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General Relativity - Physics Consensus
There is broad consensus in physics that general relativity is the correct classical description of gravitation. Published by Albert Einstein in 1915, the theory describes gravity not as a force in the Newtonian sense but as a curvature of spacetime caused by mass and energy. Its core predictions - including the precession of planetary orbits, gravitational lensing, gravitational redshift, and gravitational waves - have each been confirmed observationally. General relativity is known to be incomplete as a fundamental theory, as it does not incorporate quantum mechanics and breaks down at singularities; the search for a quantum theory of gravity remains an open problem.
Evidence Base
General relativity rests on two core principles. The equivalence principle holds that the effects of gravity are locally indistinguishable from those of acceleration, and that all objects fall at the same rate in a gravitational field regardless of their composition. The principle of general covariance holds that the laws of physics take the same form in all reference frames. From these principles, Einstein derived that mass and energy curve the fabric of spacetime, and that gravitational attraction is the motion of objects following geodesics - the straightest possible paths - through that curved spacetime.
The theory's predictions differ from Newtonian gravity in measurable ways, each of which has been confirmed:
- Precession of Mercury's perihelion - A known anomaly before 1915, explained exactly by general relativity.1)
- Gravitational lensing - The bending of light around massive objects, first confirmed during the solar eclipse of 29 May 1919.2)
- Gravitational redshift - Light climbing out of a gravity well loses energy, shifting toward longer wavelengths, confirmed by multiple experiments.3)
- Gravitational waves - Ripples in spacetime predicted by the theory, directly detected in September 2015 by the LIGO collaboration, a century after Einstein's prediction.4)
General relativity also predicts the existence of black holes - regions where spacetime curvature becomes extreme enough that nothing, including light, can escape beyond the event horizon. It forms the theoretical basis for modern physical cosmology, including the expanding-universe models derived from its field equations by Alexander Friedmann in 1922 and independently by Georges Lemaitre in 1927.5)6)
Limits and Open Questions
General relativity is known to be incomplete as a fundamental theory. It does not incorporate quantum mechanics, and its field equations predict singularities - points of infinite density and curvature - inside black holes and at the origin of the Big Bang. Whether these singularities represent genuine physical features or the breakdown of the theory's applicability is unresolved.
The cosmological constant, which Einstein introduced into his field equations and later retracted, has been reintroduced to account for observed accelerating cosmic expansion attributed to dark energy. The theoretical basis for this constant - and the nature of dark energy itself - remains an active area of research.
The consensus that general relativity is the correct classical description of gravity does not settle the question of what theory supersedes it at quantum scales.
Dissenting Viewpoints
Interpretation of singularities - Whether singularities represent real physical features or indicate the limits of the theory remains disputed among physicists. See Viewpoint: Singularities as Physical and Viewpoint: Singularities as Theoretical Breakdown.
Status of quantum gravity - Competing research programs - including string theory, loop quantum gravity, and others - disagree on the correct framework for unifying general relativity with quantum mechanics. No empirical test has yet distinguished between them. See Quantum Gravity Debate.
Cosmological constant - The reintroduction of the cosmological constant to account for accelerating expansion remains theoretically contested. See Dark Energy and Cosmological Constant Debate.