The Fascinating Secrets Of Quantum Physics & Gravity With Jim Al-Khalili

Beneath the complexities of everyday life, the rules of our universe seem reassuringly simple. This solid bridge supports my weight. The water flowing underneath always goes downhill. And when I throw this stone, it always flies through the air following a predictable path.

But as scientists peered deep into the tiny building blocks of matter, all such certainty vanished. They found the weird world of quantum mechanics. Deep down inside everything we see around us, we found a universe completely unlike our own. To paraphrase one of the founders of quantum mechanics, everything we call real is made up of things that cannot be themselves regarded as real.

Around a 100 years ago, some of the world's greatest scientists began a journey down the rabbit hole into the strange and the bizarre. They found that in the realm of the very small, things could be in two places at once. That their fates are dictated by chance. And that reality itself defies all common sense. And at stake, that everything we thought we knew about the world might turn out to be completely wrong.

The story of our descent into scientific madness begins with the most unlikely object. Berlin, 1890. Germany is a new country, recently unified and hungry to industrialize. In this newly unified Germany, a number of new engineering companies were founded. They spent millions buying the European patent for Edison's new invention, the light bulb. The light bulb was the epitome of modern technology, a great optimistic symbol of progress. Engineering companies quickly realized there were fortunes to be made building street lights for the new German Empire. But what they didn't realize was that they would also unleash a scientific revolution.

Strangely enough, this humble object is responsible for the birth of the most important theory in the whole of science, quantum mechanics. A theory that I've spent my life studying. And that's because back in 1900, the light bulb presented a rather strange problem. Engineers knew that if you heated the filament with electricity, it glowed. The physics that underpinned this though was completely unknown. But something as basic as the relationship between the temperature of the filament and the color of light it produces was still a complete mystery. A mystery they were obviously keen to solve. And with the help of the new German state, they saw how to steal a march on their competitors.

In 1887, the German government invested millions in a new technical research institute here in Berlin, the Physical Technicia Rashenstat or PTR. Then in 1900 they enlisted a bright if somewhat straight lace scientist to help work here. His name was Max Plank. Plank took on a deceptively simple problem. Why the color of the light changes as the filament gets hotter.

To get a sense of the puzzle facing plank, I'm going to ride this bicycle with an old-fashioned lamp powered by an old-fashioned dynamo. Obviously, the faster I go, the brighter the light. The more I pedal, the more electricity the dynamo produces, the hotter the filament in the lamp, and the brighter the light. But the light the bulb makes isn't just getting brighter. It's changing color, too. As I speed up, the color shifts from red to orange to yellow. Right now, I'm going to really belt it. Now the bulb's filament is getting even hotter. But although it certainly gets brighter, the color seems to stay the same. Yellow, white. Why doesn't the light get any bluer?

To investigate, Plank and his colleagues built this, a black body radiator. It's a special tube they could heat to a very precise temperature and a way to measure the color or frequency of the light it produced. Nowadays, over a 100 years later, the PTR still do exactly this kind of measurement, just much more accurately. The temperature inside here is 841° centigrade. I can feel the heat coming off and it's glowing with a lovely orangey red color. It's about the same color as my bike light when I'm cycling slowly, but I want to see something hotter still. The temperature inside here is about 2,000° centigrade. and it's glowing with a much brighter whiter colored light. To produce light of this intensity in color requires a power of about 40 kW. Now that's equivalent to about 400 m on a bike cycling very fast or the combined output of the entire tour to France. Although the light is whiter, it's red white. There's very little blue. Why is blue so much harder to make than red? And further up the spectrum, beyond blue, the so-called ultraviolet, is hardly produced at all. Even when we look at things as hot as the sun, even the sun at a temperature of 5 12,000° centigrade produces mostly white visible light and makes remarkably little ultraviolet light given how hot it is. Why is this? Why is ultraviolet light so hard to make? This remarkable failure of common sense so perplexed scientists of the late 19th century that they gave it a very dramatic name. They called it the ultraviolet catastrophe.

Planck took a crucial first step to solving this. He found the precise mathematical link between the color of light, its frequency, and its energy. But he didn't understand the connection. However, it was another weird anomaly that would really put the cat amongst the pigeons. In the late 19th century, scientists were studying the then their newly discovered radio waves and how they were transmitted. And to do that, they were building experimental rigs very similar to this one. Basically, by spinning this disc, they could generate huge voltages that cause sparks to jump across the gap between the two metal spheres. But in doing so, they discovered something very unexpected to do with light. They found that by shining a powerful light source on the spheres, they could make the sparks jump across more easily. This suggested a mysterious and unexplained connection between light and electricity.

To understand what was happening, scientists use this. It's called a gold leaf electroscope. It's basically a more sensitive version of the spark gap apparatus. Now, first of all, I have to charge it up. What I'm doing is adding an excess of electrons that are pushing the two gold leaves apart. Now, first I take red light and shine it on the metal surface and nothing happens. Even if I increase the brightness of the light, still the gold leaves aren't affected. Now I'll try this special blue light rich in ultraviolet. Immediately the gold leaves collapse. Light can clearly remove static electric charge from the leaves. It can somehow knock out the electrons I added to them. But why is ultraviolet light so much better at doing this than red light? This new puzzle became known as the photo electric effect.

The ultraviolet catastrophe and the photoelectric effect were big problems for physicists because neither could be understood using the best science of the time. The science that said quite unequivocally that light was a wave. All around us we see light behaving in a perfectly common sense wavy way. Look at the shadow of my hand. It's fuzzy around the edges. We understand this as the light hitting the sides of my hand and bending and smearing out slightly, just like water waves around an obstruction. Perfectly common sense wavelike behavior. And here's something else, something rather beautiful. Look at these soap bubbles. shine a light on them and gorgeous colored patterns emerge from nowhere. And this was easily explained if you accept that light was a wave reflecting off the outer and inner layers of the thin soap film and breaking up into the colors of the rainbow. Rather like ripples on the surface of water, light was simply ripples of energy spreading through space. And this was as firmly accepted as the fact that the earth was round. But although this wave theory worked perfectly well for shadows and bubbles, when it came to the ultraviolet catastrophe and the photoelectric effect, the wheels started coming off. The problem was this. How could light do this? To truly grasp how absurd this phenomenon was, it might be useful to consider how waves in water behave.

This is the wave tank at the RNLI's headquarters in Dorset. It's used to train lifeboat teams to deal with a range of different kinds of water waves. First, small waves just 30 cm high. These waves don't have enough energy, hardly enough energy to knock this top pan off the other. But when the waves grow to over a meter and a half, it's a very different proposition. And they're really throwing me about. There's no way I can keep this pan balanced on the top. It's clear what water waves are telling us. Bigger, more intense waves have more power. They easily knock me and the cans around. So, if light was a wave, more intensity should knock out more electrons. But that's not what happened. Remember, no matter how intense the red light was, it still didn't budge electrons from the metal. But, weirdly, weak ultraviolet worked within seconds. So, thinking of light as a wave just wasn't adding up.

To resolve this, someone needed to think the unthinkable. And in 1905, someone did. You may well have heard of him. His name was Albert Einstein.

This is the Akenhold Sternvark Observatory in Berlin. Perched on top is a strange huge iron and steel construction, but it's not a gun. It's actually a telescope. Built in 1896, the telescope was one of the largest of its kind in the world and made the observatory the go-to place to engage and astound the public in new science. Albert Einstein gave a very famous public lecture here on his theory of relativity, which is of course what he's most famous for, but it's not the work that won him the Nobel Prize. In 1905, he'd also come up with a new theory to explain the photoelectric effect. And what he suggested was revolutionary and even heretical. He argued that we have to forget all about the idea that light is a wave and think of it instead as a stream of tiny bullet-like particles. The term he used to describe a particle of light was a quantum. To Einstein, a quantum was a tiny lump of energy. And although in 1905 the word wasn't new, the idea that light could be a quantum seemed crazy. And yet, following Einstein's heretical line of thought to its logical conclusion, solved all the problems with light at a single stroke.

I'll try to explain how this helps using a rough analogy. Of course, like all analogies, it's far from perfect, but hopefully it'll give you a sense of the physics to help you understand why thinking of light as a stream of particles solves the mystery of the photoelectric effect. In this analogy, these red balls represent Einstein's light quanta. And those cans over there are the electricity held in the metal. Now, in the original experiment, they made electricity flow from the surface of the metal by shining light on it. In my analogy, I'm going to try and knock those tin cans over using these red balls. absolutely no effect. That's just like red light. According to Einstein, each particle of red light carries very little energy because red light has a low frequency. So even a very bright red light with many red light particles can't dislodge any electrons from the metal plate just like the red balls. Now I'm going to use heavier balls like these blue golf balls and I'm going to try and knock off the tinkans with these. They are like the ultraviolet lights in the experiment. Now each individual light particle carries more energy because ultraviolet light is higher frequency. Just a few of them like a dim ultraviolet light are enough to knock the electrons out of the metal plate and collapse the gold leaf. So Einstein's idea that light is made up of tiny particles or quanta is a wonderful explanation of the photoelectric effect. I remember when I first learned about this being blown away by its sheer elegance and simplicity. But what's more, Einstein's nifty idea also helped solve Planck's mystery of the light bulb. There was more red than ultraviolet because ultraviolet quanta took so much more energy to make about 100 times more energy. No wonder there were so few of them.

That moment at the beginning of the 20th century signaled a genuine revolution because it demonstrated that the kind of uh physical science of people were doing right back to Newton and Llass and people like that that you needed a completely new approach. Physics has never recovered from that moment in the sense that it's built on that moment. That's where modern physics really began. But Einstein's theory also left physicists with a dizzying paradox defying all common sense. Light was definitely a wave which explained shadows and bubbles. And now it was definitely a particle too. Einstein's quanta explaining the photoelectric effect and the ultraviolet catastrophe. Then just a few years after Einstein's brilliant crazy idea, the paradox got a lot deeper and a whole lot weirder. Because what seemed to be a curious mystery about light was about to become a battleground about the nature of reality itself.

1922. The Western world is in the grip of a revolution, a cultural revolution. James Joyce's Ulyses is published. Stravinsky is at the height of his powers and Chaplain has just released his first serious movie. The Ottoman Empire collapses. Europe is still recovering from the war to end all wars in which millions of men lost their lives. Russia is newly communist. Meanwhile, America is exporting jazz to the world. Thank you. In arts, politics, literature, economics, there was an insatiable appetite for change. This was the birth of modernism.

So uncertain, but and I might get into trouble for saying this, I would argue that the upheaval that took place in physics at this time would eclipse them all and have far longerlasting consequences. It had begun with the discovery of the weird and contradictory wave particle nature of light. It ended up as an epic battle fought between the greatest minds in science for the highest possible stakes. The nature of reality itself.

On one side, a new wave of modernist revolutionary scientists and their leader, the brilliant Danish physicist Neils Bore. On the other side, the voice of reason, Albert Einstein, at the height of his powers and now world famous, a formidable adversary. The battle raged for decades. Actually, in some ways, it still does. It was fought across the world in universities, at conferences, in bars and cafes. It would reduce grown men to tears. And it began with a deceptively simple experiment.

But weirdly, it was an experiment that wasn't even about light. It was about the particles that make electricity. somebody else. In the mid 1920s, an experiment was carried out at Bell Laboratories in New Jersey in America, which uncovered something entirely unexpected about electrons. Now, at the time, it was accepted without question that electrons were these tiny lumps of matter, small but solid particles, like miniature billyard balls. In the experiment, they fired a beam of electrons at a crystal and watched how they scattered. Now, that's entirely equivalent to taking a beam of electrons, say from an electron gun, and firing it at a screen with two slits in it, so that the electrons pass through the slits and hit another screen at the back. What the Bell scientists found shocked the physics world to the core. To understand why, consider a similar experiment with water waves. I've set up a simple experiment. I have a water ripple tank placed on top of an overhead projector. I have a generator producing waves that pass through two narrow gaps. The projector beams the image of the waves onto the back wall. You can see as the waves come in from the left and squeeze through the two gaps, they spread out on the other side and interfere with each other. What this means is that when you get the crest from one wave meeting the crest from another, they add up to make a higher wave. But when the crest from one meets a trough, they cancel out. This gives rise to these characteristic lines leading to the signature wave pattern bands of light and dark. Whenever you see these light and dark bands, the signature wave pattern, you know without doubt that you've got wavelike behavior.

So guess what they saw in New Jersey. Now it seemed that firing electrons, tiny solid particles through the two gaps, produced exactly the same kind of pattern. Bands of light and dark. First light, for a long time believed to be a wave, was found to sometimes behave like particles. And now electrons for a long time believed to be particles were behaving like waves. But it was actually stranger than that. The wave pattern wasn't merely some result of the entire beam of electrons. More recently, this experiment has been repeated in labs around the world by firing one electron at a time through the slits onto the screen.

At first, each electron seems to land randomly on the screen. But gradually, a pattern forms, the signature wave pattern. Let me be quite clear about just how weird this is. Remember from the wave tank experiment where the signature wave pattern only exists because each wave passes through both slits and then its two pieces interfere with each other. But here every individual electron, each single particle is passing alone through the slits before it hits the screen. And yet each single electron is still contributing to the signature wave pattern. Each electron has to be behaving like a wave.

