
12:41It's used to train lifeboat teams to deal with a range of different kinds of water waves.
12:47First, small waves, just 30 centimetres high.
12:50These waves don't have much energy.
12:53Hardly enough energy to knock this top pan off the other.
13:00But when the waves grow to over a metre and a half,
13:04it's a very different proposition.
13:07And they're really throwing me about.
13:10There's no way I can keep this pan balanced on the top.
13:22It's clear what water waves are telling us.
13:25Bigger, more intense waves have more power.
13:31They easily knock me and the cans around.
13:39So if light was a wave, more intensity should knock out more electrons.
13:45But that's not what happened.
13:48Remember, no matter how intense the red light was,
13:51it still didn't budge electrons from the metal.
13:54But, weirdly, weak ultraviolet worked within seconds.
13:59So thinking of light as a wave just wasn't adding up.
14:07To resolve this, someone needed to think the unthinkable.
14:11And in 1905, someone did.
14:14You may well have heard of him.
14:15His name was Albert Einstein.
14:27This is the Aachenhold Sternwark Observatory in Berlin.
14:33Perched on top is a strange, huge iron and steel construction.
14:39But it's not a gun.
14:41It's actually a telescope.
14:47Built in 1896, the telescope was one of the largest of its kind in the world,
14:52and made the observatory the go-to place to engage and astound the public in new science.
15:00Albert Einstein gave a very famous public lecture here on his theory of relativity,
15:05which is, of course, what he's most famous for.
15:08But it's not the work that won him the Nobel Prize.
15:18In 1905, he'd also come up with a new theory to explain the photoelectric effect.
15:25And what he suggested was revolutionary, and even heretical.
15:34He argued that we have to forget all about the idea that light is a wave,
15:39and think of it instead as a stream of tiny bullet-like particles.
15:44The term he used to describe a particle of light was a quantum.
15:51To Einstein, a quantum was a tiny lump of energy.
15:55And although in 1905 the word wasn't new,
15:59the idea that light could be a quantum seemed crazy.
16:05And yet, following Einstein's heretical line of thought to its logical conclusion,
16:11solved all the problems with light at a single stroke.
16:22I'll try to explain how this helps using a rough analogy.
16:26Of course, like all analogies, it's far from perfect.
16:28But hopefully, it'll give you a sense of the physics,
16:31to help you understand why thinking of light as a stream of particles
16:36solves the mystery of the photoelectric effect.
16:43In this analogy, these red balls represent Einstein's light quanta.
16:49And those cans over there are the electricity held in the metal.
16:54Now, in the original experiment, they made electricity flow from the surface of the metal by shining light on it.
17:00In my analogy, I'm going to try and knock those tin cans over using these red balls.
17:17Absolutely no effect.
17:22That's just like red light.
17:27According to Einstein, each particle of red light carries very little energy
17:33because red light has a low frequency.
17:36So even a very bright red light, with many red light particles,
17:41can't dislodge any electrons from the metal plate.
17:45Just like the red balls.
17:47Now I'm going to use heavier balls, like these blue golf balls,
17:52and I'm going to try and knock off the tin cans with these.
17:59They're like the ultraviolet light in the experiment.
18:05Now, each individual light particle carries more energy,
18:11because ultraviolet light is higher frequency.
18:22Just a few of them, like a dim ultraviolet light, are enough to knock the electrons out of the metal
18:28plate and collapse the gold leaf.
18:33So, Einstein's idea that light is made up of tiny particles, or quanta, is a wonderful explanation of the photoelectric
18:40effect.
18:41I remember when I first learnt about this, being blown away by sheer elegance and simplicity.
18:49But what's more, Einstein's nifty idea also helped solve Planck's mystery of the light bulb.
18:56There was more red than ultraviolet, because ultraviolet quanta took so much more energy to make.
19:03About a hundred times more energy.
19:06No wonder there were so few of them.
19:11That moment, at the beginning of the 20th century, signalled a genuine revolution.
19:17Because it demonstrated that the kind of physical science that people were doing right back to Newton and Laplace and
19:24people like that,
19:25that you needed a completely new approach.
19:30Physics has never recovered from that moment, in the sense that it's built on that moment.
