Several hundred laboratories around the world are active in radiometric dating.
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.
In a related article on geologic ages (Ages), we presented a chart with the various geologic eras and their ages.
In a separate article (Radiometric dating), we sketched in some technical detail how these dates are calculated using radiometric dating techniques.
If two or more radiometric clocks based on different elements and running at different rates give the same age, that's powerful evidence that the ages are probably correct.
Along this line, Roger Wiens, a scientist at the Los Alamos National Laboratory, asks those who are skeptical of radiometric dating to consider the following (quoted in several cases from [Wiens2002]): All of the different dating methods agree--they agree a great majority of the time over millions of years of time.Over a thousand papers on radiometric dating were published in scientifically recognized journals in the last year, and hundreds of thousands of dates have been published in the last 50 years.Essentially all of these strongly favor an old Earth.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).The differences actually found in the scientific literature are usually close to the margin of error, usually a few percent, not orders of magnitude!