Others have suggested that cities look and operate like biological organisms, but that is not the case, says Bettencourt. “A city is a bunch of people, but more importantly, it’s a bunch of people interacting, so hence the social network,” he explains. “What’s important are the properties of this social network: the scaling was giving us clues. But then when you think of this superlinearity, which means the socioeconomic outputs are the result of those interactions, are expressed as growing superlinear functions of populations, the only system that I could think of in nature is a star. A star does have this property – it’s essentially a nuclear reactor sustained by gravity and shines brighter (has greater luminosity) the larger its mass. So there’s a sense that this behavior that is sustained by and created by attractive interactions and whose output is proportional to rate of interactions, is what a city is and a star is, and so in that sense they are analogous.”…
The result is this “special social reactor” that adheres to four main assumptions about city dynamics and scaling:
1) There are “mixing populations”: basically, cities have attractive interactions and social outputs are the results of those, which leads to more social interactions.
2) There is “incremental network growth”: notably, the networks themselves and the supporting infrastructure develop gradually as the city grows. The infrastructure is decentralized as are the networks themselves. This is very different from an organism, says Bettencourt, whose internal “infrastructure” (analogous to a vascular system for example) develops basically all at once and has a centralized node.
3) “Human effort is bounded”: as he writes in his paper, “The increasing mental and physical demand from their inhabitants has been a pervasive concern to social scientists. Thus this assumption is necessary to lift an important objection to any conceptualization of cities as scale-invariant systems.” In other words, “The costs imposed on people by living in the city do not scale up,” he says, because as the number of social interactions increase, one doesn’t have to necessarily travel more to get to these interactions. “The city comes to you as it becomes denser,” he notes.
4) “Socioeconomic outputs are proportional to local social interactions”: this gives us an interesting snapshot of exactly what a city is – not just a conglomeration of individuals, but rather a concentration of social interactions.
Sounds interesting. Cities are both agglomerations of social interactions as well as have unique infrastructures (physical and social) that gives shape to and is shaped by these interactions.