Summary of the Fundamental Forces

There are 4 fundamental forces that have been identified. In our present Universe they have rather different properties.

• The strong interaction is very strong, but very short-ranged. It acts only over ranges of ordercm – the size of the nuclues - and is responsible for holding the nuclei of atoms together. It is basically attractive, but can be effectively repulsive in some circumstances.

• The electromagnetic force causes electric and magnetic effects such as the repulsion between like electrical charges or the interaction of bar magnets. It is long-ranged, much weaker than the strong force. It can be attractive or repulsive – charges are positive or negative – like charges repel, unlike charges attract, like currents attract, unlike currents repel - and acts only between pieces of matter carrying electrical charge or materials carrying elective currents On large scales electric charges tend to neutralise each other, so that larger bodies tend to be more neutral, and electric forces are not significant at the largest scales.

• The weak force is responsible for radioactive decay and neutrino interactions. It has a very short range and, as its name indicates, it is very weak. The existance of this force forces free neutrons to have a lifetime of 15 minutes or so.

• The gravitational force is weak, but very long ranged. Furthermore, it is always attractive, and acts between any two pieces of matter in the Universe since mass is its source.

All the forces are associated with particles which carry the forces between affected particles.

The strong force is carried by gluons.

The electromagnetic force is carried by photons.

The weak force is carried by W and Z bosons.

The gravitational force is carried by gravitons.

The shortest range forces are carried by the heaviest particles, which are restricted in range because of the uncertainty principle. The graviton and phton have no mass, hence infinite range.

On the largest scale gravitation is most important. This is because it is long ranged and always attractive. Although gravitation is extremely weak, it always wins over the largest distances and therefore is the most important force for the understanding of the large scale structure and evolution of the Universe. It is gravity that forces the clumping of matter into stars and galaxies, and forces matter in the centres of stars to undergo nuclear fusion. The other forces tend to cancel each other out or are restricted in range by the nature of the particles that carry the force.