For me； no signals； no one calling out to us is a depressing prospect。 〃plete silence；〃 said Jean…Jacques Rousseau in a different context； 〃induces melancholy； it is an image of death。〃 But I'm with Henry David Thoreau： 〃Why should I feel lonely？ Is not our planet in the Milky Way？〃
The realization that such beings exist and that； as the evolutionary process requires； they must be very different from us； would have a striking implication： Whatever differences divide us down here on Earth are trivial pared to the differences between any of us and any of them。 Maybe it's a long shot； but the discovery of extraterrestrial intelligence might play a role in unifying our squabbling and divided planet。 It would be the last of the Great Demotions； a rite of passage for our species and a transforming event in the ancient quest to discover our place in the Universe。
In our fascination with SETI； we might be tempted； even without good evidence； to succumb to belief but this would be self…indulgent and foolish。 We must surrender our skepticism only in the face of rock…solid evidence。 Science demands a tolerance for ambiguity。 Where we are ignorant； we withhold belief。 Whatever annoyance the uncertainty engenders serves a higher purpose： It drives us to accumulate better data。 This attitude is the difference between science and so much else。 Science offers little in the way of cheap thrills。 The standards of evidence are strict。 But when followed they allow us to see far； illuminating even a great darkness。
CHAPTER 21 TO THE SKY！
The stairs of the sky are let down for him that he may ascend thereon to heaven。 O gods； put your arms under the king： raise him； lift him to the sky。 To the sky！ To the sky！
—HYMN FOR A DEAD PHARAOH （EGYPT； CA。 2600 B。C。）
When my grandparents were children； the electric light； the automobile； the airplane； and the radio were Stupefying technological advances； the wonders of the age。 You might hear wild stories about them； but you could not find a single exemplar in that little village in Austria…Hungary； near the banks of the river Bug。 But in that same time； around the turn of the last century； there were two men who foresaw other； far more ambitious； inventions—Konstantin Tsiolkovsky； the theoretician； a nearly deaf schoolteacher in the obscure Russian town of Kaluga； and Robert Goddard； the engineer； a professor at an equally obscure American college in Massachusetts。 They dreamt of using rockets to journey to the planets and the stars。 Step by step； they worked out the fundamental physics and many of the details。 Gradually； their machines took shape。 Ultimately； their dream proved infectious。
In their time； the very idea was considered disreputable； or even a symptom of some obscure derangement。 Goddard found that merely mentioning a voyage to other worlds subjected him to ridicule； and he dared not publish or even discuss in public his long…term vision of flights to the stars。 As teenagers； both had epiphanal visions of spaceflight that never left them。 〃I still have dreams in which I fly up to the stars in my machine；〃 Tsiolkovsky wrote in middle age。 〃It is difficult to work all on your own for many years； in adverse conditions without a gleam of hope； without any help。〃 Many of his contemporaries thought he was truly mad。 Those who knew physics better than Tsiolkovsky and Goddard—including The New York Times in a dismissive editorial not retracted until the eve of Apollo 11—insisted that rockets could not work in a vacuum； that the Moon and the planets were forever beyond human reach。
A generation later； inspired by Tsiolkovsky and Goddard； Wernher von Braun was constructing the first rocket capable of reaching the edge of space； the V…2。 But in one of those ironies with which the twentieth century is replete； von Braun was building it for the Nazis—as an instrument of indiscriminate slaughter of civilians； as a 〃vengeance weapon〃 for Hitler； the rocket factories staffed with slave labor； untold human suffering exacted in the construction of every booster； and von Braun himself made an officer in the SS。 He was aiming at the Moon； he joked unselfconsciously； but hit London instead。
Another generation later； building on the work of Tsiolkovsky and Goddard； extending von Braun's technological genius； we were up there in space； silently circumnavigating the Earth； treading the ancient and desolate lunar surface。 Our machines—increasingly petent and autonomous—were spreading through the Solar System； discovering new worlds； examining them closely； searching for life； paring them with Earth。
This is one reason that in the long astronomical perspective there is something truly epochal about 〃now〃—which we can define as the few centuries centered on the year you're reading this book。 And there's a second reason： This is the first moment in the history of our planet when any species； by its own voluntary actions； has bee a danger to itself—as well as to vast numbers of others。 Let me recount the ways：
·We've been burning fossil fuels for hundreds of thousands of years。 By the 1960s； there were so many of us burning wood； coal； oil； and natural gas on so large a scale； that scientists began to worry about the increasing greenhouse effect； the dangers of global warming began slowly slipping into public consciousness。
·CFCs were invented in the 1920s and 1930s； in 1974 they were discovered to attack the protective ozone layer。 Fifteen years later a worldwide ban on their production was going into effect。
·Nuclear weapons were invented in 1945。 It took until 1983 before the global consequences of thermonuclear war were understood。 By 1992； large numbers of warheads were being dismantled。
