我们必须从那些研究过生命起源的学者的研究成果中得出结论：只要存在一颗与我们地球大体相似的行星，生命就几乎必然会出现。
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.
在我们太阳系的所有行星中，我们现在已相当确定地球是唯一一颗生命能够存活的星球。
Of all the planets in our solar system, we are now pretty certain the Earth is the only one on which life can survive.
火星太过干燥且缺乏氧气，金星温度极高，水星亦然，而外侧行星的温度接近绝对零度，大气层以氢气为主。
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.
但其他太阳——天文学家称之为"恒星"——势必拥有与我们地球相似的行星；而由于宇宙中恒星数量如此庞大，这种可能性已近乎成为必然。
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.
仅在我们自己的银河系中就有一千亿颗恒星，而宇宙中还有三十亿个其他银河系（即星系）。
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.
因此，我们已知存在的恒星数量现已估计约为 3 亿亿亿颗。
So the number of stars that we know exist is now estimated at about 300 million million million.
尽管在某处已经萌生的生命中，也许只有 1% 会演化为高度复杂的智慧形态，但行星数量如此之多，智慧生命势必将成为宇宙中自然存在的一部分。
Although perhaps only 1 percent 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.
那么，如果我们已如此确信宇宙中存在其他智慧生命，为何我们至今尚未有来自外太空的访客呢？
If then we are so certain that other intelligent life exists in the universe, why have we had no visitors from outer space yet?
首先，他们可能在数千乃至数百万年前就曾到访过我们这颗星球，但发现我们当时盛行的原始状态，对于他们自身先进的知识体系而言，毫无趣味。
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.
美国著名射电天文学家罗纳德·布雷斯韦尔教授在《自然》杂志上论证道：如此一个高等文明，在造访我们太阳系时，可能已留下了一个自动信使，以等待某个先进文明可能出现的觉醒时刻。
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.
这样的信使一旦接收到我们的无线电和电视信号，很可能会将其转发回其母星——尽管其他文明由此对我们所形成的印象，最好还是不说为妙。
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.
但在此处，我们遭遇了与其他行星上的人们建立联系的所有障碍中最为艰巨的一个——将我们彼此隔绝的天文学尺度的距离。
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.
作为一个合理的估计，这些（智慧生命所在的行星）平均而言可能相距我们约100光年之远。
As a reasonable guess, they might, on an average, be 100 light years away.
（一光年是光以每秒 18.6 万英里的速度在一年内所行进的距离，即 6 万亿英里。）
(A light year is the distance which light travels at 186,000 miles per second in one year, namely 6 million million miles.)
无线电波同样以光速传播；假设这样一个自动信使接收到了我们20世纪20年代的最初广播，那么传往其母星的信息如今才刚走完一半路程。
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.
同样，我们目前原始的化学火箭，尽管足以将人类送入轨道，却根本没有可能将我们运送到最近的另一颗恒星——距离四光年之遥——更不用说数十乃至数百光年的距离了。
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.
幸运的是，存在一种让我们与其他智慧生命沟通的"唯一理性途径"——正如沃尔特·沙利文在其出色的著作《我们并不孤独》中所阐述的。
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.
这取决于 21 厘米波长的精确无线电频率，即每秒 1420 兆周。
This depends on the precise radio frequency of the 21-cm wavelength, or 1420 megacycles per second.
它是宇宙中氢原子的自然辐射频率，并于1951年被我们发现；宇宙中任何类型的射电天文学家都必然知晓这一频率。
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.
一旦这一波长的存在被发现，不久之后便有人提出将其作为星际通信中唯一可辨识的广播频率加以使用。
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.
若没有这样的东西，在其他行星上寻找智慧生命，就如同在没有预先约好地点的情况下试图在伦敦与朋友相遇，荒唐地在街头漫游，寄望于偶然相逢。
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.