The Philosophy of Everything 2e — Part II, Ch 7: Energy An Abstract

There is no architect. Every star, every tree and every thought in your head is what happens when one stuff, energy, keeps trying to fall quiet and keeps failing.

In brief: In this chapter I trade the small picture most of us carry of energy, a current in a wire, for a much bigger one. Energy is the one stuff everything is made of, moving under laws nobody wrote. I follow it as it builds atoms, stars and galaxies, and I explain why I think the cosmos has no beginning, no end and only one force. Where my reading leaves the textbook, I tell you so plainly.

For most of my life, when somebody used the word energy, I pictured electricity. A current in a wire. The slow glow inside a battery. The thing that made a fridge cold or a phone bright. The picture was useful. It was also small. Energy turns out to be the stuff every other thing in the universe is made out of. That includes the wire and the battery and the fridge, and the person standing in front of them looking puzzled. The little picture in my head had captured one corner of it. The rest had been hiding in plain sight my whole life.

What I had to do, slowly, was stop thinking of energy as one of the things in the world and start thinking of it as what the world is. The trees. The stars. The thoughts moving across the inside of your head right now. All of them are arrangements, briefly held, of the same one stuff. The arrangements look very different. The stuff is the same. Suppose you wanted to take a person apart, all the way down, past the body and the cells and the molecules and the atoms. You would not find any new ingredient at the bottom. You would find energy, in some unimaginably small and still form, doing what it does. Whatever else the universe is, it is energy in motion. There is nothing else for it to be.

Let me plant one more thing before the chapter begins, because the rest of the chapter sits on it.

Energy alone does not make a world. A world is made by energy and one other thing. That other thing is invisible, unchanging and much harder to point at. The old rules. The eternal laws of how energy is allowed to behave. Neither alone is enough. Energy without laws is noise. Laws without energy are blueprints with nothing to build them. Together they make everything we see, including the seeing.

There is an old image from the East that I have come to find useful for holding the cooperation of these two in one head. Picture two travellers on a road. One can walk but cannot see. The other can see but cannot walk. The blind man carries the lame man on his back. The lame man calls out the turns: where the road bends, where a tree has fallen, when a curve opens up. The blind man takes the steps. Without the legs there is no journey. Without the eyes there is no direction. Either alone gets nowhere. Together they cross continents.

Energy is the legs. The eternal laws are the eyes. The whole universe, every galaxy, every blade of grass, every paragraph of this book, is the journey those two have taken together over a very long time. They had no plan. They had no map. The road threw at them whatever the road threw, and they adjusted, step by step. What fell out of all those steps is what we now call the cosmos. Once that picture is in your head, the rest of this chapter sits behind it.

One thing about what follows, said plainly so you are not misled. A good deal of what I am going to tell you here is not the picture you will find in a physics textbook. It is the cosmos as it looks from inside the philosophy I have been building. It is my own reading of how energy behaves, tested against my own sense of how the world hangs together. On some of it the people who do this for a living would agree with me. On some of it they would not, and I will tell you where. I am not asking you to take any of it on my word. I am asking you to do with it what this whole book asks you to do with everything. Look at it, weigh it against what you can check, and keep it only if it holds. This is one man's account of what is under the world. It is not the last word, and it is not pretending to be.

The first thing I had to learn about energy is that it has two faces, and the two are always in tension.

One face is moving. Doing something. Pushing, falling, flowing, burning, lighting up. The old word for it is kinetic, which is just a long word for moving. The other face is sitting still, but ready. Stored. Held. Waiting. The water at the top of the dam, not yet released. The book on the high shelf, not yet falling. The arrow on the bowstring, not yet let go. The old word for it is potential, which is just a long word for what could happen. Most of what looks dead in the world is actually full of potential energy, waiting for some small change in the surroundings to wake it up.

These two faces are not different substances. They are the same stuff in two arrangements. The water at the top of the dam, when it falls, has not become a different thing. The energy that was sitting in it as potential has shifted face, and is now moving. The arrow that flies has not been given anything new. The energy in the drawn bow has changed face. This swap can run either way. The boulder that rolls down the hill and stops in the valley is now sitting there full of potential energy again. That is because if anything pushes it the other way, it will roll back up. Stillness and motion are not opposites. They are the two seats one thing keeps swapping between. The Eternal Law of Duality is what makes this swap happen. Every active thing tilts towards the inactive. Every inactive thing tilts towards the active. Neither can win for long.

