The process by which the radioactive decay of the isotope [V] carbon 14 or 14C can be used to date the point in time when a particular piece of vegetable or animal matter died was invented by Willard Libby (1908–1980), an American physicist, in about 1947. He had worked on methods of separating uranium isotopes for the construction of nuclear weapons, and in the postwar period he turned his attention to carbon isotope decay. Carbon 14 is continually formed by the interaction of neutrons with nitrogen 14 in the atmosphere, with the neutrons produced by the interaction of cosmic rays with the atmosphere. Plants absorb the 14C with carbon dioxide from the atmosphere, and it is passed from plants to animals as they eat vegetable matter.
Once a plant dies, it ceases to absorb carbon from the atmosphere, and the 14C in the tissue begins to decay to nonradioactive carbon. The isotope has a half-life of about 5,730 years. Thus if a piece of wood is found in a structure with half the normal distribution of 14C, the tree
from which the wood was cut died about 5,730 years ago. With careful measurement it is possible to determine quite accurately the age of ancient remains. Libby and his team applied the technique to Egyptian relics. They were able to use the technique to date the age of artifacts and remains up to 50,000 to 60,000 years old. The technique has been widely used as a tool in a number of sciences, including paleontology, archaeology, Pleistocene geology, and by historians.
However, when findings through radiocarbon dating contradict fondly held assumptions or theories regarding the dating of a particular artifact, defenders of the alternate chronology have challenged the reliability of the method, leading to heated controversies. For example, in the case of the Shroud of Turin, a large piece of fabric that carbon 14 dated to A.D.1300 to 1400, some researchers had assumed the cloth to be the shroud in which the historical Jesus was wrapped after the Crucifixion. The cloth bears a strange photographlike image that is difficult to explain. Defenders of the view that the shroud could be dated to the time of Christ suggested that contamination of the sample by later impregnation of atmospheric carbon, perhaps as the result of exposure to smoke, would account for the later dating by the carbon 14 method.