🚀 go-pugleaf

How is radioactive dating done?

 I understand that by knowing the current % (say Z) of X in Y,
and the half-life of X, that we can project backwards
and figure out the % (say Z') of X in Y at any previous time.
So to figure out the age of Y  we need to know the original
percentage of X in Y.  But how is that known?

 Let me put it in a more concrete form. Suppose we use carbon
dating to measure the age of a fossilized plant. I don't
understand why we aren't measuring the age of the carbon,
created I presume at the some time before the age of the earth,
rather than the age of the plant.

dennis