We constrain the post-Newtonian gravity parameter \gamma on kiloparsec scales by comparing the masses of 15 elliptical lensing galaxies from the Sloan Lens ACS Survey as determined in two independent ways .
The first method assumes only that Newtonian gravity is correct and is independent of \gamma , while the second uses gravitational lensing which depends on \gamma .
More specifically , we combine Einstein radii and radial surface-brightness gradient measurements of the lens galaxies with empirical distributions for the mass concentration and velocity anisotropy of elliptical galaxies in the local universe to predict \gamma -dependent probability distributions for the lens-galaxy velocity dispersions .
By comparing with observed velocity dispersions , we derive a maximum-likelihood value of \gamma = 0.98 \pm 0.07 ( 68 % confidence ) .
This result is in excellent agreement with the prediction of general relativity that has previously been verified to this accuracy only on solar-system length scales .