FORCES IN NATURE

Our present-day understanding of the physical world includes four distinct classes of forces. Two are familiar in everyday experience. The other two are concerned with interactions of fundamental particles. We cannot observe the latter with the unaided senses; studying them requires sophisticated and elaborate experiments.

Of the two familiar classes of force, gravitational interactions were the first to be studied in detail. The weight of a body results from the earth's gravitational attraction acting on it. The sun's gravitational force on the earth is responsible for making the earth move in a nearly circular orbit instead of following a straight-line path as it would do if there were no force. Indeed, one of Newton's great achievements was to recognize that both the motions of the planets around the sun and the free fall of objects on earth are manifestations of gravitational forces.

The second familiar class of forces, electromagnetic interactions, includes electric and magnetic forces. When you run a comb through your hair, you can then use the comb to pick up bits of paper or fluff; this interaction is the result of electric charge on the comb. We encounter magnetic forces in interactions between magnets or between a magnet and a piece of iron. These may seem to fall in a different category, but detailed study shows that magnetic interactions are actually the result of electric charges in motion. An electromagnet causes magnetic interactions as a result of an electric current in a coil of wire. We will study electric and magnetic interactions in detail in the second half of this book.

These two familiar kinds of interactions differ enormously in their strength. For example, the electrical repulsion between two protons at a given distance apart is stronger than the gravitational attraction by a factor of the order of 10³⁵. Thus gravitational forces play no significant role in determining the microscopic structure of atoms, molecules, and materials. But in bodies of /astronomical size, protons and electrons are ordinarily present in nearly equal numbers; their charges are opposite and nearly cancel out. Gravitational interactions are the dominant influence in the motion of planets and also in the /internal structure of stars.

The other two classes of interactions are less familiar. One, the strong interaction, is responsible for holding the nuclei of atoms together. Nuclei contain electrically neutral and positively charged particles. The charged particles repel each other; a nucleus could not be stable if it were not for the presence of an attractive force of a different kind that counteracts the repulsive electrical interactions. In this context the strong interaction is also called the nuclear force. It has shorter range than electrical interactions, but within its range it is much stronger.

Finally, there are the weak interactions; they play no direct role in the behavior of ordinary matter but are of vital importance in interactions among fundamental particles. The weak interaction is responsible for the emission of electrons (beta particles) from radioactive nuclei. In a beta-emitting nucleus a neutron is converted into a proton, an electron, and a third particle called a neutrino. The weak interaction is also responsible for the decay of many unstable particles produced^-in high-energy collisions of fundamental particles. Since about 1970, attempts have been made to understand all four classes of interactions on the basis of a single unified theory called grand unified theory. Such theories are still very speculative, as yet none has been completely successful. In 1983, however, evidence was found that supports a unified theory of the electromagnetic and weak interactions. The entire area is a very active field of present-day theoretical and experimental research.

Date: 2015-01-12; view: 1311