Zero point energy of a conducting spherical shell

The zero-point energy of a conducting spherical shell is evaluated by imposing boundary conditions on the potential A_{mu}, and on the ghost fields. The scheme requires that temporal and tangential components of A_{mu} perturbations should vanish at the boundary, jointly with the gauge-averaging functional, first chosen to be of the Lorenz type. Gauge invariance of such boundary conditions is then obtained provided that the ghost fields vanish at the boundary. Normal and longitudinal modes of the potential obey an entangled system of eigenvalue equations, whose solution is a linear combination of Bessel functions under the above assumptions, and with the help of the Feynman choice for a dimensionless gauge parameter. Interestingly, ghost modes cancel exactly the contribution to the Casimir energy resulting from transverse and temporal modes of A_{mu}, jointly with the decoupled normal mode of A_{mu}. Moreover, normal and longitudinal components of A_{mu} for the interior and the exterior problem give a result in complete agreement with the one first found by Boyer, who studied instead boundary conditions involving TE and TM modes of the electromagnetic field. The coupled eigenvalue equations for perturbative modes of the potential are also analyzed in the axial gauge, and for arbitrary values of the gauge parameter. The set of modes which contribute to the Casimir energy is then drastically changed, and comparison with the case of a flat boundary sheds some light on the key features of the Casimir energy in noncovariant gauges.