#6.     Electromagnetic Waves

Perhaps the greatest theoretical achievement of physics in the 19th century was the discovery of electromagnetic waves. The first hint was an unexpected connection between electric phenomena and the velocity of light.

Electric forces in nature come in two kinds. First, there is the electric attraction or repulsion between (+) and (-) electric charges. It is possible to use this to define a unit of electric charge, as the charge which repels a similar charge at a distance of, say, 1 meter, with a force of unit strength (actual formulas make this precise).

But second, there is also the attraction and repulsion between parallel electric currents. One could then define the unit of current, as the current which, when flowing in a straight wire, attracts a similar current in a parallel wire 1 meter away with a force of unit strength, for every meter of the wires' length.

But electric current and charge are related! We could have just as well based the unit of current on the unit of charge--say, as the current in which one unit of charge passes each second through any cross section of the wire. This second definition turns out to be quite different, and if meters and seconds are used in all definitions, the ratio of the two units of current turns out to be the speed of light, 300,000,000 meters per second.

In Faraday's time the speed of light was known, although not as accurately as it is today. It was first derived around 1676 by Ole (Olaus) Roemer, a Danish astronomer working in Paris. Roemer tried to predict eclipses of Jupiter's moon Io (mentioned later here in an altogether different connection) and he found a difference between actual and predicted eclipse times, which grew and then decreased again as the Earth circled the Sun. He correctly guessed the reason, namely, as the Earth moved in its orbit, its distance to Jupiter also went up and down, and light needed extra time to cover the extra distance.

But what was the meaning of the link between electricity and light?

Faraday had already found that a changing magnetic field could produce a changing electric field: that is the principle behind the electric transformers which change electricity from the high voltage (that is, the high electric pressure) of the power lines to the safer low voltage used in homes. But "field" originally only meant the force a magnet or an electric charge could expect at a point in space, if it were placed there. If no magnet or charge was placed at the point, all that existed there was just empty space.

Faraday's idea of the field changed all that--the idea that even empty space could take on special properties near magnets and near electric charges. James Clerk Maxwell, a brilliant young Scotsman, proposed in 1861 a symmetry between the fields--that perhaps a changing electric field could also create a magnetic field. That required a small modification of the equations of electricity, predicting new effects which however became significant only with very fast changes.

But that small modification also had a far-reaching consequence, for it suggested that a spreading "electromagnetic wave" could exist in space empty of matter. In such a wave, an oscillating electric field, alternating quickly between two opposite directions, creates an oscillating magnetic field, which in turn generates a new oscillating electric field further away, and so on. The wave spreads with the speed of light, and Maxwell suggested that this indeed was light.

Maxwell's theory explained many observed features of light. Then Heinrich Hertz in Germany showed that an electric current bouncing back and forth in a wire (nowadays it would be called an "antenna") could be the source of such waves. (The current also produces a magnetic field in accordance with Ampere's law, but that field decreases rapidly with distance.) Electric sparks create such back-and-forth currents when they jump across a gap--hence the crackling caused by lightning on AM radio--and Hertz in 1886 used such sparks to send a radio signal across his lab. Later the Italian Marconi, with more sensitive detectors, extended the range of radio reception, and in 1903 detected signals from Europe as far as Cape Cod, Massachussets.

It was presumed that light from the hot wire of a lightbulb was emitted because the heat caused electrons to bounce back and forth rapidly, turning each into a tiny antenna. When physicists tried to follow that idea, however, they found that the familiar laws of nature had to be modified on the scale of atomic sizes. That was how quantum theory originated.

Gradually other electromagnetic waves were found The wave nature of light causes different colors to be reflected differently by a surface ruled in fine parallel scratches--which is why a compact laser disk (for music or computer use) shimmers in all colors of the rainbow. The orderly rows of atoms in a crystal also form parallel lines but spaced much more closely, and they turned out to have the same effect on X-rays, showing that X-rays, like light, also were electromagnetic waves, but of a much shorter wavelength. Later it was found that beams of electrons in a magnetic field, inside a vacuum tube, could become unstable and emit waves longer than light: the magnetron tube where this occured was a top-secret radar device in World War II, and it later made the microwave oven possible.

Electromagnetic waves led to radio and television, and to a huge electronic industry. But they are also generated in space--by unstable electron beams in the magnetosphere, as well as at the Sun and in the far-away universe, telling us about energetic particles in distant space, or else teasing us with unresolved mysteries. You can find more about this in the section on high energy particles.

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Last updated March 13, 1999