The Demon-Haunted World_ Science as a Candle in the Dark - Carl Sagan [187]
Maxwell then asked himself a strange question: what would these equations look like in empty space, in a vacuum, in a place where there were no electrical charges and no electrical currents? We might very well anticipate no electric and no magnetic fields in a vacuum. Instead, he suggested that the right form of the Maxwell equations for the behaviour of electricity and magnetism in empty space is this:
x E = 0
• B = 0
x E = -Ò
x B = ÜÕÌ
He set á equal to zero, indicating that there are no electrical charges. He also set j equal to zero, indicating that there are no electrical currents. But he didn’t discard the last term in the fourth equation, ÜÕÌ feeble displacement current in insulators.
Why not? As you can see from the equations, Maxwell’s intuition preserved the symmetry between the magnetic and electric fields. Even in a vacuum, in the total absence of electricity, or even matter, a changing magnetic field, he proposed, elicits an electric field and vice versa. The equations were to represent Nature, and Nature is, Maxwell believed, beautiful and elegant. (There was also another, more technical reason for preserving the displacement current in a vacuum, which we pass over here.) This essentially aesthetic judgement by a nerdish physicist, entirely unknown except to a few other academic scientists, has done more to shape our civilization than any ten recent presidents and prime ministers.
Briefly, the four Maxwell equations for a vacuum say (1) there are no electrical charges in a vacuum; (2) there are no magnetic monopoles in a vacuum; (3) a changing magnetic field generates an electrical field; and (4) vice versa.
When the equations were written down like this, Maxwell was readily able to show that E and B propagated through empty space as if they were waves. What’s more, he could calculate the speed of the wave. It was just 1 divided by the square root of Õ times Ü. But Õ and Ü had been measured in the laboratory. When you plugged in the numbers you found that the electric and magnetic fields in a vacuum ought to propagate, astonishingly, at the same speed as had already been measured for light. The agreement was too close to be accidental. Suddenly, disconcertingly, electricity and magnetism were deeply implicated in the nature of light.
Since light now appeared to behave as waves and to derive from electric and magnetic fields, Maxwell called it electromagnetic. Those obscure experiments with batteries and wires had something to do with the brightness of the Sun, with how we see, with what light is. Ruminating on Maxwell’s discovery many years later, Albert Einstein wrote, To few men in the world has such an experience been vouchsafed.’
Maxwell himself was baffled by the results. The vacuum seemed to act like a dielectric. He said that it can be ‘electrically polarized’. Living in a mechanical age, Maxwell felt obliged to offer some kind of mechanical model for the propagation of an electromagnetic wave through a perfect vacuum. So he imagined space filled with a mysterious substance he called the aether, which supported and contained the time-varying electric and magnetic fields - something like a throbbing but invisible Jell-O permeating the Universe. The quivering of the aether was the reason that light travelled through it - just as water waves propagate through water and sound waves through air.
But it had to be very odd stuff, this ether, very thin, ghostly, almost incorporeal. The Sun and the Moon, the planets and the stars had to pass through it without being slowed down, without noticing. And yet it had to be stiff enough to support all these waves propagating at prodigious speed.
The word ‘aether’ is still, in a desultory fashion, in use - in English mainly in the adjective ethereal, residing in the aether. It has some of the same connotations as the more modern ‘spacy’ or ‘spaced out’. When, in the