How is half-life used in carbon dating?

Half-life is used in carbon dating to determine the age of ancient artefacts and fossils.

In carbon dating, the half-life of carbon-14 (C-14) is particularly important. Carbon-14 is a radioactive isotope of carbon that is present in the atmosphere and absorbed by living organisms. When an organism dies, it stops absorbing carbon-14, and the isotope begins to decay at a known rate, which is its half-life. The half-life of carbon-14 is approximately 5,730 years.

By measuring the amount of carbon-14 left in a sample, scientists can calculate how many half-lives have passed since the death of the organism, and thus determine the age of the sample. This is done by comparing the ratio of carbon-14 to carbon-12 (C-12), a stable isotope of carbon that does not decay. The ratio of C-14 to C-12 in a sample from a dead plant or animal, such as a piece of wood or a fragment of bone, provides information about when that plant or animal died.

For example, if a sample has half the amount of C-14 that you would expect in a living organism, one half-life has passed and the sample is approximately 5,730 years old. If only a quarter of the C-14 remains, two half-lives have passed and the sample is around 11,460 years old, and so on.

However, carbon dating is not infallible. It assumes that the ratio of C-14 to C-12 in the atmosphere has remained constant over time, which is not the case. Changes in the Earth's magnetic field or in the amount of cosmic rays hitting the Earth can affect the production of C-14. Therefore, scientists use calibration curves to adjust their results and improve the accuracy of carbon dating.