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The object's approximate age can then be figured out using the known rate of decay of the isotope.
For organic materials, the comparison is between the current ratio of a radioactive isotope to a stable isotope of the same element and the known ratio of the two isotopes in living organisms.
From left to right, the isotopes are protium ( Isotopes are variants of a particular chemical element which differ in neutron number.
The fact that each isotope has one proton makes them all variants of hydrogen: the identity of the isotope is given by the number of neutrons.
Wiens has a Ph D in Physics, with a minor in Geology.
If one knows how much of this radioactive material was present initially in the object (by determining how much of the material has decayed), and one knows the half-life of the material, one can deduce the age of the object.