gravity

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.