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s； 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。
……
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 ishistory and lots of it—the stars not as they are now but as they were when their light leftthem。 for all we know； the north star； our faithful panion； might actually have burnedout last january or in 1854 or at any time since the early fourteenth century and news of it justhasn’t reached us yet。 the best we can say—can ever say—is that it was still burning on thisdate 680 years ago。 stars die all the time。 what bob evans does better than anyone else whohas ever tried is spot these moments of celestial farewell。
by day； evans is a kindly and now semiretired minister in the uniting church in australia；who does a bit of freelance work and researches the history of nineteenth…century religiousmovements。 but by night he is； in his unassuming way； a titan of the skies。 he huntssupernovae。
supernovae occur when a giant star； one much bigger than our own sun； collapses and thenspectacularly explodes； releasing in an instant the energy of a hundred billion suns； burningfor a time brighter than all the stars in its galaxy。 “it’s like a trillion hydrogen bombs going offat once；” says evans。 if a supernova explosion happened within five hundred light…years of us；we would be goners； according to evans—“it would wreck the show；” as he cheerfully puts it。
but the universe is vast； and supernovae are normally much too far away to harm us。 in fact；most are so unimaginably distant that their light reaches us as no more than the faintesttwinkle。 for the month or so that they are visible； all that distinguishes them from the otherstars in the sky is that they occupy a point of space that wasn’t filled before。 it is theseanomalous； very occasional pricks in the crowded dome of the night sky that the reverendevans finds。
to understand what a feat this is； imagine a standard dining room table covered in a blacktablecloth and someone throwing a handful of salt across it。 the scattered grains can bethought of as a galaxy。 now imagine fifteen hundred more tables like the first one—enough tofill a wal…mart parking lot； say； or to make a single line two miles long—each with a randomarray of salt across it。 now add one grain of salt to any table and let bob evans walk amongthem。 at a glance he will spot it。 that grain of salt is the supernova。
evans’s is a talent so exceptional that oliver sacks； in an anthropologist on mars； devotesa passage to him in a chapter on autistic savants—quickly adding that “there is no suggestionthat he is autistic。” evans； who has not met sacks； laughs at the suggestion that he might beeither autistic or a savant； but he is powerless to explain quite where his talent es from。
“i just seem to have a knack for memorizing star fields；” he told me； with a franklyapologetic look； when i visited him and his wife； elaine； in their picture…book bungalow on atranquil edge of the village of hazelbrook； out where sydney finally ends and the boundlessaustralian bush begins。 “i’m not particularly good at other things；” he added。 “i don’tremember names well。”
“or where he’s put things；” called elaine from the kitchen。
he nodded frankly again and grinned； t