A technique for measuring the age of organic remains based on the rate of decay of carbon 14. Because the ratio of carbon 12 to carbon 14 present in all living organisms is the same, and because the decay rate of carbon 14 is constant, the length of time that has passed since an organism has died can be calculated by comparing the ratio of carbon 12 to carbon 14 in its remains to the known ratio in living organisms. Also called carbon-14 dating
A Closer Look In the late 1940s, American chemist Willard Libby developed a method for determining when the death of an organism had occurred. He first noted that the cells of all living things contain atoms taken in from the organism's environment, including carbon; all organic compounds contain carbon. Most carbon consists of the isotopes carbon 12 and carbon 13, which are very stable. A very small percentage of carbon, however, consists of the isotope carbon 14, or radiocarbon, which is unstable. Carbon 14 has a half-life of 5,780 years, and is continuously created in Earth's atmosphere through the interaction of nitrogen and gamma rays from outer space. Because atmospheric carbon 14 arises at about the same rate that the atom decays, Earth's levels of carbon 14 have remained fairly constant. Once an organism is dead, however, no new carbon is actively absorbed by its tissues, and its carbon 14 gradually decays. Libby thus reasoned that by measuring carbon 14 levels in the remains of an organism that died long ago, one could estimate the time of its death. This procedure of radiocarbon dating has been widely adopted and is considered accurate enough for practical use to study remains up to 50,000 years old.