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Which Creation? Already in vigorous operation is radio astronomy, a postwar newcomer that may prove more important than its optical older brother. Already, it has drawn a new map of the heavens, finding strong "radio stars" where nothing can be seen in visible light. Some of these mysterious sources have turned out to be pairs of galaxies in collision, which are of especial importance to cosmologists in their struggle to figure out how the universe was formed. They are fairly common, and they seem to extend indefinitely into the depths of space, rushing away faster and faster in proportion to their distance from the earth. Radio astronomy may be able to chase them close to the "edge of the knowable universe," where they will be moving away so fast that their light and radio waves cannot reach the earth at all. Long before this point is attained, the cosmologists should have evidence enough to decide whether the universe was created in one place at the same time or whether it is being created continuously in the form of virgin hydrogen atoms in the empty spaces between the galaxies.
At the farthest end of the space science spectrum is a project to listen for messages sent by intelligent creatures living on planets revolving around other stars than the sun. This project was made plausible by Harvard's Physics Professor Edward Purcell, who was the first to detect the 21-cm. waves from cold hydrogen throughout space. Purcell explains that if intelligent aliens send messages to the earth, they will use a sort of reversed cipher that is deliberately made easy to translate. Their first problem will be to select the proper radio frequency: there is no use picking one at random. Unless listening earthlings know how to tune their receivers, they will hear nothing. Therefore, says Purcell, the aliens will select the 21-cm. waves, which are the sharpest and most universal radio waves that flash through space. The aliens will reason that if earthlings are bright enough to have an electronic technology, they will know about the 21-cm. waves and will tune to them.
A further subtlety, says Purcell. is that when the aliens turn their transmitter toward the sun, they will know the speed at which their star is approaching the solar system or receding from it. They will therefore allow for the slight shift of frequency caused by this motion. They may also allow for the motion of their planet on its orbit, but cannot know the earth's orbital motion. This final fine tuning will have to be done at the receiver on earth.
What message will the aliens send if they want to be understood by earthlings? Purcell suggests that a simple on-off signal will be easiest to detect, and is most likely to be sent. But he speculates that many messages of varying difficulty may be sent simultaneously, which is not hard to do. Aliens on a planet of Epsilon Eridani, a likely star, will not expect to get an answer from the solar system in less than 22 years. But by sending simultaneous messages, they can educate their earthside listeners quickly. Besides simple number series, says Purcell, the messages will probably contain other mathematical relationships. Words and logical concepts can be taught in the same way, growing more and more complicated as the many-layered message is deciphered.