The Sun

The accelerant of entropy for everything that lives here.

Nearly everything alive on this planet is running on sunlight. The energy in the food you ate today, the oil that moved it to you, the wind, the rain, the warmth of the air, the muscle that carried you here, can almost all be traced back, through a chain of a few steps, to light that left the surface of the Sun eight minutes before it arrived. The Sun is not one feature of the biosphere among others. It is the engine. Everything else on these pages, the air’s circulation, the ocean’s currents, the soil’s slow building and breaking, is in large part the Sun’s energy moving through the system on its way from concentrated light to dispersed heat. This page is about that engine, and about the strange and exact sense in which the Sun is.

I. The Furnace and the Gradient

The Sun is a vast sphere of hydrogen and helium, so massive that its own gravity crushes its core to temperatures and pressures at which hydrogen nuclei fuse into helium, releasing energy in the process. This nuclear fusion has been running for around four and a half billion years and will run for billions more. The energy it releases works its way out and streams off into space in all directions as light and heat, and a tiny fraction of that outpouring, a few billionths of the total, happens to fall on the Earth. That sliver is enough to power the entire living world.

The Sun is a small, intensely hot point in a vast, cold sky. Energy arrives from it concentrated, in high-quality, low-entropy form (a narrow stream of hot sunlight from one direction), and the Earth eventually radiates the same amount of energy back out to space in degraded, high-entropy form (dispersed, cooler infrared heat, spread in all directions). The Earth does not, on balance, keep the Sun’s energy; it passes it through, taking it in concentrated and releasing it dispersed. And that difference, that flow from concentrated to dispersed, is the gradient that every process on the planet’s surface feeds on.

We imagine the Sun gives life energy to fight decay. The deeper truth, from the thermodynamics in Entropy, is the reverse: life exists because it speeds the Sun’s energy on its way from order to disorder. A living thing, a forest, the whole biosphere, is a structure that arises in the flow precisely because it accelerates the dispersal of the solar gradient, dissipating that concentrated energy faster and more thoroughly than bare rock would. The Sun is the accelerant of entropy, not because it destroys things, but because it pours out the gradient that life organises itself to dissipate. Far from being a fight against the second law, life is one of the universe’s most effective ways of obeying it.

II. The Gateway: Photosynthesis

For all that solar energy to enter the living world, something has to catch it and turn it into a form life can use, and on this planet, almost the entire job is done by one process: photosynthesis. It is the single most important gateway in the biosphere, the narrow door through which nearly all energy passes from the physical world into the living one.

The mechanism is, at its core, the capture of light by a pigment, chlorophyll, which absorbs particular wavelengths of sunlight (reflecting the green ones, which is why the living world is coloured the way it is) and uses that captured energy to drive chemistry: combining carbon dioxide from the air and water into energy-rich sugars, releasing oxygen as the by-product that, as The Air described, remade the atmosphere. In that moment, a photon of sunlight becomes chemical energy stored in a molecule, and the energy is now inside the living system, available to be eaten, burned, built with, and passed on. Every green plant, every algae, and a host of bacteria are running this capture, and together they are the point of entry for essentially all the energy in the food web. When you eat, you are eating stored sunlight, captured by chlorophyll, however many steps back down the chain it was caught.

With very few exceptions, every living thing you will ever encounter is either photosynthesising or eating something that did, or eating something that ate something that did. The deep-sea vent communities from The Ocean, running on the Earth’s own chemical energy rather than sunlight, are the rare and instructive exception that proves the rule: they show that life on starlight is not strictly necessary in principle, while underlining how nearly universal it is in practice. The biosphere is, with those few exceptions, a single solar-powered system, and chlorophyll is the turnstile every unit of its energy passes through.

III. Captured Sunlight, Flowing and Fading

Once solar energy is captured, it flows through the living world in a particular pattern, and understanding that pattern is understanding how every ecosystem is built, because every ecosystem is, at bottom, an arrangement for moving captured sunlight around before it escapes as heat.

The energy enters through the photosynthesisers, the plants and algae, which ecologists call producers because they produce the living world’s energy supply from light. They are eaten by plant-eaters, which are eaten by predators, which may be eaten by predators in turn, and all of them, when they die, are broken down by the decomposers of The Ground. At each step, the captured solar energy is passed along, and here is the rule that shapes the whole structure: at every transfer, most of the energy is lost, dissipated as heat. As a rough rule of thumb, only around a tenth of the energy at one level makes it into the next. This single fact explains the shape of life. It is why there are vastly more plants than plant-eaters, and more plant-eaters than predators; why top predators are rare and need huge territories; why food chains are short, rarely more than four or five steps, because after a few transfers there is almost no captured sunlight left to pass on. The pyramid shape of every ecosystem, broad base of plants narrowing to a few predators at the top, is a direct consequence of solar energy degrading as it flows through the living world. (The full structure of these ecosystems and the cycling of matter through them is taken up in The Earth; the point here is that what is flowing through them is sunlight, fading at every step.)

