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  3. Half a century ago a radio astronomer called Frank Drake thought of a way to calculate the likelihood of establishing contact wi

Half a century ago a radio astronomer called Frank Drake thought of a way to calculate the likelihood of establishing contact wi

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问题     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.
    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.
    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.
    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.
    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 civilizations, analogous to the past 50 years on Earth, during which noisy electromagnetic signals are generated in large amounts.
    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.
    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.
    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. Now the Chinese are building a 500-metre telescope, known as FAST, in Guizhou province, and an international collaboration called the Square Kilometer Array is trying, as its name suggests, to build a grid of radio-telescopes over a square kilometer 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.
    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.
What difficulties are there in scientists’ search for alien lives through radio waves?
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答案The difficulties for scientists to search for alien lives through radio waves are as follows．Firstly，radio telescopes must chop the spectrum into fine portions to study it，like tuning into a signalon a car radio． Secondly，the trade off between a telescope’s field of view and its magnificationneeds to be achieved．Thirdly，the rise of spread-spectrum communication makes those signalshard to hear．
解析 (第三段介绍了使用无线电波探测外星生物面临的其中两个困难。第四段介绍了第三个困难，并且在第五段对此困难进行了详细说明。)
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