Temples, Tombs & Hieroglyphs_ A Popular History of Ancient Egypt - Barbara Mertz [15]
In 1945, Willard F. Libby of the University of Chicago was studying the effect of cosmic ray neutrons upon the nitrogen of the atmosphere. The result of the meeting was a genuine, if tiny, nuclear reaction; the product was radioactive carbon. Libby argued that since its chemical behavior is the same as that of ordinary carbon, this carbon 14, or radiocarbon, should form carbon dioxide molecules and mix in with the ordinary carbon dioxide of the atmosphere. Every high school student of biology knows that carbon dioxide is taken in by plants in the process of photosynthesis. Since animals live off plants, the conclusion was logical, though rather startling: all living matter should be weakly radioactive, from the tiny proportion of carbon 14 that it absorbs.
The first verification of Libby’s theory came from a decidedly inglorious source—the methane gas given off by the city of Baltimore’s sewage. Not only did this decaying organic material give off radioactivity, but it contained exactly the proportion of carbon 14 that Libby had predicted. Subsequent tests were performed on samples of wood, oil, and other material from all over the world. The proportions were as predicted.
This was a good confirmation of the theory, but it was more than that. Libby immediately saw the possible application of the process to dating. Among his samples had been wood from the tombs of Snefru and Djoser, kings of the Fourth Dynasty. The dates given by radiocarbon checked out with the calculations Egyptologists had made independently.
How does it work? Obviously the laboratory apparatus did not contain a neon coil that lit up and read 4,500 years. Before the laboratory results could be translated into years of time, a lot of work had to be done.
Let’s take a specific organic object as an example—an oak tree, perhaps. When the tree died, it of course stopped taking in carbon 14. As it lay in the earth, or in the walls of a building in the form of planks, the radiocarbon it contained at its demise, being unstable, began to disintegrate. Libby calculated that the rate is about one percent each eighty years. The process of decay is exponential; that is, in the first eighty years one percent of the total decays, in the next eighty years one percent of the remaining total, and so on. Scientists talk about decay rate in terms of its “half-life”—the length of time it takes for half the original radioactive content to decay. At the latest measurement, the half-life of carbon 14 is 5,568 years.
Thus, by measuring the amount of carbon 14 remaining in our oak tree, or any piece of it, we can calculate (and if that sounds simple, it is not) how many years have passed since the tree stopped living. Truly, the process is brilliantly conceived. But it has certain limitations.
These limitations arise from various causes. One is the problem of the increase of error. You may have seen radiocarbon dates given in various publications; they look something like this: 3,325 years + - 150. The “plus or minus” indicates the range of possible error. The older the date given, the greater the range. Why the lack of precision? Well, for one thing, it is very difficult to get an uncontaminated sample, free of modern organic substances. If the sample we are working with is fairly recent in age, it still contains a large part of the original radiocarbon; hence, the intrusion of a chunk of modern carbon 14 represents only a small proportion of the total and does not affect the results too much. But if our object is thirty thousand years old, it has lost all but a tiny amount of the carbon 14 it contained at its demise; the amount is so small that it is hard to detect, even with precise laboratory instruments, and any intrusion, however minute, affects the results enormously. The problem