The essential new aspect of nuclear matter which accounts for all its unusual properties is the strong nuclear force, and the feature that makes this force so unique is its extremely short range. It acts only when the nucleons come very near to each other, that is, when their distance is about two to three times their diameter. At such a distance, the nuclear force is strongly attractive, but when the distance becomes less the force becomes strongly repulsive so that the nucleons cannot approach each other any closer. In this way, the nuclear force keeps the nucleus in an extremely stable, though extremely dynamic equilibrium.
The picture of matter which emerges from the study of atoms and nuclei shows that most of it is concentrated in tiny drops separated by huge distances. In the vast space between the massive and fiercely boiling nuclear drops move the electrons. These constitute only a tiny fraction of the total mass, but give matter its solid aspect and provide the links necessary to build up the molecular structures. They are also involved in the chemical reactions and are responsible for the chemical properties of matter. Nuclear reactions, on the other hand, generally do not occur naturally in this form of matter because the available energies are not high enough to disturb the nuclear equilibrium.
This form of matter, however, with its multitude of shapes and textures and its complicated molecular architecture, can exist only under very special conditions, when the temperature is not too high, so that the molecules do not jiggle too much. When the thermal energy increases about a hundredfold, as it does in most stars, all atomic and molecular structures are destroyed. Most of the matter in the universe exists, in fact, in a state which is very different from the one just described. In the centre of the stars exist large accumulations of nuclear matter, and nuclear processes which occur only very rarely on earth predominate there. They are essential for the great variety of stellar phenomena observed in astronomy, most of which arise from a combination of nuclear and gravitational effects. For our planet, the nuclear processes in the centre of the Sun are of particular importance because they furnish the energy which sustains our terrestrial environment. It has been one of the great triumphs of modern physics to discover that the constant energy flow from the Sun, our vital link with the world of the very large, is a result of nuclear reactions, of phenomena in the world of the infinitely small.
In the history of penetrating into this submicroscopic world, a stage was reached in the early 1930s when scientists thought they had now finally discovered the ‘basic building blocks’ of matter. It was known that all matter consisted of atoms and that all atoms consisted of protons, neutrons and electrons. These so-called ‘elementary particles’ were seen as the ultimate indestructible units of matter: atoms in the Demo-critean sense. Although quantum theory implies, as mentioned previously, that we cannot decompose the world into independently existing smallest units, this was not generally perceived at that time. The classical habits of thought were still so persistent that most physicists tried to understand matter in terms of its ‘basic building blocks’, and this trend of thought is, in fact, quite strong even today.
Two further developments in modern physics have shown, however, that the notion of elementary particles as the primary units of matter has to be abandoned. One of these developments was experimental, the other theoretical, and both began in the 1930s. On the experimental side, new particles were discovered as physicists refined their experimental techniques and developed ingenious new devices for particle detection. Thus the number of particles increased from three to six by 1935, then to eighteen by 1955, and today we know over two hundred elementary’ particles. The two tables opposite, taken from a recent publication,11 show most of the particles known today. They illustrate convincingly that the adjective ‘elementary’ is no longer very attractive in such a situation. As more and more particles were discovered over the years, it became clear that not all of them could be called ‘elementary’, and today there is a widespread belief among physicists that none of them deserves this name.
This belief is enforced by the theoretical developments which paralleled the discovery of an ever-increasing number of particles. Soon after the formulation of quantum theory, it became clear that a complete theory of nuclear phenomena must not only be a quantum theory, but must also incorporate relativity theory. This is because the particles confined to dimensions of the size of nuclei often move so fast that their speed comes close to the speed of light. This fact is crucial for the description of their behaviour, because every description of natural phenomena involving velocities close to the speed of light has to take relativity theory into account. It has to be, as we say, a ‘relativistic’ description. What we need, therefore, for a full understanding of the nuclear world is a theory which incorporates both quantum theory and relativity theory. Such a theory has not yet been found, and therefore we have as yet been unable to formulate a complete theory of the nucleus. Although we know quite a lot about nuclear structure and about the interactions between nuclear particles, we do not yet understand the nature and complicated form of the nuclear force on a fundamental level. There is no complete theory of the nuclear particle world comparable to quantum theory for the atomic world. We do have several ‘quantum-relativistic’ models which describe some aspects of the world of particles very well, but the fusion of quantum and relativity theory into a complete theory of the particle world is still the central problem and great challenge of modern fundamental physics.
Relativity theory has had a profound influence on our picture of matter by forcing us to modify our concept of a particle in an essential way. In classical physics, the mass of an object had always been associated with an indestructible material substance, with some ‘stuff’ of which all things were thought to be made. Relativity theory showed that mass has nothing to do with any substance, but is a form of energy. Energy, however, is a dynamic quantity associated with activity, or with processes. The fact that the mass of a particle is equivalent to a certain amount of energy means that the particle can no longer be seen as a static object, but has to be conceived as a dynamic pattern, a process involving the energy which manifests itself as the particle’s mass.
This new view of particles was initiated by Dirac when he formulated a relativistic equation describing the behaviour of electrons. Dirac’s theory was not only extremely successful in accounting for the fine details of atomic structure, but also revealed a fundamental symmetry between matter and antimatter. It predicted the existence of an anti-electron with the same mass as the electron but with an opposite charge. This positively charged particle, now called the positron was indeed discovered two years after Dirac had predicted it. The symmetry between matter and antimatter implies that for every particle there exists an antiparticle with equal mass and opposite charge. Pairs of particles and antiparticles can be created if enough energy is available and can be made to turn into pure energy in the reverse process of annihilation. These processes of particle creation and annihilation had been predicted from Dirac’s theory before they were actually discovered in nature, and since then they have been observed millions of times.
The creation of material particles from pure energy is certainly the most spectacular effect of relativity theory, and it can only be understood in terms of the view of particles outlined above. Before relativistic particle physics, the constituents of matter had always been considered as being either elementary units which were indestructible and unchangeable, or as composite objects which could be broken up into their constituent parts; and the basic question was whether one could divide matter again and again, or whether one would finally arrive at some smallest indivisible units. After Dirac’s discovery, the whole question of the division of matter appeared in a new light. When two particles collide with high energies, they generally break into pieces, but these pieces are not smaller than the original particles. They are