An ecosystem is a structure through which solar energy flows, getting more dispersed at every stage, until what entered as concentrated sunlight leaves as low-grade heat radiated back to the cold sky. Life does not store the Sun’s gradient. It rides it down, building intricate structures, food webs, and bodies on the way, all of it temporary, all of it ultimately handing the energy on toward dispersal. The Sun pours out the energy; the biosphere is the elaborate, beautiful machinery for spreading it out.

IV. Light as Information

Sunlight is not only fuel; it is the most reliable piece of information in the environment, the steady beat of light and dark and the turning of the seasons, and life has wired itself to that beat at the deepest level.

Because the Earth turns, every organism evolved under a relentless daily cycle of light and darkness, and almost all life carries an internal clock tuned to it: the circadian rhythm, developed practically in Sleep & Circadian Rhythm. This is a genuine molecular oscillator, present in creatures from bacteria to humans, that keeps roughly twenty-four-hour time and is reset each day by the light entering the eye. Your sleep, your hormone cycles, your body temperature, your alertness, your digestion, all of it rises and falls on a daily rhythm anchored to the Sun. The light is the master signal, and when the signal is disrupted by shift work, by artificial light at night, by crossing time zones, the body’s timing degrades, and health suffers in measurable ways. The seasonal swing of light matters too, shaping mood and physiology over the year, the winter dip in light lowering mood in many people. You are tuned to a rhythm that has been steady for as long as there has been life on a turning planet.

That morning light exposure supports healthy circadian timing, that adequate sunlight supports vitamin D production, and that natural light affects mood and alertness.

V. Holding It Together: Gravity

One more thing the Sun does for the biosphere, easy to overlook because it never changes: it holds the whole system in place. The Sun’s immense gravity is what keeps the Earth in its orbit, at the steady distance that keeps the planet in the narrow range of temperatures where water is liquid, and life is possible. Drift much closer, and the oceans would boil; much further and they would freeze. The stability of the Earth’s orbit, the near-constancy of its distance from the Sun over billions of years, is part of what gave life the long uninterrupted runway it needed to develop. Gravity also links the Sun and the Earth and the Moon into the system that drives the tides, the slow daily breathing of the oceans against the land, which shaped the rich zone between sea and shore where so much life, possibly including our own coastal ancestors, has flourished. The Sun’s gravity is the quiet, constant condition underneath all the energy and all the light: the thing that keeps the Earth in the habitable place long enough for any of the rest to matter.

VI. You Are Running on Sunlight

You are, quite literally, running on sunlight a few steps removed. The food that powers every beat of your heart and every thought is chemical energy that chlorophyll caught from the Sun, days or weeks or (in the case of the fossil fuels behind your food system) hundreds of millions of years ago. The oxygen you burn it with was made by that same photosynthesis. Your daily rhythm is set by the Sun’s light; your mood and sleep move with it; your body makes vitamin D from it. You are warm because of it. You are, in the most material sense, a temporary knot of captured sunlight, an arrangement that the solar gradient flows through and animates for a while before the energy moves on and the matter returns to the cycles of The Ground and The Ocean.

Almost every human culture grasped some version of this and built its highest reverence around it, which is no accident: people living close to the land could hardly miss that the Sun was the source of life. The Egyptians had Ra, the sun god from whom creation flowed. Many cultures aligned their greatest monuments to the solstices, marking with enormous effort the turning points of the solar year. Sun worship, in one form or another, appears across the ancient world more widely than almost any other, and the great seasonal festivals that survive into the present are, at root, markings of the Sun’s annual death and return. They were an accurate intuition, expressed in the language available, of a real and total dependence. The Sun genuinely is the source of nearly all the energy in your life, the setter of your rhythms, the condition of the planet’s habitability. Calling it the giver of life was not superstition so much as a true observation wearing the clothes of myth. We have replaced the myth with the physics, and the physics, if anything, makes the dependence more total and more astonishing.

So the Sun resolves into the one indispensable thing: a fusion furnace pouring out a gradient of concentrated energy, of which the Earth intercepts a sliver and life organises itself to ride down toward dispersal; the source, through the single gateway of photosynthesis, of nearly all the energy in the living world; the signal that times every organism on the planet; and the gravitational anchor that holds the Earth in the one place all of this is possible. You are one of the countless temporary forms its outpouring energy takes on its long way from the furnace to the cold and final dark.

VII. Cross-Links

• The Biosphere for the section overview
• The Air for the solar heat engine that drives weather
• The Ocean for the solar energy stored and moved by the sea
• The Ground for where the captured sunlight is finally broken back down
• The Earth for the ecosystem structure the solar energy flows through
• Resources for the reading list