19:34That's where modern physics really began.
19:39But Einstein's theory also left physicists with a dizzying paradox, defying all common sense.
19:47Light was definitely a wave, which explained shadows and bubbles.
19:53And now, it was definitely a particle too.
19:57Einstein's quanta, explaining the photoelectric effect and the ultraviolet catastrophe.
20:03Then, just a few years after Einstein's brilliant, crazy idea, the paradox got a lot deeper.
20:11And a whole lot weirder.
20:15Because what seemed to be a curious mystery about light,
20:19was about to become a battleground about the nature of reality itself.
20:341922. The Western world is in the grip of a revolution. A cultural revolution.
20:41James Joyce's Ulysses is published.
20:44Stravinsky is at the height of his powers.
20:47And Chaplin has just released his first serious movie.
20:50The Ottoman Empire collapses.
20:53Europe is still recovering from the war to end all wars, in which millions of men lost their lives.
20:59Russia is newly communist.
21:01Meanwhile, America is exporting jazz to the world.
21:14In art, politics, literature, economics, there was an insatiable appetite for change.
21:21This was the birth of modernism.
21:23You got a heart that there's no way of knowing.
21:28Can see where you are, but can't see where you're going.
21:32And I'm stuck here still.
21:35I'm tangled up with you.
21:42This whole world can be so uncertain.
21:47But, and I'm not getting into trouble for saying this, I would argue that the upheaval that took place in
21:54physics at this time would eclipse them all and have far longer lasting consequences.
22:00It had begun with the discovery of the weird and contradictory wave-particle nature of light.
22:07It ended up as an epic battle fought between the greatest minds in science for the highest possible stakes.
22:14The nature of reality itself.
22:18I know I deserve you.
22:21I know you're my savior.
22:23But when I observe you, you change your behavior.
22:27On one side, a new wave of modernist revolutionary scientists and their leader, the brilliant Danish physicist Niels Bohr.
22:37On the other side, the voice of reason, Albert Einstein, at the height of his powers and now world famous,
22:44a formidable adversary.
22:51The battle raged for decades.
22:54Actually, in some ways, it still does.
22:56It was fought across the world in universities, at conferences, in bars and cafes.
23:02It would reduce grown men to tears.
23:05And it began with a deceptively simple experiment.
23:10This whole world can be so uncertain.
23:15But weirdly, it was an experiment that wasn't even about light.
23:19It was about the particles that make electricity.
23:28In the mid-1920s, an experiment was carried out at Bell Laboratories in New Jersey in America, which uncovered something
23:36entirely unexpected about electrons.
23:38Now, at the time, it was accepted without question that electrons were these tiny lumps of matter, small but solid
23:46particles like miniature billiard balls.
23:48In the experiment, they fired a beam of electrons at a crystal and watched how they scattered.
23:54Now, that's entirely equivalent to taking a beam of electrons, say, from an electron gun and firing it at a
24:02screen with two slits in it, so that the electrons pass through the slits and hit another screen at the
24:12back.
24:14What the Bell scientists found shocked the physics world to the core.
24:21To understand why, consider a similar experiment with water waves.
24:27I've set up a simple experiment.
24:29I have a water ripple tank placed on top of an overhead projector.
24:34I have a generator producing waves that pass through two narrow gaps.
24:39The projector beams the image of the waves onto the back wall.
24:44You can see, as the waves come in from the left and squeeze through the two gaps, they spread out
24:50on the other side and interfere with each other.
24:53What this means is that when you get the crest from one wave meeting the crest from another, they add
24:58up to make a higher wave.
25:00But when the crest from one meets a trough, they cancel out.
25:06This gives rise to these characteristic lines leading to the signature wave pattern.
25:15Bands of light and dark.
25:19Whenever you see these light and dark bands, the signature wave pattern, you know without doubt that you've got wave
25:27-like behaviour.
25:35So, guess what they saw in New Jersey?
25:39Now it seemed that firing electrons, tiny solid particles through the two gaps, produced exactly the same kind of pattern.
25:48Bands of light and dark.
25:53First, light, for a long time believed to be a wave, was found to sometimes behave like particles.
25:59And now electrons, for a long time believed to be particles, were behaving like waves.
