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-., 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
proposed theories that unified this interaction with the electromagnetic force, just as Maxwell had unified electricity and
magnetism about a hundred years earlier. They suggested that in addition to the photon, there were three other spin-1
particles, known collectively as massive vector bosons, that carried the weak force. These were called W+ (pronounced
W plus), W- (pronounced W minus), and Zo (pronounced Z naught), and each had a mass of around 100 GeV (GeV
stands for gigaelectron-volt, or one thousand million electron volts). The Weinberg-Salam theory exhibits a property
known as spontaneous symmetry breaking. This means that what appear to be a number of completely different particles
at low energies are in fact found to be all the same type of particle, only in different states. At high energies all these
particles behave similarly. The effect is rather like the behavior of a roulette ball on a roulette wheel. At high energies
(when the wheel is spun quickly) the ball behaves in essentially only one way – it rolls round and round. But as the wheel
slows, the energy of the ball decreases, and eventually the ball drops into one of the thirty-seven slots in the wheel. In
other words, at low energies there are thirty-seven different states in which the ball can exist. If, for some reason, we
could only observe the ball at low energies, we would then think that there were thirty-seven different types of ball!
In the Weinberg-Salam theory, at energies much greater than 100 GeV, the three new particles and the photon would all
behave in a similar manner. But at the lower particle energies that occur in most normal situations, this symmetry
between the particles would be broken. WE, W, and Zo would acquire large masses, making the forces they carry have a
very short range. At the time that Salam and Weinberg proposed their theory, few people believed them, and particle
accelerators were not powerful enough to reach the energies of 100 GeV required to produce real W+, W-, or Zo particles.
However, over the next ten years or so, the other predictions of the theory at lower energies agreed so well with
experiment that, in 1979, Salam and Weinberg were awarded the Nobel Prize for physics, together with Sheldon
Glashow, also at Harvard, who had suggested similar unified theories of the electromagnetic and weak nuclear forces.
The Nobel committee was spared the embarrassment of having made a mistake by the discovery in 1983 at CERN
(European Centre for Nuclear Research) of the three massive partners of the photon, with the correct predicted masses
and other properties. Carlo Rubbia, who led the team of several hundred physicists that made the discovery, received the
Nobel Prize in 1984, along with Simon van der Meer, the CERNengineer who developed the antimatter storage system
employed. (It is very difficult to make a mark in experimental physics these days unless you are already at the top! )
The fourth category is the strong nuclear force, which holds the quarks together in the proton and neutron, and holds the
protons and neutrons together in the nucleus of an atom. It is believed that this force is carried by another spin-1 particle,
called the gluon, which interacts only with itself and with the quarks. The strong nuclear force has a curious property
called confinement: it always binds particles together into combinations that have no color. One cannot have a single
quark on its own because it would have a color (red, green, or blue). Instead, a red quark has to be joined to a green and
a blue quark by a “string” of gluons (red + green + blue = white). Such a triplet constitutes a proton or a neutron. Another
possibility is a pair consisting of a quark and an antiquark (red + antired, or green + antigreen, or blue + antiblue = white).
Such combinations make up the particles known as mesons, which are unstable because the quark and antiquark can
annihilate each other, producing electrons and other particles. Similarly, confinement prevents one having a single gluon
on its own, because gluons also have color. Instead, one has to have a collection of gluons whose colors add up to white.
Such a collection forms an unstable particle called a glueball.
The fact that confinement prevents one from observing an isolated quark or gluon might seem to make the whole notion
of quarks and gluons as particles somewhat metaphysical. However, there is another property of the strong nuclear
force, called asymptotic freedom, that makes the concept of quarks and gluons well defined. At normal energies, the
strong nuclear force is indeed strong, and it binds the quarks tightly together. However, experiments with large particle
accelerators indicate that at high energies the strong force becomes much weaker, and the quarks and gluons behave
almost like free particles.