To explain this strange result, Neilsbore and his colleagues created quantum mechanics, a crazy theory of light and matter that embraced contradiction and didn't care that it was almost impossible to understand. As Neils Boore himself said, "Anyone who isn't shocked by quantum theory hasn't understood it." So viewers, I'm going to take our tiny electron and use it to delve deep into the heart of reality. And yes, prepare to be shocked because this is the only way to explain what we observe when a single electron travels through the slits and hits the screen. Quantum mechanics says this. We can't describe what's traveling as a physical object. All we can talk about are the chances of where the electron might be. This wave of chance somehow travels through both slits, producing interference just like the water wave. Then when it hits the screen, what was just the ghostly possibility of an electron mysteriously becomes real.

Let me try and capture just how weird this is with an analogy. If I spin this coin, then all the time it's spinning, it's a blur. I can't tell if it's heads or tails. But if I stop it, I force it to decide and its heads. So before it was sort of not heads or tails, but a mixture of both. But as soon as I've stopped it, I've forced it to make up its mind. This is what Bore and his supporters claimed was happening with our electrons. In a sense, as it spins, the coin is both heads and tails. Similarly, the electron's wave of chance passes through both slits, two paths at the same time. Our coin then stops at heads. The ethereal wave of probability hits the screen and only then becomes a particle.

The quantum world was unlike anything ever seen before. It's hard to overstate just how crazy this is. Bore was effectively claiming that one can never know where the electron actually is at all until you measure it. And it's not just that you don't know where the electron is. It's weirdly as though the electron itself is everywhere at once. Bear in mind that electrons are among the commonest and most basic building blocks of reality. And yet here's Bore saying that only by looking do we actually conjure their position into existence. It's like there's a curtain between us and the quantum world. And behind it, there is no solid reality, just the potential for reality. Things only become real when we pull back the curtain and look. And this view, ladies and gentlemen, became known as the Copenhagen interpretation.

Persuasive as it might seem, many people couldn't stomach Neils Ball's outlandish ideas, and they found a natural leader in the most powerful man in science. Albert Einstein hated this interpretation with every fiber of his being. He famously said, "Does the moon cease to exist when I don't look at it?" He was very unhappy because it uh gave limits to knowledge that he didn't think should be final. He thought there should be a better underlying theory.

Over the next 10 years, Einstein and Bore would argue passionately about whether quantum mechanics meant giving up on reality or not. Then with two other scientists, Nathan Rosen and Boris Podolski, Einstein thought they'd found a way to win the argument. He was convinced he'd found a fatal flaw in the Copenhagen interpretation and its claim that reality was summoned into existence by the act of looking at it. At the heart of Einstein's argument was an aspect of quantum mechanics called entanglement. Now entanglement is this special incredibly close relationship between a pair of quantum particles whose fates are intertwined. For example, if they were created in the same event. Let me try and explain this by imagining the two particles are spinning coins. Imagine these coins are two electrons created from the same event and then moved apart from each other. Quantum mechanics says that because they're created together, they're entangled. And now many of their properties are forever linked wherever they are. Remember, the Copenhagen interpretation says that until you measure one of the coins, neither of them is heads or tails. In fact, heads and tails don't even exist. And here's where entanglement makes this weird situation even weirder. When we stop the first coin and it becomes heads because the coins are linked through entanglement, the second coin will simultaneously become tails. And here's the crucial thing. I can't predict what the outcome of my measurement will be, only that they will always be opposite.

Einstein seized on this because it meant that something was happening between the two coins that was almost too crazy to imagine. It's as if the two coins are secretly communicating. Communicating instantaneously across space and time, even if the first coin was on Earth and the other was on Pluto. Einstein refused to believe this instantaneous faster than light communication. His theory of relativity said that nothing could travel that fast, not even information. So, how could one coin instantaneously know how the other would land? He disparagingly called it spooky action at a distance and claimed it was a fatal flaw in the Copenhagen interpretation. What's more, he had a better idea. Einstein believed there was a simpler interpretation that somehow the destiny of the two coins, whether or not they ended up heads or tails, was already fixed long before we observe them. He said that although it seemed the coin was deciding to be say heads at the moment of observation, actually that decision was taken long before It was just hidden from us. In Einstein's mind, quantum particles were nothing like spinning coins. They were more like, say, a pair of gloves, left and right, separated into boxes. We don't know which box contains which glove until we open one. But when we do and find say a right-handed glove, then immediately we know that the other box contains a left-handed glove. But crucially, this requires no spooky action at a distance. Neither glove has been altered by the act of observation. Both of them were either left or right-handed glove from the beginning. And the only thing that has changed is our knowledge. So which is the true description of reality? BS coins which only become real when we look at them and then magically communicate to each other or Einstein's gloves which are hidden from us but are definitely left or right from the beginning. In other words, is there an objective reality as Einstein believed or not?

As Bore maintained, in the late 1930s, as the world plunged into war, there was no way to answer this question. The battle to understand the nature of reality was deadlocked. The war rolled across Europe and many of the leading scientists fled to the United States. Then, as the Second World War led inexorably to the Cold War, American science, backed by dollar bills and a new vision of the future, boomed. Remember, after the war, physicists came back raring to go and try to apply uh the uh ideas of of quantum theory to uh to to atoms, the interaction between electrons and and light and what have you. You didn't need to worry about the philosophical side of things. uh to make progress with that. So as you say it really took a backseat.

Quantum mechanics led to a profound understanding of semiconductors which helped create the modern electronic age. It produced lasers revolutionizing communications, breathtaking new medical advances and breakthroughs in nuclear power. Quantum mechanics was so successful that most working physicists deliberately chose to ignore Einstein's objections. It simply didn't matter to them because it worked. They even coined a phrase for it, shut up and calculate. And the price for this success was that Bor and Einstein's debate on the reality of the quantum world was simply brushed under the carpet. And amidst all this success and pragmatism, there were few who still worried what it all meant.

But as the 50s rolled headlong into the 60s, one lone dissenter worked out how to settle the argument once and for all. John Bell, I think it's fair to say, isn't well known to the general public. But to physicists like me, he's well a hero. He was an original thinker with real courage in his convictions. And the story of his rise to become one of the greats of physics is made even more remarkable when you consider how he started. He was born in Belfast in the 1920s into a poor working-class family. His father was a horse dealer and they really struggled to get him into Queens University Belfast to study physics. In fact, he was the only one in his family to even finish school. This, I believe, made him insatiably curious, fiery, and stubborn. I remember meeting John Bell in 1989, a year before he died. We were both at a conference in America and we happened to be sharing a lift just after both attending a talk on quantum mechanics. Keen to say something to the great John Bell, I said I thought the the the speaker's conclusions were completely crazy. He he stared at me with his piercing blue eyes and for a moment I thought my fledgling physics career was going down the drain. But as the lift doors opened and he was about to leave, he said, "Yes, I completely agree with you. Haven't they heard of the helium problem? To this day, I'm not quite sure what the helium problem is, but I was just so relieved that John Bell and I agreed.

For many years he worked here at Britain's atomic energy research center Harwell who built this early experimental nuclear reactor called Daido. It was here that he started pondering the deep and worrying questions that quantum mechanics raised. Did the quantum world only exist when it was observed? Or was there a deeper truth out there waiting to be discovered? In fact, he was so troubled he began to wonder if there was a problem at the heart of quantum mechanics. He famously said, "I hesitate to think it might be wrong, but I know it is rotten."

And so, in the early 1960s, Belle decided to try and resolve the crisis at the heart of quantum physics. It was an epic challenge. After all, how do you check if something is real? that something is or isn't there all without looking? How do you look behind the curtain without pulling it open? But John Bell came up with a brilliant way of doing exactly that. I think this is one of the most ingenious ideas in the whole of physics. It's certainly one of the most difficult to understand and explain, but I'm going to try and have a go. And yes, I'm afraid I'm going to use another analogy. This time I'm going to play a game of cards, but it's one for the highest possible stakes, the nature of reality itself. The card game is against a mysterious quantum dealer. The cards he deals represent any subatomic particles or even quant of light photons. And the game we'll play will ultimately tell us whether Einstein or Bore was right. Now, the rules of the game are deceptively simple. The dealer is going to deal two cards face down. If they're the same color, I win. If they're different colors, I lose. So, I have a red. So, I need another red to win. That's black. I lose again. Opposite colors. I've lost both those. That's four in a row. That's six pairs in a row that I've lost. Okay, I think I know what the deal is doing here. Clearly, the deck has been rigged in advance so that every pair come out as opposite colors. But there's a simple way to catch the dealer out. So, what we can do now is change the rules of the game. This time, if they are the opposite color, I win. But once again, every time my evil quantum opponent beats me, but again, I can see what the crafted dealer could have done. Maybe while I wasn't looking, he's switched the pack and and and rigged it so that it always lands in his favor. Now every pair is the same color. Rig decks, remember, were what Einstein thought was really happening in the entanglement experiment. He said that just like the gloves were already placed in the box. So the evil dealer stacked the cards before we played. But Neil's B's idea was very different. He said, "Red and black don't even exist until you turn them over." Belle's genius was that he came up with a way of deciding once and for all who was right, Einstein or Bore. This is how he did it. I'm now not going to tell the dealer which game I want to play, same color wins or different color wins, until after he's dealt the cards. Now, because he can never predict which rules I'm going to play by, he can never stack the deck correctly. Now, he can't win. Or can he? So, now the rules are different wins the same. Okay. Same color wins. This gets to the very heart of Belle's idea. If we now start playing and I win as many as I lose, then Einstein was right. The dealer is just a trickster with a gift for slight of hand. Reality may be tricky, but it does have an objective existence. But what if I lose? Well, then I'm forced to admit that there is no sensible explanation. Each card must be sending secret signals to the other across space and time in defiance of everything we know. I'm forced to accept that at the fundamental quantum level, reality is truly unknowable.

Bell reduced this idea into a single mathematical equation that tells us once and for all what seemed unanswerable, how reality really is. John Bell published his idea in 1964 and the extraordinary thing is at the time the entire physics community ignored him. Total radio silence. It seems the world simply wasn't ready. Perhaps it was because his equation seemed untestable or just because nobody thought it was worth investigating. But that was about to change. And the change would come from a very unexpected place.

America was in crisis over Vietnam, Watergate, feminism, the Black Panthers. And while all this was going on, a small group of hippie physicists were working at the University of Berkeley in California. They did all the hippie things. They smoked dope. They popped LSD. They debated things like Buddhism and telepathy. And they loved quantum mechanics. In its weird version of reality, they saw parallels with their own esoteric beliefs. Their hippie new age style physics also caught the attention of the public who read their crazy hippie books that mixed quantum mechanics with eastern mysticism. books like The Tower of Physics, The Dancing Wooy Masters, and my personal favorite, Spacetime and Beyond, towards an explanation of the unexplainable. But more importantly for our story, the story of quantum mechanics, these hippie physicists also turn their attention to Einstein's now famous thought experiment and what it told us about the nature of reality. They saw Neil's Bor's secret signaling as proof that physics supported their own ideas. Because if two particles could spookily communicate across space, then ESP, telepathy, and clairvoyance were probably true as well. If only they could prove it really existed.

Then in 1972, they realized that with a bit of mathematical slight of hand, they could take Bell's equation and experimentally test it. One of their group, John Clauser, borrowed some equipment from the lab he was working in and set up the first genuine and ultimate test of quantum mechanics. This is a picture of that first experiment built of leftovers and stolen equipment. Over the next few years, it was improved by a team led by Alan Aspect in Paris, making its results more reliable. Over 10 years after Bell first proposed his equation, finally it could be put to the test.

This is a modern version of the experiment first carried out by John Clauser and then Alan aspect. Here a crystal converts laser light into pairs of entangled light quanta photons making two very precise beams. These photons are passed round and bent back again until they pass through these detectors. The two photons are like the two cards the evil dealer places in front of me. We'll measure a property of the photons called polarization, which is equivalent to the color of the playing cards in my game. So for instance, winning with two matching red cards might be the same as two photons with matching polarization. But because this is quantum mechanics, it's more complicated than my simple card game. And these dials here allow me to measure a second property of the photons as well. Now that's equivalent to me not only trying to guess the color of the face of the cards, but also trying to guess the color of the back of the cards. Okay, so we're now going to switch on the laser and start the experiment. So this number here gives me the number of photon pairs coming through the experiment. That's equivalent to the pairs of cards in my game. The graph here dropping down gives me the probability that I can win, that I'm guessing right. The more photons, the more accurate it becomes. I'll stop as an uncertainty of about 1%. And the final answer is.56. So if I put that into my equation, I now need to run the experiment three more times corresponding to the four different settings of these dials. Each run is now like a different set of rules for the quantum dealer. And when I add them up and get the answer, if it's less than two, then Einstein was right. If it's greater than two, then bore was right. Okay, so now for the second setting, just remember what the experiment will show. If the numbers come out less than two, then it's proof the dealer has been stacking the deck. This was Einstein's view. Okay, so the number I get this time is 0.82. Now reset for run three. But if the result is greater than two then the deck cannot be stacked and something else is at work. Okay, so the run three result is minus.59. And finally run four. This last number will finally reveal if the world follows common sense or something much more bizarre. Okay, so our final result is in. And it's 0.56. So if we turn the laser off, right, I better just work out the answer. And there we have it. 2.53. It's a number greater than two. Absolute proof that Albert Einstein was wrong and Neil's Bore was right.

