Four basic forces

Quarks and leptons are the building blocks of matter, but they require some sort of mortar to bind themselves together into more-complex forms, whether on a nuclear or a universal scale. The particles that provide this mortar are associated with four basic forces that are collectively referred to as the fundamental interactions of matter. These four basic forces are gravity (or the gravitational force), the electromagnetic force, and two forces more familiar to physicists than to laypeople: the strong force and the weak force.

On the largest scales the dominant force is gravity. Gravity governs the aggregation of matter into stars and galaxies and influences the way that the universe has evolved since its origin in the big bang. The best-understood force, however, is the electromagnetic force, which underlies the related phenomena of electricity and magnetism. The electromagnetic force binds negatively charged electrons to positively charged atomic nuclei and gives rise to the bonding between atoms to form matter in bulk.

Gravity and electromagnetism are well known at the macroscopic level. The other two forces act only on subatomic scales, indeed on subnuclear scales. The strong force binds quarks together within protons, neutrons, and other subatomic particles. Rather as the electromagnetic force is ultimately responsible for holding bulk matter together, so the strong force also keeps protons and neutrons together within atomic nuclei. Unlike the strong force, which acts only between quarks, the weak force acts on both quarks and leptons. This force is responsible for the beta decay of a neutron into a proton and for the nuclear reactions that fuel the Sun and other stars.

Field theory

Since the 1930s physicists have recognized that they can use field theory to describe the interactions of all four basic forces with matter. In mathematical terms a field describes something that varies continuously through space and time. A familiar example is the field that surrounds a piece of magnetized iron. The magnetic field maps the way that the force varies in strength and direction around the magnet. The appropriate fields for the four basic forces appear to have an important property in common: they all exhibit what is known as gauge symmetry. Put simply, this means that certain changes can be made that do not affect the basic structure of the field. It also implies that the relevant physical laws are the same in different regions of space and time.

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At a subatomic, quantum level these field theories display a significant feature. They describe each basic force as being in a sense carried by its own subatomic particles. These “force” particles are now called gauge bosons, and they differ from the “matter” particles—the quarks and leptons discussed earlier—in a fundamental way. Bosons are characterized by integer values of their spin quantum number, whereas quarks and leptons have half-integer values of spin.

The most familiar gauge boson is the photon, which transmits the electromagnetic force between electrically charged objects such as electrons and protons. The photon acts as a private, invisible messenger between these particles, influencing their behaviour with the information it conveys, rather as a ball influences the actions of children playing catch. Other gauge bosons, with varying properties, are involved with the other basic forces.

In developing a gauge theory for the weak force in the 1960s, physicists discovered that the best theory, which would always yield sensible answers, must also incorporate the electromagnetic force. The result was what is now called electroweak theory. It was the first workable example of a unified field theory linking forces that manifest themselves differently in the everyday world. Unified theory reveals that the basic forces, though outwardly diverse, are in fact separate facets of a single underlying force. The search for a unified theory of everything, which incorporates all four fundamental forces, is one of the major goals of particle physics. It is leading theorists to an exciting area of study that involves not only subatomic particle physics but also cosmology and astrophysics.