  We study the response of granular materials to external stress using
experiment, simulation, and theory. We derive an entropic, Ginzburg-Landau
functional that enforces mechanical stability and positivity of contact forces.
In this framework, the elastic moduli depend only on the applied stress. A
combination of this feature and the positivity constraint leads to stress
correlations whose shape and magnitude are extremely sensitive to the applied
stress. The predictions from the theory describe the stress correlations for
both simulations and experiments semiquantitatively.

Statistical Mechanics;Soft Condensed Matter