Discovering a unifying law of design in nature was not on my to-do list when I traveled to Nancy, France, in late September 1995. I was a forty-seven-year-old professor of mechanical engineering at Duke University who had come to deliver a lecture at an international conference on thermodynamics. Giving you a sense of how steeped my career was in mechanical engineering, I remember that I had brought flyers announcing the publication of my seventh book, Entropy Generation Minimization.
My work took a fateful turn during the prebanquet speech delivered by the Belgian Nobel laureate Ilya Prigogine. Echoing the scientific community’s conventional wisdom, this famous man asserted that the tree-shaped structures that abound in nature—including river basins and deltas, the air passages in our lungs, and lightning bolts—were aléatoires (the result of throwing the dice). That is, there is nothing underlying their similar design. It’s just a cosmic coincidence.
When he made that statement, something clicked, the penny dropped. I knew that Prigogine, and everyone else, was wrong. They weren’t blind; the similarities among these treelike structures are clear to the naked eye. What they couldn’t see was the scientific principle that governs the design of these diverse phenomena. In a flash, I realized that the world was not formed by random accidents, chance, and fate but that behind the dizzying diversity is a seamless stream of predictable patterns.
As these thoughts began to flow, I started down a long, uncharted, and wondrously exciting path that would allow me to see the world in a new, and better, light. In the sixteen years since, I have shown how a single law of physics shapes the design of all around us. This insight would lead me to challenge many articles of faith held by my scientific colleagues, including the bedrock beliefs that biological creatures like you and me are governed by different principles from the inanimate world of winds and rivers and the engineered world of airplanes, ships, and automobiles. Over time, I would develop a new understanding of evolutionary phenomena and the oneness of nature that would reveal how design emerges without an intelligent designer. I would also offer a new theory for the history of Earth and what it means to be alive.
In addition, I and a growing number of scientists around the world would begin finding new ways to make life easier: better ways to design roads and transport systems; to predict the evolution of civilization and science, of cities, universities, sports, and the global use of energy. We would unravel the mysteries of Egypt’s Pyramids and the genius of the Eiffel Tower while demonstrating how governments are designed like river basins and how businesses are as interdependent as the trees on the forest floor.
All that lay in the future when I boarded the plane for the trip home. High over the Atlantic, I opened my notebook (the old-fashioned kind, with paper) and wrote down the constructal law:
For a finite-size flow system to persist in time (to live), its configuration must evolve
in such a way that provides easier access to the currents that flow through it.
I was writing in the language of science, but the fundamental idea is this: Everything that moves, whether animate or inanimate, is a flow system. All flow systems generate shape and structure in time in order to facilitate this movement across a landscape filled with resistance (for example, friction). The designs we see in nature are not the result of chance. They arise naturally, spontaneously, because they enhance access to flow in time.
Flow systems have two basic features (properties). There is the current that is flowing (for example, fluid, heat, mass, or information) and the design through which it flows. A lightning bolt, for example, is a flow system for discharging electricity from a cloud. In a flash it creates a brilliant branched structure because this is a very efficient way to move a current (electricity) from a volume (the cloud) to a point (the church steeple or another cloud). A river basin’s evolution produces a similar architecture because it, too, is moving a current (water) from an area (the plain) to a point (the river mouth). We also find a treelike structure in the air passages of lungs (a flow system for oxygen), in the capillaries (a flow system for blood), and the dendrites of neurons in our brains (a flow system for electrical signals and images). This treelike pattern emerges throughout nature because it is an effective design for facilitating point-to-area and area-to-point flows. Indeed, wherever you find such flows, you find a treelike structure.
Since human beings are part of nature and governed by its laws, the point-to-area and area-to-point flows we construct also tend to have treelike structures. These include the transportation routes we follow to work (a flow system for moving people and goods), which include many smaller driveways and neighborhood paths flowing into a few larger roads and highways. So, too, do the flowing networks of information, material, employees, and customers that keep those businesses afloat. The engineered world we have built so that we can move more easily does not copy any part of the natural design; it is a manifestation of it. That said, once we know the principle, we can use it to improve our designs.
