Types of interactions

In chapter 2 we have seen that the nuclear interaction is very much stronger than the electromagnetic interaction; their relative strengths are characterised by the difference in the ‘coupling constants’ which are discussed in more detail below. We may recognise at this stage two other forms of interaction, gravitational forces and the so-called ‘weak’ interaction. One of the most notable milestones of recent years has been the unification of the weak and electromagnetic interactions as different aspects of the same underlying force, in the theory of Glashow, Weinberg and Salam, and the discovery of the W and Z particles, the additional quanta of the unified interaction, with the properties predicted. This topic is treated in more detail in chapter 11.

We may summarise as follows:

Strong interactions are responsible for the interactions between nucleons, nucleons and mesons and a number of other particles. The mesons act as the quanta of the strong interaction on the nuclear scale. These interactions reflect the interaction between quarks due to the exchange of gluons on the sub-nucleon level.

Electromagnetic interactions are responsible for the force between electrically-charged particles and are mediated by the exchange of photons.

Weak interactions are responsible for many particle decays such as radioactive decay (the basic process n → p + e- + vg), pion and muon decay and a number of other decay processes.

Gravitational interactions exist between all particles having mass and are believed to be mediated by the so far undetected gravitons.