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The fundamental forces of nature

There are at present only four certainly known types of force, and these are listed below. The relative importance of each force in an interaction depends on the type of interaction being considered.

(a) The gravitational force
This acts between all particles with mass and is responsible for holding planets in orbit around the Sun. Range infinite varying as 1/d2.
(b) The electromagnetic force
This acts between all charged particles, and is the binding force of atoms and molecules. Range infinite varying as 1/d2
(c) The weak force
This is responsible for radioactive decay and the change in quark flavour. It acts between all particles. It is seen in lepton reactions such as the reaction between a neutrino and a muon. Range about 10-3 fm
(d) The strong force
This holds neutrons and protons together in a nucleus. It acts between hadrons since they contain quarks. Range about 1 fm.

Note: 1 fm = 1 fermi = 10-15 m about the diameter of a small atomic nucleus.

These fundamental forces can be explained by describing them in terms of exchange particles and these are shown in the following table.

Force (interaction) Particle name Charge Mass (proton = 1)
Electromagnetic photon 0 0
Strong gluon 0 0
  W+ +e 89
Weak W- -e 89
  Zo 0 99

Attempts have been made to simplify this theory by combining two or more of these forces. This is called unification. In 1979 Glashow, Salam and Weinberg succeeded in producing a theory which combines the electromagnetic and weak forces the so called electroweak force.

If we travel back in time towards the Big Bang then before about 10-35 s the strong nuclear force was indistinguishable from the electroweak force. Physicists have tried to produce a theory which combines the electroweak force and the strong force but no one has yet succeeded. This theory is called The Grand Unified Theory or GUT.

© Keith Gibbs 2010