Storm in a Teacup: The Physics of Everyday Life(34)
This rock is granite. It has not moved or changed in human memory. But four hundred million years ago there was a giant volcano in the southern hemisphere, and magma from below squeezed into the gaps in the volcanic rock. Over the following millennia the magma cooled, separating slowly into crystals of different types, and became hard unyielding granite. As more time has passed, the rocky leviathan has been ground down by ice ages, chipped away by plants and ice, polished by rain. While the volcano was wearing away, it was also traveling. Since the giant explosion that finished it, this chunk of continent has been creeping north. On top of it, species and geological eras came and went as the machinery of the planet shunted the ill-fitting jigsaw pieces of its surface together and apart. Today, a tenth of the total lifetime of our planet later, all that is left of the original dramatic volcano is the sorry remains of its exposed guts. We call it Ben Nevis, the highest mountain in the British Isles.
When you and I look at either the mountain or the raindrop, we notice very little change. But that’s just because of our own perception of time, not because of what we’re looking at.
We live in the middle of the timescales, and sometimes it’s hard to take the rest of time seriously. It’s not just the difference between now and then, it’s the vertigo you get when you think about what “now” actually is. It could be a millionth of a second, or a year. Your perspective is completely different when you’re looking at incredibly fast events or glacially slow ones. But the difference hasn’t got anything to do with how things are changing; it’s just a question of how long they take to get there. And where is “there”? It is equilibrium, a state of balance. Left to itself, nothing will ever shift from this final position because it has no reason to do so. At the end, there are no forces to move anything, because they’re all balanced. The physical world, all of it, only ever has one destination: equilibrium.
Imagine a lock gate in a canal. Locks were invented for the most ingenious of reasons: to allow boats on a canal to climb hills. They work because boats can propel themselves forward against water flow, but only if that water flow is really slow. No canal boat can power up a waterfall, but with the help of a lock, a boat can still climb a hill. A lock consists of two sets of gates which form a complete bottleneck in a canal, trapping an isolated pool of water between them. On one side of the lock the water is higher; on the other, it’s lower. Anything wanting to go up or down the canal has to go through the lock. Let’s say there’s a boat waiting at the bottom. The water in between the gates is initially at the same height as the canal at the bottom. The lower gates open, our boat chugs into the lock, and the lower gates close. Now the top gate is opened, just slightly, and water flows into the lock. This is the important bit. When the top gates were closed, the water above the lock had no reason to go anywhere. It was sitting in the lowest place it could be, in equilibrium. There was nowhere better for it to be, and it would stay put there indefinitely. But as soon as a gap is opened that connects it with the pool of water between the gates, this changes. Suddenly, there’s a route to somewhere better. Gravity is always pulling the water downward, and we’ve just opened the door for the water to respond to gravity’s pull and move itself even farther downward. So it flows in to join the boat, and it keeps filling the lock up until the water height inside it is the same as the water height above the lock. No one had to do anything other than provide the route to a new equilibrium. But now the boat is at the same height as the top part of the canal, and once the gates are fully opened it can chug along on its way upstream, against the very slow canal flow. Behind it, once the gates are closed again, everything is in equilibrium. The water between the locks will stay there indefinitely because it has nowhere better to be. All the forces are balanced. Then at some point a boat will enter the lock from upstream, someone opens the lower gate, and the water is allowed to flow out into the downstream canal, where it will continue on its way to a new equilibrium.
The lesson of all this is that you can get a lot done in the world by controlling where the equilibrium position is. Left to themselves, things shuffle around until everything is balanced and then they stay there. The way to get things done is to be in control of where equilibrium is. If you can move the goalposts on demand, you can make sure that things flow in the direction you want them to go in, and only when you say so.
The idea that the physical world will always move toward balance—that hot and cold liquids will mix until everything is the same temperature, or that a balloon will expand until the pressure is equal inside and out—is related to the concept that time only flows one way. The world can’t run backward. Water is never going to flow by itself through a lock from the lower level to the higher level. That means that you can tell which way is forward by looking for systems moving toward equilibrium. While moving things by brute force will cost you a lot of energy, influencing the speed of the slide to equilibrium often costs very little. It is also often extremely useful.
The Hoover Dam is one of the biggest civil engineering achievements of the last century. Driving toward it from Las Vegas, you weave through a red rocky landscape where it seems impossible that anything large could be hidden. The only clues that there might be something unusual nearby come from occasional glimpses of sparking blue water completely out of place in the middle of a desert. And then you turn a corner and there it is, all 7.5 million tons of it: a giant concrete stopper lodged in the middle of this rugged American landscape.