  Light traveling through the vacuum interacts with virtual particles similarly
to the way that light traveling through a dielectric interacts with ordinary
matter. And just as the permittivity of a dielectric can be calculated, the
permittivity $\epsilon_0$ of the vacuum can be calculated, yielding an equation
for the fine-structure constant $\alpha$. The most important contributions to
the value of $\alpha$ arise from interactions in the vacuum of photons with
virtual, bound states of charged lepton-antilepton pairs. Considering only
these contributions, the fully screened $\alpha \cong 1/(8^2\sqrt{3\pi/2})
\cong 1/139$.
