Carbon-14 dating uses the ratio of radioactive carbon-14 to non-radioactive carbon-12 to determine if the ratio is the same as in living organisms, or if it is lower, indicating that the carbon-14 has decayed during a period of thousands of years. Carbon-14 has a half-life of 5,730 years, which is the amount of time that it takes for half of a given sample of carbon-14 to decay.
Carbon-14 is only a small percentage of the total carbon in the environment and originates in the atmosphere when cosmic rays and high-speed particles from space hit nitrogen atoms. Although the physics are more complicated than this, essentially the nitrogen atoms lose a proton and gain a neutron to become carbon-14.
Plants take up carbon-14 when they convert CO2 into sugar and build cellular structures. Animals take up carbon-14 when they eat plants or other animals. These organisms no longer take in new carbon-14 once they've died, so the carbon-14 isn't replenished as it decays back into nitrogen.
Carbon-14 dating currently uses a technique called accelerator mass spectrometry, and it's able to determine the ratio of carbon-14 to carbon-12 in a sample. The older technique takes longer and relies on the detection of beta radiation from the decaying carbon-14. Measure the number of decay events, and then compare the number of decays per unit mass per unit time to the decay activity in living organisms.
Carbon-14 dating has a 95 percent accuracy when determining the age of organic material up to 50,000 years old. After 50,000 years, however, the amount of carbon-14 remaining is too low for an accurate reading.