Introduction

Our present understanding of the physical phenomena in nature and the lawsgoverning them is based on the assumption that quarks and leptons are thebasic constituents of matter which interact with each other through strong(quarks only), weak, electromagnetic, and gravitational interactions. One ofthe major aims of scientists in the physics community has been to formulatea unified theory of all these fundamental interactions to describe thenatural phenomenon. The first and earliest step in this direction was tounify electromagnetic and gravitational interactions, both of them beinglong range interactions, that is, proportional to; many attempts were madeto unify them in the early twentieth century. It was then believed thatthese were the only two fundamental interactions and the interactions couldbe described by field theories based on the principle of invariance undercertain transformations called local gauge transformations because of theirexplicit dependence on space–time coordinates. In this type of fieldtheories, the electromagnetic interaction between two charged particles isdescribed by the exchange of a massless vector fieldAμ (x) asproposed by Weyl [40], while the gravitational interaction between the twoobjects is described by the exchange of a tensor fieldgμν (x)as proposed by Weyl [322] and Einstein [323]. Later, after the discovery ofthe atomic nucleus and the experimental studies of the structure of nucleiand the phenomenon of nuclear radioactivity, two more fundamentalinteractions, viz., strong and weak interactions were revealed. Theexistence of strong interaction is responsible for binding neutrons andprotons together and the weak interaction enables them to decay inside thenucleus. Both the interactions were found to be of short range. The need wasfelt to formulate a unified theory of all the four fundamental interactions,viz., the electromagnetic, strong, weak, and gravitational interactions.