As mentioned above, the isochron dating method boils down to plotting multiple data points, after some calculation, on a graph, which, if the measurements and calculations are done properly, should lie on a straight line, or very nearly on a straight line.The slope of this line, after another simple calculation, then gives the age.Quantum mechanics is one of two cornerstones of modern physics (the other is general relativity), and has been precisely confirmed in thousands of very exacting experiments.
Some of the most commonly used radiometric schemes are [Dalrymple1991, pg. 55]: Each method has its own particular range of applicability, which derives from the half-life of the particular radioactive decay involved.
Uranium-thorium dating, for instance, can be used to date specimens up to about 500,000 years old (since the half-life of the U-Th decay is 75,000 years), but Rubidium-Strontium dating can be used to date specimens billions of years old (since the half-life of the Rb-Sr decay is 48.8 billion years).
Fortunately, scientists have developed several methods that not only circumvent the difficulty of not knowing the original amounts, but also provide a very reliable means of statistical validity checking.
For example, the rubidium-strontium isochron method, one of the most widely used schemes, is based on the radioactive decay of rubidium-87 into strontium-87 by the emission of a high-energy electron.
Of course, in real scientific research, scientists do not rely on manually drawing points on graph paper to determine a best-fit straight line or to determine the line's slope or y-intercept.