雙語：尋找外星人或應另闢蹊徑(圖)

尋找「外星人」或應另闢蹊徑

　　Fifty years ago this week, on April 8, 1960, a little-knownastronomer named Frank Drake sat at the controls of a radio telescope atan 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 themost ambitious scientific experiment in history。

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

　　If a civilization endures for long enough, it might seek to migratebeyond its planetary system and colonize, or at least explore, thegalaxy. The Milky Way is huge—about 100,000 light years across—andcontains 400 billion stars, but given enough time, a determinedcivilization could spread far and wide. Our solar system is about 4.5billion years old, but the galaxy is much older; there were stars andplanets around long before Earth even existed. There has been plenty oftime for at least one of those expansionary civilizations to reach ourgalactic neighborhood—a prospect that once led the physicist EnricoFermi 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 byaliens during the brief span of human history. On purely statisticalgrounds any visitation is likely to have been a very long time ago. Topluck a figure out of midair, imagine that an alien expedition passedour way 100 million years ago. Would any traces remain?

　　Not many. However, some remnants might still persist. Buriednuclear waste could be detectable even after billions of years.Large-scale mineral exploitation such as quarrying leaves distinctivescars that, in the case of Earth, would eventually become obscured byoverlying strata but would still show up in geological surveys. Spaceprobes parked in orbit round the sun might lie dormant yet intact for animmense period of time. Scientists could look for such hallmarks ofalien technology on Earth and the moon, in near space, on Mars and amongthe asteroids。

　　Another physical object with enormous longevity is DNA. Our bodiescontain some genes that have remained little changed in 100 millionyears. An alien expedition to Earth might have used biotechnology toassist with mineral processing, agriculture or environmental projects.If they modified the genomes of some terrestrial organisms for thispurpose, or created their own micro-organisms from scratch, the legacyof this tampering might endure to this day, hidden in the biologicalrecord。

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

　　1960年4月8日，一個名叫弗蘭克·德雷克的天文學家在美國西弗吉尼亞州的一家天文台上，操控著一台射電望遠鏡，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科學實驗的開始。

　　然而，到如今，經過了50年的耐心傾聽，天文學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隻？或者是我們一直在錯誤的時間、錯誤的地點找尋著錯誤的目標？

　　把注意力放在無線電波信號上，這樣的搜尋是極為有限的。如果一個文明存在了足夠長的時間，它也許會設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10萬光年，擁有4000億顆恆星，不過只要有充足的時間和堅定的決心，一個文明可以走得很遠。我們所在的太陽系已經存在了約45億年，但銀河系的歷史要早得多；早在地球形成之前，銀河系就已經存在了恆星和行星。這樣的可能性曾讓著名的物理學家費米提出那個著名的問題：「其他人都在哪裡？」

　　我們怎麼知道它們是否已經來過這裡？

　　如果外星人曾在地球有過人類存在的這短暫時期造訪過這裡，那將是驚人的巧合。單純從統計學上看，這樣的造訪可能出現在很久以前。隨便說個時間，就說一支外星人探險隊曾在1億年前來過我們這裡。還會找到它們留下的蹤跡嗎？

　　不會有很多。不過有些痕迹仍會存在。埋在地下的核廢物就算在幾十億年後依然可以探測得到。大規模的採礦也會留下痕迹。停在太陽周圍軌道的太空探測器也許依然完好。科學家可以在地球和月球以及周圍區域尋找地外技術的標誌。

　　另外一個可以長久存在的是DNA。我們體內的一些基因歷經多年，基本沒發生變化。來地球探險的外星人也許利用生物技術來從事礦物處理以及農業或環境方面的活動。如果它們為此改變了陸地生物的基因，或者從無到有地製造了與自己類似的生物體，那麼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麼信息，它們可以用陸地生物那些沒用的DNA來承載這些數據，就好像是將紙條放入漂流瓶。

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