“Considering the city as a complex system leads to reducing individuals to atoms”

Tribune. We often read that cities should be thought of as “complex systems” in order to better manage their management. Thus, thanks to the pandemic, for example, the concept has been used to emphasize the supposedly uncontrollable effects of global urban growth, which may have favored the spread of the virus on a global scale. But, if we want to go beyond the accepted idea that the city is complicated, we must make this notion operational, and first attempt to define it precisely.

Most scientific definitions of complex systems bring them back to the idea of ​​systems composed of many interacting entities, which spontaneously emerge a “whole” that is “greater” than themselves, in the sense that it presents unexpected properties.

This characterization first opposes complex systems to “simple” systems, in which the interactions are weak and do not generate surprising properties at the collective level. Thus, apart from rare shocks, the atoms of a gas follow random trajectories, independently of each other, and the overall properties of the gas can be calculated quite simply.

The definition also differentiates complex systems from “complicated” systems, in which the whole does not emerge spontaneously but following the overall plan of an architect, as in the case of an airplane or a computer. On the contrary, the construction of the anthills and their multiple galleries is a canonical example of a complex system, because it is carried out by the workers, without central coordination by a queen who would hold the plan.

A complex model for understanding urban segregation

When it comes to cities, one of the most famous “complex” models was proposed in 1971 by economist Thomas Schelling to understand ethnic segregation in the United States. Imagine a chessboard where the squares represent as many dwellings. Some are occupied by whites, others by blacks, and some, finally, are temporarily unoccupied. Suppose then that all the inhabitants prefer to have a mixed neighborhood, having as many neighbors of both types.

Each day, a resident is chosen at random and offered to move to an empty apartment, also chosen at random. If this move increases the satisfaction of the inhabitant, he accepts it, otherwise he refuses. The next day, we take, always at random, another inhabitant and another empty hut and we start again.

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