Scientists often use the value of 10 half-lives to indicate when a radioactive isotope will be gone, or rather, when a very negligible amount is still left.This is why radiocarbon dating is only useful for dating objects up to around 50,000 years old (about 10 half-lives).
However, once the organism dies, the amount of carbon-14 steadily decreases.
By measuring the amount of carbon-14 left in the organism, it's possible to work out how old it is.
Radioactive carbon-14 is continually formed in the atmosphere by the bombardment of cosmic ray neutrons on nitrogen-14 atoms.
After it forms, carbon-14 naturally decomposes, with a half-life of 5,730 years, through beta-particle decay.
Then the radiocarbon dating measures remaining radioactivity.
By knowing how much carbon-14 is left in a sample, the age of the organism and when it died can be worked out.
Libby was awarded the Nobel Prize in chemistry for his work in 1960.
Carbon dioxide in the atmosphere contains a constant amount of carbon-14, and as long as an organism is living, the amount of carbon-14 inside it is the same as the atmosphere.
This technique works well for materials up to around 50,000 years old.