\begin{definition}[Definition:Radiometric Dating]
'''Radiometric dating''' is a technique whose purpose is to work out the age $T$ of a physical object $B$.
The pieces of information are known:
:$(1): \quad$ The ratio $r_0$ of a radioactive isotope $E_R$ of a chemical element $E$ to its stable isotope $E_S$ in $B$ at the time it came into being
:$(2): \quad$ The ratio $r$ of $E_R$ to $E_S$ at the time now
:$(3): \quad$ The half-life of the radioactive isotope $E_R$
It is known from scientific investigation what $r_0$ is when a physical object is created.
It is also known from scientific investigation what the rate of radioactive decay of $E_R$ is.
Hence it can be worked out by use of the First-Order Reaction how long it would take for the ratio of $E_R$ to $E_S$ to reach its current ratio $r$.
\end{definition}