Indian philosophy named these two halves long before anyone called them potential and kinetic. It called them Śiva and Śakti: the still and the dancing, the silent and the moving. Different traditions reached the same observation. The names changed. The pair did not.

The thing that drives this swap is older than energy itself, and stranger. Energy wants to settle. The whole behaviour of energy, in every part of the cosmos, can be described in one sentence: it is trying to get to a state where there is nothing left to do. A flat, undisturbed level. The same temperature everywhere. The same pressure. No pushing, no pulling, no falling, no rising. We could call this state nothing. It would also be the end of everything. If energy ever fully reached it, the lights of the universe would go out and not come back on.

Energy never reaches it. The closer it gets, the harder it becomes to keep getting closer. There is an old principle that does this work. It is the Eternal Law of Energy, which says that energy cannot destroy itself. It cannot leave the room. The route out is closed. So when energy can no longer dissipate, that is, spread itself out any thinner, it does the next available thing. A tiny crest somewhere turns itself inside out and becomes a tiny trough somewhere else. The seed of a new disturbance is planted. A little movement starts. A wave forms. The wave runs into another wave and produces a bigger one. The bigger one cools down into something steady. The steady thing later wakes up into motion. And so on, forever, in every part of the cosmos, at different speeds, in different forms. The thing energy wants, total quiet, turns out to be the thing it cannot have. Its inability to get there is what produces every star and every conversation and every species and you and me.

I want to be plain about this, because it sounds like a small philosophical observation and it is in fact the engine room of the whole book. Nothing is in charge. There is no architect. The universe is the side-effect of one stuff trying, and always failing, to fall quiet. The whole work of being alive, every bit of it, is what happens in the failing.

Energy expresses itself through a small number of properties, and acts across what physicists call fields. These are two words worth slowing down for, because the rest of physics rides on them and most of us were never told what they actually are.

A property is something energy is. Its mass. Its spin. Its charge. Its frequency. Its temperature. Whether it is moving and how fast. Whether it lives a long time before it changes into something else, or only a short while. The colour it gives off, if any. The pull it exerts on the things around it. Some properties are basic. They are present in even the simplest arrangements of energy. Others are built up out of basic ones. Area is length combined with itself. Volume is area combined with length again. Density is mass arranged over volume. Each step up the ladder is properties interacting to give birth to new properties. By the time you reach something as elaborate as a person, the number of properties involved in describing what they are is staggering. But each one started from a small handful of basic ones. They were combined and recombined and combined again until something complicated emerged.

A field is the part of space where a property is at work. The pull of the Earth has a field. You can feel it under your feet right now, and an astronaut a few hundred miles above can no longer feel it the same way. The magnetic field of a magnet ends somewhere. Iron filings inside the field move. Iron filings outside it do not. Every property has a field. The field is where the property is actually doing something. Outside the field, the property is not in play. Two pieces of energy reach each other only through the fields they share.

When I described the universe in the last chapter as a football match, this is where the picture earns its keep. The players are the energies. Their positions are the properties: the goalkeeper, the full-back, the wing-back, the defender, the sweeper, the midfielder, the striker. Each position has a particular way of being on the field. Each carries its own area of influence. The fields, in our sense, are those areas. They are the parts of the ground where each player can actually act on the ball or on the other players. The goalkeeper's field is narrow and intense, close to the goal. The striker's field is wide and far away. The midfielder reaches across both. Where two fields overlap, the players can pass, block, tackle. Where two fields do not overlap, the players are operating in different parts of the match and might as well be in different games. The whole of physics, once you understand that energy works through fields, becomes a much smaller and more orderly thing than the textbooks make it look.

A great deal of what we used to call magic in the old days was actually the experience of two fields overlapping in a way nobody could yet describe. A magnet reaching invisibly through a piece of wood to pull a pin. Light reaching across the dark from one star to another. The pull of the moon on the tide. None of this is mystical once you know that fields exist. It is just energy doing work through fields, exactly as it has always done.

The next thing to understand is how energy is laid out across the universe, because the layout is not even, and the unevenness is where almost all the interesting things happen.

