  We consider the Casimir-Polder interaction energy between a metallic
nanoparticle and a metallic plate, as well as the Casimir interaction energy
between two macroscopic metal plates, in terms of the many-body dispersion
interactions between their constituents. Expressions for two- and three-body
dispersion interactions between the microscopic parts of a real metal are first
obtained, both in the retarded and non-retarded limits. These expressions are
then used to evaluate, a compare each other, the overall two- and three-body
contributions to the macroscopic Casimir-Polder and Casimir force, by summing
up the contributions from the microscopic constituents of the bodies (metal
nanoparticles), for two geometries: metal nanoparticle/half-space and
half-space/half-space, where all the materials are assumed perfect conductors.
In the case of nanoparticle/half-space, our results fully agree with those that
can be extracted from the corresponding macroscopic results, and explicitly
show the non-applicability of the pairwise approximation for the geometry
considered. In both cases, we find that, while the overall two-body
contribution yields an attractive force, the overall three-body contribution is
repulsive. Also, they turn out to be of the same order, consistently with the
known nonapplicability of the pairwise approximation. The issue of the rapidity
of convergence of the many-body expansion is also briefly discussed.
