Question

What is gravity? What is accelaration?

Answer 1

Gravity is the natural force that causes things to fall toward the earth.

Acceleration is the rate of change of velocity of an object with respect to time.

Answer 2

Gravity (from Latin gravis 'heavy'[1]) is a physical connection between space and matter that is precisely described by Einstein’s 1915 geometric theory of gravity known as General Relativity. Put simply, that theory says, "Space acts on matter, telling it how to move. In turn, matter reacts back on space, telling it how to curve."[2] Here and below, the word space is equivalent to spacetime.

File:Apollo 15 feather and hammer drop.ogvPlay media

Hammer and feather drop: astronaut David Scott (from mission Apollo 15) on the Moon enacting the legend of Galileo's gravity experiment. (1.38 MB, oggformat).

Gravity is then the physical connection between spacetime and matter that causes a curvature of spacetime. Here a curvature of spacetime is the same as a geometry of spacetime. A most remarkable feature of General Relativity is that it regards a geometry of spacetime as a new physical entity, with degrees of freedom and a dynamics of its own.[2]

An extreme example of a curvature of spacetime (or a geometry of spacetime) is a black hole, from which nothing—not even light—can escape once past the black hole's event horizon.[3]

Current models of particle physics imply that the earliest instance of gravity in the Universe, possibly in the form of quantum gravity, supergravity or a gravitational singularity, along with ordinary space and time, developed during the Planck epoch (up to 10−43 seconds after the birth of the Universe), possibly from a primeval state, such as a false vacuum, quantum vacuum or virtual particle, in a currently unknown manner.[4] Attempts to develop a theory of gravity consistent with quantum mechanics, a quantum gravity theory, which would allow gravity to be united in a common mathematical framework (a theory of everything) with the other three fundamental interactions of physics, are a current area of research.