The significance of this result is simply enormous. Just remember what it means. Einstein's version of reality cannot be true. No amount of clever jiggory pokery with our experiment can cheat nature. The two entangled photons properties couldn't have been set from the beginning, but are summoned into existence only when we measure them. Something strange is linking them across space. Something we can't explain or even imagine other than by using mathematics. And weirder, photons do only become real when we observe them. In some strange sense, it really does suggest the moon doesn't exist when we're not looking. It truly defies common sense. No wonder towards the end of his life Einstein wrote all these 50 years of conscious brooding have brought me no nearer to the question what are like quanta every Tom Dick and Harry thinks he knows it but he is mistaken the experiment only confirms this whatever is happening we just don't understand it but it doesn't mean we should stop looking.

While it's true that Einstein's dream of finding a reasonable common sense explanation was shattered for good, my own personal view is that this doesn't necessarily banish physical reality. Like Einstein, I still believe there might be a more palatable explanation underlying the weird results of quantum mechanics. But one thing is clear. Whether there are physical spooky connections, whether there are parallel universes, whether we bring reality into existence by looking, whatever the truth is, the weirdness of the quantum world won't go away. It'll rear its ugly head somewhere.

120 years ago, the greatest scientific revolution ever was brought about by a light bulb. And scientists are still using powerful light sources like X-rays to unlock nature's mysteries. This is the diamond light source. It's Britain's single largest science facility. The X-rays produced here are 10 billion times more powerful than a hospital X-ray. With that sort of power, scientists can slice into matter and glimpse those quantum secrets inside. Researchers here are using this powerful light beam to investigate new materials which may have the potential to bring about an electronics breakthrough as great as any before.

Just as the quantum pioneers of the 20s and 30s ended up bringing about a scientific and technological revolution, so this generation of physicists are set to usher in a new quantum era. An era where Einstein's hated quantum entanglement now produces unbreakable computer security. new kinds of communication systems, super fast computers, and other advances we can't yet even imagine. And this is why quantum mechanics thrills and frustrates me. It's capricious. It's counterintuitive. It even sometimes feels just plain wrong. And yet it still surprises us every day. And I for one believe that our knowledge of the quantum world is still far from complete. That there are greater truths about nature yet to be discovered. And that's still what keeps me awake at night.

Next week, join me as my journey into the quantum world gets even more surprising. I investigate how its weird rules are crucial for life and how the bizarre behavior of subatomic particles might even influence evolution itself. Welcome to a new and very strange world of nature. It's being taken over by the weird subatomic particles of quantum physics. As a physicist, I've spent my working life studying how these particles behave in the laboratory. But now I'm heading out into the natural world. I'm on a mission to prove that quantum physics can solve the greatest mysteries in biology. This is a real adventure for me. I'm very much out of my comfort zone trying to apply the very careful ideas I'm familiar with in a physics laboratory to the messy world of living things. I believe that quantum physics could hold many of life's secrets. That deep in the cells of animals, particles glide through walls like ghosts. that when plants capture sunlight, their cells are invaded by shimmering waves that can be everywhere at the same time. And that even our human senses are tuning into strange quantum vibrations.

In the fantastic world of quantum biology, life is a game of chance played by quantum rules. This is what I hope to convince you of to show you that quantum mechanics is essential in explaining many of the important processes in life and potentially that quantum mechanics may even underpin the very existence of life itself. My quest begins with one of the most majestic sites in nature, migration. Every winter, barnacle geese arrive right on queue at the same Scottish river. The end of an epic 2,000mi voyage from Swalbard high above the Arctic Circle. Of course, many birds head south for winter, then back home for summer. But for decades, exactly how birds navigated with such accuracy was one of the greatest mysteries in biology.

So the most recent discovery has caused a sensation. In the past few years, one species of bird has helped create a scientific revolution. I was one of many physicists who were shocked to discover that it navigates using one of the strangest tricks in the whole of science. It utilizes a quirk of quantum mechanics, one that bamboozled even the greatest of physicists from Richard Fineman to Albert Einstein himself. So you might be surprised to discover the identity of this mysterious creature. Say hello to the quantum robin.

This is the European robin. Every year she migrates from northern Europe to the tip of Spain and back. In this laboratory in the woods, biologist Henrik Muritson is trying to solve the mystery of how she does it. But he's found himself in my world, the strange world of quantum mechanics. Quantum mechanics describes the very weird behavior of subatomic particles. Down in this realm of the very small, we have to abandon common sense and intuition. Instead, this is a world where objects can spread out like waves. Quantum particles can be in many places at once and send each other mysterious communications. I set out to understand how the bird finds its way. But it just turned out that the data more and more pointed towards this as the only explanation that could bring all the different results together.

Henrik's investigating a long-standing theory that robins navigate by the Earth's magnetic field. His laboratory is an ingenious magnetic bird cage. And these plastic cones lined with scratch sensitive paper provide the key measurements. Henrik's artificial magnetic field is like the Earth's except that he can point it in any direction he likes. Inside their cones, the robins always respond to the field, leaving scratches in a single direction. The big mystery is how. The Earth's magnetic field is incredibly weak. Far too weak for any living creature to detect. But Henrik has found an intriguing clue by giving the quantum Robin a mask. We have little leather hoods similar to what you put on a falcon, you know, but just for Robin. And you have then a hole in front of one eye or a hole in front of the other eye. But what we can see is that if you cover up the right eye, you turn off the magnetic compass processing in the left part of the brain. If you cover up this eye, you turn the compass off in this part of the brain. The robin's magnetic compass seems to be in her eyes. I can show you what's going on using my own eye. Now, we use our eyes for vision, but we also have a second light detecting mechanism. If I shine this torch into my eye, you can see that my pupil closes down. It's basically a defense mechanism to protect my eyes. My eye is responding to particles of light or photons. The energy provided by the photons is clearly enough to activate chemical reactions. After all, that's what controls my eye muscles. Light must be causing similar chemical reactions in the robin's eyes. In fact, it's the power supply for a unique form of magnetic compass inside her cells in the weird world of subatomic particles. A place where only quantum physics can explain what's going on.

To see why, imagine the chemical reactions in the robin's eye taking place in mountains and valleys of energy. To get a reaction to start, you have to push molecules to the top of a mountain. Thanks to Henrik's experiments, we now know that light does most of the hard work. But when it reaches the very peak, the molecule becomes incredibly sensitive to the slightest touch. The key point here is that the Robin's chemical compass is now balanced on an energy peak between two valleys. Going one way produces one set of chemical products. the other a different set. Now, even a tiny change in the Earth's magnetic field can tip the molecule over the top. But the way this happens defies common sense. The final piece of the puzzle depends on one of the truly mind-boggling ideas in physics. But don't worry if you find it hard to understand. Even Albert Einstein called it spooky. The idea is called quantum entanglement. It involves particles that seem to communicate faster than the speed of light. In 1935, Einstein published a famous paper arguing that it was impossible. But Einstein was wrong. In recent years, extremely delicate experiments have shown that subatomic particles really are entangled. It means they can subtly and instantaneously influence each other across space. And now it seems the same thing is going on inside the robin's eye.

When a photon enters the robin's eye, it creates what's called an entangled pair of electrons. Here's how it works. Each electron has two possible states. For simplicity, I'm choosing to call them red and green. Now, here's the weird thing. Until I measure it, it's neither one nor the other, but both at the same time. Think of the electrons like spinning discs. They are simultaneously red and green. But by firing a dart, I can force the first electron to be one or the other. So far, it's just a game of chance. I don't know what I'll get until I try it. So, I know my first electron is red. Suppose I now measure the second electron. You'd think I'd have a 50/50 chance of getting red or green. After all, that's what you'd expect in the normal everyday world. But you'd be wrong. In quantum entanglement, the electrons are mysteriously linked. For example, if I get red on the first, I always get red on the second. It's not a game of chance anymore. It's as if the first electron is telling the second one what to do. That's why Einstein called it spooky. The electrons seem to know that they should both have the same color no matter how far apart they are. The really important part is that the two electrons needn't be the same color. they can be entangled in a different way. So that if the first electron is red, the second one is always green. It seems that this mysterious

Connection is the ultimate secret of the quantum Robin's compass because the direction of the Earth's magnetic field can influence the outcome. Near the equator, they may be more likely to be red, but near the pole, they may be more likely to be red green. And that's the vital factor that finally tips the balance of the robin's chemical compass.

Tiny variations in the Earth's magnetic field change the way electrons in the Robin's eye are entangled, and that's just enough to trigger her compass. Now, finally, we can see how something as weak as the Earth's magnetic field can tip that balance one way or the [Music] other. If the message changes, the chemical reaction tips a different way, changing the Robin's compass reading. Suddenly, it looks like it's a fundamentally quantum mechanical phenomenon in birds. It would be one of the first, if not the first, in biology. Biologists better get used to the weirdness of physics. The Robin is navigating by spooky quantum entanglement. To see subtle quantum effects even in a controlled austere environment of a physics lab is really difficult. And yet here's the robin doing it with ease. These experiments are real and verifiable. And yet even though I'm seeing them with my own eyes, I still find it hard to believe. [Music] Bird navigation has brought physics and nature together as the science of quantum [Music] biology. There's a whole new world to explore. But its pioneers have found that it doesn't just affect birds. It affects every single one of us. Because the latest experiments say you're doing quantum physics right now. And believe it or not, you're doing it with your [Music] nose.

Hello, Jim. Hello. Hello, little girl. Our sense of smell is remarkable and quite different from our other senses of sight and hearing. Among the thousands of scents that we can recognize, many of them may well trigger very powerful memories and emotions. It's as though our sense of smell is wired directly to our inner consciousness. It's also different in another way. The other senses of sight and hearing rely on us detecting waves, light, and sound. But our sense of smell involves detecting particles, chemical molecules. Recently, scientists have begun to realize that when it comes to our sense of smell, something very mysterious is going on. For decades, biologists thought they knew exactly how our noses sniffed out different chemicals. But physicists like Jenny Brooks think there could be a new ingredient in the mix. And it smells like quantum mechanics. A lot of people speak of the sense of smell and of faction and the science of affection as being a problem that's been solved and we know all about it. And we do know a lot about it. We know about the ingredients. We know about the equipment that we use to smell. But I would argue that there's um a little bit more to understand. To understand more, I need someone to help me with a smell test. And Jem is going to sniff him [Music] out. Every human being gives off a cocktail of chemicals. Jem's nose could detect a single gram of it dissolved over an entire [Music] city. So, she has no trouble finding the man I'm looking for. Meet Colin the gardener. A man who's used to smelling the flowers. Right then, Colin, I'm going to put your sniffing skills to the test. Cool. I've got a selection of chemicals here, and I want you to tell me what what they remind you of. Okay. I'll start you off easily. Oh, that's like a minty minty vapor rub. It is. Yeah. Sort of thing. Yeah. What you menthol? Yep. But it's that that essence. Right. Here's the next one. Uh you should be able to recognize this one. That's baking with my daughter. Mhm. Um icing sugar sort of thing. Vanilla. Vanilla. Yeah.

When our noses detect a chemical, they fire a nerve signal to our [Music] brains. But different chemicals create different sensations. The standard explanation for this is to do with the shape of the molecules. The conventional theory that goes back to the 1950s says that the scent molecule has a particular shape that allows it to fit in to the receptor molecules in our [Music] nose. If it has the right shape, it's like a hand in a glove or a key in a lock. In fact, it's called the lock and key mechanism. With the wrong shape, it won't fit into the receptor. But with the right shape, it fits into the receptor, triggering that unique smell sensation. Different receptors are wired to different parts of our brains. So when a menthol molecule locks into its specific receptor, it triggers that minty fresh sensation. [Music]

