cation of when the human species is ready to consider terraforming seriously is when we have put our own world right。 We can consider it a test of the depth of our understanding and our mitment。 The first step in engineering the Solar System is to guarantee the habitability of the Earth。
Then we'll be ready to spread out to asteroids； ets； Mars； the moons of the outer Solar System； and beyond。 Jack Williamson 's prediction that this will begin to e about by the twenty…second century may not be far off the mark。
THE NOTION OF OUR DESCENDANTS living and working on other worlds； and even moving some of them around for their convenience； seems the most extravagant science fiction。 Be realistic； a voice inside my head counsels。 But this is realistic。 We're on the cusp of the technology； near the midpoint between impossible and routine。 It's easy to be conflicted about it。 If we don't do something awful to ourselves in the interim； in another century terraforming may seem no more impossible than a human…tended space station does today。
I think the experience of living on other worlds is bound to change us。 Our descendants； born and raised elsewhere； will naturally begin to owe primary loyalty to the worlds of their birth； whatever affection they retain for the Earth。 Their physical needs； their methods of supplying those needs； their technologies； and their social structures will all have to be different。
A blade of grass is a monplace on Earth； it would be a miracle on Mars。 Our descendants on Mars will know the value of a patch of green。 And if a blade of grass is priceless； what is the value of a human being？ The American revolutionary Tom Paine； in describing his contemporaries； had thoughts along these lines：
The wants which necessarily acpany the cultivation of a wilderness produced among them a state of society which countries long harassed by the quarrels and intrigues of governments had neglected to cherish。 In such a situation man bees what he ought to be。 He sees his species 。 。 。 as kindred。
Having seen at first hand a procession of barren and desolate worlds； it will be natural for our spacefaring descendants to cherish life。 Having learned something from the tenure of our species on Earth； they may wish to apply those lessons to other worlds—to spare generations to e the avoidable suffering that their ancestors were obliged to endure； and to draw upon our experience and our mistakes as we begin our open…ended evolution into space。
CHAPTER 20 DARKNESS
Far away； hidden from the eyes of daylight； there are watchers in the skies。
—EURIPIDES； THE BACCHAE （CA。 406 B。C。）
As children； we fear the dark。 Anything might be out。 here。 The unknown troubles us。 Ironically； it is our fate to live in the dark。 This unexpected finding of science is only about three centuries old。 Head out from the Earth in any direction you choose； and—after an initial flash of blue and a longer wait while the Sun fades—you are surrounded by blackness； punctuated only here and there by the faint and distant stars。
Even after we are grown； the darkness retains its power to frighten us。 And so there are those who say we should not inquire too closely into who else might be living in that darkness。 Better not to know； they say。
There are 400 billion stars in the Milky Way Galaxy。 Of this immense multitude； could it be that our humdrum Sun is the only one with an inhabited planet？ Maybe。 Maybe the origin of life or intelligence is exceedingly improbable。 Or maybe civilizations arise all the time； but wipe themselves out as soon as they are able。
Or； here and there； peppered across space； orbiting other suns； maybe there are worlds something like our own； on which other beings gaze up and wonder as we do about who else lives in the dark。 Could the Milky Way be rippling with life and intelligence—worlds calling out to worlds—while we on Earth are alive at the critical moment when we first decide to listen？
Our species has discovered a way to municate through the dark； to transcend immense distances。 No means of munication is faster or cheaper or reaches out farther。 It's called radio。
After billions of years of biological evolution—on their planet and ours—an alien civilization cannot be in technological lockstep with us。 There have been humans for more than twenty thousand centuries； but we've had radio only for about one century。 If alien civilizations are behind us； they're likely to be too far behind to have radio。 And if they're ahead of us； they're likely to be far ahead of us。 Think of the technical advances on our world over just the last few centuries。 What is for us technologically difficult or impossible； what might seem to us like magic； might for them be trivially easy。 They might use other； very advanced means to municate with their peers； but they would know about radio as an approach to newly emerging civilizations。 Even with no more than our level of technology at the transmitting and receiving ends； we could municate today across much of the Galaxy。 They should be able to do much better。
If they exist。
