Page 38 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 5

Figure 5:1
All the known particles in the universe can be divided into two groups: particles of spin ½, which make up the matter in
the universe, and particles of spin 0, 1, and 2, which, as we shall see, give rise to 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 can-not exist in the same state; that is, they cannot have both the same position and the same
velocity, within the limits given 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 that 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, form
separate, well-defined atoms. They would all collapse to form a roughly uniform, dense “soup.”

A proper understanding of the electron and other spin-½ particles did not come until 1928, when a theory was proposed
by Paul Dirac, who later was elected to the Lucasian Professorship of Mathematics at Cambridge (the same
professorship that Newton had once held and that I now hold). Dirac’s theory was the first of its kind that was consistent
with both quantum mechanics and the special theory of relativity. It 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 revolutions. It also predicted that the electron should have a partner: an anti-electron, or positron. The
discovery of the positron in 1932 confirmed Dirac’s theory and led to his being awarded the Nobel Prize for physics in
1933. We now know that every particle has an antiparticle, with which it can annihilate. (In the case of the force-carrying
particles, the antiparticles are the same as the particles themselves.) 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. The question of why there seem to be so many more particles than antiparticles around us is extremely

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