Memory effects in granular materials

We present a combined experimental and theoretical study of memory effects in vibration-induced compaction of granular materials. In particular, the r esponse of the system to an abrupt change in shaking intensity is measured. At short times after the perturbation a granular analog of aging in glasse s is observed. Using a simple two-state model, we are able to explain this short-time response. We also discuss the possibility for the system to obey an approximate pseudo-fluctuation-dissipation theorem relationship and rel ate our work to earlier experimental and theoretical studies of the problem .