But the lock and key theory has always had a problem. And Colin's next test will show you why. Okay. How about this one? Quite strong smell. Oh, that's Yeah. What does it What does it remind you of? What does it conjure up? What memories? I think Christmas cake. Yeah. Masipan. M. Yeah, that's it. Y very Yeah. Colin identified the smell of marzipan or almonds. In fact, it's due to a scent molecule called benzaldihide. What I didn't give him to smell was this other chemical, cyanide. Both benzaldihide and cyanide have the same smell. They both smell of almonds. But these molecules are both very different shapes. So the lock and key mechanism as an explanation for how we smell can't be the whole story. So, why would two molecules with different shapes smell the same? Quantum biology has a head spinning explanation. It says our noses aren't smelling chemical [Music] molecules, they're listening to them. It's not just the shape of a scent molecule that matters. Let's take a closer look at this model of a cyanide molecule. The white ball here is a hydrogen atom, and the gray sticks are the bonds that hold it together with the carbon and nitrogen. But the reality isn't as simple as that. I can give you a better sense of what's going on if we look at this larger white ball. You see, atoms don't just sit still. The bonds that hold them together are like vibrating strings. And that gives us a whole new way of thinking about smell. The bizarre new quantum theory of smell is all about vibrating [Music] bonds. Chemical molecules are playing music for our noses. Imagine a receptor molecule in my nose is like my guitar. Before it can make a sound, a scent molecule has to enter my nose. And when that scent molecule is in place, its chemical bonds provide the strings, and it's ready to be played. The receptor molecules contain quantum particles, electrons. As they leap from one atom to another, they vibrate the bonds of the scent molecule, like my fingers plucking a guitar [Music] string. What's remarkable about this theory is that it tells us our sense of smell is about the vibrations of molecules or wavelike behavior and not so much about the shape of a particular scent molecule. Our sense of smell may be much more like our sense of hearing. [Music] A particular molecule, say that of grass, will vibrate at a particular [Music] frequency. But a different molecule, say that of mint, will vibrate at a different frequency. [Music] [Music] This would explain why cyanide smells like almonds. The two molecules have different shapes, but their chemical bonds just happen to vibrate at the same frequency. The quantum vibration in the odorant is almost literally like a particle of sound. So yeah, with saying that the process of smell could be exactly like an acoustic resonance event. It could be very analogous to um hearing and seeing actually. But can we really be listening with our noses? A bizarre theory needs a bizarre experiment to test it. Here's how it works. Scientists used a molecule that smells fruity like orange blossom. But if the theory is right, then I should be able to change its smell by changing its vibrations. The molecule contains lots of hydrogen atoms like this bonded to carbon atoms. But what if I were to replace all these atoms with a different form of hydrogen called dutyium? Now, it won't change the shape of the molecule, but it will change the way it vibrates. And here's why. Dutyium is twice as heavy as normal hydrogen, and so it vibrates more slowly. Now, different vibrations mean different smells. So, if I were to make a new form of this chemical, all packed with dutyium atoms instead of normal hydrogen, it should smell different. Quantum biologists found a unique way to carry out this experiment. A smell comparison using the real experts in fruity aromomas. Fruit flies. First, the flies were trained to avoid the modified version of the fruity molecule. To be honest, I haven't got a clue how you go about training a fruitly, but apparently you can. In the laboratory, the flies had to pass through a kind of maze. They were then given a choice. Go right for the nice fruity smell or left for the nasty modified [Music] version. They could definitely smell the difference. They always preferred the original and turned right. [Music] The fruitfly experiment gives hard evidence that quantum smell theory really [Music] works. But ultimately it works in harmony with the lock and key theory. First the scent molecule fits into the receptor. Then those molecular vibrations take over. Incredible as it seems, flies, humans, and dogs may be smelling the sound of quantum biology. Our sense of smell is fascinating and mysterious as it is. But to think that when I encounter a particular scent and that sets off a whole wave of memories and emotions in my mind that it's underpinned that it's triggered by quantum mechanics, I think makes it even more remarkable. The mysterious influence of quantum physics reaches into every corner of the natural [Music] world. In fact, it inhabits the walls of every living cell on Earth. Because the latest experiments suggest a magical solution to one of the greatest mysteries of nature, the miracle of [Music] metamorphosis. The transformation of a tadpole into a frog has never been fully explained. In little more than 6 weeks, the tadpole breaks down, then reassembles in its adult form. But the big mystery is how it happens so fast. When you think about it, there's nothing more extraordinary than a tadpole turning into a frog. Take its tail for example. Over a period of several weeks, it gets reabsorbed into the body, and the proteins and fibers that make up the flesh get recycled to form the frog's new limbs. But for this to happen, trillions and trillions of chemical reactions work together, breaking molecules, forming new ones in a carefully orchestrated dance. But the fibers that hold flesh together are very, very strong. They're a bit like these ropes holding my raft together. In order to dismantle the raft, I'd have to undo these very tight [Music] knots. You can think of it like this. A tadpole is held together by long ropes of proteins knotted together by chemical bonds. The bonds are so strong that they should last for years, much longer than the tadpole's entire lifespan. So, how can it turn into a frog in just a few weeks? The explanation involves one of the most important molecules of life. Tiny widgets in all our cells called enzymes. The enzymes are the actual machinery of the cell. They're actually the the little machines inside cells that do the chemical transformations that are involved in everyday life. They're absolutely crucial. And the reason they're so crucial is because what they're able to do is to accelerate chemical reactions by enormous amounts. Let me show you just how quickly enzymes get to work. Inside this bottle is a substance called hydrogen peroxide. You're probably most familiar with it as the chemical used to bleach hair. In fact, I obtained this sample from my local hairdressers. Hydrogen peroxide is also produced in the body and it's the job of the liver to get rid of it. Now the way it does that is using an enzyme which breaks down hydrogen peroxide into water and oxygen. Now to show you just how quickly this enzyme works, I'm going to do a quick demonstration. I've got some liver here which I've chopped up in order to release the enzyme. Now watch what happens when I add this liver mixture containing the enzyme to the hydrogen peroxide. Watch how quickly the oxygen is released. Just 100 g of liver fired my rocket nearly 20 ft. Liver enzymes make the breakdown of hydrogen peroxide incredibly efficient. It happens a trillion times faster. That's a million million times faster than it would otherwise. In metamorphosis, it's enzymes that dismantle the tadpole's tail. And that means breaking down an incredibly tough protein called collagen. Collagen is one of the most important proteins for the biological world. It's the protein which actually gives that resilience, that elasticity to tendons, to cartilage, and and of course to to our skin as well. And in the tail of the tadpole, it provides the kind of scaffold, right? That's that supports that structure. Now, when the tadpole is transformed into the frog, what you need to do is to essentially have an enzyme collagenise, which will literally snip the collagen down into small pieces and thereby take that scaffold apart. But how do enzymes break chemical bonds apart so incredibly fast? Let me show you why it's a problem only quantum biology can solve. Think of it this way. All these different parts of the knot are like subatomic particles, electrons, protons that hold the different parts of the molecule together. Now to untie the knot, enzymes have to move protons about. But as you can see, this takes quite a bit of effort and a lot of time. If there are many, many knots to unpick. Physicists have a fancy way of saying put in effort to get something done. They say you have to overcome an energy barrier. [Music] Okay, here's my energy barrier. And here's my proton. To break a bond apart, it needs enough energy to get over the barrier. The trouble is, when we work out how long this would take, it's much too slow to break down a tadpole's tail. But this is where protons turn into ghosts. I wouldn't blame you for thinking that this is an idea that a clever theoretician has come up with that it's just mere speculation, something that we have no proof of, but we do. It takes place all the time. In the quantum world, protons don't have to go over barriers. They can tunnel straight through. Tunneling strikes at the very heart of what is most strange about quantum mechanics. It's like nothing we see in our everyday world. A quantum particle can tunnel from one place to another even if it has to pass through an impenetrable barrier. They're not solid objects like balls in our everyday world. They have spread out fuzzy wavelike behavior that allows them to leak through an energy barrier. A particle can disappear on one side of the barrier and instantaneously reappear on the other. In nuclear physics, this effect is a proven fact. Without quantum tunneling, the sun simply wouldn't shine. But I never thought I'd see it in a tadpole. It's hard to stress just how weird this process is. It's as though I would approach a solid brick wall and like a phantom disappear from one side and reappear on the other. The most important advantage of tunneling is its speed. It happens incredibly quickly, much faster than if protons go over the barrier. As a nuclear physicist, quantum tunneling is my bread and butter. Subatomic particles like protons do it all the time. But what has this got to do with biology? The answer is that without quantum ghosts, the metamorphosis of a tadpole would be impossible. Remember, chemical bonds are basically knots. Tunneling unties them fast. Have a look at these two knots. Now, on the face of it, they look identical, but there's a subtle difference. This knot has the two short ends of the rope on the same side, whereas this one has the two short ends on opposite sides. Now, you think that wouldn't make a difference, but it does. You see, this knot is very hard to break, whereas this one is easy. Quantum tunneling turns strong knots into weak ones. So in a tadpole, the entire collagen scaffold breaks apart easily. And finally, other enzymes rebuild it in the shape of a frog. The quantum tunneling of particles is one of those weird features of the subatomic world that a physicist like me is very familiar with. After all, it's responsible for radioactive decay. And it goes on inside the sun. It's the reason why the sun and all stars shine. But to discover that it's going on inside every cell of every living organism on the planet, because every cell contains enzymes, now that I find truly amazing. [Music]

Quantum biology casts its spell over every living creature. We've seen that birds, mammals, insects, and amphibians are governed by the strangest laws in science. But the most dramatic recent breakthrough concerns the single vital process on which all these forms of life depend. the conversion of air and sunlight into plants. This fine specimen is a lar decidua or European larch. It's about 100 ft high. And right at this moment, passing just this side of the planet Venus, is a bullet with this tree's name on it. The bullet is a photon nearing the end of its long journey from the sun. Its ultimate destiny is to kickstart a series of chemical reactions that underpins all life on Earth. Photosynthesis. Every second of every day, 16,000 tons of new plant life are created on Earth. And for me, it's incredible to think that our existence on this planet depends on what happens in the next trillionth of a second. The crucial first stage of photosynthesis is the capture of energy from the sun. It's nearly 100% efficient, vastly superior to any human [Music] technology. But the way that every plant on Earth achieves this is one of the great puzzles in biology. When it turned out that quantum weirdness might hold the answer, physicists could hardly believe it. It was like a revelation. It was very exciting. uh because I was used to work on problems that were quite abstract experiments. It was I'm a theoretician but I always related my theory to experiments that were very clean in the lab things that you can control. But now finding out that the things that I knew can help me to understand better how nature works really I don't know scientifically it was like a a new inspiration to my life as a scientist. So I I really I would say I fell in love with this field. Textbook biology says the color of green plants comes from chlorophyll molecules. Inside the living cells, they absorb light from the sun. This energy is then transferred incredibly quickly to the food making factory at the heart of the cell. The entire event takes just a millionth of a millionth of a second. When the photon hits the cell, it knocks an electron out of the middle of a chlorophyll molecule. This creates a tiny packet of energy called an exiton. The exon then bounces its way through a forest of chlorophyll molecules until it reaches what's called the reaction center. Now that's where its energy is used to drive chemical processes that create the allimportant biomolelecules of life. The problem is the exiton needs to find its way to the reaction center in the first place. Textbook biology can't explain how the exaton does this because of course it doesn't know where it's going. [Music] It just bounces around like a pinball in a process called a random walk. Sooner or later, it'll pass through every single part of the cell. [Music] But this isn't the most efficient way to get [Music] around because when the exiton eventually does reach the reaction center, it's by pure chance. If the exiton just blindly and randomly hops between the chlorophyll molecules, it would take too long to reach the reaction center and would have lost its energy as waste heat. But it doesn't. Something very different must be going on. The vital clue comes from recent experiments that stunned the world of science. Chemists fired lasers at plant cells to simulate the capture of light from the sun. They confirmed the exatom wasn't bouncing along a haphazard route through the cell. This original understanding didn't explain what we were observing in the lab. So the mystery realizing okay so then what is the explanation for what we are observing in the lab? The solution is that plants obey the most famous law in all of quantum mechanics. The uncertainty principle. It says you can never be certain that the exaton is in one specific place. Instead, it behaves like a quantum wave smearing itself out across the cell. The exaton doesn't simply move from A to [Music] B. In a bizarre but very real sense, it's heading in every direction at the same time. It's spreading itself out as a wave so that it can explore all possible routes simultaneously. This strikes at the very heart of what's so strange about quantum mechanics. The exiton wave isn't just going this way or that way. It's following all paths at the same time. That's what gives it such incredible efficiency. [Music] The beauty of it is if the exaton is trying every route to the reaction center at once, it's bound to find the fastest possible way to deliver its energy. It's hard to express how incredible this discovery seems to physicists like me. Biological cells are full of the random jiggling of billions of atoms and molecules. But somehow exatons maintain their form as beautiful, perfect quantum waves, transporting the energy that guarantees life on Earth. It opened a whole new scientific path for me and I I really enjoy the fact that to be able to understand fully what is happening there or in the plants you have to interact with scientists that have completely different approaches like biologist and chemist. Uh but we we all have to come together to actually understand what is the relevant of this the relevance of this. So for me this is one of the most exciting parts of this field. Real scientific experiments leave no [Music] doubt. The strange hand of quantum mechanics has shaped the entire living world. It's not a surprise that you should find quantum tricks being used in biological systems. The reason is because they're better. Quantum entanglement is normally seen in the tightly controlled conditions of the physics lab, but now we know that robins use it to navigate with extraordinary [Music] precision. Quantum vibrations mean our noses listen to chemicals, enhancing our perception of the world around us. [Music] The living cells of all animals depend on protons that vanish and reappear like ghosts, speeding up the vital processes of [Music] life. And photosynthesis reveals the big picture. a shimmering world where quantum waves capture the sun's energy in an instant. Sometimes people say, "Ah, but physicists have been looking for this for decades." Well, biology has had millions of years. The ultramodern science of quantum mechanics is an ancient fact of life. For the end of my journey, I want to take these ideas to their logical conclusion. Of course, as a scientist, any speculations I have have to be backed up by careful experiments. So, I want to concoct a thought experiment that helps me to answer the biggest biological question I can think of. Does quantum physics play any role in the mechanism of evolution itself? In 1859, Charles Darwin stunned the world with his theory of evolution by natural selection. He went on to explain the differences between humans and other [Music] apes. 150 years later, there's no doubt that Darwin's theory accounts for every living organism on land and sea. But I'd like to explore the latest extraordinary interpretation of his ideas. Could there be a quantum theory of evolution? [Music] [Applause]

