The Confrontation between General Relativity and Experiment

TL;DR

General relativity has endured extensive experimental scrutiny across weak- and strong-field regimes, with the Einstein equivalence principle (EEP) well supported by WEP, LLI, and LPI tests. Theoretical frameworks such as the PPN formalism and SME provide structured means to quantify deviations, while alternative metric theories (scalar-tensor, f(R), TeVeS) predict distinct PN and strong-field signatures. Gravitational-wave observations and binary pulsar timing now probe strong gravity with high precision, and results to date (e.g., Cassini γ bounds, Hulse–Taylor damping, Nordtvedt constraints) corroborate GR, placing tight limits on many alternative theories. The coming decade promises further tests of strong-field gravity and gravitational waves, potentially revealing new physics or strengthening GR’s status as the correct description of gravitation in the regimes accessible by humanity.

Abstract

The status of experimental tests of general relativity and of theoretical frameworks for analyzing them are reviewed and updated. Einstein's equivalence principle (EEP) is well supported by experiments such as the Eotvos experiment, tests of local Lorentz invariance and clock experiments. Ongoing 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, the Nordtvedt effect in lunar motion, and frame-dragging. 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 a growing family of other binary pulsar systems is yielding new tests, especially of strong-field effects. Current and future tests of relativity will center on strong gravity and gravitational waves.

Figures (10)

• 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 Eöt-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 $370 \mathrm{\ km\ s}^{-1}$ 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.