There are two basic patterns. In some places, energy is spread out fairly evenly. The amount in any one cubic metre is more or less the same as the amount in any other cubic metre nearby. This is the pattern in the enormous, mostly empty stretches between galaxies. There, what is left over from the explosions of old stars drifts about quietly: hydrogen, helium and a few other things, at very low density. We cannot see this directly with our eyes. We know it is there because of how it pulls on light passing through, and because of how the rest of the universe behaves at its boundaries.

In other places, energy is bunched up. Most of what we call matter is bunching. A galaxy is a bunching. A star is a bunching inside that bunching. A planet is a bunching inside that. We are a bunching of a bunching of a bunching. The places where the bunching is densest pour out so much light that we see them easily. The places where the bunching is thin we miss, even though by volume there is far more of it.

The interesting thing (and what science has spent the last fifty years arguing about) is what happens at the boundary between bunched and unbunched. The relatively even distribution outside the galaxy presses gently inward. It does not have the density to form anything new. But its faint presence at the edges of galaxies pulls on the outer stars, making them orbit faster than they would otherwise. The orbits at the rim do not look quite right. Modern physics has tried to make sense of this in two ways. One is by inventing new invisible substances no one has ever seen. The other is by proposing an imaginary anti-gravitational force pushing the outer stars outward.

I do not think either is correct. I think what is being seen is the boundary effect of a fairly even outside distribution pressing against an uneven inside distribution. The numbers come out right without needing a new substance and without needing an opposing force. We just have to be honest that there is more energy out there than we had counted, in a form we cannot easily see, doing exactly what energy always does at boundaries.

And here I am not alone, which matters more than my saying it by myself. There are physicists now, working with the standard equations rather than against them, who have shown that the cosmos can be accounted for without inventing dark matter or dark energy at all. They reached it by the hard road, through the mathematics. The fact that careful people arrive at the same place by a route entirely different from mine is the strongest reason I have to think the place is real. I should be fair here, though: most cosmologists do not yet accept that this has been shown. For them, dark matter still best explains many observations at once, from how clusters of galaxies bend light to the pattern in the oldest light in the sky.

When two galaxies cross paths and one of them is small enough, they merge into a larger galaxy. There is a limit, though, to how large a galaxy can grow. Past that limit its own gravity can no longer hold the parts together, and it breaks back apart into smaller pieces. A galaxy cluster is a group of these held together by their shared outer rims of leftover energy. The galaxies inside the cluster bounce around the centre like children on a jumping trampoline, never escaping it, never quite settling. The whole machinery is governed not by any plan but by the same two faces of energy doing their slow back-and-forth.

The biggest of these bunchings, the very densest concentrations of energy anywhere, are black holes. A black hole forms when the centre of a giant star collapses under its own weight. It can grow further when two galaxies merge and their cores fall together. Even light cannot escape from one, which is what makes them dark. But they are not permanent. They are radiating heat slowly back outward, layer by layer. As each layer goes, its surface charge flips from positive to negative or back. Given enough time (and the time required is unimaginable), a black hole reduces itself to nothing.

The flipping charges of its outer layers reach across the cosmos and flip the magnetic poles of nearby stars. All the stars in a galaxy are in some kind of magnetic entanglement with the dense things at the centre, which is to say their magnetism is tangled up with it. And our own sun flips its poles on a schedule we call the solar cycle. That cycle is not the sun's private rhythm. It is part of a much larger conversation between dense things in our galactic neighbourhood, in which the sun is one voice. Here, too, I should tell you where the experts stand: solar physicists trace the sun's cycle of roughly eleven years to hot, charged gas churning inside the sun itself. They see no sign that it is driven from the centre of the galaxy.

There is a question I want to come to now, because it took me the longest to answer, and because I think most adults answer it wrongly when they answer it at all.

Where did all this come from? When did it start?

The story I was given came in slightly different versions, from the science textbooks of my youth and from the religious teachers of my childhood. It was the same story dressed in different clothes. There was a beginning. Before the beginning, there was nothing. Then, all at once, there was something. The religious version had a being who started it. The scientific version had a singularity: a tiny, dense point that exploded into everything that followed. Both versions agreed that the universe was an event with a start. Both versions assumed an eventual ending. Both versions, in their hearts, are wrong.

