New constraints on gravity-induced birefringence

A wide class of gravitation theories predicts gravity-induced birefringence. For Moffat’s NGT, the prototypical theory of this type, Gabriel, Haugan, Mann, and Palmer used the predicted gravitational birefringence and observations of solar polarization to constrain the Sun’s nonsymmetric charge ${\mathit{l}}_{\mathrm{\odot }}$. We improve on this constraint by making use of improved knowledge of the solar source of polarization and of a refined analysis procedure. We obtain ${\mathit{l}}_{\mathrm{\odot }}^2$${)}^2$. © 1996 The American Physical Society.