Lesson54
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 own solar system we arc 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 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 recent 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
参考译文
我们从生命起源研究者的成果中必然会得出以下结论：已知行星只要近似于地球，生命就几乎肯定会开始。我们现已相当肯定太阳系诸行星中只有在地球上生命才能生存。火星太干燥且缺少氧气，金星实在太热，水星也一样，外圈行星的
湿度接近绝对零度且大气主要由氢气组成。但天文学家称之为恒星的其他太阳肯定有像我们的太阳那样的行星，而且由于宇宙中恒星数目巨大，这种可能性实际上已成为确定性，单是我们的银河系就有1千亿颗恒星，而宇宙中还有30亿个别的
银河系（星系）。因此我们现在知道恒星的数目估计约为3万亿亿颗。
虽然开始形成生命处可能只有百分之一会发展成高度复杂的和智能的模式，但由于行星数目极多，因此智能生命肯定是宇宙的一个天然组成部分。
如果我们对宇宙中有别的智能生命已这么肯定，那又为什么至今尚没有来自外空间的拜访者呢？首先，他们可能于千百万年之前就来过地球，发现那时普遍的原始状态完全不能对他们自己的先进知识产生兴趣。杰出的美国射电天文学Ronald Bracewell
教授在《自然》杂志上发表文章，认为访问过我们太阳系的高级文明可能留下了一种自动信使以等待先进文明的可能觉醒。这种信使接收到我们的无线电和电视信号时很可能将其转播回它的母行星，虽然最好不要谈论别的文明会由此得到什么印象。
可是在这里我们遇上了同其他行星上的人们进行联系时最难以克服的障碍——隔开我们的天文学距离。作为合理的猜测，他们可能平均远在100光年以外（一光年指速度为每秒186000英里的光传播一年所通过的距离，即6万亿英里）。无线电波
传播速度同光带一样，假设这种信使接收到我们于本世纪20年代的初次广播，则送往其母行星的信息现在仅走过一半路程。类似地，我们自己目前的原始化学火箭虽然已能把人送入地球轨道，却根本无法将我们送到4光年以外的最近的恒星上去，更
不用说百十光年以上的距离了。
幸运的是，我们有同其他智能生物通讯的某种“独特的合理途径”，正如Walter Sullivan在他杰出的新作《我们并不孤单》里提出的那样。这种通讯依靠的是21厘米波段（或1420兆周）的精确无线电频率。它是人们于1951年发现的太空氢原子
的天然发射频率；宇宙间任何一种射电天文学家都必定知道它。
一旦发现这种波长的存在，不久之后就有人提出可把它用为星际通讯的可辨认的广播频率。要是没有这一类手段，那么，寻找其他行星上的智能就好像不事先约定而想在伦敦会友，并可笑地在街上徘徊以期偶尔一遇。
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