Satellite images of Earth at night evoke ambiguous feelings: While on a ground level our cities appear as purely cultural artifacts, a traveler from outer space might just as well marvel at them as beautifully glowing organic fungi-like structures that sprouted on our planet. Less than a millennium ago, the Earth at night was all dark. Today it is all glowing and blossoming.
Scientists think the laws governing the structure of galaxies in outer space are the same laws underlying the growth of cities. Henry Lin and Abraham Loeb at the Harvard-Smithsonian Centre for Astrophysics have used models for showing how galaxies evolve based on matter density to propose a unifying theory for scaling laws of human populations.
“We treat the population density as the fundamental quantity, thinking of cities as objects that form when the population density exceeds a critical threshold,” Lin and Loeb wrote in their research paper (pdf). “The situation is therefore conceptually and mathematically analogous to the formation of galaxies in the universe.”
Using known scaling-laws describing the spread of galaxies, the astrophysicists showed they can also describe structures of urban sociology, such as an inverse-rank law that says the probability that one person will be friends with another in a large city is inversely proportional to the number of people who live closer to the first person than the second.
Next they simplified it to see how far their law would stretch—and the answer was pretty far. “We derive a simple statistical model that explains all of these scaling laws based on a single unifying principle involving the random spatial growth of clusters of people on all scales,” they write.
All of which suggests that the underlying laws that govern some fairly complex human behaviors are the same as those that determined the formation of the very galaxy we live in. Which is surprising, at least for those who see cities as entirely unnatural artifacts.
"Nature likes to hide itself", Ancient Greek philosopher Heraclitus already said. And apparently some of the same natural forces behind the shaping of galaxies are also active in the formation of cities.