26:04But it was actually stranger than that.
26:06The wave pattern wasn't merely some result of the entire beam of electrons.
26:12More recently, this experiment has been repeated in labs around the world, by firing one electron at a time through
26:21the slits onto the screen.
26:26At first, each electron seems to land randomly on the screen.
26:33But gradually, a pattern forms.
26:36The signature wave pattern.
26:39Let me be quite clear about just how weird this is.
26:43Remember from the wave tank experiment where the signature wave pattern only exists because each wave passes through both slits
26:52and then its two pieces interfere with each other.
26:55But here, every individual electron, each single particle, is passing alone through the slits before it hits the screen.
27:04And yet, each single electron is still contributing to the signature wave pattern.
27:12Each electron has to be behaving like a wave.
27:20To explain this strange result, Niels Bohr and his colleagues created quantum mechanics.
27:27A crazy theory of light and matter that embraced contradiction and didn't care that it was almost impossible to understand.
27:35As Niels Bohr himself said, anyone who isn't shocked by quantum theory hasn't understood it.
27:40So, viewers, I'm going to take our tiny electron and use it to delve deep into the heart of reality.
27:49And, yes, prepare to be shocked because this is the only way to explain what we observe when a single
27:55electron travels through the slits and hits the screen.
27:59Quantum mechanics says this.
28:02We can't describe what's travelling as a physical object.
28:07All we can talk about are the chances of where the electron might be.
28:13This wave of chance somehow travels through both slits, producing interference just like the water wave.
28:23Then, when it hits the screen, what was just the ghostly possibility of an electron mysteriously becomes real.
28:34Let me try and capture just how weird this is with an analogy.
28:39If I spin this coin...
28:46Then, all the time it's spinning, it's a blur. I can't tell if it's heads or tails.
28:51But, if I stop it, I force it to decide, and it's heads.
28:56So, before, it was sort of not heads or tails, but a mixture of both.
29:01But as soon as I've stopped it, I've forced it to make up its mind.
29:05This is what Bohr and his supporters claimed was happening with our electrons.
29:14In a sense, as it spins, the coin is both heads and tails.
29:20Similarly, the electron's wave of chance passes through both slits, two paths at the same time.
29:29Our coin then stops at heads.
29:34The ethereal wave of probability hits the screen and only then becomes a particle.
29:42The quantum world was unlike anything ever seen before.
29:48Bohr.
29:49It's hard to overstate just how crazy this is.
29:53Bohr was effectively claiming that one can never know where the electron actually is at all until you measure it.
30:00And it's not just that you don't know where the electron is.
30:04It's weirdly as though the electron itself is everywhere at once.
30:10Bear in mind that electrons are among the commonest and most basic building blocks of reality.
30:16And yet, here's Bohr saying that only by looking do we actually conjure their position into existence.
30:24It's like there's a curtain between us and the quantum world.
30:29And behind it, there is no solid reality.
30:34Just the potential for reality.
30:40Things only become real when we pull back the curtain and look.
30:45And this view, ladies and gentlemen, became known as the Copenhagen interpretation.
30:58Persuasive as it might seem, many people couldn't stomach Niels Bohr's outlandish ideas.
31:04And they found a natural leader in the most powerful man in science.
31:10Albert Einstein hated this interpretation with every fibre of his being.
31:16He famously said, does the moon cease to exist when I don't look at it?
31:21He was very unhappy because it gave limits to knowledge that he didn't think should be final.
31:28He thought there should be a better underlying theory.
31:35Over the next ten years, Einstein and Bohr would argue passionately about whether quantum mechanics meant giving up on reality
31:44or not.
31:48Then, with two other scientists, Nathan Rosen and Boris Podolsky, Einstein thought they'd found a way to win the argument.
31:57He was convinced he'd found a fatal flaw in the Copenhagen interpretation.
32:01And it's claimed that reality was summoned into existence by the act of looking at it.
32:07At the heart of Einstein's argument was an aspect of quantum mechanics called entanglement.
32:13Now, entanglement is this special, incredibly close relationship between a pair of quantum particles whose fates are intertwined.
32:22For example, if they were created in the same event.
32:30Let me try and explain this by imagining the two particles are spinning coins.
