Post-Newtonian gravity: its theory--experiment interface

Abstract

The local non-gravitational physics of matter possesses, to very high degree, two observational features: (1) cosmological constancy of its dimensionless numbers in Earth laboratories as well as throughout spacetime, and (2) rotational invariance. This strongly implies that cosmological--gravitational physics is metric. We review the properties of metric gravity---which represents a broad class of theories of gravity including general relativity, a highly unique metric theory which also fulfils the strong equivalence principle, i.e., spacetime invariance of local gravitational physics, as well. Although the more general parametrized post-Newtonian (PPN) gravitational metric field is considered, and its empirical status is reviewed in the appendix, particular attention is given to the most viable degrees of freedom in metric gravity which are parametrized by two of the PPN coefficients and , both presently experimentally determined to a part in to be in accord with general relativity's predictions. A new determination of and using existing radar ranging data to Viking Mars landers is described which involves the simultaneous fit of two post-Newtonian effects---Shapiro time delay of radar propagation and polarization of inner planet orbits if the Sun's gravitational-to-inertial mass ratio differs from 1. Methods for measuring these aspects of gravity to a part in in the future are discussed, and the special theoretical significance in reaching this precision is outlined. Second post-Newtonian order renormalizations of first post-Newtonian order gravity effects at the level (the Sun's fractional gravitational self-energy) are illustrated as new measurement objectives for the future.