ABSTRACT
There are four fundamental interactions in nature: the strong, weak, electromagnetic and gravitational interactions. The rst three of these interactions are explained as gauge theories while the gravitational interaction is explained by the general theory of relativity, classically. The superstring theory promises to be a unied theory of all of these interactions at very high energy, when all the interactions become strong. It is also emerging as a consistent quantum theory of gravity, which would ensure that the ultimate unied theory is also a quantum theory of all the interactions.
