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ay or a 61…watt light that is 119 feet； 8 inches away。 ontop of that you must make allowances for distortions caused by variations in the earth鈥檚atmosphere； by intergalactic dust； contaminating light from foreground stars； and many otherfactors。 the upshot is that your putations are necessarily based on a series of nestedassumptions； any of which could be a source of contention。 there is also the problem thataccess to telescopes is always at a premium and historically measuring red shifts has beennotably costly in telescope time。 it could take all night to get a single exposure。 inconsequence； astronomers have sometimes been pelled （or willing） to base conclusionson notably scanty evidence。 in cosmology； as the journalist geoffrey carr has suggested； wehave 鈥渁 mountain of theory built on a molehill of evidence。鈥潯r as martin rees has put it：
鈥渙ur present satisfaction ＇with our state of understanding＇ may reflect the paucity of the datarather than the excellence of the theory。鈥�
this uncertainty applies； incidentally； to relatively nearby things as much as to the distantedges of the universe。 as donald goldsmith notes； when astronomers say that the galaxy m87is 60 million light…years away； what they really mean （鈥渂ut do not often stress to the generalpublic鈥潱s that it is somewhere between 40 million and 90 million light…years away鈥攏ot2you are of course entitled to wonder what is meant exactly by 〃a constant of 50〃 or 〃a constant of 100。〃 theanswer lies in astronomical units of measure。 except conversationally； astronomers dont use light…years。 theyuse a distance called the parsec （a contraction of parallax and second）； based on a universal measure called thestellar parallax and equivalent to 3。26 light…years。 really big measures； like the size of a universe； are measuredin megaparsecs： a million parsecs。 the constant is expressed in terms of kilometers per second per megaparsec。
thus when astronomers refer to a hubble constant of 50； what they really mean is 〃50 kilometers per second permegaparsec。〃 for most of us that is of course an utterly meaningless measure； but then with astronomicalmeasures most distances are so huge as to be utterly meaningless。
quite the same thing。 for the universe at large； matters are naturally magnified。 bearing allthat in mind； the best bets these days for the age of the universe seem to be fixed on a range ofabout 12 billion to 13。5 billion years； but we remain a long way from unanimity。
one interesting recently suggested theory is that the universe is not nearly as big as wethought； that when we peer into the distance some of the galaxies we see may simply bereflections； ghost images created by rebounded light。
the fact is； there is a great deal； even at quite a fundamental level； that we don鈥檛 know鈥攏otleast what the universe is made of。 when scientists calculate the amount of matter needed tohold things together； they always e up desperately short。 it appears that at least 90 percentof the universe； and perhaps as much as 99 percent； is posed of fritz zwicky鈥檚 鈥渄arkmatter鈥濃�攕tuff that is by its nature invisible to us。 it is slightly galling to think that we live ina universe that； for the most part； we can鈥檛 even see； but there you are。 at least the names forthe two main possible culprits are entertaining： they are said to be either wimps （for weaklyinteracting massive particles； which is to say specks of invisible matter left over from the bigbang） or machos （for massive pact halo objects鈥攔eally just another name for blackholes； brown dwarfs； and other very dim stars）。
particle physicists have tended to favor the particle explanation of wimps； astrophysiciststhe stellar explanation of machos。 for a time machos had the upper hand； but not nearlyenough of them were found； so sentiment swung back toward wimps but with the problemthat no wimp has ever been found。 because they are weakly interacting； they are （assumingthey even exist） very hard to detect。 cosmic rays would cause too much interference。 soscientists must go deep underground。 one kilometer underground cosmic bombardmentswould be one millionth what they would be on the surface。 but even when all these are addedin； 鈥渢wo…thirds of the universe is still missing from the balance sheet；鈥潯s one mentatorhas put it。 for the moment we might very well call them dunnos （for dark unknownnonreflective nondetectable objects somewhere）。
recent evidence suggests that not only are the galaxies of the universe racing away fromus； but that they are doing so at a rate that is accelerating。 this is counter to all expectations。 itappears that the universe may not only be filled with dark matter； but with dark energy。
