Radioactive and radiocarbon dating
In living organisms, which are always taking in carbon, the levels of carbon 14 likewise stay constant.But in a dead organism, no new carbon is coming in, and its carbon 14 gradually begins to decay.The rate of decay (given the symbol λ) is the fraction of the 'parent' atoms that decay in unit time.
The unstable or more commonly known radioactive isotopes break down by radioactive decay into other isotopes.
"Everything which has come down to us from heathendom is wrapped in a thick fog; it belongs to a space of time we cannot measure.
We know that it is older than Christendom, but whether by a couple of years or a couple of centuries, or even by more than a millenium, we can do no more than guess." [Rasmus Nyerup, (Danish antiquarian), 1802 (in Trigger, 19)].
Carbon 14 is continually being created in the Earth's atmosphere by the interaction of nitrogen and gamma rays from outer space.
Since atmospheric carbon 14 arises at about the same rate that the atom decays, the Earth's levels of carbon 14 have remained constant.