  Many properties of solids such as the glass state are commonly treated as
nonequilibrium phenomena, which involve many conceptual difficulties. However,
few studies have addressed the problem of understanding equilibrium itself.
Equilibrium is commonly assessed based on the assumption that its thermodynamic
state should be determined solely by temperature and pressure. However, this
assumption must be reappraised from the beginning through a rigorous definition
of equilibrium because no rigorous proof for this assumption exists. Previous
work showed that for solids, the equilibrium positions of all constituent atoms
of the solid are state variables (TC). In this study, this conclusion is
further elaborated starting from the principles of solid-state physics. This
theory has been applied to glass materials. Results show that first, the glass
state is an equilibrium state. Accordingly, the properties of a glass can be
solely described by the present positions of the atom, irrespective of their
previous history. Second, the glass transition, although a nonequilibrium
phenomenon, is well described using TCs if the thermal part of the energy is be
assumed to be well separated from the structural part. The only dynamic
parameter involved in this approach is the relaxation time, which is uniquely
determined using the present values of the TCs. Therefore, complicated
functions describing the past history are unnecessary. This implies that the
activation energy for the structural relaxation strongly depends on TCs. This
finding provides a reasonable understanding of the large deviations from the
Arrhenius law. The unrealistic values of the activation energy and of the
pre-exponential factor of the relaxation time can be resolved on this basis.
The theory is particularly suitable for experiments that do not involve
hypothetical quantities and models; therefore, all quantities are measurable.
