Society has many laws, science only a few. There are only a handful of principles such as Galileo's law of gravity or Newton's laws of motion that nature always obeys.

In Adrian Bejan's field, there are just two great laws of thermodynamics. So the 59-year-old professor of mechanical engineering at Duke is making no small claim when he says that he has discovered another.

He calls it the Constructal Law and says it predicts how everything -- from lightning bolts and rivers to plants, people and money -- flows through time and space. It explains why birds and airplanes can fly, how cities grow and epidemics spread. It unravels the mysteries of snowflakes and Egypt's pyramids, the evolution of written languages.

By applying its principles, we can build better houses and roads; derive deeper understandings of the challenges posed by illegal immigration and climate change. Through the constructal law, we can make the world a better place.

Bejan's revolutionary idea challenges the age-old assumption that nature is without design. Just as scientists say life developed thanks to a series of random mutations, they reject the notion that an organizing principle informs and shapes the movement of materials.

Where others saw disorder, Bejan found pattern. "One of the basic aims of design is to get the most amount of work done with the least amount of effort," he explained. "What we as engineers try to accomplish through our plans, nature achieves on its own. Everywhere you look, animate [people, trees] and inanimate [rivers, mud cracks] phenomena, if given freedom, organize themselves into patterns and shapes that help them get from here to there in an easier manner."

He pauses, as a smile spreads across his chiseled face. "Like the old saying, they go with the flow."

It is no accident, for example, that blood vessels and pulmonary airways have round cross-sections just like the pathways carved by earthworms and ants. This shape is the most efficient way for them to flow. A river's width is proportional to its depth because this, too, maximizes its flow.

Or consider lava. As it begins its journey from the volcano's belly, it organizes itself into a series of concentric circles. 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 ground helps it flow. No one, of course, is telling the lava to do this -- there is no lava flow manager saying you here, you there. It just does it, Bejan says, because of the natural tendency of moving matter to organize itself into patterns that increase the efficiency of its flow.

As it makes its way down the mountain, something else happens: The lava does not just ooze, which is an inefficient way to flow. Instead it forms a channel, from which treelike branches spread. This, it turns out, is the best way for it to flow.

In his book-lined office at Duke, Bejan displays an aerial view of flowing lava. Then he spreads out pictures of river basins and snowflakes, of the air passages in lungs and the coronary arteries of the heart. All display the same treelike design. "They look alike because they display this universal tendency of things that flow to seek great access," Bejan said. "As time goes on, so long as they are not overly constrained by outside forces, they will organize themselves into even more efficient design flows."

From hunch to law

Bejan did not shout "Eureka!" on that momentous day in 1995 when he discovered the constructal law. He was, after all, in a crowded conference room in France. But inspiration came in the proverbial flash when he heard a Nobel laureate proclaim that the treelike flow structures that characterize lightning bolts and river basins are "nondeterministic."

"When he said that I heard the click," Bejan recalled. "I knew he was wrong."

Bejan granted that no two trees -- like no two snowflakes -- are identical. Nature does not produce carbon copies. "Their similarities are far more important than their slight differences, just as cars may have different designs but they are all still cars."

On the plane ride home, he 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."

That moment proved a turning point in his distinguished career. Bejan was already one of the world's most influential engineers -- to date he has published 460 scholarly articles and 23 books including "Shape and Structure, from Engineering to Nature" and "Thermal Design and Optimization." He has won the highest honors in his field, including the Max Jakob Memorial Award in 1999, which is considered the Nobel Prize of thermal science.

Since 1995, Bejan has devoted his energies to applying the constructal law to various phenomena. For example, one of the basic goals of any design is to accomplish the most work with the least energy. Now consider a bird, which expends energy in two ways: First, vertically, as it lifts and maintains itself in the air. Second, horizontally, as it battles air friction to move forward. Bejan's research -- which involves a great deal of math and physics -- found that birds are "designed" to achieve "an optimal balance" between these two types of energy.

He discovered the same thing studying how fish swim and animals run.

Scientists, of course, have long studied animal locomotion. But, Bejan noted, they have generally worked backward from observations in the field. They watch birds fly and fish swim, then develop explanations that fit those results. They didn't apply their conclusions about flying birds to swimming fish.

The constructal law makes that connection. It does not begin with observation but an idea -- the general tendency of things, if given freedom, to flow more easily. It then uses this single universal principle to predict a wide range of phenomena.

"This can have practical results," Bejan added, "as we see that the design goals of fish and birds can help people make better boats, airplanes and robots."

One more example: The constructal law predicts that flow systems try to balance the amount of time it takes to go slow and fast. For example, the amount of time it takes to draw the breath into your lungs -- a relatively long distance -- is the same amount of time it takes that air to fill the alveoli in the lungs -- a relatively short distance. This same dynamic is everywhere in nature. The time it takes rain to seep into the ground is equal to the time it spends flowing in the stream, which is the same as the time it spends in the river.

