Gravitational lensing and the variability of G

The four observables associated with gravitational lensing of distant quasars by intervening galaxies -image splittings, relative amplifications, time delays, and optical depths - provide separate measures of the strength of the gravitational constant G at cosmological distances. These allow one, in principle, to factor out unknown lensing parameters to directly probe the variation of G over cosmological time. We estimate constraints on G which may be derivable by this method both now and in the future. The limits one may obtain can compete or exceed other direct limits on G today, but unfortunately extracting this information is not independent of the effort to fix other cosmological parameters such as H_o and Ω_o from lensing observations.