重力

Gravity — Grokipedia Fact-checked by Grok 28 days ago Gravity Ara Eve Leo Sal 1x Gravity is a fundamental interaction that causes mutual attraction between all objects with mass or energy . As the weakest of the four fundamental forces, it dominates large-scale structures in the universe due to its long-range action and cumulative effects. [1] This attraction keeps planets in orbit around stars, governs the motion of celestial bodies, and on Earth produces the downward pull that gives objects weight, retains the atmosphere, and generates tides. [2] [3] Isaac Newton's law of universal gravitation, formulated in 1687, states that every particle attracts every other with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centers: F = G m 1 m 2 r 2 F = G \frac{m_1 m_2}{r^2} F = G r 2 m 1 ​ m 2 ​ ​ , where G G G is the gravitational constant ( 6.67430 × 1 0 − 11 m 3 k g − 1 s − 2 6.67430 \times 10^{-11} \, \mathrm{m^3 kg^{-1} s^{-2}} 6.67430 × 1 0 − 11 m 3 k g − 1 s − 2 ). This law unified terrestrial and celestial mechanics , explaining planetary orbits, tides, and falling objects. [4] [5] In 1915, Albert Einstein's general relativity redefined gravity as the curvature of spacetime caused by mass and energy, with objects following geodesic paths in this curved geometry. The theory predicts light bending around massive bodies, gravitational time dilation, the precession of Mercury's orbit, and gravitational waves —ripples in spacetime from accelerating masses—directly detected in 2015. [6] [7] [8] Gravity shapes the expansion of the universe , galaxy formation, and black holes. Reconciling general relativity with quantum mechanics remains a major challenge, motivating research into quantum gravity theories such as string theory and loop quantum gravity . On Earth, precise gravity measurements support geodesy , resource exploration, and climate studies. [9] [10] [11] Characterization Definition and Fundamental Role Gravity is one of the four fundamental interactions in nature, alongside electromagnetism , the strong nuclear force , and the weak nuclear force . [1] [12] It acts as an attractive force between any two objects that possess mass or energy , with no repulsive counterpart observed in this interaction. [1] [13] In contrast to electromagnetism , which can be either attractive or repulsive depending on charges, gravity consistently draws masses toward each other. [13] This force manifests in everyday phenomena, such as causing objects to fall toward Earth's surface, and on cosmic scales, it maintains the stability of planetary orbits around stars by counterbalancing centrifugal tendencies. [2] [14] Gravity also plays a pivotal role in shaping the large-scale structure of the universe , clumping matter into galaxies, clusters, and vast filaments through its cumulative pull on distributed masses. [15] [16] Gravity possesses an infinite range, extending across the observable universe without diminishment by distance in principle, though its effects weaken with separation. [1] [12] It is the weakest of the fundamental forces by many orders of magnitude, yet it dominates on astronomical scales because the other forces tend to cancel out—such as electromagnetism in neutral cosmic plasmas—while gravity accumulates additively over vast assemblies of matter. [1] [17] In daily life, the sensation of weight represents the gravitational attraction exerted by Earth on an object's mass, pulling it downward toward the planet's center. [18] [3] Strength and Universal Constant The gravitational constant , denoted $ G $, is a fundamental physical constant that quantifies the strength of gravitational attraction in Newton's law of universal gravitation : F = G m 1 m 2 r 2 F = G \frac{m_1 m_2}{r^2} F = G r 2 m 1 ​ m 2 ​ ​ where G G G is the proportionality factor, m 1 m_1 m 1 ​ and m 2 m_2 m 2 ​ are the masses, and r r r is the distance between them. The currently accepted value, recommended by the Committee on Data for Science and Technology (CODATA) in 2022, is G = 6.67430 × 1 0 − 11 m 3 k g − 1 s − 2 G = 6.67430 \times 10^{-11} \, \mathrm{m}^3 \mathrm{kg}^{-1} \mathrm{s}^{-2} G = 6.67430 × 1 0 − 11 m 3 kg − 1 s − 2 with a relative standard uncertainty of 22 parts per million. [19] This value allows calculation of gravitational forces across a wide range of scales, from planetary orbits to galactic structures. Gravity is the weakest of the four fundamental forces, approximately $ 10^{38} $ times weaker than the strong nuclear force when compared via their dimensionless coupling constants at typical interaction scales. The strong nuclear force, which binds quarks into protons and neutrons and holds atomic nuclei together, has a coupling constant near 1. In contrast, gravity’s effective coupling constant is around $ 10^{-39} $. [20] This extreme disparity occurs because gravity couples universally to mass-energy but with a very small constant, while the strong force acts with immense