Radiometric dating is the method for establishing the age of objects by measuring the levels of radioisotopes in the sample. It decays to nitrogen 14 with a half life of 5730 years.
Measuring the ratio of C14 to C12 and C13 therefore dates the organic matter for periods back to about eight half-lives of the isotope, 45,000 years.After a long enough time the minority isotope is in an amount too small to be measured.There are about two dozen decay pairs used for dating.Uranium 235 decay to lead has a half-life of 713 million years, so it is well suited to dating the universe.other isotope pairs cover intermediate time periods between the spans for carbon 14 and uranium.
Some radiometric dating methods depend upon knowing the initial amount of the isotope subject to decay.For example, the C14 concentration in the atmosphere depends upon cosmic ray intensity.To take this into account, a calibration curve is developed using other dating methods to establish the C14 levels over time.Other methods do not require knowing the initial quantities.For example, potassium decays into two different isotopes of argon having different half-lives.Argon/argon dating works using only the ratio of the concentration of the argon isotopes. For the purposes of this debate, "accurate" means that 95% of the dating errors are within 10% of the measured date, within the time span for which the isotope pair is utilized.