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Tonight, we are going to travel somewhere very close. Closer than the room around you. Closer than your own hand. We are going to travel inward, into the smallest things there are. We will begin with something you could hold between two fingers, a single grain of sand. And then, slowly, we will let it grow, until we can see what it is made of, and what that is made of, and what lies beneath that, all the way down to the smallest distance that has any meaning at all. Along the way we will meet the smallest insect, the smallest living cells, the first photograph of a single molecule, and the strange, quiet world inside the atom. And if you would like more slow journeys like this one, you are always welcome to subscribe. Now, let us begin. There is nothing to remember and nothing to follow closely. The world we are visiting tonight is all around you already, and inside you, very still, and very small. You only have to listen, and let yourself grow smaller with me. Imagine a single grain of sand, resting on the tip of your finger. It is about half a millimetre across. Small enough to be lost in a crease of your skin, large enough to catch the light. Look closely, and it is not a perfect sphere. It is rounded, and slightly frosted, like a tiny pebble of cloudy glass. Most grains of sand are made of quartz. And most of them began life inside a mountain, as part of a slab of granite, perhaps hundreds of millions of years ago. Slowly, rain and frost and wind broke the rock apart. Rivers carried the pieces down to the sea. Waves rolled them back and forth along the shore, again and again, wearing away their sharp edges, for longer than there have been people. So this one tiny grain, so light you could not feel it on your finger, has travelled a very long way, and for a very long time, to rest here. Somewhere near you right now, there is a speck of dust too small to see clearly. It might be a tenth the width of a grain of sand, drifting slowly in the air, lit for a moment by a lamp. And yet that tiny speck is made of more atoms than there are stars in the whole of the Milky Way. Our galaxy holds a few hundred billion stars. The speck of dust holds tens of trillions of atoms, perhaps far more. Count every star in our galaxy, and you would still fall far short. The whole of a galaxy, in one speck of dust. As we grow smaller, we soon reach the edge of what a human eye can see. On a good day, in good light, the smallest thing most of us can make out is about a tenth of a millimetre across. That is about the width of a single hair from your head. Hold a hair up against a bright window, and you can just see it, a fine dark line against the light. Make it any thinner, and it would vanish. Below this size, there is a whole world, as busy and as varied as our own, that no one could see at all for almost all of human history. Creatures that live their whole lives in a single drop of water. Grains and threads and crystals of astonishing shape. All of it was always there, in every handful of soil and every drop of rain. It was simply too small for us. So let us step past that edge now, quietly, into the world below the width of a hair. The first person to really look into that hidden world was a cloth merchant in the Dutch town of Delft. His name was Antonie van Leeuwenhoek. In his spare time, he made tiny lenses, each no bigger than a lentil, ground and polished by hand and set between two small metal plates. Some of them could magnify things more than two hundred times. In the sixteen seventies, he began to look through them at everything he could find. A drop of rain water. A scraping from his own teeth. Water that had stood for a few days with peppercorns soaking in it. And there, in every drop, he found life. Tiny creatures swimming, spinning and darting about, far too small to see. He called them animalcules, little animals, and wrote long, careful letters about them to the Royal Society in London. At first, some people did not believe him. But others looked, and saw them too. A whole living world had been hiding in every drop of water, all along. Drifting through that hidden world are grains of pollen. Each grain is a tiny capsule made by a flower or a tree, usually somewhere between a hundredth and a tenth of a millimetre across. To the eye, pollen is only a yellow dust. But under a microscope, every kind of plant has its own shape. Some grains are smooth and round. Some are covered in spikes, like tiny sea urchins. Pine pollen has two little air sacs on its sides, like a pair of floats, so that it can drift on the wind for many kilometres. And their outer walls are made of one of the toughest natural materials known, a substance called sporopollenin. It can survive heat, and acid, and time. Pollen from long ago is still found today, buried in the mud at the bottom of lakes and in ancient rocks, perfectly preserved. By counting it, layer by layer, scientists can tell which plants grew in a place thousands, and even millions of years ago. A dusting of yellow on a spring morning, carrying a quiet record of every forest that came before. Look closely at a spider's web, glistening with dew in the early morning. Each of its threads is only a few thousandths of a millimetre thick, many times thinner than a human hair. So thin that, in most light, you cannot see a single strand at all, only the shine of the dew along it. And yet, for its weight, spider silk is stronger than steel. The silk of one spider, Darwin's bark spider of Madagascar, is among the toughest materials ever found in nature. It spins threads long enough to cross whole rivers. A single strand of silk long enough to stretch all the way around the Earth would weigh less than a kilogram. Inside each thread, long chains of protein are folded and lined up side by side, some stiff and some stretchy, woven by the spider in a single, silent motion as it walks. A tiny, perfect bridge, built in the dark, in a single night. Every cloud you have ever
