到这个时候，可靠的年代最远只达埃及的第一王朝──公元前3000年左右。例如，谁也没有把握说出，最后一批冰盖是在什么时候退缩的，法国的克罗马农人是在过去什么时候装饰拉斯科山洞的。

　　铅的危害

　　Libby's idea was so useful that he would be awarded a Nobel Prize for it in 1960. It was based on the realization that all living things have within them an isotope of carbon called carbon-14, which begins to decay at a measurable rate the instant they die. Carbon-14 has a half-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 good fix on the age of an object—though only up to a point. After eight half-lives, only 1/256 of the original radioactive carbon remains, which is too little to make a reliable measurement, so radiocarbon dating works only for objects up to forty thousand or so years old.

　　利比的方法用途很广，他因此获得了1960年的诺贝尔奖。这种方法基于一种认识：生物内部都有一种碳的同位素──名叫碳－14，生物一死，该同位素马上以可以测定的速度开始衰变。碳－14大约有5600年的半衰期──即任何样品消失一半所需的时间──因此，通过确定某种特定的碳样的衰变程度，利比就可以有效地锁定一个物体的年代──虽然是在一定限度以内。经过八个半衰期以后，原先的放射性碳只剩下0.39%。这个量太小，无法进行可靠的测算，因此碳－14年代测定法只适用于年代不超过4万年左右的物体。

　　Curiously, just as the technique was becoming widespread, certain flaws within it became apparent. To begin with, it was discovered that one of the basic components of Libby's formula, known as the decay constant, was off by about 3 percent. By this time, however, thousands of measurements had been taken throughout the world.

　　有意思的是，随着这项技术的广泛使用，有些疵点也日渐显露出来。首先，人们发现，利比公式里有个名叫衰变常数的基本成分存在3%的误差。而到了这个时候，全世界已经进行了数千次计算。