Astrophysics (Index)About


(theory that mass attracts)

In physics, gravity (or gravitation) is the name given to a force that attracts masses together generally according to the product of their masses and the reciprocal of the square of the intervening distance (law of gravitation). It was theorized by Isaac Newton who observed the force drawing objects toward Earth and that drawing planets toward the Sun and moons toward planets could all be explained by a single law, according to his estimates of feasible masses of the Sun, planets, and moons. Newton posited it as universal, i.e., that in other situations masses would affect each other according to the same law, its effect between everyday objects being negligible because of their relatively tiny mass.

Albert Einstein recast the theory as space itself being sucked into each mass (general relativity, GR), calibrating his formula to virtually match Newton's excepting extreme circumstances, but with some consequences, such as the effect of a massive object on passing EMR: GR predicted a degree of light bending unexplainable by Newton's laws. The term Newtonian gravity is used when it is necessary to distinguish it from GR.

These theories are phenomenally successful: for example, their reliability and precision has made space navigation as we know it possible, and the theorized degree to which it bends light has been observed. Yet they have failed to explain some observations:

Scientists have sufficient faith in gravitational theory that they cite it to assert galaxies and galaxy clusters must include matter that has yet to be detected (dark matter), and that something otherwise-undetected must be providing an outward force throughout the universe (dark energy). Alternately, attempts have been made to further refine gravitational theory to explain these observations (such as modified Newtonian dynamics and DGP gravity).

The term gravity has a different, but related use: in studies of the detail of the effects of the gravity of Earth and/or other bodies, the terms gravity and gravitation are often used with distinct meanings: gravitation to indicate the universal force and gravity to indicate a body's gravitational effects, i.e., to indicate the downward force-per-unit-mass (which amounts to acceleration) experienced at different positions in relation to the body, such as at a particular point on its surface. In this usage, the word gravity is also meant to include the effects of inertia from the body's rotation (centrifugal force), the two together being what would be measured by an accelerometer. Gravimetry is the measurement of this acceleration.

Further reading:

Referenced by pages:
accretion disk
alternative cosmologies
atmospheric escape
atmospheric tide
baryon acoustic oscillations (BAO)
black hole thermodynamics
binary star
binding energy
Birkhoff's theorem
black hole (BH)
broad-line region (BLR)
Bouguer anomaly
Brunt-Väisälä frequency
cold dark matter (CDM)
Chandrasekhar limit
complex crater
computational astrophysics
conic section
convection zone
corotation torque
cosmic string
cosmic web
cosmological constant (Λ)
Coulomb's law
critical density (ρc)
Darcy velocity field
dark energy
dark flow
dark matter
dark matter annihilation
dark matter halo
DGP gravity
dark matter filament
Doppler shift
Effelsberg 100-m Radio Telescope
Einstein-de Sitter model
electron degeneracy
escape velocity (Ve)
extra-solar planet
star formation feedback
free-air anomaly
free-fall time
fuzzy dark matter (FDM)
galactic tide
galaxy cluster (CL)
Goddard gravity model (GGM)
gravitational instability (GI)
giant star
general relativity (GR)
gravitational collapse
gravitational constant (G)
gravitational instability model
gravitational lensing
gravitationally bound
gravitational potential (Φ)
gravity anomaly
gravity sounding
gravity wave
gravitational wave (GW)
GW detection (GW)
Hale Telescope
helium rain
hypermassive neutron star (HMNS)
Hubble expansion
Hubble time (tH)
hydrodynamic equations
hydrodynamic escape
hydrostatic equilibrium
internal gravity wave
inverse square law
ion engine
isolation mass
isothermal core
Jeans length
Jeans parameter (λ)
Keplerian disk
Keplerian orbit
Kepler's laws
Kelvin-Helmholtz mechanism
Kelvin-Helmholtz timescale (KH timescale)
Lagrangian point
Lambda-CDM model (ΛCDM)
Lane-Emden equation
liquid mirror telescope
long-period comet
Mach's principle
maximum iron fraction
Maxwell-Boltzmann distribution
mirror support cell
mixing length theory
modified Newtonian dynamics (MOND)
multi-messenger astronomy
N-body problem
N-body simulation
neutron star (NS)
open cluster (OC)
orbital resonance
orbital speed
pointing error (PE)
planetary differentiation
planet formation
potential energy (PE)
power law
radio science
repulsive dark matter (RDM)
redshift (z)
reduced mass
red-giant branch (RGB)
Richardson number (Ri)
Roche limit
supercritical fluid (SCF)
Schrödinger-Poisson equation
gravitational singularity
Solar and Heliospheric Observatory (SOHO)
Solar Orbiter (SolO)
speed of light (c)
spherical harmonics
spiral density wave
stellar cluster (SC)
stellar core
stellar dynamics
stellar evolution
stellar mass determination
stellar stream
stellar structure
string theory
strong-field gravity
strong force
surface gravity
symmetry breaking
Trace Gas Orbiter (TGO)
tidal capture
tidal force
timescale (t)
time standard
Theory of Everything (TOE)
theory of figures (TOF)
Toomre Q parameter (Q)
topological defect
terrestrial time (TT)
T-Tauri star (TTS)
transit timing variations (TTV)
variable star
virial parameter
virial theorem
wide binaries (WB)
wavefront error (WFE)