Searches for the role of spin and polarization in gravity: a five-year update

TL;DR

This paper reviews recent five-year progress in understanding the role of spin and polarization in gravity, focusing on experimental constraints, theoretical developments, and the implications of spacetime structure on fundamental particles and interactions.

Contribution

It provides an updated synthesis of experimental and theoretical advances in spin-related gravitational research since the last review, emphasizing constraints on spacetime metrics and particle interactions.

Findings

High-precision constraints on spacetime constitutive tensor from nonbirefringence.

Laboratory and astrophysical limits on axion and dilaton fields.

Progress in measuring gyrogravitational ratios and spin interactions.

Abstract

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.