Extraterrestrial Civilizations - Isaac Asimov [136]
In a way that was not surprising. The layers of charged atoms in the upper atmosphere that reflected Earth-made radio waves and kept them in the neighborhood of the surface instead of allowing them to pass outward into space would also serve to reflect spacemade radio waves and keep them away from Earth’s surface.
In 1931, however, the American radio engineer Karl Guthe Jansky (1905–1950), working for Bell Telephone Laboratories, detected an odd signal when he was trying to determine the source of static that interfered with the developing technique of radio telephony. It turned out that the signal was coming from the sky. That was the first indication that there was a wide band of short-wave radio waves, called microwaves, that could easily penetrate Earth’s atmosphere. There were two types of electromagnetic radiations that we could get from the sky: a narrow band of visible light and a broad band of microwaves.
By December 1932, it was demonstrated that Jansky had detected radio waves from the Galactic center, and that made front-page headlines in the New York Times. Some astronomers, such as Jesse Leonard Greenstein (1909–) and Fred Lawrence Whipple (1906–), at once appreciated the potentialities of the discovery, but there was little that could be done about it. There were no decent instruments for detecting such radiation. One American radio engineer, Grote Reber (1911–), did take it seriously, however. He built a device to detect radio waves from the sky (a “radio telescope”) and from his back yard, beginning in 1938, studied as much of the sky as he could reach in order to measure the intensity of radio-wave reception from different areas.
During World War II, the development of radar changed everything. Radar made use of microwaves so that microwave technology advanced rapidly, and after the war, radio astronomy quickly became a giant, revolutionizing the science as it had been revolutionized by Galileo’s optical telescope 3½ centuries before.
In just a few decades, radio telescopes have been built that can detect microwaves far more delicately than light can be detected. Sources of microwave radiation could be detected at distances too great for us to make out light radiation of anything like equivalent energy. In fact, we can right now detect microwaves from any star in the Galaxy, even though those microwaves are sent out with no more energy than we ourselves could dispose of.
Then, too, the sources of microwaves can be located with great precision, and the varieties of microwaves can be differentiated with great ease. Every molecule emits or absorbs its own specific wavelength, so that the chemical constitution of interstellar gas clouds can be determined with great precision. Microwaves are not blanked out by background radiation. In most parts of the sky, microwaves are not radiated with the intensity of light, and even where microwaves are plentiful, it would be easy for a civilization to send out a specific wavelength that would be far stronger than the natural background for that wavelength.
It amounts to this: If any civilization is trying to send out messages, it would surely come to the conclusion that microwaves are a better, cheaper, and more natural medium for those messages than light—or, in fact, than anything.
We finally have what looks like the answer. To send, or receive, messages across the interstellar gulfs, we must make use of microwaves.
But at what energy level, or wavelength, ought we to expect the message to come? Receivers can be tuned to receive some specific wavelength, and if the message is being sent at another wavelength, it will be missed. On the other hand, to try to tune in all possible wavelengths would enormously increase the difficulty and expense of listening. But can we read the extraterrestrial mind and guess the wavelength it would choose to use?
During World