原文
We must conclude from the work of those who have studied the origin of life, that given a planet only approximately like our own, life is almost certain to start. Of all the planets in our solar system, we ware now pretty certain the Earth is the only one on which life can survive. Mars is too dry and poor in oxygen, Venus far too hot, and so is Mercury, and the outer planets have temperatures near absolute zero and hydrogen-dominated atmospheres. But other suns, start as the astronomers call them, are bound to have planets like our own, and as is the number of stars in the universe is so vast, this possibility becomes virtual certainty. There are one hundred thousand million starts in our own Milky Way alone, and then there are exist is now estimated at about 300 million million.
Although perhaps only 1 per cent of the life that has started somewhere will develop into highly complex and intelligent patterns, so vast is the number of planets, that intelligent life is bound to be a natural part of the universe.
If then we are so certain that other intelligent life exists in the universe, why have we had no visitors from outer space yet? First of all, they may have come to this planet of ours thousands or millions of years ago, and found our then prevailing primitive state completely uninteresting to their own advanced knowledge. Professor Ronald Bracewell, a leading American radio astronomer, argued in Nature that such a superior civilization, on a visit to our own solar system, may have left an automatic messenger behind to await the possible awakening of an advanced civilization. Such a messenger, receiving our radio and television signals, might well re-transmit them back to its home-planet, although what impression any other civilization would thus get from us is best left unsaid.
But here we come up against the most difficult of all obstacles to contact with people on other planets -- the astronomical distances which separate us. As a reasonable guess, they might, on an average, be 100 light years away. (A light year is the distance which light travels at 186,000 miles per second in one year, namely 6 million million miles.) Radio waves also travel at the speed of light, and assuming such an automatic messenger picked up our first broadcasts of the 1920's, the message to its home planet is barely halfway there. Similarly, our own present primitive chemical rockets, though good enough to orbit men, have no chance of transporting us to the nearest other star, four light years away, let alone distances of tens or hundreds of light years.
Fortunately, there is a 'uniquely rational way' for us to communicate with other intelligent beings, as Walter Sullivan has put it in his excellent book, We Are not Alone. This depends on the precise radio frequency of the 21-cm wavelength, or 1420 megacycles per second. It is the natural frequency of emission of the hydrogen atoms in space and was discovered by us in 1951; it must be known to any kind of radio astronomer in the universe.
Once the existence of this wave-length had been discovered, it was not long before its use as the uniquely recognizable broadcasting frequency for interstellar communication was suggested. Without something of this kind, searching for intelligences on other planets would be like trying to meet a friend in London without a pre-arranged rendezvous and absurdly wandering the streets in the hope of a chance encounter.
--ANTHONY MICHAELIS Are There Strangers in Space? from The Weekend Telegraph--
译文
我们必须从那些研究生命起源的学者们的工作中得出结论：只要有一颗行星大致类似于我们的地球，生命几乎肯定会开始。在我们太阳系的所有行星中，我们现在相当肯定，地球是唯一能维持生命的地方。火星过于干燥且氧气不足，金星过于炎热，水星也是如此，而外行星的温度接近绝对零度，且大气以氢为主。
但是，其他的恒星——天文学家称之为“恒星”的那些——必然会有类似于我们自己的行星；由于宇宙中恒星的数量极其庞大，这种可能性几乎成了必然的确定性。我们自己的银河系中就有十万亿颗恒星，此外，现估计存在的星系约有三亿亿个。
尽管或许只有1%的在某处开始的生命会发展成高度复杂且智能的形式，但由于行星的数量如此众多，以致智能生命注定会成为宇宙中自然的一部分。
如果我们如此确信宇宙中存在其他智能生命，那为什么迄今还没有外星访客呢？首先，他们可能在数千或数百万年前就曾造访过我们的星球，却发现我们当时的主导状态太过原始，对他们先进的知识毫无吸引力。美国著名射电天文学家罗纳德·布雷斯韦尔教授曾在《自然》杂志上论证道，这样的高级文明在访问我们太阳系时，可能会留下一台自动信使，以等待先进文明的可能觉醒。这类信使在接收到我们的无线电和电视信号后，很可能会将它们转播回其母星，尽管其他文明由此对我们的印象最好还是不言而喻。
但在这里，我们遇到了与外星人接触的最大障碍——那就是天文距离的隔阂。合理推测，它们平均可能相隔100光年。（光年是光以每秒186,000英里的速度在一年内行进的距离，即600万万英里。）无线电波也以光速传播，假设那样一台自动信使捕捉到了我们1920年代的首次广播，那么发往其母星的信息现在才刚刚走完一半路程。同样，我们目前原始的化学火箭，虽然足以让人类进入轨道，却根本无法将我们送往最近的其他恒星——那颗恒星距离我们四光年之遥，更不用说数十或数百光年的距离了。
幸运的是，正如沃尔特·沙利文在其优秀著作《我们并不孤单》中所言，我们有一种“独一无二的理性方式”来与其他智能生物沟通。这依赖于21厘米波长的精确无线电频率，或1420兆赫每秒。它是太空中的氢原子自然发射的频率，于1951年被我们发现；它必定为宇宙中任何射电天文学家所知晓。
一旦这种波长的存在被发现，不久便有人建议将其用作星际通信中独一无二的可识别广播频率。没有这类方法，搜寻其他行星上的智能生命就如同在伦敦试图与朋友不经事先约定就邂逅一般荒谬——漫无目的地在街头游荡，寄希望于偶遇。
——安东尼·迈克利斯 《太空中有陌生人吗？》 选自《周末电讯报》