Can quantum evolution explain how the snail eat its shell? The snails I'm used to seeing in my back garden tend to have rather bland, boring shells. So have a look at this beauty. The patterns on its shell very perfectly match the lines on the stem. It's called a banded snail. Sepia nemerales. And the pattern isn't there by accident. Come and have a look at this. Less well adapted snails are more likely to be found here. This stone is called a thrush's anvil. The song thrush is the snail's main predator. It catches a snail and smashes its shell against the stone to get to the snail. Now, what I can see here is that there aren't many banded snail shells, suggesting that its colors camouflage it very well, hiding it from the bird. Darwin's theory says that evolution depends on variation within a [Music] species. Snails with camouflage are more likely to survive and reproduce, passing on their shells to the next generation so that the species as a whole becomes better adapted. So variation, the random differences between snails is the driving force behind their evolution. Now all species evolve and adapt to their environment. But the question I'd like to explore is whether quantum mechanics plays a role in this. The only way to find out is by scientific experiments. So, my adventures in quantum biology finally bring me [Music] home to the University of Surrey. Here in the laboratories, I'm planning a new analysis of the most celebrated molecule in science, deoxy ribboucleic acid or DNA. Its double helix holds the genetic code for every living organism. It's a remarkable fact that Darwin himself had no idea what created variation in the species. The structure of DNA wasn't discovered until 1953 by Francis Crick and James Watson. The most famous feature of DNA is of course its beautiful double helix structure. But that's just scaffolding. The real genetic secret lies in between. The four different colored molecules are called bases. The color code on one side, say blue, red, blue, forms a gene that parents pass on to their offspring. A gene is a bit like a jigsaw puzzle. It fits together like this. A full strand of the double helix forms a colored pattern, but the other strand always pairs up the same way. A blue base always goes with yellow and green always goes with red because only those colors have the right shape to fit together. What Crick and Watson realized was that this provides a mechanism for passing on the genetic code. When cells reproduce, the two strands of DNA separate, ready to be copied, but red still goes with green and yellow still goes with blue. So bit by bit, the cell creates two new strands. two perfect copies of the entire genetic code. So far, there's no genetic variation. This new copy is identical to the original. But here's the interesting bit. During the copying process, something very important can happen. Sometimes mistakes creep in. They're called mutations. Let's have a look at these two bases here. The two prongs that hold them together are subatomic particles. They're protons. They're basically the bonds between the strands of DNA. These protons can jump across to the other side. If the strands split, when the protons have jumped across, they find themselves in the wrong position. Now, this red base will no longer bind to a green base. Instead, it has to bond to a yellow base. Slotting this back in, we see that now this copy is no longer identical to the original because I have a yellow base here instead of a green one. We've brought in a genetic mutation. Jumping protons would change the snail's DNA. It could make a new gene for camouflage shells. The question is, how do protons jump? It's my belief that quantum spookiness can take over. Now, for these mutations to take place, the protons have to overcome an energy barrier. And if you remember what happened with enzymes, well, you can probably guess what's coming next. Protons can behave as if barriers don't exist. They tunnel straight through. But does this ghostly effect really happen? My colleagues in biology are already looking for the very first evidence of quantum mutations. Biologists didn't really even know about quantum mechanics. So when you tell them that, you know, particles can be in two places at once, they kind of say, well, not in my cells, they can't. Our experiment involves samples of bacteria. The first sample is prepared in normal water containing hydrogen nuclei or protons. When the bacteria reproduce, we simply count the mutations. But if our theory is correct, then we should be able to change the rate at which mutations occur. Remember how we tested the quantum theory of smell? What if I replace the proton with its big brother, the duteron? This is the nucleus of an atom of dutyium. Now, crucially, a duterron is twice as heavy as a proton. And this should influence how easy it is for the duterron to quantum tunnel. Quantum mechanics is full of surprises. Protons tunnel easily. Deuterons don't. [Music] These heavier particles are much more likely to bounce straight back. So the second sample of bacteria is prepared in heavy water which is full of duterrons. Our theory says you should get far fewer mutations and so far the results are extremely encouraging. the preliminary experiments that we've done um gives us uh a hint that the mutation rate is indeed depressed in duterrated water. We find that it is lowered. So, uh my hunch is that we're right. Uh but um we'll have to wait a little while before we're sure. Final proof lies in the future, even if we're right. Quantum tunneling is a rare form of mutation. But our results promise hard evidence for a new explanation of one of the most fundamental processes of life. Even the meest possibility of a new quantum mechanism for evolution itself is tremendously exciting. In fact, the story of quantum biology is only just beginning. What the frog, the robin, the fruitfly, and the tree have shown us is that real quantum effects are going on in nature all the time. And if there's anything we've learned from the history of quantum mechanics, it's this. We can never be certain where new discoveries will take us [Music] next. Quantum biology is a revolution in science. But it's time I got back to the physics department. [Music]

Would you like to lose a little bit of weight without doing any exercise or dieting? Would you like to age just a bit more slowly than your friends? Well, you might be surprised to hear the laws of physics can help. The key to unlocking these everyday questions is gravity. It sculpts the universe. It warps space and time. It's a fundamental force of nature. But gravity's strange powers discovered by Albert Einstein also affect our daily lives in the most unexpected [Music] ways. In this film, we'll be using cuttingedge scientific techniques to investigate how gravity changes your weight. It's gone up. your height. I really have shrunk and even your posture. And with the help of thousands of volunteers, I'll show you how gravity makes us all age at different rates. Throughout the day, I've just been logging on to the phone, logging on to the app. As a physicist, gravity is central to my work. Oh, wow. And in exploring it, I'll be challenged on how I understand this most mysterious force. Wow. Okay, I need to go and write this one down. And I'll have to tackle the very nature of reality itself. gravity. It binds together all the matter in the universe and it makes our existence here possible. But in the end, it all boils down to one simple question. What happens if I drop an object? Gravity's many mysteries are all contained in this single action. How an object falls. Here's the first puzzle. Why does a hammer fall faster than a feather? You might think it's because the hammer is heavier, but that's not the real reason. The answer is air resistance. It's not the weight of the objects that matters. is their shape. And I can demonstrate this very easily with these two umbrellas. They both have exactly the same weight. But if I open one of them, you can be pretty sure it'll drop more slowly than the other [Music] one. In fact, all objects would fall at the same rate if you could only remove the air. The first person to realize this was the 16th century mathematician Galileo Galile. Famously, it said he worked it out by dropping objects off the leaning tower of Pisa. And he was spectacularly proven right in an experiment carried out on the moon in 1971. Well, in my left hand, I have a a feather. in my right hand a hammer and I'll drop the two of them here and hopefully they'll hit the ground at the same time. It worked perfectly. How about that? Proves that Mr. Galileo was correct in his findings. Now, Galileo was obsessed with a second question, too. When you drop an object, it's actually quite hard to tell if it falls at a constant speed or picks up speed as it [Music] drops. Even in slow motion, it's pretty hard to [Music] tell. But Galileo realized this. First, drop an object a very short distance. It lands with very little impact. But of course, drop it from higher up. This time, the ball easily breaks the tile, which means it must have accelerated, gaining in speed and momentum as it dropped. Galileo had identified something fundamental to all falling objects. They accelerate. He realized there might be a way to measure how much falling objects gain in speed. What he devised was the first ever attempt to measure gravity itself. He built a long wooden ramp rather like this that he had sloping at a shallow angle. The idea was to roll balls down the ramp and measure their acceleration. The crucial thing is that the ramp had to be at this shallow angle to reduce the effects of wind resistance. It also meant that the balls would roll down slowly enough to give him time to measure their speed. But the big problem was this. How do you measure time accurately in an age when there were no accurate time pieces, let alone stopwatches? Well, Galileo came up with an ingenious idea involving the flow of water. Essentially measuring time from the amount of water collected in a cup. So, we're going to try and repeat Galileo's experiment. I say we because I have a couple of willing volunteers, Gavin and Yanna. Three, two, one, go. And stop. Okay, there's one. Now, if you come down a quarter of the way down the ramp, go. Stop. Okay, so now half of the way down. Go. Stop. Just in time. Okay. And then 3/4 of the way down. Go and stop. Right. Turn the tap off. Okay. So, we have our four measurements and I can see a progression from fuller to emptier. But what we need to do now is find the mathematical pattern by weighing carefully the water in each glass. Weighing the water should give us an idea of how long each roll took. And in our experiment, these were the results. Now, there's one immediate thing you can tell. The ball really sped up the longer it rolled. In fact, our results seem to show that the time it took to cover the first quarter of the ramp was about the same time it took to cover the next three quarters. Right? So, we have a strong hint of a mathematical pattern. Now, we'll see if we're right by placing bells along the ramp at intervals which are based on the results. Yeah, this. Okay, this arrangement looks a bit strange because the gap between the first two bells is much shorter than the gap between the third and fourth bells. But that's okay because if we've got our calculations right, the ball starts off slowly, so it covers a shorter distance, and as it picks up pace, it'll cover longer and longer distances. So, we should hear the bells ringing at equal intervals in time. Go. [Music] Beautiful. So, what does this all mean? What's the mathematical formula? Well, this is something that Galileo worked out. Let's say from the start, the ball covers a distance of 1 meter in the first second. After 2 seconds, it'll have covered 4 m. After 3 seconds, 9 m. After 4 seconds, 16 m. and so on. If you recognize this progression, you'll see that distance goes like the square of time. Galileo had found the rate at which gravity speeds up objects. And he'd found another fundamental principle. You can measure the strength of gravity by how much it causes falling objects to accelerate. Detecting gravity has become exceptionally sophisticated these days, but still uses exactly the same principle. This is Hurst Monso Castle in Sussex, and in its grounds lies the Space Godysy facility. Here, Vicki uses an astonishingly sensitive instrument to detect the exact strength of gravity on this one spot. Okay. So, Vicki, tell me about this incredible gravity meter that you work with. Okay. So, this is the dropping chamber and a stripped down version. So, essentially what happens is you've got a cart that gets raised to the top and then the cart accelerates away from a mass in the middle and so this section here lifts off and as it drops it drops under free fall. So, this component in the middle as it drops is basically just Newton's apple falling to the ground. Yes. So, this is a stripped down version but but that's the real thing. This is the real thing. How does it actually work in here? It's a vacuum. So, there's no wind resistance. So, there's no wind resistance. Inside, a laser is used to measure exactly how fast the mass is accelerating. This is the 21st century version of Galileo's ramp and the balls rolling down. So, can we get it going? Of course. If you just like to press the button on the laptop. This one? Yep. Okay. So, it's now communicating with it. Oh, here we go. There we go. So, it waits 5 seconds, then takes a measurement of gravity and Oh, and and and I've you can see the uh all the the results appearing now. Yep. Each of those green dots is a measurement of gravity with the actual number that it's getting for each one. The unit Vicky uses has a familiar ring. I see that the the number up at the top here. So, you've got this unit micro gal. Yes. A gal is essentially 1 cm/s squared. The gal was named after Galileo. So we've just taken the measurement of gravity here today and it's this highly accurate number 9811247 micro gals. The reading means that the Earth's gravity speeds up a falling object by around 9.81 81 m/ second for every second it drops. Vicki tells me something intriguing. She takes a reading here every week and she's found that the strength of gravity changes by tiny amounts over time. Heavy rainfall, for example, can cause gravity to increase slightly. Presumably, if gravity is changing here in one spot, it it'll have different values all around the world and so you can have a gravity map of the entire planet. That's right. Yeah. So, what's the reason for these strange fluctuations? That's what I want to investigate next. So, gravity changes as we move across the surface of the Earth. Well, this lies at the heart of a challenge that I've set two young volunteers. I've given them a task to try and find the place in Britain where gravity is at its weakest. So, where objects would weigh the least and I've given them just 3 days to try and find it. The volunteers are Australa Sendra, a PhD student. I've been living in London for five, six years and I'm originally from Seville in Spain. I'm very interested in taking part in this project because I would really like to know more about how this world works. And Poppy Beum, a journalist who lives in London. I did my degree in biomedical science. Um, and I did biology and chemistry for my A levels, but I haven't done any physics since I left school. I'm fascinated to find out more about gravity, and I actually enjoy a puzzle. I like a challenge. Now the team can't just weigh themselves to see changes in gravity. Body weight fluctuates naturally by a couple of kilos over the course of a day. Whereas changes due to gravity as they travel around the country are going to be tiny in comparison, a matter of a few grams. So they're going to have to use sophisticated scientific methods if they want to measure gravity accurately. And that's why the volunteers will be joined by three specialists in gravity science. PhD student Sonac Bose. He'll be in charge of some very sensitive measuring apparatus from the National Physical Laboratory. Sha Hughes, a geologist who'll be using a portable gravity meter, and Andrew Ponson, a cosmologist at University College London, who will help interpret the results. We've taken a collective weight for the team before they set off. It's 380 kg. So, can they find the place in Britain where that'll decrease? They're setting out in Snowonia National Park in North Wales. The railway climbs from here to the thousand m summit of Snowden. Sha takes his first gravity reading. The inside is a mass on a beam and you turn this counter this dial until you get the beam central. By counting the number of turns of the dial, Sha can calculate the downward pull of gravity acting on the mass inside the machine. Sonac has a simpler method. So, inside the box is a 2 kg mass. And it's supposed to be sort of as perfectly 2 kg as it's possible to get. All right. And place it here. Ooh, it's just coming under, isn't it? 1998. 2 g. So, it was 2 kilos in the laboratory, but now here it's a bit less. It's the first puzzle. Why does a 2 kilo mass tip the scales at just under 2 kilos? And it's one which gets straight to the heart of what the challenge is really about. Mass is often confused with a related quantity, weight. The mass of these dumbbells is fixed. It doesn't change. It's a measure of how much stuff they contain. Weight is different. It's a measure of the effect of gravity on these dumbbells. The downward force pulling them to the ground in the same way that it's keeping my feet firmly stuck to the ground. The crucial difference is this. If I was holding these dumbbells on the moon, they'd still have exactly the same mass, but they would weigh six times less because the moon's gravity is so much weaker than the Earth's. So that's why Sonax bringing along the 2 kilo mass. If it changes weight, then this should mean that gravity itself has changed. Ahead of them is the summit of the highest mountain in England and Wales, famed for its stunning scenery. Or it would be stunning if you could see it. And uh this is what we came all the way up here for this amazing view at the top of Snowden. You wouldn't know it, but honestly, we are here. So, we're now near the summit of Snowden, and I've set up the gravimeter again, and we're going to see what the difference in the reading is. Um, he has to turn the dial again and again to try and get a reading. It's clear gravity has changed, but which way has it got stronger or weaker? The team leaves Sha to work out his results and tries to position the scales as close as possible to the summit. But the reading is all over the place. Oh, it's gone up. It's fluctuating quite a lot due to the wind. Oh, wow. I have to say, this is what science is always like, isn't it? It's never quite what you want it to be. So they head inside to the cafe next to the summit. The wind was being a bit naughty, but hopefully in 0, so it should be all right. 1998.2 down there. 1 1997.8. There you go. We've got it. That's 4 of a gram off. The mass weighs a tiny bit less. It's lost about 5,000th of its weight. And Sean's found that gravity itself has reduced. At the top of the meas the mountain, we took the measurement and we discovered that the grav pull of gravity had gone down. Uh it had gone down equivalent of 206 turns of the dial. And we worked out that that's equivalent to 219 mg. So, it's clear from the team's measurements, gravity weakens as you go higher and you get a bit lighter. It's just an excuse to say where are we like the lightest? Who cares? But in the sense that it's actually really interesting. It's like an illustrative example of of seeing how this is actually fluctuating depending on different factors. Yeah, absolutely. And that we could measure it and we could see it with our own eyes. It actually makes you think about gravity in a very active way. It's such a fundamental force phenomenon in nature, but we don't know much about it. But why does gravity change with altitude? To understand that question, you have to get to grips with the extraordinary discoveries of the next scientific giant in our story, Isaac Newton. Born in England in the middle of the 17th century, he spent his life wrestling with so many apparently separate questions from why things fall to the ground to why planets orbit the sun. It took the genius of Newton to realize that there was one single equation that could answer all these questions. And here it is, his famous law of gravity. It might look complicated, but this is one of the most important equations in the whole of science. F here is the force. Now, Newton said there's an attractive force between any two objects in the universe. On this side of the equation, G we call the gravitational constant. Now, Newton knew it had to be there, but he didn't know what its value was. M1 and M2 represent the two objects, and R is the distance between them. Now the equation tells us that the more massive the objects are, the bigger m1 and m2, the greater the attractive force. But the further apart they are, the bigger the value of r here, the weaker the gravitational force. With Newton, what was once mysterious now became clear. Newton's equation describes why an object falls to the ground, including his famous apple. But its true genius is that it applies to any object anywhere in the universe. So it's a very simple and elegant way of describing some of the seemingly most complicated phenomena in the cosmos. His law of gravitation can still be used today to explain how orbits work to predict when a comet will return to describe why galaxies spin or to slingshot spacecraft around planets. Newton tells us to look for the underlying simplicity in natural phenomena. For instance, how the moon orbits the Earth. If I let go of this apple, it'll

