This book is about design in nature as a scientific discipline, centered on a physics law of design and evolution: the constructal law. This law sweeps the entire mosaic of nature from inanimate rivers to animate designs, such as vascular tissues, locomotion, and social organization.
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 (Figure 1).
Figure 1. The phenomenon of design in nature unites the inanimate with the animate. The left side shows the delta of the Lena River in northern Siberia. The right side shows a cast of the human lung.
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 (Figure 2).
Figure 2. Animate and inanimate phenomena of generation-of-flow configuration in nature, which have been predicted based on the constructal law. Top row: river drainage basins, bronchial trees, and round-duct and open-channel cross sections. Middle row: cracks in shrinking solids, snowflake solidification, and splat versus splash when a liquid droplet hits a wall. Bottom row: laminar versus turbulent flow and animal locomotion (flying, running, and swimming).
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 (Figure 3).
Figure 3. Mud cracks on the banks of the Luangwa River, Zambia.
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 tendency 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 erupting 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 (Figure 4). 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.
Figure 4. Rice volcanoes: the regular pattern of vertical ducts constructed by the flow of steam during the boiling of rice.
Figure 5. The free fall of a piece of toilet paper makes visible the constructal design phenomenon of turbulence. When the fall is fast enough, eddies of air are configured on both sides of the paper, because this is the more efficient way of transferring vertical motion (momentum) from the paper to the surrounding air. The momentum is transferred laterally, away from the falling airstream. The paper is highly flexible and makes the turbulent eddies visible, looping around them like a skier through slalom gates.
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 (Figure 5). Or when you pour a glass of dark beer, regularly spaced eddies emerge around the rim (Figure 6). 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.
Figure 6. More constructal design of turbulence in a glass of dark beer. The momentum from the falling liquid is transferred more effectively to the body of stationary liquid by a design of eddies regularly spaced around the rim. The bubbles gather on the surface only above the regions that correspond to downward flow.
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.
As a first principle, the constructal law does not start from observation. It is a pure theory, a purely mental viewing of how things should be. We don’t catalog and measure every river (or bird, tree, lightning bolt, etc.). Instead, we discover mentally just one of them, and one is enough—it is the cat out of the bag; it keeps us awake until we assure ourselves that nature is the way in which the principle painted it for us in the mind, in the dark of the night. In its streamlined form, our use of the scientific method has three steps:
To appreciate an important advantage of this theory, consider the work of Robert Elmer Horton (1875–1945), the soil scientist whose achievements were so great that the Horton Medal, the highest honor bestowed by the American Geophysical Union, was named in his honor. One of his achievements was the study of the number of tributary streams that feed each larger river channel. He and his associates spent years poring over empirical data, studying maps, and counting river channels to find that the average number of daughter streams flowing into the mother stream is a number between 3 and 5.
Three colleagues and I found the same scaling rule with pencil and paper using the constructal law. We imagined a very simple river basin and asked what flow structure (in this case, how many tributaries) posed less and less resistance for a given volume rate of water input (the streams) to the territory (river basin). The answer we arrived at was four. No doubt, Horton’s empirical work made it easy for us to verify our findings. But had he known about the constructal law, he would not have had to perform innumerable measurements to reach the same conclusion.
Indeed, once we recognize that the constructal law governs design in nature, we can predict all configurations using only our minds. Such is the power of theory.
In the sixteen years since the conference in Nancy, I and many other researchers have not found a single flow system that cannot be predicted by the constructal law. Specialists are using it to illuminate a wide range of subjects, including linguistics and sociology, nuclear decontamination, globalization, finance, warfare, patterns of residential segregation, and human mortality. The applications are so numerous that the constructal law is still in its infancy. You, dear reader, are in on the cutting edge of an emerging idea that has only just started to flow on the globe and into books.
