Our present understanding of the physical phenomena in nature and the laws governing them is based on the assumption that quarks and leptons are the basic constituents of matter which interact with each other through strong (quarks only), weak, electromagnetic, and gravitational interactions. One of the major aims of scientists in the physics community has been to formulate a unified theory of all these fundamental interactions to describe the natural phenomenon. The first and earliest step in this direction was to unify electromagnetic and gravitational interactions, both of them being long range interactions, that is, proportional to; many attempts were made to unify them in the early twentieth century. It was then believed that these were the only two fundamental interactions and the interactions could be described by field theories based on the principle of invariance under certain transformations called local gauge transformations because of their explicit dependence on space–time coordinates. In this type of field theories, the electromagnetic interaction between two charged particles is described by the exchange of a massless vector field Aμ (x) as proposed by Weyl , while the gravitational interaction between the two objects is described by the exchange of a tensor field gμν (x) as proposed by Weyl  and Einstein . Later, after the discovery of the atomic nucleus and the experimental studies of the structure of nuclei and the phenomenon of nuclear radioactivity, two more fundamental interactions, viz., strong and weak interactions were revealed. The existence of strong interaction is responsible for binding neutrons and protons together and the weak interaction enables them to decay inside the nucleus. Both the interactions were found to be of short range. The need was felt to formulate a unified theory of all the four fundamental interactions, viz., the electromagnetic, strong, weak, and gravitational interactions.