按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
ppy。 It's the kind of life that forged us。 So after a brief； only partially successful sedentary experiment； we may bee wanderers again…more technological than last time； but even then our technology； stone tools and fire； was our only hedge against extinction。
If safety lies in isolation and remoteness； then some of our descendants will eventually emigrate to the outer ets of the Oort Cloud。 With a trillion etary nuclei； each separated from the next by about as much as Mars is from Earth； there will be a great deal to do out there。*
* Even if we are not in any particular hurry； we may be able by then to make small worlds move faster than we can make spacecraft move today。 If so； our descendants will eventually overtake the two Voyager spacecraft—launched in the remote twentieth century—before they leave the Oort Cloud； before they make for interstellar space。 Perhaps they will retrieve these derelict ships of long ago。 Or perhaps they will permit them to sail on。
The outer edge of the Sun's Oort Cloud is perhaps halfway to the nearest star。 Not every other star has an Oort Cloud； but many probably do。 As the Sun passes nearby stars； our Oort Cloud will encounter； and partially pass through； other et clouds； like two swarms of gnats interpenetrating but not colliding。 To occupy a et of another star will then be not much more difficult than to occupy one of our own。 From the frontiers of some other solar system the children of the blue dot may peer longingly at the moving points of light denoting substantial （and well…lit） planets。 Some munities—feeling the ancient human love for oceans and sunlight stirring within them—may begin the long journey down to the bright； warm； and clement planets of a new sun。
Other munities may consider this last strategy a weakness。 Planets are associated with natural catastrophes。 Planets may have pre…existing life and intelligence。 Planets are easy for other beings to find。 Better to remain in the darkness。 Better to spread ourselves among many 〃small and obscure worlds。 Better to stay hidden。
ONCE WE CAN SEND our machines and ourselves far from home； far from the planets—once we really enter the theater of the Universe—we are bound to e upon phenomena unlike anything we've ever encountered。 Here are three possible examples：
First： Starting some 550 astronomical units （AU） out—about ten times farther from the Sun than Jupiter； and therefore much more accessible than the Oort Cloud—there's something extraordinary。 Just as an ordinary lens focuses far…off images； so does gravity。 （Gravitational lensing by distant stars and galaxies is now being detected。） Five hundred fifty AU from the Sun—only a year away if we could travel at 1 percent the speed of light—is where the focus begins （although when effects of the solar corona； the halo of ionized gas surrounding the Sun； are taken into account； the focus may be considerably farther out）。 There； distant radio signals are enormously enhanced； amplifying whispers。 The magnification of distant images would allow us （with a modest radio telescope） to resolve a continent at the distance of the nearest star and the inner Solar System at the distance of the nearest spiral galaxy。 If you are free to roam an imaginary spherical shell at the appropriate focal distance and centered on the Sun； you are free to explore the Universe in stupendous magnification； to peer at it with unprecedented clarity； to eavesdrop on the radio signals of distant civilizations； if any； and to glimpse the earliest events in the history of the Universe。 Alternatively； the lens could be used the other way； to amplify a very modest signal of ours so it could be heard over immense distances。 There are reasons that draw us to hundreds and thousands of AU。 Other civilizations will have their own regions of gravitational focusing； depending on the mass and radius of their star； some a little closer； some a little farther away than ours。 Gravitational lensing may serve as a mon inducement for civilizations to explore the regions just beyond the planetary parts of their solar systems。
Second： Spend a moment thinking about brown dwarfs； hypothetical very low temperature stars； considerably more massive than Jupiter； but considerably less massive than the Sun。 Nobody knows if brown dwarfs exist。 Some experts； using nearer stars as gravitational lenses to detect the presence of more distant ones； claim to have found evidence of brown dwarfs。 From the tiny fraction of the whole sky that has so far been observed by this technique； an enormous number of brown dwarfs is inferred。 Others disagree。 In the 1950s； it was suggested by the astronomer Harlow Shapley of Harvard that brown dwarfs—he called them 〃Lilliputian stars〃—were inhabited。 He pictured their surfaces as warm as a June day in Cambridge； with lots of area。 They would be stars that humans could survive on and explore。
Third： The physicists B。 J。 Carr and Stephen Hawking of Cambridge University have shown that fluctuations in the density of matter in the earliest stages of the Universe could have generated a wide variety of small black holes。 Primordial black holes—if they exist—must decay by emitting radiation to space； a consequence of the laws of quantum mechanics。 The less massive the black hole； the faster it dissipates。 Any primordial black hole in the final stages of decay today would have to weigh about as much as a mountain。 All the smaller ones are gone。 Since the abundance—to say nothing of the existence—of primordial black holes depends on what happened in the earliest moments after the Big Bang； no one can be sure that there are any to be found； we certainly can't be sure that any lie nearby。 Not very restrictive upper limits on their abundance have been set by the failure so far to find short gamma ray pulses； a ponent of the Hawking radiation。
In a separate study； G。 E。 Brown of Caltech and the pioneering nuclear physicist Hans Bethe of Cornell suggest that about a billion non…primordial black holes are strewn through the Galaxy； generated in the evolution of stars。 If so； the nearest may be only 10 or 20 lightyears away。
If there are black holes within reach—whether they'