Storm in a Teacup: The Physics of Everyday Life(85)
As we walk, we use fluid tucked inside our skulls to help with our balance. The fluid sloshing around deep inside the tiny cavity of our inner ear keeps going when we stop and lags behind when we start. Sensors on the walls of that cavity feed that information into the giant connected network of our brain, helping with the decisions about which muscle to move next.
On this occasion, we reach a door, push it open with our free hand, and step outside.
Earth
Outside, we can look sideways through the invisible atmosphere, at the rest of the world. Our planet is a system made of five interacting components: rocks, atmosphere, oceans, ice, and life. Each one has its own rhythm and dynamics, but the sumptuous variety that we see on Earth is the result of the timeless dance that connects them. The same forces drive them all, and there are similarities in surprising places. As we look out through the invisible molecules that fill the sky, packets of air are shifting according to their buoyancy. Air that has been warmed by the building we’ve just stepped out of is rising because it’s less dense than the air around it. Columns of rising air from warm ground could be a few miles tall, taking perhaps five minutes or so to rise each mile. Cooler, denser air is flowing in beneath to take its place, hauled downward by Earth’s gravity. These patterns of convection stretch out across the landscape we’re looking at. Air is never completely still.
If we were looking sideways across the surface of the deep ocean, our view might encompass similar buoyancy flows, also invisible. Cold, salty water in the North Atlantic sinks downward toward the center of the Earth, just like the cooler, denser air. Once it reaches the ocean floor, it flows sideways across the seabed until it warms or mixes with less salty water and floats back to the surface. In the sky, one cycle of floating up and sinking down might take a few hours. In the ocean, one cycle might take four thousand years, and the water will be carried around half the globe as it happens.
And then, down below our feet right now, the rocks themselves are also moving. The Earth’s mantle makes up most of the planet, a thick layer between the outer core and the thin crust floating on top. It’s liquid but viscous, sluggish and slow. This melt is being heated, both by the Earth’s hot core and by the slow decay of radioactive elements buried deep inside it. This shunting of energy around the deep rocks is happening now, beneath all of us. As the hot mantle rock becomes buoyant, it floats upward and cooler rock sinks down to take its place. But molten rock at these temperatures and pressures takes time to move. Deep below us, a mantle plume might take a year to float upward by three-quarters of an inch. A full cycle from the bottom to the top and back again might take 50 million years. But the center of the Earth bows to the same physics as the atmosphere and the ocean, continuously shifting heat from within to without.
A vast amount of heat energy is continuously moving outward from the center of the Earth, but it’s utterly insignificant compared to the amount of light energy from the Sun striking our planet. And in almost every environment on Earth, tucked away in corners or dominating the landscape, there is green. It might be a furtive veneer of moss on a brick wall or the luxuriant biological architecture of a rainforest, but plants are everywhere. Each leaf is the support structure for layers of chlorophyll-stuffed cells, each one a tiny molecular factory turning sunlight and carbon dioxide into sugar and oxygen. A fraction of the energy in the flood of light washing over each leaf is captured and stashed away as sugar: fuel for the future. Even on the calmest of sunny days, in a field where everything looks still and unchanging, the plants are busy. One molecule at a time, they are producing the oxygen that we breathe, enough to keep all the other living things on Earth alive, enough to maintain an atmosphere that is 21 percent oxygen. These tiny molecular machines are continuously remaking a fifth of our entire planet’s atmosphere. As we look sideways through the air, we’re looking through the jostling molecular output of millions of ferns, trees, algae, grasses, and more, produced over thousands of years: the bounty of a green army.
From our position on the ground, just outside our home, we can see only a small fraction of our planet. Suppose we can levitate, and we’ll see far more. As we rise through the atmosphere, the air molecules spread out. Gravity is pulling them downward, and it can only hold a very thin layer to the Earth’s surface. As we rise past the top of the largest thunderstorm, approximately 12 miles up, 90 percent of the molecules in the atmosphere are beneath us. The deepest point in the ocean is 7 miles below sea level, and below that, there is dense rock for about 4,000 miles before you get to the center. Without a rocket, we humans are confined to a vertical range of a measly 18 miles, playing on the edge of the giant planet we call home. A layer of paint coating a ping-pong ball has the same thickness relative to the sphere it covers.
At a height of 62 miles, we are officially at the boundary between Earth and outer space, and we can see the globe rolling past beneath us—green, brown, white, and blue, spinning in the blackness of space. From up here, the scale of the ocean is shocking: a planet-sized shell made of one simple molecule repeated over and over again. Water is the canvas for life, but only in the Goldilocks zone,* the energy range within which the molecules move about as a liquid. Give those molecules extra energy, and their vibrations will shake apart any complex molecules that they house. More energy still, and they will float away as a gas, useless for protecting fragile life. At the lower end of the Goldilocks range, as you reduce the energy, the vibrations slow until the molecules must slot themselves into an ice lattice. Immobility like that is the enemy of life. Even the process of building these inflexible ice crystals can burst any living cell that contains them. Our planet is special not just because it has water, but because that water is mostly liquid. From our vantage point here on the edge of space, Earth’s most precious asset dominates the view.