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 Poincaré 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, Poincaré 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. An investigation on the implications and interrelations of equivalence principles led to a possible pseudoscalar or vector interaction, and led to the proposal of weak equivalence principle II (WEP II) which includes rotation in the universal free-fall motion. Evidence for WEP II is discussed and compiled. Cosmological searches for photon-polarization coupling test the possibility of violation of Einstein's equivalence principle and the existence of cosmic pseudoscalar/vector interaction and may reveal a potential influence to our presently observed universe from a larger arena. In relativistic gravity, there is a Lense–Thirring frame dragging on a rotating body with angular momentum. Analogous to gyromagnetic ratio in electromagnetism, one can define gyrogravitational ratio. A profound search for the role of spin in gravitation is to measure the gyrogravitational ratio of particles. This could lead us to probe and understand the microscopic origins of gravity. We discuss the strategies to perform such experiments.