The Confrontation between General Relativity and Experiment: A 1998 Update

TL;DR

This 1998 update synthesizes the status of experimental tests of general relativity (GR) and frameworks for analyzing them, highlighting the strong support for the Einstein Equivalence Principle and the tight constraints on post-Newtonian parameters, especially γ. It surveys both weak-field solar-system tests and strong-field regimes accessed through binary pulsars, showing GR’s predictions—such as gravitational radiation damping—agreeing with observations, while also outlining the potential of upcoming gravitational-wave astronomy to challenge GR in dynamical, strong-gravity environments. The work emphasizes the utility of formalisms like the PPN and THεμ/C^2 frameworks in translating experiments into theory-space constraints and discusses how scalar-tensor theories remain viable only in certain limits, with pulsar data offering particularly stringent strong-field tests. Finally, it points to future prospects, including space-based tests (STEP, GAIA), precision redshift and inverse-square-law tests at short ranges, and the forthcoming era of gravitational-wave observations with LIGO/VIRGO and beyond, which will illuminate GR’s validity and guide potential new physics.

Abstract

The status of experimental tests of general relativity and of theoretical frameworks for analysing them are reviewed. Einstein's equivalence principle (EEP) is well supported by experiments such as the Eötvös experiment, tests of special relativity, and the gravitational redshift experiment. Future tests of EEP will search for new interactions arising from unification or quantum gravity. Tests of general relativity have reached high precision, including the light deflection, the Shapiro time delay, the perihelion advance of Mercury, and the Nordtvedt effect in lunar motion. Gravitational wave damping has been detected to half a percent using the binary pulsar, and new binary pulsar systems promise further improvements. When direct observation of gravitational radiation from astrophysical sources begins, new tests of general relativity will be possible.

Figures (9)

• Figure 1: Selected tests of the Weak Equivalence Principle, showing bounds on $\eta$, which measures fractional difference in acceleration of different materials or bodies. Free-fall and Eöt-Wash experiments originally performed to search for fifth force. Hatched line shows current bounds on $\eta$ for gravitating bodies from lunar laser ranging (LURE).
• Figure 2: Selected tests of local Lorentz invariance showing bounds on parameter $\delta$, which measures degree of violation of Lorentz invariance in electromagnetism. Michelson-Morley, Joos, and Brillet-Hall experiments test isotropy of round-trip speed of light, the latter experiment using laser technology. Centrifuge, two-photon absorption (TPA) and JPL experiments test isotropy of light speed using one-way propagation. Remaining four experiments test isotropy of nuclear energy levels. Limits assume speed of Earth of 300 km/s relative to the mean rest frame of the universe.
• Figure 3: Selected tests of local position invariance via gravitational redshift experiments, showing bounds on $\alpha$, which measures degree of deviation of redshift from the formula $\Delta \nu / \nu = \Delta U/c^2$.
• Figure 4: Geometry of light deflection measurements.
• Figure 5: Measurements of the coefficient $(1 + \gamma )/2$ from light deflection and time delay measurements. General relativity value is unity. Arrows denote anomalously large values from 1929 and 1936 eclipse expeditions. Shapiro time-delay measurements using Viking spacecraft and VLBI light deflection measurements yielded agreement with GR to 0.1 percent. Hipparcos denotes the optical astrometry satellite.