The Thermodynamics of Empire

Energy is the lifeblood of all societies. Just as we can understand fundamental things about the human body by tracing its flow of blood, we can understand much about a society's activities by tracing its flows of energy.

Through the ages, the ways in which human beings have produced and used energy have sporadically advanced, and each advance has defined a new stage in technology and the civilization it sustains.8 Ancient Romans, for instance, not only lacked rock-cutting machines, they also had no dump trucks or hoists powered by electricity, gasoline, or diesel fuel. Almost all the work they applied to the job of cutting, moving, and lifting stone and other materials was done by human and animal muscles; the energy fueling those muscles came of course from food, and the food was mainly grain and hay, such as wheat, barley, millet, rye, spelt, and alfalfa grown by Roman farmers (most Romans ate only very small amounts of meat).9

We readily agree that energy is vital, but it's harder to grasp its role in our affairs.10 When we think of energy, we usually think of the gasoline that fuels our cars, the electricity that lights our homes, and maybe the natural gas and coal that we burn in our power plants. In other words, we usually think of energy as fuel, and we tend to think that it's useful because of the immediate services this fuel provides—services like transport, light, and heat.

Of course, these are critical services, but energy's role in our lives is actually much more fundamental, essential, and subtle. We extract energy from our environment to create order out of disorder and complexity out of simplicity.11 We often use this order and complexity, in turn, to help us solve the problems we face—for instance, to shelter ourselves from our harsh environment and to protect ourselves from attack. Put simply, societies with access to lots of energy are generally more adaptive, resilient, and better at solving problems.

To understand the link between energy and complexity, we need to explore some of the scientific principles that govern energy's behavior. Although people usually think of energy as a tangible thing, like gasoline, it's actually not a material substance at all. It's really a property of things. A handful of wheat kernels, a lead-acid battery, a stream of photons of light, or a rushing river all have the property of energy. This property can move from one place to another, and when it moves we can sometimes use the resulting flow of energy to do what physicists call "work"—that is, to change things in our physical world. For example, the energy in a rushing river can do work for us when it moves to another system like a water mill or turbine. The energy in a lead-acid battery does work only when it flows to a light bulb or an electric motor.

Some sources of energy are more useful for doing work than others, provided that we know what to do with the energy available in them. The high-quality energy available from gasoline, for instance, is better for doing work than the low-quality energy of the diffuse heat present in the ground under our feet, which isn't much good for anything, except perhaps heating other things, like our buildings and homes.12 So sources of high-quality energy, such as oil, natural gas, and rushing rivers, are extraordinarily valuable, because we can use them to provide many different types of service.13

There are, fundamentally, only two forms of energy: kinetic energy, which is the energy of material in motion, and potential energy, which is energy that's trapped in something. We can convert potential energy to kinetic energy and vice versa. When we burn gasoline to power a car engine, we're converting the potential energy in the gasoline's chemical bonds to the kinetic energy of the engine's motion. On the other hand, when we use a river's rushing water to turn an electric turbine and then use the turbine's electricity to charge a battery, we're transferring the river's kinetic energy to the turbine to do work, and then we're using this work to create potential energy in the battery.

Heat is a special type of kinetic energy. It's stored in a material like the walls of our house or the air around us by the vibrations and other motion of the atoms and molecules making up that material. We often use heat as an intermediary form of energy to run our machines. For example, when gasoline is ignited in the cylinders of a car engine, high-temperature gases are created, and the heat of these gases provides the kinetic energy that drives the engine's pistons. Heat also drives our jet and rocket engines. Understanding the nature of heat can tell us important things about the flow of energy in natural and human-made systems, such as how much work a given system can do.

This is the province of the sub-field of physics called thermodynamics. The nineteenth-century discovery of the laws of thermody-namics—among the most important in all science—was a stunning breakthrough. These laws tell us two essential things about our natural world. The first says that energy can't be created or destroyed: the total energy of any system and its surroundings (generously defined) stays constant, whether the system is a mechanical device like a car engine, a biological system like a human body, or a social system like ancient Rome's.14 The second law says that during the normal operation of most systems, energy degrades in quality: high-quality energy degrades to progressively lower-quality energy, with the end result being simply low-grade heat.15 It's as if energy always flows down a slope from forms that we can use to do lots of work—like hoisting rocks in the Colosseum—to forms that aren't very useful to us at all. And every time we use energy to do work, we further degrade its quality. As a system's energy degrades, physicists say its "entropy"— often described as its disorder or randomness—increases.

To see all of this better, imagine you're sealed inside a big box with a newspaper, a chair, and a battery wired to a light bulb. Let's assume that the box is isolated from the rest of the world, so energy can't flow into or out of it. Inside the box, high-quality chemical energy is concentrated in the battery; it's also concentrated in you in the form of the sugars and proteins that you've obtained from food and that power your body. Physicists say that such energy is "coherent" and "ordered." Sitting inside the box, you can read the newspaper because the battery converts its chemical energy into electrical energy that the bulb then transforms into light energy. That light—speeding away from the bulb—hits the newspaper, walls, and other things in the box where it's absorbed and degraded into low-quality heat. The light's energy is essentially spread around the box as heat, slightly increasing the incoherent and disordered vibrations of the molecules making up the things in the box. But the chemical energy in the battery is finite, so eventually the battery is exhausted, and the light goes out.

The light won't last forever in a closed system.

