An Overview of Solar System Gravitational Physics: The Theory—Experiment Interface

Abstract

If the gravitational metric tensor field of Einstein’s General Relativity is supplemented by other long range, very weakly coupled interaction fields, then General Relativity’s Equivalence Principle foundations are violated, or its post-Newtonian (1/c ²order) structure is altered, or both. Space experiments test for and measure such possibilities; presently universality of free fall is confirmed to about a part in 10¹², and no deviations of post-Newtonian metric gravity from general relativity are seen down to the few parts in 10⁴ level. Future experiments in space can significantly increase the precision to which fundamental physical law is probed. In particular, transponded interplanetary laser ranging can measure presence of metrically coupled scalar fields in gravity with two or three orders of magnitude higher precision than past experiments, and can begin to measure the second post-Newtonian (1/c ⁴) structure of gravity. A space-based experiment of the universality of free-fall (STEP) can detect additional interactions of a non-metric nature with five or six orders of magnitude higher precision than today’s experiments.