Cities are Urban Social Reactors: One Part Star, One Part Network

Cities grow and change, sprawling out into suburbs or remaining compact and planned. In fact, they've often been likened to organisms, ant colonies and river networks. Now, though, scientists have suggested that cities are like a new kind of complex system in nature--a sort of social reactor that is part star and part network.

In order to actually examine the growth of cities, researchers derived a series of mathematical formulas that describe how cities' properties vary in relation to their population size. In order to create these formulas, though, they had to gather masses of data from the cities of today around the world. They included general statistical patterns such as land use, urban infrastructure and rates of socioeconomic activity. Using this data, they laid the foundation for a quantitative theory of cities.

In fact, the theory is mainly composed on "scaling" relationships. This predicts, based on the city's size, the average numerical characteristics of a settlement, including everything from the number of patents it produces to the total length of its roads to the number of social interactions that its inhabitants enjoy. These relationships and the related equations, models and network analyses provide the basis for the theoretical framework.

"A city is first and foremost a social reactor," said Luis Bettencourt, the lead researcher, in a news release. "It works like a star, attracting people and acceleration social interaction and social outputs in a way that is analogous to how stars compress matter and burn brighter and faster the bigger they are."

Cities aren't just like stars, though. They're also like massive social networks. They're not so much made up of people as they are their contacts and interactions. These social interactions happen inside other networks-social, spatial and infrastructural. Together, they allow people, things and information to meet across urban space. How these networks knit together often determines how productive or prosperous a city is.

This framework also has practical applications. It could help city planners and policy makers to keep these social reactors working optimally--or improve them. Creating urban policies that create positive social interactions is crucial for a "healthy" city.

"Rapid urbanization is the fastest, most intense social phenomenon that ever happened to humankind, perhaps to biology on Earth," said Bettencourt in a news release. "I think we can now start to understand in new and better ways why this is happening everywhere and ultimately what it means for our species and for our planet."

The findings are published in the journal Science.