  We investigate the dispersion of a classical electromagnetic field in a
relativistic ideal gas of charged bosons using scalar quantum electrodynamics
at finite temperature and charge density. We derive the effective
electromagnetic responses and the electromagnetic propagation modes that
characterize the gas as a left-handed material with negative effective index of
refraction $n_{\rm eff}=-1$ below the transverse plasmon frequency. In the
condensed phase, we show that the longitudinal plasmon dispersion relation
exhibits a roton-type local minimum that disappears at the transition
temperature.
