Astronomers have found no evidence suggesting that intelligent life exists on any of earth’s sister planets. Most scientists would agree, however, that life of some kind probably does exist in other parts of our galaxy and in other galaxies. Even if this probability were certainty and space travel were possible over the vast distances we measure in light years, the chance that earthman and alien will ever establish physical contact remains infinitesimally small. An explorer (whether from earth or from a planet of another sun) would have to begin by locating, among the millions of stars in the heavens, a particular star that had a family of life-bearing planets. If he were able to identify one of these needles in the cosmic haystack, he would next have to find out which of the planets supported living, intelligent organisms. If he could find the planet and set down his spaceship, the explorer would then have to try to identify and to communicate with creatures that might be unimaginably strange--so strange that he would not recognize them as either living or intelligent. At present, only light waves and radio waves can bridge the immensities of space. Physical travel to other star systems is not now and may never be possible. Nevertheless, men are making attempts to find out whether other intelligent beings do exist outside the solar system and, if so, where. The earliest effort, known as Project Ozma, started a few years ago at the National Radio Astronomy Observatory at Green Bank, West Virginia. As the first step in a systematic search, the astronomers began to listen for possible radio signals from the neighborhood of certain stars. Tau Ceti, Epsilon Eridani, and 61 Cygni were chosen as the first targets because they lie within range of our radio telescopes--ten to eleven light years distant--and because they resemble our own sun in age and type and therefore might have planetary systems not unlike our own. So far, the radio telescopes have detected no phenomena that might be interpreted as artificial signals. The problems involved are incredibly difficult. A background of radio noise--“swishes,” “whistles,” “tweeks”--comes in constantly from the universe at large. Deliberate signals, if they occurred, would be hard to distinguish from the random noise. Even if signals came in and were detected, they might still be indecipherable just as the written records of some early civilizations on our own planet remain a mystery. Egyptian hieroglyphs were meaningless pictures for millennia until the Rosetta stone provided the key, less than 200 years ago. The many pages of text and pictures left by the Mayan Indians cannot yet be read, except for some dates and a few astronomical symbols. Hundreds of inscriptions exist in the Etruscan language, written in an alphabet that resembles the familiar Greek, but scholars have deciphered only a few words.