  The Euclidean formulation of SU(2) Yang-Mills thermodynamics admits periodic,
(anti)selfdual solutions to the fundamental, classical equation of motion which
possess one unit of topological charge: (anti)calorons. A spatial coarse
graining over the central region in a pair of such localised field
configurations with trivial holonomy generates an inert adjoint scalar field
$\phi$, effectively describing the pure quantum part of the thermal ground
state in the induced quantum field theory. Here we show for the limit of zero
holonomy how (anti)calorons associate a temperature independent electric
permittivity and magnetic permeability to the thermal ground state of
SU(2)$_{\tiny\mbox{CMB}}$, the Yang-Mills theory conjectured to underlie photon
propagation.
