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  IN 1990,A year into the journey to Jupiter of an American spacecraft called Galileo,Carl Sagan,a well-known astronomer,turned the probe’s instruments back towards Earth.He wanted to find out whether it was possible to detect evidence of life on the planet from a distance.


  Galileo took spectrographic measurements of sunlight streaming through Earth’s atmosphere and found methane and oxygen,both indicators of living processes.The probe also took photographs of Earth at different wavelengths and uncovered something called the“red edge”—a sharp change in the reflectance of the planet at red wavelengths,which Sagan ascribed to the presence of photosynthetic plant life on the surface.


  There was,however,a third clue—and an indicator that life not only exists on Earth,but has also developed intelligence.This came from the narrowband electromagnetic radiation that was streaming from Earth’s surface—in other words television and radio channels leaking into space.“That,as far as we know,is an unmistakable indicator of technology and an unmistakable indicator of life,”says Andrew Siemion,an astrophysicist at the Berkeley SETI Research Centre in California.“And indeed,it is the most detectable signature of life on this planet as viewed from a distant vantage.”Sagan’s experiment thus confirmed what those engaged in the search for extraterrestrial intelligence(which is what SETI stands for)had suspected—that the best way to find aliens is to look for unnatural radio signals.

  然而,还有第三条线索——它表明生命不仅存在于地球上,而且还发展了智能。这来自于从地球表面流出的窄带电磁辐射,换句话说,是电视和无线电频道泄漏到太空中。加州伯克利SETI研究中心的天体物理学家Andrew Siemion说:“就我们所知,这是一个明确无误的技术指标,也是一个明确无误的生命指标。”的确,从遥远的角度看,这是地球上最容易探测到的生命迹象。因此,萨根的实验证实了那些搜寻地外文明(SETI的缩写)的人的猜测——寻找外星人的最佳方法就是寻找非自然的无线电信号。

  That search is accelerating.As Dr Siemion told this year’s meeting of the American Association for the Advancement of Science(AAAS),in Seattle,better telescopes,faster computers,the discovery of thousands of planets circling stars other than the sun,and an influx of money and scientific talent are transforming the field.What was once a fringe activity has now become mainstream.The next decade,he reckons,will allow astronomers to improve their search for signals from outer space a thousandfold.


  SETI began 60 years ago with Project Ozma.This was led by Frank Drake,an astronomer at Cornell University.It was an attempt to listen for radio waves that might be coming from intelligent life in the vicinity of two nearby sun-like stars,Tau Ceti and Epsilon Eridani.Further projects along this line followed,but no little green men were detected.By the turn of the 21st century people were starting to lose interest and financial support for the endeavour was dwindling.

  SETI始于60年前的奥兹玛计划。这项研究由康奈尔大学的天文学家弗兰克·德雷克领导。这是一次试图监听来自附近两颗类似太阳的恒星Tau Ceti和Epsilon Eridani附近的智能生命的无线电波的尝试。沿着这条路线又有了新的项目,但没有发现小绿人。到了21世纪之交,人们开始对这项努力失去兴趣,对它的财政支持也在减少。

  The tide turned,though,in 2009 with the launch of Kepler,an American probe designed to look for planets orbiting other stars.It found them.In spades.Astronomers have now confirmed the existence of more than 4,000 such exoplanets.Moreover,their models suggest that virtually every star has a planetary system.With so many potential habitats,and more being discovered every month,the prospects of finding alien life suddenly brightened.
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  They will brighten further with the launch,in 2021,of the James Webb Space Telescope and the opening of new observatories on the ground,such as the Extremely Large Telescope(ELT)in Chile.These will be able to repeat,for some nearby exoplanets,the first of Sagan’s experiments with Galileo—the search for biosignatures.But,even if they find them,exciting as that would be,it would say nothing about the crucial final letter in SETI,the“I”for intelligence.


  The search for intelligence was boosted in 2015 with the launch of Breakthrough Listen,a project paid for by Yuri Milner,a software billionaire with a background in physics.This enterprise,on which Dr Siemion is chief scientist,is planned to last ten years and expected to cost$100m.It is intended to survey the 1m nearest stars in detail,scan the entire plane of Earth’s home galaxy,the Milky Way,and also listen for messages from the 100 closest galaxies.It is paying for astronomers interested in SETI to have time on more than a dozen big radio observatories around the world,and it collects in a day an amount of data that used previously to take a year to gather.


  Nor is Breakthrough Listen alone.On February 13th the SETI Institute,a not-for-profit organisation in California,and America’s National Radio Astronomy Observatory,announced a collaboration.The observatory operates the Very Large Array(VLA),a radio telescope in New Mexico that features in the film“Contact”.The collaboration allows SETI researchers to tap the entire stream of data the VLA records as it carries out its routine experiments.

  突破不仅仅是听。2月13日,位于加州的非营利组织SETI研究所和美国国家射电天文台宣布了一项合作计划。该天文台运行着甚大阵(VLA),这是一架位于新墨西哥州的射电望远镜,电影《接触》中使用了它。这项合作使得SETI的研究人员可以在VLA进行常规实验时,利用VLA记录的整个数据流。 纽约时报中英文网 http://www.qqenglish.com

  The partnership will become even more useful for SETI researchers if the VLA gets a proposed upgrade that would greatly enhance its sensitivity.This next-generation instrument would be good enough to detect electromagnetic radiation leaking from planets,similar to the television and radio channels that Sagan measured leaking from Earth.



  Nor,despite the importance of radio waves,will future SETI searches stick to that part of the spectrum.Alien civilisations might,for example,use laser flashes as optical beacons to signal their presence.If a nanosecond-long pulse of such light were pointed in the direction of Earth,it could be made energetic enough to seem brighter to human instruments than any of the stars in the same part of the sky.


  Moreover,as observatories get better,astronomers will also be able to spot small,potentially pertinent features in and around exoplanets.These might be rings of satellites above a planet’s equator—or even structures surrounding a planetary system’s star,put there to capture that star’s light as a source of energy.



  Even a single space station,were it big enough,might be detectable using methods similar to those employed to spot exoplanets.A space station the size of Earth’s moon(admittedly,a huge space station)would stop enough of the light of its star from reaching Earth to produce a perceptible dip in that light.This“transit”method is how Kepler spotted its prey and,as instruments improve,should one day be sensitive enough to spot exomoons too.The way to tell the difference between a moon and a moon-sized space station,says Dr Siemion,would be that the former would orbit regularly whereas the latter might be able to move around at will.


  More standard searches for chemical biosignatures could also be useful for SETI.A system of particular interest is Trappist-1,40 light-years from Earth,where seven rocky worlds orbit a red dwarf.Three of these are within the star’s“habitable zone”—a region where planets could have liquid water on their surfaces.


  The next generation of instruments will reveal whether the planets in Trappist-1 have atmospheres or oceans.If there are atmospheres,they will also reveal what gases are within them—possibly showing biosignatures of the sort Sagan spotted on Earth.


  Meanwhile,the Breakthrough Listen dataset continues to grow.In a study discussed in Seattle,a team of astronomers used it to look for radio transmissions from 20 stars in the so-called“Earth transit zone”—the part of the sky from which Earth itself would be detectable by the transit method.The hypothesis is that,if intelligent beings do exist in that region,they might thus have discovered Earth and be aiming transmissions at it.So far,the skies have been quiet.But SETI researchers are patient people.And they live in hope.



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