Relativistic deflection of radio signals in the solar gravitational field measured with VLBI

Abstract

For an observer on Earth the general theory of relativity (GR) predicts an apparent outward displacement of a star seen at the Sun's limb of 1.75 arc s. A generalized formulation for gravitational deflection of light¹ includes a parameter γ which ranges from 1 (GR) to −1 (no deflection). Early total eclipse measurements² constrained γ by 20–40%, permitting alternative gravitational theories (for example, ref. 3). Radio interferometer measurements of deflections of extragalactic objects^4,5 and timing measurements of spacecraft signals⁶ have all been consistent with GR with increasing accuracy. We now report the most sensitive measurement of deflection so far achieved, in which very long baseline interferometry (VLBI) observations yield a value of γ of 1.008 with a 1 σ formal standard error of ±0.005.