Age-dating a rock using its radioactive isotopes only works by assuming that the rate at which that 'clock' ticks was constant in the past and essentially identical to that in the present. Not long ago, scientists discovered excess helium in crystals and 'orphaned' polonium radiohalos, both of which imply that the decay rates of isotopes commonly used to date earth rocks were dramatically accelerated in the past. Even today, researchers are finding small but significant changes in isotope decay rates, and these add credibility to the idea that isotopic processes were once very different from today's processes.
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