virtual particles which can never be directly detected; but if one electron moves past another, real photons may be given off, which we detect as light waves.
Force-carrying particles can be grouped into four categories according to the strength of the force that they carry and the particles with which they interact. It should be emphasized that this division into four classes is man-made; it is convenient for the construction of partial theories, but it may not correspond to anything deeper. Ultimately, most physicists hope to find a unified theory that will explain all four forces as different aspects of a single force. Indeed, many would say this is the prime goal of physics today. Recently, successful attempts have been made to unify three of the four categories of force- and I shall describe these. The question of the unification of the remaining category, gravity we shall leave till later.