  We show that the interpretation of $\mathbf{D}=\varepsilon_{0} \mathbf{E}$ as
vacuum polarization is consistent with quantum electrodynamics. A free
electromagnetic field polarizes the vacuum but the magnetization and
polarization currents cancel giving zero source current. The speed of light is
a universal constant while the fine structure constant that couples the EM
field to matter runs. In that sense, the quantum vacuum can be understood as a
modern Lorentz invariant ether.
