Gravity §
Gravity is the interaction by which mass-energy curves spacetime, producing the attractive effect we observe as weight, orbital motion, and the large-scale structure of the universe. It is relevant whenever mass is present: engineering (structural loads, fluid pressure), orbital mechanics (satellites, spacecraft trajectories), geophysics (tidal forces, gravitational anomalies), astrophysics (stellar evolution, black holes), and cosmology (expansion, structure formation).
Authoritative sources
Brief history §
- 1687: Newton formulates the inverse-square law, unifying falling apples and planetary orbits.
- 1915: Einstein publishes general relativity, identifying gravity with spacetime curvature.
- 1919: Eddington’s eclipse expedition confirms light deflection, validating GR.
- 2015: LIGO detects gravitational waves from a binary black-hole merger, opening a new observational window.
Categorization §
The five classes separate gravity by the regime or first-principle aspect it isolates: the Newtonian force law, the geometric theory, radiative degrees of freedom, equilibrium/collapse solutions, and the largest-scale behavior. General relativity is the parent theory: the last four rows are its weak-field limit, its radiative solutions, its strong-field stationary solutions, and its homogeneous-isotropic limit.
| Page | Class | Regime / principle | Governing law(s) | Typical application | Relation to other classes |
|---|---|---|---|---|---|
| Newtonian gravitation | Weak field, slow motion (v ≪ c, Φ/c2 ≪ 1) | ∇2 Φ = 4π G ρ, F = −m∇Φ | Orbital mechanics, structural loads, tides, fluid hydrostatics | Leading-order limit of general relativity; reduces to a single scalar potential | |
| General relativity | Full geometric theory; equivalence principle | Gμν = 8π G Tμν/c4 | GPS correction, black-hole physics, gravitational lensing, cosmology | Contains Newtonian gravitation as the weak-field limit; source is the full stress-energy tensor | |
| Gravitational waves | Radiative (time-varying quadrupole) degrees of freedom | Linearized Einstein equations in transverse-traceless gauge; hμνTT | LIGO/Virgo/KAGRA detection, binary pulsar timing, cosmological distance ladder | Propagating solutions of GR at c; carry energy and angular momentum away from sources | |
| Compact objects | Strong-field equilibrium or collapse (Φ/c2 ∼ 1) | TOV equation (neutron stars); Schwarzschild / Kerr metrics (black holes) | Pulsar timing, accretion-disk spectroscopy, event-horizon imaging | Exact or near-exact GR solutions where Newtonian gravity is insufficient | |
| Cosmological gravity | Largest scales (≳ Mpc); homogeneous and isotropic average | Friedmann equations; ä/a = −4π G(ρ+3p)/3c2 | Expansion history, CMB anisotropies, large-scale structure, dark energy | GR applied to a homogeneous fluid; gravity dominates over all other interactions on these scales |
Interactive simulation §
The simulation below shows a test particle orbiting a central mass. Drag the Relativistic compactness slider: at small values the orbit is a closed Newtonian ellipse (the deformed spacetime grid is nearly flat); as the central compactness grows, the grid curves visibly and the orbit’s periapsis precesses, exactly as observed for Mercury and for pulsars in binary systems.
Details §
Details
From Newton to Einstein §
Newton’s law is exact in the limit of weak fields and low velocities. It treats gravity as a force acting instantaneously at a distance through a scalar potential . General relativity replaces this picture: mass-energy tells spacetime how to curve (Einstein field equations), and curved spacetime tells matter how to move (geodesic equation). The Newtonian potential is recovered as the component of the metric in the weak-field, slow-motion limit:
Why gravity is unique §
Among the four fundamental interactions, gravity is the only one that has not been quantized in a complete, experimentally verified theory. It couples to the stress-energy tensor (which includes the gravitational field’s own energy), making it non-linear and non-renormalizable in perturbation theory. This is why the other three forces are described by quantum gauge theories while gravity remains classical (geometric) in all practical applications — not because a quantum theory of gravity is impossible, but because none has yet been experimentally confirmed.
Coupling hierarchy §
The gravitational coupling between two protons is times the electromagnetic coupling. Gravity is negligible at particle scales but dominates at astronomical scales because it is purely attractive and has infinite range, so it accumulates without cancellation.
Boundary with Standard-Model cosmology §
This topic covers the gravitational dynamics of the universe (Friedmann equations, expansion, structure formation); the matter/energy content and thermal history of the early universe (nucleosynthesis, recombination, CMB anisotropy physics) belong to the Cosmology page under the Standard model.