The theory of relativity was developed in the 1900s by Albert Einstein. The theory aimed to solve the problems which could not be solved by the traditional classical physics in the 19^{th} century. The theory of relativity contains two interconnected theories by Albert Einstein which is about the special relativity and special relativity.

Special relativity explains and describes all physical phenomena related to elementary particles and their interactions with each other, except gravity. General relativity works on explaining the law of gravitation and how it relates to the other forces of nature. This also applies to the astrophysical realm, cosmology and includes astronomy.

Albert Einstein gave special relativity in 1905 which was based on the findings of many other scientists. And he developed the general relativity in 1916 through various contributions by others. Special relativity was except by the physicists by 1920 and it went on to become a vital tool for the theorists and the experimentalists in fields of nuclear physics, atomic physics and even quantum mechanics.

By these standards, the general relativity did not serve any purpose except correcting the Newtonian gravitation theory. The calculations were difficult and could not be understood by many. By 1960, the general relativity became related to astronomy and physics.

##### THE THEORY OF SPECIAL RELATIVITY

This theory first appeared in 1905 in a paper titled “On the Electrodynamics of Moving Bodies” by Einstein. This is a theory on the structure of the spacetime. This is based on two postulates:

- The principle of relativity states that the laws of physics are same for each and every observer in the uniform motion relative to each other.
- Regardless of the motion of the light source or the relative motion of the observers, the speed of light in the vacuum is same for each and every observer.

Some of the consequences of the theory includes time dilation, relativistic mass, length contradiction, the relativity of simultaneity, mass-energy equivalence etc.

##### THE THEORY OF GENERAL RELATIVITY

The equivalence principle stated that the state of accelerated motion and being at rest in the gravitational field is physically identical. This meant that free fall is inertial motion which meant that the objects free fall without any force acting on them which is contrary to the belief that gravitational pull is responsible for the fall of the object.

This was incompatible with special relativity which stated that the inertial objects can’t accelerate with respect to each other but this does happen with the objects in free fall. To solve this, Einstein said that the spacetime is curved. He gave Einstein field equations for relating the curvature of the spacetime with energy, mass, and momentum.

Some of the consequences of this theory are gravitational time dilation, bending of light rays in presence of gravity etc.