Difference between revisions of "Carbon dating"

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(Principles: Move image up)
(Limits of Carbon Dating: Add a bit about calibration.)
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Carbon dating, like other [[radiometric dating]] methods, requires certain assumptions that cannot be scientifically proved.
 
Carbon dating, like other [[radiometric dating]] methods, requires certain assumptions that cannot be scientifically proved.
 
These include the starting conditions, the constancy of the rate of decay, and that no material has left or entered the sample.
 
These include the starting conditions, the constancy of the rate of decay, and that no material has left or entered the sample.
 +
 +
Unlike other radiometric dating techniques where it is not possible to [[calibration|calibrate]] the method against historically-known dates, limited calibration is possible for carbon dating.
 +
That is, samples with dates known from historical records can be used to check the accuracy of the method.
 +
Despite this, however, caution is still necessary in accepting dates derived from carbon dating.
  
 
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.{{Fact}}  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>  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.{{Fact}}  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>  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).

Revision as of 11:09, September 17, 2008

Carbon dating, or carbon-14 dating, is a method for comparing the ages of organic materials such as bones or things made from anything that once lived.

Principles

The first-order decay curve of carbon-14 based on the half-life of 5730 years.

The technique is based on comparing the levels of 14C and 12C isotopes in the sample. 14C is produced in the atmosphere by cosmic ray neutrons replacing a proton in nitrogen (14N), producing 14C.

14C is unstable and decays back to 14N, at the rate of 50% every 5,730 years (so after 11,460 years 25% will be left, after 17,190 years 12.5% will be left, and so on).

In the meantime, however, the 14C will combine with oxygen in the atmosphere to form carbon dioxide, which enters the food chain via photosynthesis in plant. By this means, most living things also have 14C and 12C in the same ratio as in the atmosphere. This ratio is about one 14C atom for every 1,000,000,000,000 12C atoms.

However, when the sample dies, it stops ingesting 14C, so as the 14C decays to 14N, the ratio of 14C and 12C changes. This ratio of 14C to 12C is measured and a calculation turns the measurement into a figure representing how long ago the sample died.

Depending on the method used to measure the 14C, after about 50,000 years or so there is not enough left to measure, although advanced techniques can possibly stretch that to 100,000 years.[1] This then becomes the maximum age that can theoretically be derived by this method.

Limits of Carbon Dating

Carbon dating, like other radiometric dating methods, requires certain assumptions that cannot be scientifically proved. These include the starting conditions, the constancy of the rate of decay, and that no material has left or entered the sample.

Unlike other radiometric dating techniques where it is not possible to calibrate the method against historically-known dates, limited calibration is possible for carbon dating. That is, samples with dates known from historical records can be used to check the accuracy of the method. Despite this, however, caution is still necessary in accepting dates derived from carbon dating.

First, it had not been proven scientifically whether the rate of decay of 14C has remained constant over hundreds or thousands of years.[Citation Needed] Some scientists have suggested, based on experimental observations, that the laws of physics do change over time.[2] 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 14C 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 14C proportions would be dated older than their true age."[3]

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 14C to 12C in the atmosphere.[4]

See also

References

  1. Nave, R., Carbon Dating, Georgia State University.
  2. "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[1]
  3. Nondestructive Testing (NDT) Resource Center, "Uncertainty in Carbon Dating"[2]
  4. http://www.talkorigins.org/indexcc/CD/CD011.html