Gravitational Light Deflection with SKA-VLBI and Its Application to Precision Tests of General Relativity

Experimental test of general relativity remains an ongoing endeavour. Radio astrometry provides a vital tool for precisely measuring the light deflection caused by the Sun, testing general relativity, and discriminating between gravitational theories. The best accuracy for the post-Newtonian relativistic parameter, gamma, achieved with very long baseline interferometry is 9 times 10-5. With 300-sec integration, SKA-VLBI can achieve a sensitivity of about 15 mu Jy at 15 GHz over a bandwidth of 0.256 GHz. This enables detection of about 36 extragalactic radio sources per square degree with flux densities of about 1.5 mJy, and potentially detecting in-beam radio sources. Single-epoch SKA-VLBI observations may achieve an astrometric precision of about 2 mu as. Utilising the Sun as a gravitational lens, 10-epoch positional tracking of extragalactic sources could improve gamma accuracy to about 10-7. Even with Jupiter as a lens, SKA-VLBI can measure gamma to about 10-4. Critically, it may conduct the first measurement of quadrupolar deflection of light caused by Jupiter, determining the physical oblateness of Jupiter, J 2, J, to within about 10-3. These advances are expected to rigorously test and improve gravitational theories or high-order parameterized post-Newtonian formalisms, while laying the foundations for (sub) mu as astrometry.