So by measuring carbon 14 levels in an organism that died long ago, researchers can figure out when it died.
A form of radiometric dating used to determine the age of organic remains in ancient objects, such as archaeological specimens, on the basis of the half-life of carbon-14 and a comparison between the ratio of carbon-12 to carbon-14 in a sample of the remains to the known ratio in living organisms. A technique for measuring the age of organic remains based on the rate of decay of carbon 14.
The carbon 14 present in an organism at the time of its death decays at a steady rate, and so the age of the remains can be calculated from the amount of carbon 14 that is left. The cells of all living things contain carbon atoms that they take in from their environment.
Back in the 1940s, the American chemist Willard Libby used this fact to determine the ages of organisms long dead.
Most carbon atoms have six protons and six neutrons in their nuclei and are called carbon 12. But a tiny percentage of carbon is made of carbon 14, or radiocarbon, which has six protons and eight neutrons and is not stable: half of any sample of it decays into other atoms after 5,700 years.
"After a robbery scam that goes bad, lovers Nikki and Al take off into the Australian outback, pursued by the police and a malevolent footballer named Zipper Doyle, and meet a number of offbeat characters." (Cover notes, DVD release) A woman off camera explains how she has difficulty trusting people, especially men, because of things she saw when she was young.
This comment introduces a prologue in which a little girl watches helplessly as a man, presumably her father, appears at the door, douses her mother with petrol and sets her alight.
The young girl, now a woman in her twenties, is Nikki Davies (Frances O'Connor).
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.