Figure 5:2
Figure 5:2 shows a photograph of a collision between a high-energy proton and antiproton. The success of the unification
of the electromagnetic and weak nuclear forces led to a number of attempts to combine these two forces with the strong
nuclear force into what is called a grand unified theory (or GUT). This title is rather an exaggeration: the resultant theories
are not all that grand, nor are they fully unified, as they do not include gravity. Nor are they really complete theories,
because they contain a number of parameters whose values cannot be predicted from the theory but have to be chosen
to fit in with experiment. Nevertheless, they may be a step toward a complete, fully unified theory. The basic idea of
GUTs is as follows: as was mentioned above, the strong nuclear force gets weaker at high energies. On the other hand,
the electromagnetic and weak forces, which are not asymptotically free, get stronger at high energies. At some very high
energy, called the grand unification energy, these three forces would all have the same strength and so could just be
different aspects of a single force. The GUTs also predict that at this energy the different spin-. matter particles, like
quarks and electrons, would also all be essentially the same, thus achieving another unification.
The value of the grand unification energy is not very well known, but it would probably have to be at least a thousand
million million GeV. The present generation of particle accelerators can collide particles at energies of about one hundred
GeV, and machines are planned that would raise this to a few thousand GeV. But a machine that was powerful enough to
accelerate particles to the grand unification energy would have to be as big as the Solar System – and would be unlikely
to be funded in the present economic climate. Thus it is impossible to test grand unified theories directly in the laboratory.
However, just as in the case of the electromagnetic and weak unified theory, there are low-energy consequences of the
theory that can be tested.
The most interesting of these is the prediction that protons, which make up much of the mass of ordinary matter, can
spontaneously decay into lighter particles such as antielectrons. The reason this is possible is that at the grand
unification energy there is no essential difference between a quark and an antielectron. The three quarks inside a proton
normally do not have enough energy to change into antielectrons, but very occasionally one of them may acquire
sufficient energy to make the transition because the uncertainty principle means that the energy of the quarks inside the
proton cannot be fixed exactly. The proton would then decay. The probability of a quark gaining sufficient energy is so
low that one is likely to have to wait at least a million million million million million years (1 followed by thirty zeros). This is
much longer than the time since the big bang, which is a mere ten thousand million years or so (1 followed by ten zeros).
Thus one might think that the possibility of spontaneous proton decay could not be tested experimentally. However, one
can increase one’s chances of detecting a decay by observing a large amount of matter containing a very large number
of protons. (If, for example, one observed a number of protons equal to 1 followed by thirty-one zeros for a period of one
year, one would expect, according to the simplest GUT, to observe more than one proton decay.)
A number of such experiments have been carried out, but none have yielded definite evidence of proton or neutron
decay. One experiment used eight thousand tons of water and was performed in the Morton Salt Mine in Ohio (to avoid
other events taking place, caused by cosmic rays, that might be confused with proton decay). Since no spontaneous
proton decay had been observed during the experiment, one can calculate that the probable life of the proton must be
greater than ten million million million million million years (1 with thirty-one zeros). This is longer than the lifetime
predicted by the simplest grand unified theory, but there are more elaborate theories in which the predicted lifetimes are
longer. Still more sensitive experiments involving even larger quantities of matter will be needed to test them.
Even though it is very difficult to observe spontaneous proton decay, it may be that our very existence is a consequence
of the reverse process, the production of protons, or more simply, of quarks, from an initial situation in which there were
no more quarks than antiquarks, which is the most natural way to imagine the universe starting out. Matter on the earth is
made up mainly of protons and neutrons, which in turn are made up of quarks. There are no antiprotons or antineutrons,
made up from antiquarks, except for a few that physicists produce in large particle accelerators. We have evidence from
cosmic rays that the same is true for all the matter in our galaxy: there are no antiprotons or antineutrons apart from a
small number that are produced as particle/ antiparticle pairs in high-energy collisions. If there were large regions of
antimatter in our galaxy, we would expect to observe large quantities of radiation from the borders between the regions of
matter and antimatter, where many particles would be colliding with their anti-particles, annihilating each other and giving
off high-energy radiation.
We have no direct evidence as to whether the matter in other galaxies is made up of protons and neutrons or antiprotons
and anti-neutrons, but it must be one or the other: there cannot be a mixture in a single galaxy because in that case we
would again observe a lot of radiation from annihilations. We therefore believe that all galaxies are composed of quarks
rather than antiquarks; it seems implausible that some galaxies should be matter and some antimatter.
Why should there be so many more quarks than antiquarks? Why are there not equal numbers of each? It is certainly
