A level revision

Interaction in fields

Fields, such as gravtional and electromagnetic fields, are just interactions between particles. This interaction is an exchange of a boson between two fermions (i.e. matter). For example in an electromagnetic field a particle at point A will emit a photon which is then absorbed by another particle at point B and vice versa.

Force

The natural question is how does the exchanging of a boson cause a force on a particle? When the bosons are exchanged energy and momentum are carried from the emitting particle to the absorbing particle. When this momentum arrives there is a net effect on each particle which can be calculated by adding up all the amplitudes of the particles involved (the boson, the absorbing particle) taking into their phases.

Charged particles interact with electromagnetic fields by exchanging a photon between them. The charge (positive or negative) on the particle also contributes to the net effect on the absorbing particles resulting in like-charges being repeled and unlike-charges being attracted.

Feynman diagrams

Feynamn diagrams, like that above, show the ways in which a process can occur, in the above example the most simple form of two electrons exchanging a photon is shown.

It is important to remember that the diagrams are just a graphical representation of what happens, they do not show events that do happen. A photon that is exchanged between two charged particles is known as a virtual particle as they are impossible to detect.