Parametrized post-Newtonian formalism (PPN)

The parametrized post-Newtonian formalism expresses a weak-field metric tensor using phenomenological parameters so that different gravitational theories can be compared with the same observations. Its spatial-curvature parameter ${\gamma }_{\mathrm{PPN}}$ is one in general relativity; it is distinct from the kinematic Lorentz factor.

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 $4GM/(b{c}^2)$.