Looking for ET: signs of life

HALF a century ago a radio astronomer called Frank Drake thought of a way to calculate the likelihood of establishing contact with aliens. He suggested the following figures should be multiplied: how many stars are formed in the galaxy in a year; what fraction of these have planets and thus form solar systems; the average number of planets per solar system that have the potential to support life; on what percentage of those where it is possible do such biospheres actually form; what percentage of such biospheres give rise to intelligent species; what percentage of intelligent life is able to transmit signals into space; and for how long could such intelligence keeps sending signals.

半个世纪前，一位名叫法兰克•德雷克的无线电天文学家发明了一种计算探寻外星生命可能性的方法。他的方法包括一系列应该纳入考虑范围的数据：银河系每年新诞生的恒星数；拥有行星并组成了星系的恒星比率；有可能会出现生命迹象的行星平均数；以上行星能够真正孕育生命的比率；演化出了智能物种的比率；能使用无线电通讯与外太空联系的智能生物比率；智能生物使用无线电通讯方式的持续时间。更多信息请访问：http://www.24en.com/

This calculation became celebrated as the Drake equation—perhaps the best attempt so far to tame a wild guess. Most of the terms remain hard to tie down, although there is a consensus that about ten stars are formed per year in the galaxy. Also, recent searches for extrasolar planets have concluded that planets are not rare.

这种计算方式便是后来著名的德雷克方程式——这也许是迄今为止对人类漫无边际的想象的最好归纳。尽管银河系每年新诞生约10颗恒星已成为共识，但是公式中还有很多要素没有具体数值。另外，近期对太阳系外行星的研究结论表示，行星数量并不算少。

At the AAAS, Dr Drake reflected on his search for alien signals. One reason this is hard is that radio telescopes must chop the spectrum into fine portions to study it, like tuning into a signal on a car radio. Another is the trade off between a telescope’s field of view and its magnification. Small telescopes see a lot of sky but can detect only strong signals. Large ones, which can detect weak signals, have a narrow focus. Astronomers therefore have difficulty looking both carefully and comprehensively.

作为美国艺术与科学院成员的德雷克博士对他的外星生命研究进行了反思。该项研究进展的困难之一是必须要用无线天文望远镜将频谱切割成精细的部分进行研究，这就像需要把车载收音机调准台一样。另一个则是需要在天文望远镜的视场和放大率之间进行取舍。小型天文望远镜视场更广但是只能探测到较强烈的信号。而大型天文望远镜，虽然可以探测到较微弱的信号，但视场却更窄。因此，天文学家们很难在视场和放大率之间鱼与熊掌兼得。

Dr Drake said there may be another difficulty. Researchers tend to look for signals similar to those now made by humanity. The Earth, though, is getting quieter because the rise of spread-spectrum communication makes stray emissions less likely than in the past.

德雷克博士曾表示，也许还会有其他的困难。研究者们倾向于寻找和人造信号相似的信号。然而，由于扩频通信技术的兴起，杂散发射减少，地球现在变得越来越“安静”了。

Spread-spectrum works by smearing a message across a wide range of frequencies. That has the advantages of combating noise and allowing many signals to be sent at once. But it also makes those signals hard for eavesdroppers to hear (which is why spread-spectrum is beloved by military men). If technologically sophisticated aliens came to the same conclusions, and thus used spread-spectrum technology, humans would have a hard time hearing them. Dr Drake suggests, therefore, that there might be only a narrow window of time in the development of civilisations, analogous to the past 50 years on Earth, during which noisy electromagnetic signals are generated in large amounts.

扩频通信利用宽频带信号传送信息。这一技术的优势在于其抗干扰性和多址信息传送性能，该技术同时还能很好的防止窃听（因此广受军事人员的青睐）。如果外星生物已经在技术上达到了这样的高度，并且也在使用扩频通信技术，那么人类想要捕获到它们发出的信号就得不是那么容易了。因此，德雷克博士表示，在人类文明发展的长河中也许只有在一个非常短暂的时间段里，譬如地球上的过去50年，人类制造了大量的电磁波信号。

It is, however, also possible that someone is actively trying to send signals to the Earth. If that were the case, the best way to do this, reckons Paul Horowitz, a physicist at Harvard, is with a laser.

但是，也可能有外星生物在不断往地球发送信号的。如果真的有，哈佛大学物理学家保罗•霍洛维茨认为，最好的发送方式便是使用激光发射。

Although radio power has changed little over the decades, the power of lasers has grown exponentially. Today’s most powerful versions can shine ten thousand times brighter than the sun, though only for a billionth of a second. If aliens have made similar progress, and point a laser towards the Earth’s solar system, such brief flashes would be detectable at a distance of many light-years. Dr Horowitz has already set up one suitable detector and this, because no huge magnification is involved, is capable of looking at broad swathes of sky.

尽管在过去的几十年里无线电功率几乎不变，但是激光的功率则呈指数增长。如今功率最大的激光机所发出的光瞬间可以比太阳所发出的光亮一万倍，即使维持的时间不超过十亿分之一秒。如果外星生物在技术上已经取得如此进步，并且向太阳系里的地球发射激光信号，那么这些激光信号在一定光年之后便很有可能会被人类探测出来。霍洛维茨博士已经建好了一个合适的探测仪，因为对放大率的要求不高，所以这个探测仪能够观测到较广的太空视场。

There is also potential for improvement on the radio side. For many years, the Arecibo telescope in Puerto Rico, which is 300 metres across, has led the search for alien life. (Sadly, its founder, William Gordon, died on February 16th.) Now the Chinese are building a 500-metre telescope, known as FAST, in Guizhou province, and an international collaboration called the Square Kilometre Array is trying, as its name suggests, to build a grid of radio-telescopes over a square kilometre of land in either South Africa or Australia. Both may be helpful. As indeed may a large new telescope in northern California built by Paul Allen, a co-founder of Microsoft.

在无线电探测方面也可能有所进展。位于波罗黎各的阿雷西博望远镜直径达300米，它在许多年里一直是外太空生命迹象探索和研究的引领者。（但不幸的是，其建造者威廉•戈登于2月16日去世了。）目前，中国也正在贵州省建造一个直径达500米的射电天文望远镜，名为FAST。而另一项名为“平方公里望远镜阵”的国际合作也即将在南非或澳大利亚展开，该项目正如其名，将在一平方公里的区域内修建起一个无线电天文望远镜阵列。这两个项目都可能会推动人类的外太空探索。同样做出贡献的还有微软创始人之一保罗•艾伦在加州北部建造的大型全新天文望远镜。

Many of the terms in the Drake equation are likely to remain elusive, so it is still impossible to predict how likely such efforts are to succeed. But even after 50 fruitless years—if the eagerness in the eyes of Dr Drake and his colleagues is any guide—it still is fun looking.

德雷克方程式中的很多要素依旧扑朔迷离，同样，要准确地预测人类探索的成功几率也不太现实。但是，即使过去的半个世纪里人类并无明显收获——从德雷克博士和他的同事们眼中的那股热忱来看——这样的探索仍然是很有意思的。