According to Wikipedia, This definition doesn't sit well with me. Flux is defined as the rate that something passes through an infinitesimal surface, divided by the infinitesimal area of that surface. For example, the current flux (or current density), when dotted with a unit vector, gives the rate that charge passes through an infinitesimal surface with that unit vector, divided by the infinitesimal area. But "rate" is not Lorentz invariant, because it involves time which is relative. So the definition above seems to be non-Lorentz invariant (or covariant or whatever). Shouldn't this be a problem?