When you think of physics, what comes to mind? Atomic energy? Gravitational waves? Maybe Newtonian equations?
Adrian Bejan thinks of those things too. But the iconoclastic energy scientist—a mechanical engineering professor at Duke University and one of the world’s foremost experts on thermodynamics—thinks of pretty much everything else as well. The same principles of physics, he argues, can be applied to all things that move, morph, and flow, from sports and technology to air currents and population growth, migratory patterns and social hierarchy.
Bejan isn’t the first person to study behavior as physics, or to use physics to describe wider systems. But his new book, The Physics of Life: The Evolution of Everything, may be the broadest consideration yet. Harking back to the original definition of the discipline—“knowledge of nature” in Greek—he ultimately concludes that “life and evolution are physics.”
To wrap our heads around this counterintuitive premise, National Geographic recently spoke with Bejan about the physics of life, the universe, and everything.
According to your book, physics describes the actions or tendencies of every living thing—and inanimate ones as well. Does that mean we can unite all behavior under physics?
Absolutely. Our narrow definition of the discipline is something that’s happened in the past hundred years, thanks to the immense impact of Albert Einstein and atomic physics and relativity at the turn of the [20th] century.
But we need to go back farther. In Latin, nature—physics—means “everything that happens.”
One thing that came directly from Charles Darwin is that humans are part of nature, along with all the other animate beings. Therefore all the things that we make—our tools, our homes, our technologies—are natural as well. It’s all part of the same thing.
In your magazine and on your TV channel, we see many animals doing this—extending their reach with tools, with intelligence, with social organization. Everything is naturally interconnected.
Your new book is premised on a law of physics that you formulated in 1996. The Constructal Law says there’s a universal evolutionary tendency toward design in nature, because everything is composed of systems that change and evolve to flow more easily.
That’s correct. But I would specify and say that the tendency is toward evolving freely—changing on the go in order to provide greater and greater ease of movement. That’s physics, stage four—more precise, more specific expressions of the same idea.
Flow systems are everywhere. They describe the ways that animals move and migrate, the ways that river deltas form, the ways that people build fires. In each case, they evolve freely to reduce friction and to flow better—to improve themselves and minimize their mistakes or imperfections. Blood flow and water flow essentially evolve the same way.
Would it be overly reductive, then, to say that everything everywhere seeks efficiency all the time?
That would be correct, except that first you need to have an understanding or definition of what efficiency is. The goal of everything is to seek greater access or easier flow and to morph freely in that direction. If you have more efficient flow—whether it’s fuel and air flowing through a combustion chamber or food matter passing through the gut of an animal—you have the correct direction of evolutionary design.
Evolution is a crucial part of how we need to define efficiency. I don’t mean evolution in the Darwinian sense. I mean that there’s a universal urge or tendency toward design and organization that changes over time in a discernible, seemingly goal-oriented direction. So it would be more accurate to call these things evolutionary design and evolutionary organization. This has nothing to do with intelligent design, by the way. It’s simply treating design and evolution as two natural scientific concepts.
In each case the urge is not toward an ideal. It is toward something better tomorrow, and to something even better the day after tomorrow—relentless improvement and refinement.
Tell me a bit about the reaction and reception to the Constructal Law over the past 20 years. I think almost anyone would acknowledge that it’s an elegant description of behavior. But why is it a law—one of the few in physics—and not a theory? How universally accepted is it? And what are the primary arguments against it?
Well, in physics there are a lot of theories but not many laws. A law has to be a summary about a phenomenon—like the laws of thermodynamics or Newton’s laws of motion. A theory, on the other hand, is a prediction of how something should be. And this prediction relies on a law, which is known. Theories are as numerous as the things the mind contemplates. The law, however, is one.
The Constructal Law brings together many interconnected phenomena and their theories, including networks, complexity, and organization. As far as I know, there’s been nothing published in the peer-reviewed literature that would qualify as a reaction against it. Absolutely nothing. That’s been a real surprise.
The amazing compliment I often get after I give a lecture is that people tell me that they’ve had the same hunch, or that they’ve known what I’m saying all along. And I think this is fantastic.
Of course, most people don’t call it the Constructal Law. They call it self-optimization, or self-organization. They refer to scaling laws and rules and animal design, even natural selection. But it’s always this idea of emergence—the tendency toward getting smarter or doing better, which means to evolve, in a universal physics sense.
So when I meet these people, I say, “Right on. And here’s why everything you know is right—and why it belongs in physics.” Because it’s true about anything and everything in the most general way: Life flows, moves, morphs, and kicks into action.
In this book you go a step beyond, say, Robert Ayres, who wrote about how thermodynamics describes developments in economics, socioeconomics, and technology. But you argue that the laws of physics can be used to explain virtually everything—from why stones roll to why humans build fires that are shaped the same way. Tell me more about that universality.
