Potassium-40 is another radioactive element naturally found in your body and has a half-life of 1.3 billion years.
Other useful radioisotopes for radioactive dating include Uranium -235 (half-life = 704 million years), Uranium -238 (half-life = 4.5 billion years), Thorium-232 (half-life = 14 billion years) and Rubidium-87 (half-life = 49 billion years).
The use of various radioisotopes allows the dating of biological and geological samples with a high degree of accuracy.
However, radioisotope dating may not work so well in the future.
Elements like potassium, uranium, lead and so on come in more than one version, depending upon how many neutrons they have in their atomic nucleus. Some isotopes are radioactive, meaning that they decay, at a fixed and known rate, into a completely different element.
Physicists understand why this happens, and they know the rate at which each isotope decays.
Trace fossils are the marks left by a living organism, such as feces, footprints or impressions of feathers or leaves.
Organisms usually need to be covered by mud, sand, tar or some other sediment as soon as possible or frozen or dessicated (dried out) for fossilization to occur.
Very occasionally, however, rocks take up some kind of permanent imprint of the body and preserve it for us to see even after millions of years. Often a fossil retains only the external shape of the body.
That's how fossils are formed, but how do we know how old they are?
The best way to date fossils is by measuring radioactive isotopes in surrounding rocks.
By looking at the ratio of carbon-12 to carbon-14 in the sample and comparing it to the ratio in a living organism, it is possible to determine the age of a formerly living thing fairly precisely. So, if you had a fossil that had 10 percent carbon-14 compared to a living sample, then that fossil would be: t = [ ln (0.10) / (-0.693) ] x 5,700 years t = [ (-2.303) / (-0.693) ] x 5,700 years t = [ 3.323 ] x 5,700 years Because the half-life of carbon-14 is 5,700 years, it is only reliable for dating objects up to about 60,000 years old.
However, the principle of carbon-14 dating applies to other isotopes as well.