Although treelike structures are a very common design in nature, they are only one manifestation of the constructal law. In a simple example, logs floating on a lake or icebergs at sea orient themselves perpendicular to the wind in order to facilitate the transfer of motion from the moving air body to the water body. A more complex example is the design of animals that have evolved to move mass better and better (to cover more distance per unit of useful energy) across the landscape. This includes the seemingly “characteristic” sizes of organs, the shape of bones, the rhythm of breathing lungs and beating hearts, of undulating tails, running legs, and flapping wings. All these designs have arisen—and work together—to allow animals, like raindrops in a river basin, to move more easily across a landscape.
The constructal law dictates that flow systems should evolve over time, acquiring better and better configurations to provide more access for the currents that flow through them. Design generation and evolution are macroscopic physics phenomena that arise naturally to provide better and better flow access to the currents that run through them. The majesty of this principle is that it occurs at every scale. Each component of an evolving flow system—each rivulet, each tree, each road—acquires evolving designs to facilitate flow access. As these elements coalesce into larger and larger structures (into evolving river basins, forests, and transport networks), the various-sized components work
together so that everything flows more easily. We see this, for example, in the shape and structure of the neural networks in the brain, of the alveoli in the lung, and the human settlements on a map. In the big picture, all the mating and morphing flows on the largest system that surrounds us, the Earth itself, evolve to enhance global flow. E pluribus unum (one out of many).
The constructal law is revolutionary because it is a law of physics—and not just of biology, hydrology, geology, geophysics, or engineering. It governs any system, any time, anywhere, encompassing inanimate (rivers and lightning bolts), animate (trees, animals), and engineered (technology) phenomena, as well as the evolving flows of social constructs such as knowledge, language, and culture. All designs arise and evolve according to the same law.
This law tears down the walls that have separated the disciplines of science by providing a new understanding of what it means to be alive. Life is movement and the constant morphing of the design of this movement. To be alive is to keep on flowing and morphing. When a system stops flowing and morphing, it is dead. Thus, river basins configure and reconfigure themselves to persist in time. When they stop flowing and morphing they become dry riverbeds, that is, the fossilized remains of earlier “live” flow systems. The solid, treelike veins of ore found underground today, for example, are fossils of the fluid streams, eddies, and meanders that flowed before solidification a long time ago. Biological creatures are alive until all their flows (blood, oxygen, locomotion, and so on) stop, after which they, too, become fossilized remains.
This unifying definition marks an advance because it removes the concept of life from the specialized domain of biology. It aligns it (or, better, it juxtaposes it) with the physics concept of the dead state, which means “equilibrium with the environment” in thermodynamics: a system that is at the same pressure, the same temperature, and so forth as its surroundings, and hence, in which nothing moves. The constructal law defines life in physics terms, and it covers all live-system phenomena. It also reframes the view that life on Earth began with the rise of primitive species some 3.5 billion years ago. As we will see, “life” began much earlier, when the first inanimate systems, such as currents of solar heat and wind, acquired evolving designs. In the big history of life on Earth, the emergence and evolution of inanimate, animate, and technological designs tell a single story. Where Darwin showed the links between biological creatures, the constructal law connects everything on the planet.
On one level, the constructal law can be expressed through the language of mathematics, physics, and engineering. My colleagues and I have published hundreds of articles in leading peer-reviewed journals. My own books for specialists—including Advanced Engineering Thermodynamics; Shape and Structure, from Engineering to Nature; and Design with Constructal Theory—use the constructal law to predict the phenomenon of design configuration. Leading universities, from Paris and Lausanne to Shanghai and Pretoria, have hosted international conferences and courses on the constructal law.
You don’t need advanced mathematics to grasp it. The constructal law is also a way of seeing. Since discovering the law, I have witnessed thousands of people—from renowned scholars and professional scientists to my students at Duke and those at high schools I’ve visited—experience a moment of discovery like the one I had in Nancy. They, too, hear the penny drop. They see it. They get it. Through this book I hope to help you recognize how the constructal law is shaping everything around—and within—you.
Seeing constructally can be thought of as a three-step process. Step one starts with Leonardo da Vinci’s insight that “motion is the cause of every life.” I love this quote because it is so expansive. And yet, Leonardo didn’t take it far enough, because he was talking only about biological creatures. In fact, not only animals but also rivers, weather patterns, snowflakes, corporations, nations, science, knowledge, culture—you name it—throb and pulse with movement.