intensity over short ranges (about $ 10^{-15} $ m). As a result, at atomic and subatomic scales—where particle masses are on the order of $ 10^{-27} $ kg or less and distances are femtometers—gravitational forces are overwhelmed by electromagnetic and nuclear forces, rendering them effectively undetectable and having no significant role in subatomic processes. At planetary and galactic scales—where masses aggregate to $ 10^{24} $ kg or more and distances span kilometers to light-years—gravity dominates due to its infinite range and cumulative effect. This scale dependence explains why gravity governs the motion of celestial bodies and the large-scale structure of the universe , while playing no meaningful part in chemical bonds or nuclear reactions. Measuring $ G $ is difficult because of the extremely small forces involved, which demand high experimental sensitivity to detect deflections on the order of microradians. The first successful measurement was performed by Henry Cavendish in 1797–1798 using a torsion balance. He suspended a light rod with small lead spheres (0.73 kg each) from a thin wire and observed the torsional deflection caused by attraction to larger stationary lead spheres (158 kg each) placed alternately on opposite sides. By measuring the equilibrium deflection and the wire’s torsion constant , Cavendish determined the Earth’s density, from which $ G $ was later calculated to be approximately $ 6.74 \times 10^{-11} , \mathrm{m}^3 \mathrm{kg}^{-1} \mathrm{s}^{-2} $. Modern measurements rely on refined torsion balances, often with cryogenic cooling and vacuum isolation to reduce environmental noise, achieving precisions of 10–20 ppm. However, results still show discrepancies at the level of about 50 ppm. Alternative methods, such as atom interferometry with laser-cooled atoms (e.g., cesium) in free fall , detect phase shifts induced by gravitational gradients and have yielded values such as $ G = 6.693 \times 10^{-11} , \mathrm{m}^3 \mathrm{kg}^{-1} \mathrm{s}^{-2} $ with uncertainties around 0.5%. These approaches offer potential for further improvement by suppressing systematic errors in quantum regimes. [21] [22] [23] Historical Development Ancient and Pre-Scientific Views In ancient Greek philosophy, particularly in Aristotle's works such as Physics and On the Heavens , the tendency of objects to fall was explained through the theory of natural motion and the four sublunary elements: earth, water, air, and fire. Each element had a natural place—earth and water moved downward toward the center of the Earth, while air and fire moved upward—due to intrinsic properties rather than any external attractive force. Motion ceased upon reaching this natural place. Aristotle's framework, which dominated Western thought for centuries, accounted for everyday falling objects without quantitative laws. [24] [25] These ideas integrated with geocentric cosmologies based on observations of falling bodies and celestial motions. Ptolemy's Almagest (2nd century CE) placed Earth at the universe's center, with planets and stars on nested rotating crystalline spheres. While focused on astronomical prediction, the model preserved Aristotelian distinctions: sublunary bodies exhibited downward tendencies, whereas celestial bodies—composed of a fifth element, ether—moved eternally in perfect circles. [26] [27] [28] Islamic scholars extended Aristotelian concepts. Ibn Sina (Avicenna, 980–1037 CE) elaborated on heaviness in Kitab al-Shifa ( The Book of Healing ), describing downward motion as an inherent tendency that accelerates as bodies near their natural place, while distinguishing it from the separate motive force driving celestial rotations. [29] [30] Medieval European thinkers introduced refinements. Jean Buridan (c. 1300–1361) developed impetus theory—an impressed force enabling sustained motion without continuous external action—and applied it to falling bodies, proposing that gravity imparts successive increments of impetus to produce acceleration. Nicole Oresme (c. 1320–1382) pioneered graphical methods in Tractatus de configurationibus qualitatum et motuum , using coordinate-like diagrams to represent motion intensities, such as plotting velocity against time to show uniform acceleration as triangular areas. [31] [32] These pre-scientific views treated gravity as a teleological tendency toward a natural place, confined to the sublunary realm and rooted in elemental natures, rather than a universal force acting between all masses. This qualitative, realm-specific perspective persisted until the shift to experimental and quantitative methods in the late 16th century. [29] Newtonian Revolution In 1687, Isaac Newton published Philosophiæ Naturalis Principia Mathematica , synthesizing Johannes Kepler's empirical laws of planetary motion with Galileo's principle of inertia into a unified framework for terrestrial and celestial mechanics. This showed that the same physical laws go