Radiometric dating is the method for establishing the age of objects by measuring the levels of radioisotopes in the sample. http://en.wikipedia.org/wiki/Radiometric_dating One example is carbon dating. Carbon 14 is created by cosmic rays in the upper atmosphere. It decays to nitrogen 14 with a half life of 5730 years. C14 is continually being created and decaying, leading to an equilibrium state in the atmosphere. When the carbon dioxide, containing C14 as well as stable C12 and C13, is taken i...