Absolute dating isotopes
Proportion 1 becomes: Stated in words, this equation says that the rate at which a certain radioisotope disintegrates depends not only on how many atoms of that isotope are present but also on an intrinsic property of that isotope represented by λ, the so-called decay constant.
Values of λ vary widely—from 10 is the time elapsed since time zero.
Given below is the simple mathematical relationship that allows the time elapsed to be calculated from the measured parent/daughter ratio.
The age calculated is only as good as the existing knowledge of the decay rate and is valid only if this rate is constant over the time that elapsed.
In short, the process of radioactive decay is immutable under all known conditions.
Although it is impossible to predict when a particular atom will change, given a sufficient number of atoms, the rate of their decay is found to be constant.
The situation is analogous to the death rate among human populations insured by an insurance company.
Even though it is impossible to predict when a given policyholder will die, the company can count on paying off a certain number of beneficiaries every month.
Of course, one must select geologic materials that contain elements with long half-lives— those for which some parent atoms would remain.
By way of explanation it can be noted that since the cause of the process lies deep within the atomic nucleus, external forces such as extreme heat and pressure have no effect.