ar X iv : g r - qc / 0 31 10 39 v 1 1 3 N ov 2 00 3 35 Years of Testing Relativistic Gravity : Where do we go from here ?

This paper addresses the motivation, technology and recent results in the tests of the general theory of relativity (GR) in the solar system. We specifically discuss Lunar Laser Ranging (LLR), the only technique available to test the Strong Equivalence Principle (SEP) and presently the most accurate method to test for the constancy of the gravitational constant G. After almost 35 years since beginning of the experiment, LLR is poised to take a dramatic step forward by proceeding from cm to mm range accuracies enabled by the new Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) currently under development in New Mexico. This facility will enable tests of the Weak and Strong Equivalence Principles with a sensitivity approaching 10, translating to a test of the SEP violation parameter, η, to a precision of ∼ 3 × 10. In addition, the v/c general relativistic effects would be tested to better than 0.1%, and measurements of the relative change in the gravitational constant, Ġ/G, would be ∼ 0.1% the inverse age of the universe. This paper also discusses a new fundamental physics experiment that will test relativistic gravity with an accuracy better than the effects of the second order in the gravitational field strength, ∝ G. The Laser Astrometric Test Of Relativity (LATOR) will not only improve the value of the parameterized post-Newtonian (PPN) γ to unprecedented levels of accuracy of 1 part in 10, it will also be able to measure effects of the next post-Newtonian order (c) of light deflection resulting from gravity’s intrinsic non-linearity, as well as measure a variety of other relativistic effects. LATOR will lead to very robust advances in the tests of fundamental physics: this mission could discover a violation or extension of general relativity, or reveal the presence of an additional long range interaction in the physical law. There are no analogs to the LATOR experiment; it is unique and is a natural culmination of solar system gravity experiments.