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.