32:40Imagine these coins are two electrons created from the same event and then moved apart from each other.
32:48Quantum mechanics says that because they're created together, they're entangled.
32:54And now, many of their properties are forever linked, wherever they are.
32:59Remember, the Copenhagen interpretation says that until you measure one of the coins, neither of them is heads or tails.
33:06In fact, heads and tails don't even exist.
33:09And here's where entanglement makes this weird situation even weirder.
33:14When we stop the first coin and it becomes heads, because the coins are linked to entanglement, the second coin
33:24will simultaneously become tails.
33:28And here's the crucial thing. I can't predict what the outcome of my measurement will be, only that they will
33:35always be opposite.
33:36Einstein seized on this because it meant that something was happening between the two coins that was almost too crazy
33:46to imagine.
33:48It's as if the two coins are secretly communicating, communicating instantaneously across space and time, even if the first coin
33:58was on Earth and the other was on Pluto.
34:01Einstein refused to believe this instantaneous, faster than light communication.
34:07His theory of relativity said that nothing could travel that fast, not even information.
34:13So how could one coin instantaneously know how the other would land?
34:20He disparagingly called it spooky action at a distance and claimed it was a fatal flaw in the Copenhagen interpretation.
34:28What's more, he had a better idea.
34:33Einstein believed there was a simpler interpretation, that somehow the destiny of the two coins, whether or not they ended
34:40up heads or tails, was already fixed long before we observed them.
34:46He said that although it seemed the coin was deciding to be, say, heads at the moment of observation, actually
34:54that decision was taken long before.
34:58It was just hidden from us.
35:03In Einstein's mind, quantum particles were nothing like spinning coins.
35:08They were more like, say, a pair of gloves, left and right, separated into boxes.
35:15We don't know which box contains which glove until we open one.
35:20But when we do and find, say, a right handed glove, then immediately we know that the other box contains
35:27the left handed glove.
35:28But crucially, this requires no spooky action at a distance.
35:33Neither glove has been altered by the act of observation.
35:36Both of them were either left or right handed glove from the beginning.
35:40And the only thing that has changed is our knowledge.
35:45So, which is the true description of reality?
35:49Bohr's coins, which only become real when we look at them,
35:55and then magically communicate to each other?
35:59Or Einstein's gloves, which are hidden from us, but are definitely left or right from the beginning?
36:05In other words, is there an objective reality, as Einstein believed, or not, as Bohr maintained?
36:13In the late 1930s, as the world plunged into war, there was no way to answer this question.
36:19The battle to understand the nature of reality was deadlocked.
36:29The war rolled across Europe, and many of the leading scientists fled to the United States.
36:38Then, as the Second World War led inexorably to the Cold War,
36:44American science, backed by dollar bills and a new vision of the future, boomed.
36:51Remember, after the war, physicists came back raring to go and try to apply the ideas of quantum theory to
36:59atoms,
37:00the interaction between electrons and light and what have you.
37:03You didn't need to worry about the philosophical side of things to make progress with that.
37:08So, as you say, it really took a back seat.
37:13Quantum mechanics led to a profound understanding of semiconductors,
37:17which helped create the modern electronic age.
37:22It produced lasers, revolutionising communications, breathtaking new medical advances,
37:32and breakthroughs in nuclear power.
37:38Quantum mechanics was so successful that most working physicists
37:43deliberately chose to ignore Einstein's objections.
37:47It simply didn't matter to them because it worked.
37:51They even coined a phrase for it,
37:54shut up and calculate.
37:59And the price for this success was that Bohr and Einstein's debate
38:03on the reality of the quantum world was simply brushed under the carpet.
38:12And amidst all this success and pragmatism, there were few who still worried what it all meant.
38:18But as the 50s rolled headlong into the 60s,
38:22one lone dissenter worked out how to settle the argument once and for all.
38:39John Bell, I think it's fair to say, isn't well known to the general public.
38:44But to physicists like me, he's, well, a hero.
38:48He was an original thinker with real courage in his convictions.
38:52And the story of his rise to become one of the greats of physics is made even more remarkable
38:58when you consider how he started.
39:01He was born in Belfast in the 1920s into a poor working class family.
