From the beginning, men have looked into the starry sky and asked, "Are we alone in this vastness?" Scientific knowledge now enables us to outline an answer to this question, and that will be our purpose here. Some interesting implications of the answer may come to mind as we proceed.
Two scientific achievements of the 1950's changed our intellectual climate and brought renewed interest in possible galactic civilizations: the development of radio astronomy provided a means of communicating with other societies, and the dramatic launching of the first Sputnik by Russia in 1958 showed that man was ready to step out into space. Several articles and books about extraterrestrial civilizations by scientists and writers appeared (4), (5), (6). A whole new branch of science—exobiology— was born. Some of what had been written, often with considerable knowledge and imagination, as "science fiction" began to appear much more plausible.
This article explores several aspects about civilizations in space: How many are there? How many in this galaxy? How many civilizations are communicative? What kind of beings inhabit other worlds? etc.