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These long time periods are computed by measuring the ratio of daughter to parent substance in a rock and inferring an age based on this ratio.
One key factor is the fact that Ar-Ar dating needs a standard of "known" age.Most scientists today believe that life has existed on the earth for billions of years.This belief in long ages for the earth and the existence of life is derived largely from radiometric dating.It was explained by the processes of heating and cooling that the soil had been through. Dalrymple, “40Ar/36Ar Analyses of Historic Lava Flows,” Earth and Planetary Science Letters,6 (1969): pp. Some rocks have been measured with negative radiometric ages, in some cases in terms of millions of years.Some rocks dated older than the 4.5 billion year evolutionary age for Earth. Isochron dating can also produce negative ages, by producing a negative slope.