Probing the Microscopic Origin of Gravity via Precision Polarization and Spin Experiments

As in other parts of physics, we advocate the interaction approach: experiments ,<-> (...are influenced and their importance is determined by...and vice versa) phenomenology <-> low-energy effective (field) theory <-> microscopic theory to probe the microscopic origin of gravity. Using the x-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 the future generation of polarization measurement of cosmic background radiation. The verification or refutal of this axionic effect will be a crucial step for constructing an effective theory and probing the microscopic origin of gravity. The interaction of spin with gravity is another important clue for probing the microscopic origin of gravity. The interplay of experiments, phenomenology and effective theory are 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.