Radiometric dating
From RationalWiki
Radiometric dating involves dating rocks or other objects by measuring the extent to which different radioactive isotopes or nuclei have decayed. Although the time at which any individual atom will decay cannot be forecast, the time in which any given percentage of a sample will decay can be calculated to varying degrees of accuracy. The time is takes for half of a sample to decay is known as the half life of the isotope. Some isotopes have half lives longer than the present age of the universe, but they are still subject to the same laws of quantum physics and will eventually decay, even if doing so at a time when all remaining atoms in the universe are separated by astronomical distances.
Various elements are used for dating different time periods — ones with relatively short half-lives like carbon-14 are useful for dating once-living objects (since they include atmospheric carbon from when they lived) from about ten to fifty thousand years old. See Carbon dating. Longer-lived isotopes provide dating information for much longer ago. The key is to measure one that has had time to decay a measurable amount, but not so much as to only leave a trace remaining. Given isotopes are useful for dating over a range from a fraction of their half life to about four or five times their half life.
Radiometric dating frequently reveals that rocks, fossils, etc. are very much older than six thousand years. Young Earth Creationists tie themselves into logical knots in an effort to reconcile the results of radiometric dating with the Biblical Account of the Earth having been created about six to ten thousand years ago.
One way Young Earth Creationists and other denialists try to discredit radiometric dating is to cite examples of occasions where a radiometric technique does not give an accurate result. This is frequently because the selected technique is used outside of its appropriate range, for example on very recent lavas.
