Grand Unified Theory

Force-carrying particles can be grouped into four catergories according to strength of the force that they carry and the particles with which they interact. Ultimately most physicists hope to find a unified theory that will explain all four forces as different aspects of a single force.

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, 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 sometimes 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 picture as being carried by a particle of spin 2 called the gravition. This has no mass of its own, so the force that carries is long range, The gravitational force between the sun and earth is ascribed to exchange of gravitions 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 gravitions make up what classical physicists would call graviattional waves, which are very weak - and so difficult to detect that they have never yet observed.

The next category is the electromagnetic force, which interacts with electrically charged particles like electrons and quarks, but not with uncharged particles like electrons and quarks, but not with uncharged particles such as gravitions. It is much stronger than the gravitional force: the electromagnetic force between two electrons is much much larger 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 negative charge. A large body such as the earth or sun, contains nearly equal numbers of positive and negative charges. Thus attractive and repulsive forces between the individual particles nearly cancel each other out, and there is very electromagnetic force. However on 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 released and a real photon is emitted - which can be observed as visible light by the human eye, if it has the right wavelength, 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 1/2, but not particles 0, 1 or 2, such as photons and gravitions. The weak nuclear force was not understood until 1967 , when Adbus Salam and Steven Weinberg proposed theories that unified this interaction with the electromagnetic force. They suggested that in addition to the photon, there were three other spin-1 particles, known collectively as massive vector bosons, that carried weak force. These were called W plus, W minus and Z naught, and each had a mass of around 100GeV(Giga). 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.

In Weinberg-Salam theory, at energies much greater than 100GeV, the three new particles and the photon would behave in a similiar manner. But at the lower particle energies that occur in most normal situations, this symmetry between the particles would be broken. w plus, w minus and z naught would acquire large masses, making the forces they carry have a very short range. Salam and proposed their theory, few people believed them and particle accelerators were not powerful enough to reach energies of 100 GeV required to produce real W plus, W minus and Z naught particles. In 1979, Weinberg and Salam were awarded the Nobel prize for Physics. The Nobel committee was spared the embarassment of having made a mistake by the discovery in 1983 at CERN (European Centre For Nuclear Research) of three massive partners of the photon, with correct predicted masses and other properties.

The fourth category is the strong nuclear force, which holds quarks together in 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 colour. One cannot have a single quark on its own because it would have a colour (red+green+blue=white). Such a triplet constitutes a proton or a neutron. Another possibility is a pair consisting of a quark and 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 has to have a colletiom of gluons also have colour. Instead, one has to have a collection of gluons whose gluons whose colours 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 glouns 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 weaker, and the quarks and gluons behave almost like free particles.

The success the unification of the electromagnetic and weak nuclear forces led to a number of attempts to combine these two with the strong nuclear force into what is called the Grand Unified Theory (GUT). This title is incorrect as the resultant theories are not at all grand, nor are they fully unified, as they do not include gravity. Nor are they really complete theories, because they contain parameters whose values cannot be predicted from the theory but have been chosen to fit in the experiment. Nevertheless, they may be a step toward a complete unified theory. the basic idea of GUTs is as follows: as we 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 higher energies. at some very high energy called the Grand Unification Energy, these three forces would have the same strength and could be different aspects of a single force. The GUTs also predict that at this energy the different spin 1/2 matter particles, like quarks and electrons, would also all be essentially the same, thus achieving another unification.

The value of Grand Unification Energy is not very well known, but it would probably have to be at least a thousand million millon 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 would be powerful enough to accelerate particles to the Grand Unification Energy would have to be the size of the solar system. Thus it is impossible to test the GUTs directly in the laboratory. However, just as in the case of electromagnetic and weak unified theory, there are low energy consequences of the theory that can be tested.

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