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seen is made of tiny droplets of water, floating in the air. Each droplet is about a hundredth of a millimetre across, so small and so light that it barely falls at all. The gentlest rising breeze is enough to hold it up. That is why clouds can float. For a droplet like this to grow into a single raindrop, about a million of them have to come together. They bump and merge, one by one, inside the cloud, until at last a drop is heavy enough to fall. So every raindrop that lands on a window, or on the roof above you, or on a leaf in the dark, is made of a million tiny cloud droplets, gathered slowly together, high in the sky. The smallest insects in the world are smaller than a single-celled amoeba. They are called fairyflies, tiny wasps that lay their eggs inside the eggs of other insects. One of them, found in Hawaii, has the delightful name Kikiki huna. It is about a sixth of a millimetre long, and it can fly. At that size, air feels thick, almost like syrup. So its wings are not flat sheets at all. They are thin sticks fringed with long bristles, like tiny feathers, and they row through the air rather than gliding. A whole insect, with eyes and legs and a brain, drifting through the world on wings of hair. Not far from the smallest insect, there is the smallest flower. On the still surface of ponds, in many warm parts of the world, there floats a plant called Wolffia. Each plant is a single tiny green grain, less than a millimetre across, with no roots, no stem and no leaves. From a distance, a pond covered in Wolffia looks as if someone has sprinkled it with green sugar. Very rarely, a Wolffia plant flowers. The flower sits in a tiny hollow on its upper side, and it is so small that it can only be seen under a microscope. And the fruit that follows is the smallest fruit in the world, not much bigger than a grain of salt. Most of the time, it does not bother with flowers at all. It simply buds, quietly making new plants from its own side, so quickly that a pond can double its covering in a day or two. In parts of Thailand, people gather it and cook it, and call it khai nam: eggs of the water. Now let us dip a single teaspoon into the ocean, and look at what it holds. To the eye, it is only a little clear water. But a teaspoon of sea water near the surface usually holds millions of single-celled living things, and tens of millions of viruses, all drifting together in the dark. There are tiny drifting algae, each one a single cell. There are bacteria, slowly spinning, feeding on whatever drifts by. And among them all, far smaller still, the viruses, quietly waiting. Across all the oceans of the world, these tiny living things add up to an almost unimaginable number. Far more, by a long way, than all the stars in the observable universe. Most of the life on Earth, by count, is like this. Too small to see, quietly going about its work, in every drop of the sea. Among the drifting cells in that teaspoon, some of the most beautiful are the diatoms. Each diatom is a single cell, living inside a tiny house of glass that it builds for itself, from silica dissolved in the sea. The house comes in two halves, one fitting over the other, like the lid on a small round box. Under a microscope, these glass houses are covered in rows of tiny pores and ribs and spokes. Some are round, like little wheels. Some are long and slender, like boats. Some are triangles, or stars. There are many thousands of kinds, each with its own pattern, as fine and regular as lace. When diatoms die, their glass houses sink, and over millions of years they gather into thick, pale layers. In the heart of the Sahara, in a place called the Bodélé Depression, there is the bed of a great lake that dried up long ago, made almost entirely of them. And each year, the desert winds lift that ancient diatom dust high into the sky, and carry it across the Atlantic Ocean, all the way to the Amazon rainforest, where it settles softly on the trees, and feeds them. Floating in that teaspoon, there may be a living thing that you have never heard of, and yet it may be the most common living thing on Earth that runs on sunlight. It is called Prochlorococcus. It is a single cell, less than a thousandth of a millimetre across, so small that it was only discovered in nineteen eighty-eight, by the oceanographer Sallie Chisholm and her colleagues. Like a plant, it lives on sunlight, turning light and water and air into food, and giving off oxygen as it does. And there are unimaginable numbers of