What ‘urban physics’ could tell us about how cities work

What ‘urban physics’ could tell us about how cities work
Boston as amorphous liquid, and other insights from an engineer

WHAT DOES A CITY LOOK LIKE? If you’re walking down the street, perhaps it looks like people and storefronts. Viewed from higher up, patterns begin to emerge: A three-dimensional grid of buildings divided by alleys, streets, and sidewalks, nearly flat in some places and scraping the sky in others. Pull back far enough, and the city starts to look like something else entirely: a cluster of molecules.

At least, that’s what it looks like to Franz-Josef Ulm, an engineering professor at the Massachusetts Institute of Technology. Ulm has built a career as an expert on the properties, patterns, and environmental potential of concrete. Taking a coffee break at MIT’s Stata Center late one afternoon, he and a colleague were looking at a large aerial photograph of a city when they had a “eureka” moment: “Hey, doesn’t that look like a molecular structure?”

With colleagues, Ulm began analyzing cities the way you’d analyze a material, looking at factors such as the arrangement of buildings, each building’s center of mass, and how they’re ordered around each other. They concluded that cities could be grouped into categories: Boston’s structure, for example, looks a lot like an “amorphous liquid.” Seattle is another liquid, and so is Los Angeles. Chicago, which was designed on a grid, looks like glass, he says; New York resembles a highly ordered crystal.


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