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The time it takes for one half of the parent atoms to decay to the daughter atoms is called the half-life.If certain things are known, it is possible to calculate the amount of time since the parent isotope began to decay.

You can use the hourglass to tell time if you know several things: the amount of sand in the top of the hourglass when it started flowing, the rate that the sand flows through the hole in the middle, and that the quantity of sand in each chamber has not been tampered with.

If any of these three conditions is not accurately known, the hourglass will give an inaccurate measure of time.

As discussed before, the assumptions influence the interpretation of the data.

There are three main assumptions that must be made to accept radiometric dating methods.

The dates must be inferred based on assumptions about the ratios.

Some of the common isotope pairs used are K-Ar, Rb-Sr, Pb-Pb, and U-Pb.

Certain types of rocks, especially those that form from magma (igneous), contain radioactive isotopes of different elements.

It is possible to measure the ratio of the different radioactive parent isotopes and their daughter isotopes in a rock, but the ratios are not dates or ages.

Even the use of isochron dating, which is supposed to eliminate some initial condition assumptions, produces dates that are not reliable.

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