Field (physics)

Field (physics), area surrounding an object, in which a gravitational or electromagnetic force is exerted on other objects. The concept of fields was first introduced by English physicist Sir Isaac Newton to explain gravitational forces (see Gravitation) and was later used by English physicist Michael Faraday to explain electromagnetic forces (see Electricity; Magnetism). Gravitational and electromagnetic forces appear to act at a distance: Massive bodies exert a force on each other, and charged particles, such as electrons or protons, exert a force 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 nonlocal 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. These lines are drawn 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. Coulomb's law describing the force between two charges, and Newton's law of gravitational attraction between two masses, actually describe the direct interaction of electromagnetic fields with charges and of gravitational fields with masses.

James Clerk Maxwell, a 19th-century Scottish physicist, further developed the field concept in his electromagnetic theory. Maxwell's equations describe the electric and magnetic fields set up by an arbitrary collection of charges. Physicist 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. Electromagnetic waves, such as visible light and radio waves, are the solutions to Maxwell's equations (see Electromagnetic Radiation). Comprehension of the nature and behavior of these 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 (see Star).

Another significant consequence of electromagnetic and gravitational fields arises in quantum theory. Quantum theory describes wave and particle behavior at the subatomic level using the concept of wave-particle duality, whereby 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 have corresponding particlelike excitations, or discrete packets of energy, called quanta. A quantum of electromagnetic energy is called a photon, and a quantum of gravitational energy is called a graviton. Because both electromagnetic and gravitational waves travel at the speed of light, their associated particles, or quanta, are massless. Wave-particle duality also predicts that each elementary particle, such as electrons and protons, must have a corresponding wavelength and quantum field. The interaction of an electromagnetic field with an electron's charge is actually the result of the interaction of this field with the quantum field of the electron. Quantum electrodynamics, the quantum field theory of electrons and photons, predicts that atomic energy levels must change in discrete steps, which have been established 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 and gluons together to form protons and neutrons.