  The force free approximation is often useful when describing tenuous plasmas
in strong cosmic magnetic fields. Time evolution of any such system is governed
by the information that can be transported along the characteristics of the
plasma modes allowed by the force free constraint. This brief article
elucidates, for the first time, the nature of the information that can be
transported along the characteristics for each of the two plasma modes, the
Alfven and fast modes. It is reassuring that these modes behave as one would
expect if the perfect magnetohydrodynamic (MHD) modes in a magnetically
dominated plasma were evaluated in the limit of zero plasma inertia (i.e., only
allowed to propagate information consistent with the force free constraint).
There are no properties of these waves that do not exist within the MHD theory,
when this additional constraint on the current is imposed. The results of the
characteristic analysis elucidates the nature of the causality violation in
force free black hole magnetospheres. The most significant result from the
standpoint of global causality is that charge and current perturbations can not
be transported along the fast mode short wavelength characteristics in the
force free limit.