If I were to add two words to the constructal law, they would be these: “given freedom.” Constraints abound in our world, preventing things from organizing themselves in more efficient ways. A dam, for instance, stops the river from flowing; bad ideas make it harder for human beings to thrive. I learned that lesson growing up in Romania during the 1950s and 1960s, when it was ruled by a Soviet-imposed government. The Russians had a crummy system and decided to force it on their free and more advanced neighbors. Like all territories, Romania is a flow system for many things, including commerce and ideas. For decades, the communist government choked off those flows, and my native country foundered. The popular uprising in what was then Czechoslovakia led to the Prague Spring in 1968, during which some restrictions were loosened. Romania held a mathematics competition: The six winners from across the nation would be allowed to apply to study abroad. I earned a top score and later was accepted by MIT, where I went on to earn all my engineering degrees. That small access to freedom enabled me—I, too, am a flow system on the landscape—to remake myself, that is, to redesign my movement on Earth.
Rigid governments lacking the ability to change are just one manifestation of the inevitable forms of resistance that obstruct flow. Instead of struggling under dictators or totalitarian governments, flow configurations evolve in one direction in time: to reduce the effects of friction and other brakes that inhibit their flow. Resistance is inevitable and unavoidable. It is why the world will never be a perfect place and why the most flow systems can accomplish is to keep getting better, that is, to be less and less imperfect. Thus the constructal law suggests the idea of progress, conveys the promise of hope: Given freedom, flow systems will generate better and better configurations to flow more easily.
In my academic life, I was particularly attuned to this phenomenon—able to see what others had missed—because I had, quite by accident, grappled with the same problem faced by rivers and trees through my research as an engineering professor at Duke University and as a consultant for industry and government. We engineers are rarely thought of as cool, but my specialty is designing smaller, more efficient systems for cooling electronics. In general, the more computational power you generate, the more heat you create. Run your hand along the bottom of your laptop or the screen of your plasma TV—you could almost fry an egg on them! For decades I used mathematics and the laws of physics to develop better designs for guiding that heat through and out of the box.
I noticed but did not think much of the fact that the drawings I was producing corresponded to the treelike flow structures that appear in nature. Before Prigogine’s speech in 1995, I had never put two and two together and seen that a universal principle explained why Mother Nature and I were arriving at similar answers. The “click” I experienced that evening made me lift my eyes from my work and consider the shape and structure of everything around me. It made me wonder: What generates all these configurations? Why does this geometry happen?
I am not the first person to ask these questions. The only thing rarer in science than the eureka moment of discovery is the lone researcher who makes a discovery completely on his own. Darwin, for example, was one of many scientists exploring the evolution of species. It was his genius to imagine mechanisms such as natural selection through which evolution occurs in biology. But knowledge is not static. The human mind persistently seeks better answers to ancient questions, better understandings to ease the flow of information.
Design in nature is generating a lot of excitement today over the entire range of science—from geophysics and biology to social dynamics and engineering. The interest is fueled by two trends:
1. A voluminous body of knowledge has accumulated, and it shows that features our minds perceive as design (configurations, rhythms, scaling rules) are present in all flow systems in nature.
2. Design phenomena are not covered by the existing laws of physics.
The empirical knowledge has far outpaced the theoretical framework that is needed to support it. This kind of mismatch is the ammunition and trigger for scientific revolution. If science is an evolving animal design, then the animal has become too heavy and has no alternative but to develop a larger skeleton for itself.
From the clash between the empirical and the theoretical comes the better science, the larger skeleton that includes a law to support all the phenomena of design and evolution in nature. Many other scientists have offered their own insights into the riddle of design in nature. To varying degrees these include fractal geometry, complexity theory, network theories, chaos theory, power laws (allometric scaling rules), and other “general models” and optimality statements (minimum, maximum, optimum), as well as Charles Darwin’s seminal work and D’Arcy Thompson’s magisterial volume On Growth and Form.
