The electromagnetic field stress tensor near dielectric half-spaces

The energy density and pressures in a vacuum region near dielectric half-spaces are investigated. First, the expectation values of the squares of the electric and magnetic fields outside a half-space filled with a uniform dispersive dielectric are examined. A positive energy density of the electromagnetic field is found which diverges at the interface despite the inclusion of dispersion in the calculation. Results for the mean squared fields, the energy density, and the pressures in the vacuum region between two parallel half-spaces are also presented. Of particular interest is the sign of the energy density. The effect of dispersion tends to increase the energy density. Nonetheless, in some cases, including physically realizable ones, energy density can be negative in the central region. The effect of finite temperature upon the energy density is also investigated. This effect also tends to increase the energy density, but negative energy density at room temperature is realizable. The null energy condition is then investigated, and it is found that it is not violated, which implies that the gravitational effect on light rays parallel to the plates is to cause focusing.