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as it is； even with the hubble telescope； we can鈥檛 see even into the oort cloud； so we don鈥檛actually know that it is there。 its existence is probable but entirely hypothetical。
*about all that can be said with confidence about the oort cloud is that it starts somewherebeyond pluto and stretches some two light…years out into the cosmos。 the basic unit ofmeasure in the solar system is the astronomical unit； or au； representing the distance from*properly called the opik…oort cloud； it is named for the estonian astronomer ernst opik； who hypothesized itsexistence in 1932； and for the dutch astronomer jan oort； who refined the calculations eighteen years later。
the sun to the earth。 pluto is about forty aus from us； the heart of the oort cloud about fiftythousand。 in a word； it is remote。
but let鈥檚 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鈥檙e a long way from anywhere now鈥攕o far fromour own sun that it鈥檚 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鈥檚 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鈥攁 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鈥檚 why we鈥檝e e out here now鈥攂ecause 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鈥攖hree or four million years atleast鈥攕o we鈥檒l leave it for now； and return to it much later in the story。
so that鈥檚 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鈥檚 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鈥檛 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鈥攅stimates range from 100 billionor so to perhaps 400 billion鈥攁nd 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鈥檚 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鈥攜et 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鈥檙e watching light that leftearth two hundred years ago。 so they鈥檙e not seeing you and me。 they鈥檙e watching the frenchrevolution and thomas jefferson and people in silk stockings and powdered wigs鈥攑eoplewho don鈥檛 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鈥檚 quite a trick。 any message we receive from themis likely to begin 鈥渄ear sire；鈥潯nd 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鈥攁 number vastlybeyond imagining。 but what is equally beyond imagining is the amount of space throughwhich they are lightly scattered。 鈥渋f we were randomly inserted into the universe；鈥潯aganwrote； 鈥渢he chances that you would be on or near a planet would be less than one in a billiontrillion trillion。鈥潯。╰hat鈥檚 1033； or a one followed by thirty…three zeroes。） 鈥渨orlds are precious。鈥�
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。
xi锛shuotx锛础锛辑
3 THE REVEREND EVANS鈥橲 UNIVERSE
灏弤璇磣t。xt‘澶﹡鍫�
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鈥攖he 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鈥檛 reached us yet。 the best we can say鈥攃an ever say鈥攊s 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。 鈥渋t鈥檚 like a trillion hydrogen bombs going offat once；鈥潯ays evans。 if a supernova explosion happened within five hundred light…years of us；we would be goners； according to evans鈥斺�渋t would wreck the show；鈥潯s 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鈥檛 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鈥攅nough tofill a wal…mart parking lot； say； or to make a single line two miles long鈥攅ach with a randomarray of salt across it。 now add one grain of salt to any ta