My work is not a response to, or critique of, their efforts. In fact, I became acquainted with this vast literature only after discovering the constructal law in 1995. What I did know at the time was thermodynamics, the science of how to convert heat into work and work into heat. Work represents movement and flow against forces that resist. Thermodynamics rests on two laws. Both are first principles: The first law commands the conservation of energy, and the second law summarizes the tendency of all currents to flow from high (temperature, pressure) to low. These two laws are about systems in the most general sense, viewed as black boxes, without shape and structure.
Not appreciated then was that the two laws of thermodynamics do not account for nature completely. Nature is not made of black boxes. Nature’s boxes are filled with configurations—even the fact that they have names (rivers, blood vessels) is due to their appearance, pattern, or design. Where the second law commands that things should flow from high to low, the constructal law commands that they should flow in configurations that flow more and more easily over time.
It occurred to me that if physics is to cover nature completely, it must be endowed with an additional first principle that accounts for the phenomenon of design generation and evolution everywhere and in everything. The constructal law is this new addition.
Previous attempts to explain design in nature are based on empiricism: observing first, thinking and explaining after. All these attempts articulate conclusions about observations that have accumulated into a body of knowledge. They are backward-looking, descriptive, and explanatory, not predictive. Darwin, for example, gathered all his observations about the evolution of biological creatures and created a convincing narrative that fit those known facts, which has been borne out by subsequent findings. Likewise, fractal geometry is descriptive, not predictive. Proponents of fractal geometry create mathematical algorithms to manufacture images that look like natural phenomena, such as snowflakes, lightning bolts, and trees. The algorithms they devise in order to draw these images are not derived from principle but from trial and error. The algorithm that the mathematician chooses in order to draw the tree in the garden is analogous to the brush and paint that the painter chooses in order to depict the same object. The mathematician shows us only the algorithms and drawings that come out right, not those that look like nothing. The painter does the same.
The constructal law does much more than explain the designs we see in nature. It articulates a law we can use to understand why designs emerge and predict how they will evolve in the future.
In recent years, many members of the scientific community have begun questioning the strictures of Darwin’s work—combating what the biologist J. Scott Turner has called, in his 2007 book The Tinkerer’s Accomplice, the “pernicious tendency for the convenient assumption to become unquestioned dogma.”
Not surprisingly, evidence of design in nature has sparked this robust inquiry. While delivering the prestigious 2007 Gifford Lecture at the University of Edinburgh, the British paleontologist Simon Conway Morris argued that evolution shows an eerie predictability, leading to the direct contradiction of the currently accepted wisdom that insists on evolution being governed by the contingencies of circumstances.
And Turner has observed “a peculiar harmony of structure and function in the devices organisms contrive to accomplish things.” Natural selection, he continues, cannot fully explain this because it is “contingent upon the past but with no view to the future, and with certainly no purposefulness or intelligence guiding the process.”
In an interview at Brown University in advance of his appearance at a 2008 symposium of the American Association for the Advancement of Science in Boston, Brown biologist Kenneth Miller said, “The idea that there is ‘design’ in nature is very appealing. People want to believe that life isn’t purposeless and random. That’s why the intelligent design movement wins the emotional battle for adherents despite its utter lack of scientific support. To fight back,” he continued, “scientists need to reclaim the language of ‘design’ and the sense of purpose and value inherent in a scientific understanding of nature.… There is, indeed, a design to life—an evolutionary design. The structures in our bodies have changed over time, as have its functions. Scientists should embrace this concept of ‘design’ and, in so doing, claim for science the sense of orderly rationality in nature to which the anti-evolution movement has long appealed.”
