The atmosphere contains many stable carbon atoms and relatively few radiocarbon atoms.
The ratio of radiocarbon to stable carbon atoms in the atmosphere has varied in the past.
This is because the amount and strength of cosmic radiation entering the earth's atmosphere has varied over time.
(This, in turn, is caused by variations in the magnetic fields of the earth and sun, for example.) Although the ratio of radiocarbon to stable carbon in the atmosphere has varied over time, it is quite uniform around the globe at any given time because the atmosphere mixes very quickly and constantly.
However, cosmic radiation constantly collides with atoms in the upper atmosphere.
Part of the result of these collisions is the production of radiocarbon (C, pronounced "c fourteen"), carbon atoms which are chemically the same as stable carbon, but have two extra neutrons.
Professor Willard Libby produced the first radiocarbon dates in 1949 and was later awarded the Nobel Prize for his efforts.
Radiocarbon dating works by comparing the three different isotopes of carbon.
The carbon in their bodies at the time of their death will remain in their bodies until they decompose, or if they become fossilized, then forever. This allows scientists to look at the amount of decay in a fossil’s radioactive carbon and determine a relative date.
Radiocarbon dating is only effective for objects and fossils that are less than 50,000 years old.
The results provide conclusive evidence that the linen of the Shroud of Turin is mediaeval.