I want to be careful here, because the wanting in me is older than the looking, and I have to admit the wanting before I describe what the looking found. I wanted the universe to have a moment of birth. I wanted there to be a before-the-beginning and a moment when nothing turned into something. The wanting came from the same place the bedtime stories of my childhood had come from: the desire for a tidy story with an opening line, a middle and an end. The science-textbook diagrams of an exploding singularity satisfied the wanting just as well as the religious story did. Both were comforting. Both were stories. Both, when I finally got around to looking carefully, turned out not to hold up.

What energy actually does, at every scale, is cycle. Take the smallest movement of energy: a tiny crest of something here, a tiny trough of something else there, the two summing to a net of zero. It does not settle into rest. It runs into something. It mixes. It builds. It breaks. It releases. It begins again. There is no single first crest from which all later crests descend. There is no last trough beyond which energy lies down and stops. Everywhere you look closely, the picture is the same. Energy moves in cycles, slowly or quickly, simply or with great complexity, never starting, never ending, only changing shape.

The water on this planet is a small picture of this. A part of the ocean evaporates into clouds. The clouds carry water inland and drop it as rain or snow. The snow becomes glaciers. The glaciers feed rivers. The rivers run back to the sea. None of this is happening all at once. Different drops are at different stages of the cycle at any given moment. A drop in your morning glass may have been falling as rain over a mountain a year ago. It may have been in a glacier a century before that. It may have been in the same ocean a million years earlier.

The cycle does not have a first drop. It does not have a last drop. It does not even have a single starting place. A fisherman on the coast would point to the sea as the beginning. A monk in the high mountains would point to the glacier. A farmer would point to the cloud. All three are right. None has the whole answer. The whole answer is that the cycle has no beginning, only points where particular eyes happen to enter it.

The universe is doing the same thing, on a much larger scale. A part of it may, at one stage, have been a super mega galaxy cluster. That is, a dense gathering of energy under enormous pressure, which began to spread out. The spreading we have observed and called the expansion of the universe is one stage of that local cycle. Smaller galaxies at the rim of the great cluster rolled away first, carrying their momentum with them. The rest followed in phases, one after another.

What we are watching now is not the whole cosmos opening like a flower from a seed. It is a small piece of the cosmos in one part of its long cycle. It continues to fall outward under the momentum it had at the start. It will go on until eventually it meets the gravitational field of something larger, and either falls into it or changes direction. Until then, the galaxies will keep drifting apart, accelerating gently, like a football kicked into space waiting for the next thing to come along. What we have called the expanding universe is, more honestly, an illusion of expansion: one part of a far larger cycle still in its outward stage. I know most cosmologists read the evidence differently: distant galaxies are moving away from us in every direction, and the further off they are, the faster they go. To them, that is the mark of space itself stretching everywhere, not of one piece of the cosmos falling outward.

But the spreading is not the beginning of anything. It is one stage of one cycle of one part of a vastly larger system. That system has been doing the same kind of thing forever, in different places, at different rates, in different forms. None of the stages is the beginning. None of the stages is the end. The cosmos is a cycle of cycles of cycles, all the way out, with no first edge anywhere.

This is harder to feel than it sounds, because every story we have ever been told has a beginning and an ending. We are built to expect the world to have one too. It does not. The closer you look, the more clearly the cycles emerge, and the less interesting the question of a first cause becomes. It turns out to be one of those questions a mind asks because the mind cannot help asking it, and not because there is an answer to it. The right move is to stop asking, and to look instead at how the cycles work.

There is one more thing here, because the people who taught me physics will dislike it. There is no heat death of the universe. The picture in which all the energy of the cosmos slowly drains into evenness, and the lights eventually go out forever, is wrong. It assumes the whole cosmos cycles all at once. The cosmos does not. The cosmos cycles in parts, the way the water on this planet cycles in parts. Some parts are in a hot phase right now. Other parts are cooling. Yet other parts are quiet, waiting to be disturbed by what arrives from somewhere else. There is no moment when the whole machine will fall silent. The machine is not a machine. It is a pattern, and patterns of this kind keep going.

One small thing about the word "universe" before we go on. What I have been calling the universe is the part of the cosmos that we can see from where we happen to be standing. If there are observers in other regions (and there is no reason to think there are not), they would see other parts of the cosmos from where they stand. What they would call the universe is what their part of the cosmos shows them. There can be, in this sense, more than one universe. They overlap in places and are separate in others, each one a kind of report from a particular location. None of them is the whole. The whole is the cosmos, and the cosmos is larger than any one universe inside it.

