More than 400,000 people moved to New York in the last 10 years. Every person had their reason, hopes, dreams, and tragedies when they arrived (even Taylor Swift). During the same period, many New Yorkers left. Maybe for a job or to be closer to family. It could be anything.
The decision to move from one city to another is highly individual. Yet one researcher is trying to create a theory of cities that teases out the patterns in them all. To Alberto Hernando de Castro, a physicist at the Ecole Polytechnique Federale de Lausanne, cities have memories, and sometimes those memories can be traumatic.
“As cells in a body, or grains in the dust, we contribute to bigger structures, bigger movements. The sum of all our decisions, of all our moves, contribute to the entity called ‘the city.’ To its pulse, its evolution, its fate,” he says.
Hernando de Castro began his work looking at population data for Spanish cities dating back for 100 years (as Co.Exist covered here). He found that a city’s future growth tended to relate heavily to its recent past–particularly, he found, the most recent 15 years of data. Another finding in his initial researcher was that two cities within 50 miles of each other tended to become “entangled,” with each playing a big role influencing its neighboring cities’ fate.
Now in a new paper, published this week in the Journal of the Royal Society Interface, Hernando de Castro turned his eye towards U.S. cities by examining U.S. Census data all the way back to 1830 through the year 2000, and found similar patterns that “confirm the behavior of cities as coherent entities.” U.S. cities have longer “memories” than European ones–25 years versus 15 years–and affect each other over longer distances–125 miles versus 50 miles–but similar patterns emerge.
The memory acts as an “inertia” that helps make predictions. For example, he says, by analyzing the evolution of New York City in the last 25 years, you can use equations to predict with high accuracy its evolution over the next 25. “This is because the city remember its path, and knowing the past path is easy to know the future one,” he says.
What was more interesting to Hernando de Castro was that certain traumatic historical events, like the U.S. Civil War or the 1929 stock market crash, seemed to erase this population-level “memory.” He calls it a “post traumatic amnesia.” “If those paths have brought the cities to an unstable situation, as wars or economical crisis, the cities can forget that path and find a new one. It is a pure act of resilience,” he says.
However, this resilient “forgetting” doesn’t kick in right away, and may take about 20 years, according to his models.
“This also means that it is not easy to predict how the population will flow after one of these crisis, and their effects are not obvious until 20 years later,” he says. “The last crisis burst in 2007, seven years ago. However, according to what we have learned, we will not completely move on until 20 years after its burst, and after that any dynamics is possible.
The moral of the story? New York City’s seemingly never-ending real estate bubble might really have an end date.