In addition to permitting more accurate dating within archaeological sites than previous methods, it allows comparison of dates of events across great distances.
Histories of archaeology often refer to its impact as the "radiocarbon revolution".
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.
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 , which is unstable.
The older a sample is, the less (the period of time after which half of a given sample will have decayed) is about 5,730 years, the oldest dates that can be reliably measured by this process date to around 50,000 years ago, although special preparation methods occasionally permit accurate analysis of older samples.
Research has been ongoing since the 1960s to determine what the proportion of in the atmosphere has been over the past fifty thousand years.
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.