Gravity is one of the four recognised fundamental interactions and is the interaction associated with energy, momentum and the geometry of spacetime. In classical Newtonian physics, gravity is described as an attractive force between masses, with the magnitude between two point masses given by F = G m1 m2 divided by r squared. In general relativity, developed by Albert Einstein, gravity is not fundamentally described as an ordinary force acting across space.
Instead, mass energy and momentum determine the curvature of spacetime, while freely falling objects follow geodesics through that curved spacetime. The two descriptions agree extremely well in weak gravitational fields and at low velocities relative to the speed of light, while general relativity provides the more comprehensive framework for strong gravitational fields, high precision timing, cosmology and relativistic motion. Gravity is unusual among the fundamental interactions because there is currently no experimentally confirmed complete quantum theory of gravity.
Newton's theory of universal gravitation established that the same gravitational interaction governs falling objects on Earth and the motion of astronomical bodies. The gravitational force decreases with the square of distance for idealised point masses or spherically symmetric mass distributions outside the body. Newton's formulation successfully explains planetary orbits, tides and many engineering applications. However, Newtonian gravity treats gravity as an instantaneous force, which conflicts with the relativistic principle that physical influences cannot propagate faster than light. Einstein's general theory of relativity resolves this issue by describing gravity through spacetime geometry. Changes in the gravitational field propagate as disturbances in the gravitational field at the speed of light, with gravitational waves providing direct evidence for this dynamical aspect of general relativity.
In general relativity, matter and energy influence the curvature of spacetime, and curved spacetime influences the motion of matter and radiation. The compact mathematical relationship is expressed through Einstein's field equations, which connect spacetime curvature with the stress energy content of the universe. The popular statement that matter tells spacetime how to curve and curved spacetime tells matter how to move is a conceptual summary rather than the complete mathematical theory.
Gravity affects light even though photons have zero rest mass because light follows null geodesics in curved spacetime. This produces phenomena such as gravitational lensing, gravitational redshift and Shapiro time delay. Massive objects can therefore bend the apparent paths of light, distort astronomical images and affect the timing of signals. These effects have been observed experimentally and provide strong evidence for general relativity.
Gravity is much weaker than the other known fundamental interactions when compared between individual elementary particles. For example, the gravitational attraction between two electrons is extraordinarily weaker than their electromagnetic repulsion. Nevertheless, gravity dominates at astronomical scales because mass and energy are always positive in the ordinary macroscopic sense, whereas electric charges can cancel between positive and negative matter. Large astronomical bodies therefore accumulate enormous gravitational effects.
Gravity controls the formation and structure of stars, planets, galaxies and galaxy clusters and governs orbital motion. It also contributes to the large scale evolution of the universe. In general relativity, gravitational systems can involve extreme spacetime curvature, such as near black holes and neutron stars, where Newtonian approximations become inadequate.
Gravity can be described using gravitational potential and gravitational fields in the Newtonian approximation. Near the surface of Earth, objects experience an approximately uniform downward acceleration of about 9.81 metres per second squared, although the exact value varies with latitude, altitude and local geological conditions. Gravitational potential energy near Earth's surface can be approximated as m g h for small height differences.
For larger distances, the Newtonian gravitational potential is described by negative G M divided by r for a test mass relative to an isolated spherical source. In general relativity, however, gravitational energy is more subtle and cannot always be represented by a unique local energy density in the same way as ordinary fields. This distinction becomes important in strong gravitational fields and cosmology.
Gravitational waves are ripples in spacetime geometry produced by accelerating asymmetric mass distributions, particularly systems involving compact massive objects such as merging black holes and neutron stars. General relativity predicts that these disturbances propagate at the speed of light. The LIGO and Virgo collaborations achieved the first direct detection of gravitational waves in 2015 from a binary black hole merger, opening gravitational wave astronomy as a new observational field.
Gravitational waves carry energy and angular momentum away from their sources and cause extremely small changes in the distances measured by sensitive interferometers. Their observation provides tests of general relativity and information about astrophysical systems that may be difficult or impossible to observe using electromagnetic radiation alone.
Gravity is also essential to cosmology. General relativity provides the theoretical foundation for modern descriptions of the expanding universe, black holes and gravitational dynamics on cosmic scales. The expansion of the universe is not simply ordinary matter moving through pre existing empty space; in standard cosmological models, spacetime itself evolves.
Dark matter contributes gravitationally to the formation of cosmic structure, while dark energy is associated in standard models with the accelerated expansion of the universe, although its fundamental nature remains unknown. Gravity also governs the formation of stars from collapsing clouds, the structure of planetary systems and the trajectories of spacecraft. These applications show that gravity is not merely the force that makes objects fall but a fundamental feature of the large scale structure and dynamics of the universe.
The major unresolved issue is the relationship between gravity and quantum mechanics. The Standard Model successfully describes the electromagnetic, weak and strong interactions using quantum field theory, but general relativity describes gravity as classical spacetime geometry. Attempts to construct quantum gravity include string theory, loop quantum gravity and other approaches, but no complete experimentally confirmed theory has yet replaced general relativity in its domain of validity.
A hypothetical graviton is sometimes proposed as the quantum excitation of a gravitational field and would be expected to have spin two, but gravitons have not been detected. Understanding quantum gravity may be necessary to describe conditions involving both extreme gravity and quantum effects, such as the deepest interior physics of black holes or the earliest moments of the universe. Gravity is therefore simultaneously a remarkably successful classical theory and one of the major open problems in fundamental physics.
