Gravity

Gravity. It is a fundamental interaction which causes mutual attraction between all things that have mass or energy. The phenomenon is most accurately described by Albert Einstein's theory of general relativity, which posits that gravity is not a force but a consequence of the curvature of spacetime caused by the uneven distribution of mass and energy. While its effects are dominant on cosmological scales, such as governing the motions of planets and galaxies, it is the weakest of the four fundamental interactions in the standard model of particle physics.

Classical mechanics

The modern quantitative description of gravitational attraction began with the work of Galileo Galilei, who conducted pioneering experiments on falling bodies. Isaac Newton subsequently formulated his law of universal gravitation in his seminal work, Philosophiæ Naturalis Principia Mathematica, which states that every particle attracts every other particle with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centers. This framework, part of Newtonian mechanics, successfully explained Kepler's laws and the motions of celestial bodies like the Moon and the planets for centuries. The gravitational constant, G, was first measured with accuracy in the Cavendish experiment conducted by Henry Cavendish.

General relativity

In the early 20th century, Albert Einstein developed his theory of general relativity, which revolutionized the understanding of gravity. This theory describes gravity not as a force but as a geometric property of spacetime, which is curved by the presence of mass and energy. Key predictions of the theory include the bending of light by massive objects, first confirmed by Arthur Eddington during the 1919 solar eclipse, and the existence of gravitational waves, directly detected in 2015 by the LIGO collaboration from the merger of two black holes. Solutions to the Einstein field equations, such as the Schwarzschild metric, describe phenomena like black holes and the gravitational lensing of distant galaxies.

Quantum gravity

A major unsolved problem in theoretical physics is the formulation of a consistent theory of quantum gravity that successfully merges general relativity with the principles of quantum mechanics. Prominent candidate theories include string theory, which posits that fundamental particles are vibrations of one-dimensional strings, and loop quantum gravity, which attempts to quantize spacetime itself. The study of the evaporation of black holes by Stephen Hawking highlighted the need for such a theory, as it involves both gravitational and quantum effects. Research in this area often focuses on the nature of spacetime at the extremely small Planck scale.

Gravitational phenomena

Gravity manifests in numerous observable phenomena across the universe. On Earth, it gives rise to weight and governs the tides, which are primarily caused by the gravitational pull of the Moon and the Sun. In astrophysics, gravity is responsible for the formation and evolution of stars, planets, and galaxies, as well as the dynamics of binary star systems and star clusters. Extreme gravitational fields produce objects like neutron stars and black holes, where the effects of general relativity are paramount. The expansion of the universe, described by the Friedmann equations, is also driven by the gravitational interplay of all its matter and energy.

Measurement and units

The strength of the gravitational field is measured as acceleration, with the standard unit being metres per second squared (m/s²). On the surface of the Earth, this acceleration, denoted g, is approximately 9.8 m/s², though it varies slightly due to factors like latitude and local geology, as mapped by missions like GRACE. The universal gravitational constant, G, is a fundamental physical constant determined experimentally, with its value set by the Committee on Data for Science and Technology. Precise measurements of gravity are crucial for fields such as geodesy, geophysics, and aerospace engineering, and are conducted using instruments like gravimeters and atomic interferometers.

Category:Fundamental interactions Category:Concepts in physics