  The static forces between electric charges and currents are modified at the
loop level by the presence of a plasma. While electric charges are screened,
currents are not. The effective coupling constant at long distances is enhanced
in both cases as compared to the vacuum, and by different amounts, a clear sign
that Lorentz symmetry is broken. We investigate these effects quantitatively,
first in a QED plasma and secondly using non-perturbative simulations of QCD
with two light degenerate flavors of quarks.