scientists sometimes also call it vacuum energy or； more exotically； quintessence。 whatever itis； it seems to be driving an expansion that no one can altogether account for。 the theory isthat empty space isn鈥檛 so empty at all鈥攖hat there are particles of matter and antimatterpopping into existence and popping out again鈥攁nd that these are pushing the universeoutward at an accelerating rate。 improbably enough； the one thing that resolves all this iseinstein鈥檚 cosmological constant鈥攖he little piece of math he dropped into the general theoryof relativity to stop the universe鈥檚 presumed expansion； and called 鈥渢he biggest blunder of mylife。鈥潯t now appears that he may have gotten things right after all。
the upshot of all this is that we live in a universe whose age we can鈥檛 quite pute；surrounded by stars whose distances we don鈥檛 altogether know； filled with matter we can鈥檛identify； operating in conformance with physical laws whose properties we don鈥檛 trulyunderstand。
and on that rather unsettling note； let鈥檚 return to planet earth and consider something thatwe do understand鈥攖hough by now you perhaps won鈥檛 be surprised to hear that we don鈥檛understand it pletely and what we do understand we haven鈥檛 understood for long。
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12 THE EARTH MOVES
灏忥伎璇磘…x…t锛垮ぉ/鍫�
in one of his last professional acts before his death in 1955； albert einstein wrote a shortbut glowing foreword to a book by a geologist named charles hapgood entitled earth鈥檚shifting crust： a key to some basic problems of earth science。 hapgood鈥檚 book was asteady demolition of the idea that continents were in motion。 in a tone that all but invited thereader to join him in a tolerant chuckle； hapgood observed that a few gullible souls hadnoticed 鈥渁n apparent correspondence in shape between certain continents。鈥潯t would appear；he went on； 鈥渢hat south america might be fitted together with africa； and so on。 。 。 。 it is evenclaimed that rock formations on opposite sides of the atlantic match。鈥�
mr。 hapgood briskly dismissed any such notions； noting that the geologists k。 e。 casterand j。 c。 mendes had done extensive fieldwork on both sides of the atlantic and hadestablished beyond question that no such similarities existed。 goodness knows what outcropsmessrs。 caster and mendes had looked at； beacuse in fact many of the rock formations onboth sides of the atlanticare the same鈥攏ot just very similar but the same。
this was not an idea that flew with mr。 hapgood； or many other geologists of his day。 thetheory hapgood alluded to was one first propounded in 1908 by an amateur americangeologist named frank bursley taylor。 taylor came from a wealthy family and had both themeans and freedom from academic constraints to pursue unconventional lines of inquiry。 hewas one of those struck by the similarity in shape between the facing coastlines of africa andsouth america； and from this observation he developed the idea that the continents had onceslid around。 he suggested鈥攑resciently as it turned out鈥攖hat the crunching together ofcontinents could have thrust up the world鈥檚 mountain chains。 he failed； however； to producemuch in the way of evidence； and the theory was considered too crackpot to merit seriousattention。
in germany； however； taylor鈥檚 idea was picked up； and effectively appropriated； by atheorist named alfred wegener； a meteorologist at the university of marburg。 wegenerinvestigated the many plant and fossil anomalies that did not fit fortably into the standardmodel of earth history and realized that very little of it made sense if conventionallyinterpreted。 animal fossils repeatedly turned up on opposite sides of oceans that were clearlytoo wide to swim。 how； he wondered； did marsupials travel from south america to australia？
how did identical snails turn up in scandinavia and new england？ and how； e to that；did one account for coal seams and other semi…tropical remnants in frigid spots likespitsbergen； four hundred miles north of norway； if they had not somehow migrated therefrom warmer climes？
wegener developed the theory that the world鈥檚 continents had once e together in asingle landmass he called pangaea； where flora and fauna had been able to mingle； before thecontinents had split apart and floated off to their present positions。 all this he put together in abook called die entstehung der kontinente und ozeane； or the origin of continents and oceans； which was published in german in 1912 and鈥攄espite the outbreak of the firstworld war in th