One brief observation before I describe how a universe builds itself out of all this.

Most of the arrangements of energy that come into existence do not last long. They flicker. They appear, react with whatever is nearby, and disappear into something else within the briefest interval. We rarely notice them, and they do not do much. Then there is a smaller set of arrangements, the stable ones, that hang around long enough to combine, recombine and build up the visible world. The atoms in your body are made of those stable ones. The flickers were the steps along the way.

A useful word for the stable arrangements is relevant. It means the kinds we can name and use to explain what is here. A useful word for the flickers is irrelevant. That is not because they do not exist. It is because they do not stay long enough to play a role we can describe. The flickers are still doing work. We just cannot trace them. The relevant arrangements are the bones of the universe we have learned how to talk about.

So how does a universe build itself, from this not-quite-equilibrium and these crests and troughs and stable and unstable arrangements? By not-quite-equilibrium I mean the near-balance that energy keeps reaching for and never quite gets. Let me walk you through what I came to see, because the layered building of it is what gave the chapter's central image back to me.

Think again of the blind man and the lame man. The blind man walks. He moves. He is energy. The lame man on his back can see, can read the road, can call out the turn: when the road bends, when a tree has fallen, when a curve opens up. The blind man does not invent the road. The lame man does not walk the road. Each step is the agreement of the two. The journey is what falls out of that agreement, in whatever direction the road happens to lead. The conditions around them at any given moment (the road, the weather, the obstacles, what other travellers are nearby) are what we have called the prevailing conditions. The laws do not change. The energy does not stop. What does change is the road. The road is what every new arrangement gets shaped against.

The first stirrings, at the bottom, are the tiniest crests and troughs of energy I described earlier. Mostly they cancel each other out. A few of them, by accident of position, combine to make something a little longer-lasting. Out of those, longer-lived waves form. Some of those waves are big enough and stable enough to start interacting with other waves through fields. Where the fields overlap, a more complicated arrangement comes into being. It is a wave with a personality of its own. It can travel further than the waves that made it, and it can interact with still other things. By a quiet law I am going to call the Eternal Law of Evolution, the arrangements that last longer get more chances to combine. The ones that combine well become longer-lasting still. It is the rule of evolution from the last chapter, given its full name. There is no scorekeeper. The persistence is its own reward. The cosmos has been getting more complicated, on average, for as long as it has been here.

From the merging of short-distance waves emerges what we have called force. A force, in the simplest description, is a self-propagating wave, that is, a wave that keeps itself going. It travels an infinite distance unless it runs into something, or meets its own opposite and cancels. It pushes on what it meets. It does not have a particle hiding inside it. It does not need a carrier. Most of physics has been trying for a century to find the carrier particles for the four "fundamental forces" of nature. I think this is a search for something that is not there. Most physicists would answer that three of the four carriers have already turned up in experiments: the photon, the gluon, and the W and Z particles. Only the carrier for gravity is still missing. There is one stuff, energy, behaving in waves, and the waves themselves are the forces. I will come back to this at the end of the chapter.

The electromagnetic wave is one of the most familiar of these. Each ripple in the electric field generates a perpendicular ripple (one at right angles to it) in the magnetic field. That ripple generates the next electric ripple, and the wave hauls itself across the cosmos at a speed of 299,792,458 metres every second. The electromagnetic spectrum (radio, infrared, visible light, ultraviolet, X-rays, gamma) is just the same wave at different lengths. Photons are the discrete packets, the separate little bundles, in which this wave is carried. None of this needed a designer. The laws are there. The energy is there. The wave appears as a matter of course.

At a higher level of complexity, the waves begin to take on a double nature. They behave like waves and they behave, also, like little particles. This is the level of the lepton. Electrons. Positrons. The neutrinos and anti-neutrinos that travel with them. The lepton is the building block of all the matter we eventually meet. Leptons combine into protons and neutrons, which combine into atoms, which combine into everything else.

