Most of us know that over time, things change, water flows, rocks fall, living creatures grow, and weather patterns shift. Scientists have long tried to explain these changes with the laws of thermodynamics—especially something called the ‘second law,’ which talks about how useful energy becomes less available (or how processes are irreversible). But there is more to the story of how nature changes (or ‘evolves’) than just energy flowing downhill.
This article introduces two important ideas, irreversibility and evolution:
- Irreversibility, guided by the second law of thermodynamics (energy flows happen in one direction, and you can’t fully undo them).
- Evolution of shape and design, guided by the constructal law (natural systems change their form over time in ways that make it easier for them to move, flow, and persist).
They are distinct, self-standing, not the same thing.
Falling rocks and flowing water
Imagine a rock falling into a pool of water. The focus is often on the rock alone, with the explanation that it sinks due to gravity and slows down because of friction (drag). However, a broader perspective reveals a larger picture—it is not just the rock but the rock and the water. As the rock moves, it pushes the water around, creating flow patterns or “designs” in the water. These patterns evolve over time as the rock slows down, highlighting the dynamic interaction between the two.
Credit. Author
Different speeds, different patterns
When the rock moves fast, the water flow around it is turbulent, creating messy-looking patterns behind it. As the rock slows, the turbulence decreases, giving way to smoother patterns. Each flow pattern helps the overall system—the rock and the water—transfer momentum more effectively from the rock to the water. This changing flow is part of an evolving design, as nature continuously finds ways to transfer movement or energy more efficiently from the rock to the surrounding water.
Two laws at work
1. Second law of thermodynamics (irreversibility)
This law commands that the rock and water should eventually come to rest in one direction of time. Power is lost to friction and related effects, heat is generated, and can’t be reversed. This process follows a one-way path, ensuring that the system moves toward a state of equilibrium.
2. Constructal law (evolution of design, or flow configuration)
The Constructal law accounts for how natural designs, such as river basins, bird wings and rhythm, and traffic patterns, evolve over time to facilitate smoother flow. In the case of a sinking rock, the “design” refers to the shape of the water flow around it, which continuously changes to enable more efficient transfer of motion from the rock to the water.
The key difference
Irreversibility dictates that isolated systems naturally change from movement (flow) move from high energy to lower energy, like a spinning top that gradually slows down. Meanwhile, the law of evolution of design rules how shapes adapt over time to facilitate more efficient movement or flow, allowing systems to transfer energy more effectively.
The two laws coexist but are not the same. One is about the ‘one-way’ flow of energy (the second law), while the other is about the constant re-shaping of flow paths (the constructal law).
Credit. Author
Beyond the rock example: nature’s inventions
Everyday ‘natural technologies’ demonstrate efficient movement and energy transfer, such as wind and ocean currents distributing heat and moisture across the planet, trees branching to transport water and nutrients, and animal bodies evolving specialized shapes—birds with wings for flight and fish with fins for swimming.
All of these shapes and patterns didn’t appear randomly. They formed over time to move better or spread something—heat, water, nutrients, or themselves. That’s the constructal law in action. Meanwhile, the energy driving these flows is always being used up in one direction (the second law).
Why this matters
Understanding these two laws helps us see nature more clearly. Many people say that ‘everything must become more disordered’ because of the second law. But nature also shows us increasing organisation—like the flow patterns around a falling rock or the branching structures in our lungs. The second law isn’t about ‘everything becoming disorganised’; it’s about flows being oriented one-way. Meanwhile, the constructal law tells us that things that flow—be it water, air, or even traffic—should evolve to get better arranged over time.
When the energy source stops, everything eventually comes to rest, as the second law says. But while energy is available, flow systems keep changing their configurations to move more easily and effectively.
Final thoughts
Irreversibility, stated as the Second Law of Thermodynamics, describes the one-way nature of energy flow, while the Constructal Law highlights how flow systems evolve their designs over time to enhance movement and efficiency.
Both are essential to understanding the changing designs that are seen in nature—from falling rocks to beating hearts. If one looks at the entire picture (the rock and the water, not just one or the other), one can see how nature’s patterns emerge, persist, and evolve—all while energy runs downhill in one irreversible direction.
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Journal reference
Bejan, A., Almahmoud, H., Gunes, U., Fakhari, H. E., & Mardanpour, P. (2024). Evolution and irreversibility: Two distinct phenomena and their distinct laws of nature. Physics of Life Reviews. https://doi.org/10.1016/j.plrev.2024.06.014
