Can Einstein's theory of gravitation be tested beyond the geometrical optics limit?

Abstract

AN upper limit of a few × 10⁻³ arc s has been placed¹ on the differential deflection of right and left circularly polarised (r.c.p. and l.c.p., respectively) radiation which passes by the Sun from the quasar 3C273. The separation of quasar images in r.c.p. and l.c.p. radiation permits a measurement of the effect of gravitation on polarised radiation. This seems to be the first experiment in which Einstein's theory of gravitation is tested beyond the geometrical optics approximation. Here, I estimate the new effect (that is, the differential gravitational deflection of polarised radiation), and point out that though, using present techniques, it is too small to be measurable for the Sun, it is, however, potentially observable for gravitationally collapsed objects.