General relativity is a theory of gravitation developed by Einstein in the years 1907?1915. The development of general relativity began with the
equivalence principle, under which the states of
accelerated motion and being at rest in a
gravitational field (for example when standing on the surface of the Earth) are physically identical. The upshot of this is that
free fall is
inertial motion: In other words an object in free fall is falling because that is how objects move when there is no
force being exerted on them, instead of this being due to the force of
gravity as is the case in
classical mechanics. This is incompatible with classical mechanics and
special relativity because in those theories inertially moving objects cannot accelerate with respect to each other, but objects in free fall do so. To resolve this difficulty Einstein first proposed that spacetime is
curved. In 1915, he devised the
Einstein field equations which relate the curvature of spacetime with the mass, energy, and momentum within it.