Radioactive potassium dating
On the surface, radiometric dating methods appear to give powerful support to the statement that life has existed on the earth for hundreds of millions, even billions, of years.
We are told that these methods are accurate to a few percent, and that there are many different methods.
We can assume that the Precambrian rocks already existed when life began, and so the ages of the Precambrian rocks are not necessarily related to the question of how long life has existed on earth.
The Cambrian period is conventionally assumed to have begun about 550 million years ago.
In order to use these methods, we have to start out with a system in which no daughter element is present, or else know how much daugher element was present initially so that it can be subtracted out.
We also need to know that no parent or daughter has entered or left the system in the meantime.
Uranium decays to lead by a complex series of steps. Thus we obtain K-Ar dating, U-Pb dating, and Rb-Sr dating, three of the most common methods.
For potassium 40, the half-life is about 1.3 billion years.
In general, in one half-life, half of the parent will have decayed.
Perhaps the earth was made from older pre-existing matter, or perhaps decay rates were briefly faster for some reason. Geologic time is divided up into periods, beginning with the Precambrian, followed by the Cambrian and a number of others, leading up to the present.
When one considers the power of God, one sees that any such conclusions are to some extent tentative. Some fossils are found in Precambrian rocks, but most of them are found in Cambrian and later periods.
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Assuming we start out with pure parent, as time passes, more and more daughter will be produced. A ratio of infinity (that is, all daughter and no parent) means an age of essentially infinity.