By measuring the ratio of the radio isotope to non-radioactive carbon, the amount of carbon-14 decay can be worked out, thereby giving an age for the specimen in question.
But that assumes that the amount of carbon-14 in the atmosphere was constant — any variation would speed up or slow down the clock.
Note that, contrary to a popular misconception, carbon dating is not used to date rocks at millions of years old.
Before we get into the details of how radiometric dating methods are used, we need to review some preliminary concepts from chemistry.
Scientists use a technique called radiometric dating to estimate the ages of rocks, fossils, and the earth.
Carbon dating is used to work out the age of organic material — in effect, any living thing.
) is only 5,730 years—that is, every 5,730 years, half of it decays away.
After two half lives, a quarter is left; after three half lives, only an eighth; after 10 half lives, less than a thousandth is left.
The clock was initially calibrated by dating objects of known age such as Egyptian mummies and bread from Pompeii; work that won Willard Libby the 1960 Nobel Prize in Chemistry.
But even he “realized that there probably would be variation”, says Christopher Bronk Ramsey, a geochronologist at the University of Oxford, UK, who led the latest work, published today in Science.