When I first got involved in the creationism/evolution controversy, back in early 1995, I looked around for an article or book that explained radiometric dating in a way that nonscientists could understand. Young-Earth creationists -- that is, creationists who believe that Earth is no more than 10,000 years old -- are fond of attacking radiometric dating methods as being full of inaccuracies and riddled with sources of error.

All these methods point to Earth being very, very old -- several billions of years old.

spectrographic dating-46

I found several good sources, but none that seemed both complete enough to stand alone and simple enough for a nongeologist to understand them.

Thus this essay, which is my attempt at producing such a source.

Contents: The half-life of a radioactive isotope is defined as the time it takes half of a sample of the element to decay.

A mathematical formula can be used to calculate the half-life from the number of breakdowns per second in a sample of the isotope.

Some isotopes have very long half-lives, measured in billions or even trillions of years.

Others have extremely short half-lives, measured in tenths or hundredths of a second.

Some isotopes can break down in more than one way -- in these cases, each different breakdown type has its own half-life.

The decay rate and therefore the half-life are fixed characteristics of an isotope. That's the first axiom of radiometric dating techniques: the half-life of a given isotope is a constant.

(Note that this doesn't mean the half-life of an element is a constant.