Brian Dennison

Improved upper limits of gravitational deflection of polarised radiation

RECENTLY Harwit et al.1 established upper limits for any polarisation dependence in the bending of radio waves by the Suns gravitational field. Any differential angular splitting between orthogonal polarisations was shown by VLBI techniques, and from the constancy of the Stokes parameters in the carrier wave from a spacecraft, to be < 10−3 and 10−6, respectively, of the total bending. Since the latter technique is not sensitive to a splitting in the tangential direction, whereas interferometry is, development of both methods is important. These experiments form a unique test of the equivalence principle1.