Hubble image
The bending of light by a massive object predicted by conventional general relativity gives rise to gravitational lensing, resulting in multiple images as seen here in the Hubble Space Telescope image of a quasar lensed by a foreground elliptical galaxy.
Gravitational birefringence produces a polarization-dependent shift D f = (f2/R2)f in the Einstein formula for the angle of deflection f = 4M/R of light with closest approach distance R to a spherically-symmetric mass M. This would be seen if f2 were characterized by an as yet unknown large scale rather than the quantum scale lc derived in Further information, and would produce a polarization dependence in the apparent position of the lensed images. Observation of this effect would be direct evidence for gravitational birefringence and imply a violation of the strong equivalence principle on astronomical scales. (Photo: Kavan Ratnatunga, Johns Hopkins University.)