How reliable is carbon dating? Is radiocarbon dating a good method to use to determine the age of things?
Carbon dating, or radiocarbon dating, can be helpful in determining the relative age of an object, but has many limitations. The testing for carbon dating relies on many factors and should be used in conjunction with other methods of dating materials. Carbon dating works only with material that was once alive. It does not work on rock, for example, but does work on wood. So, an old spear can be tested at the wooden shaft, but not the sharp stone head. The test will identify about how long ago the wood was cut from a living tree, but cannot tell when it was made into a spear or when the stone head was attached.
Sometimes, archaeologists will date an object by carbon dating another object nearby. This method of dating obviously relies on assumptions about the relationship between the object and the actual tested material.
Also, the difficulty of using carbon dating increases as objects grow older. Living things have concentrations of carbon-14 in them that are identical to the concentrations of carbon-14 in the atmosphere at the time they die. When such material does die, the material stops absorbing carbon-14 and the carbon-14 within it begins to decay. Carbon-14 makes up about 1 trillionth of the earth's atmosphere, so these minuscule quantities are what scientists rely upon.
Eventually the remaining carbon-14 becomes so little as to be nearly undetectable. Tiny variations within a sample can significantly skew results. Scientists use enrichments and enhancements to make small quantities of carbon-14 easier to detect, but this, too, can skew results. For this reason, carbon dating is only plausible for objects less than about 40,000 years old.
Another complicating factor is that carbon dating is based on the loss of carbon-14. So obtaining an accurate age depends on the assumption of how much carbon-14 was in the atmosphere at the time the organism died. Complicating things further, the Earth's concentration of carbon-14 changes based on a variety of factors.
As with any scientific testing, contamination can drastically impact the results of tests. Repeatability is another challenge. Inconsistent results are often discarded under the claim of contamination. Multiple tests, and various testing, must be done to arrive at plausible results.
Also common is problematic results being thrown out because they are inconsistent with expected results. This is not necessarily unusual in science, but one must keep in mind how assumptions and interpretations are related. At worst, this may lead to circular and self-confirming dating, though other methods of dating can reduce the risk.
In short, carbon dating is as useful as any other technique, so long as it's done properly and the results are objectively interpreted. It is not, however, an inherently error-free or black-and-white method for dating objects.
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