The Principles of Relativity along with Quantum Mechanics are the two most important achievements of the 20th century physics. Based on some earlier developments by Minkowski and using the mathematics discovered by Riemann in 1800s, Albert Einstein developed and formulated the principles of Relativity in 1905 and 1915. These came to be known as Special and General Theory of Relativity. Both theories have been tested and verified experimentally many times in the past decades.

**Special Theory of Relativity**

Formulated by Albert Einstein in 1905, it describes that the laws of the universe are independent of the inertial frame of reference. Special relativity deals with flat four-dimensional space-time. Therefore, the laws of the nature or the universe are the same for all observers, no matter how they move in the absence of gravity. The famous equation E=mC^{2} is also a consequence of this theory. The theory limits the speed of the light in all the inertial frames.

**General Theory of Relativity**

Since its development in 1915 by Albert Einstein, this theory has changed our notion of space and time. General relativity deals with curved space-time and formulates how gravity bends space-time. Its principles are used to explain and formulate the laws of the large-scale universe. According to General Relativity space and time are no longer separate and fixed entities. They are rather interconnected to each other and are flexible. Einstein equations are dynamical set of equations that describe how casual structure of the universe evolves.