fall straight down because of the pull of Earth's gravity. But if I throw it to begin with, it travels in a horizontal direction. That's a direction of travel, but Earth's gravity is still pulling it downwards. So, it ends up following a curved path.

Now, if I throw it harder, it'll travel further before it hits the ground. And in principle, if I could throw it hard enough, I could put it into orbit. And that's exactly what's happening with the moon in orbit around the Earth. It's a combination of wanting to travel in a straight line, but also being pulled down by the Earth's gravity. So, it ends up constantly falling around the Earth and constantly missing.

Newton's famous equation also explains the strange effects which the road trip team has discovered that objects get lighter as you gain in altitude. When I weigh myself, I'm represented by the first mass m1. The second mass m2 is the earth itself. And the force pulling me down, my weight depends on the distance between me and the center of the earth. And that's the secret of the road trip. If you want to find the place where you weigh the least, then you have to get as far away as you can from the Earth's [Music] core.

So, it's the afternoon of day one and the road trip team have to work out where to go next. Poppy and Australia have a good idea. Find somewhere higher than Mount Snowden. from the measurements that we that you guys did at Mount Snowden. Altitude clearly plays an important part in gravity. So with that in mind, we've got to go to the highest point in the UK, which is Ben Nevice. Okay. But there's just one thing that we haven't shown you so far. We actually brought along an extra experiment. So can we please show you this first before you make the final decision? Yes. Uh Sonac actually has the other part of this experiment. Uh we always carry around some power tools as physicists always do. So let's start off nice and gentle. Okay. And then try and pick up some pace. Proven.

The point is that when something is spinning, it kind of gets flung outwards. Um and you can actually use that to make a nice flat piece of pizza. Um but this also applies to the earth. The Earth isn't perfectly round. It's what's known as an oblate spheroid. It bulges at the equator where the spin is greatest. We kind of got two competing effects now. Um we're trying to get away from the the center, the actual core of the Earth, the point at the very center of this ball. But now we can do it in two ways. We can either kind of go up something tall or we can just go down towards the equator. This is what we find when we're doing gravity surveys is that as you move south, there tends to be an effect from latitude, which is often usually larger than the effect from altitude. So, the closer to the equator you go, the further you get from the Earth's core and the lighter you get.

So, guys, the sun's setting just behind me here. Mhm. This is north. Mhm. From the conversations we've just had, it sounds like we've got to go that way down south. Is that right? Yep. Yep. Okay, let's go. Let's go.

The team is starting to uncover the reasons why gravity changes as you cross the surface of the Earth. Our planet is defined and shaped by the complicated forces which act upon it. and detecting tiny fluctuations in its gravity field can give us important clues. It can help us understand how our world is changing.

The space godsy facility at Hmon Monso is one small part in an enormous global network which uses satellites to detect the tiniest of changes in the Earth's gravity field. Tell me what exactly your job is here. What we're doing with this telescope is measuring very accurately the distances of satellites uh from here. So we're using very short laser pulses which we direct towards the satellite. On the satellite there are reflectoring cubes which return some of that light to us. And we measure how long it takes the light to go to the satellite and back. And how far away is the satellite? What we're tracking now is one of the Galileo satellite which is about 20,000 km. 20,000 km away. Yes. Okay. So, we've got it aimed at the Galileo satellite, and you're going to turn the laser on now. Yes. Oh, wow. And that laser beam that's being fired up towards the satellite. Yeah. The time it'll take to get there and come back again, it's a fraction of a second, isn't it? It is. It's about 150,000 of a second, 150 milliseconds. And we're sending about 1,000 of those per second.

This strange looking object is based on satellite readings. It's a highly exaggerated representation of how Earth's gravity field varies over time. Fluctuations like these can give us important insights into climate change. Ice caps melting, sea levels rising, changes in groundwater. All of these have an effect on the local strength of gravity. So something as important as climate change in order to understand it and do something about it, we need to know the distribution of the gravitational field of the earth very accurately. Absolutely. Yes. And it's a global measure that we need for the road trippers.

is the start of day two and they're heading for the south coast. They're stopping off in Herafordshire. It's a good location as it's the same altitude as the base of Snowden, but they've moved about 80 mi further south. So, if they find gravity changes here, it must be due to latitude. It's not a huge difference, but it's noticeable. Our counter reading at the bottom of the mountain was 4,840. Yeah. Our counter reading here is 4,717. All right. So, we do get to see a difference. So, we're actually at the same altitude as the base of Mount Snowden, but because we've traveled further down south overnight, gravity's less here. Yeah.

They push on and by sunset they reach Sidmouth on the south coast. Sha takes the second gravity reading of the day and Poppy improvises a map. Well, sort of a map. Can we write not to scale at the top there? So, I drew this map. Scotland's a bit squashed. Wales is quite high up and Cornwell is get the idea is there. But you get the idea. So, Sean, we we've been traveling with you. You've done quite a few gravity meter readings. Can you can you plot them on this not to scale badly drawn map, please? Sure. So, if you remember, we started off in Mount Snowden about here, and that was the zero measurement for our survey. And then we've come all the way down here to the south coast. The difference from the base of Snowden is minus 212 millig. Wow. So the difference between going measuring gravity at the base of the mountain and the top of the mountain is about the same as here at this latitude and down here at this latitude. They're quite clearly at sea level. Yet gravity here is roughly the same as it is at the top of Snowden.

But where next? We are here. If we want to find out where we are the lightest, why don't we travel all the way to the most southernly point in the UK, which is here. But altitude can also help us. So why not find a place in the country that is both low in latitude but also is high in altitude in terms of height above sea level cuz that will get us somewhere that is really far away from the core of the earth while staying within the country. [Music]

So the answer to the puzzle lies in a combination of two factors. How much further south should they go and how much [Music] higher? At the end of day two, Sha's results show that the team weighs about 80 g lighter in total than back at the base of Snowden. [Music]

The way that weight changes is just one example of Newton's famous equation in [Music] action. But Newton had left his masterpiece incomplete. He didn't know the value of G, the gravitational constant, which sets the size of the force. To harness the full power of the equation, you need to know G. And the vital clue came with an incredible experiment conducted in London at the end of the 18th century. It was an attempt to work out the mass of the earth itself and it was carried out by an eccentric, extravagantly rich aristocrat, Henry Caendish.

Cavendish was a chronically shy, deeply solitary man living in total isolation in his house in Clappam. The story goes that one day he accidentally bumped into a female servant on his staircase. He was so traumatized by this event that he had a new staircase built just for him so this horrible incident could never happen again. Caendish had inherited vast fortunes and was able to dedicate his life to devising pioneering experiments, including one particularly extraordinary piece of equipment. He set up something a bit like this. It's called a torsion balance. It involves four lead spheres, two large heavy ones which are held fixed in place and suspended by a very thin wire is a wooden rod 6 ft long with two smaller balls on either end. Now the crux of the experiment is the relationship between the large ball and the small ball. Now of course there's a gravitational pull downwards on both of the balls due to the earth's gravity. But Newton also tells us that there should be a very weak gravitational pull between the balls. And this is effectively what Cavendish was trying to measure. Any slight movement of the small ball towards the large one should cause a twist in the torsion wire. And that's what Caendish was trying to detect. Of course, this is all much easier said than done. The experiment was incredibly sensitive. The tiniest of vibrations, the slightest breeze, changes in temperature could all influence the measurements. So, Caendish had to isolate the apparatus inside a box and the box within a shed. He even realized that his mere presence next to the apparatus could influence things. So, he had to remove himself outside the shed. What he then did was sit outside the shed and through a small hole in the shed wall look through a telescope to detect the tiniest of twists in the wire. It was an incredibly difficult process, but after many months he finally felt confident enough that he had a reliable result. [Music]

Caendish found that the small balls did move a tiny 4 mm. He calculated his results by comparing the density of the balls with the density of water. In the end, the result of Caendish's experiment and subsequent calculations was that the density of the earth was about 5 1/2 times that of water. Or put another way, the mass of the earth was 5.9 trillion trillion kg. What's most remarkable is that Caendish got this number right to within an accuracy of 1%. With Caendish's astonishing result, scientists were able to work out G. Then the equation could be used to determine the mass of any celestial body in orbit around [Music] another. So astronomers were able to calculate the mass of the sun and the planets and the moon and eventually even distant galaxies.

And of course, back on Earth, we never escape gravity. Over the course of the day, it actually squeezes your spine, an effect you can see for yourself if you use a measuring rod. Okay, so it's 7:00 in the morning. I've just got up and I'm going to see how tall I am before gravity drags me down. [Music] That's 178 cm or just over [Music] 5'10. Over the course of the day, gravity compresses the fluids in your [Music] spine. Right, it is just past 11 p.m. I've been standing up for most of the day. So, let's see if gravity has had an effect on my height. That is 176 cm. So, I really have shrunk by just over half an inch over the course of today. [Music]

In the longer term, gravity can affect your posture permanently, but there are exercises you can do to counteract this effect. Part of my research has been looking at the effects of gravity on the human body. So, people might not be aware or they might not always think about the effect of gravity on our physical state, on our health, and particularly on our posture. However, because it's such a constant force, gravity has a massive impact over the course of our lifetime. As you get older, you can develop a stoop, which is damaging to your mobility. Goal here has actually got very good posture, but I'd like you to just show not so good posture. So, when um poor posture is really rounded shoulders and then loss of the uh the curve in the back as well, I can't ask you to raise up your arms when you're in that posture. So hard. No. And then just come back down. Shoulders back into and then raise your arms. You can see the the effect of posture on function. Ironically, the exercises which many gymgoers do actually make your posture worse. That's if you only exercise the frontal muscles like the chest and abdominals. So, it's recommended you exercise the back muscles just as much to straighten you out and counteract the effects of [Music] gravity.

Meanwhile, it's the end of day two for the road trip and they've reached Sidmouth on the south coast looking for the place in Britain where they'll weigh the least. They've worked out the answer lies in a combination of two factors. The right mix of going south and being higher up. And for the final leg of the journey, I'm going to meet up with them. I asked them to drive a short distance west to one of the most remote areas in mainland Britain, Dartmore National Park. It's only 40 miles from the southernmost tip of Britain. Hello. Hi, Andrew. Nice to see you. And it's very high, very hilly territory, Jim. The team got to the south coast yesterday where we to find gravity at its weakest. But we haven't quite figured out whether it's altitude or latitude. Do we go further south or do we go higher up? You're right to ask, do we go as far south as possible or as high as possible? That's why I've brought you here to Dartmore and we've charted the most important points on this map here. All right, let's have a look. So, we are here two bridges. Yeah, these four dots represent these hills up there behind us which are at about 500 m above sea level. So, that's what we want to check out. These hills are close to the south coast and they're also the highest in the whole of the south of England. So logic suggests there must be the right combination of latitude and altitude. Well, there's another reason why this makes perfect sense. One which we haven't looked at yet, and that is the effect of the underlying rocks on gravity. And I've got a map here that shows you're going to trump my map, aren't you? Here we are down here. And these blue areas are the lowest areas uh according to the density of the rocks underneath. The rocks around here are made of granite which will make gravity weaker still. So that's helping as well as the altitude and the fact that we're further south. Yep. It's also playing a part. Well, we have a plausible theory, but now we need to test it. If I'm right, then at the top, our gravity reading should be by far the lowest reading of the trip. Of course, there's another effect of gravity to deal with. Now, it's knackering when you head uphill. Okay, so I think this is pretty much the start of the hills we've located on the map. So, let's see if this is the lightest place. Sean, if you want to get the gravity meter out and we'll take another reading here. Yep.

