Suppose 2 inertial frames with relative relativistic velocities. To discuss phenomena in other frame, we need to use Lorentz Transform for length and time. I got question: do we have to also use LT to talk about permittivity and permeability of other IRF? This seems strange. Permittivity has units of distance, which should be LT'ed. Thus we would conclude that permittivity of other IRF is different than that of ours? Are we to suppose that vacuum around other IRF has changes in permittivity just because it is in relative motion? If so, then how comes it is called "constant"? If not, then c must appear different in other IRF for us?