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but let’s pretend again that we have made it to the oort cloud。 the first thing you mightnotice is how very peaceful it is out here。 we’re a long way from anywhere now—so far fromour own sun that it’s not even the brightest star in the sky。 it is a remarkable thought that thatdistant tiny twinkle has enough gravity to hold all these ets in orbit。 it’s not a very strongbond； so the ets drift in a stately manner； moving at only about 220 miles an hour。 fromtime to time some of these lonely ets are nudged out of their normal orbit by some slightgravitational perturbation—a passing star perhaps。 sometimes they are ejected into theemptiness of space； never to be seen again； but sometimes they fall into a long orbit aroundthe sun。 about three or four of these a year； known as long…period ets； pass through theinner solar system。 just occasionally these stray visitors smack into something solid； likeearth。 that’s why we’ve e out here now—because the et we have e to see hasjust begun a long fall toward the center of the solar system。 it is headed for； of all places；manson； iowa。 it is going to take a long time to get there—three or four million years atleast—so we’ll leave it for now； and return to it much later in the story。
so that’s your solar system。 and what else is out there； beyond the solar system？ well；nothing and a great deal； depending on how you look at it。
in the short term； it’s nothing。 the most perfect vacuum ever created by humans is not asempty as the emptiness of interstellar space。 and there is a great deal of this nothingness untilyou get to the next bit of something。 our nearest neighbor in the cosmos； proxima centauri；which is part of the three…star cluster known as alpha centauri； is 4。3 light…years away； a sissyskip in galactic terms； but that is still a hundred million times farther than a trip to the moon。
to reach it by spaceship would take at least twenty…five thousand years； and even if you madethe trip you still wouldn’t be anywhere except at a lonely clutch of stars in the middle of avast nowhere。 to reach the next landmark of consequence； sirius； would involve another 4。6light…years of travel。 and so it would go if you tried to star…hop your way across the cosmos。
just reaching the center of our own galaxy would take far longer than we have existed asbeings。
space； let me repeat； is enormous。 the average distance between stars out there is 20million million miles。 even at speeds approaching those of light； these are fantasticallychallenging distances for any traveling individual。 of course； it is possible that alien beingstravel billions of miles to amuse themselves by planting crop circles in wiltshire orfrightening the daylights out of some poor guy in a pickup truck on a lonely road in arizona（they must have teenagers； after all）； but it does seem unlikely。
still； statistically the probability that there are other thinking beings out there is good。
nobody knows how many stars there are in the milky way—estimates range from 100 billionor so to perhaps 400 billion—and the milky way is just one of 140 billion or so othergalaxies； many of them even larger than ours。 in the 1960s； a professor at cornell namedfrank drake； excited by such whopping numbers； worked out a famous equation designed tocalculate the chances of advanced life in the cosmos based on a series of diminishingprobabilities。
under drake’s equation you divide the number of stars in a selected portion of the universeby the number of stars that are likely to have planetary systems； divide that by the number ofplanetary systems that could theoretically support life； divide that by the number on whichlife； having arisen； advances to a state of intelligence； and so on。 at each such division； thenumber shrinks colossally—yet even with the most conservative inputs the number ofadvanced civilizations just in the milky way always works out to be somewhere in themillions。
what an interesting and exciting thought。 we may be only one of millions of advancedcivilizations。 unfortunately； space being spacious； the average distance between any two ofthese civilizations is reckoned to be at least two hundred light…years； which is a great dealmore than merely saying it makes it sound。 it means for a start that even if these beings knowwe are here and are somehow able to see us in their telescopes； they’re watching light that leftearth two hundred years ago。 so they’re not seeing you and me。 they’re watching the frenchrevolution and thomas jefferson and people in silk stockings and powdered wigs—peoplewho don’t know what an atom is； or a gene； and who make their electricity by rubbing a rodof amber with a piece of fur and think that’s quite a trick。 any message we receive from themis likely to begin “dear sire；” and congratulate us on the handsomeness of our horses and ourmastery of whale oil。 two hundred light…years is a distance so far beyond us as to be； well；just beyond us。
so even if we are not really alone； in all practical terms we are。 carl sagan calculated thenumber of probable planets in the universe at large at 10 billion trillion—a number vastlybeyond imagining。 but what is equally beyond imagining is the amount of space throughwhich they are lightly scattered。 “if we were randomly inserted into the universe；” saganwrote； “the chances that you would be on or near a planet would be less than one in a billiontrillion trillion。” （that’s 1033； or a one followed by thirty…three zeroes。） “worlds are precious。”
which is why perhaps it is good news that in february 1999 the international astronomicalunion ruled officially that pluto is a planet。 the universe is a big and lonely place。 we can dowith all the neighbors we can get。
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3 THE REVEREND EVANS’S UNIVERSE
小！说
when the skies are clear and the moon is not too bright； the reverend robert evans； aquiet and cheerful man； lugs a bulky telescope onto the back deck of his home in the bluemountains of australia； about fifty miles west of sydney； and does an extraordinary thing。 helooks deep into the past and finds dying stars。
looking into the past is of course the easy part。 glance at the night sky and what you see