But other timekeeping methods exist and are still used in the modern world, circumventing the easy processing of dates and history between cultures.Throughout history, time has been defined in a variety of ways: by everything from the current ruler, or empire, or not defined at all.A commonly used radiometric dating technique relies on the breakdown of potassium (Ar in an igneous rock can tell us the amount of time that has passed since the rock crystallized.If an igneous or other rock is metamorphosed, its radiometric clock is reset, and potassium-argon measurements can be used to tell the number of years that has passed since metamorphism.Carbon-14 is a method used for young (less than 50,000 year old) sedimentary rocks.This method relies on the uptake of a naturally occurring radioactive isotope of carbon, carbon-14 by all living things.
This fact should always be remembered when using radiocarbon dates.
The University of Chicago professor developed radiocarbon dating in the late 1940s and won the 1960 Nobel Prize in chemistry for it.
When plants or animals die, they no longer exchange their carbon with fresh atoms from their environment.
The dating process is always designed to try to extract the carbon from a sample which is most representative of the original organism.
In general it is always better to date a properly identified single entity (such as a cereal grain or an identified bone) rather than a mixture of unidentified organic remains.