  The energy of fluctuating electromagnetic field is investigated for the
thermal Casimir force acting between parallel plates made of real metal. It is
proved that for nondissipative media with temperature independent dielectric
permittivity the energy at nonzero temperature comprises of the (renormalized)
energies of the zero-point and thermal photons. In this manner photons can be
considered as collective elementary excitations of the matter of plates and
electromagnetic field. If the dielectric permittivity depends on temperature
the energy contains additional terms proportional to the derivatives of
dielectric permittivity with respect to temperature, and the quasiparticle
interpretation of the fluctuating field is not possible. The correlation
between energy and free energy is considered. Previous calculations of the
Casimir energy in the framework of the Lifshitz formula at zero temperature and
optical tabulated data supplemented by the Drude model at room temperature are
analysed. It is demonstrated that this quantity is not a good approximation
either for the free energy or the energy. A physical interpretation of this
hybrid quantity is suggested. The contradictory results in the recent
literature on whether the zero-frequency term of the Lifshitz formula for the
perpendicular polarized modes has any effective contribution to the physical
quantities are discussed. Four main approaches to the resolution of this
problem are specified. The precise expressions for entropy of the fluctuating
field between plates made of real metal are obtained, which helps to decide
between the different approaches. The conclusion is that the Lifshitz formula
supplemented by the plasma model and the surface impedance approach are best
suited to describe the thermal Casimir force between real metals.
