Establishing the relative age of various materials with the use of carbon-14. This involves measuring the amount of ¹⁴C and of ¹²C and comparing the measured ratio to the one established by the production of ¹⁴C in the upper atmosphere by cosmic rays. When an organism is alive, the ¹⁴C/ ¹²C ratio in its biomass is constant (because of constant atmospheric ¹⁴C production, diffusion to the lower atmosphere, absorption and incorporation by organisms, and because of constant ¹⁴C radioactive decay); however, when the organism dies, the ratio begins to change---at a very predictable rate---because the (dead) organism is no longer incorporating new carbon. Therefore knowing the carbon-14/carbon-12 ratio now in some artefact gives a very good measure of how long the carbon has been "dead. " The object however must obviously have organic material either in it or on it for this dating method to work. Also the length of time one can "look back" in time is limited because the amount of carbon-14 must be detected with some certainty for the age to be known with confidence, and the longer the sample has been dead the less ¹⁴C is present because it is continually decaying.