Storm in a Teacup: The Physics of Everyday Life(35)
A hundred years ago, the Colorado River ran unfettered through its narrow canyon. Rain from high up in the Rocky Mountains and the vast plains to the east was funneled downhill through a series of valleys and out to the Gulf of California. The problem for the farmers and city dwellers downstream wasn’t the amount of water—there was plenty of it—but the timing of its arrival. In the spring, huge floods could wash away fields, but by the autumn only a feeble trickle was left, not enough for the growing population. The water was always going to start in the same mountains and plains, and end up in the same bit of ocean. But what farmers and townspeople alike really needed was to control when it got there,§ and especially to stop it all arriving at once. And so the stopper was built.
A drop of water that’s made its way off the Rockies and all the way down through the Grand Canyon now finds itself in Lake Mead, the giant reservoir that has built up behind the dam. It hasn’t got anywhere else to go, at least not for a while. The key thing here is that the droplet is held where it is—high up—because it can’t go down any farther. In 1930, a droplet leaving the Grand Canyon would have trickled 150 meters downward before it came to rest. But after 1935, when the dam was completed, that same drop could reach this point and still be 500 feet above the valley floor. The amazing thing is that it doesn’t take any energy to keep it there, just a carefully placed obstacle to stop it going anywhere else. It’s in human-created equilibrium and it’s staying put.
Until, of course, humans decide that they want it to go somewhere else. Humans can control the flow through the dam, rationing the water that feeds the rest of the Colorado River. There are no more floods downstream, and the river never stops flowing completely. And there’s another benefit. As the marshaled water flows past the dam, the huge pressure that has built up turns turbines that produce hydroelectric power. The consequences of this water shunting are that hundreds of thousands of people can live and work in the arid deserts of the American Southwest.
The Hoover Dam was built to control the timing of water flow, but the principle it demonstrates goes far beyond water use. When it comes to harvesting energy, all we are ever really doing is providing a few obstacles to energy that was already on its way from somewhere to somewhere else. The physical world will always move toward equilibrium, but sometimes we can control where the nearest equilibrium is and how quickly something in the world can get there. By controlling that flow, we also control the timing of energy release. Then we make sure that as the energy flows through our artificial obstacles on its way toward equilibrium, it does something useful for us. We don’t create energy and we don’t destroy it. We just move the goalposts and divert it.
Like many civilizations before ours, we face the problem of limited resources. Fossil fuels are made up of plants that built themselves using energy from the Sun, diverting that energy from its alternative outlet: gentle warmth, which is the equivalent of the bottom of the river when it comes to usefulness. Fossil fuels are the energy equivalent of dams, a form that stores energy in a temporary equilibrium. When we come along, dig them up, and provide the right kick, we’re choosing the timing of energy release by providing a route to another accessible equilibrium, via a flame and chemical decomposition to carbon dioxide and water. The problem we have is that there are only so many “upstream” resources in the form of fossil fuels, and in a few human lifetimes we have released energy that took millions of years to accumulate. The fossil fuel reservoirs are being emptied, and they will not be refilled for millions of years more. Renewable energy, like the hydroelectricity from the Hoover Dam and many others like it, diverts the waterfall of solar energy that is flowing through our world now. The game facing our civilization remains the same: How do we stop and start the energy flow efficiently, so that we can do what we want without changing our world too much?
Next time you turn on something that is battery-powered, you’re choosing the time of energy release from the battery by opening an electrical gate, and guiding the energy through the circuits of the device to help you do something useful. After that, it’ll end up as heat, which it would have done anyway. This is what the switches in our world are, all of them. They’re the gatekeepers controlling the timing of a flow, and the flow is only ever going one way: toward equilibrium. If we let the flow whoosh through all at once we get one result; if we slow it down, letting it trickle through at times that suit us, we get an entirely different result. Time matters here because it’s only ever going in one direction: By choosing when the flow toward equilibrium happens, and the speed of that flow, we give ourselves enormous control over the world. But it’s not always the case that things reach equilibrium and then stop. If they’re going really fast as they approach the balance point, they may well just keep going and fly right through. This opens the door to a whole new set of phenomena, including some problems.
Mid-afternoon tea break is an essential part of my working day. But I noticed recently that even acquiring a mug of tea forces me to slow down, and it’s not just about the time taken to boil the kettle. My office at University College London is at one end of a long corridor, and the tearoom is at the other end. The journey back to my office, accompanied by a full mug of tea, happens at the slowest pace of my entire day (my normal walking speed at work is somewhere between “brisk” and “race pace”). It’s not that there’s too much tea in the mug; the problem is the sloshing. Every step makes it worse. Any sensible person would accept that slowing down is a reasonable solution. But any physicist would do some experiments first, just to see whether that’s the only solution. You never know what you might find. And I wasn’t going to give in to the obvious without a fight.