Here’s the notion. A few years ago, Dirk Brockmann, a theoretical physicist from Germany, was visiting his American friend Dennis, and they got talking about population mobility. Dirk knew Americans move around a lot, but he wondered how to capture where they go, who they talk to. His friend said, “Have you ever heard of Where’s George? Dirk hadn’t.
It’s a website that tracks the movement of dollar bills. Thousands of people participate. All you do is take a bill out of your wallet, type the denomination, serial number, the date and your zip code onto the Where’s George? site, and then, with a pen or a stamp, deface the bill with the words “WheresGeorge.com.” After which (and this is key), you spend it. So now your bill is moving from business to business, person to person, and if and when another Where’s George volunteer discovers it, she or he will note where, note when and spend it again. Since dollar bills pass between people, Dennis suggested why not us the “Where’s George?” data to get a sense of where people go, and, just as interesting, where they don’t go?
That’s what Dirk did. After checking 1,033,095 reports (describing the movement of 464,670 bills), he came up with this map. In the Seattle area, for example, his team found that over two weeks, only 7.8 percent of the bills moved more than 500 miles away. Most of the money stayed close. More interestingly, Dirk’s team began to notice virtual borders, lines that the money rarely crossed. In this map, you can see the territory marked by the Canadian border to the north, a bit of California at the southern end, and Idaho to the east. Oregon and Washington seem undifferentiated. But at the edges, a blue border seems to capture and contain most of the cash (and the people?) moving within. Those lines, Dirk marked deep blue.