Solar-system tests of the relativistic gravity
  In 1859, Le Verrier discovered the Mercury perihelion advance anomaly. This anomaly turned out to be the first relativistic-gravity effect observed. During the 157 years to 2016, the precisions and accuracies of laboratory and space experiments, and of astrophysical and cosmological observations on relativistic gravity have been improved by 3-4 orders of magnitude. The improvements have been mainly from optical observations at first followed by radio observations. The achievements for the past 50 years are from radio Doppler tracking and radio ranging together with lunar laser ranging. At the present, the radio observations and lunar laser ranging experiments are similar in the accuracy of testing relativistic gravity. We review and summarize the present status of solar-system tests of relativistic gravity. With planetary laser ranging, spacecraft laser ranging and interferometric laser ranging (laser Doppler ranging) together with the development of drag-free technology, the optical observations will improve the accuracies by another 3-4 orders of magnitude in both the equivalence principle tests and solar-system dynamics tests of relativistic gravity. Clock tests and atomic interferometry tests of relativistic gravity will reach an ever-increasing precision. These will give crucial clues in both experimental and theoretical aspects of gravity, and may lead to answers to some profound issues in gravity and cosmology. 
Wei-Tou Ni
gr-qc astro-ph.EP astro-ph.SR