A The primary reason for the search is basic curiosity - the same curiosity about the natural world that drives all pure science. A 这种探寻的最根本原因是人类本能的好奇心，正是这种对大自然的好奇推动了一切纯科学的进步。 We want to know whether we are alone in the Universe. 我们想知道人类是不是宇宙中唯一存在的生命体。 We want to know whether life evolves naturally if given the right conditions, or whether there is something very special about the Earth to have fostered the variety of life forms that we see around us on the planet. 我们想知道如果具备了合适的条件，生命体是否会自然进化，我们还想知道地球上是否存在着某种特殊物质，孕育了我们星球上的千姿百态的生命体。 The simple detection of a radio signal will be sufficient to answer this most basic of all questions. 一个简单的无线电信号监测就可以充分回答这些最根本的问题。 In this sense, SETI is another cog in the machinery of pure science which is continually pushing out the horizon of our knowledge. 从这个意义上来讲，SETI 是持续推动纯科学发展并进而拓宽人类知识范畴的另一个重要因素。 However, there are other reasons for being interested in whether life exists elsewhere. 然而，人类对于是否存在其他生命体这件事感兴趣其实还有另外的原因。 For example, we have had civilisation on Earth for perhaps only a few thousand years, and the threats of nuclear war and pollution over the last few decades have told us that our survival may be tenuous. 例如，我们地球上的人类文明可能只有数千年，过去几十年来自核战争和污染的威胁提醒着我们，生命是脆弱的。 Will we last another two thousand years or will we wipe ourselves out? 我们是否能在下一个两千年存活，还是会自我毁灭？ Since the lifetime of a planet like ours is several billion years, we can expect that, if other civilisations do survive in our galaxy, their ages will range from zero to several billion years. 我们的地球有数十亿年的寿命，据此我们可以推测，如果在我们的银河系中还有其他文明存在，它们的历史可能是从零到数十亿年不等。 Thus any other civilisation that we hear from is likely to be far older, on average, than ourselves. 因此我们只要收到其他文明的无线电信号，那么它们的平均历史就可能比人类历史要久远得多。 The mere existence of such a civilisation will tell us that long-term survival is possible, and gives us some cause for optimism. 只要这种文明存在，我们就可以得知生命体是可以长期生存的，这也给我们带来了一些保持乐观的理由。 It is even possible that the older civilisation may pass on the benefits of their experience in dealing with threats to survival such as nuclear war and global pollution, and other threats that we haven't yet discovered. 甚至有可能这些古老的文明会把他们在解决生存问题中的经验传授给我们，比如如何应对核战争或全球污染，以及如何解决我们尚未发现的其他威胁。

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B In discussing whether we are alone, most SETI scientists adopt two ground rules. B 在讨论我们是否是宇宙中唯一的生命体时，大部分SETI 的科学家会遵循两条基本规则。 First, UFOs (Unidentified Flying Objects) are generally ignored since most scientists don’t consider the evidence for them to be strong enough to bear serious consideration (although it is also important to keep an open mind in case any really convincing evidence emerges in the future). 第一条，他们通常不会考虑 UFOs（ 不明飞行物），因为大部分科学家认为现有的证据不足以支撑他们展开深入的研究（尽管保持一种开放的思维是很重要的，说不定未来真的会出现任何令人信服的证据）。 Second, we make a very conservative assumption that we are looking for a life form that is pretty well like us, since if it differs radically from us we may well not recognise it as a life form, quite apart from whether we are able to communicate with it. 第二条，我们做了一个保守的假设，那便是，我们正在寻找的生命体和人类非常相似，因为如果完全不同，我们可能不会把它们确认为生命体，更不用提我们是否可以与它进行交流了。 In other words, the life form we are looking for may well have two green heads and seven fingers, but it will nevertheless resemble us in that it should communicate with its fellows, be interested in the Universe, live on a planet orbiting a star like our Sun. 也就是说，我们正在寻找的生命体也许会有两个绿色的脑袋和七根手指，但是他们和人类一样，可以与同类进行交流，对宇宙充满好奇，生活在一个围绕恒星公转的星球上，像我们的地球围绕着太阳转动一样。 And perhaps most restrictively, have a chemistry, like us, based on carbon and water. 也许更加严格地讲，这些生命体和我们一样，是由基本化学物质碳和水构成的。

