Carbon dating, or carbon-14 dating, is a method for comparing the ages of organic materials such as bones or artifacts made from anything that once lived. Unlike many other radiometric dating methods, carbon dating has been calibrated for historical periods and within that range can give reliable results.
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 plants. 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. This then becomes the maximum age that can theoretically be derived by this method.
Therefore, any sample with too little 14C to measure must, in theory, be older than 50,000 to 100,000 years, and it is not possible to determine how much older. Similarly, any sample with measurable 14C must be younger than 50,000 to 100,000 years, assuming that adequate precautions have been taken to eliminate contamination.
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.
Claims have been made of the method being calibrated back to 10,000 years using dendrochronology, however these older dates derived via dendrochronology have themselves been derived with the assistance of carbon dating, making this circular reasoning.
Not all living things do have 14C:12C ratios the same as the atmosphere. For example it is well known that carbon dating cannot be used on many types of marine life due to reservoirs of "old" carbon held in sedimentary rocks. This problem is especially severe in shellfish or anything that eats shellfish; it is not unusual for freshly-killed seals to be dated as several thousand years old.
...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-14 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 C-14 proportions would be dated older than their true age."
The method relies on the assumption that we know how much 14C is in the atmosphere, but this has been known to change. Nuclear testing since the 1950s has resulted in a large increase in the amount of 14C in the atmosphere, but because the levels have been measured since the 1950s, calculations can be adjusted for these changing levels, meaning that dating of recent samples is possible.
Dating laboratories do not make any allowance for the change in atmospheric levels that would have occurred as a result of Noah's Flood. This means that radio-carbon dates cannot be used to prove that the Flood did not occur, because it assumes that it did not occur.
Scientists at Brookhaven National Laboratory and the Physikalish-Technische-Bundesandstalt in Germany have reported "unexplained periodic fluctuations in the [nuclear] decay rates of Si-32 and Ra-226... strongly correlated in time, not only with each other, but also with the distance between the Earth and the Sun." It is likely that similar discrepancies and fluctuations occur with other nuclear decay rates, such as that of 14C. It remains for scientists to perform experiments to explore these emerging issues. 
Carbon Dating was developed by Willard F. Libby and his team of scientists at the University of Chicago in 1949. They initially measured the 'half-life' (a term that Libby coined) as 5568±30 years, and this became known as the Libby half-life. It was later measured more accurately to be 5730±40 years, now known as the Cambridge half-life.
Many people believe that carbon dating has proved that the Earth is millions or billions of years old, much older than the biblically derived date of around 6,000 years. However, as explained above, carbon dating is incapable of providing dates in the range of millions or billions of years, and some atheistic scientists turn to more speculative forms of dating to try to derive such extraordinary time periods.
Some also argue that carbon dating should only be used on samples that fall within the range over which it can measure. However, this prompts the question of how one might determine this prior to using carbon dating to determine the age. They further argue that dating much older items will result in anomalous dates, which might fall within the range that carbon dating can measure. This is incorrect. Any sample that is older than the range that carbon dating will measure will record zero 14C, and can therefore not be confused with younger samples.
- Higham, Thomas, Introduction, Radiocarbon web-info.
- Nave, R., Carbon Dating, Georgia State University.
- Batten, Don, Tree ring dating (dendrochronology) (Creation Ministries International).
- Nondestructive Testing (NDT) Resource Center, Carbon-14 Dating"
- Higham, Thomas? K-12 Radiocarbon web-info.
- Batten, Don, (Ed.) What about carbon dating? Chapter 4 of the Creation Answers Book.
- Evidence for Correlations Between Nuclear Decay Rates and Earth-Sun Distance, Jenkins et al. Abstract: Unexplained periodic fluctuations in the decay rates of Si-32 and Ra-226 have been reported by groups at Brookhaven National Laboratory (Si-32), and at the Physikalisch-Technische-Bundesandstalt in Germany (Ra-226). We show from an analysis of the raw data in these experiments that the observed fluctuations are strongly correlated in time, not only with each other, but also with the distance between the Earth and the Sun. Some implications of these results are also discussed, including the suggestion that discrepancies in published half-life determinations for these and other nuclides may be attributable in part to differences in solar activity during the course of the various experiments, or to seasonal variations in fundamental constants.