>> “An assumption of a maximum age of 6,000 years requires an assumption of a decay rate that has never been observed.” <<
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No, it merely requires a more reasonable assumption of the original distribution of the elements that would better match the requirements of life on Earth.
This wouldn’t apply to tl, but it might to any “old earth creationists” that you might talk to.
How old would Adam have appeared to be an hour after he was created from the dust?
So, how old should a created earth appear to be now?
Why would Uranium consistently be distributed along with all the daughter elements of radiometric decay, in ratios consistent with billions of years worth of radiologic decay, at the molecular level? Stable elements do not exhibit this kind of homogenous distribution. What is reasonable about assuming that this distribution of radioisotopes is a requirement of life on Earth?