Traveling with dollars

A trio of theoretical physicists from the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany, and the University of California at Santa Barbara analyzed human travel movements using dollar bills as a proxy. With data on the travels of almost a half-million dollar bills within the continental US, the researchers discovered that the movement of dollar bills resembles superdiffusive motion with a power-law distribution reminiscent of Lévy flights (see “Beyond Brownian Motion,” Physics Today, February 1996, page 33), but attenuated by long waiting times. With such a mathematical formalism in hand, the spread of human infectious diseases may now be open to a new kind of quantitative analysis. (D. Brockmann, L. Hufnagel, T. Geisel, Nature   439 , 462, 2006  https://doi.org/10.1038/nature04292 .)