Carl%20Sagan%20-%20The%20Demon%20Haunted%20World

THE DEMON-HAUNTED WORLD
The fourth describes the converse - how a changing electric
field (or an electrical current) induces a magnetic field.
The four equations are essentially distillations of generations of
laboratory experiments, mainly by French and British scientists.
What I've described here vaguely and qualitatively, the equations
describe exactly and quantitatively.
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:
He set p 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,
the 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
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