Even things that seem just to sit there are, in fact, flow systems. Take that quintessence of stagnation, the mud puddle. There it sits, murky and soupy. And yet, when the sun emerges after the rain, dry air begins to draw moisture from it because of the natural tendency toward equilibrium (in this case, of wet and dry). Before long, the puddle is gone. Soon the dirt begins to crack in telltale, treelike patterns in order to facilitate the flow of moisture from the ground to the air. That puddle is, in fact, a vibrant, morphing flow system. If we trained a movie camera on it, we’d see plenty of action.
Human beings are also flow systems, similar to but more complex than mud cracks. Internally, the flow of blood carries oxygen and food through a treelike network of blood vessels to organs whose size and shape are just right to enable us to move efficiently per amount of useful energy derived from food. The design of our bodies—just like that of every other animal from sharks to antelopes to great blue herons as well as that of trucks on the highway—has evolved to enable us to cover greater distances per unit of useful energy (food, fuel). And, like trees in the forest, we are also part of other, much larger, flow systems on Earth. When we get in cars, we enter the flow of traffic. In the office, the work we produce flows along with that of coworkers to reach customers through various channels. At the supermarket, tea that flowed from farmers and distributors in Sri Lanka settles into our shopping baskets. As we will see, all these seemingly independent designs are morphing and mating to facilitate our movement.
Step two is to recognize that all flow systems have the ten- dency to endow themselves with a characteristic that was not recognized until the constructal law—design. This property includes the flow system’s configuration (the architecture, geometry, shape, and structure) and its rhythm (the predictable rate at which it pulses and moves).
Design does not emerge willy-nilly. To know why things look the way they do, first recognize what flows through them and then think of what shape and structure should emerge to facilitate that flow. The configuration of a flow system is not a peripheral feature. It is the defining characteristic. In later chapters we will illustrate this by showing how the shape and structure of seemingly disparate phenomena—including rivers, fish, sprinters, economies, universities, and the Internet—are predicted by the constructal law.
Step three turns our drawing into a movie because designs evolve. Flow systems configure and reconfigure themselves over time. This evolution occurs in one direction: Flow designs get measurably better, moving more easily and farther if possible. Of course, there will be bumps and mistakes: Every trial involves error. But in broad terms, tomorrow’s system should flow better than today’s.
This is the natural phenomenon covered by the constructal law: the generation, ceaseless morphing, and improvement of flow design. This mental viewing enables us to recognize that people, birds, and other animals are flow systems that carry mass on the surface of the globe; that trees and mud cracks are flow systems for moving water from the ground to the air; that universities, newspapers, and books are flow systems for spreading knowledge across the globe. All generate designs that should evolve to better facilitate the flow of these currents. This insight allows us to recognize pattern in phenomena long dismissed as accident.
Consider the snowflake. The prevailing view in science is that the intricate crystals formed by the snowflake have no function. This is wrong. In fact, the snowflake is a flow design for dispersing the heat—called the latent heat of solidification—generated on its surfaces during freezing. As water vapor condenses and freezes it throws off its excess heat. When the ice crystal first forms, its spherical bead is the shape that grows faster than other shapes, the shape that facilitates rapid solidification. When the bead is large enough, needles emerge and enhance solidification (that is, produce ice) faster than the sphere. To facilitate solidification even more, larger snowflakes morph into shapes with more needles that disperse heat. Complexity is finite (modest), and is part of the constructal design that emerges. Complexity is a result, not an objective; not an artist’s wish; and, contrary to current dogma based in fractal geometry, it is certainly not “maximized.”
Now let’s take a closer look at the organized fury of an erupt- ing volcano—a flow system of lava. As it begins its journey through the shaft, the concentration of the mixture of molten rock is such that lava organizes itself into a series of concentric sheaths. In the center is lava of high viscosity (less runny); on the outside is lava of low viscosity (runnier). The low-viscosity lava that touches the solid rock helps it flow. When lava pours out of the volcano, another remarkable phenomenon occurs: The lava seems to select between two flow options, choosing the better way to move at any given time. If the molten rock is moving slowly, it oozes out of the volcano. If it is moving quickly, it generates a different flow configuration—a treelike structure with channels and branches—because this is the better way to move quickly. And, if we know the size of the area that the lava will spread across, we can predict the number of channels that will be generated.
