尋找“外星人”或應(yīng)另辟蹊徑

　　Fifty years ago this week, on April 8, 1960, a little-known astronomer named Frank Drake sat at the controls of a radio telescope at an observatory in Green Bank, W.Va., and began to sweep the skies, looking for a signal from an alien civilization. It was the start of the most ambitious scientific experiment in history。

　　After five decades of patient listening, however, all the astronomers have to show for it is an eerie silence. Does that mean we are alone in the universe after all? Or might we be looking for the wrong thing in the wrong place at the wrong time?

　　If a civilization endures for long enough, it might seek to migrate beyond its planetary system and colonize, or at least explore, the galaxy. The Milky Way is huge—about 100,000 light years across—and contains 400 billion stars, but given enough time, a determined civilization could spread far and wide. Our solar system is about 4.5 billion years old, but the galaxy is much older; there were stars and planets around long before Earth even existed. There has been plenty of time for at least one of those expansionary civilizations to reach our galactic neighborhood—a prospect that once led the physicist Enrico Fermi to famously utter "Where is everybody?"

　　How do we know they haven't been here already?

　　It would be an incredible coincidence if Earth had been visited by aliens during the brief span of human history. On purely statistical grounds any visitation is likely to have been a very long time ago. To pluck a figure out of midair, imagine that an alien expedition passed our way 100 million years ago. Would any traces remain?

　　Not many. However, some remnants might still persist. Buried nuclear waste could be detectable even after billions of years. Large-scale mineral exploitation such as quarrying leaves distinctive scars that, in the case of Earth, would eventually become obscured by overlying strata but would still show up in geological surveys. Space probes parked in orbit round the sun might lie dormant yet intact for an immense period of time. Scientists could look for such hallmarks of alien technology on Earth and the moon, in near space, on Mars and among the asteroids。

　　Another physical object with enormous longevity is DNA. Our bodies contain some genes that have remained little changed in 100 million years. An alien expedition to Earth might have used biotechnology to assist with mineral processing, agriculture or environmental projects. If they modified the genomes of some terrestrial organisms for this purpose, or created their own micro-organisms from scratch, the legacy of this tampering might endure to this day, hidden in the biological record。

　　Which leads to an even more radical proposal. Life on Earth stores genetic information in DNA. A lot of DNA seems to be junk, however. If aliens, or their robotic surrogates, long ago wanted to leave us a message, they need not have used radio waves. They could have uploaded the data into the junk DNA of terrestrial organisms. It would be the modern equivalent of a message in a bottle, with the message being encoded digitally in nucleic acid and the bottle being a living, replicating cell。

　　1960年4月8日，一個(gè)名叫弗蘭克·德雷克的天文學(xué)家在美國西弗吉尼亞州的一家天文臺上，操控著一臺射電望遠(yuǎn)鏡，對著天空搜索，尋找來自地外文明的信息。這是歷史上最雄心勃勃的一項(xiàng)科學(xué)實(shí)驗(yàn)的開始。

　　然而，到如今，經(jīng)過了50年的耐心傾聽，天文學(xué)家展示的卻依然是一片死寂。這是否意味著我們在宇宙中形單影只？或者是我們一直在錯(cuò)誤的時(shí)間、錯(cuò)誤的地點(diǎn)找尋著錯(cuò)誤的目標(biāo)？

　　把注意力放在無線電波信號上，這樣的搜尋是極為有限的。如果一個(gè)文明存在了足夠長的時(shí)間，它也許會設(shè)法走出自己所在的星球，向銀河系開拓。銀河系廣袤無邊，跨度達(dá)10萬光年，擁有4000億顆恒星，不過只要有充足的時(shí)間和堅(jiān)定的決心，一個(gè)文明可以走得很遠(yuǎn)。我們所在的太陽系已經(jīng)存在了約45億年，但銀河系的歷史要早得多；早在地球形成之前，銀河系就已經(jīng)存在了恒星和行星。這樣的可能性曾讓著名的物理學(xué)家費(fèi)米提出那個(gè)著名的問題：“其他人都在哪里？”

　　我們怎么知道它們是否已經(jīng)來過這里？

　　如果外星人曾在地球有過人類存在的這短暫時(shí)期造訪過這里，那將是驚人的巧合。單純從統(tǒng)計(jì)學(xué)上看，這樣的造訪可能出現(xiàn)在很久以前。隨便說個(gè)時(shí)間，就說一支外星人探險(xiǎn)隊(duì)曾在1億年前來過我們這里。還會找到它們留下的蹤跡嗎？

　　不會有很多。不過有些痕跡仍會存在。埋在地下的核廢物就算在幾十億年后依然可以探測得到。大規(guī)模的采礦也會留下痕跡。停在太陽周圍軌道的太空探測器也許依然完好?？茖W(xué)家可以在地球和月球以及周圍區(qū)域?qū)ふ业赝饧夹g(shù)的標(biāo)志。

　　另外一個(gè)可以長久存在的是DNA。我們體內(nèi)的一些基因歷經(jīng)多年，基本沒發(fā)生變化。來地球探險(xiǎn)的外星人也許利用生物技術(shù)來從事礦物處理以及農(nóng)業(yè)或環(huán)境方面的活動。如果它們?yōu)榇烁淖兞岁懙厣锏幕?，或者從無到有地制造了與自己類似的生物體，那么這樣的遺贈也許留存至今，藏在紛繁浩淼的生物記錄中。

　　甚至可以有一個(gè)更加大膽的想法。地球上的生命在DNA中儲存了基因信息。然而許多DNA似乎是沒用的。如果外星人很久以前想給我們留下什么信息，它們可以用陸地生物那些沒用的DNA來承載這些數(shù)據(jù)，就好像是將紙條放入漂流瓶。