Field (physics)

Field (physics), area surrounding an object in which a gravitational or electromagnetic force is exerted on other objects. The 19th-century British physicist Michael Faraday developed the concept of the field to describe and explain electromagnetic forces (see Electricity; Magnetism). Gravitational and electromagnetic forces appear to act at a distance: bodies with mass exert forces on each other (see Gravitation), and charged particles, such as electrons or protons, exert forces on other charged particles, all without direct contact. Charged particles and massive bodies are therefore said to be the sources of electromagnetic and gravitational fields. These fields extend throughout space and exert an electromagnetic or gravitational force on other objects. The apparently non-local interaction between two charged bodies or two massive bodies is actually a local interaction between the field set up by one of the bodies and the charge or mass of the other body.

The strength and direction of the force that a field exerts can be represented with field lines. The direction of the field line at a given point shows the direction of the force that would be experienced by a very small charge or mass at that point. These lines are closely spaced near the source of the field, where the force is stronger, and more widely spaced farther from the source, where the force is weaker. The strength of the electromagnetic field of a point charge (a charge that has a very small physical size compared to its distance from other objects) is proportional to the value of the charge and decreases in inverse proportion to the square of the distance from the charge. Similarly, the strength of the gravitational field of a point mass is proportional to the value of the object’s mass and decreases in inverse proportion to the square of the distance from the object.

In the 19th century the British physicist James Clerk Maxwell further developed the field concept in his theory of electromagnetism. Maxwell’s equations describe the electric and magnetic fields set up by an arbitrary collection of charges. Albert Einstein developed an analogous set of equations for the gravitational fields that result from an arbitrary distribution of masses. Both sets of equations have wavelike solutions, with waves that travel at the speed of light. Maxwell deduced that light consists of electromagnetic waves, and later other types of electromagnetic wave, such as radio waves and x-rays, were discovered (see Electromagnetic Radiation). The prediction and discovery of electromagnetic waves is one of the most important consequences of using the concept of fields. Although there is no direct evidence for gravitational waves, their existence is indirectly confirmed by astronomical phenomena associated with binary pulsars.

Another significant consequence of electromagnetic and gravitational fields arises in quantum theory. Quantum theory describes wave and particle behaviour at the subatomic level, using the concept of wave-particle duality: that waves of a given wavelength correspond to particles of a given momentum, with the product of the wavelength and the momentum being fixed by Planck’s constant. According to quantum theory, the waves associated with electromagnetic and gravitational fields correspond to particles. The particles associated with electromagnetic fields are called photons, and those associated with gravitational fields are called gravitons.

Wave-particle duality also predicts that each elementary particle, such as an electron or proton, can be described by a quantum field with a corresponding wavelength. The interaction of an electromagnetic field with, for example, an electron’s charge is actually the result of the interaction of this field with the quantum field of the electron. Quantum electrodynamics is the quantum field theory of the electromagnetic interaction between charged particles and photons. It makes predictions—for example, about shifts in atomic energy levels—that have been confirmed experimentally with very high accuracy. Analogous quantum field theories have been developed to describe other fundamental processes in nature, such as the forces that bind quarks together to form protons and neutrons. Each field is associated with its own type of “carrier” particle, analogous to the photon and graviton. See Standard Model.