Tests of general relativity

Tests of general relativity compare its predictions about gravity, spacetime, and motion with experimental measurements and astronomical observations.

Perihelion precession of Mercury: The gradual rotation of Mercury’s orbital ellipse, including a component unexplained by Newtonian planetary perturbations. Einstein’s theory accounts for the roughly 43 arcseconds per century left unexplained by Newtonian gravity.
Equivalence principle: The principle that inertial and gravitational effects are locally indistinguishable under specified conditions. Universality of free fall is a foundation tested by precision experiments.
Gravity Probe B: A NASA satellite experiment that used gyroscopes to measure geodetic and frame-dragging effects. Its gyroscope precession provided a direct test of two relativistic effects around Earth.
Newtonian gravity: A theory that describes gravitational attraction as a force between masses in space and time. It succeeds in weak fields but misses effects such as Mercury’s residual perihelion precession.
Gravitational deflection of light: The change in a light ray’s direction as it passes through curved spacetime near a mass. The 1919 eclipse observations tested whether the Sun bends starlight as general relativity predicts.
Gravitational redshift: The change in a signal’s frequency as it moves between regions of different gravitational potential. Clock and light-frequency comparisons test how gravity affects time and energy.
Lunar Laser Ranging: The measurement of the Earth–Moon distance by timing laser pulses reflected from lunar retroreflectors. Long-term ranging tests relativistic orbital dynamics and the strong equivalence principle.
Scalar–tensor theory: A class of gravity theories in which a scalar field supplements the spacetime metric. Its deviations from general relativity can be constrained by solar-system and pulsar tests.
Shapiro time delay: The extra travel time experienced by a signal passing through a gravitational field. Radar echoes from planets and spacecraft test the theory’s predicted delay near the Sun.
Frame-dragging: The relativistic dragging of spacetime by the rotation of a massive body. Gyroscope and satellite-orbit measurements probe this prediction around Earth.

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