  In Yang-Mills theory massless point sources lead naturally to shock-wave
configurations. Their magnetic counterparts endow the vacuum of the
four-dimensional compact abelian model with a Coulomb-gas behaviour whose
physical implications are briefly discussed. (Contribution to ``Quark
Confinement and the Hadron Spectrum'', Como 20-24 June 1994. Revised version.)

High Energy Physics - Theory;Condensed Matter;High Energy Physics - Lattice