The Confrontation between General Relativity and Experiment

TL;DR

This review assesses the experimental status and theoretical frameworks of general relativity, tracing the evolution from foundational tests of the Einstein equivalence principle to precise post-Newtonian measurements and strong-field tests. It details the main experimental pillars—WEP, LLI, LPI, and the PPN formalism—alongside metric and non-metric theories, and then surveys strong-field probes via binary pulsars and gravitational-wave observations. By integrating solar-system tests, pulsar timing, and gravitational-wave data, the paper highlights general relativity’s triumphs and outlines upcoming missions (e.g., MICROSCOPE, Cassini-era improvements, GAIA, LATOR, LISA) that could tighten constraints or reveal new physics. The work emphasizes the continued relevance of precision tests for probing unification, quantum gravity effects, and the possible existence of new interactions in the gravitational sector. It positions gravitational-wave astronomy as a transformative arena for testing GR in the strong-field regime and for exploring the fundamental structure of spacetime.

Abstract

The status of experimental tests of general relativity and of theoretical frameworks for analysing them is reviewed. Einstein's equivalence principle (EEP) is well supported by experiments such as the Eotvos experiment, tests of special relativity, and the gravitational redshift experiment. Future tests of EEP and of the inverse square law are searching for new interactions arising from unification or quantum gravity. Tests of general relativity at the post-Newtonian level 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 in an amount that agrees with general relativity to better than half a percent using the Hulse-Taylor binary pulsar, and other binary pulsar systems have yielded other tests, especially of strong-field effects. When direct observation of gravitational radiation from astrophysical sources begins, new tests of general relativity will be possible.

Figures (8)

• Figure 1: Selected tests of the weak equivalence principle, showing bounds on $\eta$, which measures fractional difference in acceleration of different materials or bodies. The free-fall and E$\hbox{\it "o}$t-Wash experiments were originally performed to search for a fifth force (green region, representing many experiments). The blue band shows evolving bounds on $\eta$ for gravitating bodies from lunar laser ranging (LLR).
• Figure 2: Selected tests of local Lorentz invariance showing the bounds on the parameter $\delta$, which measures the degree of violation of Lorentz invariance in electromagnetism. The Michelson-Morley, Joos, Brillet-Hall and cavity experiments test the isotropy of the round-trip speed of light. The centrifuge, two-photon absorption (TPA) and JPL experiments test the isotropy of light speed using one-way propagation. The most precise experiments test isotropy of atomic energy levels. The limits assume a speed of Earth of 370 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 $\Delta \nu / \nu = \Delta U/c^2$. In null redshift experiments, the bound is on the difference in $\alpha$ between different kinds of clocks.
• Figure 4: Geometry of light deflection measurements.
• Figure 5: Measurements of the coefficient $(1 + \gamma )/2$ from light deflection and time delay measurements. Its GR value is unity. The arrows at the top denote anomalously large values from early eclipse expeditions. The Shapiro time-delay measurements using the Cassini spacecraft yielded an agreement with GR to $10^{-3}$ percent, and VLBI light deflection measurements have reached 0.02 percent. Hipparcos denotes the optical astrometry satellite, which reached 0.1 percent.