Fundamental Forces

Fundamental Forces, four types of interaction in terms of which all forces between material objects can be described. Two of these forces, gravitation and electromagnetism, have a long range, and are familiar in the everyday world. The other two forces, called the strong and weak nuclear forces, operate only on a subatomic scale.

Gravitation was the first force to be described by a mathematical theory (by Isaac Newton in the 17th century). It is the weakest of the four forces. If the strength of the strong nuclear force is set at 1 unit, the strength of the electromagnetic force (first described mathematically by James Clerk Maxwell in the 19th century) is 10^-2, that of the weak force 10^-6, and that of gravitation a mere 10^-40. The reason why gravitation is so noticeable is that the strength of the force adds up as more and more matter is put in one place. In addition, the force extends far from its source, decreasing as the inverse square of the distance—that is, at twice the distance the gravitational force is reduced to a quarter, and so on. So the gravitational influence of a large amount of matter like the Sun is considerable, and extends far enough to keep all the planets in the solar system in their orbits. The range of gravitation is in principle infinite.

Even though electromagnetism is so much stronger than gravity, it cannot extend its influence far because electricity occurs in opposite charges (positive and negative), which cancel each other out. So the Earth, for example, has no overall charge and exerts no electromagnetic influence on the other planets in the solar system. To put the forces in perspective, it takes all of the gravitational attraction of the matter that makes up the Earth to overcome the electromagnetic forces between atoms in the stem of an apple that hold the apple to its tree.

The existence of the two nuclear forces became apparent only in the 1930s, when physicists began to probe the structure of the atom. The weak nuclear force is responsible for the process of radioactive decay that occurs inside atomic nuclei, and for the decay of neutrons into protons and electrons (beta decay).

The strong force holds atomic nuclei together. Nuclei are made of a mixture of positively charged protons and electrically neutral neutrons, and so have an overall positive charge. Without the strong force the repulsion between the protons would blow nuclei apart, and make matter unstable.

Albert Einstein attempted to find a single unified field theory that would describe gravitation and electromagnetism. He failed, but physicists still strive to formulate one theory that can account for all the fundamental forces. In the 1960s, physicists succeeded in finding a mathematical description, or model, of the electromagnetic and weak forces in one framework, known as the electroweak theory, reducing the number of separate forces to three. There is a real possibility of combining the description of the strong force with electroweak theory to make a single mathematical description of the forces, excluding gravity, called a grand unification theory. It is proving much more difficult to fit gravitation in with the other forces, although superstring theory suggests ways in which this might be done.

Each of the fundamental forces is “carried” by particles that are exchanged between the particles that interact. Electromagnetic forces involve the exchange of photons; the weak nuclear force involves the exchange of particles called W and Z bosons, while the strong nuclear force involves particles called gluons. Gravitation is believed to be carried by gravitons, which would be associated with gravitational waves.

See also Fundamental Constants; Magnetism; Physics; Standard Model.