Okay, Sean sets up his equipment one more time. What's the news? Well, at the bottom of Mount Snowden was our zero for this test. We found we lost a certain amount by going up to the top of Mount Snowden. We found we lost a certain amount coming south to the south coast. Not only have we beaten that, we've smashed it. Brilliant. We were minus 219 milligals lower at the top of Mount Snowden. Here on Dartmore, we're minus 347 millig. Brilliant. So, it is a combination of three things. We're we're far south, so it's the latitude. We're at altitude. We got high up. And we're surrounded by all this granite rock, which is low density. Anyway, I hope you all think it was worth the climb up here. Anyway, there you go. Boom. Science.

Now, we already know that the altitude of these hills takes us much further from the Earth's core than anywhere else further south in Britain. So, gravity must be weakest here. There's extra evidence, too. The British Geological Survey has compiled tens of thousands of gravity readings made in the UK and the lowest readings ever recorded were all taken around here on the high hills of Dartmore. What do we do to celebrate? We weigh ourselves, of course. BFX weight, imagine it's all them Nutella pancakes for breakfast. I need to lose weight. I can tell you that you should weigh something like 20 gram less than you did at the base of Mount Snowden. Guys, I'm guessing something like 25 to 30 g less. So, if you want to weigh as little as possible, this is the place in Britain to come. But in any case, it's such a tiny amount that it it's going to be wiped out entirely by whatever it was you had for breakfast this morning. [Music]

In episode two, I'll investigate how Albert Einstein's theory of gravity completely changed our understanding of space and time. [Music] [Music] I'll travel to Louisiana in the USA to listen to the sound of black holes [Music] colliding. Biggest source of energy in the universe. One of the biggest events you'd ever measure. And we just barely saw it. And with the help of thousands of volunteers and a smartphone app, I'll explore how the science of gravity can help you age slower. [Music] Would you like to lose a little bit of weight without doing any exercise or dieting? Would you like to age just a bit more slowly than your friends? Well, you might be surprised to hear the laws of physics can help. The key to unlocking these everyday questions is gravity. It sculpts the universe. It warps space and time. It's a fundamental force of nature. But gravity's strange powers discovered by Albert Einstein also affect our daily lives in the most unexpected ways. Gravity. What goes up must come down. All of our lives, we abide by its rules. It dominates our every action. But there's one select group of humans who know what it's like to live free of gravity. Two one zero. Liftoff. [Music]

Everybody's used to gravity. We're used to the oppression of it. Gravity is the ultimate oppressor. It grinds us under its heel 24/7 with no release until you're in space and then suddenly you're free from gravity. You are you're weightless in orbit. Canadian astronaut Chris Hadfield spent five months on board the International Space Station. You can pull your knees up to your chest and just tumble. Or if you take a a wet cloth and you get it dripping wet and everybody on Earth knows what'll happen when you ring it out. All the water will fall inevitably. If you do that in weightlessness, the water stays there and it actually because of the surface tension starts crawling up your arms. [Music] It's a little bit mesmerizing and hypnotic to be in weightlessness. If you're weightless, you don't need a bed. You don't need a mattress. You don't need a pillow. Your body is floating completely suspended like [Music] magic. Movement becomes effortless. You can push off with one finger and and fly and tumble. You don't need to hold yourself where you are with with muscle. You you can just with a delicate fingertip pressure. You can stay where you are. Okay. Separation confirmed. Timer is on. Backing away at a rate of just a little over 1/10enth of a meter per second. Re-entering gravity is a punishing experience. To come back to Earth is violent. It can be five times the force of gravity or eight times the force of gravity crushing you down into the floor of the ship for quite a long time. Then of course you hit the ground and tumble and roll to a stop and and now you're the victim of your past. You're the the victim of your decision-making. Lying there trying to shake your head and get used to being in gravity again. I remarked at the time that I had forgotten that my lips have weight and my tongue has weight. You don't think about it, but if you try and talk articulately standing on your head, you'll notice that you have to sort of control your lips and your tongue a little differently just because gravity's pushing them the other way. And it's the same sort of thing. Raising your arm, holding your head up, b turning your head when everything wants to tumble. Uh just keeping your balance, all of those things. It's a little bit like like relearning to walk again like like an infant.

Gravity shapes our bodies and molds our planet. Nothing happens on Earth without its power and influence. Sir Isaac Newton explained so many of its effects using one simple equation. And in the centuries that followed, his laws of physics led to breakthrough after breakthrough, spurring on the industrial revolution. But in the first decade of the 20th century, the next genius in our story challenged the very foundations of our understanding of gravity. A young German scientist called Albert Einstein was churning something over in his mind. He thought that something in Newton's laws didn't quite add [Music] up.

Imagine I'm the sun and this tennis ball is the earth in orbit around me. Newton's laws can describe very precisely the path the earth takes around the sun in terms of the mutual gravitational attraction between the two bodies. But what Newton can't explain is what connects them. In reality, of course, there is no invisible string between the Earth and the Sun holding the two together. There's just empty space, a complete void. And yet, according to Newton, the Earth and Sun pull on each other instantaneously across a vast distance. How can gravity act in this way when there's nothing to connect it or transmit [Music] it?

After years puzzling over this, Einstein had a blinding flash of inspiration. Just like Galileo in his ramp or Newton with his apple, Einstein's breakthrough came because he was thinking about one simple action. What happens when something falls? To explain, I'm visiting this 400 ft high tower in Northampton built to safety test lifts. [Music] One day in 1907, Einstein had what he called the happiest thought of his [Music] life. What if I was standing in a stationary lift, completely isolated from the outside world, not feeling anything apart from the pull of gravity on my feet? What if then the lift cable breaks and I start falling? What are the forces that I will feel as I'm plummeting to the [Applause] ground? Well, I'm not going to try that. Fortunately, there's another way to test this without me having to plunge down a lift shaft. Sorry to disappoint you. This little device here that I've strapped to this plastic toy is an industrial accelerometer. So, it measures acceleration. Now, I've got it connected to my laptop and it's showing a measurement of 1g. Now, that's the downward acceleration due to the pull of Earth's gravity. So, basically, it works just like a gravity meter. But what happens if I were to drop it? Presumably, it'll carry on measuring 1g because it's falling in Earth's gravity. Okay. Okay. Well, let's try that and [Music] [Applause] see. So, you can see here along this line at the bottom, that's when I was holding it still and it's measuring an acceleration of 1g. These oscillations here is when I stood up and there's a bit of disturbance. But this spike along here is the moment I released it. And this short duration along here is the time it was falling. And you see while it was falling, it was registering an acceleration of zero. Now, if you think about it, this is really odd. The accelerometer is accelerating downwards. It's plummeting in the full grip of Earth's gravity. And yet it's measuring no acceleration at all. It's as though gravity has completely disappeared.

Einstein's insight was that when something falls, it no longer feels the pull of gravity. In fact, falling is like floating in empty space. This is the essence of Einstein's happy thought and what we now call his principle of equivalence. Einstein's point is that when the man in the lift falls, he doesn't just feel weightless, he is weightless. Einstein said the man feels no force pulling on him because there is no force pulling on him. Gravity doesn't act on him. It acts on the space and time around him. What we now call the geometry of spaceime. [Music]

This was a radical redefinition. Einstein says, "Forget the idea of gravity as a force acting mysteriously between two objects. Now we have to think of it as the shape of spaceime [Music] changing. You see, Newton saw space and time as independent, fixed, and immutable. that three-dimensional space is the stage in which things happen. But time is separate. It ticks by at the same rate everywhere in the universe. According to Newton, an object would travel through space in a straight line unless acted upon by a force like gravity that will cause it to deviate from that path. But Einstein said that space and time aren't fixed and immutable. They're interconnected, meshed together in what is known as spacetime. And he said that spacetime can be warped, that matter curves space and time around it. So after Einstein, we no longer see gravity as an invisible string pulling objects together. Instead, a body like the Earth warps the structure of space and time around it. And an object in orbit follows a path which is as straight as possible through that spaceime. It's a fundamental part of Einstein's vision of reality. Space and time can't be disentangled. You can't talk about space separately from time. So matter warps time as well as space. It's known as gravitational time dilation. And it's possibly the strangest of all of Einstein's [Music] discoveries.

I've got two identical clocks here. Now, because the clock lower down is closer to the center of the Earth, it feels ever so slightly a stronger gravitational pull than the clock higher up. Einstein's theory says that the lower clock will tick by at a slightly slower rate than the higher clock. Basically, gravity slows time down. It's an extraordinary conception of reality that Einstein describes. Space is being curved and time is being distorted. So why can't we perceive this in our everyday lives? Einstein had a rather nice way of explaining it. Most of us have had the experience as children of trying to work out what our parents do for a living. Well, imagine your father as Albert Einstein. When he was about 12 years old, young Edward Einstein asked his father why he was so famous, what he discovered. Well, this put Einstein senior on the spot. But he came up with a beautifully simple analogy. Einstein told his son, "When a blind beetle crawls over the surface of a curved branch, it doesn't notice that the track it has covered is curved." I was lucky enough to notice what the beetle didn't notice. This is what Einstein meant. The beetle is free to move in any direction on the branch. It can move forwards, backwards, left, and right, but it has no concept of a direction up off the branch. It's as though for the beetle, the universe is missing the third dimension. The beetle may think it's moving in a straight line along the branch, but we can see that the surface it's walking on is itself curving and twisted. Einstein's point was that what we see as the twists and curves of the branch feel to the beetle like forces pushing and pulling it. Okay, so consider this rather strange example. Imagine we have two beetles perched on this pumpkin and for whatever reason they want to walk up towards the top. Now if they start at the equator pointing due north as they walk they will begin by moving parallel to each other that means their paths should never meet. But as they get closer to the top their paths get closer together. Now if they're clever beetles they might try and figure out what's going on. and they could imagine that there's some mysterious force that's pulling them closer together. But for us, from our perspective, we can see there is no such force. All they're doing is following straight paths over a curved surface. [Music]

Just as the Beatles have no sense that the surface of the branch is curved, we completely fail to perceive the bizarre ways that gravity shapes the reality we live in. Einstein's problem was proving that he was right. After years more thought, he realized that there was a way by looking far out into the solar system. Incredibly, here in the grounds of Hurst Monso Castle is housed one of the original telescopes that were used to prove Einstein was correct. In 1915, when Einstein developed his general theory of relativity, it was just that. It was a theory. It had no proof. In fact, many people found it completely outlandish. But then just four years later in 1919, this telescope, and allow me to geek out a bit here, and I'll give it its correct name. This is the 13in astrographic refractor. This telescope proved that Einstein was in fact right, that gravity does curve space itself. [Music]

Since then, observation after observation have confirmed that matter curves space and slows down time. So, the simple question of why things fall the way they do has led us deeper and deeper into the very nature of space and time itself. Gravitational science shows us how galaxies, stars, and planets form. By measuring gravity, we've discovered the existence of dark matter, that 80% of the mass of our universe is invisible, and we don't know what it's made of. And we've detected exotic objects with extreme gravity, like neutron stars, which have more mass than our sun, yet are only 20 km across. But it's another mysterious aspect of Einstein's universe that I want to explore in my next gravity project. Here at the University of Surrey, some colleagues and I have been working on it for months. What we're doing is devising a nationwide citizen science project. We're developing a smartphone app that uses the GPS contained on your phone to explore one of the strangest properties of gravity. How it affects the rate at which we age. I formulated the equations myself. So we can build a map of the and a small team of computer scientists and software developers is using them to devise the app. I didn't realize gravity. Einstein discovered that as gravity changes, so does the rate that time ticks. This means the strength of gravity you feel affects how quickly or slowly you age. The aim of my app is to demonstrate this effect. It works by using a phone's GPS data to estimate your local gravity and it also calculates the average speed at which you move because this too affects the rate at which you age. It then uses the equations I've written which are based on Einstein's theory of relativity to calculate overall how fast or slowly you're aging. Once the app is ready, I tweet about [Music] it. Thousands of people download it and we start to gather results from across the country. Some people send me videos giving me their results. How fast they're aging compared with how time ticks out in space in zero gravity. Over the past day, I have aged less by about uh 172 microsconds. I have aged less by 10 10.02 milliseconds. Uh so since downloading the app, I have aged less by 1.14 milliseconds. Since opening time warp, I have aged less by 2.6 milliseconds. Our aim is to use their results to build up a map of how time flows because of gravity. My smartphone project provides just one insight into the space and time which Einstein's theories describe. [Music]

