#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.

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

    Remember the idea of Faraday which evolved into the "magnetic field" concept--that space in which magnetic forces may be observed is somehow changed? Faraday also showed that a magnetic field which varied in time--like the one produced by an alternating current (AC)--could drive electric currents, if (say) copper wires were placed in it in the appropriate way. That was "magnetic induction," the phenomenon on which electric transformers are based.

    So, magnetic fields could produce electric currents, and we already know that electric currents produce magnetic fields. Would it perhaps be possible for space to support a wave motion alternating between the two? Sort of:

    With that term added (the "displacement current"), the equations of electricity and magnetism allowed a wave to exist, propagating at the speed of light. The drawing below illustrates such a wave--green is the magnetic part, blue the electric part--the term Maxwell added. The wave is drawn propagating just along one line. Actually it fills space, but it would be hard to draw that.

Electromagnetic Wave (see text above)

    Maxwell proposed that it indeed was light. There had been earlier hints--as noted above, the velocity of light had appeared unexpectedly in the equations of electricity and magnetism--and further studies confirmed it. For instance, if a beam of light hits the side of a glass prism, only part of it enters--another part gets reflected. Maxwell's theory correctly predicted properties of the reflected beam.

    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.