Radiometric dating is a method that scientists use to measure the age of things. The most widely known form of radiometric dating is that based on carbon-14. It works like this. Carbon-12 is the most common form of carbon, and carbon-14 is what is called an isotope of it. (An isotope is a variation of the normal atom of an element, in that it has more or less neutrons than the standard atom.) Carbon-12 has six protons and six neutrons at its nucleus, and is therefore said to have an atomic weight of 12. The component of an atom that determines its character is the number of electrons in orbit around its nucleus. In carbon’s case there are six.
In the high atmosphere, the sun’s rays knock out neutrons from the nuclei of atoms. These neutrons in turn bump into other atoms in the lower atmosphere. Nitrogen makes up about 78% of the atmosphere, so nitrogen atoms are prime targets for being bumped. A nitrogen atom has seven protons and seven neutrons in its nucleus, along with seven electrons spinning around the nucleus. The stray neutrons dislodged by the sun’s rays knock one electron off a small fraction of nitrogen atoms. Because the number of electrons determine the character of an atom, and these “bumped” atoms now have only six electrons, these nitrogen atoms become carbon-14. Because carbon-14 is an unstable isotope, it will in time radioactively decay back to normal nitrogen. This decay rate of carbon-14 back to nitrogen-14 is measurable.
The standard measurement in radioactive decay is called a half-life. This is how long it would take a certain amount of a substance to decay to half its weight (a gram to half a gram, for instance). The half-life of carbon-14 is 5,730 years (plus or minus 40 years). To measure the age of things, scientists make the supposition that the ratio of carbon-12 to carbon-14 in the atmosphere has remained constant over time.
Carbon-12 and carbon-14 are both absorbed by living things such as animals and plants, in the form of carbon dioxide. Once the living thing dies, no more carbon is absorbed. The carbon-14 content of the dead animal or plant then immediately starts to radioactively decay back to nitrogen, and escapes as a gas. The carbon-12, on the other hand, does not decay. Thus, if the amount of carbon-14 in relation to the amount of carbon-12 in a sample from a carcass can be measured, it should be able to give a fairly good estimate as to when the living thing died.
That all sounds good in theory, but the problem lies in the original supposition that the ratio of carbon-12 to carbon-14 in the atmosphere has remained constant. This has not been the case. Remember that carbon-14 does not come from carbon-12, but rather from nitrogen in the atmosphere. With the advent of the Industrial Revolution and the burning of fossil fuels, much more carbon-12 has been injected into the atmosphere with no corresponding increased levels of nitrogen turning into carbon-14. Although above-ground nuclear tests have discharged some more carbon-14 into the atmosphere, this has not compensated for the difference.
Also, it has been discovered that plants discriminate against carbon dioxide containing carbon-14 atoms, so that the absorption rate is lower than for normal carbon-12. Additionally, the rate of carbon-14 formation in the atmosphere has fluctuated due to Earth’s weakening magnetic field. And finally, the Flood would have made a huge difference in that a vast quantity of carbon, in the form of plant and animal life, was pulled out of the biosphere and buried under the sediment deposited as the waters receded.
The net effect of all these conditions is that ancient specimens will test much older than they are.
Furthermore, the limit of carbon-14 dating is set at around 50,000 years, as after that time there would be no detectable amount of carbon-14 left in a sample. So with regard to dating fossils that are supposedly millions of years old, carbon-14 dating is useless.
Other forms of radiometric dating are even more subject to error.