Researchers at the University of California at Santa Barbara and the Max Planck Institute for Dynamics and Self Organization have used the Web game "Where's George?," which monitors the geographic circulation of dollar bills by their serial numbers, to would spread from human to human.
The physicists based their research on the idea that like viruses, money is transported by people from place to place.
"Since we can't track people with tracking devices, like we do animals, we needed to get data that provided us with millions of movements of individuals," said Lars Hufnagel, a post-doctoral fellow at the Kavli Institute for Theoretical Physics at UCSB.
"We use these dollar bills as radar devices for humans," he said. Hufnagel co-wrote an article on the research in this week's Nature journal.
Threat of a human "superflu" has lorded over people's minds around the world since the spread of bird flu in Asia and SARS (severe acute respiratory syndrome) overseas. Modern transportation and travel make it much easier for a disease in one region of the world to spread to another quickly. In contrast, centuries-ago pandemics moved slower across geographical regions because people typically traveled only a few kilometers daily.
It's difficult to forecast how an epidemic might spread today, given that there's no quantitative analysis on how people move around in a country, or from country to country. Hufnagel's research, conducted with two other scientists, aims to create a statistical model for how people move about despite the mode of transportation, but tracking dollar bills is not conclusive.
Hufnagel said that the group's findings were surprising because the bills moved along a simple mathematical model, called universal scaling laws. In this case, scaling laws describe movement from local areas to regional to long-distance lengths, but they have been applied to biology and physical systems.
Using data from half a million bills tracked on the site, the scientists developed a scaling law theory that describes the observed movements of travelers over distances from just a few kilometers to a few thousand. For example, most money travels locally, but there is a small likelihood it will move across the country.
In modeling epidemics, Hufnagel said, the scale becomes specific to the disease. A more severe epidemic might travel differently than a less severe one, for example.