Gauge/gravity duality leads to a simple, analytical, and phenomenologically compelling nonperturbative approximation to the full light-front QCD Hamiltonian. This approach, called “Light-Front Holography”, successfully describes the spectroscopy of light-quark meson and baryons, their elastic and transition form factors, and other hadronic properties. The bound-state Schrödinger and Dirac equations of the soft-wall AdS/QCD model predict linear Regge trajectories which have the same slope in orbital angular momentum L and radial quantum number n for both mesons and baryons. Light-front holography connects the fifth-dimensional coordinate of AdS space z to an invariant impact separation variable ζ in 3 + 1 space at fixed light-front time. A key feature is the determination of the frame-independent light-front wavefunctions of hadrons – the relativistic analogs of the Schrödinger wavefunctions of atomic physics which allow one to compute form factors, transversity distributions, spin properties of the valence quarks, jet hadronization, and other hadronic observables. One thus obtains a one-parameter color-confining model for hadron physics at the amplitude level. AdS/QCD also predicts the form of the nonperturbative effective coupling αsAdS (Q) and its β-function with an infrared fixed point which agrees with the effective coupling αg1 (Q2 ) extracted from measurements of the Bjorken sum rule below Q2 < 1 GeV2 . This is consistent with a flux-tube interpretation of QCD where soft gluons with virtualities Q2 < 1 GeV2 are sublimated into a color-confining potential for quarks. We discuss a number of phenomenological hadronic properties which support this picture.