39:06His father was a horse dealer.
39:08And they really struggled to get him into Queen's University Belfast to study physics.
39:13In fact, he was the only one in his family to even finish school.
39:16This, I believe, made him insatiably curious, fiery and stubborn.
39:28I remember meeting John Bell in 1989, a year before he died.
39:32We were both at a conference in America.
39:35And we happened to be sharing a lift just after both attending a talk on quantum mechanics.
39:41Keen to say something to the great John Bell, I said, I thought the speaker's conclusions were completely crazy.
39:50He stared at me with his piercing blue eyes.
39:52And for a moment, I thought my fledgling physics career was going down the drain.
39:56But as the lift doors opened and he was about to leave, he said, yes, I completely agree with you.
40:02Haven't they heard of the helium problem?
40:04To this day, I'm not quite sure what the helium problem is, but I was just so relieved that John
40:10Bell and I agreed.
40:22For many years, he worked here at Britain's Atomic Energy Research Centre at Harwell,
40:27who built this early experimental nuclear reactor called Dido.
40:36It was here that he started pondering the deep and worrying questions that quantum mechanics raised.
40:43Did the quantum world only exist when it was observed?
40:47Or was there a deeper truth out there waiting to be discovered?
40:52In fact, he was so troubled, he began to wonder if there was a problem at the heart of quantum
40:58mechanics.
41:00He famously said, I hesitate to think it might be wrong, but I know it is rotten.
41:08And so in the early 1960s, Bell decided to try and resolve the crisis at the heart of quantum physics.
41:14It was an epic challenge.
41:16After all, how do you check if something is real, if something is or isn't there, all without looking?
41:23How do you look behind the curtain without pulling it open?
41:28But John Bell came up with a brilliant way of doing exactly that.
41:36I think this is one of the most ingenious ideas in the whole of physics.
41:41It's certainly one of the most difficult to understand and explain.
41:44But I'm going to try and have a go, and yes, I'm afraid I'm going to use another analogy.
41:48This time, I'm going to play a game of cards, but it's one for the highest possible stakes, the nature
41:57of reality itself.
41:59The card game is against a mysterious quantum dealer.
42:04The cards he deals represent any subatomic particles, or even quanta of light, photons.
42:12And the game we'll play will ultimately tell us whether Einstein or Bohr was right.
42:20Now, the rules of the game are deceptively simple.
42:23The dealer is going to deal two cards, face down.
42:27If they're the same colour, I win.
42:30If they're different colours, I lose.
42:39So, I have a red, so I need another red to win.
42:44And it's black. I lose.
42:50Again, opposite colours. I've lost both of those.
43:00That's four in a row.
43:09That's six pairs in a row that I've lost.
43:12OK, I think I know what the dealer is doing here.
43:15Clearly, the deck has been rigged in advance so that every pair come out as opposite colours.
43:22But there's a simple way to catch the dealer out.
43:26So, what we can do now is change the rules of the game.
43:30This time, if they are the opposite colour, I win.
43:40But once again, every time, my evil quantum opponent beats me.
43:53But again, I can see what the crafted dealer could have done.
43:57Maybe while I wasn't looking, he's switched the pack and rigged it so that it always lands in his favour.
44:03Now, every pair is the same colour.
44:10Rigged decks, remember, were what Einstein thought was really happening in the entanglement experiment.
44:16He said that just like the gloves were already placed in the box,
44:21so the evil dealer stacked the cards before we played.
44:26But Niels Bohr's idea was very different.
44:30He said red and black don't even exist until you turn them over.
44:36Bell's genius was that he came up with a way of deciding once and for all who was right, Einstein
44:43or Bohr.
44:44This is how he did it.
44:46I'm now not going to tell the dealer which game I want to play,
44:50same colour wins or different colour wins, until after he's dealt the cards.
45:00Now, because he can never predict which rules I'm going to play by, he can never stack the deck correctly.
45:09Now, he can't win.
45:12Or can he?
45:14So now the rules are different wins.
45:21The same, okay.
45:24Same colour wins.
45:29This gets to the very heart of Bell's idea.
45:32If we now start playing and I win as many as I lose, then Einstein was right.