them. Something like three octillion, a three followed by twenty-seven zeros, drifting through the sunlit waters of the world's oceans. Together, they make a real share of the oxygen in the air. So a little of every breath you take tonight, slow and quiet, was made by a living thing far too small to see, floating in a sea you may never visit. Let us grow smaller still. Some of the smallest living cells we know of are bacteria called Mycoplasma. They are only about a quarter of a thousandth of a millimetre across. Line up four thousand of them, side by side, and they would barely span a single millimetre. They are so simple that they do not even have a cell wall. Each one is a tiny, soft bubble of life, holding just enough instructions to keep itself alive, and to make another. Biologists have long wondered: how small can a living thing be, and still be alive? A cell has to hold its instructions, and the machinery to read them, and a way to take in food. Somewhere around this size, there is simply no room for anything less. These tiny cells sit very near that edge. As small as life can be, and still be life. Around here, we reach a strange limit. Not the limit of our eyes, but the limit of light itself. In eighteen seventy-three, the German physicist Ernst Abbe worked out that no ordinary microscope, however perfect its lenses, can show anything much smaller than about half the length of a wave of light. Below about a five-thousandth of a millimetre,
everything simply blurs together. Light, it turns out, is too big to see the smallest things. So, in nineteen thirty-one, two young scientists in Berlin, Ernst Ruska and Max Knoll, built a microscope that used a beam of electrons instead of light. Electrons have far shorter waves. And suddenly, a whole new layer of the world came into view. More than fifty years later, in nineteen eighty-six, Ernst Ruska shared the Nobel Prize in Physics for that first electron microscope. Beneath the smallest cells are the viruses. A virus is not quite a living thing, and not quite a non-living one. It is a tiny package of instructions, wrapped in a coat of protein, often built in beautiful geometric shapes: spheres made of triangles, like tiny twenty-sided dice, or slender rods, or shapes like little landing craft with spindly legs. Most are between about a fiftieth and a third of a thousandth of a millimetre across. The smallest known are only about seventeen millionths of a millimetre wide. But there are surprises. In nineteen ninety-two, in a water cooling tower in the English city of Bradford, scientists found something that looked like a bacterium. It was only years later, in two thousand and three, that they realised it was a virus, one so large it was bigger than some living cells. They called it the mimivirus. The smallest world is still full of things we did not expect. Now let us look at a grain of ordinary table salt. To the eye, it is a tiny white cube. And that shape is not an accident. If you could zoom into it, again and again, you would find that it is cubes all the way down. Salt is made of two kinds of atoms, sodium and chlorine, arranged in a perfect, repeating pattern. One sodium, one chlorine, one sodium, one chlorine, in every direction, in rows and columns and layers, like the squares of a three-dimensional chessboard. Because the atoms stack in cubes, the crystal grows in cubes too. The neat little shape you see on your fingertip is the shape of its atoms, repeated many millions of times over. It is one of the rare places where the hidden order of the smallest things shows itself, plainly, in the world we can see. Let us go further down, into the world of molecules. A molecule is a small group of atoms, held together, sharing their outer parts. A molecule of water is made of one atom of oxygen and two of hydrogen, bent together in a gentle angle, like a small boomerang. A single water molecule is about three ten-millionths of a millimetre across. That is so small that a single glass of water holds more molecules than there are glasses of water in all the oceans of the Earth. Many hundreds of times more. And those molecules are never still. Even in a glass of water standing quietly on a table, they tumble and turn, and bump against each other, billions of times every second, each one jostling its neighbours, endlessly, all night long. That endless jostling was first glimpsed almost two hundred years ago. In eighteen twenty-seven, the Scottish botanist Robert Brown was studying grains of pollen in water, under his microscope. He noticed that tiny particles from inside the grains never stayed still. They jiggled and wandered, this way and that, without any reason he could see. At first he wondered whether they might be alive. But he found the same restless dance in tiny specks of dust and crushed stone, which certainly were not. For almost eighty years, nobody could fully explain it. Then, in nineteen oh five, a young Albert Einstein showed what was happening. The specks were being nudged, from every side, by the invisible molecules of the water around them, bumping into them countless times every second. A few years later, the French physicist Jean Perrin measured this dance with great care, and the numbers matched. It was some of the clearest evidence yet that