SOURCE: The New York Times
By ANDREW ADAMATZKY and ANDREW ILACHINSKI
THE United States interstate highway system is often celebrated as a simple yet highly efficient transportation scheme — one that, starting in the 1950s, transformed the American economy and lifestyle. Its network of roads was designed to maximally span the country with a minimal number of links, while still allowing for enough redundancy to help drivers overcome wrong turns and missed exits.
But it’s worth remembering that the highway system was created by mere humans, using only human intelligence. To find out if it’s optimally designed, we need to consult a higher authority. Namely, slime mold.
Let us explain.
There is a slime mold known as Physarum polycephalum that lives in forests around the world. It feeds on various kinds of microscopic particles. As it forages for food, protoplasmic tubes of slime extend out and bifurcate like tree branches; whenever it happens upon a source of nutrients, it gathers into a bloblike formation. The whole thing — blobs connected by tubes — is a single organism, and the network serves to transport nutrients throughout its “body.”
An interesting fact about this slime mold is that it is highly intelligent — or at least it behaves as if it is. In locating food in its environment, it builds networks that have been shown to be optimally efficient in transporting the nutrients over the area in question. If placed in a maze, for instance, with a source of food outside the maze, the slime mold will discover the shortest path out.
The Japanese researcher Toshiyuki Nakagaki and his colleagues have demonstrated that the slime mold’s foraging behavior can be used to perform sophisticated computations, as long as the problems are represented spatially. Problems solved by the slime mold include not only the shortest path out of a maze, but also other complex mathematical challenges (like creating a Voronoi diagram and a Delaunay triangulation).
Despite its ability to solve an array of problems, the slime mold was designed by evolution to solve just one problem: how to build an optimal transport network (for its nutrients). So we decided to investigate how the slime mold, when presented with the task of connecting the major urban areas of the United States, would design a transport system. Would its design resemble that of the United States highway system, or would the slime mold propose a superior one?
Here’s how our experiment worked. As we detail in a forthcoming article in the journal Complex Systems, we took a large dish in the shape of the United States and placed rolled oats (a food for the slime mold) in the locations of 20 major urban areas. Then we put the slime mold on the rolled oats representing the New York area. The slime mold propagated out from New York toward the other urban areas and eventually spanned them all with its network of protoplasmic tubes. We performed this experiment a number of times.
What did the resulting network look like? It looked remarkably like the United States interstate highway system.
We found that the slime mold approximated almost all interstates. Links from Dallas to Houston, from Chicago to Milwaukee and from New York to Boston were reproduced by the slime mold in almost all experiments. We also found that in three out of four experiments, the slime mold approximated the routes from the San Jose, Calif., area to Las Vegas; the chain of links connecting Denver to Albuquerque to the Phoenix area to the Los Angeles area; and the chain of links connecting Kansas City, Mo., to Oklahoma City to the Dallas area to the Houston area.
It also approximated two chains — one connecting Milwaukee to the Chicago area to Nashville to Memphis; the other connecting Boston to the New York area to Charlotte, N.C., to Atlanta to Jacksonville, Fla. — that are bridged by a link from the Chicago area to New York. (Routes that weren’t approximated were those directly connecting Denver to the San Jose area, the Houston area to Albuquerque and Jacksonville, the New York area to Nashville, and Boston to the Chicago area.)
From the slime mold’s point of view, Interstates 10 and 20 represent the core backbone of the United States transport network: when these interstates are removed, the network separates into disconnected western and eastern parts.
The United States is not alone in having an interstate highway system that gets the slime mold’s seal of approval. We have done similar experiments with various colleagues, using slime mold to approximate the highway systems of Britain, Mexico, the Netherlands, Canada and Brazil.
For all of these countries, we found that the slime mold network matched, at least partly, the network of human-made transport systems, though the closeness of fit varied from country to country. The United States highway system, for instance, is less slime-mold-like than that of Canada.
How Americans feel about that fact depends, presumably, on how they feel about the wisdom of slime.
Andrew Adamatzky is a professor of computer science at the University of the West of England in Bristol. Andrew Ilachinski is the principal research scientist at the Center for Naval Analyses.
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