RTB's correspondence team is asked repeatedly to explain methods for dating the age of the universe and earth. The most frequent question has to do with the reliability of carbon 14 dating.
I make no mention of carbon 14 in my books for six reasons: First, the carbon 14 dating method measures the time since a living organism has died. Thus, it is useless for measuring anything that has never been alive, such as a rock. Second, compared with many other radiometric elements, carbon 14 decays quickly. It is useless for dating anything older than about 50,000 years. Third, because carbon 14 forms from cosmic ray bombardment of nitrogen 14 (and decays back into nitrogen 14 through the release of beta particles, i.e., electrons) the effect of variations in cosmic radiation intensity (caused by altitude, depth below the earth's surface, and astronomical events) can be difficult to calibrate. Fourth, a specimen's contamination by carbon from surrounding soil, water, vegetation, and animal matter can seriously undermine accuracy of tests on a given sample. Fifth, the release of carbon dioxide from fossil fuel burning significantly dilutes carbon 14, and researchers have no accurate way to calibrate this dilution factor. Sixth, and perhaps most significant, astronomy provides much simpler, more consistent, and more direct methods for measuring the date for creation.
With all these strikes against it, you might wonder why carbon 14 dating draws so much attention. Of all the dating methods that scientists use, carbon 14 has received the most publicity. Why? Perhaps because it's the method commonly used in dating popular artifacts, such as the Shroud of Turin.
Also, many young-earth creationists have attempted to discredit all scientific tools for age-determination by demonstrating how unreliable carbon 14 can be. In their book, Reasons Skeptics Should Consider Christianity (Here's Life Publishers, 1981), Josh McDowell and Don Stewart poke fun at a Yale University study yielding three different ages for an antler -- 5,340 years, 9,310 years, and 10,320 years. Their derision is unwarranted, however, for the researchers explained what caused variations in result: limestone contamination of the sample. The 5,340 years is the carbon 14 date for the limestone contaminant; the 10,320 years is the antler's date; and the 9,310 years is the date for the antler and limestone combined.
People without training in science may not understand that any radiometric dating method can only be trusted for samples with ages close to the half-life of the element in question. Carbon 14's half-life is 5,770 years. This means it takes 5,770 years for half of the carbon 14 to decay into nitrogen 14. It takes 11,540 years for three fourths of the carbon 14 to decay, 17,310 years for seven-eighths of the carbon to decay, and 23,080 years for fifteen-sixteenths of the carbon to decay. Thus, the half-life of carbon 14 makes it a useful dating tool only for specimens between about 500 and 25,000 years old. Because of carbon 14's effectiveness range, it has been an excellent tool for giving us accurate dates of ancient Bible manuscripts. These dates have proved invaluable for settling disputes about authorship of various books of the Bible. Likewise, the carbon 14 dating of archeological artifacts has repeatedly vindicated biblical texts against the criticism of skeptics. As with other tools, it proves beneficial if and when it is applied to the right task.
Here are some brief guidelines for evaluating the trustworthiness of a carbon 14 test. First, always look at the age range to insure that it is appropriate--somewhere between 500 and 25,000 years. Second, look for the error bar. The size of possible error indicates the degree of contamination of the specimen tested or inadequacy of the sample's size. Third, be extra cautious about measurements dating back to eras when cosmic ray intensity was significantly less than it is today. More specifically, a dramatic supernova (massive star explosion) occurred near Earth (about a thousand light years distant) between 8,000 and 15,000 years ago. That event dramatically increased Earth's level of cosmic ray bombardment.
Even in cases of reasonable doubt about the date-determining accuracy of carbon 14, the measure may still have value. It can be helpful in showing that one sample is older or younger than another and by roughly how much. Sometimes relative dates are all we need to answer an important question.