Tests of General Relativity

Core Idea

General relativity has been tested across an extraordinary range of scales and field strengths, from laboratory experiments to cosmological observations, passing every test with remarkable precision. The classical solar-system tests include: perihelion precession of Mercury (43 arcsec/century, confirmed to 0.1%), deflection of light by the Sun (1.75 arcsec, confirmed to 0.01% via VLBI), gravitational redshift (Pound-Rebka, GPS, confirmed to 10⁻⁵), and Shapiro time delay (confirmed to 0.001%). Strong-field tests include binary pulsar orbital decay (PSR B1913+16, matching the gravitational wave quadrupole formula to 0.2%), direct gravitational wave detection (LIGO/Virgo, confirming the nonlinear strong-field regime), black hole shadow imaging (Event Horizon Telescope), and frame-dragging measurements (Gravity Probe B, LAGEOS). Cosmological tests include the expansion history consistent with the Friedmann equations and the spectrum of CMB anisotropies. No confirmed deviation from GR has been found.

Explainer

General relativity makes precise, quantitative predictions that can be tested against observation, and it has passed every test conducted over more than a century. The classical solar-system tests — perihelion precession, light deflection, gravitational redshift — were the first confirmations and remain among the most precise. Mercury's anomalous perihelion precession of 42.98 arcsec/century was the first quantitative test (1915). Light deflection by the Sun (1.75 arcsec at the limb) was confirmed in 1919 and has since been verified to 0.01% precision using VLBI radio observations. Gravitational redshift has been confirmed from the Pound-Rebka experiment (10%) through Gravity Probe A (0.007%) to modern optical atomic clocks (which detect the redshift over centimeter height differences). The Shapiro time delay, confirmed to 0.001% by the Cassini spacecraft, tests the spatial curvature component of the metric independently of the other effects.

Binary pulsars transformed GR testing by providing access to strong-field, high-velocity, and radiative gravity. The Hulse-Taylor binary pulsar PSR B1913+16 (discovered 1974) consists of two neutron stars in a tight, eccentric orbit. Its periastron advance (4.2°/year — 35,000 times Mercury's rate), gravitational time dilation, and Shapiro delay test strong-field gravity. Most dramatically, the cumulative orbital phase shift from gravitational wave energy loss matches the GR quadrupole formula prediction to 0.2% over four decades of observation — indirect proof that gravitational waves exist and carry energy. The double pulsar PSR J0737-3039 (discovered 2003) provides five or more independent tests of GR in a single system, all consistent.

The direct detection of gravitational waves by LIGO in 2015 opened a new frontier. The first signal (GW150914) matched the numerical-relativity prediction for a binary black hole merger — two 30-solar-mass black holes spiraling together, merging, and ringing down to a single Kerr black hole — with extraordinary precision. This tested GR in the most extreme regime possible: velocities approaching c, gravitational fields at the Planck curvature scale, and the fully nonlinear dynamics of merging horizons. Subsequent detections have confirmed the GR predictions for binary neutron star mergers (GW170817, also observed electromagnetically across the spectrum) and black hole-neutron star mergers. The gravitational wave speed was constrained to equal the speed of light to within 10⁻¹⁵, ruling out many modified gravity theories.

Additional tests continue to accumulate. The Event Horizon Telescope imaged the shadows of the supermassive black holes M87* and Sgr A*, confirming that the shadow size and shape are consistent with the Kerr metric. Gravity Probe B confirmed frame dragging (Lense-Thirring precession) in Earth orbit. Cosmological observations — the CMB spectrum, baryon acoustic oscillations, Type Ia supernovae — are consistent with the Friedmann equations derived from GR. Laboratory tests of the equivalence principle (torsion-balance experiments) confirm the equality of gravitational and inertial mass to 10⁻¹³ precision. Despite this extraordinary success, physicists expect GR to break down at the Planck scale, where quantum gravitational effects become important — a regime not yet accessible to experiment.