Changes

First, it had not been proven scientifically whether the rate of decay of <SUP>14</SUP>C has remained constant over hundreds or thousands of years until recently (with the study of very short lived isotopes): nor had it been proven that it has ''not'' remained constant.  (Notably, the decay of other isotopes that have half-lives over the course of days to microseconds all follow the characteristic first-order kinetics; hundereds of isotopes follow this pattern, and their properties extenstively have been studied.)  Some scientists have suggested, based on uncertainty theories, 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 footnotes 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'' This belief, however, has never been widely accepted in the scientific community  The "change" discussed here - in a study that is still very controversial - occurred over billions of years, though, and not in the near enough future to have had an effect on the utility of carbon dating (since carbon dating is only useful to date life within the past 50,000 years, only a change within 50,000 years would affect the dating method).

First, it had not been proven scientifically whether the rate of decay of <SUP>14</SUP>C has remained constant over hundreds or thousands of years until recently (with the study of very short lived isotopes): nor had it been proven that it has ''not'' remained constant.  (Notably, the decay of other isotopes that have half-lives over the course of days to microseconds all follow the characteristic first-order kinetics; hundereds of isotopes follow this pattern, and their properties extenstively have been studied.)  Some scientists have suggested, based on uncertainty theories, 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 footnotes 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'' This belief, however, has never been widely accepted in the scientific community  The "change" discussed here - in a study that is still very controversial - occurred over billions of years, though, and not in the near enough future to have had an effect on the utility of carbon dating (since carbon dating is only useful to date life within the past 50,000 years, only a change within 50,000 years would affect the dating method).

Second, "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 <SUP>14</SUP>C is excluded in plants also means that the apparent age of a living animal may be affected by an animal's diet.  An animal that ingested plants with  relatively low <SUP>14</SUP>C proportions would be dated older than their true age."<ref>Nondestructive Testing (NDT) Resource Center, "Uncertainty in Carbon Dating"[http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/carbondating.htm]</ref>However, this problem is rare.

Second, "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 <SUP>14</SUP>C is excluded in plants also means that the apparent age of a living animal may be affected by an animal's diet.  An animal that ingested plants with  relatively low <SUP>14</SUP>C proportions would be dated older than their true age."<ref>Nondestructive Testing (NDT) Resource Center, "Uncertainty in Carbon Dating"[http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/carbondating.htm]</ref>

== Known Limits in Carbon Dating ==

== Known Limits in Carbon Dating ==

Carbon dating 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 <SUP>14</SUP>C to <SUP>12</SUP>C in the atmosphere. Instead, isotopes like uranium 238(half-life 4.6 billion years) have been used to prove the age of the earth is around 4.5 billion years.<ref>http://www.talkorigins.org/indexcc/CD/CD011.html</ref>

Carbon dating 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 <SUP>14</SUP>C to <SUP>12</SUP>C in the atmosphere. <ref>http://www.talkorigins.org/indexcc/CD/CD011.html</ref> Instead, isotopes like uranium 238(half-life 4.6 billion years) have been used to prove the age of the earth is around 4.5 billion years.

Since the half-life of <SUP>14</SUP>C is only 5730 years, after 50,000 years only about 1/500th of the <SUP>14</SUP>C remains - and since even initially it is only present as a minute proportion of the whole (0.0057% of all Carbon atoms), measuring the exact quantity present with precision enough to be of use for dating purposes is extremely difficult and requires expensive equipment. 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 and have been be used to prove the age of the earth is roughly 4.5 billion years.

Since the half-life of <SUP>14</SUP>C is only 5730 years, after 50,000 years only about 1/500th of the <SUP>14</SUP>C remains - and since even initially it is only present as a minute proportion of the whole (0.0057% of all Carbon atoms), measuring the exact quantity present with precision enough to be of use for dating purposes is extremely difficult and requires expensive equipment. 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 and have been be used to prove the age of the earth is roughly 4.5 billion years.