interatomic events.

Consider an external excitation of a material which responds with a specific change in its state. The decay or fading of the response constitutes the relaxation process. The decay can be described by an exponential function (within certain limits) $e^{t/\tau_{0}}$ where $t$ is time and $\tau_{0}$ is the time constant characterizing the rate of relaxation. Usually $\tau_{0}$ becomes smaller with an increase in temperature or decrease in pressure. Phosphorous fades more slowly at colder temperatures, for example.

In section 2 we discuss the kinematics of a media with a variable Riemannian metric $G$ in a 4-D material space-time $P$, embedded into 4-D Absolute (Newton’s) space-time $M^4$ with the Euclidean metric $H$. Thus, the 3-D “ground state” metric tensor is introduced together with the proper time lapse function and the material shift vector field. We consider mass conservation law in section 3. Elastic and inelastic strain tensors $E^{el};E^{in}$ are introduced in section 4 as a measure of deformation and the “unstrained state” respectively. The Lagrangian describing inelastic and elastic processes in media is discussed in section 5. A variational formulation of aging theory and the Euler-Lagrange equations (equations of elasticity coupled with the aging equations) are considered in section 