Scientific definition of radiometric dating
Another method is to make age measurements on several samples from the same rock unit.
The simplest means is to repeat the analytical measurements in order to check for laboratory errors.The slope of the line determines the date, and the closeness of fit is a measure of the statistical reliability of the resulting date.Technical details on how these dates are calculated are given in Radiometric dating. As with any experimental procedure in any field of science, these measurements are subject to certain "glitches" and "anomalies," as noted in the literature.Some [skeptics] make it sound like there is a lot of disagreement, but this is not the case.The disagreement in values needed to support the position of young-Earth proponents would require differences in age measured by orders of magnitude (e.g., factors of 10,000, 100,000, a million, or more).Several hundred laboratories around the world are active in radiometric dating.
Their results consistently agree with an old Earth.
Creationist Henry Morris, for example, criticizes this type of "uniformitarian" assumption [Morris2000, pg. But numerous experiments have been conducted to detect any change in radioactivity as a result of chemical activity, exceedingly high heat, pressure, or magnetic field. Scientists have also performed very exacting experiments to detect any change in the constants or laws of physics over time, but various lines of evidence indicate that these laws have been in force, essentially the same as we observe them today, over the multi-billion-year age of the universe.
None of these experiments has detected any significant deviation for any isotope used in geologic dating [Dalrymple1991, pg. Note, for instance, that light coming to earth from distant stars (which in some cases emanated billions of years ago) reflects the same patterns of atomic spectra, based in the laws of quantum mechanics, that we see today.
A recent survey of the rubidium-strontium method found only about 30 cases, out of tens of thousands of published results, where a date determined using the proper procedures was subsequently found to be in error.
One question that sometimes arises here is how can scientists assume that rates of radioactivity have been constant over the great time spans involved.
Here is one example of an isochron, based on measurements of basaltic meteorites (in this case the resulting date is 4.4 billion years) [Basaltic1981, pg. Skeptics of old-earth geology make great hay of these examples.