"This is a universal principle," Bejan says, "which can be used, for example, when designing roads. Ideally, the time we spend driving down our little streets should equal the time we spend on the access roads, which should equal the time we're driving on the highway."

Application beyond nature

While applying his law, Bejan has also worked to disseminate it. He has written books and papers on the subject and teaches a course on it at Duke. Through grants from the National Science Foundation he has organized symposiums to share his findings with scholars from around the world -- not just other engineers, but biologists, social scientists, economists and linguists.

Some of their findings are included in the new book, "Constructal Theory of Social Dynamics," which Bejan edited with a colleague at Duke, the sociologist Gilbert W. Merkx.

Bejan said these contributions have been particularly gratifying, because they show how the constructal law applies to areas outside his field. The book's essays cover a wide array of topics, including the flow of people through life (aging and mortality), the flow of social networks (including interlocking corporate boards), the flow of capital and of immigrants from Mexico to the United States.

In his paper, Merkx describes the profound sweep of Bejan's work. He notes that since the time of the 18th-century philosopher Immanuel Kant, intellectuals have divided knowledge into two realms -- the natural sciences and the study of culture and social behavior. In fact, he says, "social networks" resemble "river networks or trees." Constructal theory, he continues, "explains that tree networks exist because they require the least amount of useful energy, and hence are the most efficient. Hence, rational actors [i.e. people] will tend to gravitate toward social networks that exhibit the same properties of efficiency shown by nature."

The idea that people are attracted to efficient systems is not new. What Bejan has done is to link it to a broader tendency in nature.

Bejan said this insight has eluded scientists for two reasons. First, since the Enlightenment, when reason began to overtake faith, scientists have rejected the idea that there is a purpose or design to nature. "Science is biased against the idea that there is any direction to things," Bejan said.

In addition, scientists have compartmentalized their work. Some look at river basins, others ant hills; some study electricity, others the vagaries of human activity. "Each field is a discipline," he explained. "There is an iron curtain separating the biologists from the geologists, those who study the animate and the inanimate."

This, he said, has precluded researchers from seeing that an underlying principle explains all flow structures. "Before constructal theory," he said, "people did not have a way of putting the river basin and animals together, of recognizing the oneness of everything."

Given his own history, it's no shock that Bejan would challenge conventional wisdom. What is surprising is the degree to which luck has shaped the destiny of this man who beckons his scientific colleagues to see order and purpose in the world.

Bejan was born in Romania in 1948. His father was a veterinarian, his mother a pharmacist. Though neither was a political leader, "they spoke the truth about the communist system being imposed on us by the Soviets," he said. "They taught me not to just accept things as they are."

The year he was born, his father was sent for several months to a re-education camp. A decade later, his mother vanished for a while during another purge.

Thinking visually

Though growing up in a police state, Bejan was able to develop his talents. When he was 6, he recalls, a motorcycle rally was held in his town. His parents were so impressed by the drawings he made that day that they sent him to art school.

"I have always been very visual," he said. "I think in drawings, which helped me see patterns."

Bejan was also a jock. "Basketball was my sport and my passion growing up," he said, noting that he eventually became a member of the national select team. "Basketball is all about the art of passing, seeing the pattern, the flow of the game. The passer is the facilitator of the flow channel."

Bejan's great break came in 1968. The Prague Spring that year offered a hint of liberty in Soviet-dominated countries. Romania's government opened a tiny window to the West by proclaiming that six students would be allowed to cross the Iron Curtain and study abroad. Bejan joined hundreds of other star students who took the qualifying exam. He aced the test and was allowed to attend MIT. He paid the price for this opportunity: He would only see his parents once more before the fall of the Berlin Wall in 1989.

Bejan worked three jobs to cover living expenses. One was as a brawny laborer -- Bejan is 6 foot 2 -- moving canisters filled with liquid nitrogen in the school's refrigeration lab. "The professor who ran that lab took me under his wing," Bejan said. "I just got lost in the work."

At bottom, Bejan said, the challenge of this field is to find ways to reduce the heat generated by machines in order to maximize the amount of cooling. "As in nature, there will always be imperfection," he said. "But if you move things around, change the design of your flow system, you minimize these imperfections by putting them in the right places."

Bejan's scholarship and consulting jobs for multinational corporations such as IBM focused on this problem after he earned his Ph.D. from MIT in 1975. He taught at the University of Colorado at Boulder before joining Duke as a tenured professor in 1984.

Through the years he began to see that he could optimize his designs by paying attention to their shape and pattern. And, wouldn't you know, they manifested a treelike structure.

While his work presented him with specific problems -- how to make this machine or that work better -- his mind saw the bigger picture. "I have relatively little patience in splitting hairs," he said. "I feel the first question, the fundamental question beneath it all, is noble."

Science has few laws because they are hard to establish. It can take decades to show that they apply universally and just as long to persuade researchers to change their way of thinking.

But Bejan is confident that his work will continue to thrive.

After all, he notes, "Good ideas flow more easily than bad ones."