atoms and molecules were real. For a long time, no one had ever truly seen a single molecule. Then, in two thousand and nine, scientists at a laboratory near Zurich, in Switzerland, did something remarkable. Using a microscope with a needle so fine that its tip ended in a single molecule, Leo Gross and his colleagues felt their way across one tiny molecule, called pentacene, lying flat on a surface. The image they made looks almost like a drawing in a chemistry book. Five small rings, joined in a row, like a little honeycomb. You can see the bonds between the atoms, as soft, pale lines. It was the first time anyone had seen, so clearly, the shape of a single molecule. Something people had only drawn, from imagination and reasoning, for more than a hundred years. It looked just the way they had always drawn it. A few years before that, people had already learned to move single atoms, one at a time. In nineteen eighty-nine, at a laboratory in California, the physicist Don Eigler and his colleague Erhard Schweizer used a very fine needle to nudge single atoms of xenon across a cold, flat surface of nickel. Atom by atom, they arranged thirty-five of them into three letters: the initials of the company they worked for. It was the smallest writing the world had ever seen. Years later, in two thousand and thirteen, the same kind of laboratory made a tiny film, using molecules of carbon monoxide, moved one by one and photographed in each new position. It shows a little figure playing with a ball, dancing and bouncing across the screen. It is called A Boy and His Atom, and it is the smallest stop-motion film ever made. Now we arrive at the atom itself. An atom is about a ten-millionth of a millimetre across. It is very hard to picture something so small. But the physicist Richard Feynman once offered a lovely way to try. Imagine an apple, he said, and imagine it growing, larger and larger, until it is as big as the whole Earth. If you did that, the atoms inside the apple would grow too. And they would end up about the size of the original apple. So the atoms are to the apple as the apple is to the Earth. Every apple, every leaf,
every grain of sand, every breath of air is made of these impossibly tiny pieces, packed together in their countless billions. Can you ever actually see a single atom? For a long time, the answer seemed to be no. But in two thousand and twelve, scientists at Griffith University in Brisbane, Australia, held a single atom of ytterbium almost perfectly still in the middle of a vacuum, and shone light on it. The atom soaked up a little of the light, and cast a shadow. They photographed that shadow: the first picture of the shadow of a single atom. And in two thousand and eighteen, at the University of Oxford, a young physicist called David Nadlinger did something even simpler. He held a single atom of strontium between two small metal points, lit it with a blue-violet laser until it glowed, and photographed it with an ordinary camera. In his picture, there is a tiny, pale dot of light, floating in the dark. It is one atom. Just one, shining on its own, small enough that millions of them would fit across a single millimetre. For a long time, people imagined atoms as tiny solid balls. Then, in nineteen oh nine, in a laboratory in Manchester, two young scientists, Hans Geiger and Ernest Marsden, tried firing tiny particles at a sheet of gold foil, so thin that light could almost pass through it. Most of the particles flew straight through, as if nothing were there. But once in a long while, one bounced almost straight back. Their teacher, Ernest Rutherford, worked out what this meant. Almost all of an atom's weight was gathered in a tiny centre, which he called the nucleus. And all the rest of the atom was almost entirely empty space. How small is the nucleus? If an atom were the size of a great football stadium, the nucleus would be about the size of a pinhead, sitting in the very middle. All the rest, the stands, the pitch, the air above, would be nearly empty. Everything you have ever touched is made of atoms like this. Mostly space, held lightly together. In a way, you have never truly touched anything. Every atom has a tiny, heavy centre, wrapped in a soft cloud of electrons. And electrons push other electrons away. So when your hand rests on a table, the atoms of your skin never press into the atoms of the wood. As they come very close, their electron clouds push against each other, gently, and that push is what you feel as touch. Your head on a pillow tonight is held up the same way, by a tiny force, across a gap far too small to see. Always almost touching, and resting there. In old drawings, electrons circle the nucleus like tiny planets around a tiny sun. But that is not quite how they are. An electron is not really in one place at a time, the way a pebble is. It is more like a soft cloud of possibilities, gathered around the nucleus, thicker in some places and thinner in others. These clouds have shapes. Some are simple spheres. Some are shaped like two balloons, joined at the middle. Some are like rings, or petals. Each shape is a different
