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Timescale of radioactive decay
Although the time at which any individual atom will decay cannot be forecast, the time in which any given percentage of a sample will decay can be calculated to varying degrees of accuracy. The time that it takes for half of a sample to decay is known as the half life of the isotope. Some isotopes have half lives longer than the present age of the universe, but they are still subject to the same laws of quantum physics and will eventually decay, even if doing so at a time when all remaining atoms in the universe are separated by astronomical distances.
Various elements are used for dating different time periods; ones with relatively short half-lives like carbon-14 (or 14C) are useful for dating once-living objects (since they include atmospheric carbon from when they were alive) from about ten to fifty thousand years old. See Carbon dating. Longer-lived isotopes provide dating information for much older times. The key is to measure an isotope that has had time to decay a measurable amount, but not so much as to only leave a trace remaining. Given isotopes are useful for dating over a range from a fraction of their half life to about four or five times their half life.
How it works
Symbolically, the process of radioactive decay can be expressed by the following differential equation, where N is the quantity of decaying nuclei and k is a positive number called the exponential decay constant. The meaning of this equation is that the rate of change of the number of nuclei over time is proportional only to the number of nuclei. This is consistent with the assumption that each decay event is independent and its chance does not vary over time.
The solution to this equation is:
For decay, the constant k in the above equation can be calculated as:
where is the half-life of the element, is the time expired since the sample contained the initial number atoms of the nuclide, and is the remaining amount of the nuclide. We can measure directly, for example by using a radiation detector, and obtain a good estimate of by analyzing the chemical composition of the sample. The half-life , specific to each nuclide, can be accurately measured on a pure sample, and is known to be independent of the chemical composition of the sample, temperature and pressure. Solving for gives us the estimated age of the sample:
You find a bone fragment and through analysis you determine that it contains 13% of its original carbon-14. The half-life of carbon-14 is approximately 5,730 years. Approximately how old is the bone?
Start with the equation for continuous growth and decay:
Since the quantity represents 13% (or 13/100ths) of , it follows that , thus:
cancels out, leaving us with:
We can take the "inverse property of logarithms" and get:
Solving for t gives us:
We can now calculate the value of k as follows:
Plug into equation and solve:
Thus the bone is approximately 17,000 years old. (Our input data had two significant figures, so reporting a more accurate result would be meaningless.)
Limitations of radiometric dating
A important limitation of radiometric dating often overlooked by layman (and not always made clear in scholarly works as well) is that any date is actually a range, following the 68–95–99.7 rule.
A proper radiometric date should read years before present (with 1950 being present) ± range/2 at x standard deviations (Xσ)', but is often reported as a single year or a year range, like 1260–1390 CE (the date for the Shroud of Turin). This leaves out important information which would tell you how precise is the dating result.
Carbon-14 dating has an interesting limitation in that the ratio of regular carbon to carbon-14 in the air is not constant and therefore any date must be calibrated using dendrochronology. Another limitation is that carbon-14 can only tell you when something was last alive, not when it was used.
Note that although carbon-14 dating receives a lot of attention, since it can give information about the relatively recent past, it is rarely used in geology (and almost never used to date fossils). Carbon-14 decays almost completely within 100,000 years of the organism dying, and many fossils and rock strata are hundreds of times older than that. To date older fossils, other methods are used, such as potassium-argon or argon-argon dating.
Radiometric dating and YEC
Radiometric dating — through processes similar to those outlined in the example problem above — frequently reveals that rocks, fossils, etc. are very much older than the approximately 6,000 to 10,000 years reckoned by young earth creationists. The oldest rock so far dated is a zircon crystal that formed 4.4-billion-years ago, which was only 200 million years or so after the Earth itself formed.  YEC biblical literalists are necessarily bound to the dogmatic religions conclusion that the Earth is of a certain age based on a particular literal interpretation of the Genesis creation myth. They tie themselves in logical knots trying to reconcile the results of radiometric dating with the unwavering belief that the Earth was created ex nihilo about 6,000 to 10,000 years ago.
One way Young Earth Creationists and other denialists try to discredit radiometric dating is to cite examples radiometric dating techniques providing inaccurate results. This is frequently because the selected technique is used outside of its appropriate range, for example on very recent lavas. In attempting to date Mt. St. Helens, creationists attempted discredit the discipline through dishonest practices. The Institute for Creation Research's RATE project aimed to show scientifically that methods of radiometric dating produced wildly inconsistent and incorrect values. Ultimately these "creation scientists" were forced to admit that even for methods they accepted as sound, the age of the Earth would be vastly greater than the 6,000 they set out to prove.
- Carbon dating
- 101 evidences for a young age of the Earth and the universe
- Evidence against a recent creation
- The exception are the nuclides decaying via electron capture, where the half-life is marginally dependent on pressure and chemical bonds.
- Crystal is 'oldest scrap of Earth crust'
- See, e.g., Science in the balance: "If the Bible says something happened, we are to believe it, up front, right away, without hesitation. We are to teach what the Bible says and let scientific research and discovery catch up to the truth of Scripture. Science is not a priority tool of biblical interpretation. … Science is a powerful, God-given tool, but only the Bible is true in every aspect; the text trumps any theory of science. Its truth does not wait for verification from us."
- See also, Geochronology kata John Woodmorappe by Steven Schimmrich, Woodmorappe Replies to Steven Schimmrich's "Geochronology kata John Woodmorappe" by "John Woodmorappe," and Schimmrich Responds to John Woodmorappe's rebuttal by Steven Schimmrich for a revealing exchange between a Ph.D. Structural Geologist and a well-known creationist crank long engaged in unsuccessfully attempting to debunk methods of radiometric dating.
- See also, Dr Kevin R. Henke exposes John Woodmorappe's fraudulent attacks on radiometric dating and reveals other creationist misrepresentations.
- Kevin R. Henke. "Young-Earth Creationist 'Dating' of a Mt. St. Helens Dacite: The Failure of Austin and Swenson to Recognize Obviously Ancient Minerals". No Answers in Genesis.