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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 advancedcivilization. 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.
计算结果，我们得出的具有典型意义的数字是有大约一百万个技术文明社会。这可是个大得令人咋舌的数目。想像一下这百万个世界的千姿百态、各种生活方式和商业往来，真令人兴奋不已。然而，银河系大约有二千五百亿颗恒星，即便有百万个文明世界，平均每二十万颗恒星中，也仅有一颗不到的恒星，其周围的一颗行星上存在着先进的文明社会。由于不知道哪些星球上存在这种可能性，我们不得不进行大量的观察。这表明，搜寻外星人需要作出巨大的努力。
Despite claims about ancient astronauts and unidentified flying objects, there is no firm evidence for past visitations of the Earth by other civilizations. We are restricted to remote signaling and, of the long-distance techniques available to our technology, radio is by far the best. Radio telescopes are relatively inexpensive; radio signals travel at the speed of light, faster than which nothing can go; and the use of radio for communication is not a short-sighted or anthropocentric activity. Radio represents a large part of the electromagneticspectrum and any technical civilization anywhere in the Galaxy will have discovered radio early -- just as in the last few centuries we have explored the entire electromagnetic spectrum from short gamma rays to very long radio waves. Advanced civilizations might very well use some other means of communication with their peers. But if they wish to communicate with backward or emerging civilizations, there are only a few obvious methods, the chief of whichis radio.
尽管有人声称见过古代的太空人和来历不明的飞行物体，但并无确切证据证实别的文明世界的来客曾经访问过地球。我们目前只限于远距离信号的使用，而在我们的技术所能掌握的长距离通讯手段中，无线电是最佳的一种。相对说来，射电望远镜并不贵;无线电能以光速发送，其他任何东西都无法做到比它更快;采用无线电作通讯工具不是短视的或以人类为宇宙中心的行为。无线电包括了大部分电磁波谱，所以银河系中任何地方的技术文明社会，该早已发现了无线电技术了——如同我们在近几个世纪中，对包括短的γ射线到长的无线电波在内的整个电磁波谱进行过探测一样。先进的文明世界相互间很可能采用别的方法进行通讯联系。可是，如果他们想和其他落后的或发展中的文明社会联络的话，就只有几种显而易见的办法，而其中为主的就是无线电。
