A critical length scale in jammed granular media

Granular media consist of macroscopic, athermal particles that ``jam'' into a solid-like state when subjected to a confining pressure. Recent studies of this jamming transition in systems of frictionless particles have shown, quite remarkably, that the jamming point has many features of a critical point, exhibiting power law scalings of various quantities nearby. We study the linear response of these jammed systems to a localized mechanical perturbation. The response fluctuates over a length scale that diverges at the jamming transition, providing a direct numerical observation of a critical length scale in jammed granular media.