gravity
(gravitation)
(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:
- Galaxies within galaxy clusters do not orbit in accordance to gravitational theory, given the apparent masses of the clusters' constituent galaxies and gas.
- Stars in galaxies do not orbit it in accordance to gravitational theory, given the apparent masses of the galaxies' constituent stars and clouds.
- Gravity would make the universe accelerate inwardly (or given its current expansion, would make that expansion decelerate), but observations suggest it is doing the opposite.
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.
(physics,relativity)
Further reading:
http://en.wikipedia.org/wiki/Gravity
http://en.wikipedia.org/wiki/Gravimetry
Referenced by pages:
accretion
accretion disk
admittance
advection
alternative cosmologies
areal coordinate system
atmosphere
atmospheric escape
atmospheric tide
baryon acoustic oscillations (BAO)
barycenter
black hole thermodynamics
binary star
binding energy
Birkhoff's theorem
black hole (BH)
broad-line region (BLR)
Bouguer anomaly
Brunt-Väisälä frequency
Castro
cold dark matter (CDM)
Ceres
Chandrasekhar limit
Chern-Simons gravity
complex crater
computational astrophysics
conic section
convection zone
corotation torque
cosmic string
cosmic web
cosmological constant (Λ)
Coulomb's law
critical density (ρc)
curvature
Darcy velocity field
dark energy
dark flow
dark matter
dark matter annihilation
dark matter halo
DGP gravity
dark matter filament
Doppler shift
eccentricity (e)
Effelsberg 100-m Radio Telescope
Einstein-de Sitter model
electron degeneracy
electroweak
escape velocity (Ve)
Europa
evaporation
extra-solar planet
star formation feedback
Fermi sea
free-air anomaly
free-fall time
f(R) gravity
fuzzy dark matter (FDM)
galactic tide
galaxy cluster (CL)
geodesic
Goddard gravity model (GGM)
gravitational instability (GI)
giant star
general relativity (GR)
GRAIL
gravimetry
gravitational collapse
gravitational constant (G)
gravitational instability model
gravitational lensing
gravitationally bound
gravitational potential (Φ)
gravitational potential model
graviton
gravity anomaly
gravity assist
gravity sounding
gravity wave
gravitational wave (GW)
GW detection (GW)
Hale Telescope
helium rain
Hill radius
hypermassive neutron star (HMNS)
homologous collapse
Hubble expansion
Hubble time (tH)
hydrodynamic equations
hydrostatic equilibrium
internal gravity wave
inverse square law
isolation mass
J2
Jeans length
Jeans parameter (λ)
JUICE
Juno
Keplerian disk
Keplerian orbit
Kepler's laws
Kelvin-Helmholtz mechanism
Kelvin-Helmholtz timescale (KH timescale)
Lagrangian point
Lambda-CDM model (ΛCDM)
Lane-Emden equation
libration
liquid mirror telescope
long-period comet
Lovelock gravity
Mach's principle
mass
maximum iron fraction
Maxwell-Boltzmann distribution
mirror support cell
mixing length theory
MMX
modified Newtonian dynamics (MOND)
multi-messenger astronomy
N-body problem
N-body simulation
neutron star (NS)
Newton's laws
nutation
open cluster (OC)
orbital resonance
orbital speed
pointing error (PE)
physical field
planet
planetary differentiation
planetesimal
planet formation
Poisson's equation
potential energy (PE)
power law
parameterized post-Newtonian formalism (PPN formalism)
precession
protostar
Psyche
radio science
repulsive dark matter (RDM)
redshift (z)
reduced mass
relativity
red-giant branch (RGB)
Richardson number (Ri)
Roche limit
Roche lobe
scalar-tensor gravity
supercritical fluid (SCF)
Schrödinger-Poisson equation
sedimentation
self-gravitation
gravitational singularity
Solar and Heliospheric Observatory (SOHO)
Solar Orbiter (SolO)
sounder
special relativity (SR)
speed of light (c)
spherical harmonics
spiral density wave
standard gravitational parameter (μ)
stationkeeping
stellar cluster (SC)
stellar core
stellar dynamics
stellar evolution
stellar mass determination
stellar stream
stellar structure
string theory
strong-field gravity
strong force
superfluid
surface gravity
symmetry breaking
Trace Gas Orbiter (TGO)
theoretical modified GR metrics
tidal capture
tidal force
tidal migration
tide
timescale (t)
time standard
Titan
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
warp
wide binaries (WB)
wavefront error (WFE)
Wigner crystal
wormhole
Index