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General relativity is a scientific theory describing how matter, energy, time, and space interact. It was first published by Albert Einstein in 1917 as an extension to his theory of special relativity. General relativity treats space and time as a single unified four-dimensional “space time”; under general relativity, matter deforms the geometry of space time, and space time deformations cause matter to move, which we see as gravity. 

The basic assumption of general relativity is that forces caused by gravity and forces caused by acceleration are equivalent. If a closed box is undergoing acceleration, no experiment done within the box can tell if the box is at rest within a gravitational field, or is being accelerated through space. This principle, that all physical laws are the same for accelerated observers and observers in a gravitational field, is known as the equivalence principle; it has been experimentally tested to more than twelve decimal places of accuracy.