It's based on the very simple principle that radioactive isotopes decay at a steady, predictable rate.
Radium, for example, has a half-life of about 1,600 years.
This technique is not restricted to bones; it can also be used on cloth, wood and plant fibers. The short half-life of carbon-14 means its cannot be used to date extremely old fossils.Carbon-14 dating has been used successfully on the Dead Sea Scrolls, Minoan ruins and tombs of the pharohs among other things. Levels of carbon-14 become difficult to measure and compare after about 50,000 years (between 8 and 9 half lives; where 1% of the original carbon-14 remains undecayed).Radiocarbon dating is one of the great tools of science that has allowed archeologists to shed new light on everything from the building of Stonehenge to the beginnings of international trade.However, a new study from the Imperial College London suggests that fossil fuel carbon emissions may be so diluting radioactive carbon isotopes that within decades it will difficult to differentiate between modern artifacts and those over a thousand years old.And finally, they count all of the tree-rings, using the matching patterns to connect all the pieces, and they determine the age of the oldest piece of wood.
This is called a "long chronology." By dating the oldest piece of wood using the Carbon dating method and comparing the two dates, scientists can make the necessary adjustments to their calculations. Aardsma, "Myths Regarding Radiocarbon Dating," Impact, No.Carbon Dating - Dendrochronology As we've already seen, in order for Carbon dating to work we need to know what the C-12 to C-14 ratio was at the time of a specimen's death.If the ratio has fluctuated throughout the unobservable past (and we can be sure that it has), how can we determine what the ratio was during the lifetime of a specimen that lived and died before we first began measuring the ratio?Carbon-14 is created from nitrogen-14 in the upper atmosphere of the earth.Radiation from the sun collides with atoms in the atmosphere.Furthermore, the presumption that radiocarbon dating is an "exact science" is erroneous and in all fairness few scientists make this claim anyway.