c=1/√(ε_0 μ_0) From the gauge theory point of view, I thought that if we consider the potential rather than the electromagnetic field as the physical reality, we would not have to consider two quantities, dielectric constant and magnetic permeability, but even if we write in potential, Maxwell's equations cannot be written down in terms of the speed of light alone. It is curious that the Aharonov-Bohm effect shows that the potential is more physically real than the electromagnetic field.