Higher spin particles look the same if one turns them through smaller fractions of a complete revolution. All this seems fairly straightforward , but the remarkable fact is that these are particles that do not look the same if one turns them through just one revolution : you have to turn them through two complete revolutions !! Such particles are said to have spin ½.
All the known particles in the universe can be divided into two groups : particles of spin ½ (Quarks) , which make up the matter of the universe , and particles of spin 0 , 1 and 2 , which we shall see , give rise to the forces between the matter particles.
The matter particles obey what is called Pauli's exclusion principle. This was discovered in 1925 by an Austrian physicist , Wolfgang Pauli for which he received the Nobel Prize in 1945. He was the archetypal theoretical physicist : it was said of him that even his presence in the same town would make experiments go wrong !! Pauli's exclusion principle says that two similar particles cannot exist in the same state , that is , they cannot have both the same position and the same velocity , within the limits give by the uncertainty principle.
The exclusion principle is crucial because it explains why matter particles do not collapse to a state of very high density under the influence of the forces , produced by the particles of spin 0 , 1 and 2. If the matter particles have very nearly the same positions , they must have different velocities , which means they will not stay in the same position for long. If the world had been created without the exclusion principle , quarks would not form separate , well – defined protons and neutrons. Nor would these , together with electrons , for separate , well – defined atoms. They would all collapse to form a roughly uniform , dense "soup".
A proper understanding of the electron , and the spin ½ particles did not come until 1928 , when a theory was proposed by Paul Dirac , who was later elected to the Lucasian Professorship of Mathematics at Cambridge ( the same professorship Newton once held , and what Stephen Hawking currently **owns) Dirac's theory was the first of its kind that was consistent with both quantum mechanics , and the special theory of relativity.
Dirac explained mathematically why the electron had spin ½ , that is , why it didn't look the same if you turned it through only one complete revolution , but did if you turned it through two complete revolutions. It also predicted that the electron should have a partner , an antielectron , or positron. We now know that every particle has an antiparticle , with which it can annihilate.
There could be whole antiworlds and antipeople , made out of antiparticles. However , if you meet your antiself , don't shake hands !! You would both vanish in a great flash of light.
It is an important property of the force – carrying particles , that they do not obey the exclusion principle. This means that there is no limit to the number that can be exchanged , and so they can give rise to strong force. However , if the force – carrying particles have a high mass , it will be difficult to produce and exchange them over a large distance. So the forces that they carry , will have only a short range.