Experimental measurement of an effective temperature for densely packed granular materials

A densely packed granular system is an example of an out-of- equilibrium system in the jammed state. It has been a longstanding problem to determine whether this class of systems can be described by concepts arising from equilibrium statistical mechanics, such as an ``effective temperature'' and ``compactivity.'' The measurement of the effective temperature is realized in the laboratory by slowly shearing a closely-packed ensemble of spherical beads confined by an external pressure in a Couette geometry. All the probe particles, independent of their characteristic features, equilibrate at the same temperature, given by the packing density of the system.