At the heart of Earth’s outer engine is the energy supply from the Sun. As it heats up the rocks, ocean, or atmosphere, or as it fuels sugar production in plants, it’s pushing the engine away from equilibrium. As long as there’s an imbalance in the distribution of energy, there is always the potential for things to change. The movement energy of falling rain can erode mountains as it splashes down on bare rock. The vast excess of heat energy at the equator drives tropical storms, battering palm trees, redistributing water from sea level to high mountains, and sending waves crashing onto beaches. The energy stored in a plant will be used to build branches, leaves, fruit, and seeds, eventually running out of usefulness as low-level heat. Only the seed will be left, a package of genetic information destined to restart the cycle with new energy from the Sun’s fountain of light. Our planet lives because of the constant injection of energy from above, feeding the engine and preventing Earth from winding down into stable, unchanging equilibrium. From up here on the edge of space, we can’t see the tiny details, but we can see the big picture: Energy flows onto Earth from the Sun, trickles down through the ocean, the atmosphere, and life, and eventually carries on out into space as the planet radiates heat away. The same amount of energy goes in and comes out. But the Earth is a gigantic dam in the energy flow, storing and using this precious resource in myriad ways before it’s released to the universe.
As we drift back down to ground level, a beach now looks like a process instead of a place, a patchwork of timescales and size scales. The ocean waves are carrying energy from storms far out at sea. As they break on the beach, they rattle the sand and rocks, grinding them together. One speck at a time, the stone is chipped away, each pebble sculpted by millions of random collisions. It takes a millisecond to remove one minuscule chip, but years of slow attrition to make the pebble smooth. In geological time, a beach is temporary. It will last only if the supply of new pebbles and sand is greater than the loss as they wash out to sea. Over months and years, sand will shift into the sea and back out again in response to the ocean. We love our tidal beaches precisely because we can see the ebb and flow reshaping the sand twice a day; it’s as though the slate is wiped clean, and we find the simplicity of the newly smoothed sand satisfying. But this daily remodeling hides the decadal shifts as our coastline grows and shrinks in front of us. The life in the rock pools thrives on change, adapted to periods of being high and dry alternating with spells of complete submersion. Though a casual glance at a rock pool can give the impression of a museum exhibit behind glass, in every pool there is a fierce battle for resources going on. The resources on offer are all ultimately very simple: access to the drips of energy oozing through the Earth’s system or the chance to gather the molecular building blocks needed to construct a life. More than anywhere else, a beach exemplifies the transience of life. When the energy and nutrients are available to support life, rock pools flourish. During the barren periods, life will be found elsewhere. Species evolve by altering their use of the physical toolbox available to them one genetic mutation at a time. Whether they’re harvesting energy, moving around, communicating, or reproducing, they are all just using the same principles in different ways.
Energy passes through, but the Earth itself is constantly recycled. Almost all the aluminum, carbon, and gold that make up our planet has been here for billions of years, shifting from one form to another. It might seem that after this long, these different substances should all be jumbled up, mixed together in a giant planetary soup. But the physical and chemical processes around us are continually sorting the pile, so that pockets of similar atoms group together. Gravity allows liquids to drain through porous solids, so that water soaks into the soil and joins vast underground aquifers while the soil stays where it is. When vast blooms of tiny calcium-based marine creatures live and then die at the ocean surface, it’s gravity that coaxes them downward so that they drift toward the ocean floor. The vast marine cemeteries that sometimes form as a result in shallow seas compress, shift, and become distinctive white limestone. Salt deposits are formed because water molecules will evaporate easily to become a gas when they are given more energy, but salts won’t. The lava produced at volcanic mid-ocean ridges is far more dense than water, so it stays on the ocean floor, building new crust. And life itself is constantly plucking materials from the world around it, reshaping and reorganizing them, and then leaving the detritus to be reused when it dies.
On a dark night, looking up at the sky, we see waves that have traveled across our solar system or our galaxy or our universe to reach our eyes. For millennia, light waves were our only connection to the rest of the universe, the only reason we knew that there was anything else out there. A couple of decades ago, we started to observe the thin streams of matter that reach us: neutrinos and cosmic rays. And then gravitational waves came along, only the third way that we have of touching the rest of the universe. In February 2016, it was finally confirmed that catastrophic astronomical events like the merging of black holes also send out waves, ripples in space itself. Gravitational waves have been passing through all of us our entire lives, and we’re finally about to discover what we’ve been missing out on. The light and gravitational waves whooshing past our planet weave a rich tapestry that lets us map out our universe and then add an arrow labeled: “We are here.”
But on an average day on Earth, there are more immediate considerations. Standing outside our home and watching the world go by is a reminder of the gigantic system that we’re part of. We are a small sliver of the life that keeps the system running in its current configuration. When Homo sapiens first emerged, each human only had two life-support systems: a body and a planet. But now there is a third.
This planet has been altered by many species, but only in the past few thousand years has a single species knowingly rebuilt its environment to suit itself. It is almost a single organism now, a sprawling planet-sized web of interconnections between individual consciousnesses. Each individual is almost entirely dependent on others in the system for survival, but still has its own contribution to make. An understanding of the laws of physics is one of the pillars holding up our society, and we could not manage our transport, resource management, communication, or decision-making without it. Science and technology make possible the greatest ever collective human achievement: our civilization.
Civilization
A candle and a book. Portable energy and portable information, available on demand but with the potential to last for centuries. These are the threads that stitch individual human lives together to build something much bigger: a cooperative society that is always building on the work of the previous generation. Energy must keep flowing through our civilization, so the candle can be stored almost indefinitely but can only be used once. Knowledge accumulates, so one book may stimulate many minds. There were candles and books two thousand years ago and there are still candles and books now. They are simple technologies, but they work. We have built the modern world by storing energy and sharing information about what to do with it.