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，一个名叫弗兰克·德雷克的天文学家在美国西弗吉尼亚州的一家天文台上，操控着一台射电望远镜，对着天空搜索，寻找来自地外文明的信息。这是历史上最雄心勃勃的一项科学实验的开始。

　　然而，到如今，经过了50年的耐心倾听，天文学家展示的却依然是一片死寂。这是否意味着我们在宇宙中形单影只？或者是我们一直在错误的时间、错误的地点找寻着错误的目标？

　　把注意力放在无线电波信号上，这样的搜寻是极为有限的。如果一个文明存在了足够长的时间，它也许会设法走出自己所在的星球，向银河系开拓。银河系广袤无边，跨度达10万光年，拥有4000亿颗恒星，不过只要有充足的时间和坚定的决心，一个文明可以走得很远。我们所在的太阳系已经存在了约45亿年，但银河系的历史要早得多；早在地球形成之前，银河系就已经存在了恒星和行星。这样的可能性曾让著名的物理学家费米提出那个著名的问题：“其他人都在哪里？”

　　我们怎么知道它们是否已经来过这里？

　　如果外星人曾在地球有过人类存在的这短暂时期造访过这里，那将是惊人的巧合。单纯从统计学上看，这样的造访可能出现在很久以前。随便说个时间，就说一支外星人探险队曾在1亿年前来过我们这里。还会找到它们留下的踪迹吗？

　　不会有很多。不过有些痕迹仍会存在。埋在地下的核废物就算在几十亿年后依然可以探测得到。大规模的采矿也会留下痕迹。停在太阳周围轨道的太空探测器也许依然完好。科学家可以在地球和月球以及周围区域寻找地外技术的标志。

　　另外一个可以长久存在的是DNA。我们体内的一些基因历经多年，基本没发生变化。来地球探险的外星人也许利用生物技术来从事矿物处理以及农业或环境方面的活动。如果它们为此改变了陆地生物的基因，或者从无到有地制造了与自己类似的生物体，那么这样的遗赠也许留存至今，藏在纷繁浩淼的生物记录中。

　　甚至可以有一个更加大胆的想法。地球上的生命在DNA中储存了基因信息。然而许多DNA似乎是没用的。如果外星人很久以前想给我们留下什么信息，它们可以用陆地生物那些没用的DNA来承载这些数据，就好像是将纸条放入漂流瓶。