way an electron can be, around an atom. And when atoms meet, their clouds overlap and blend, and share their electrons. That sharing is what joins atoms together into molecules, into salt and water and silk and sand. Everything that holds together, in the whole world, is held together by these quiet, shared clouds. The way electrons move even gives things their colour. When light falls on something, its electrons can take in some colours and give back others. And the colours that are given back are the colours we see. Gold is a beautiful example. Most metals, like silver and iron and tin, look silvery grey. Gold should be silvery too. But gold atoms are very heavy, with a great many electrons, and the innermost ones move astonishingly fast, a large part of the speed of light. At those speeds, Einstein's theory of relativity begins to matter. It changes the electrons' clouds just enough that gold soaks up a little of the blue in white light, and gives back warm yellows and reds. So the warm glow of a gold ring is, in a way, Einstein's relativity, shining quietly from inside every atom. The simplest atom of all is hydrogen. A single electron, wrapped around a single tiny centre. It is by far the most common atom in the universe. And it is old. The centres of almost all the hydrogen atoms there are were made in the very first moments of the universe, nearly fourteen billion years ago, long before there were any stars. There is hydrogen in every molecule of water. Two atoms of it, in each small bent molecule. So when you drink a glass of water, much of what you are drinking is nearly as old as the universe itself. Those tiny pieces have drifted through clouds of gas, and the hearts of stars, and the oceans of the early Earth. And tonight, some of them are in a glass, beside someone, quietly waiting. Now we go inside the nucleus. It is made of two kinds of particles, packed tightly together. Protons, which carry a small positive charge. And neutrons, which carry none at all. A nucleus is astonishingly small, tens of thousands of times smaller than its atom. Ten trillion of them, lined up in a row, would stretch only about a centimetre. And yet almost all the weight of everything is gathered here. The mountains, the oceans, the trees, your own body as it rests tonight: nearly every bit of that weight sits in these tiny, hidden centres, deep inside every atom. If you could squeeze all the empty space out of every person on Earth, we would all fit in a sugar cube. Every atom in your body is almost entirely empty. Nearly all its weight sits in a tiny centre, the nucleus, far smaller than the atom around it. So imagine taking every atom in every person alive, and pressing out all of that empty space, until only the nuclei were left, packed tightly together. All of humanity would shrink down to something about the size of a single sugar cube. A very heavy one, weighing hundreds of millions of tonnes. Eight billion people, in a sugar cube. There is a puzzle
inside every nucleus. Protons all carry the same kind of charge, and things with the same charge push each other away. Packed together so tightly, they should fly apart in an instant. But they do not. Something holds them together, and it is called the strong force. Up close, it is far stronger than the push between the protons. Many times stronger. It holds protons and neutrons together so tightly that the nucleus is, in its own way, the most solid thing there is. And yet the strong force barely reaches beyond the edge of the nucleus. Step away by even the width of a single proton, and it fades away almost completely. So the strongest force in nature is also one of the most private. It works only in the tiniest spaces, quietly holding the centre of every atom together, and we never feel it at all. For a long time, protons and neutrons were thought to be the smallest pieces of the nucleus. But in the nineteen sixties, physicists began to suspect that they were made of something smaller still. In nineteen sixty-four, the physicist Murray Gell-Mann gave these smaller pieces a name. He called them quarks, borrowing the word from a line in James Joyce's novel, Finnegans Wake: three quarks for Muster Mark. At almost the same time, another young physicist, George Zweig, had the same idea, and called them aces. A few years later, in nineteen sixty-eight, experiments in California fired electrons deep into protons, and found that they bounced off tiny, hard points inside. The quarks were really there. A proton is made of three of them: two of a kind called up, and one called down. A neutron is the other way round. There is a kind of particle that can never be alone. It is called a quark. Every proton and neutron, at the heart of every atom, is made of three of them. And quarks are bound together by a force that does something strange. With most forces, the further apart things are, the weaker the pull. But with quarks, the pull grows stronger. Try to pull a single quark free, and the energy you put in turns into brand new quarks instead. No one has ever seen a quark on its own. Always in company. Never alone, all the way down. Here is something lovely about protons. You might think that a proton weighs what its three quarks weigh, added together. But it does not. The three quarks inside a proton make up only about one per cent of its weight. The rest comes from the energy of the quarks themselves, racing about inside the proton, and