gravity

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.