45:38The dealer is just a trickster with a gift for sleight of hand.
45:43Reality may be tricky, but it does have an objective existence.
45:51But what if I lose?
45:53Well, then I'm forced to admit that there is no sensible explanation.
46:01Each card must be sending secret signals to the other, across space and time, in defiance of everything we know.
46:10I'm forced to accept that at the fundamental quantum level, reality is truly unknowable.
46:18What if I lose?
46:22Bell reduced this idea into a single mathematical equation that tells us, once and for all, what seemed unanswerable.
46:32How reality really is.
46:35John Bell published his idea in 1964, and the extraordinary thing is, at the time, the entire physics community ignored
46:45him.
46:46Total radio silence.
46:47It seems the world simply wasn't ready.
46:52Perhaps it was because his equation seemed untestable, or just because nobody thought it was worth investigating.
47:00But that was about to change, and the change would come from a very unexpected place.
47:09This is the dawning of the age of Aquarius. Age of Aquarius. Aquarius. Aquarius.
47:31America was in crisis over Vietnam, Watergate, feminism, the Black Panthers.
47:37And while all this was going on, a small group of hippie physicists were working at the University of Berkeley
47:42in California.
47:43They did all the hippie things. They smoked dope, they popped LSD, they debated things like Buddhism and telepathy.
47:56And they loved quantum mechanics. In its weird version of reality, they saw parallels with their own esoteric beliefs.
48:15Their hippie New Age-style physics also caught the attention of the public, who read their crazy hippie books that
48:23mixed quantum mechanics with Eastern mysticism.
48:26Books like The Tau of Physics, The Dancing Wu Li Masters, and my personal favorite, Space, Time and Beyond, Towards
48:35an Explanation of the Unexplainable.
48:39But more importantly for our story, the story of quantum mechanics, these hippie physicists also turned their attention to Einstein's
48:48now famous thought experiment,
48:49and what it told us about the nature of reality.
48:54They saw Niels Bohr's secret signaling as proof that physics supported their own ideas.
49:01Because if two particles could spookily communicate across space, then ESP, telepathy and clairvoyance were probably true as well.
49:11If only they could prove it really existed.
49:14Then, in 1972, they realized that with a bit of mathematical sleight of hand, they could take Bell's equation and
49:24experimentally test it.
49:26One of their group, John Clauser, borrowed some equipment from the lab he was working in and set up the
49:32first, genuine and ultimate test of quantum mechanics.
49:39This is a picture of that first experiment, built of leftovers and stolen equipment.
49:45Over the next few years, it was improved by a team led by Allen Aspect in Paris, making its results
49:52more reliable.
49:54Over ten years after Bell first proposed his equation, finally it could be put to the test.
50:02This is a modern version of the experiment first carried out by John Clauser and then Allen Aspect.
50:12Here, a crystal converts laser light into pairs of entangled light quanta, photons, making two very precise beams.
50:26These photons are passed round and bent back again until they pass through these detectors.
50:32The two photons are like the two cards the evil dealer places in front of me.
50:39We'll measure a property of the photons called polarisation, which is equivalent to the colour of the playing cards in
50:46my game.
50:47So, for instance, winning with two matching red cards might be the same as two photons with matching polarisation.
50:56But because this is quantum mechanics, it's more complicated than my simple card game.
51:00And these dials here allow me to measure a second property of the photons as well.
51:05Now, that's equivalent to me not only trying to guess the colour of the face of the cards,
51:10but also trying to guess the colour of the back of the cards.
51:14OK, so we're now going to switch on the laser and start the experiment.
51:23So, this number here gives me the number of photon pairs coming through the experiment.
51:29That's equivalent to the pairs of cards in my game.
51:32The graph here dropping down gives me the probability that I can win, that I'm guessing right.
51:37The more photons, the more accurate it becomes.
51:41I'll stop at an uncertainty of about 1%.
51:45And the final answer is 0.56.
51:48So, if I put that into my equation, I now need to run the experiment three more times, corresponding to
51:57the four different settings of these dials.
52:01Each run is now like a different set of rules for the quantum dealer.
52:06And when I add them up and get the answer, if it's less than two, then Einstein was right.
52:12If it's greater than two, then Bohr was right.
