  We study the dynamics of charged pions in the nuclear medium via the ratio of
differential $\pi^-$- and $\pi^+$-spectra in a coupled-channel BUU (CBUU)
approach. The relative energy shift of the charged pions is found to correlate
with the pion freeze-out time in nucleus-nucleus collisions as well as with the
impact parameter of the heavy-ion reaction. Furthermore, the long-range Coulomb
force provides a 'clock' for the expansion of the hot nuclear system. Detailed
comparisons with experimental data for $Au + Au$ at 1 GeV/A and $Ni + Ni$ at
2.0 GeV/A are presented.
