  The properties of the vacuum are described by quantum physics including the
response to external fields such as electromagnetic radiation. Of the two
parameters that govern the details of the electromagnetic field dynamics in
vacuum, one is fixed by the requirement of Lorentz invariance $c=
1/\sqrt{\varepsilon_{0} \mu_{0}}$. The other one, $Z_{0}=
\sqrt{\mu_{0}/\varepsilon_{0}} = 1/(c\varepsilon_{0})$ and its relation to the
quantum vacuum, is discussed in this contribution. Deriving $\varepsilon_{0}$
from the properties of the quantum vacuum implies the derivation of the fine
structure constant.
