James Clerk Maxwell

James Clerk Maxwell the Scottish physicist and mathematician (1831-1879) whose formulation of the classical theory of electromagnetic radiation united electricity, magnetism and light into a single framework. His four equations, later named in his honour, are regarded as among the most important achievements of nineteenth-century physics and laid the foundation for relativity and quantum theory.

Early Life and Education

James Clerk Maxwell was born on 13 June 1831 in Edinburgh, the only child of a comfortable middle-class family. His mother died when he was young, and he was raised chiefly by his father, who encouraged his curiosity about the workings of things. He showed remarkable mathematical talent as a boy, and at the age of fourteen he presented a paper on the geometry of oval curves to the Royal Society of Edinburgh.

He studied at the University of Edinburgh and then at Cambridge, where he took his degree in 1854 and was made a fellow of Trinity College. At Cambridge he read widely in philosophy and science and proved himself a mathematician of exceptional power and originality. In 1856 he was appointed professor of natural philosophy at Marischal College in Aberdeen, and in 1860 he moved to King's College London. In 1865 he retired to his family estate at Glenlair in Scotland to write, and in 1871 he returned to Cambridge as the first Cavendish Professor.

Colour Vision and the Kinetic Theory of Gases

Maxwell's early researches ranged across several fields. He studied colour vision and demonstrated with a colour top that white light could be produced by mixing three primary colours, work that helped to establish the three-colour theory and was honoured by the Royal Society. He also took a keen interest in the rings of Saturn, showing that they must consist of many small particles rather than a solid or fluid body.

In the 1860s he made fundamental contributions to the kinetic theory of gases, deriving the distribution of molecular speeds that now bears his name and relating the viscosity, diffusion and thermal conductivity of a gas to the motion of its molecules. This work gave strong support to the atomic view of matter and is a cornerstone of statistical mechanics. He also shared with Boltzmann the task of founding the statistical interpretation of thermodynamics, showing how the irreversible behaviour of bulk matter emerges from the reversible motion of its countless molecules.

Electromagnetism and the Field Equations

Maxwell's greatest achievement was the theory of the electromagnetic field. Building on the experiments of Michael Faraday, whose intuitive idea of lines of force he expressed in precise mathematical form, he developed a set of equations describing how electric and magnetic fields vary in space and time and how they generate one another. He published the mature version in his Treatise on Electricity and Magnetism in 1873. These equations remain the foundation of every branch of engineering that deals with electricity, magnetism and light, and they provided the language in which Einstein later framed the special theory of relativity.

A striking consequence of his equations was the prediction that changing electric and magnetic fields would propagate as waves through space at a speed he calculated to match the measured speed of light. He therefore concluded that light itself is an electromagnetic wave, uniting optics with electricity and magnetism in a single theory. This unification is one of the great syntheses in the history of science. It also implied that other forms of radiation, invisible to the eye, might exist at different wavelengths, a prediction confirmed when Hertz later produced radio waves. In the form later given them by Oliver Heaviside and others, Maxwell's equations are four in number, describing how electric charges produce electric fields, how magnetic fields arise from currents, how changing fields induce one another, and how magnetic field lines never end.

Thermodynamics and the Demon

Maxwell also contributed to thermodynamics and the theory of heat. With Ludwig Boltzmann he developed the statistical interpretation of the second law of thermodynamics, and he devised the celebrated thought experiment known as Maxwell's demon, imagining a tiny being able to sort fast and slow molecules and so apparently reverse the increase of entropy.

The paradox was not a serious challenge to the second law but a device for clarifying what the law really means. Maxwell saw that the demon would need information about the molecules, and the resolution of the puzzle, in terms of the cost of acquiring and erasing that information, became an important theme in the later study of information and thermodynamics.

Later Years and Legacy

In 1871 Maxwell became the first Cavendish Professor of Physics at Cambridge, where he organised the new Cavendish Laboratory and edited the unpublished papers of Henry Cavendish. He died of abdominal cancer on 5 November 1879 at the age of forty-eight, and his great Treatise was still the leading statement of electromagnetic theory at his death.

His equations were confirmed by Heinrich Hertz, who generated and detected electromagnetic waves in the 1880s, and they were later shown to be consistent with the special theory of relativity and to survive, in modified form, into quantum electrodynamics. Maxwell is ranked with Newton and Einstein among the greatest theoretical physicists, and his unification of light with electromagnetism remains a model of how physics advances.

See also

• Maxwell's equations
• Electromagnetic radiation
• Kinetic theory of gases
• Maxwell's demon