Extraterrestrial Civilizations - Isaac Asimov [135]
With laser light we come closer to a practical signaling device than anything yet mentioned, but even a laser signal originating from some planet would, at great distances, be drowned out by the general light of the star the planet circles.
One possibility that has been suggested is this—
The spectra of Suntype stars have numerous dark lines representing missing photons—photons that have been preferentially absorbed by specific atoms in the stars’ atmospheres. Suppose a planetary civilization sends out a strong laser beam at the precise energy level of one of the more prominent dark lines of the star’s spectrum. That would brighten that dark line.
If we studied the spectrum of a star and discovered that it was missing one of the dark lines characteristic of a certain group of atoms in the star’s atmosphere, but that other dark lines also characteristic of that group were present, we would have to conclude that the missing energy level had been supplied by artificial means. That would mean the presence of a civilization.
Nothing like that has been observed—but before feeling depressed over that, let us see if perchance there are still simpler ways of signaling. After all, no civilization would be expected to use the harder method when a simpler is available.
MICROWAVES
Early in the nineteenth century, electromagnetic radiation outside the range of visible light was first discovered. In 1800, William Herschel discovered the infrared range of sunlight by the manner in which a thermometer was affected beyond the red limit of the range of visible light. In 1801, the German physicist Johann Wilhelm Ritter (1776–1810) discovered the ultraviolet range of sunlight by the manner in which chemical reactions were brought about beyond the violet limit of the range of visible light.
These discoveries did not affect astronomy very much, however. Most of the range of ultraviolet and infrared could not penetrate the atmosphere, so that little of it reached us from the Sun and the stars.
Beginning in 1864, Maxwell (who had worked out the kinetic theory of gases) developed the theory of electromagnetism. This first identified light as an electromagnetic radiation and predicted the existence of many octaves of such radiation on either side of the visible light range.
In 1888, the German physicist Heinrich Rudolf Hertz (1857–1894) detected lightlike radiation with wavelengths a million times longer than light and with energy levels that were, therefore, only a millionth as high. The new radiation came to be spoken of as radio waves.
Radio waves, because of their low energy content, turned out to be easy to produce, and despite their low energy content, easy to receive. Radio waves could penetrate all sorts of material objects as light could not. Radio waves could bounce off layers of charged particles in the upper atmosphere as light could not, so that radio waves could, in effect, follow the curve of Earth’s surface. Radio waves could easily be produced in coherent fashion, so that a tight beam could go long distances, and could easily be modified to carry messages.
For all these reasons radio waves were clearly ideal for longrange communication, and that, too, without the wires that telegraphs and cables required. The first to make practical use of radio waves in this way was the Italian electrical engineer Guglielmo Marconi (1874–1937). In 1901, he sent a radio-wave signal across the Atlantic Ocean, a feat generally recognized as the invention of radio.
From that day on, with further improvements and refinements, radio became a more and more important means of communication. It was clear to many people that any technological civilization would surely make use of radio communication in preference to anything else.
Therefore,