CC image from Other Think

Several months ago, I picked up a copy of Design in Nature as an impulse buy at the bookstore. I was purchasing a gift and the cover caught my eye. A quick perusal of the jacket and a few pages of the introduction was enough for me to fork over the cash.  I didn’t get around to reading it until I had several airline flights this summer (with the accompanying missed connections) to dig into the book.

The basic premise of the book is that the similarities we see in nature (why trees and lightning bolts and river deltas share the same branch-like architecture) isn’t a coincidence, and it certainly isn’t the result of divine inspiration.  Rather, these similarities are explained via thermodynamics and the ‘constructal law‘ as coined by the author, Adrian Bejan.  The law states: “For a finite-size system to persist in time (to live), it must evolve in such a way that it provides easier access to the imposed currents that flow through it.”

In short, the flow systems and the laws of physics that govern them influence those similarities in design. From the wiki summary:

The constructal law represents three steps toward making “design in nature” a concept and law-based domain in science:

1. Life is flow: all flow systems are live systems, the animate and the inanimate.
2. Design generation and evolution is a phenomenon of physics.
3. Designs have the universal tendency to evolve in a certain direction in time.^[2]

The constructal law is a first principle of physics that accounts for all design and evolution in nature. It holds that shape and structure arises to facilitate flow. The designs that arise spontaneously in nature reflect this tendency: they allow entities to flow more easily – to measurably move more current farther and faster for less unit of useful energy consumed.^[3] Rain drops, for example, coalesce and move together, generating rivulets, streams and the mighty river basins of the world because this design allows them to move more easily. The constructal law asks the question: Why does this design arise at all? Why can’t the water just seep through the ground? The constructal law provides this answer: Because the water flows better with design. The constructal law covers the tendency of nature to generate designs to facilitate flow.

Reading the book, I thought back to previous examples of similar observations of cities:

• The similarities of subway networks across multiple cities (linked previously here)
• The work of Geoffrey West on a universal theory of cities (also here), economies of scale and the benefits of agglomeration
• Jarrett Walker’s analogies of transit systems as rivers (both here and here), particularly with the usefulness of drawing out key principles (e.g. ‘branching divides frequency’).
• Any number of urban economic studies of agglomeration, innovation, and human capital – studying the flows of information in cities (examples here, here, and here, among many others)

Jarrett Walker’s recent Email of the Month post sparked me to write this.  Walker’s emailer, Kenny Easwaran, notes:

At the time, I was thinking of the various transportation systems we know of that aren’t designed by humans.  The main examples I could think of were things within the human body, and I noticed that things like the circulatory systems of animals and plants, and the digestive system of animals, seem to follow somewhat different trajectories from grids.  In particular, they either have a branching tree structure, or something more like an extended linear structure.

Having recently finished Adrian Bejan’s book on constructal theory, the analogy to tree-like systems immediately caught my eye. For me, Bejan’s description of all of these phenomena as flow systems ruled by common principles of physics helps shape my thinking, even if it is a bit vague.  Walker’s analogies of transit networks to rivers is a similar case.