Radiocarbon dating

Radiocarbon dating, also known as the C14 dating method, is a way of telling how old an object is. It is a type of radiometric dating. 
The method uses the radioactive isotope carbon-14. Most organic matter contains carbon. Carbon has different isotopes, which are usually not radioactive. 14C is the radioactive one, its half-life (time it takes to reduce its radioactivity by half) is about 5,730 years. This makes it possible to tell the age of substances that contain carbon. The method works to about 60,000 years old. Dates obtained are usually written as "before present" ('present' is 1950). 
Plants take up atmospheric carbon dioxide by photosynthesis, and are eaten by animals, so every living thing is constantly exchanging carbon-14 with its environment as long as it lives. Once it dies, however, this exchange stops.
In 1958 Hessel de Vries showed that the concentration of carbon-14 in the atmosphere varies with time and locality. The relatively short-lived 14C is constantly renewed by cosmic ray bombardment on atmospheric nitrogen. Since the bombardment is slightly variable, and for other reasons, the 14C taken into organic matter is also slightly variable. That leads to errors in the chronology. However, under about 20,000 years the results can be compared with dendrochronology, based on tree rings. For the most accurate work, variations are compensated by means of calibration curves. 
The method was developed by Willard Libby and his colleagues at the University of Chicago in 1949. In 1960, he was awarded the Nobel Prize in Chemistry for this work. He first demonstrated the accuracy of radiocarbon dating by accurately estimating the age of wood from an ancient Egyptian royal barge of which the age was known from historical documents.