Warped Passages - Lisa Randall [76]
You can see evidence of a magnetic field by sprinkling iron filings in the vicinity of a magnet. The particles organize themselves in patterns according to the strength and direction of the field. You can also experience a field by holding two magnets close together. You’ll feel the magnets’ mutual attraction or repulsion well before they touch each other. Each is responding to the field that permeates the region between them.
The ubiquity of electric fields was brought home to me one day when I was finishing a climb on a ridge near Boulder, Colorado, with a partner who was new to climbing but had a lot of hiking experience.
An electrical storm was approaching rapidly, and I didn’t want to make him nervous, so I encouraged him to move quickly without pointing out that the rope was crackling and his hair was standing on end. When we were safely down at the bottom happily reviewing our adventure, much of which had been a delightful climb, my partner told me that of course he had known we were in danger: my hair had been visibly standing on end too! The electric field wasn’t only in one place—it was everywhere around us.
Before the nineteenth century, no one described electricity and magnetism in terms of fields. People conventionally used the term action at a distance to describe these forces. Action at a distance is the expression you might have learned in elementary school which describes how an electrically charged object instantly attracts or repels any other charge, no matter where it is. This might not seem mysterious, since it’s what we’re accustomed to. However, it would be extraordinary if something in one place could instantly affect another object some distance away. How would the effect be communicated?
Although it might sound like just a matter of semantics, there really is an enormous conceptual difference between a field and action at a distance. According to the field interpretation of electromagnetism, a charge doesn’t affect other regions of space immediately. The field needs time to adjust. A moving charge creates a field in its immediate vicinity, which seeps (albeit very rapidly) throughout space. Objects learn of the motion of the distant charge only after light (which is composed of electromagnetic fields) has had time to reach them. The electric and magnetic fields therefore change no faster than the finite speed of light allows. At any given point in space, the field adjusts only after sufficient time has elapsed for the effect of the distant charge to reach that point.
However, despite the critical importance of Faraday’s electromagnetic fields, they were more heuristic than mathematical. Perhaps because of his spotty education, math was not Faraday’s strength. But another British physicist, James Clerk Maxwell, incorporated Faraday’s field idea into classical electromagnetic theory. Maxwell was a brilliant scientist who counted among his many interests optics and color, the mathematics of ovals, thermodynamics, the rings of Saturn, measuring latitude with a bowl of treacle, and the question of how cats land upright while conserving angular momentum when dropped upside down.*
Maxwell’s most important contribution to physics was the set of equations that describe how to derive the values of electric and magnetic fields from a distribution of charges and currents.14† From these equations, he deduced the existence of electromagnetic waves—the waves in all forms of electromagnetic radiation, as in your computer, television, microwave oven, and the many other conveniences of the modern era.
However, Maxwell made one mistake. Like all other physicists of his day, he took the field idea too materially. He assumed that the field arose from the vibrations of an aether—an idea that Einstein, as we have seen, ultimately debunked. Nonetheless, Einstein credited Maxwell with the origin of the special theory of relativity: Maxwell’s electromagnetic