Adrian Bejan is a firm believer in omens. He also likes poking conventional wisdom in the eye.
A few years ago, as the only engineer speaking to a packed house of biologists at an animal design conference, he showed how his constructal law predicteded the basic characteristics of animal flight. After the talk, during the first coffee break, he was approached by eminent Penn State biologist James Marden, who wondered if the same principle could be applied to running.
By the next coffee break, Bejan and Marden had figured it out on the proverbial napkin. Naturally, they then began to wonder if these same principles would apply to swimming. However, this answer took a little more time and thought. The commonly held belief was that fish locomotion was different than other animal locomotion. Since they live in water, the conventional wisdom held, fish were different because they would not be subject to the laws of gravity.
Four months later, while driving to Duke, the naked explanation struck him out of the blue. Moments later, a news story came on the radio about the 100th anniversary of the Wright brothers’ first flight on Kitty Hawk.
“It was then I knew I was on the right track,” Bejan recalled. “That was more than just a coincidence.”
The way he saw it, birds and animals could be seen as weight-lifters, since their means of locomotion required effort with an unyielding base – the ground – and a limitless top – the air. His eureka moment in the car was that as fish swim, they too have an unyielding floor – the sea bed. Hence, water flowed over and around them like the air over runners and flyers. So, fish too were weight-lifters.
“And all these forms of locomotion are predicted by the constructal law,” he said.
“Our finding that animal locomotion adheres to constructal law tells us that – even though you couldn’t predict exactly what animals would look like if you started evolution over on earth, or it happened on another planet – with a given gravity and density of their tissues, the same basic patterns of their design would evolve again,” Marden added.
The law, which Bejan started describing in 1996, is the principle that flow systems evolve in time to balance and minimize imperfections, reducing friction or other forms of resistance, so that they flow more and more easily in time. He is fond of using illustrations to make his point. For example, he has used images ranging from the branching symmetries of the lungs, river basins and trees touching top-to-top.
Now, with the help of Marden, Bejan believes that he has now unified both the biological and geophysical principles of nature’s design through the constructal law, which can also be termed, to his mind, the physics law of evolution. Just like that Wright brothers “ah-hah” moment in the car, Bejan finds certain satisfaction that his latest – and most sweeping – cliam with the constructal law is published in the 200th anniversary year of the birth of Charles Darwin.
Not only was the paper published in this symbolic year, but also in the prominent journal Physics of Life Reviews. Not a biology journal, not an engineering journal.
“I see it as a badge of honor that such a paper was published in a physics journal,” Bejan said. “It sits at the interface of biology and physics, with neither side particularly thrilled.”
When people speak of evolution and Darwin, they mean the animlas and trees. Too bad, Bejan said, because design features are everywhere in nature. The apparent similarities and proportions of river basins all over the world have been known since 1930s. Bejan and Marden have shown that the design of river basins comes from the same constructal law as the design of all animals and vegetation.
In an article published recently in the International Journal of Heat and Mass Transfer commemorating Bejan’s 60th birthday, colleagues and former students of Bejan described the constructal law as an animated movie, where one screen is replaced by another screen on which the currents flow with greater ease.
“The constructal law is the time direction of the movie: toward flow configurations (designs, drawings) that flow more easily,” they wrote. “Adrian sees the constructal law as a universal principle of evolution, which applies in many fields, from physics to economics.
“Indeed, the constructal literature of the past decade has focused on showing that the constructal law covers ‘natural design’ phenomena across the board, from biology and geophysics to social dynamics and technology evolution,” they write. “He sees it (constructal law) as a compact summary of common observations, the tape of evolution running in one direction, which may be expressed in physics terms simply as: time and configuration.”
Bejan does not expect his latest publication to be warmly embraced. It has something to annoy just about every scientific camp. In it, he uses the constructal law to predict such diverse phenomena as human organ sizes, turbulence and river basins.
“This is an exciting development for physicists, but it should also resonate with biologists,” Bejan said. “The idea that organic evolution is analogous to the way form evolves in inanimate flow systems is a novel concept that has the potential to unite perspectives and approaches across disparate disciplines. We suggest that the constructal law provides a powerful tool for examining and understanding variation in both the animate and inanimate compartments of nature.”