(Note that this doesn't mean the half-life of an element is a constant.
Different isotopes of the same element can have substantially different half-lives.) It's important to understand that the half-life is a purely statistical measurement. A sample of U238 ten thousand years old will have precisely the same half-life as one ten billion years old.
All these methods point to Earth being very, very old -- several billions of years old.
Radiometric dating methods are the strongest direct evidence that geologists have for the age of the Earth.
When I first became interested in the creation-evolution debate, in late 1994, I looked around for sources that clearly and simply explained what radiometric dating is and why young-Earth creationists are driven to discredit it.
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.