Page 39 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 5
important, and I shall return to it later in the chapter.

In quantum mechanics, the forces or interactions between matter particles are all supposed to be carried by particles of
integer spin – 0, 1, or 2. What happens is that a matter particle, such as an electron or a quark, emits a force-carrying
particle. The recoil from this emission changes the velocity of the matter particle. The force-carrying particle then collides
with another matter particle and is absorbed. This collision changes the velocity of the second particle, just as if there had
been a force between the two matter particles. 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 a 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. On the other hand, if the
force-carrying particles have no mass of their own, the forces will be long range. The force-carrying particles exchanged
between matter particles are said to be virtual particles because, unlike “real” particles, they cannot be directly detected
by a particle detector. We know they exist, however, because they do have a measurable effect: they give rise to forces
between matter particles. Particles of spin 0, 1, or 2 do also exist in some circumstances as real particles, when they can
be directly detected. They then appear to us as what a classical physicist would call waves, such as waves of light or
gravitational waves. They may sometimes be emitted when matter particles interact with each other by exchanging virtual
force-carrying particles. (For example, the electric repulsive force between two electrons is due to the exchange of virtual
photons, which can never be directly detected; but if one electron moves past another, real photons may be given off,
which we detect as light waves.)

Force-carrying particles can be grouped into four categories according to the strength of the force that they carry and the
particles with which they interact. It should be emphasized that this division into four classes is man-made; it is
convenient for the construction of partial theories, but it may not correspond to anything deeper. Ultimately, most
physicists hope to find a unified theory that will explain all four forces as different aspects of a single force. Indeed, many
would say this is the prime goal of physics today. Recently, successful attempts have been made to unify three of the
four categories of force – and I shall describe these in this chapter. The question of the unification of the remaining
category, gravity, we shall leave till later.

The first category is the gravitational force. This force is universal, that is, every particle feels the force of gravity,
according to its mass or energy. Gravity is the weakest of the four forces by a long way; it is so weak that we would not
notice it at all were it not for two special properties that it has: it can act over large distances, and it is always attractive.
This means that the very weak gravitational forces between the individual particles in two large bodies, such as the earth
and the sun, can all add up to produce a significant force. The other three forces are either short range, or are sometimes
attractive and some-times repulsive, so they tend to cancel out. In the quantum mechanical way of looking at the
gravitational field, the force between two matter particles is pictured as being carried by a particle of spin 2 called the
graviton. This has no mass of its own, so the force that it carries is long range. The gravitational force between the sun
and the earth is ascribed to the exchange of gravitons between the particles that make up these two bodies. Although the
exchanged particles are virtual, they certainly do produce a measurable effect – they make the earth orbit the sun! Real
gravitons make up what classical physicists would call gravitational waves, which are very weak – and so difficult to
detect that they have not yet been observed.

The next category is the electromagnetic force, which interacts with electrically charged particles like electrons and
quarks, but not with uncharged particles such as gravitons. It is much stronger than the gravitational force: the
electromagnetic force between two electrons is about a million million million million million million million (1 with forty-two
zeros after it) times bigger than the gravitational force. However, there are two kinds of electric charge, positive and
negative. The force between two positive charges is repulsive, as is the force between two negative charges, but the
force is attractive between a positive and a negative charge. A large body, such as the earth or the sun, contains nearly
equal numbers of positive and negative charges. Thus the attractive and repulsive forces between the individual particles
nearly cancel each other out, and there is very little net electromagnetic force. However, on the small scales of atoms
and molecules, electromagnetic forces dominate. The electromagnetic attraction between negatively charged electrons
and positively charged protons in the nucleus causes the electrons to orbit the nucleus of the atom, just as gravitational
attraction causes the earth to orbit the sun. The electromagnetic attraction is pictured as being caused by the exchange
of large numbers of virtual massless particles of spin 1, called photons. Again, the photons that are exchanged are virtual
particles. However, when an electron changes from one allowed orbit to another one nearer to the nucleus, energy is
released and a real photon is emitted – which can be observed as visible light by the human eye, if it has the right
wave-length, or by a photon detector such as photographic film. Equally, if a real photon collides with an atom, it may
move an electron from an orbit nearer the nucleus to one farther away. This uses up the energy of the photon, so it is
absorbed.

The third category is called the weak nuclear force, which is responsible for radioactivity and which acts on all matter
particles of spin-½, but not on particles of spin 0, 1, or 2, such as photons and gravitons. The weak nuclear force was not
well understood until 1967, when Abdus Salam at Imperial College, London, and Steven Weinberg at Harvard both

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