In this hour of Radiolab, we take to the street to ask what makes cities tick.

There's no scientific metric for measuring a city's personality, but step out on the sidewalk and you can see and feel it. Two physicists explain one tidy mathematical formula that they believe holds the key to what drives a city. Yet math can't explain most of the human-scale details that make urban life unique. So we head out in search of what the numbers miss, and meet a reluctant city dweller, a man who's walked 700 feet below Manhattan, and a once-thriving community that's slipping away.

CORRECTION: An earlier version of this program indicated that the Dow Jones Industrial Average originated in the 1920s. In fact, it originated in the 1890s; during the 1920s it was expanded to include 30 companies, the number it includes today. The audio has been adjusted in consideration of this fact.

In the first segment of this episode, we listed a German census as one of West and Bettencourt’s sources. This was incorrect, a German census did not appear in their data set, and the audio has been updated to reflect this correction.

In the second segment, we used the term “watts” incorrectly. The audio has been updated to recognize this error.