Carbon dating is a technique to determine the age of archeological objects, fossils, or any remnant from the past. The phenomenon of radioactivity makes it possible. This was developed by Willard Libby and his associates, in 1949. The impact of this technique on the world of science, led to Libby being awarded the Nobel prize in chemistry, in 1960.
This dating method is based on comparison of ratios of two carbon isotopes, which are C-12 and C-14. C-14 is radioactive, while C-12 is not. Isotopes are elements with the same atomic number, but different atomic mass numbers.
C-14 has a half life of approximately 5730 years. By comparing the amount of radioactive carbon present now, with the amount at the beginning, an age estimate can be made, based on laws of radioactive decay.
Why was carbon chosen for the dating method, out of all the radioactive elements? One reason is the fact that it has a radioactive isotope and second reason is that life is based on carbon compounds. This made age determination of carbonaceous life possible.
Limitations of the Technique
Every scientific method has its limitations. This is because the fundamental assumptions or axioms, on which a method like carbon dating is based, are only approximately true or accurate. All the physical laws we know have limits of validity.
The fact that scientific methods like these, fail beyond a certain domain of approximation, doesn't make them redundant. It only means that these need to be used with caution and with a knowledge of the limits of their accuracy.
C-14/C-12 Ratio is Not Absolute
The accuracy of this method is largely dependent on an assumed absolute ratio of C-14 to C-12, which is supposed to have remained constant over time. However, in reality the ratio varies with time and place.
C-14 is created in the upper layers of the atmosphere, by the bombardment of Nitrogen with cosmic rays. It then enters the lower atmosphere and gets distributed, forming carbon dioxide in the process.
This carbon dioxide, made of radioactive carbon, is assimilated by plants, through the process of photosynthesis and it enters into food cycle. Carbon forms the backbone of life on Earth.
This ratio of carbon-14 to carbon-12 is taken to be the one detected, in 1950, which is taken as the 'Present' or the point in time, up to which the age is determined.
Hence, the ages are specified as 3400 BP (3400 years BP). Where 'BP' stands for before present. The flaw in this method is that it assumes that the carbon-14/carbon-12 ratio has been the same till 1950, which is not the case in reality.
However, this problem is taken care of, by calibration curves, which account for the change in the ratio. Still, this changing ratio of carbon isotopes, does make a minuscule dent in the calculation accuracy.
There is another possibility, which may affect the accuracy of this dating method. It is necessary that the dating sample be as uncontaminated as possible, to give it an accurate age. However, that may not be possible for samples, which are too old.
The possibility of contamination of very ancient samples, is unavoidable. Contamination disturbs the ratio of carbon isotopes found in situ, in the sample. This may lead to an inaccurate determination of age. There is nothing we can do in this case.
However, error in determination of age, by carbon dating methods can be compensated by using other dating techniques, which use radioactive elements, other than carbon.
This dating technique is inaccurate at times but its error levels are very low and certainly not inaccurate enough to be called wrong. Accuracy of this dating technique is not perfect, but it has given remarkably precise results, when tested to calculate age of artifacts, whose age was known, by other methods.