Forces always come in pairs (Remember Newton's third law). If you jump up in the air the Earth moves (slightly) in the other direction. After reaching your maximum height the gravitational force causes you to accelerate downwards and at the same time the Earth accelerates upwards to meet you. The forces are the result of an interaction between yourself and the Earth.
On the smallest scale forces between fundamental particles are usually referred to as interactions.
There are four fundamental interactions in nature:
Gravitational interaction affects particles with mass. It is the weakest interaction but governs the large scale structure of the universe because of its infinite range.
Weak interaction affects all particles. It is, as its name suggests, very weak and its range is extremely short. It is responsible for beta decay as we shall see.
Electromagnetic interaction affects particles with charge. It has an infinite range like gravity but is much stronger than both it and the weak interaction.
Strong interaction affects hadrons (i.e. particles made from quarks). It binds the quarks together but a residual effect of this is to bind the nucleons together in the nucleus. It is the strongest interaction but it has a very short range.
To see how such interactions arise imagine two astronauts drifting slowly towards each other in space. One of them throws a heavy ball towards the other and immediately recoils. Upon catching the ball the second astronaut also recoils. The net result is that an interaction has taken place between the astronauts due to the exchange of the ball. (See diagram)
Similarly the electromagnetic interaction, which results in the repulsion between two electrons, can be explained by the continual exchange of virtual photons. The exchange particles are virtual in the sense that they cannot be observed.
The diagram below represents the exchange of a virtual photon between two electrons and is known as a Feynman diagram.
Such diagrams are not meant to show actual trajectories of particles but only the relationships between the particles.
All four fundamental interactions can be explained as due to the exchange of virtual particles (exchange particles or gauge bosons).
|Interaction||Exchange particles||Range (m)|
|strong||gluon (between quarks)||10-15|
|mesons (between hadrons)|
|weak||W-, W+, Z0||10-18|
You may be wondering where the virtual particles come from and indeed where they go to. They do in fact only exist during the interaction. The energy for their creation is 'borrowed' from the universe. Although this seems to violate the energy conservation law it is allowed as long as the borrowed energy is 'paid back' quickly enough.