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they die。 carbon…14 has ahalf…life—that is； the time it takes for half of any sample to disappear1—of about 5；600 years；so by working out how much a given sample of carbon had decayed； libby could get a goodfix on the age of an object—though only up to a point。 after eight half…lives； only 1/256 of theoriginal radioactive carbon remains； which is too little to make a reliable measurement； soradiocarbon dating works only for objects up to forty thousand or so years old。
curiously； just as the technique was being widespread； certain flaws within it becameapparent。 to begin with； it was discovered that one of the basic ponents of libby’sformula； known as the decay constant； was off by about 3 percent。 by this time； however；thousands of measurements had been taken throughout the world。 rather than restate everyone； scientists decided to keep the inaccurate constant。 “thus；” tim flannery notes； “everyraw radiocarbon date you read today is given as too young by around 3 percent。” theproblems didn’t quite stop there。 it was also quickly discovered that carbon…14 samples can beeasily contaminated with carbon from other sources—a tiny scrap of vegetable matter； forinstance； that has been collected with the sample and not noticed。 for younger samples—those under twenty thousand years or so—slight contamination does not always matter somuch； but for older samples it can be a serious problem because so few remaining atoms arebeing counted。 in the first instance； to borrow from flannery； it is like miscounting by a dollarwhen counting to a thousand； in the second it is more like miscounting by a dollar when youhave only two dollars to count。
libby’s method was also based on the assumption that the amount of carbon…14 in theatmosphere； and the rate at which it has been absorbed by living things； has been consistentthroughout history。 in fact it hasn’t been。 we now know that the volume of atmosphericcarbon…14 varies depending on how well or not earth’s magnetism is deflecting cosmic rays；and that that can vary significantly over time。 this means that some carbon…14 dates are more1if you have ever wondered how the atoms determine which 50 percent will die and which 50 percent willsurvive for the next session； the answer is that the half…life is really just a statistical convenience…a kind ofactuarial table for elemental things。 imagine you had a sample of material with a half…life of 30 seconds。 it isntthat every atom in the sample will exist for exactly 30 seconds or 60 seconds or 90 seconds or some other tidilyordained period。 each atom will in fact survive for an entirely random length of time that has nothing to do withmultiples of 30； it might last until two seconds from now or it might oscillate away for years or decades orcenturies to e。 no one can say。 but what we can say is that for the sample as a whole the rate ofdisappearance will be such that half the atoms will disappear every 30 seconds。 its an average rate； in otherwords； and you can apply it to any large sampling。 someone once worked out； for instance； that dimes have ahalf…life of about 30 years。
dubious than others。 this is particularly so with dates just around the time that people firstcame to the americas； which is one of the reasons the matter is so perennially in dispute。
finally； and perhaps a little unexpectedly； readings can be thrown out by seeminglyunrelated external factors—such as the diets of those whose bones are being tested。 onerecent case involved the long…running debate over whether syphilis originated in the newworld or the old。 archeologists in hull； in the north of england； found that monks in amonastery graveyard had suffered from syphilis； but the initial conclusion that the monks haddone so before columbus’s voyage was cast into doubt by the realization that they had eaten alot of fish； which could make their bones appear to be older than in fact they were。 the monksmay well have had syphilis； but how it got to them； and when； remain tantalizinglyunresolved。
because of the accumulated shortings of carbon…14； scientists devised other methods ofdating ancient materials； among them thermoluminesence； which measures electrons trappedin clays； and electron spin resonance； which involves bombarding a sample withelectromagnetic waves and measuring the vibrations of the electrons。 but even the best ofthese could not date anything older than about 200；000 years； and they couldn’t date inorganicmaterials like rocks at all； which is of course what you need if you wish to determine the ageof your planet。
the problems of dating rocks were such that at one point almost everyone in the world hadgiven up on them。 had it not been for a determined english professor named arthur holmes；the quest might well have fallen into abeyance altogether。
holmes was heroic as much for the obstacles he overcame as for the results he achieved。
by the 1920s； when holmes was in the prime of his career； geology had slipped out offashion—physics was the new excitement of the age—and had bee severely underfunded；particularly in britain； its spiritual birthplace。 at durham university； holmes was for manyyears the entire geology department。 often he had to borrow or patch together equipment inorder to pursue his radiometric dating of rocks。 at one point； his calculations were effectivelyheld up for a year while he waited for the university to provide him with a simple addingmachine。 occasionally； he had to drop out of academic life altogether to earn enough tosupport his family—for a time he ran a curio shop in newcastle upon tyne—and sometimeshe could not even afford the ￡5 annual membership fee for the geological society。
the technique holmes used in his work was theoretically straightforward and arose directlyfrom the process； first observed by ernest rutherford in 1904； in which some atoms decayfrom one element into another at a rate predictable enough that you can use them as clocks。 ifyou know how long it takes for potassium…40 to bee argon…40； and you measure theamounts of each in a sample； you can work out how old a material is。 holmes’s contributionwas to measure the decay rate of uranium into lead to calculate the age of rocks； a