Shock Waves and the Vacuum Structure of Gauge Theories
  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.) 
M. Martellini, A. Sagnotti and M. Zeni