I see my contribution, the Constructal Law, as something that’s not limited to physics. I think it lives in the most permanent room in the house of science, which is thermodynamics.
You can learn what is strongest, what is most efficient, simply by looking at a dung beetle or a tree. And before you know it, you learn how to make better and more impressive tools and dwellings.
The short of it is this: Life comes from engines. I define a live system as one that flows and morphs freely in order to persist in time, because that’s basically what living is: Nothing moves unless it is driven, unless it is pushed.
That pushing—another word for power—comes from engines. And these engines are innumerable; they’re everywhere in the animate realm, but also in the inanimate. Oceanic circulation is an engine. So is atmospheric circulation.
And all engines generate power. But the power is invisible, because it is instantly destroyed. It dissipates in the course of flowing, whether it’s the jet stream in the atmosphere or the animals in the bushes. They’re all behaving the same way. Earth itself is constantly flowing and morphing because its engine is driven by the sun.
I want to home in for a second on hierarchy. “The evolution of anything that moves on earth,” you write, “including humans, leads to hierarchy in movement, naturally. We see this hierarchy occurring naturally everywhere, from traffic in the city, to oxygen transport in the lung, to fast and slow thinking in the flow architecture of the brain.” Can you say more about that?
I presented this idea in the book in two ways. One is that economy of scale is an idea rooted in physics. The bigger flow system is inherently more efficient, because it has bigger openings—and more possibilities—for things to flow through it. You can think of that as less resistance.
So the bigger things tend to be the more efficient carriers. Freight is cheaper on a big truck. Economy of scale, then, is reality. It’s nature. According to the Constructal Law, the tendency is toward greater ease of flowing, which implies that over time, things should become bigger.
But that’s only half the story. The other half is that big things cannot possibly flow over areas smaller than they are. They can’t reach into the tiniest pockets.
Everything on Earth depends on flow. And if you have a river basin or a human lung, you have a three-dimensional thing called volume. All flow systems on Earth have volume.
The architecture of any efficient system must comprise many smaller volume elements and branches, like in fractal geometry. The word for that is hierarchy—the architecture that emerges naturally. It serves the whole—in this case the area or the volume—better and better over time, evolving and improving itself in the process. In my book, I use a rolling stone and a whirl of turbulence to show that the bigger a live entity is, the longer it will keep moving in time and space.
This is why we now have people in physics—not just biology—estimating, explaining, and anticipating life span. We finally know why the bigger things should live longer and travel farther. And why they have to live together with many smaller, shorter-living entities, which travel shorter distances, in order to exist.
In the past, you’ve used the Constructal Law to argue that birds and airplanes should increase their efficiency in similar ways. You’ve also said that the nature-inspired works of the Spanish architect Antoni Gaudí, such as the Sagrada Família, are early examples of biomimetic engineering. That human-nature connection makes a lot of intuitive sense to me. Why don’t more people study the world in this way?
I don’t know. I really don’t know. But there are many people who use nature in order to help themselves and their fellow man. That’s where art comes from. And religion. And, most recently, science. They all come from this.
You can learn what is strongest, what is most efficient, simply by looking at a dung beetle or a tree. And before you know it, you learn how to make better and more impressive tools and dwellings. Taller, stronger, faster. Improvement.
Our narrow definition of physics is something that’s happened in the past hundred years. But we need to go back farther. In Latin, nature—physics—means 'everything that happens.'
Last month two art exhibits opened—one in Spain, one in Chile—that were inspired by the Constructal Law. I had nothing to do with them; I’ve never met either of the artists. But in their own way, they’re both looking at nature frankly, with the eyes and mind of Gaudí.
The Constructal Law has been in play for a long time now. But the public may be hearing about it for the first time through this book. That’s the way that science flows. It’s the natural way. The source—in this case me—has to constantly improve the narrative and disseminate this new law of physics.
So what’s the next step—for you and for this notion?
The next step for me is to think of new ways to demonstrate to the public why this law of physics is essential to us. My objective in this book was to show why it’s important—why there is an answer to the famous question “What is life?” Now it is clear what life is—and the answer is coming from physics.
But metaphorically, the most important thing is to learn why this law of physics is important to us—to predict the future of social organization, the evolution of science and scientific inquiry.
My own thinking evolved while I was writing this book. I conclude with the idea that science itself is an evolutionary design that empowers humans. So in that way alone, I think, the Constructal Law has a lot to say—and a lot of eyes to open.
The bottom line is that many of the things that we take for granted need to be explained in a better way. As you and I know, they can disappear overnight if we take our eye off the ball. Or, worse, the ball will hit us in the face. Wake up! I say to readers. If you don’t wake up, the ball will wake you up.
This interview was edited for length and clarity.