“Urban physicists” are claiming that cities are like molecules

Ulm in front of the photo that inspired him. Image: Len Rubenstein.

Franz-Josef Ulm, an engineering professor at MIT, was looking at an aerial photo of a city with a colleague, trying to work out why it seemed so familiar. Then suddenly, they got it: with its regular, grid-like pattern, it looked a little like the molecular structure of cement.

That this was the first comparison to spring to mind is less unlikely than it sounds – Ulm is a specialist in the material’s structure, and the director of something called the Concrete Sustainability Hub. His realisation about cement’s similarity to the layout of a city set him wondering: do all cities have ordered or disordered structures, like physical materials do? And if so, how does it affect them?

To find out, Ulm and some other researchers collected together data on the locations of buildings and streets in 12 US cities, and used computer modelling to create simplified structures for each city. They compared these structures to the molecular makeup of different substances, then divided the cities into categories according to which material they most resembled. 

They found that New York has the highly-ordered structure of crystal:

Chicago has a glass-like structure –so still regular, but less regimented:

And Boston, Seattle and Los Angeles are like “amorphous liquids”. Here’s Boston:

So why does it matter if New York has all the structural variation of squared paper, while Boston and Seattle are freewheeling sprawls of streets and buildings?

Well, Ulm and his team believe that just as crystals, glasses and liquids all have different physical properties, cities with different building patterns will respond differently to atmospheric conditions. Tight crystal structures, for example, tend to retain more heat: this might explain why New York bakes in the summer. The researchers believe that structure could help predict how cities will react to global warming, or withstand extreme weather.  

This new field, which Ulm and his team are calling “urban physics”, has yet to make it into a scientific journal. But Ulm’s research on city structure and temperature is currently under peer review and should be unveiled to the scientific world soon.

Reception from other physicists, however, has so far been mixed. As Ulm admit in a press release from MIT:

"They said, 'You crazy guys, you're jumping 12 orders of magnitude from one field [molecular physics] to another [urban planning]...[But] as we move forward, we're finding city patterns that are amazing, and we really believe they could have a major impact for 21st-century urban planners."

Crazy guys, indeed. 

Images: New York: Getty; Chicago: A den Tex, published under Creative Commons; Boston: David Wilson at Wikimedia Commons.


CityMetric is now City Monitor! Come see us at our new home

City Monitor is now live in beta at citymonitor.ai.

CityMetric is now City Monitor, a name that reflects both a ramping up of our ambitions as well as our membership in a network of like-minded publications from New Statesman Media Group. Our new site is now live in beta, so please visit us there going forward. Here’s what CityMetric readers should know about this exciting transition.  

Regular CityMetric readers may have already noticed a few changes around here since the spring. CityMetric’s beloved founding editor, Jonn Elledge, has moved on to some new adventures, and a new team has formed to take the site into the future. It’s led by yours truly – I’m Sommer Mathis, the editor-in-chief of City Monitor. Hello!

My background includes having served as the founding editor of CityLab, editor-in-chief of Atlas Obscura, and editor-in-chief of DCist, a local news publication in the District of Columbia. I’ve been reporting on and writing about cities in one way or another for the past 15 years. To me, there is no more important story in the world right now than how cities are changing and adapting to an increasingly challenging global landscape. The majority of the world’s population lives in cities, and if we’re ever going to be able to tackle the most pressing issues currently facing our planet – the climate emergency, rising inequality, the Covid-19 pandemic ­­­– cities are going to have to lead the way.

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Our team will continue to grow in the coming weeks, and we’ll also be collaborating closely with our editorial colleagues across New Statesman Media Group. In fact, we’re launching a whole network of new publications, covering topics such as the clean energy transition, foreign direct investment, technology, banks and more. Many of these sectors will frequently overlap with our cities coverage, and a key part of our plan is make the most of the expertise that all of these newsrooms combined will bring to bear on our journalism.

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As for CityMetric, some of its archives have already been moved over to the new website, and the rest will follow not long after. If you’re looking for a favourite piece from CityMetric’s past, for a time you’ll still be able to find it here, but before long the whole archive will move over to City Monitor.

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Sommer Mathis is editor-in-chief of City Monitor.