Local stationarity for lattice dynamics in the harmonic approximation
  We consider the lattice dynamics in the harmonic approximation for We consider the lattice dynamics in the harmonic approximation for a simple hypercubic lattice with arbitrary unit cell. The initial data are random according to a probability measure which enforces slow spatial variation on the linear scale $\epsilon^{-1}$. We establish two time regimes. For times of order $\epsilon^{-\gamma}$, $0<\gamma<1$, locally the measure converges to a Gaussian measure which is space-time stationary with a covariance inherited from the initial (in general, non-Gaussian) measure. For times of order $\epsilon^{-1}$ this local space covariance changes in time and is governed by a semiclassical transport equation. 
T.V. Dudnikova, H. Spohn