George Murphy wrote:
.............
> Suppose you were looking at abundances of a radioisotope with 1/2 life 2 billion
> years. The abundance in your rock A would be down by a factor (1/2)^(25.5/2) ~
> (1/2)^(13) ~ 1/8000 of its initial value. OTOH, the abundance in rock C would be
> (1/2)^(.125/2) = (1/2)^(1/16) ~ .96 of its original value. Whether you could
> distinguish 1/8000 from 0, or .96 from 1, would depend on how good your instrumentation
> was. There are no _absolute_ limits, "too old" or "too young".
For rock C we would actually have (1/2)^(.000125/2) ~ (1/2)^(.00006) ~ .99995
which illustrates even better the possible difficulty of distinguishing the result from
unity.
Apologetically,
George
George L. Murphy
gmurphy@raex.com
http://web.raex.com/~gmurphy/