Goldstein et. al.

"A general continuum theory for the distribution of hairs in a bundle is developed, treating indi-
vidual fibers as elastic fi laments with random intrinsic curvatures. Applying this formalism to the
iconic problem of the ponytail, the combined eff ects of bending elasticity, gravity, and orientational
disorder are recast as a di fferential equation for the envelope of the bundle, in which the compress-
ibility enters through an `equation of state'. From this, we identify the balance of forces in various
regions of the ponytail, extract a remarkably simple equation of state from laboratory measurements
of human ponytails, and relate the pressure to the measured random curvatures of individual hairs."

Swedish Finance Minister Anders Borg.

Abstract of the paper, "The Shape of a Ponytail and the Statistical Physics of Hair Fiber Bundles," which won the Ig Nobel Prize in Physics.