52:15OK, so now for the second setting.
52:17Just remember what the experiment will show.
52:21If the numbers come out less than two, then it's proof the dealer has been stacking the deck.
52:28This was Einstein's view.
52:30OK, so the number I get this time is 0.82.
52:39Now, reset for run three.
52:45But if the result is greater than two, then the deck cannot be stacked.
52:50And something else is at work.
52:52OK, so the run three result is minus 0.59.
52:56And finally, run four.
53:02This last number will finally reveal if the world follows common sense or something much more bizarre.
53:10OK, so our final result is in, and it's 0.56.
53:14So if we turn the laser off...
53:18Right, I better just work out the answer.
53:26And there we have it.
53:282.53.
53:30It's a number greater than two.
53:33Absolute proof that Albert Einstein was wrong and Niels Bohr was right.
53:47The significance of this result is simply enormous.
53:52Just remember what it means.
53:54Einstein's version of reality cannot be true.
53:56No amount of clever jiggery-pokery with our experiment can cheat nature.
54:02The two entangled photons' properties couldn't have been set from the beginning,
54:07but are summoned into existence only when we measure them.
54:14Something strange is linking them across space.
54:17Something we can't explain or even imagine other than by using mathematics.
54:23And weirder, photons do only become real when we observe them.
54:29In some strange sense, it really does suggest the moon doesn't exist when we're not looking.
54:36It truly defies common sense.
54:41No wonder, towards the end of his life, Einstein wrote,
54:44all these 50 years of conscious brooding have brought me no nearer to the question,
54:50what are light quanta?
54:52Every Tom, Dick and Harry thinks he knows it, but he is mistaken.
54:58The experiment only confirms this.
55:01Whatever is happening, we just don't understand it.
55:06But it doesn't mean we should stop looking.
55:11While it's true that Einstein's dream of finding a reasonable, common sense explanation was shattered for good,
55:19my own personal view is that this doesn't necessarily banish physical reality.
55:24Like Einstein, I still believe there might be a more palatable explanation
55:29underlying the weird results of quantum mechanics.
55:33But one thing is clear.
55:34Whether there are physical, spooky connections,
55:38whether there are parallel universes,
55:40whether we bring reality into existence by looking,
55:44whatever the truth is,
55:46the weirdness of the quantum world won't go away.
55:50It'll rear its ugly head somewhere.
55:56120 years ago, the greatest scientific revolution ever was brought about by a light bulb.
56:06And scientists are still using powerful light sources, like X-rays, to unlock nature's mysteries.
56:17This is the diamond light source. It's Britain's single largest science facility.
56:23The X-rays produced here are 10 billion times more powerful than a hospital X-ray.
56:30With that sort of power, scientists can slice into matter and glimpse those quantum secrets inside.
56:45Researchers here are using this powerful light beam to investigate new materials,
56:51which may have the potential to bring about an electronics breakthrough as great as any before.
57:01Just as the quantum pioneers of the 20s and 30s ended up bringing about a scientific and technological revolution,
57:09so this generation of physicists are set to usher in a new quantum era.
57:15An era where Einstein's hated quantum entanglement now produces unbreakable computer security.
57:23New kinds of communication systems, super-fast computers, and other advances we can't yet even imagine.
57:39And this is why quantum mechanics thrills and frustrates me.
57:44It's capricious, it's counterintuitive, it even sometimes feels just plain wrong.
57:50And yet, it still surprises us every day.
57:55And I, for one, believe that our knowledge of the quantum world is still far from complete.
58:00That there are greater truths about nature yet to be discovered.
58:05And that's still what keeps me awake at night.
58:13Next week, join me as my journey into the quantum world gets even more surprising.
58:18I investigate how its weird rules are crucial for life,
58:22and how the bizarre behaviour of subatomic particles might even influence evolution itself.
58:34Coming up next tonight, higher than the tallest building, higher than the biggest plane.
58:39Higher than everyone, except for two actual astronauts.
58:41James May goes to the edge of space in just a moment.
58:44Then at half past ten, we're off to Hyde Park for some summer sunshine with the Rolling Stones.
58:50The
58:51The
58:51The
58:51The
58:51I'm tangled up with you
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