I digress, but I too have questions about the validation of radiometric dating methods that are too old to verify by observation.
Perhaps it might help to list the observations that do verify radiometric dating.
1. The observed decay rate of isotopes, and the constancy of the physical laws that govern radiometric decay as seen across the entire universe verify the validity of using decay rates as clocks. Even a small amount of an unstable isotope can produce thousands of decay events in a few minutes, even for isotopes with very long half lives. This means that measuring their decay rates does not require us to sit around for billions of years. We also know that the stability of isotopes is governed by atomic forces, the very same atomic forces that govern the power output of stars and the spectra that they produce. We observe that stars all have the same power output throughout the universe, and have the same spectra.
2. The observed properties of newly formed rocks verify the validity of the models used for radiometric dating. For example, we observe that the chemistry of zircon formation results in the inclusion of uranium and the exclusion of lead. Therefore, we can know that any Pb found in a zircon got there from the known decay products of U.
3. The cross correlation of different radiometric methods is another independent test of the methods validity. There are many different radioisotopes used in dating, and they each have a different stable product. Three examples are K/Ar, U/Pb, and Rb/Sr dating. Different isotopes have different decay rates, and there are different types of decay processes (e.g. beta and gamma). Therefore, if radiometric dating didn't work, then there would be no reason why dates from completely different isotope pairs would produce the same dates, but they do. Different methods using different isotopes give us the same dates.
4. Correlation of radiometric dating with non-radiometric dates is also a very strong source of validation. For example, if radiometric dating really does work then we should see a strong correlation between radiometric dates and specific fossils and sediments. That is exactly what we see. This is exactly what they did with radiometric dating and the famous K/T boundary which marks the end of the dinosaurs. The K/T boundary had already been defined at several places around the globe well before radiometric dating came along. When they used radiometric dating on that boundary it returned the same date at every location.
If you want to argue that radiometric dating doesn't work, then you need to explain all of these correlations, be they correlations with fossils or correlations between different radiometric dating methods. In science, this type of consilience
(a very important term to understand) is the gold standard for validating a method.
Edited by Taq, : No reason given.