In physics, gravity (or gravitation)
is the name given to a force that attracts masses together
(gravitational force)
generally according to the product of their masses and the reciprocal of the
square of the intervening distance (law of gravitation).
The law 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 masses.
Albert Einstein re-cast 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 his model from GR.
These theories are phenomenally successful: for example,
the spacecraft-navigation methods we use would utterly fail
unless these laws precisely describe gravity throughout
the solar system, and the theorized degree to which
gravity bends light has been observed.
Yet they have failed to explain some astronomical 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 indicate the universe 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 often 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.