the energy of the force that binds them, all held in that tiny space. Einstein showed that energy and weight are, at heart, the same thing. And so, deep inside every proton, that endless motion shows up as weight. Which means that almost all of the weight of your own body, as it rests here tonight, is not really the weight of tiny pieces of matter. It is the energy of motion, held still, inside every atom you are made of. Now let us return to the electron, and look at it more closely. It was discovered in eighteen ninety-seven, by the physicist J. J. Thomson, in Cambridge, the first particle ever found that was smaller than an atom. And as far as anyone can tell, an electron has no size at all. Unlike a proton, it does not seem to be made of anything smaller. However closely we look, it behaves like a single point, with weight and charge, and nothing inside. Experiments have even tested how round its charge is, and found it to be more perfectly round than anything we can measure. Rounder, in its way, than any ball, any planet, or any drop of water. The smallest thing, perhaps, that is not made of anything else. There are particles even lighter than the electron. They are called neutrinos, and they are so light that, for a long time, people thought they weighed nothing at all. We now know they have a little weight. But only a very little. In twenty twenty-two, an experiment in Germany called KATRIN weighed them more carefully than ever before. It found that a neutrino weighs, at most, less than a millionth of the weight of an electron. They pass through almost everything, barely noticing the matter around them, crossing whole planets as if they were mist. The lightest pieces of matter that we know of. Drifting through the universe in vast numbers, so faint and so light that they barely touch anything at all. To catch something so faint, you need something very large, and very quiet. Deep beneath the South Pole, there is a block of ice one kilometre wide on every side, a whole cubic kilometre of clear, ancient Antarctic ice. Strung through it are more than five thousand small glass spheres, each one watching the darkness. It is called IceCube. Very rarely, a neutrino passing through the ice strikes an atom, and a faint blue flash of light spreads through the darkness. The spheres catch the flash, and from its shape, scientists can tell where the neutrino came from. In twenty seventeen, one of those flashes was traced back to a galaxy called T X S zero five oh six plus zero five six, about four billion light-years away. A single, almost weightless particle had crossed half the universe, and ended its journey in a faint blue glimmer, deep in the ice at the bottom of the world. Why does anything have weight at all? For a long time, this was one of the deepest questions in physics. In nineteen sixty-four, the physicist Peter Higgs, and several others at the same time, suggested an answer. Perhaps, they said, all of space is filled with something invisible, a kind of field, like an endless calm sea. Some particles pass through it untouched, and have no weight. Others are slowed by it, a little or a lot, and that slowing is what gives them their weight. If that sea was there, it should be possible to make a tiny ripple in it. A particle. In two thousand and twelve, after nearly fifty years of searching, scientists at CERN, near Geneva, announced that they had found it: the Higgs boson. Peter Higgs, by then in his eighties, was in the audience, and quietly wiped
away a tear. We often think of empty space as truly empty. Nothing at all. But at the very smallest scales, empty space is never perfectly still. It shimmers. Tiny ripples of energy rise and fade, too briefly and too faintly to see. In nineteen forty-eight, the Dutch physicist Hendrik Casimir worked out something surprising. Place two smooth, flat plates very close together, in a perfect vacuum, with nothing between them. They will be gently pushed together, by the shimmer of empty space itself. Decades later, in nineteen ninety-seven, the physicist Steve Lamoreaux measured that tiny push, and found it was just as Casimir had said. So even nothing is something. Even the most perfect emptiness, deep down, is very quietly alive. How far down can we go? Is there a smallest distance, a size below which nothing smaller can exist? Many physicists think there may be. It is called the Planck length, after the German physicist Max Planck, who first worked it out in eighteen ninety-nine. It is so small that it is almost impossible to picture. If a single atom were blown up to the size of the entire observable universe, the Planck length would grow to be about the height of a tall building. At that scale, our ideas about space and time may simply stop working. Space itself might not be smooth at all, but grainy, or foamy, or something stranger still, that no one has yet imagined. No experiment can come anywhere near it. It may be the very bottom of the world. Or it may be another door, leading somewhere we cannot yet see. What might be waiting down there, at the very smallest scale? No one knows. But there are ideas. One idea is that every particle, the electron, the quark, all of them, might really be a tiny vibrating string, far too small to ever see. Different notes, played