The Demon-Haunted World_ Science as a Candle in the Dark - Carl Sagan [188]
The whole idea of light and matter moving through the aether was to lead in another forty years to Einstein’s Special Theory of Relativity, E = mc2, and a great deal else. Relativity, and experiments leading up to it, showed conclusively that there is no aether supporting the propagation of electromagnetic waves, as Einstein writes in the extract from his famous paper that I reproduced in Chapter 2. The wave goes by itself. The changing electric field generates a magnetic field; the changing magnetic field generates an electric field. They hold each other up, by their bootstraps.
Many physicists were deeply troubled by the demise of the ‘luminiferous’ ether. They had needed some mechanical model to make the whole notion of the propagation of light in a vacuum reasonable, plausible, understandable. But this is a crutch, a symptom of our difficulties in reconnoitring realms in which common sense no longer serves. The physicist Richard Feynman described it this way:
Today, we understand better that what counts are the equations themselves and not the model used to get them. We may only question whether the equations are true or false. This is answered by doing experiments, and untold numbers of experiments have confirmed Maxwell’s equations. If we take away the scaffolding he used to build it, we find that Maxwell’s beautiful edifice stands on its own.
But what are these time-varying electric and magnetic fields permeating all of space? What do and mean? We feel so much more comfortable with the idea of things touching and jiggling, pushing and pulling, rather than ‘fields’ magically moving objects at a distance, or mere mathematical abstractions. But, as Feynman pointed out, our sense that at least in everyday life we can rely on solid, sensible physical contact to explain, say, why the butter knife comes to you when you pick it up, is a misconception. What does it mean to have physical contact? What exactly is happening when you pick up a knife, or push a swing, or make a wave in a waterbed by pressing down on it periodically? When we investigate deeply, we find that there is no physical contact. Instead, the electrical charges on your hand are influencing the electrical charges on the knife or swing or waterbed, and vice versa. Despite everyday experience and common sense, even here, there is only the interaction of electric fields. Nothing is touching anything.
No physicist started out impatient with common-sense notions, eager to replace them with some mathematical abstraction that could be understood only by rarified theoretical physics. Instead, they began, as we all do, with comfortable, standard, common-sense notions. The trouble is that Nature does not comply. If we no longer insist on our notions of how Nature ought to behave, but instead stand before Nature with an open and receptive mind, we find that common sense often doesn’t work. Why not? Because our notions, both hereditary and learned, of how Nature works were forged in the millions of years our ancestors were hunters and gatherers. In this case common sense is a faithless guide because no hunter-gatherer’s life ever depended on understanding time-variable electric and magnetic fields. There were no evolutionary penalties for ignorance of Maxwell’s equations. In our time it’s different.
Maxwell’s equations show that a rapidly varying electric field (making large) ought to generate electromagnetic waves. In 1888 the German physicist Heinrich Hertz did the experiment and found that he had generated a new kind of radiation, radio waves. Seven years later, British scientists in Cambridge transmitted radio signals over a distance of a kilometre. By 1901, Guglielmo Marconi of Italy was using radio waves to communicate across the Atlantic