Otto Stern, the Gdh Sum Rule and Various Spin Crises

The history of spin and anomalous magnetic moment is full of puzzles and "crises" from the first observations in the 1920's to the present day. The Gerasimov-Drell-Hearn sum rule connects the anomalous magnetic moment with the helicity dependent cross section for photoproduction, and as such checks the internal consistency of our understanding of the nucleon spin structure. Various generalizations of the sum rule have been proposed for the case of virtual photons, thus interpolating from the real photon point to deep inelastic scattering. A series of recent and newly proposed experiments with beam and target/recoil polarization will study this transition between the coherent spin-dependent response and the incoherent response of the partons in the scaling region.