The United Nations has declared 2015 the “International Year of Light.” One impetus for that designation is that this year is the 150th anniversary of a revolutionary scientific paper: “A Dynamical Theory of the Electromagnetic Field,” by Scottish physicist James Clerk Maxwell.
Published in June 1865 in the Philosophical Transactions of the Royal Society, the article described how electricity and magnetism are as closely connected as interlocking gears. Drive one and the other must respond; change one and the other can pop into existence, even where there was none before.
In perhaps the most spectacular achievement of all, the paper shows how their combined action creates a dynamo known as electromagnetic waves. Maxwell further demonstrated that these waves can propagate through any medium, including perfectly empty space. He proved that if the waves travel through pure vacuum, they must move at the speed of light. Finally, he made the colossal leap that light, in all its forms, is electromagnetic waves of various frequencies (rates of oscillation) and amplitudes (maximum wave heights).
Einstein, who was born in 1879, the year that Maxwell died, often acknowledged a debt to the Scottish physicist. As he once wrote:
We may say that, before Maxwell, Physical Reality, in so far as it was to represent the process of nature, was thought of as consisting in material particles, whose variations consist only in movements governed by partial differential equations. Since Maxwell’s time, Physical Reality has been thought of as represented by continuous fields, governed by partial differential equations, and not capable of any mechanical interpretation. This change in the conception of Reality is the most profound and the most fruitful that physics has experienced since the time of Newton.
Maxwell became interested in the forces of nature while growing up in the rugged countryside of Scotland. He was a loner, and loved to walk by himself marveling at the beauty of the wild. At Edinburgh Academy, where he attended school, he was teased by his fellow students for spending so much time alone, rather than playing typical games. They gave him the cutting nickname “Dafty” — ironic indeed for a boy destined for brilliant accomplishments.
After graduating high school, Maxwell enrolled at Edinburgh University, where he became adept at both theoretical and experimental science. He completed his education at Cambridge. By the 1850s, he turned his keen mind to the workings of electricity and magnetism.
At that time, there were two ways of looking at electric and magnetic forces. One approach, based upon Newtonian “action at a distance,” imagined “invisible threads” connecting charges and currents remotely. Maxwell did not find this idea intuitive; rather it was too abstract.
The other method, pioneered by self-taught English scientist Michael Faraday, envisioned space being filled with a kind of fluid that conveys force. In the case of electricity, lines of such a “fluid” were generated by positive charges and absorbed by negatively charges. Maxwell became a solid proponent and developer of this idea which, in modified form, came to be known as the field method.
A little known side of Maxwell was that he also wrote poetry, including the following lines declaring the triumph of the field method over action over a distance:
Thy reign, O Force! is over. Now no more Heed we thine action; Repulsion leaves us where we were before, So does attraction.
Maxwell soon discovered deep connections between electric and magnetic fields. It was already known that charges produce electric fields, and moving charges or currents generate magnetic fields. Faraday had shown that changing magnetic fields generate electric fields. Maxwell completed the picture by demonstrating that changing electric fields produce magnetic fields. This led to a kind of domino effect in which moving charges created changing electric and magnetic fields that propagated through space perpendicular to each other.
In mathematical form, later simplified by Oliver Heaviside, they became known as Maxwell’s equations of electromagnetism.
By solving these equations, Maxwell found wavelike solutions that propagated through any kind of media, including pure vacuum. He was astonished and delighted to discover that they move at the known speed of light. Therefore, he reached the revolutionary conclusion: electromagnetic waves are the same thing as light, in all of its forms and frequencies.
Maxwell’s discovery soon found use in transmission of radio signals. By collecting an audio signal, amplifying it and using it to modulate the movement of charges in a broadcasting antenna, electromagnetic waves can be produced at radio frequencies. These propagate through space and can be picked up by receiving antennas that convert the signal back to patterns of moving charges, which, in a radio receiver, can be used to generate sounds. Hence, it was less than half-a-century after Maxwell’s discovery that Guglielmo Marconi broadcast the first radio transmission across the Atlantic Ocean, pioneered a means of continuous relay, and the age of wireless communications commenced.
Today physicists are searching for a “theory of everything” that unites all of the natural forces: gravitation and the strong and weak nuclear forces, along with electromagnetism. Considerable progress was made in the 1960s and 1970s, when Steven Weinberg, Abdus Salam and Sheldon Glashow each contributed to the Standard Model that included electroweak unification. For their brilliant work, they were awarded the Nobel Prize. The discovery of the Higgs boson cemented their work by offering the last major particle predicted by their theory.
Today, hope for further unification leads us down the theoretical paths of Grand Unified Theories, where the strong force unifies with the electroweak, and even further, where gravity unifies with the other three forces under a single framework.
But electromagnetism was the first true unification of forces. Its formulation showed that nature offers deep mathematical connections between varied phenomena. For that, we owe an outstanding debt to the genius of Maxwell.