The next great leap in our understanding of unificationthat of electricity and magnetismtook place two hundred years later, in the mid-1860s, during the American Civil War. While the United States was thrown into chaos by that devastating war, across the Atlantic the world of science was also in a period of great turmoil. Experiments being performed in Europe pointed to the unmistakable fact that magnetism, under certain circumstances, can turn into an electric field, and vice versa.
For centuries it was thought that magnetism, the force that guides the compass needles of navigators while at sea, and electricity, the force that creates everything from lightning bolts to the shock upon touching a doorknob after walking across a carpet, were distinct forces. However, by the mid-1800s, this rigid separation was falling apart as scientists realized that vibrating electric fields could create magnetic ones, and vice versa.
This effect can be demonstrated easily. For example, simply by shoving a bar magnet into a coil of wire we can generate a small electric current within the wire. Thus, a changing magnetic field has created an electric field. Similarly, we can reverse this demonstration by running an electric current through this coil of wire, thereby producing a magnetic field around the coil. Thus, a changing electric field has now created a magnetic field.
This same principlethat changing electric fields can produce magnetic fields and vice versais the reason why we have electricity in our homes. In a hydroelectric plant, water falling over a dam rotates a huge wheel connected to a turbine. The turbine contains large wire coils that spin rapidly in a magnetic field. Electricity is created by the spinning motion of these coils as they move in the magnetic field. This electricity, in turn, is sent over hundreds of miles of wires into our homes. Thus, a changing magnetic field (created by the dam) is transformed into an electric field (which brings electricity into our homes through our wall sockets).
In 1860, however, this effect was understood poorly. An obscure thirty-year-old Scottish physicist at Cambridge University, James Clerk Maxwell, challenged the prevailing thinking of the day andclaimed that electricity and magnetism were not distinct forces but two sides of the same coin. In fact, he made the most astonishing discovery of the century when he found that this observation could unlock the secret to the most mysterious phenomenon of all: light itself.
Electric and magnetic fields, Maxwell knew, could be visualized as force fields that permeate all space. They can be represented by an infinite array of arrows emanating smoothly from an electric charge. For example, the force fields created by a bar magnet reach into space like a spiderweb and can ensnare nearby metallic objects.
Maxwell went further than this, however, and argued that it might be possible for electric and magnetic fields to vibrate together in precise synchronization, so that they generated a wave that could travel by itself in space without assistance.
One can visualize the following scenario: What would happen if a vibrating magnetic field created an electric field, which in turn vibrated and created yet another magnetic field, which in turn vibrated and created still another electric field, et cetera? Wouldn’t such an infinite chain of vibrating electric and magnetic fields travel by itself, much like a wave?
As with Newton’s laws of gravitation, the essence of the idea is simple and pictorial. Think, for example, of a long line of dominoes. Tipping over the first domino, of course, will trigger a cascading wave of falling dominoes. Let’s say, however, that this line of dominoes consists of two types, colored black and white, and that these colored dominoes alternate along the line. If we remove the black dominoes, leaving only the white ones in the line, then a wave can no longer travel. We need both the white and the black dominoes in order to have a traveling wave. In summary, it is the interplay of white and black dominoes, with each one tipping over the next, that makes possible the wave of falling dominoes.
Similarly, Maxwell discovered that the interplay between vibrating magnetic and electric fields created the wave. He found that electric or magnetic fields alone could not create this wavelike motion, similar to the analogy with only black or white dominoes. Only the delicate interplay between electric and magnetic fields could produce this wave.