Changes

The technique is based on comparing the levels of <SUP>14</SUP>C-14 and <SUP>12</SUP>C-12 isotopes in the sample. When the sample was living, the ratio of C-<SUP>14 </SUP>C to <SUP>12</SUP>C-12 in the material is the same as in the atmosphere. But after death the <SUP>14</SUP>C-14 isotope decays into N-14, while <SUP>12</SUP>C-12 does not. The loss in <SUP>14</SUP>C-14 is then used to estimate how long ago the sample died, which is typically the same as how long the sample existed.

The rate of decay of C-<SUP>14 </SUP>C is such that 50% of the C-<SUP>14 </SUP>C in the sample will decay in 5730 years: 50% of the C-<SUP>14 </SUP>C in the sample is left, after 11460 years 25% will be left, after 17190 years 12.5% will be left, after 50,000 years only about 1/500th of the C-<SUP>14 </SUP>C remains - and since even initially it is only present as a minute proportion of the whole (0.0000000001% of all Carbon atoms)<ref>http://www.enviroliteracy.org/article.php/1006.html</ref>, measuring the exact quantity present accurately enough to be of use for dating purposes is extremely difficult. For this reason, scientists do not generally attempt to carbon date material that is believed to be older than about 50,000-60,000 years old.

[[Image:C-14decay.JPG|framed|The first-order decay curve of carbon-14 based on the half-life of 5730 years. Because of the small amount of C-<SUP>14 </SUP>C remaining after 50,000 years, the rate of decay becomes small.]]

First, it had not been proven scientifically whether the rate of decay of C-<SUP>14 </SUP>C has remained constant over hundreds or thousands of years. Some scientists have suggested, based on experimental observations, that the laws of physics do change over time.<ref>"The idea that nature's laws change over time was proposed in the 1930s by one of the titans in the history of physics, Paul Dirac of England. According to Dirac's large numbers hypothesis, the force of gravity changed over time." See Keay Davidson, "Recent study forces scientists to rethink basic law of physics," San Francisco Chronicle[http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2005/05/09/MNG5LCLEU41.DTL]</ref>

Second, estimating the age of material using carbon dating assumes that the ratio of C-<SUP>14 to </SUP>C-to <SUP>12 </SUP>C was the same in the past as today, which is also impossible to prove scientifically. "[T]he proportions of C-14 in the atmosphere in historic times is unknown. The C-14:C-12 atmospheric ratio is known to vary over time and it is not at all certain that the curve is 'well behaved.'"<ref>Nondestructive Testing (NDT) Resource Center, "Uncertainty in Carbon Dating"[http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/carbondating.htm]</ref>

Third, "various plants have differing abilities to exclude significant proportions of the C-14 in their intake. This varies with environmental conditions as well. The varying rates at which C-<SUP>14 </SUP>C is excluded in plants also means that the apparent age of a living animal may be affected by an animals animal's diet. An animal that ingested plants with relatively low C-<SUP>14 </SUP>C proportions would be dated older than their true age."<ref>''Id.''</ref>

Due to these fundamental limitations, the only scientific conclusion that can be drawn from carbon dating is that material X is older than material Y when X has lost more C-<SUP>14 </SUP>C than Y has. It is not possible to determine, with any degree of scientific certainty, how much older X is than Y.

Even the staunchest advocates of carbon dating admit that it the technique cannot give accurate numbers if the sample is younger than 150 years or older than 50,000 years. The inaccuracy for material younger than 150 years is ascribed to changes in the ratio of C-<SUP>14 to </SUP>C-to <SUP>12 </SUP>C in the atmosphere.<ref>http://www.talkorigins.org/indexcc/CD/CD011.html</ref>

Since the half-life of C-<SUP>14 </SUP>C is only 5715 years, after 50,000 years only about 1/500th of the C-<SUP>14 </SUP>C remains - and since even initially it is only present as a minute proportion of the whole (0.0000000001% of all Carbon atoms), measuring the exact quantity present with precision enough to be of use for dating purposes is extremely difficult. For this reason, scientists do not generally attempt to carbon date material that is believed to be older than about 50,000-60,000 years old. However, isotopes with longer half-lives may be used.