  In the physical vacuum of QCD, the energy density of light-quark fields
strongly coupled to slowly varying gluon fields can be negative. The states
that drive this energy density lowest are condensates of pairs of quarks and
antiquarks of nearly opposite momenta. These quark-antiquark condensates break
chiral symmetry. They may also affect other features of hadronic physics, such
as the range of the strong force and the confinement of color.
