The constancy of G and other gravitational experiments

Abstract

Traditionally, theories of gravitation have received their most demanding tests in the solar-system laboratory. Today, electronic observing technology makes possible solar system tests of substantially increased accuracy. We consider how these technologies are being used to study gravitation with an emphasis on two questions: (i) Dirac and others have investigated theories in which the constant of gravitation, G , appears to change with time. Recent analyses using the Viking data yield | G / G | < 3 x 10 ^-11 per year. With further analysis, the currently available ensemble of data should permit an estimate of G/G with an uncertainty of 10 ^-11 per year. At this level it will become possible to distinguish among competitive theories. (ii) Shapiro’s time-delay effect has provided the most stringent solar-system test of general relativity. The effect has been measured to be consistent with the predictions of general relativity with a fractional uncertainty of 0.1%. An improved analysis of an enhanced data set should soon permit an even more stringent test. Technology now permits new kinds of tests to be performed. Among these are some that measure relativistic effects due to the square of the (solar) potential and others that detect the Earth’s ‘gravitomagnetic’ field (the Lense-Thirring effect). These experiments, and the use of astrophysical systems are among the experimental challenges for the coming decades.