Gravitation

Gravitation is the natural phenomenon of attraction between masses or energy distributions. It is one of the four fundamental interactions in nature and governs the motion of celestial bodies, planets, stars, galaxies, and even light in general relativity. In classical physics, gravitation is a force acting at a distance, while in modern physics it is described as the curvature of spacetime caused by mass and energy.

In Newtonian gravity, the gravitational force between two point masses is given by F = G m1 m2 / r^2, where G is the gravitational constant and r is the separation. The corresponding gravitational field g = GM/r^2 describes the acceleration of a small test mass. This inverse-square law explains planetary orbits as ellipses (Kepler's laws) and underpins tidal effects, satellite motion, and many engineering calculations.

General relativity, proposed by Albert Einstein, recasts gravitation as the geometry of spacetime. Mass-energy tells spacetime how to curve, and curved spacetime tells matter how to move. This theory accounts for phenomena such as gravitational time dilation, light deflection by mass, the precession of Mercury’s orbit, and gravitational waves—ripples in spacetime detected directly in 2015 and repeatedly since.

Observationally, gravity is observed across scales, from lab Cavendish measurements of G to orbital dynamics of planets and galaxies. Gravitational interactions influence structure formation, black holes, and cosmology. Although extremely well tested in many regimes, gravity remains unsuccessfully reconciled with quantum mechanics, and a quantum theory of gravity is an active area of research.