The New York Times magazine has a very provocative and enlightening article on the work of physicist Geoffrey West on cities "A Physicist Solves the City" (December 17, 2010). Along with co-author (and fellow physicist Luis Bettencourt), West claims to have literally solved some of the most important "equations" explaining the importance of cities.
I won't go into the detail (but the article is well worth reading), but I think the last line of the article and quote from West sums it up:
“Think about how powerless a mayor is,” West says. “They can’t tell people where to live or what to do or who to talk to. Cities can’t be managed, and that’s what keeps them so vibrant. They’re just these insane masses of people, bumping into each other and maybe sharing an idea or two. It’s the freedom of the city that keeps it alive.”
It's also why, he notes, we don't necessarily want private corporations running cities.
Jane Jacobs lives in the laws of urban physics.