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

Understand

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.