Gauge boson exchange

Physicists today currently believe that all forces in nature can be ultimately described in terms of 3 fundamental forces, these are: the force of universal gravitation, the electroweak force, and the strong force (often referred to as the QCD force). Even as recently as 20 years ago these forces were not necessarily the forces that were described as fundamental. It has only been through the steady development of the theory of unification of certain forces, (such as electricity and magnetism), that we now believe all other forces derive from these three fundamental ones.

The electromagnetic force, which is part of the electroweak force, is responsible for most of the secondary forces, including tension, friction, drag, normal forces, and spring forces. But how exactly do these forces work? For instance we know that two particles processing the same charge (i.e. electrons) experience a force which makes them repel from one another, but why is this? Physicists believe that all of these forces (or interactions) are mediated by a particle known as a "gauge boson". Unlike the fundamental particles described earlier, these particles have no mass and are called 'virtual particles' (although the term is slightly misleading as experiments have shown that they do exist). One of the gauge bosons will already be familiar to you, this is the photon (the particle responsible for carrying the electromagnetic force). Other gauge bosons are the W+, W-, and the Z0 gauge boson. As well as these gluons, the particles responsible for the forces between quarks are also gauge bosons. Finally theory predicts the existence of a gauge boson known as the Higgs boson, which we have yet to observe in experiment let alone fully understand.

Although physicists are confident that gauge bosons are responsible for mediating the interactions which give rise to the three fundamental forces from which all other forces arise, it can be difficult to visualise how the exchange of these virtual particles can result in many of the forces we observe in the world around us.

Going back to the example of the repulsive force that exists between two electrons when they are brought into close proximity. A metaphor that is often used to help visualise repulsive forces such as these is to think of the electrons as two ice skaters moving parallel to one another on an ice rink. If the skaters (the electrons) now start to throw a heavy object, such as a medicine ball, back and forth between them the force of the impact of the ball each time it is caught by one of the skaters, combined with the virtually frictionless surface of the rink, will mean that the two skaters will start to move away from one another.

This picture is one way to think of repulsive forces, such as the one that exists between like charges, except that the 'medicine ball' is actually a gauge boson, and in real life the 'ball' is transparent, essentially invisible to us. The fact that gauge bosons are invisible to us, and that we can only observe the effects they produce rather then the particles themselves, has led to the phrase "action at a distance" to describe the way in which they work. Effects such as when one magnet causes another to move without contact even being made, seem to almost be some form of 'magic', however intuitively we know that this cannot be the case. Although you cannot see them, it is through the exchange of gauge bosons that all these effects come about.