Morris, Miller, Turner, and others have the right hunch: Design in nature does not arise by accident. Their comments underscore the fact that we are living in revolutionary times, when fundamental assumptions are being challenged. But most scientists are willing to go only so far with their iconoclasm. Even as they question some tenets of Darwin and his followers, they hold on to the idea that biological organisms are different from everything else. The celebrated science writer Richard Dawkins articulated this view in his acclaimed book The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe Without Design, when he asked “how [complicated things] came into existence and why they are so complicated.” He argued, “the explanation … is likely to be broadly the same for complicated things everywhere in the universe; the same for us, for chimpanzees, worms, oak trees and monsters from outer space.” Just when it seems he is going to offer a universal outlook, he pulls back. “On the other hand, it will not be the same for…‘simple’ things, such as rocks, clouds, rivers, galaxies and quarks. These are the stuff of physics. Chimps and dogs and bats and cockroaches and people … are the stuff of biology.”
This fundamental division between physics and biology is false. It does not result from a broad view of how the world works but from that ancient adage: Your answers are only as good as the assumptions underlying your questions. Darwin and his followers heroically helped remove God from the scientific equation. And, to the discomfort of many, they took human beings down a peg or two when it comes to our place in the cosmos. But they couldn’t completely break from the past, couldn’t see beyond the idea that biological life is special.
The remnants of this old worldview aren’t the only things that have hindered understanding. At its best, science encompasses everything—it seeks to provide a rational basis for all that is. However, especially during the last two hundred years, its practitioners have tended to slice and dice the universe into smaller and smaller pieces, all the way to the infinitesimal. Some people study rocks, others look at birds; some study space, others focus on human beings. You may have noticed the same phenomenon when you seek medical treatment—one doctor specializes in kidneys, another in colons, another in the heart; no one can manage all your care.
Because scientists have focused on ever-smaller questions, and ever-smaller dimensions, most have failed to see the big picture. This has prevented even those who are aware of the overarching tendencies of design in nature from taking the imaginative leap to see that the broad evolutionary tendencies we observe in living creatures also shape inanimate phenomena that do not possess DNA subject to random mutation, such as rivers, global weather patterns, and everything else that moves.
I took this step in 1996. While writing my second paper on the constructal law for an international journal, I noted:
A lot has been written about natural selection and the impact that thermodynamic efficiency has on survival. In fact, to refer to living systems as complex power plants has become routine. The tendency of living systems to become optimized in every building block and to develop optimal associations of such building blocks has not been explained; it has been abandoned to the notion that it is imprinted in the genetic code of the organism.
If this is so, then what genetic code might be responsible for the development of equivalent structures in such nonliving systems as rivers and lightning?…Whose genetic code is responsible for the societal “trees” that connect us, for all the electronic circuits, telephone lines, air lines [routes], assembly lines, alleys, streets, highways, and elevator shafts in multistory buildings?
I am not disputing the role of genetics in the origin of species—just as I don’t discount the pivotal role of soil erosion in the formation of river basins. But mechanism is not law. It may explain what has happened but not why it should happen. Indeed, in view of the constructal law, we see that the search for mechanism has been monumentally unproductive for the understanding of design in nature. There is no single mechanism that generates design in river basins and biological organisms. Instead there is a single principle of physics that governs the design-generating action of soil erosion or genetics. On one level these two phenomena couldn’t be more different—yet both create shape and structure that facilitate flow. Natural selection, random mutation, and soil erosion are not the endgame. They are just three of the many morphing mechanisms we find in nature that serve the unifying principle for all evolutionary phenomena, the constructal law.
The constructal law also challenges another idea that has become dogma since Darwin—that there is no overarching direction to evolution. Proponents of that view claim that adaptations make species better able to survive, but they never explain why these changes should occur and what they mean by “better.” The closest they come is through a piece of circular logic that says: A change is better if it aids survival; any change that aids survival is better. The constructal law, by contrast, predicts that evolution should occur because of the tendency of all flow systems to generate better and better designs for the currents that flow through them. It expresses the meaning of “better” in unambiguous physics terms—change that facilitates faster, easier movement. As we will see, not only do river basins and forests improve in time but so do biological creatures—the rise of species from single-cell organisms to fish, birds, and humans is the story of better, more efficient flow of animal mass on the landscape. In big history, all these designs have emerged because they enhance the movement, mixing, and churning of energy and mass on the planet.
