  As in other parts of physics, we advocate the interaction approach:
experiments <--> phenomenology <--> low-energy effective (field) theory <-->
microscopic theory to probe the microscopic origin of gravity. Using chi-g
phenomenological framework, we discuss the tests of equivalence principles. The
only experimentally unconstrained degree of freedom is the axion freedom. It
has effects on the long-range astrophysical/cosmological propagation of
electromagnetic waves and can be tested/measured using future generation of
polarization measurement of cosmic background radiation. The verification or
refutal of this axionic effect will be a crucial step for constructing
effective theory and probing the microscopic origin of gravity. The interaction
of spin with gravity is another important clue for probing microscopic origin
of gravity. The interplay of experiments, phenomenology and effective theory is
expounded. An ideal way to reveal the microscopic origin of gravity is to
measure the gyrogravitational ratio of particles. Three potential experimental
methods are considered.
