On the 20th June 1933 Professor Einstein addressed a large and
enthusiastic audience in the Victorian Gothic Bute Hall of the
University of Glasgow. Einstein spoke 'About the Origins of the General Theory of Relativity'.
In 1905 Einstein had changed the face of physics forever with the
publication of his radical new ideas on special relativity. His general
theory of relativity was introduced to the world in 1915. However in
1933, Einstein faced another challenge—survival in a world of change. The theory of relativity, or simply relativity, refers specifically to two theories: Albert Einstein's special relativity and general relativity. However, "relativity" can also refer to Galilean relativity.
The term "theory of relativity" was coined by Max Planck in 1908 to emphasize how special relativity (and later, general relativity) uses the principle of relativity.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.
Some of the consequences of general relativity are:
Time goes slower at lower gravitational potentials. This is called gravitational time dilation.Orbits precess in a way unexpected in Newton's theory of gravity. (This has been observed in the orbit of Mercury and in binary pulsars).Even rays of light (which are weightless) bend in the presence of a gravitational field.The Universe is expanding, and the far parts of it are moving away from us faster than the speed of light. This does not contradict the theory of special relativity, since it is space itself that is expanding.Frame-dragging, in which a rotating mass "drags along" the space time around it.
Technically, general relativity is a metric theory of gravitation whose defining feature is its use of the Einstein field equations. The solutions of the field equations are metric tensors which define the topology of the spacetime and how objects move intertially.