I have to be straight with you here, because this is one of the places where I leave the textbook behind. The standard account does not build protons and neutrons out of leptons at all. It builds them out of other particles called quarks, and it has real evidence on its side. Fire a particle hard enough into a proton, and something inside scatters it back as though there were solid pieces in there. What I am laying out is my own reconstruction, shaped to fit the rest of the picture in this chapter, and I find it cleaner. It may turn out to be wrong. I would rather you know it is mine, and that the people who do this for a living see it differently, than have you take it for settled fact.

Let me walk this slowly, because the picture is what matters and the picture is unfamiliar.

An electron and an anti-electron neutrino make one kind of pair, balanced together with a net negative charge. A positron and an electron neutrino make the other kind, balanced together with a net positive. The two kinds of pair then balance each other. A negative pair and a positive pair join to make a neutral grouping that has no net charge. A neutron is many of these neutral pairs of pairs, held in a cyclical arrangement. In shape, it resembles the way carbon atoms loop into a ring of benzene, the six-sided ring that chemists draw.

Inside the nucleus of an atom, lepton pairs are circulating between the protons and the neutrons, much like the second hand of a clock travels round its face. When the cycle is steady, with every neutron handing off and receiving its pair on time, the nucleus is stable. When there is too much or too little for the cycle to balance, the nucleus is unstable. It will eventually shed or absorb material to come back into balance. That instability is what we call radioactivity. That balance is what we used to call the strong force when nobody knew what it was. The "strong force" is not a special force. It is the cycle of lepton pairs going smoothly. The "weak force" is the same cycle with hiccups.

This is most visible in the lightest atoms. A neutron sometimes loses its outermost negative lepton pair. What is left behind is a proton: a positive nucleus that grabs an electron from the neighbourhood to balance itself. That is hydrogen, the simplest atom there is. One proton, one electron orbiting it.

A hydrogen atom can absorb a neutron to become deuterium, an atom with one proton and one neutron. The pairing inside the nucleus now circulates cleanly between the two, like a clock with two faces sharing one second hand. If the deuterium then absorbs another neutron, it becomes tritium (one proton between two neutrons), but the cycle no longer balances. There is an excess pair sitting at the ends. The unstable arrangement sheds an electron pair, and the leftover positron in the nucleus pulls in an electron from the orbit. The atom that began as hydrogen now has two protons and one neutron, which is helium. As soon as it picks up another neutron, the cycle settles. Hydrogen becomes deuterium becomes tritium becomes helium, by very small adjustments at the level of the lepton pairs. From there, by similar small adjustments, proton by proton and neutron by neutron, the rest of the table of elements builds itself. Every element heavier than helium is the same machinery running for longer.

The opposite arrangement is an atom made of anti-protons and positrons, the so-called anti-atom. It does not actually thrive in nature. Negative nuclei cannot share their lepton pairs with each other to make stable extended arrangements. They fall apart on contact with the positive matter around them. The world made of matter is the world that the laws actually allow. The anti-version was on the table at the start, in equal amounts. The laws have selected for one kind, slowly, over time. We are walking around inside what survived the selection.

Once you have atoms, the world starts to build upward through clear steps. I want to name the steps, because they recur throughout the rest of the book. Several lepton pairs make an arrangement: a stable cycle inside a proton or a neutron. Several arrangements make a network. An atom is a network. A compound is a network. An ore is a network. Several networks make an organisation. There are two kinds of organisation, depending on what the networks happen to be doing. The inanimate kind, the non-living, is the rocks and the rivers and the clouds and the mountains. The animate kind, the living, is the bacteria and the viruses and the plants and the animals. Both are organisations of networks. The line between them is real but thinner than we have usually been told.

Several organisations interacting make a system. A forest is a system. A city is a system. A human family is a system. The web of life on this planet is a system. Several systems make a cluster. The solar system is part of a galactic cluster. Galactic clusters are organised, in their turn, into superclusters and beyond. The same logic that builds an atom from lepton pairs builds a galaxy cluster from systems. It is the same machinery scaled up.

Each step up this ladder happens by the same mechanism. Something assembles by chance under particular conditions. Most assemblies do not last. The ones that last are the ones where both sides of the assembly are getting something out of being together. The network gives the organisation what it needs, and the organisation gives the network back what it needs in return.

