  A complementary approach, derived from (a) higher dimensional anti--de Sitter
(AdS) space, (b) light-front quantization and (c) the invariance properties of
the full conformal group in one dimension leads to a nonperturbative
relativistic light-front wave equation which incorporates essential
spectroscopic and dynamical features of hadron physics. The fundamental
conformal symmetry of the classical QCD Lagrangian in the limit of massless
quarks is encoded in the resulting effective theory. The mass scale for
confinement emerges from the isomorphism between the conformal group and
SO(2,1). This scale appears in the light-front Hamiltonian by mapping to the
evolution operator in the formalism of de Alfaro, Fubini and Furlan, which
retains the conformal invariance of the action. Remarkably, the specific form
of the confinement interaction and the corresponding modification of AdS space
are uniquely determined in this procedure.
