(1831-1879) Scottish Theoretical Physicist (Classical Electromagnetism, Optics, Thermodynamics)
James Clerk Maxwell is considered the greatest theoretical physicist of the 19th century. He brilliantly synthesized the key findings of his predecessors in four equations that unified the description of electromagnetic processes. In so doing he discovered that light consists of electromagnetic waves and led the way to exploration of the whole spectrum of electromagnetic radiation. A multifaceted genius who made fundamental contributions to the study of every problem he addressed, he also formulated the statistical kinetic theory of gases, solved the mystery of Saturn’s rings, and discovered the principles governing color vision.
Maxwell was born in Edinburgh, Scotland, on November 13, 1831, but spent his boyhood on his family’s country estate in Glenlair. From his earliest years he showed a high degree of curiosity and a passion for finding out how things worked. After his mother died, plans for his home education were dropped and the boy was sent to Edinburgh Academy, where he studied from age 10 to age 16. A shy, solitary boy, absorbed in mathematical and mechanical pursuits, Maxwell quickly proved to be a precocious, prize-winning student. When he was only 14, he discovered an original method for drawing a perfect oval based on his generalization of the definition of an ellipse and submitted his work, in his first paper, to the Royal Society of Edinburgh in 1846.
Upon graduating from the academy in 1847, he entered the University of Edinburgh and, still an undergraduate, began doing independent research on the theory of color. In 1849, building on the work of thomas young and hermann ludwig ferdinand von helmholtz, he demonstrated that colors could be built up from mixtures of the three primary colors—red, green, and blue—by spinning disks containing sectors of these colors in various sizes. He would develop this work for many years, inventing a color box in which the primary colors could be selected from the Sun’s spectrum and combined. This model explained how all colors are produced by adding and subtracting the primary colors. Turning to the problem of color vision, he confirmed Young’s theory that the eye has three kinds of receptors sensitive to the primary colors and showed that color blindness is due to defects in the receptors. The culmination of his investigations would occur in 1861, when he produced the first color photograph to use a three-color process.
In 1850, Maxwell went on to study at Cambridge. He graduated with a degree in mathematics from Trinity College in 1854 and was awarded a fellowship allowing him to continue his work at Cambridge. Maxwell’s genius for pursuing several research problems at once was already evident. In 1856, Maxwell moved to Scotland in order to be near his ailing father and became professor of natural philosophy at Marischal College, Aberdeen. He competed for the Adams Prize offered by Cambridge on 1857, awarded to whoever could offer a satisfactory explanation for the rings of Saturn that would result in a stable structure. He won the prize by showing that whereas a solid ring would collapse and a fluid ring would break up, stability could be achieved if the rings consisted of numerous small solid particles, an explanation now confirmed by the Voyager spacecraft.
In 1859, he married Katherine Mary Dewar, daughter of the head of Marischal College; he then moved to London in 1860 as professor of natural philosophy and astronomy at King’s College. There he continued the work begun at Cambridge, in 1855-1856, when he had established the foundation for his most important work by providing a mathematical formulation for michael faraday’s theory that electric and magnetic effects result from field lines of force that surround conductors and magnets. His seminal paper, “On Faraday’s Lines of Force,” had compared the behavior of the lines of force with the flow of an incompressible fluid. This model implied the existence of a medium in which the field lines were established.
Over the next 15 years, Maxwell would publish a series of papers in which, step by step, he developed the four field equations that describe the physics of electrodynamics. He continued to develop the incompressible fluid model for the medium, assuming that it contained rotating vortices corresponding to magnetic fields separated by cells corresponding to electric fields. By considering how the motion of the vortices and cells could produce magnetic and electric fields, he explained all previously known effects of electro-magnetism. In Maxwell’s formulation, Coulomb’s law required two equations for the electric field, and Ampere’s law and Faraday’s law required two more for the magnetic field. Thus, Maxwell’s equations showed that (1) unlike charges attract each other; like charges repel (Coulomb’s law); (2) there are no single, isolated magnetic poles (if there is a north, there will be a corresponding south pole) (Ampere’s law); (3) electrical currents can cause magnetic fields (Ampere’s law); and (4) changing magnetic fields can cause changing electric fields (Faraday’s law).
However, Maxwell noted that these equations were not symmetrical in the manner in which electric and magnetic fields entered into them. To correct this, he postulated that if a changing magnetic field could produce a changing electric field, then symmetry required that a changing electric field could produce a changing magnetic field. This implied a new phenomenon, namely, that an oscillating combination of transverse electric and magnetic fields could propagate through space at the speed of light. This led Maxwell to claim that light, in fact, is a form of electromagnetic radiation:
We can scarcely avoid the conclusion that light consists in the transverse undulations of the same medium, which is the cause of electric and magnetic phenomena.
Maxwell’s equations demonstrated that electricity and magnetism are aspects of a single electromagnetic field, and that light itself is a variety of this field. In this way he unified what had been the separate studies of electricity, magnetism, and optics. In his 1873 Treatise on Electricity and Magnetism he summarized all of his work on the subject, establishing that light has a radiation pressure and suggesting that a whole family of electromagnetic radiations must exist, of which light is only one. This prediction was confirmed in 1888 when heinrich rudolf hertz discovered the existence of radio waves, which move at the speed of light.
At the same time that he was developing his ideas on electromagnetic theory, Maxwell continued work begun at Aberdeen in 1860 on the kinetic theory of gases. He built on the work of rudolf julius emmanuel clausius, who in 1857-1858 had shown that a gas must consist of molecules in constant motion colliding with each other and the walls of the container. He arrived at a formula to express the distribution of velocity in gas molecules, relating it to temperature and thus demonstrating that heat resides in the motion of molecules.
His kinetic theory did not fully explain heat conduction, and it was modified by ludwig boltzmann in 1868, resulting in what became known as the Maxwell-Boltzmann distribution law. Maxwell also accurately estimated the size of molecules and invented a method of separating gases in a centrifuge. In addition, the kinetic theory had an important impact on the question of the validity of the second law of thermodynamics, which states that heat cannot spontaneously flow from a cooler body to a hotter one. For example, when two connected containers of gases have the same temperature, it is statistically possible for the molecules to diffuse spontaneously so that the faster-moving molecules all concentrate in one container while the slower molecules gather in the other, making the first container hotter and the second colder. Maxwell conceived this hypothesis, known as Maxwell’s demon. Even though this process is statistically possible it is highly unlikely. In this context, the second law is not absolute but only highly probable.
In 1865, Maxwell’s father died, and he returned to his family estate in Glenlair, Scotland, and devoted himself to research. He made periodic trips to Cambridge. In 1871, he was persuaded to move to Cambridge, where he became the first professor of experimental physics and set up the Cavendish Laboratory, which opened in 1874. He continued his lectures there until 1879, when he contracted cancer. That summer, he returned to Glenlair to be with his wife, who was also ill. He died at age 48, on November 5, 1879, in Cambridge.
The four partial differential equations now known as Maxwell’s equations are among the great achievements of 19th-century mathematics. The year before Maxwell died, he suggested an experiment for measuring the effect of the ether. This inspired albert abraham michel-son and Edward Morley to carry out their famous experiment in the 1880s, the results of which disproved the existence of the ether, the medium in which light waves were thought to be propagated. The discovery that there was no ether did not discredit Maxwell’s work. His equations and descriptions of electromagnetic waves remained valid even though the waves require no medium. They paved the way for the discovery of special relativity and of the spectrum of electromagnetic radiation, such as X rays and gamma rays, that is at the core of modern physics.