The old word for this is mutualism. It is the simple fact that arrangements where each side feeds the other outlast arrangements where one side takes and the other gives. They also outlast arrangements where neither gives anything. Mutualism is what abides by what we have called the Eternal Law of Association. There is no scorekeeper. The persistence is its own reward. The ladder is not climbed by ambition. It is climbed by the fact that mutual benefit, on the long view, is the only thing that stays in place long enough to become anything. This is something the next several chapters of the book will keep coming back to. It is one of the most important things to know about the world.

From the bottom to the top, the same picture holds. Energy, doing what energy does, under laws that do not change, building one ladder rung at a time, with no plan, no architect, and a great deal of time on its hands. The cosmos as we have it is what the road has thrown the two travellers, so far.

A small detour, because we use two words constantly and most of us have never examined them.

When I drop a stone, it falls down. I have called that direction "down" for as long as I can remember. But step outside the picture for a moment. Imagine the same stone dropped by a friend in Australia. To my eyes she is upside-down, and her stone falls in the opposite direction from mine. Both of us are right. Both of us are calling "down" something quite different in space.

Down, it turns out, is not a fixed direction. Down is wherever the centre of the nearest large bunching of energy is. It is the place where the gravitational pull is strongest. On Earth, the centre of the Earth is down. Up is away from the centre of the Earth. On the moon, the centre of the moon is down. Up is away from the moon. For the moon orbiting the Earth, the Earth is "down." For the Earth orbiting the Sun, the Sun is "down." For the Sun orbiting the centre of the galaxy, the galactic centre is "down." There is no absolute up and down in the cosmos. There are only directions towards and away from concentrations of energy.

If you wanted the deepest version of this, it would go like this. Down is always towards the lower-energy state. Up is always towards the higher-energy state. Things fall towards less energy. They rise towards more. This is not poetry. It is the actual definition once you have walked far enough out from the small picture of our own planet.

The last move I want to walk you through is about force, because force is where so many of the bad pictures live.

A force, in the cleanest description I have found for it, is a wave. Not a particle. Not a thing exchanged between objects. A wave. It travels outward from wherever it is generated, exerts a push on what it meets, and either keeps going or is absorbed. A bomb in a city sends a wave outward through the air, and the shape of the wave is what shatters the glass in windows three streets over. The wave is the force. The glass is what the force met. A meteor falling into the atmosphere sends a much larger wave, which can shatter glass over thousands of square kilometres. The bigger the source, the further the wave reaches.

Different sources generate force waves of different shapes and intensities. The waves themselves are made of the same energy as everything else. They are not separate substances. They do not need separate carriers. No force-carrier particle has to be invented to explain how a push gets from one body to another. The push is the wave.

What we have called gravity is two of these waves working against each other. The Sun is rotating. The Earth is rotating. The rotation of each generates a wave (the old technical word for it is torque, the rotational push of a turning body) that travels outward in every direction. Where the Sun's torque meets the Earth, it presses inward on the Earth. Where the Earth's torque meets the Sun, it presses inward on the Sun. The two waves keep meeting, and they balance each other. The result of that balance is that the Earth holds its orbit at the distance where the two are even. This is what we have been calling gravity. There is no mysterious pull at a distance. There are two rotating bodies pushing back on each other, and the orbit is the line at which the pushes match.

This too is my own picture, and here the textbook has a fair objection you deserve to hear. Two bodies that are not spinning at all still pull on each other. This has been measured carefully, with still weights in a room. A push that comes from spinning cannot by itself explain that. I think the deeper account is the one I am giving. But I am telling you plainly that it runs against a measured fact, and that it is mine and not the settled view. You should hold the two up against each other and decide for yourself, rather than take it on my say-so.

And the cooperation is not one-sided. The Earth holds its orbit because the Sun's torque finds it. The Sun holds its own position in the galactic neighbourhood partly because the Earth's torque finds it. So do all the other waves coming from all the other planets and from the rest of the system. Each gives the other something. This is true of the moon and the Earth, the Earth and the Sun, the Sun and the centre of the galaxy. The old word for it, again, is mutualism, and it is not a property only of living things. The inanimate world is built of it too. The whole architecture of the cosmos is one long arrangement of bodies keeping each other in place by keeping each other slightly pushed.

