🚀 go-pugleaf

In article <9204080901.aa00587@Paris.ics.uci.edu>, kibler@turing.ICS.UCI.EDU (Dennis Kibler) writes:
|> 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.

Carbon 14 dating is relatively short lived (could stretch it
to a few hundred thousand years).

The question you raise is not trivial.  There has been a lot of
research about this question, as it has been raised in the
context of trying to figure out if some cultural traits spread
through diffusion or emerged in parallel.  One of the pioneers in
the question seems to have been Colin Renfrew (a historian, not a
physicist).  One of the techniques they used in trying to
calibrate the amount of C14 was to match dates with tree-ring
data; they took samples from around the globe, and were able to
reconstruct a fairly continuous set of tree-ring matches at
several sites.  The results seemed to show that C14 was fairly
homogenous in the time scale of fluctuations; the fluctuations
were not small (its large enough that extrapolations backwards
sometimes come up with multiple roots to the equations).  If
there is ambiguity, it must be resolved in some other way
(pottery, geology, etc).  Renfrew wrote a book on this in the
early '70's.


Dan