To theoretical physicist Dirk Brockmann, the borders of the United States are out of date.
"Some are kind of arbitrary like New Mexico, Arizona: They’re just kind of drawn on the map," says Brockmann. "Often, they no longer correlate with our behavior."
Specifically, they no longer correlate with how we move.
Brockmann was doing research on human mobility in 2005, and struggling to find useful sources of data, when on the way back from a conference in Canada, he stopped by the home of his old friend Dennis Derryberry in the Green Mountains of Vermont. Over a beer on the porch, he told Derryberry about his research. Derryberry asked: "Do you know about WheresGeorge.com?"
You can think of WheresGeorge.com as a primitive FourSquare for $1 bills. "Georgers"—as users call themselves—"check in" their bills by entering the zip codes and serial numbers, then write or stamp "wheresgeorge.com" on the bill. If someone finds the bill and enters it again, they get a "hit." The top Georger—an ammunition dealer who goes by the handle Wattsburg Gary—has entered more than 2 million bills and has nearly half a million hits.
This was, according to Brockmann’s account, the beginning of "Where’s George?" research: "Forming a mental image of millions of these dollar bill journeys in my head, I was convinced that analyzing this data would reveal essential properties of human mobility, the driving force behind the dispersal of bank notes."
Brockmann has, in fact, used the dollar bill data to reveal certain "essential properties" (specifically, that our travels follow a Power Law), and also to model the Swine Flu epidemic. But one of his coolest research projects is his work on "effective boundaries."
Brockmann took data for how the dollar bills traveled, and used network theory to draw lines where dollar bills are less likely to cross. In places they follow state borders, but not always; Missouri is divided into East and West, as is Pennsylvania. The "Chicago catchment area" includes a big chunk of both Indiana and Wisconsin.
The resulting map shows how "effective communities" don’t necessarily follow state lines. "I don’t know so much about the culture of the U.S.," says Brockmann, who grew up in Germany. "But when I give talks on this, normally someone in the audience says, 'Oh, this makes perfect sense."