国外精模板248.pptVIP

PowerPoint Template /see161 Contents Hot Tip How do I incorporate my logo to a slide that will apply to all the other slides? On the [View] menu, point to [Master], and then click [Slide Master] or [Notes Master]. Change images to the one you like, then it will apply to all the other slides.  Diagram Cycle Diagram Diagram Diagram Diagram 3-D Pie Chart Diagram Diagram Cycle Diagram Diagram Diagram Diagram Diagram Table 3-D Pie Chart Diagram Some scientists working on the question of extraterrestrial intelligence, myself among them, have attempted to estimate the number of advanced technical civilizations -- defined operationally as societies capable of radio astronomy -- in the Milky Way Galaxy. Such estimates are little better than guesses. They require assigning numerical values to quantities such as the numbers and ages of stars; the abundance of planetary systems and the likelihood of the origin of life, which we know less well; and the probability of the evolution of intelligent life and the lifetime of technical civilizations, about which we know very little indeed. When we do the arithmetic, the sorts of numbers we come up with are, characteristically, around a million technical civilizations. A million civilizations is a breathtakingly large number, and it is exhilarating to imagine the diversity, lifestyles and commerce of those million worlds. But the Milky Way Galaxy contains some 250 billion stars, and even with a million civilizations, less than one star in 200,000 would have a planet inhabited by an advanced civilization. Since we have little idea which stars are likely candidates, we will have to examine a very large number of them. Such considerations suggest that the quest for extraterrestrial intelligence may require a significant effort. Despite claims about ancient astronauts and unidentified flying objects, there is no firm evidence for past visitation of the Earth by other civilizations. We are restricted to remote signaling and,