Searches for the role of spin and polarization in gravity
  Spin is fundamental in physics. Gravitation is universal. Searches for the
role of spin in gravitation dated before the firm establishment of the electron
spin in 1925. Since mass and spin or helicity in the case of zero mass are the
only invariants of the Poincare group and mass participates in universal
gravitation, these searches are natural steps to pursue. Here we review both
the theoretical and experimental efforts in searching for the role of
spin/polarization in gravitation. We discuss torsion, Poincare gauge theories,
teleparallel theories, metric-affine connection theories and pseudoscalar
(axion) theories. We discuss laboratory searches for electron and nucleus
spin-couplings -- the weak equivalence principle experiments for
polarized-bodies, the finite-range spin-coupling experiments, the spin-spin
coupling experiments and the cosmic-spin coupling experiments. The role played
by angular momentum and rotation is explicitly discussed. We discuss
astrophysical and cosmological searches for photon polarization coupling.
Investigation in the implications and interrelations of equivalence principles
led to a possible pseudoscalar or vector interaction, and led to the proposal
of WEP II (Weak Equivalence Principle II) which include rotation in the
universal free-fall motion. Evidences for WEP II are discussed and compiled.