on the same kind of string, would make different particles, the way one violin string can sing many notes. Another idea is that space itself is woven from tiny loops, like the threads of an impossibly fine cloth. These are beautiful ideas. But they are still only ideas. No one has yet found a way to test them. And perhaps that is fine. At the bottom of everything, there may be a little mystery left, waiting quietly for someone in the future to find it. Now let us turn around, and look back up. We have travelled from a grain of sand, down past the edge of seeing, through cells and molecules and atoms, all the way to the Planck length. And if we looked the other way, out into space, we could travel just as far upward, to the edge of the observable universe. If you were to set out all of these sizes, from the very smallest to the very largest, and find the point exactly halfway between them, you might expect it to be somewhere near the size of a person. But it is not. The middle of everything, it turns out, is much smaller. It is about a tenth of a millimetre. About the width of a single hair, or a grain of pollen, or a fairyfly. At the very edge of what our eyes can see, sits the quiet
middle of the whole universe. Here is perhaps the most wonderful thing of all. In its very first moment, the universe was smaller than an atom. And in that tiny space, the same faint shimmer that fills empty space was there too, tiny ripples of energy, rising and fading. Then the universe grew, enormously, in a tiny fraction of a second. And, according to our best ideas, those tiny ripples were stretched along with it, until they became gentle differences, slightly thicker here, slightly thinner there, across all of space. Over billions of years, gravity gathered those slightly thicker places together. And they became the galaxies. If that idea is right, then the great spirals of stars, and the vast threads of galaxies that cross the sky, all began as ripples smaller than anything we can ever hope to see. The largest things in the universe, grown from the smallest. Even the Sun shines because of the smallest things. Deep in its core, protons crowd together in great heat, and when two of them join, a little energy is set free. That is what makes sunlight. But there is a puzzle. Even at fifteen million degrees, the Sun's core is not quite hot enough. The protons push each other away so strongly that, by the old rules, they should almost never come close enough to join. The answer lies in the strange rules of the very small. A proton is not quite a tiny ball in one place. It is a little blurred, a little spread out. And so, just occasionally, it can slip through the wall that should keep it out, as if passing through a closed door. Physicists call this tunnelling. It happens very rarely, to any one proton. But there are so many of them that the Sun can shine, steadily, for billions of years. Every warm morning, every sunrise you have ever seen, depends on that quiet slipping through. Now let us come back to you. Your body is made of something like seven billion billion billion atoms. A seven followed by twenty-seven zeros. Far more than the number of stars in the observable universe. Most of them are hydrogen, the oldest atoms there are. Then oxygen, carbon and nitrogen, and small amounts of calcium, and iron, and dozens of others. And they do not stay. Slowly, quietly, many of the atoms in your body are replaced, by the food you eat and the water you drink and the air you breathe. Many of the atoms in you tonight were not part of you a few years ago. And yet you remain. You are not really the atoms themselves. You are the pattern they make together, held gently in place, renewed day after day, like a river that stays the same river, even as its water flows on. So here we are, at the end of our journey inward. Inside you, right now, molecules are tumbling and turning. Electrons are spread in their soft clouds. Quarks are racing inside their protons, never alone. Neutrinos are passing quietly through, untouched. All of that motion, deep down, in every part of you. And yet, from the outside, you are lying still, and warm, and quiet. That is how the world is made. Endless small movements, adding up to something calm. Like the sea, which is made of countless tumbling waves, and yet from far above looks perfectly smooth. Like the night sky, made of fierce, burning stars, and yet so still and peaceful when you look up at it. The smallest things are always busy. And they are always holding you. Let us come home now, slowly, from all of it. The grain of sand, resting on your fingertip. The pollen, drifting on the wind. The thread of silk, shining with dew in the early morning. The fairyfly, rowing through the air on its feathered wings. The tiny cells, floating in the sunlit sea. The crystal of salt, cubes all the way down. The empty atom, with its tiny heart. The three quarks, never alone. The faint blue flash, far beneath the snows of Antarctica. All of it is far too small to see. And here, the night is calm, and everything is held together, gently, from the smallest pieces up. You are made of the same small things as the stars. Let them rest now, all of them, all the way down. You are warm, and safe, and it is time to sleep. Sleep well.