We know that the forces are carried by the exchange of virtual particles. As part of his theory of quantum electrodynamics, Richard Feynman developed a method of illustrating the electromagnetic force using Feynman diagrams.
Picture 6.4 We can show the force between two electrons using a Feynman diagram.
Basic Feynman diagrams
Feynman diagrams are useful for showing interactions between two (or more) particles. They are graphs with time on the y-axis and position on the x-axis (though sometimes you will see the axes swapped around). Different particles are represented by different lines in this domain. For example, an electron is represented by a straight line with an arrow (picture 6.4). The arrow is pointing upwards to show it is moving forwards in time.
We can show an electron giving out a virtual photon as a wavy line moving to the right. Notice that the virtual photon has a shallower gradient (this is because it is travelling faster at the speed of light). Also, notice that the electron recoils when it gives out the virtual photon this is in line with the conservation of momentum.
Now let's imagine that the virtual photon is absorbed by another electron. The second electron also recoils. So this diagram shows the repulsive force between two electrons. They are pushed apart as they move forwards through time.
Sometimes the photons might decay to produce a new particle/antiparticle pair such as a muon and anti-muon. This process is called pair production. Notice that the positron and the anti-muon are both travelling backwards in time.
Picture 6.6 Exchange of gluon carrying the force between two quarks. Notice that the quarks change colour in the exchange.
Feynman diagrams can also be used to describe the strong nuclear force. This is the force between quarks and is carried by the gluon which is shown as a looping line. The force between quarks depends on a property called colour (of course the quarks do not have colour it is just a property given to them - sometimes called colour charge see page 23). The theory that describes this is called quantum chromodynamics. Notice that the gluon carries colour charge itself (unlike the virtual photon that doesn't carry electric charge in the electromagnetic force). This means that the quarks' colour charges change when they emit and absorb the gluon.