The ground state energy of a boundary quantum field theory is derived in
planar geometry in D+1 dimensional spacetime. It provides a universal
expression for the Casimir energy which exhibits its dependence on the boundary
conditions via the reflection amplitudes of the low energy particle
excitations. We demonstrate the easy and straightforward applicability of the
general expression by analyzing the free scalar field with Robin boundary
condition and by rederiving the most important results available in the
literature for this geometry.Comment: 10 pages, 2 eps figures, LaTeX2e file. v2: A reference is added, some
minor modifications made to clarify the text. v3: 9 pages, 3 eps figures,
LaTeX2e file, revtex style. Paper throughly restructured and rewritten. Much
more details are given, but essential results and conclusions are unchanged.
Version accepted for publicatio