When I jump up, the Earth's torque is what pushes me back. When an astronaut on the Moon picks up a rock and lets go of it, the Moon's torque is what brings the rock back to the Moon's surface. Think, too, of light from a distant star that bends around a heavy object on its way to my eye. The bending is the light's electromagnetic wave being deflected (turned aside) by the rotational push coming off the heavy object. The torque of a sufficiently dense thing, a black hole for example, can loop the surrounding light permanently around it. None of this needs a new force. There is just energy, in waves, doing what energy does.

The same is true at every other scale. Inside an atom, the nucleus is rotating, and so are the electrons orbiting it. Their two rotational waves balance. The electrons stay at the distance where the push from the nucleus matches their own push outward. When the electron absorbs energy, it pushes outward harder, and the balance shifts to a wider orbit. When the electron emits energy, the balance shifts back inward. We have been calling this quantum gravity. It is the same arrangement as the planet around the sun. Only the scale is different. I should be honest that physicists would not use that name: for them, what holds an electron near its nucleus is the electric pull between opposite charges. "Quantum gravity" is their name for a separate, unfinished search into how gravity works at the very smallest scales.

There is, I have come to think, only one force in nature. Different expressions of it have been given different names: strong, weak, electromagnetic, gravitational. That happened because for a long time we did not see that they were all the same thing. They are. Energy in motion, through fields, under the eternal laws. That is it. Everything you have ever experienced as a push or a pull, in your body or in the world outside, is one of those waves arriving at you.

The same energy that lights a sun lights the thought you just had. The same laws that hold a planet in orbit hold the electron in its place inside one of your atoms. It is what makes the mind think and the lungs and the plants breathe. It is what makes the heart and the sun tick, the bodies move, and the planets keep their slow orbits. There is no place where it is not.

The next question, once I had seen this, was the one I had been dodging for years. If everything is energy moving under laws that nobody wrote, and we are made of that energy, where does that leave my will? Am I doing anything, or am I being done? Was this paragraph going to be written no matter what I thought I was choosing? That was the question waiting for me on the other side of this one.

Words to Keep

• energy: the one stuff that every star, tree, body and thought is made of. Whatever else the universe is, it is energy in motion.

• the blind man and the lame man: an old image from the East. Energy is the legs that walk, and the eternal laws are the eyes that see the road.

• kinetic and potential: the two faces of energy, moving and stored. They are the same stuff in two arrangements, always swapping seats.

• Śiva and Śakti: the old Indian names for the still and the dancing halves of reality, the same pair as potential and kinetic.

• property: something energy is, such as its mass, spin, charge or temperature. Elaborate properties are built from a small handful of basic ones.

• field: the part of space where a property is at work. Two pieces of energy reach each other only through the fields they share.

• cycle: what energy does at every scale. It builds, breaks, releases and begins again, with no first start and no last end.

• universe and cosmos: a universe is the part of the cosmos visible from one place, so there can be many. The cosmos is the whole.

• relevant and irrelevant: relevant arrangements of energy last long enough to name and use. Irrelevant ones are flickers too brief to trace.

• the ladder: the steps by which energy builds upward, from lepton pairs to arrangements, networks, organisations, systems and clusters.

• mutualism: an arrangement where each side feeds the other. Such arrangements outlast all others, in the living world and the non-living alike.

• force: in this book, a wave of energy that pushes on what it meets, not a particle. Gravity is two such waves in balance.

Sources and Further Reading

• Most of this chapter is my own reading of how energy behaves, worked out from my own observation and the physics I was taught, not from any one book.

• Īśvarakṛṣṇa, Sāṃkhyakārikā. The classic text of the Sāṃkhya school, whose picture of the lame man riding on the blind man's back is the old image from the East at the heart of this chapter.

• The Indian tradition of Śiva and Śakti, the still and the dancing, which named the two faces of energy long before physics called them potential and kinetic.

• Henry Cavendish, Experiments to Determine the Density of the Earth (1798). The careful measurement, with still weights in a room, of the pull between bodies that are not spinning.

• Edwin Hubble, A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae (1929). The classic evidence for the spreading of the galaxies, which this chapter reads as one stage of a local cycle.

• Vera Rubin and W. Kent Ford, Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions (1970). Early measurements of outer stars orbiting faster than expected, the puzzle behind the dark matter debate.

• Jerome Friedman, Henry Kendall and Richard Taylor, the electron-scattering experiments at SLAC, the Stanford linear accelerator in California (late 1960s). The evidence for quarks inside the proton, the place where I leave the textbook behind.