·The first asteroid was discovered in 1801。 More or less serious proposals to move them around were floated beginning in the 1980s。 Recognition of the potential dangers of asteroid deflection technology followed shortly after。
·Biological warfare has been with us for centuries； but its deadly mating with molecular biology has occurred only lately。
·We humans have already precipitated extinctions of species on a scale unprecedented since the end of the Cretaceous Period。 But only in the last decade has the magnitude of these extinctions bee clear； and the possibility raised that in our ignorance of the interrelations of life on Earth we may be endangering our own future。
Look at the dates on this list and consider the range of new technologies currently under development。 Is it not likely that other dangers of our own making are yet to be discovered； some perhaps even more serious？
In the littered field of discredited self…congratulatory chauvinisms； there is only one that seems to hold up； one sense in which we are special： Due to our own actions or inactions； and the misuse of our technology； we live at an extraordinary moment； for the Earth at least—the first time that a species has bee able to wipe itself out。 But this is also； we may note； the first time that a species has bee able to journey to the planets and the stars。 The two times； brought about by the same technology； coincide—a few centuries in the history of a 4。5…billion…year…old planet。 If you were somehow dropped down on the Earth randomly at any moment in the past （or future）； the chance of arriving at this critical moment would be less than 1 in 10 million。 Our leverage on the future is high just now。
It might be a familiar progression； transpiring on many worlds—a planet； newly formed； placidly revolves around its star； life slowly forms； a kaleidoscopic procession of creatures evolves； intelligence emerges which； at least up to a point； confers enormous survival value； and then technology is invented。 It dawns on them that there are such things as laws of Nature； that these laws can be revealed by experiment； and that knowledge of these laws can be made both to save and to take lives； both on unprecedented scales。 Science； they recognize； grants immense powers。 In a flash； they create world…altering contrivances。 Some planetary civilizations see their way through； place limits on what may and what must not be done； and safely pass through the time of perils。 Others are not so lucky or so prudent； perish。
Since； in the long run； every planetary society will be endangered by impacts from space； every surviving civilization is obliged to bee spacefaring—…not because of exploratory or romantic zeal； but for the most practical reason imaginable： staying alive。 And once you're out there in space for centuries and millennia； moving little worlds around and engineering planets； your species has been pried loose from its cradle。 If they exist； many other civilizations will eventually venture far from home。*
* Might a planetary civilization which has survived its adolescence wish to encourage others struggling with their emerging technologies？ Perhaps they would make special efforts to broadcast news of their existence； the triumphant announcement that it's possible to avoid self…annihilation。 Or would they at first be very cautious？ Having avoided catastrophes of their own making； perhaps they would fear giving away knowledge of their existence； lest some other； unknown； aggrandizing civilization out there in the dark is looking for Lebensraum or slavering to put down the potential petition。 That might be a reason for us to explore neighboring star systems； but discreetly。
Maybe they would be silent for another reason： because broadcasting the existence of an advanced civilization might encourage emerging civilizations to do less than their best efforts to safeguard their future— hoping instead that someone will e out of the dark and save them from themselves。
A MEANS HAS BEEN OFFERED of estimating how precarious our circumstances are—remarkably； without in any way addressing the nature of the hazards。 J。 Richard Gott III is an astrophysicist at Princeton University。 He asks us to adopt a generalized Copernican principle； something I've described elsewhere as the Principle of Mediocrity。 Chances are that we do not live in a truly extraordinary time。 Hardly anyone ever did。 The probability is high that we're born； live out our days； and die somewhere in the broad middle range of the lifetime of our species （or civilization； or nation）。 Almost certainly； Gott says； we do not live in first or last times。 So if your species is very young； it follows that it's unlikely to last long—because if it were to last long； you （and the rest of us alive today） would be extraordinary in living； proportionally speaking； so near the beginning。
What then is the projected longevity of our species？ Gott concludes； at the 97。5 percent confidence level； that there will be humans for no more than 8 million years。 That's his upper limit； about the same as the average lifetime of many mammalian species。 In that case； our technology neither harms nor helps。 But Gott's lower limit； with the same claimed reliability； is only 12 years。 He will not give you 40…to…1 odds that humans will still be around by the time babies now alive bee teenagers。 In everyday life we try very hard not to take risks so large； not to board airplanes； say； with 1 chance in 40 of crashing。 We will agree to surgery in which 95 perc