What we are seeing is the mindless lava self-organizing into flow patterns to ease movement. This process happens everywhere in nature. Depending on its size and speed, a falling drop of liquid, for example, will become a splat (round disk) or a splash (crown shape). Smaller and slower droplets come to rest as splats. Larger and faster droplets come to rest as splashes. This phenomenon is well established. Your ink-jet printer, for example, depends on it, emitting specific quantities of ink at just the right speed in order to produce precise images. So does the forensic science of blood splatter popularized through TV crime shows. Before the constructal law, no one knew why this splat versus splash happens. As we will explore later in this book, these two shape-generating ways of flowing—slow and short, fast and long—are ubiquitous. In fact, most systems, including every beat of your heart, every breath you take, and the circuits that power your computer and brain, involve both types of flows. Striking the balance between them is a hallmark of natural design.
The constructal law also teaches us that evolution can be observed at all timescales, including during our own lifetime. When we speak of rivers and animals evolving to increase flow access, we are describing very gradual changes. But when lava generates design, droplets of liquid splash and splat, lightning bolts crackle in the summer heat, and snowflakes form against the winter sky, we are witnessing evolution right before our eyes. We can also watch it occur at home. For instance, if you throw some rigatoni into a pot of boiling water, you can watch the tubes tumble around in a disorganized fashion. After a few minutes, something amazing happens. Instead of lying flat, they begin to stand up straight, organizing themselves into a chimneylike pattern to facilitate the flow of heat and steam. If you prefer rice to pasta, boil some of that. When the water level drops enough, you will see equally spaced chimneys of steam escaping the entire mushy body. An exquisite tapestry of little volcanoes with round shafts is the easiest way for the heat to come out of the boiling mass, and they form every time. In both cases, the riddle of design is solved by asking what is flowing. The answer is not rigatoni or rice but heat and steam.
Similarly, if you drop a piece of toilet paper from the top of a tall ladder, it undulates so that it falls like a meandering river. Or when you pour a glass of dark beer, regularly spaced eddies emerge around the rim. In both cases, it is not the toilet paper or beer that is generating design but the momentum created when these objects fall. Because of the natural tendency toward equilibrium, the momentum (the movement) is transferred laterally to the surrounding still air and water through the design phenomenon of turbulence. In all instances, design emerges because things flow better with configuration.
Of course, there is no conscious intelligence behind these patterns, no Divine Architect churning out brilliant blueprints. To preempt any confusion, let me make this perfectly clear: The constructal law is not headed toward a creationist argument, and in no way does it support the claims of those who promulgate the fantasy of intelligent design. Anyone who takes excerpts from this book to suggest that I am arguing for a spiritual sense of “designedness” is engaging in an intentional act of dishonesty.
Instead, just as other impersonal, naturally arising phenomena such as gravity, the freezing points of fluids, and thermodynamics make things operate in a certain way, flow systems generate better and better flowing designs. Until now, we could only observe the patterns. The constructal law tells us why those patterns arise and empowers us to predict how they should change in the future. It reveals that it is not love or money that makes the world go round but flow and design.
This raises the question: How come? What causes the constructal law? The short answer: We don’t know. The constructal law is what is known in science as a first principle, an idea that cannot be deduced or derived from other laws (if it could, it would be a theorem). It just is—a law of physics that governs the emergence of macroscopic shape and structure in nature. Like all scientific laws, it is a concise summary that encompasses billions of observations of natural phenomena of the same kind. It addresses two of the biggest questions in science: Why does designedness” (configuration, rhythm, scaling rules) happen everywhere in animate and inanimate systems alike? Why does the design-generation phenomenon persist in time?
The constructal law is a shout from the rooftops: Everything that flows and moves generates designs that evolve to survive (to live). This is not a desire or objective but the natural tendency, that is, the physics phenomenon.
Excerpted from Design in Nature by Adrian Bejan and J. Peder ZaneCopyright © 2012 by Adrian Bejan. Excerpted by permission of Anchor, a division of Random House LLC. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.