Introduction

As indicated in Chapters 1 and 2, we are faced in nature with several types of fundamental interaction or field between particles. Each field has its distinct characteristics, such as space–time transformation properties (vector, tensor, scalar etc.), a particular set of conservation rules that are obeyed by the interaction and a characteristic coupling constant that determines the magnitude of the collision cross-sections or decay rates mediated by the interaction.

The fact that the strength of the gravitational interaction between two protons, for example, is only 10−38 of their electrical interaction has always been a puzzle and a challenge, and many attempts have been made to understand the interrelation between the different fields. In the last decades, the belief has grown that the strong, weak, electromagnetic and gravitational interactions are but different aspects of a single universal interaction, which would be manifested at some colossally high energy. At the everyday energies met with in laboratory studies in particle physics, it is necessary to assume that this symmetry is badly broken, at these mass or energy scales which are puny relative to the unification energy.

The first successful attempt to unify two apparently different interactions was achieved by Clerk Maxwell in 1865. He showed that electricity and magnetism could be unified into a single theory involving a vector field (the electromagnetic field) interacting between electric charges and currents.