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C Even when we make these assumptions, our understanding of other life forms is still severely limited. C 即使我们做了这些假设，我们对于其他生命体的了解还是非常有限。 We do not even know, for example, how many stars have planets, and we certain^ do not know how likely it is that life will arise naturally, given the right conditions. 就像我们甚至不知道有多少颗恒星是有行星围绕的；我们当然也不知道即使有了合适的条件，生命体自然形成的几率有多大。 However, when we look at the 100 billion stars in our galaxy (the Milky Way), and 100 billion galaxies in the observable Universe, it seems inconceivable that at least one of these planets does not have a life form on it; in fact, the best educated guess we can make, using the little that we do know about the conditions for carbon-based life, leads us to estimate that perhaps one in 100,000 stars might have a life-bearing planet orbiting it. 然而，当我们观察银河系的1,000 亿颗恒星及宇宙中可探测到的1,000亿个星系的时候，我们很难相信这些恒星中没有任何生命体存在。事实上，凭借我们仅有的一点点对于碳基生命体的知识，我们可以做出的最有根据的推测就是：也许每十万个恒星中就会有一个可能孕育生命的行星围绕着它在运转。 That means that our nearest neighbours are perhaps 100 light years away, which is almost next door in astronomical terms. 那也就意味着我们最近的邻居离我们也许只有100 光年，从天文学的角度理解，这几乎是隔壁的邻居了。

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D An alien civilisation could choose many different ways of sending information across the galaxy, but many of these either require too much energy, or else are severely attenuated while traversing the vast distances across the galaxy. D 一个外星文明可以在银河系中选择多种不同的方式来传递信息，但是这些方式需要耗费巨大的能量，要不然就会在银河系长距离的传播过程中损耗殆尽。 It turns out that, for a given amount of transmitted power, radio waves in the frequency range 1000 to 3000 MHz travel the greatest distance, and so all searches to date have concentrated on looking for radio waves in this frequency range. 实验证明，在一定的发射功率下，频率在1,000 兆赫到3,000 兆赫之间的无线电波传播距离最远，所以到现在我们所有的电波搜寻频率都集中在这个范围以内。 So far there have been a number of searches by various groups around the world, including Australian searches using the radio telescope at Parkes, New South Wales. 截至目前，世界各地有许多不同的团体进行了多次搜寻，这当中包括了在澳大利亚新南威尔士的帕克斯用无线电天文望远镜进行的搜寻。 Until now there have not been any detections from the few hundred stars which have been searched. 然而，在已经搜寻到的上百颗恒星中，研究人员尚未发现有任何生命体的存在。 The scale of the searches has been increased dramatically since 1992, when the US Congress voted NASA $10 million per year for ten years to conduct a thorough search for extra-terrestrial life. 自1992 年美国国会计划在未来10 年里每年为NASA（美国国家航空航天局）投入1,000 万美元用于彻底搜寻外星生命体以来，搜寻的规模开始大幅增长。 Much of the money in this project is being spent on developing the special hardware needed to search many frequencies at once. 项目中的很多资金用来研发可以同时搜寻多个频率的特殊硬件上。 The project has two parts. 该项目分为两个部分。 One part is a targeted search using the world's largest radio telescopes, the American-operated telescope in Arecibo, Puerto Rico and the French telescope in Nancy in France. 其中一部分是用全世界最大的无线电天文望远镜进行的针对性搜寻，这些望远镜分别是来自由美国操控的位于波多黎各阿雷西沃的望远镜及法国南锡的法国望远镜。 This part of the project is searching the nearest 1000 likely stars with high sensitivity for signals in the frequency range 1000 to 3000 MHz. 这部分项目搜寻的是对频率在1,000至3,000 兆赫之间的信号有高度敏感性的最近的1,000 颗恒星。 The other part of the project is an undirected search which is monitoring all of space with a lower sensitivity, using the smaller antennas of NASA's Deep Space Network. 该项目的另外一个部分是使用NASA（美国国家航空航天局）深空探测网络的小型天线，来监控整个太空中不太活跃的恒星的自发搜寻。

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E There is considerable debate over how we should react if we detect a signal from an alien civilisation. E 在监测到来自外星的信号后我们应该做何回应， 是一个备受争议的话题。 Everybody agrees that we should not reply immediately. 所有人都认为我们不应该立即做出回应。 Quite apart from the impracticality of sending a reply over such large distances at short notice, it raises a host of ethical questions that would have to be addressed by the global community before any reply could be sent. 先不提马上向如此遥远的地方回复是不切实际的，这还会引发许多在做出回应之前必须经过国际社会共同解决的道德问题。 Would the human race face the culture shock if faced with 8 superior and much older civilisation? 假如我们面对的是一个更加优越、更加古老的文明，人类会不会直面文化冲击？ Luckily, there is no urgency about this. 幸运的是，这并不是一个亟待解决的问题。 The stars being searched are hundreds of light years away, so it takes hundreds of years for their signal to reach us, and a further few hundred years for our reply to reach them. 这些我们正在搜寻的恒星距离我们数百光年，所以它们的信号传送到我们这里需要数百年，而我们做出回应也需要另外一个数百年才能到达这些恒星。 It's not important, then, if there's a delay of a few years, or decades, while the human race debates the question of whether to reply, and perhaps carefully drafts a reply. 当人类在争论是否做出回应或精心起草回应内容的时候，延误几年甚至几十年也都无关紧要。