The search for Extra-terrestrial intelligence

有外星人么？——寻找外星智能生物

The question of whether we are alone in the Universe has haunted humanity for centuries, but we may now stand poised on the brink of the answer to that question, as we search for radio signals from other intelligent civilisations. This search, often known by the acronym SETI (search for extra-terrestrial intelligence), is a difficult one. Although groups around the world have been searching intermittently for three decades, it is only now that we have reached the level of technology where we can make a determined attempt to search all nearby stars for any sign of life.

我们在宇宙中究竟是不是唯一的智能生物这一问题已经困扰人类几个世纪了，不过我们现在可能已经准备好要能够回答那个问题了，因为我们正在寻找来自其他智能文明的无线电信号。这一通常被人们当作 SETI（Search for Extra-terrestrial intelligence）而熟知的探索并不容易。虽然全世界的各个组织已经间或地搜索三十年了，也只是最近才具有合格程度的技术，用以做出确定性的对于附近各星体上生命迹象的努力。

A 这一搜索的首要原因是人们基本的好奇心——这一好奇心和推动所有纯科学发展的那种对自然界的好奇心一样。我们想知道我们在宇宙中是否是唯一的智能生物。我们想知道如果给定合适的条件，生命体是否能自然地发展产生。我们还想知道是否地球能够孕育我们身边所见的各种生命形式，这是不是非常特别的。对于无线电信号的简单探索足够回答所有这些问题的最基本层面。从这一意义上说，SETI在不断超出我们知识界限的纯科学这一机械系统中是另一个钝齿。然而，对这一问题感兴趣还有别的原因。比如，我们在地球上只有大约几千年的文明，而过去几十年里的核子战争和污染问题的威胁告诉我们我们的生存状况是非常脆弱的。我们是能够继续存活两千年还是将会自我毁灭呢？既然向我们的行星一样的星球的一生寿命有几十亿年，我们可以想见如果在我们的银河中确实存在其他文明，他们的存在年代的范围应该在0——几十亿年之间。因此任何其他我们能够听到的文明平均起来都可能比我们要古远得多。这种文明的存在将会告诉我们长久存活是有可能实现的，并且让我们有理由乐观起来。甚至有可能我们接触到的更古老的文明会把他们处理生存威胁（如核子战争和全球污染以及我们还未发现的其他威胁）的经验传授给我们使我们受益。

AThe primary reason for the search is basic curiosity 一 the same curiosity about the natural world that drives all pure science. We want to know whether we are alone in the Universe. 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. The simple detection of a radio signal will be sufficient to answer this most basic of all questions. In this sense, SETI is another cog in the machinery of pure science which is continually pushing out the horizon of our knowledge. However, there are other reasons for being interested in whether life exists elsewhere. 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. Will we last another two thousand years or will we wipe ourselves out? 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. Thus any other civilisation that we hear from is likely to be far older, on average, than ourselves, the mere existence of such a civilisation will tell us that long-term survival is possible, and gives us some cause for optimism. 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.

B 在探讨我们是不是唯一的智能生物这一问题时，大部分的 SETI 科学家采取两套基本规则。第一种是说UFO通常会被忽略因为大部分科学家不认为他们相关的证据足够可靠，因而不值得认真对待（虽然还是要保持一种开放的思维方式，一旦未来出现足够可靠的证据就要接受）。第二种是一个非常保守的假设，即我们正在寻找的生命形式是一种与我们非常像的，因为如果它与我们太过于不同的话我们就无法将它识别为一种生命形式，更别说与它们交流了。换句话说，我们正在寻找的生命形式很可能有两个绿色的头或者七只手指头，但是它们会和我们很像因为它们能够与同伴交流，能够对宇宙产生兴趣，能够在一个围绕像太阳一样的恒星周转运行的行星上生活，并且严格意义上来说有像我们一样的给予碳和水的化学构造。

C 甚至当我们做出这些假设时，我们对于其他生命形式的理解都是非常有限的。我们甚至不知道，比如说，究竟有多少颗恒星有行星环绕，我们当然也不知道在合适的条件下生命究竟有多大的可能性能够自然地产生。然而，当我们看到我们星系中100000000000颗恒星以及宇宙中可以观察到的100000000000个星系时，下结论说这些行星中没有一颗有生命体似乎是不合情理的。事实上，我们可以利用我们对于以碳为基础的生命的状况的微薄知识做的最成熟的假设能引导我们估计出约有1/100000的恒星可能会有一颗载有生命的行星围绕着它。这意味着我们最近的邻居大约有100光年远，这在天文学概念中几乎就是贴门住。

BIn discussing whether we are alone, most SETI scientists adopt two ground rules. 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). 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. 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, and perhaps most restrictively, have a chemistry, like us, based on carbon and water.

D 一个外星文明可能会选择许多方法来在穿越星系发送信息，但是这些方法中的大部分都需要很多能量，或者会在穿过很长的距离后严重衰减。事实上，在给定传输的能量后，频率在1000——3000兆赫的无线电波能传送最常的距离，所以目前所有的搜索都着眼于寻找这个范围内的无线电波。到不前为止，世界范围内的不同组织已经进行了多次搜索，包括澳大利亚在新南威尔士的派克斯镇使用无线电望远镜那一次。到目前为止对于已经搜索的那几百个恒星中并没有任何结果。搜索的规模自从1992年以来大幅度增加了，其间美国国会十年间每年投票拨给 NASA（美国航空航天局）10000000美元用于对地外生物的全面搜查。这一项目中大部分的钱都用与发展一次能搜索多个频率的技术所需的特殊硬件。这一工程有两部分。其一是有目标的搜查，使用了世界上最大的无线电望远镜，美国在波多黎各的阿雷西波和法国的南斯所安置和操作的无线电望远镜。项目的这一部分是以对于1000——3000兆赫的频率有高度敏感性的功能来搜索最近的1000颗可能恒星。项目的另一部分是一个无目标的搜索，这一搜索将监视敏感度低下的所有区域，使用NASA的深度空间网状系统的更小的天线。

E 针对我们应该如何应对可能探测到的外太空信号人们有巨大争论。每个人都同意我们不应该立刻做出答复。除了立刻进行如此长距离的答复的不可行性之外，人们还提出了一系列道德问题，这些问题将必须等国际社会达成共识来解决。人类如果遇到的是一个更为发达更加古远的文明是否会遇到文化冲击呢？幸运的是，这个问题并不亟待解决。被搜索的恒星离我们有好几百光年远，因此它们的信号要传到我们这里也得几百年，而我们答复过去也要几百年。因此如果我们耽误个几年或几十年来争辩是否答复并谨慎地起草出答复内容的话，不会有什么大碍的。

CEven when we make these assumptions, our understanding of other life forms is still severely limited. We do not even know, for example, how many stars have planets, and we certainly do not know how likely it is that life will arise naturally, given the right conditions. 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. That means that our nearest neighbours are perhaps ^ 00 light years away, which is almost next door in astronomical terms.

DAn 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. 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. 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. Until now there have not been any detections from the few hundred stars which have been searched. 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. Much of the money in this project is being spent on developing the special hardware needed to search many frequencies at once. The project has two parts. 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. 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. 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.

EThere is considerable debate over how we should react if we detect a signal from an alien civilisation. Everybody agrees that we should not reply immediately. 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. Would the human race face the culture shock if faced with a superior and much older civilisation? Luckily, there is no urgency about this. 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. 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.