Searches for the role of spin and polarization in gravity

TL;DR

This paper reviews theoretical and experimental efforts to understand the role of spin and polarization in gravity, exploring various theories, laboratory experiments, and astrophysical observations related to spin-gravity interactions.

Contribution

It provides a comprehensive overview of the historical, theoretical, and experimental investigations into spin's role in gravitation, including new discussions on WEP II and related evidence.

Findings

Laboratory experiments constrain spin-gravity couplings.

Astrophysical observations test photon polarization effects.

Evidence supports extensions of equivalence principles including rotation.

Abstract

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.