On the Zero-Point Energy of a Conducting Spherical Shell

TL;DR

This paper evaluates the zero-point energy of a conducting spherical shell using boundary conditions on the potential and ghost fields, revealing gauge invariance and the cancellation of ghost contributions, and compares covariant and non-covariant gauges.

Contribution

It provides a detailed analysis of the eigenmodes and boundary conditions affecting Casimir energy in spherical geometries, including gauge invariance and ghost mode contributions.

Findings

Ghost modes cancel the contribution from transverse and temporal modes.

Normal and longitudinal modes agree with Boyer's results for TE and TM modes.

Gauge choice significantly alters the set of modes contributing to the Casimir energy.

Abstract

The zero-point energy of a conducting spherical shell is evaluated by imposing boundary conditions on the potential, and on the ghost fields. The scheme requires that temporal and tangential components of perturbations of the potential should vanish at the boundary, jointly with the gauge-averaging functional, first chosen 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 the potential, jointly with the decoupled normal mode of the potential. Moreover, normal and longitudinal components of the potential 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 non-covariant gauges.