43 · 宇宙中有外星人吗？
Are There Strangers in Space?
区分观点、依据、例子与限制，用自己的话准确重建论述。
时代语境：本课的星系数量、生命推断和“现在”“已经走到半途”等判断均以成文年代为参照，不能直接当作今天已知的天文学事实。原稿这里已写作 megahertz（兆赫）；若另一个版本用 megacycles per second，两者在此都是每秒百万次的频率单位。
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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 are 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, stars as the astronomers call them, are bound to have planets like our own, and as the number of stars in the universe is so vast, this possibility becomes virtual certainty. There are one hundred thousand million stars in our own Milky Way alone, and then there are three thousand million other Milky Ways, or galaxies, in the universe.So the number of stars that we know exist is now estimated at about 300 million 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 1,420 megahertz. 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.
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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 are 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, stars as the astronomers call them, are bound to have planets like our own, and as the number of stars in the universe is so vast, this possibility becomes virtual certainty. There are one hundred thousand million stars in our own Milky Way alone, and then there are three thousand million other Milky Ways, or galaxies, in the universe.So the number of stars that we know exist is now estimated at about 300 million 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 1,420 megahertz. 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.
根据那些研究生命起源的学者所做的工作，我们必须得出这样的结论：只要一颗行星与地球大致相似，生命几乎肯定会在那里出现。在太阳系所有行星中，我们现在已相当确信，地球是唯一能让生命存活的行星。火星太干燥、氧气太少；金星太热，水星也一样；外层行星的温度则接近绝对零度，大气主要由氢构成。不过，其他太阳——也就是天文学家所说的恒星——必定也有类似地球的行星。宇宙中的恒星数量如此庞大，这种可能性实际上就成了近乎确定的事。仅我们所在的银河系，就有一千亿颗恒星；宇宙中另外还有三十亿个类似银河系的星系。因此，目前估计我们已知存在的恒星约有三万亿亿颗。
在某处已经出现的生命中，也许只有百分之一会发展成高度复杂、具有智慧的形态。但行星的数量实在太大，智慧生命必定会成为宇宙的自然组成部分。
那么，既然我们如此确信宇宙中存在其他智慧生命，为什么至今没有天外来客呢？首先，他们可能早在几千年或几百万年前就来过地球，只是以他们先进的知识来看，当时这里普遍存在的原始状态毫无吸引力。美国著名射电天文学家罗纳德·布雷斯韦尔教授曾在《自然》杂志上提出：这样的高级文明如果访问过太阳系，可能会留下一个自动信使，等待先进文明有朝一日在这里苏醒。这个信使收到我们的广播和电视信号后，很可能会把它们转发回自己的母星。至于其他文明会因此对我们形成什么印象，还是不说为妙。
不过，说到与其他行星上的生命联系，我们便遇到了最难克服的障碍：彼此相隔的距离实在太远，简直是天文数字。合理地猜测，他们与我们的平均距离可能是一百光年。一光年，就是光以每秒十八万六千英里的速度传播一年所走的距离，即六万亿英里。无线电波也以光速传播。假设这样一个自动信使接收到了我们在20世纪20年代最早发出的广播信号，那么，它传回母星的消息才勉强走到半途。同样，我们目前这种原始的化学火箭，虽然足以把人送上环绕地球的轨道，却根本无法把我们送到四光年外最近的另一颗恒星，更不用说几十乃至几百光年外的地方了。
幸运的是，我们有一种“独一无二的合理办法”，可以与其他智慧生命通信。沃尔特·沙利文在他的佳作《我们并不孤独》中就是这样说的。这种办法利用的是波长二十一厘米所对应的特定无线电频率，即一千四百二十兆赫。这是太空中氢原子发射辐射的自然频率，1951年为我们发现；宇宙中任何一位射电天文学家，都应该知道这个频率。
这种波长一经发现，没过多久，就有人建议把它作为星际通信中能够明确辨认的广播频率。如果没有这样的共同依据，寻找其他行星上的智慧生命，就像想在伦敦见一个朋友，却既未约好地点，又荒唐地在街头乱逛，指望碰巧遇上。
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译文依据整理稿，由本站按中文表达重新校订；解析据吴应时整理资料及首轮审校稿；词伙与练习提示为本站学习设计。原PDF第191—196页，2022年2月第2版。