The constructal law is the latest advance in our ever-evolving understanding of nature. Yet, on a basic level, my work is connected to those who came before, both in and out of science, who have tried to describe the flowing world around them. The novelist William Faulkner, for example, hinted at my new definition of life when he wrote, “ ‘living’ is motion, and ‘motion’ is change and alteration and therefore the alternative to motion is un-motion, stasis, death.…”
Although Faulkner spoke of human beings, not rivers, his quote suggests that people have long understood a basic truth of the constructal law that is encompassed by the old sayings “going with the flow,” “taking the path of least resistance,” and “doing the most with the least.” The American transcendentalist Henry David Thoreau expressed this as a philosophy of life in 1853 when he wrote: “Dwell as near as possible to the channel in which your life flows.” The nineteenth-century American economist Henry George articulated this principle as well when he observed: “The fundamental principle of human action…is that men seek to gratify their desires with the least exertion.”
The idea that nature organizes itself to move more easily has a long pedigree in the sciences, too. In the first century CE, Heron of Alexandria intuited that a ray of light bouncing off a mirror and traveling between two points follows the shortest path. From this mental viewing he predicted the shape of the reflected ray, that is, that the angle of incidence should be equal to the angle of reflection. In the seventeenth century, Pierre Fermat had a similar vision, the concept of minimum travel time, when he predicted the shape of the refracted ray, that is, the broken ray when light passes from air into water.
The great scientists who developed mechanics and calculus three centuries ago (Newton, Leibniz, Euler, the Bernoullis, Maupertuis, Lagrange) began to question design in nature by thinking that nature optimizes things. Variational calculus emerged as a technique for identifying “optimal” paths—ultimate drawings, “destined” to satisfy specific objectives when constraints (aka reality) are also taken into account. Close, but no cigar. Nature does not produce optima, or “end designs” or “destiny.” Nature is governed by the tendency to generate shapes and design that evolve in time to reduce imperfection. Design evolution never ends.
The constructal law is not about destiny (or optimum, maximum, minimum, most, least, best, worst, etc.). Yet the insights from the eighteenth century suggest one of the powers of the constructal law: It offers a scientific confirmation, a rational, testable basis for our intuition that there is a direction in time to the evolution of all around us, a purpose, a direction toward flow performance in all that goes on around us.
The constructal law also helps us see another fact that people have long intuited—the harmony in nature. Rivers are lovely for many reasons, and one of them is that they follow geometric rules predicted by the constructal law: Their depth is proportional with their width—big streams are wide and deep; small streams are narrow and shallow. This, of course, is good for the flow of water. This and the myriad other scaling laws we find in nature are only surface reflections of a far deeper harmony. As we will see, our ideas of beauty take practical form when we see how they are often reflected in natural designs.
The constructal law teaches us that nothing operates in isolation; every flow system is part of a bigger flow system, shaped by and in service to the world around it. The flow system we call a tree is also part of the larger flow system (that also includes rivers and weather patterns) for moving water from the ground to the air in order to achieve an equilibrium of moisture locally and globally. At the end of the day, the tree, like every other flow system, exists in order to facilitate nature’s tendency to flow with configuration. Its shape and structure reflect the tendency to generate designs to do this efficiently. This interdependence, born of thermodynamics and the constructal law, is the true source of harmony, balance, and oneness in nature.
In my professional capacity I see the constructal law as a powerful scientific tool. As a human being, I also appreciate its metaphysical implications. Poets have long celebrated the balance and harmony of the world, the oneness of nature. But this has been hard to prove rationally. Until now. By identifying a principle that joins the animate and inanimate worlds, that links the flow of rivers to the flow of cities and the flow of money, the design of our lungs and blood vessels to trees and lightning bolts, the constructal law brings science in line with poetry. It reveals our deep connection. It illuminates the tendency that unites everything that moves.
