General relativity is Albert Einstein's geometric theory of gravitation, published in its final form in November 1915. It describes gravity not as a force acting between masses — as in the Newtonian framework it superseded — but as a curvature of spacetime caused by the presence of mass and energy. Objects in free fall, including planets in orbit, follow the straightest possible paths (geodesics) through this curved spacetime. The theory encompasses and extends special relativity (1905) and reduces to Newtonian gravity in the limit of weak fields and low velocities. General relativity governs phenomena at the largest scales: planetary motion, black holes, gravitational waves, and the structure and evolution of the universe as a whole.
Einstein developed the theory over approximately a decade, drawing on the mathematics of Riemannian geometry and the contributions of mathematicians including Marcel Grossmann. The core equations — the Einstein field equations — relate the curvature of spacetime (described by the Einstein tensor) to the distribution of mass-energy (described by the stress-energy tensor). They are ten coupled, nonlinear partial differential equations. Exact analytical solutions exist only for idealized cases; the most important include the Schwarzschild solution (describing spacetime outside a spherically symmetric mass), the Kerr solution (for rotating black holes), and the Friedmann-Lemaître-Robertson-Walker (FLRW) metric (describing a homogeneous, isotropic expanding universe).
For a detailed account of the theory's development, early reception, and experimental confirmation, see General Relativity — History.
General relativity has accumulated a substantial body of experimental confirmation. The precession of Mercury's perihelion, unexplained under Newtonian mechanics, matched Einstein's prediction precisely. The deflection of starlight by the Sun was measured during the solar eclipse of 29 May 1919, consistent with the relativistic prediction. Gravitational redshift has been confirmed in laboratory experiments (Pound–Rebka, 1959) and in precise satellite clock corrections required for the Global Positioning System. The Hulse–Taylor binary pulsar provided indirect evidence for gravitational wave emission beginning in the 1970s. Direct detection of gravitational waves was achieved by the LIGO collaboration on 14 September 2015, confirmed and announced in February 2016, from the merger of two black holes. The Event Horizon Telescope produced the first image of a black hole shadow in 2019 (M87*) and of the Milky Way's central black hole (Sagittarius A*) in 2022.
General relativity is not a complete theory of physics. It is incompatible with quantum mechanics at a foundational level: the theory is classical and continuous, while quantum mechanics is discrete and probabilistic. No experimentally confirmed quantum theory of gravity exists. The behavior of spacetime at the Planck scale (approximately 1.616 × 10⁻³⁵ meters) remains outside the scope of general relativity. Several competing frameworks — including loop quantum gravity, string theory, and causal dynamical triangulations — seek to reconcile the two theories; none has achieved experimental confirmation. See Quantum Gravity — Debate.
General relativity also requires, when applied to galactic rotation curves and cosmological structure, the postulation of entities not directly observed: dark matter and dark energy. These account for approximately 95% of the total mass-energy content of the universe under the standard cosmological model (ΛCDM). Whether these represent genuine physical substances, modifications to the gravitational law, or artifacts of another incomplete assumption is an active area of research and dispute. See Dark Matter — Debate and Dark Energy — Debate.
Broad consensus exists within physics that general relativity is the correct classical theory of gravitation in the regimes in which it has been tested. See General Relativity — Physics Consensus.
Consensus is absent on the theory's extension to quantum scales, on the physical interpretation of dark matter and dark energy, and on what, if anything, preceded or caused the initial conditions described by relativistic cosmology. These questions have dedicated debate and viewpoint pages linked below.
Standard interpretation (geometric spacetime): The majority position in contemporary physics holds that general relativity accurately describes gravity as spacetime curvature, that its predictions have been confirmed across many independent experiments, and that it should be taken as a correct description of nature in its domain of applicability. See General Relativity — Standard Geometric Interpretation Viewpoint.
Modified gravity alternatives: A minority of physicists and cosmologists argue that the observational data attributed to dark matter or dark energy can be better explained by modifying the gravitational law itself — for example, through Modified Newtonian Dynamics (MOND), Tensor-Vector-Scalar gravity (TeVeS), or f(R) gravity theories — rather than by postulating undetected substances. See Modified Gravity — MOND Viewpoint.
Quantum gravity approaches: Researchers disagree substantially on how to reconcile general relativity with quantum mechanics. Loop quantum gravity, string/M-theory, causal set theory, and emergent gravity represent distinct and mutually contested research programs. See Quantum Gravity — Loop Quantum Gravity Viewpoint and Quantum Gravity — String Theory Viewpoint.
Interpretive and philosophical positions: Questions about the ontological status of spacetime — whether it is a substance, a relation among objects, or a useful mathematical fiction — remain live in the philosophy of physics. See General Relativity — Spacetime Substantivalism Viewpoint and General Relativity — Relationism Viewpoint.