The Arrow of Time: Past and Future of Grains

In a granular fluid the balance between energy fluxes entering and leaving the system establishes a non-equilibrium stationary state. Therefore time-reversal symmetry is broken and this affects the statistical features of many observables. A few examples are discussed here, pertaining to the two main paradigms studied in previous chapters: tracer's dynamics and hydrodynamics. In both cases the choice of a reduced number of degrees of freedom appears as a contraction of information. Sometimes this reduction affects non-equilibrium properties.