Gravity - Wikiparency

# Gravity Gravity is the universal attraction between masses — by an enormous margin the weakest of the fundamental interactions, and yet the one that architects the universe, because it is the only one that is always attractive, cannot be screened, and reaches to infinity. In Newton's law the [[Force]] between masses is F = Gm₁m₂/r², with G = 6.674×10⁻¹¹ N·m²·kg⁻²; in [[General_relativity]] the same phenomenon is reread as curvature of spacetime. Between those two descriptions sits most of the history of [[Physics]]: gravity fixed the shape of the [[Earth]], lights the [[Sun]] by compressing [[Hydrogen]] until [[Nuclear_fusion]] ignites, collapses stars into [[Black_hole|black holes]], and still refuses to merge cleanly with [[Quantum_mechanics]] — the deepest open seam in physical law. ## One law from apple to Moon [[Isaac_Newton]]'s 1687 synthesis was an act of unification: the fall of an object at Earth's surface and the orbit of the [[Moon]] are the same motion under one inverse-square law. The check is arithmetic worth repeating: the Moon rides ~60 Earth radii out, and its centripetal acceleration is g/60² ≈ 0.0027 m/s² — exactly what a 9.8 m/s² surface field diluted by distance-squared predicts. [[Johannes_Kepler]]'s three planetary laws, empirical until then, drop out of the same equation, tying [[Newton's_laws_of_motion]] to the sky. The constant G is the hard part — gravity is so feeble that measuring the attraction between laboratory masses took until Cavendish's torsion balance in 1798. Surface gravity itself wanders from 9.78 m/s² at the equator to 9.83 at the poles ([[Earth]]'s spin and equatorial bulge), a variation mapped today by [[Pendulum]] descendants and satellite gradiometry, and used to prospect for [[Density]] anomalies underground. ## The odd one among the forces Compare two [[Proton|protons]]: their gravitational attraction is ~10³⁶ times weaker than their [[Coulomb's_law|Coulomb]] repulsion. Gravity wins the universe anyway, on three structural advantages. It has one sign — masses only attract, so there is no gravitational analog of the charge cancellation that neutralizes matter electrically at large scales. It cannot be shielded. And it is long-range, falling only as 1/r². So while electromagnetism dominates at molecular scale, summing over planetary masses hands gravity the last word: it is why the [[Atmosphere_of_Earth]] stays put (mostly — light [[Helium]] atoms reach escape speed and leak to space, which is why terrestrial helium must be mined, not skimmed from air), why [[Water]] finds sea level and the [[Mississippi_River]] drains a continent, and why 11.2 km/s is the price of leaving [[Earth]] entirely. The [[Gravitational_field]] formalism — g = GM/r², fields superposing linearly — is Newton's law packaged for [[Engineering|engineers]] and remains exact enough for every mission NASA has flown on Newtonian navigation. ## Einstein's revision: geometry, not force General relativity (1915) begins from the equivalence principle — free fall is locally indistinguishable from no gravity at all, a fact tested to about one part in 10¹⁵ by the MICROSCOPE satellite (final results 2022). Einstein's conclusion: gravity is not a force acting in spacetime but the shape of spacetime, a four-dimensional [[Manifold]] whose curvature, encoded in a metric [[Tensor]] and read with [[Differential_geometry]], is sourced by mass-energy; free bodies simply follow [[Geodesic|geodesics]]. The receipts came fast: the theory accounted for Mercury's anomalous perihelion advance (43 arcseconds per century), predicted the 1.75″ deflection of starlight confirmed in the 1919 eclipse expedition, and requires clocks to run slow deeper in a field — a correction of ~38 microseconds per day for navigation satellites which, if ignored, would corrupt [[Radio_navigation|satellite positioning]] by kilometres per day. Where fields are weak the theory hands back Newton; where they are strong it predicts objects Newtonian physics cannot: a mass compressed inside the Schwarzschild radius r_s = 2GM/c² (9 mm for Earth, 3 km for the Sun) becomes a [[Black_hole]]. In September 2015 the LIGO interferometers measured the spacetime strain — 10⁻²¹ — of two ~30-solar-mass black holes merging: gravity observed as ripples in geometry itself, and the 2017 neutron-star merger added a spectroscopic bonus, kilonova debris consistent with r-process [[Nucleosynthesis]] of [[Gold]] and [[Uranium]]. ## Gravity builds everything Every large structure is gravity's compound interest on tiny density fluctuations. Clouds of primordial [[Hydrogen]] and [[Helium]] in the [[Interstellar_medium]] pass the Jeans threshold and collapse; compression heats the core to ~1.5×10⁷ K, [[Nuclear_fusion]] ignites, and the star settles into hydrostatic equilibrium — gravitational crush balanced by [[Plasma_(physics)|plasma]] pressure, with the [[Sun]] burning ~600 million tonnes of hydrogen per second on that ledger, feeding the element factory behind [[Abundance_of_the_chemical_elements]]. Leftover disk material aggregates into a [[Planetary_system]]; self-gravity rounds any body above a few hundred kilometres — [[Ceres_(dwarf_planet)]] is a sphere while smaller asteroids stay lumpy — and tidal gradients from [[Moon]] and Sun knead oceans and moons alike ([[Jupiter]]'s tides power volcanoes on Io). On the home planet gravity is the quiet operating system: it stratifies the [[Atmosphere_of_Earth|atmosphere]], drives buoyancy and [[Heat_transfer|convection]] (no candle flame teardrop without it), sets [[Drag_(physics)|terminal velocity]], loads every bone and bridge, and hands [[Fluid_dynamics]] its body forces. ## The seam that won't close Gravity remains the interaction without a quantum theory. [[Quantum_mechanics]] and general relativity are each superbly confirmed, and jointly inconsistent where both matter — at the Planck length √(ℏG/c³) ≈ 1.6×10⁻³⁵ m, inside black holes, at the first instant of cosmic expansion. Naive estimates of vacuum [[Zero-point_energy]] overshoot the observed cosmic acceleration by some 120 orders of magnitude — the worst prediction in physics. Hawking's 1974 semiclassical result that black holes radiate thermally, tying horizon area to [[Entropy]], is the one firm plank across the gap, and the information-loss puzzle it raised is still being argued. Even gravity's weakness is unexplained — why 10³⁶? The force that was mathematized first, by [[Isaac_Newton]], is on present evidence the one physics understands least. **On the spine:** [[Isaac_Newton]] · [[General_relativity]] · [[Gravitational_field]] · [[Black_hole]] · [[Nuclear_fusion]].  *Connected to the Apex Spine:* Gravity → [[Superfluidity|Superfluidity]] — [[WT!Thury_Hydrodynamics_Compendium|Compendium]] section 13, *Superfluidity*.  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Gravity) : [Wikitube](https://en.wikitube.io/wiki/Gravity) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Hydrogen]], [[PORTAL_Helium]], [[PORTAL_Physics]]. --- *Repopulated 2026-08-12 · redlink fill · 0 deletions.*