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！
string theory has further spawned something called “m theory；” which incorporatessurfaces known as membranes—or simply “branes” to the hipper souls of the world ofphysics。 i’m afraid this is the stop on the knowledge highway where most of us must get off。
here is a sentence from the new york times； explaining this as simply as possible to a generalaudience： “the ekpyrotic process begins far in the indefinite past with a pair of flat emptybranes sitting parallel to each other in a warped five…dimensional space。 。 。 。 the two branes；which form the walls of the fifth dimension； could have popped out of nothingness as aquantum fluctuation in the even more distant past and then drifted apart。” no arguing withthat。 no understanding it either。 ekpyrotic； incidentally； es from the greek word for“conflagration。”
matters in physics have now reached such a pitch that； as paul davies noted in nature； it is“almost impossible for the non…scientist to discriminate between the legitimately weird andthe outright crackpot。” the question came interestingly to a head in the fall of 2002 when twofrench physicists； twin brothers igor and grickha bogdanov； produced a theory of ambitiousdensity involving such concepts as “imaginary time” and the “kubo…schwinger…martincondition；” and purporting to describe the nothingness that was the universe before the bigbang—a period that was always assumed to be unknowable （since it predated the birth ofphysics and its properties）。
almost at once the bogdanov paper excited debate among physicists as to whether it wastwaddle； a work of genius； or a hoax。 “scientifically； it’s clearly more or less pletenonsense；” columbia university physicist peter woit told the new york times； “but thesedays that doesn’t much distinguish it from a lot of the rest of the literature。”
karl popper； whom steven weinberg has called “the dean of modern philosophers ofscience；” once suggested that there may not be an ultimate theory for physics—that； rather；every explanation may require a further explanation； producing “an infinite chain of more andmore fundamental principles。” a rival possibility is that such knowledge may simply bebeyond us。 “so far； fortunately；” writes weinberg in dreams of a final theory； “we do notseem to be ing to the end of our intellectual resources。”
almost certainly this is an area that will see further developments of thought； and almostcertainly these thoughts will again be beyond most of us。
while physicists in the middle decades of the twentieth…century were looking perplexedlyinto the world of the very small； astronomers were finding no less arresting an inpletenessof understanding in the universe at large。
when we last met edwin hubble； he had determined that nearly all the galaxies in our fieldof view are flying away from us； and that the speed and distance of this retreat are neatlyproportional： the farther away the galaxy； the faster it is moving。 hubble realized that thiscould be expressed with a simple equation； ho = v/d （where ho is the constant； v is therecessional velocity of a flying galaxy； andd its distance away from us）。 ho has been knownever since as the hubble constant and the whole as hubble’s law。 using his formula； hubblecalculated that the universe was about two billion years old； which was a little awkwardbecause even by the late 1920s it was fairly obvious that many things within the universe—not least earth itself—were probably older than that。 refining this figure has been an ongoingpreoccupation of cosmology。
almost the only thing constant about the hubble constant has been the amount ofdisagreement over what value to give it。 in 1956； astronomers discovered that cepheidvariables were more variable than they had thought； they came in two varieties； not one。 thisallowed them to rework their calculations and e up with a new age for the universe offrom 7 to 20 billion years—not terribly precise； but at least old enough； at last； to embrace theformation of the earth。
in the years that followed there erupted a long…running dispute between allan sandage； heirto hubble at mount wilson； and gérard de vaucouleurs； a french…born astronomer based atthe university of texas。 sandage； after years of careful calculations； arrived at a value for thehubble constant of 50； giving the universe an age of 20 billion years。 de vaucouleurs wasequally certain that the hubble constant was 100。
2this would mean that the universe wasonly half the size and age that sandage believed—ten billion years。 matters took a furtherlurch into uncertainty when in 1994 a team from the carnegie observatories in california；using measures from the hubble space telescope； suggested that the universe could be as littleas eight billion years old—an age even they conceded was younger than some of the starswithin the universe。 in february 2003； a team from nasa and the goddard space flightcenter in maryland； using a new； far…reaching type of satellite called the wilkinsonmicrowave anistropy probe； announced with some confidence that the age of the universe is13。7 billion years； give or take a hundred million years or so。 there matters rest； at least forthe moment。
the difficulty in making final determinations is that there are often acres of room forinterpretation。 imagine standing in a field at night and trying to decide how far away twodistant electric lights are。 using fairly straightforward tools of astronomy you can easilyenough determine that the bulbs are of equal brightness and that one is； say； 50 percent moredistant than the other。 but what you can’t be certain of is whether the nearer light is； let ussay； a 58…watt bulb that is 122 feet away 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’satmosphere； 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 “a mountain of theory built on a molehill of evidence。” or as martin rees has put it：
“our present satisfaction ＇with our state of understanding＇ may reflect the paucity of the datarather than the excellence of the theory。”
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 （“but do not often stress to the generalpublic”） is that it is somewhere between 40 million and 90 million light…years away—not2you 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 convers