Searches for the role of spin and polarization in gravity: a five-year
  update
  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 Casimir invariants of the Poincar\'e group and
mass participates in universal gravitation, these searches are natural steps to
pursue. In this update, we report on the progress on this topic in the last
five years after our last review. We begin with how is Lorentz/Poincar\'e group
in local physics arisen from spacetime structure as seen by photon and matter
through experiments/observations. The cosmic verification of the Galileo
Equivalence Principle for photons/electromagnetic wave packets (Universality of
Propagation in spacetime independent of photon energy and polarization, i.e.
nonbirefringence) constrains the spacetime constitutive tensor to high
precision to a core metric form with an axion degree and a dilaton degree of
freedom. Hughes-Drever-type experiments then constrain this core metric to
agree with the matter metric. Thus comes the metric with axion and dilation. In
local physics this metric gives the Lorentz/Poincar\'e covariance. Constraints
on axion and dilaton from polarized/unpolarized laboratory/astrophysical/cosmic
experiments/observations are presented. In the end, we review the theoretical
progress on the issue of gyrogravitational ratio for fundamental particles and
the experimental progress on the measurements of possible long
range/intermediate range spin-spin, spin-monopole and spin-cosmos interactions.
