(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 electromagnetic radiation passing near a massive
object being affected just as a passing object would be.
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 seen (dark matter), and that there must be an as-yet-unexplained
outward force at work in the universe (dark energy).
Alternately, attempts have been made to further refine gravitational
theory to explain these observations (modified Newtonian dynamics and DGP gravity).
In studies of the detail of the effects of the gravity of Earth
and/or other bodies, the words gravity and gravitation are often
used with distinct meanings: gravitation to indicate the universal
force and gravity to indicate a body's gravitation's 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 affects 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.
baryon acoustic oscillations (BAO)
black hole (BH)
cold dark matter (CDM)
cosmological constant (Λ)
critical density (ρc)
Darcy velocity field
dark matter annihilation
dark matter halo
Effelsberg 100-m Radio Telescope
Einstein-de Sitter model
escape velocity (Ve)
star formation feedback
galaxy cluster (CL)
Goddard gravity model (GGM)
gravitational instability (GI)
giant molecular cloud (GMC)
general relativity (GR)
gravitational instability model
gravitational potential (Φ)
gravitational wave (GW)
GW detection (GW)
hypermassive neutron star (HMNS)
internal gravity wave
inverse square law
Jeans parameter (λ)
Lambda-CDM model (ΛCDM)
liquid mirror telescope
maximum iron fraction
mirror support cell
mixing length theory
modified Newtonian dynamics (MOND)
pointing error (PE)
potential energy (PE)
red-giant branch (RGB)
Richardson number (Ri)
Solar and Heliospheric Observatory (SOHO)
speed of light (c)
spiral density wave
stellar cluster (SC)
stellar mass determination
Theory of Everything (TOE)
Toomre Q parameter (Q)
T-Tauri star (TTS)
wavefront error (WFE)