Radiometric dating--the process of determining the age of rocks from the decay of their radioactive elements--has been in widespread use for over half a century.
There are over forty such techniques, each using a different radioactive element or a different way of measuring them.
You cannot predict exactly when any one particular grain will get to the bottom, but you can predict from one time to the next how long the whole pile of sand takes to fall.
Once all of the sand has fallen out of the top, the hourglass will no longer keep time unless it is turned over again.
Many Christians have been led to distrust radiometric dating and are completely unaware of the great number of laboratory measurements that have shown these methods to be consistent.
Further evidence comes from the complete agreement between radiometric dates and other dating methods such as counting tree rings or glacier ice core layers.This paper describes in relatively simple terms how a number of the dating techniques work, how accurately the half-lives of the radioactive elements and the rock dates themselves are known, and how dates are checked with one another.In the process the paper refutes a number of misconceptions prevalent among Christians today. Wiens has a Ph D in Physics, with a minor in Geology.His Ph D thesis was on isotope ratios in meteorites, including surface exposure dating.