Radiometric dating is based on the idea that
If certain things are known, it is possible to calculate the amount of time since the parent isotope began to decay.
For example, if you began with 1 gram of carbon-14, after 5,730 years you would be left with 0.50 g and only 0.25 g after 11,460 years.
Some of the common isotope pairs used are K-Ar, Rb-Sr, Pb-Pb, and U-Pb.
Carbon-14 dating is another common technique, but it can only be used on carbon-containing things that were once alive.
Uniformitarian geologists use so-called dating methods to determine the ages of the surrounding rocks.
Certain types of rocks, especially those that form from magma (igneous), contain radioactive isotopes of different elements.
The dates must be inferred based on assumptions about the ratios.
Most scientists and many Christians believe that the radiometric dating methods prove that the earth is 4.5 billion years old.