 Parametrized post-Newtonian light deflection

The leading Schwarzschild light deflection of a distant ray passing an isolated spherical mass is $\alpha =2(1+{\gamma }_{\mathrm{PPN}})GM/(b{c}^2)$. Thus measurements of angular displacement can constrain ${\gamma }_{\mathrm{PPN}}$ rather than merely detect a nonzero gravitational field. In general relativity this becomes the Schwarzschild light deflection $4GM/(b{c}^2)$.