Extraterrestrial Civilizations - Isaac Asimov [137]
The individual hydrogen atom undergoes the change only very rarely but, considering all the hydrogen atoms in space, great numbers are undergoing the change at every moment, so that if Van de Hulst’s calculations were correct, the microwaves produced by hydrogen atoms should be detectable. In 1951, the American physicist Edward Mills Purcell (1912–) did detect them.
The hydrogen atom is predominant in the space between the stars, and the 21-centimeter wavelength is therefore a universal radiation that would be received anywhere. Any civilization that had reached our technological level would certainly be radio astronomers, and we can be certain they would have instruments equipped to receive the 21-centimeter wavelength even if they bothered with nothing else. Surely they would transmit messages over a wavelength they could themselves receive and one that they would be certain that all other civilizations would be tuned to.
In 1959, therefore, the American physicist Philip Morrison and the Italian physicist Giuseppe Cocconi (1914–) suggested that if signals from extraterrestrials were searched for, they should be searched for at 21-centimeter wavelengths.
That is the microwave wavelength, however, in which the background radiation is strongest and potentially the most obscuring—particularly in the region of the Milky Way. There is some feeling, therefore, that we ought to look somewhere else, perhaps at 42 centimeters or 10.5 centimeters, since doubling or halving the obvious choice is the simplest way of using 21 centimeters as the basis for the message without using that wavelength itself.
Another suggestion is to make use of hydroxyl, the 2-atom combination of hydrogen and oxygen, which, next to hydrogen itself, is the most widespread emitter of microwaves in interstellar space. Its microwave emission has a wavelength of 17 centimeters (6.7 inches).
Since hydrogen and hydroxyl together make water, the stretch of microwaves from 17 to 21 centimeters in wavelength is sometimes called the waterhole. The name is particularly apt, because the hope is that different civilizations will send and receive messages in this region as different species of animals come to drink at literal water-holes on Earth.
In 1960, the first real attempt was made to listen to the 21-centimeter wavelength in the sky in the hope of detecting messages from extraterrestrial civilization. It was carried through in the United States under the direction of Frank Drake, who called it Project Ozma. Ozma was Princess of Oz, the distant land in the sky of the well-known children’s adventure series. After all, the astronomers were trying to gain evidence of occupied lands even farther in the sky than Oz is.
The listening began at 4 A.M. on April 8, 1960, with absolutely no publicity, since the astronomers feared ridicule. It continued for a total of 150 hours through July, and the project then came to an end. The listeners were on the alert for anything with a very narrow range of wavelengths that seemed to flicker in a way that was neither quite regular nor quite random. They detected nothing of the sort.
Since Project Ozma, there have been six or eight other such programs, all at a level even more modest than the first, in the United States, in Canada, and in the Soviet Union. There have been no positive results, but the fact is that the search has been very brief and superficial so far.
Astronomers remain alive to the possibility of accidental discoveries, of course. When, in 1967, pulsars (very tiny, very dense, very rapidly rotating stars that were remnants of collapse following supernova explosions) were discovered, for just a short while the surprising detection of pulses of microwaves gave