Gravity and its strange ways have given us astonishing insights into the dark secrets of our universe. Perhaps the weirdest objects in the universe are black holes. Collapsed stars whose gravity is so strong that not even light can escape their grip. Now, for the first time ever, their effects have been felt on Earth, and they've been detected through the medium of gravity itself. It's a story that has revolutionized the study of modern cosmology. 1.3 billion years ago in a galaxy far far away, two black holes swirled around each other, drew closer and closer together until they finally collided with incredible violence. In that final fraction of a second, at the precise moment that they merged, a disturbance was created that sent ripples out through the universe. [Music]

Gravitational waves are a key prediction of Einstein's theory. Matter doesn't just curve spacetime. It can cause waves, ripples which expand outwards, exactly like a stone dropped in water. This particular wave was unimaginably large. The energy released was greater than all the light being given out by all the stars in the universe. The wave rippled through space at the speed of light. In 1.3 billion years, it covered a distance of over 10 billion trillion [Music] km until on the morning of the 14th of September 2015, it arrived here. The streets and cafes of New Orleans, in fact, everything in America and on Earth, expanded and contracted very, very slightly as the wave passed through. No one noticed as by the time it arrived here, the distortion was phenomenally tiny. Except that one science laboratory did notice and I'm going to see [Music] it. A thousand scientists across the world are collaborating on it. It's the culmination of over 50 years of effort and is one of the most sophisticated experiments ever devised by humanity. So, I'm pretty excited to see it. It's a rather unusual setting. Here I am in the middle of rural Louisiana, about an hour's drive outside New Orleans. I don't expect to find such a multi-million dollar cuttingedge research facility as this. And yet, this is the place where recently one of the most important scientific discoveries in human history was made. This is LIGO. The laser interpherometer gravitational wave observatory is an enormous construction shaped like an L with a sophisticated laser system bouncing up and down the two arms. So, we're standing on top of one of LIGO's two arms. This is the first LIGO arm. And in that tube, we there's a laser beam um that we bounce back and forth between a mirror in the end station and a mirror in this building. And the other bit goes that way four kilometers perpendicular to the arm we first saw. This is the L shape. It's a big L on the ground. So the light bounces back and forth in that arm and bounces back and forth in this arm. And what we actually measure with LIGO is the length of this arm as measured by the light between the two mirrors and the length of that arm as measured by the light between two mirrors. And then the laser interferometer measures the difference between those two arm lengths. So as the gravitational wave passed through, the lasers picked it up. They detected that LIGO's two arms changed in length to a very very tiny degree. [Music]

The signal that we saw was just a few thousandth of the size of the um of the atomic nucleus is the biggest the signal ever got. So far far smaller than the size of a single atom. Oh, much much smaller. Yeah, that would and you need something this huge to pick that up. That's right. And so this is one of the most biggest this biggest source of energy in the universe, one of the biggest events you'd ever measure. And we just barely saw it. The LIGO scientists turned the gravitational waves into sound waves. So what you're about to hear is, in a very real sense, the sound of two black holes colliding. It was the first observation of any kind of pairs of stellar mass black holes. Stellar mass means, you know, several or a bunch of of of suns in weight. Um and and so we learned that they exist. We learned that there are enough of them that occasionally they run into each other and and coalesce. Um and um and we also learned by comparing the waveform we observed with general relativity calculations that general relativity is is as far as we know dead on [Music] right.

The long concrete bunker to my left houses the beam line, one of the LIGO's laser arms. The detail and and the effort that's gone into isolating the beam from the outside environment reminds me very much of Caendish's famous experiment. He too had to worry about isolating his his experiment from external disturbances. Only of course LIGO takes things to a far far greater degree. Inside the arm is one of the largest and purest vacuums in the world. Atmospheric pressure in there has been reduced to 1 trillionth of the pressure outside. The mirrors inside are so reflective that they only absorb 1 in 3 million photons. And at the end of my little trip lies a British success [Music] story. Well, I made it all the way to the end of one of the LIGO arms. To be honest, it took me a bit longer than I thought, especially in that thing. But housed inside this building is one of the reflecting mirrors that bounces the laser beam all the way back down the 4 km arm to the main control center. And the technology that went into developing these mirrors is quite remarkable. It was developed in the UK at the University of Glasgow. [Music] This is what the mirror looks like. Its surface is extraordinarily smooth. No bump bigger than a few billionth of a meter high. Equally amazing are these fused silica fibers a few times the thickness of a human hair. Designed by the University of Glasgow in conjunction with scientists from other British universities. They isolate the mirror completely so it hangs perfectly still. You could say that in there is the quietest place on Earth. Despite this, outside events do sometimes interfere with the work here, as I witnessed for myself. I've wandered into the control room here at LIGO because I'm told something kicked off a few hours ago. Uh, and they're all very busy. The image that's flickering up there is not meant to be like that. Essentially, what they've picked up is a seismic disturbance, an earthquake. Now, that's not an earthquake down the road. It started on the other side of the planet in Japan. So it just gives us a sense of the tremendous challenges faced by LIGO and the team here and the level of sensitivity needed that an earthquake on the other side of the earth can disrupt their measurements and they and have to reset everything all over again. One of the scientists involved in developing this extraordinary place put it quite succinctly. Once we were blind, but now we can see.

Throughout the entire history of astronomy, we've studied gravity and how it affects matter in the universe and how it warps spaceime. But only by looking at the light that enters our telescopes. Now for the first time we can study the universe in a different way. The discovery of gravitational waves means we can see objects that cause extreme warping of spaceime and its effect on gravity directly. This essentially opens up a new era in astronomy. It gives us a new way of looking out at the universe. [Music]

Professor Sheila Rowan was one of the scientists who spearheaded the British effort for LIGO. For her and her colleagues, gravitational wave detection is just in its infancy. New instruments, even more sensitive than LIGO, are now being developed. There's so much that we don't understand about the universe that we live in. And this has suddenly given us a new tool, a new way to probe the dark processes in the universe. Because every time we make the the observatories more sensitive, we can sense gravitational wave signals from further away from farther out in the universe, from further back in cosmic history. Things like super massive black holes spiraling in to collide. small black holes orbiting around super massive black holes, tracing out the dense and spaceime of those super massive objects. A long-term goal is to probe back further towards what we think of as the Big Bang, the earliest moments that we understand of the universe as we know it. [Music]

If you think about it, time and time again in the history of science, unlocking the mysteries of gravity have led to a deeper understanding of the universe. Galileo and his ramp, Newton and his apple, Einstein and the falling man in the lift. Each of these characters challenged the scientific consensus of the day. And even today, understanding the true nature of gravity remains one of the biggest challenges in science. Which brings me back to the smartphone app. And it's at this point that our story, for me at least, takes a completely unexpected turn. Unfortunately, it's all gone a bit pear-shaped. Okay, so here's what's happened. A couple of months ago, we launched the app and it was all going really well. Thousands of people downloaded it and have been sending us their results. We've been collecting the data to create this nationwide map to show how time flows at different rates for different people around the country. Unfortunately, I've just realized there's a big problem. You see, I was going over the scientific literature and I came across this subtle point about relativity which basically made me sit bolt upright. There was this horrible dawning realization that I've made a mistake in the equations that get fed into the app. So, what this means is all the results we've been gathering are wrong.

The issue lies in the strange and subtle effects of Einstein's theories of relativity and it's fundamental to the way time flows across the surface of the globe. Now, what if I use my smartphone app where I live here on the south coast of England and then go and spend a few days down near the equator, say here on the west coast of Africa. Now, we know from the road trip that gravity is weaker by the equator. So, that means time ticks faster there. But there's another important factor we have to take into account. Movement. You see, when I'm here near the equator, I'm moving more quickly than I was back in Britain because of the rotation of the Earth. Einstein says movement slows down time, so clocks will tick slower at the equator. This is where the error crept in. You see, I had taken into account these two effects, but I'd missed a crucial point. They cancel each other out exactly. In fact, the Earth bulges out exactly the right amount for its rotational speed to make sure they cancel out. So all clocks on the surface of the Earth at sea level tick at exactly the same rate. So now I'm having to go right back to square one and completely rewrite the equations for the [Music] app. And to test if it's working, I'm going to use it over the course of a normal working week. This is where I live. This is Portsmouth. Uh, which means I'm very close to sea level. And this is how I start most mornings, catching the train to work. The app records my speed as I'm on the train and calculates how this slows down my personal clock. I think the train journey should have slowed my time down by a tiny a few trillionth of a second. I'm heading for the BBC's headquarters in central London, and gravity should be a bit weaker here. I'm a few meters above sea level, I guess, here, and so there'll be a speed up of my time because of altitude. The app compares the way my time flows with a stationary clock at sea level. So, what's my result? On an average day, my movement makes me age slower by a third of a nancond. That's a third of a billionth of a second. But the weaker gravity I'm in means I age faster. Overall, half a nancond faster. I've also given the app to some other volunteers to compare how they age over an average day. Nick flies cargo planes. He flies from Chicago to Germany. Tomorrow morning we have to leave to go to first to Milan and then onto Tokyo. His travel slows down his aging, but much weaker gravity at high altitude speeds his clock up by just a bit more. Overall, he's aging 5 nconds faster than a stationary clock at sea level. Vanessa runs a pub in the Yorkshire Dales. I'm going to take you outside and see the weather conditions here. So, here we are outside the Tan Hill in. We live right in the middle of the national park on the mall. The Tanhill Inn is famous as Britain's highest altitude pub at over 500 m above sea level. We don't have any neighbors. We just have sheep. Her altitude means she ages faster every day by around 4 nconds compared to someone at sea level. There's Kevin, a mountaineer in the highlands. I'm on a mountain in Glen Core called Scoria. I've been at an altitude generally of between 2,000 to 3,000 ft for a lot of the day. Throughout the day, I've just been logging onto the phone, logging onto the app, and just checking it out and having a look, and I've been watching it get bigger, watching the value get bigger and bigger. So, it's been quite a lot of fun. On an average day of climbing, Kevin's personal clock goes faster by one nanoc. [Music] Gary works for a Scottish water retailer. My job takes me all over the UK dealing with energy consultants and energy brokers as far up north as Inesse, as far down south as London. approximately do about a thousand miles a week, sometimes more depending on the number of meetings I have. Gary's car journeys do slow his time down a bit, but being above sea level means he still ages faster by 34 of a nancond. Our final volunteer is Walter. He lives

close to sea level at the iconic Jono Gros. I run a tourism business and I started about 50 years ago. So when people come here, they can actually physically speak to someone who's been born in Ton Gro. And if they ask questions, I can tell them all sorts of useless information because I'm fully useless information.

So our final results show that if you want to age more slowly, try to live near sea level like Walter. Or there is another way to do it. Get a job on the International Space Station. It's 17,000 mph orbit will give you a boost. We did the math for the astronauts. Every month you are about 1 millisecond younger. So you know a thousandth of a second. So after 6 months you are that much younger than people on Earth. So I'm younger than I should be. I hope I hope I look it.

Of course, for us on Earth, time dilation is so utterly minuscule, a few billionth of a second between us, you might think it's too frivolous to even bother [Music] about. And yet, in the long and difficult process of designing the app, I've come to an extraordinary conclusion. The different ways that time flows may not be some quirky byproduct of gravity. It may actually be gravity. It may be the cause of gravity, the reason why objects [Music] fall.

One of the colleagues I've been consulting is Kip Thorne. He's one of the world's leading theoretical physicists and a driving force behind the creation of LIGO. While I was going back over some of the basic physics behind the app, I came across an intriguing idea of his. It's a very interesting and different way of describing gravity. This is what Kip says. Everything likes to live where it'll age the most slowly and gravity pulls it there.

Kip's based at Caltech in California and is one of the most respected theoretical physicists in the world. Firstly, Kip, a serious thank you for for helping out with the uh debacle over the Well, I I sympathize. I've made so many errors in my own over the years that I am totally sympathetic. One of the things that that that struck me uh thinking about this is something you wrote, Kip. You said, "Everything likes to live where it'll age the most slowly and gravity pulls it there." Was this a way of explaining something uh that you felt was a a neat explanation or is there something deeply profound about that?

I think there is something deeply profound in some sense but it it's a lovely description of Einstein's first major insight about gravity in 1912. He realized that gravity that we feel on earth is due to a slowing of time on earth. So time comes before gravity in that sense. You know on on the earth's surface time runs more slowly and that accounts for why gravity wants to keep us there. Well I think in a very deep sense this is true. Objects want to fall that that the flow of time or the rate of flow of time is the thing that produces the gravity. It is the thing that is ultimately responsible for the fall. So somehow it's in the nature of all objects to move towards a region where time runs slower.

Kip's formulation works anywhere in the universe where the gravitational field is such as on earth. The difference in the rate of flow of time is tiny. At high altitude and on the surface of the earth, the difference in the rate of flow of time is one second in 100 years. That's not very much, but that is enough. That is precisely the right amount uh to produce the gravitational pull that we feel and produce the accelerations we're talking about. Wow. Okay, I need to go and write this one down.

So, my investigation deep into the weird ways of gravity has finally left me face to face with one of the greatest mysteries in all of physics, the nature of time itself. It sounds like such a simple question. Why does the apple fall? And yet hundreds of years of scientific inquiry investigating this single action have led us to completely redefine the way we think about the very nature of space and time. And now I've been presented with this extraordinary proposition that somehow in some profound way the apple falls because it's seeking out the place where time runs the slowest. So, does gravity dictate the flow of time, or does time itself define gravity? Could this hint to fundamental new laws of physics as yet undiscovered? I think I'm going to have to think about this a bit more. [Music] I know Ive you know you're my observe you change your behavior so I'm stuck here still I'm tangled up with [Music] My name [Music] [Music] [Music] [Music] [Music]