Exact Casimir interaction of perfectly conducting three-spheres in four euclidean dimensions

Exploiting conformal symmetry, we derive a simple exact formula for the classical electromagnetic Casimir interaction of two perfectly conducting three-spheres, including the sphere-plate geometry as a special case, in four Euclidean dimensions. We verify that the short distance expansion of the Casimir energy agrees to leading order with the proximity force approximation (PFA), while the next-to-leading order is in agreement with a recently proposed derivative expansion of the Casimir energy. At the next-to-next-to-leading order we find a nonanalytic correction to PFA, which for a sphere-plate system is of the order of ðd=RÞ3=2 logðd=RÞ, where d is the separation and R the sphere radius.