But our fear of the dark rebels。 The idea of alien beings troubles us。 We conjure up objections：
〃It's too expensive。〃 But； in its fullest modern technological expression； it costs less than one attack helicopter a year。
〃We'll never understand what they're saying。〃 But； because the message …is transmitted by radio； we and they must have radio physics； radio astronomy； and radio technology in mon。 The laws of Nature are the same everywhere； so science itself provides a means and language of munication even between very different kinds of beings—provided they both have science。 Figuring out the message； if we're fortunate enough to receive one； may be much easier than acquiring it。
〃It would be demoralizing to learn that our science is primitive。〃 But by the standards of the next few centuries； at least some of our present science will be considered primitive； extraterrestrials or no extraterrestrials。 （So will some of our present politics； ethics； economics； and religion。） To go beyond present science is one of。 the chief goals of science。 Serious students are not monly plunged into fits of despair on turning the pages of a textbook and discovering that some further topic is known to the author but not yet to the student。 Usually the students struggle a little； acquire the new knowledge； and； following an ancient human tradition； continue to turn the pages。
〃All through history advanced civilizations have ruined civilizations just slightly more backward。〃 Certainly。 But malevolent aliens； should they exist； will not discover our existence from the fact that we listen。 The search programs only receive； they do not send。*
* Surprisingly many people； including New York Times editorialists； are concerned that once extraterrestrials know where we are； they will e here and eat us。 Put aside the profound biological differences that must exist between the hypothetical aliens and ourselves； imagine that we constitute an interstellar gastronomic delicacy。 Why transport large numbers of us to alien restaurants？ The freightage is enormous。 Wouldn't it be better just to steal a few humans； sequence our amino acids or whatever else is the source of our delectability； and then just synthesize the identical food product from scratch？
THE DEBATE IS； for the moment； moot。 We are now； on an unprecedented scale； listening for radio signals from possible other civilizations in the depths of space。 Alive today is the first generation of scientists to interrogate the darkness。 Conceivably it might also be the last generation before contact is made—and this the last moment before we discover that someone in the darkness is calling out to us。
This quest is called the Search for Extraterrestrial Intelligence （SETI）。 Let me describe how far we've e。
The first SETI program was carried out by Frank Drake at the National Radio Astronomy Observatory in Greenbank； West Virginia； in 1960。 He listened to two nearby Sun…like stars for two weeks at one particular frequency。 （〃Nearby〃 is a relative term： The nearest was 12 light…years—70 trillion miles—away。）
Almost at the moment Drake pointed the radio telescope and turned the system on； he picked up a very strong signal。 Was it a message from alien beings？ Then it went away。 If the signal disappears； you can't scrutinize it。 You can't see if； because of the Earth's rotation； it moves with the sky。 If it's not repeatable； you've learned almost nothing from it—it might be terrestrial radio interference； or a failure of your amplifier or detector 。 。 。 or an alien signal。 Unrepeatable data； no matter how illustrious the scientist reporting them； are not worth much。
Weeks later； the signal was detected again。 It turned out to be a military aircraft broadcasting on an unauthorized frequency。 Drake reported negative results。 But in science a negative result is not at all the same thing as a failure。 His great achievement was to show that modern technology is fully able to listen for signals from hypothetical civilizations on the planets of other stars。
Since then there've been a number of attempts； often on time borrowed from other radio telescope observing programs； and almost never for longer than a few months。 There've been some more false alarms； at Ohio State； in Arecibo； Puerto Rico； in France； Russia； and elsewhere； but nothing that could pass muster with the world scientific munity。
Meanwhile； the technology for detection has been getting cheaper； the sensitivity keeps improving； the scientific respectability of SETI has continued to grow； and even NASA and Congress have bee a little less afraid to support it。 Diverse； plementary search strategies are possible and necessary。 It was clear years ago that if the trend continued； the technology for a prehensive SETI effort would eventually fall within the reach even of private organizations （or wealthy individuals）； and sooner or later； the government would be willing to support a major program。 After 30 years of work； for some of us it's been later rather than sooner。 But at last the time has e。
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