You (the person in the box) are much like the battery: you convert your chemical energy into other forms of energy, such as kinetic energy as you move and lift things. If there's a bicycle generator in the box, you can hop on board and use it to drive the light and finish reading your newspaper, warming up the box even more as the heat from your exertion is carried away from your body by your sweat. But just like the battery, you too will run down and eventually die. All the high-quality energy in the battery and your body will be then degraded, and the box itself will wind down—just the way a mechanical clock winds down—to a black three-dimensional space of uniform temperature.

We'll see later that societies that don't have access to enough high-quality energy are likely to disintegrate. The laws of thermodynamics tell us that despite the fact that energy can't be created or destroyed, it does inevitably degrade, which makes it progressively less useful for work. And if it's less useful for work, it's less useful for maintaining a society's complexity and resilience.

Our light-in-a-box illustration is entirely imaginary and artificial. It's a "closed" system, which means it's sealed off from the outside world. Scientists use such imaginary systems to tease out implications of their theories. But in the real world almost all physical, biological, technological, and social systems are "open." They interact with their surroundings. Most important, these systems often extract high-quality energy from their environment to do work or to reduce disorder at their core, and they expel waste heat and material back into that environment. A city like ancient Rome, for example, imported from its hinterland timber, fresh water, and energy in the form of food and released back into its surroundings heat, sewage, and garbage (at its peak population, Rome probably produced around one million cubic meters of human waste each year).16 A modern car engine takes in fuel to do the work of moving the car, and it expels heat and exhaust gases into the atmosphere. A steel mill takes in raw materials like iron ore and high-quality energy (in the form of coking coal, for instance) to create coherent and ordered materials like steel rods; in the process, the mill discards heat, carbon dioxide, and pollution into nearby air and water.

At first glance, open systems appear to violate the classical thermodynamic principle that disorder, randomness, and entropy always increase. After all, the mill produces low-entropy steel rods. And over time Rome's internal arrangements became more ordered and complex, as its various social and technological parts became more diverse, specialized, and interdependent.

The principle that a system's entropy must always increase, scientists eventually realized, applies only when its boundaries are defined to encompass virtually all its interactions with its surrounding environment. The system of a steel mill then includes the entire technological infrastructure that produces the iron ore and coke it uses as well as the atmosphere and waterways into which it expels its pollutants. And the system of ancient Rome included the solar energy provided by the sun and the city's entire natural hinterland of land, forest, water, and air. Within this generous boundary, the average quality of the system's energy always declines, and entropy always increases.

All the same, there can be parts of the broader system that have a very high degree of order: Rome was a zone of low entropy within its larger system. In fact, things like cities, ecosystems, and even our human bodies can create order and complexity spontaneously, decreasing their entropy even further in the process.17 Cities build elaborate transportation, water, and energy infrastructures; ecosystems become more biologically diverse as new species evolve; and human embryos develop into people, with all their complex organs and structure. How do such amazing things happen?

Scientists still aren't sure. But they now know that systems like cities, ecosystems, or human bodies are, as they say, "far from thermodynamic equilibrium."18 They can spontaneously create order inside themselves. But maintaining this order is a bit like holding a marble on the side of a bowl with your finger: the marble wants to sit at the bottom of the bowl—that's its equilibrium point; so holding the marble on the side takes a constant input of energy. Similarly, cities, ecosystems, and human bodies must have a constant input of high-quality energy to maintain their complexity and order—their position far from thermodynamic equilibrium—in the face of nature's relentless tendency toward degradation and disorder. And, as the system gets larger and more complex, more and more energy is needed to keep it operating.

All these ideas can help us grasp why the Roman empire fell and, ultimately, discern the fate of our own societies. The Romans employed astonishing technological prowess to construct buildings like the Colosseum. Less obviously but just as critically, they needed considerable social prowess to assemble themselves into work units, coordinate the efforts of these units, encourage specialization skills, and provide themselves with public services like governance, tax collection, and security. Codified laws regulated everything from money and debt to property rights, corporate organization, guilds, and the employment of laborers and slaves.19

Complex social organization doesn't appear out of thin air. Courts must be staffed, functionaries paid, and armies fed and supplied with weapons. More fundamentally, to create and sustain organizations, rules, and laws, people must move around, communicate, discuss, argue, and negotiate with one another; educate and train one another; and record—in some stable medium like rock, parchment, or CD ROM— basic rules, contracts, and bargains. All these activities once again require high-quality energy.

So the Romans used farms to capture the sunlight falling on wide swathes of land around the Mediterranean basin. Some of the farms existed before the Romans arrived. Especially in the eastern Mediterranean—for instance, in modern-day Lebanon and Syria—the expanding empire often simply took over existing cities and their food-production and tax systems, while in northwestern Europe, in places like the Rhône valley, new land was sometimes converted into farms. But wherever the farms were located, they played a role in the Roman energy economy similar to that of solar battery chargers: they converted sunlight into a form of high-quality potential energy, especially fodder and grain, that was storable and transportable.

The Romans then focused this energy—they used their food batteries, so to speak—to create a productive, resilient, and phenomenally complex system of public buildings, manufacturing facilities, housing, roads, aqueducts, and social organization. And here's the punch line: recent research (which we'll come to later) shows that the Roman empire was eventually unable to generate enough high-quality energy to support its technical and social complexity. This shortfall—more than proximate events like incompetent emperors and invasion by Visigoths—was the fundamental cause of Rome's fall. In other words, the empire's loss of internal order, coherence, and complexity was, in significant part, a thermodynamic crisis. The empire tipped into irreversible decline precisely because it couldn't feed its energy hunger.20

This was Rome's fate. Will it be ours as well? A closer look at energy's role in ancient Roman society will help us find out.

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