From Ben in Muncie, IN
I have a nagging question about radiometric decay that I'm having trouble finding an answer to. In order to calculate something's age, we need to know what the C-14 to C-12 ratio was at the time when it stopped exchanging carbon with the environment. But how exactly do we know what the environmental ratio was, say 50,000 years ago? I know carbon-14 is produced in the upper atmosphere. Are we at some kind of equilibrium, where it is produced at the exact same rate at which it decays? Otherwise, how would we know what that original concentration was? I know we can calibrate with events we know about, like Mt. Vesuvius' eruption, but that only takes us back about 2,000 years. Do we simply assume that the environmental ratio tens of thousands of years ago is the same as today? If so, how do we support that assumption? (I have the same question about other forms of radiometric decay.)
I really appreciate all that you guys do for His Kingdom! Keep up the good work!
You identified an important point regarding radioisotope dating: How do we determine the proper starting amount of the radioactive element? The answer encompasses three points.
First, a good rule of thumb to remember is that a specific form of radioisotope dating works best for dates within a factor of 10 of the specific element's half-life. So, for example, carbon-14 has a half-life of 5,730 years; thus it provides good dates for things that lived (because living things ingest carbon-14) between 500 and 50,000 years ago.
Second, as neutrons from cosmic rays hit Earth's upper atmosphere, they convert nitrogen into carbon-14. The cosmic-ray rate is remarkably uniform, making the amount of carbon-14 in the atmosphere also rather uniform. However, the size and strength of the Sun's magnetic field and Earth's magnetic field do affect the cosmic-ray rate—and thus the production of carbon-14. There may even be small changes in the half-life of carbon-14. Even with all these (relatively small) uncertainties, scientists can calibrate their effects using tree rings, foraminifera, and corals so that radiocarbon dating gives reliable dates back to 50,000 years.
Third, even more difficulties arise when trying to date organisms that don't breathe air directly (like marine animals). However, a vibrant scientific community exists—composed of both Christians and non-Christians—that works to address all the irregularities in order to provide reliable radiocarbon dates.
For more information, including links to further details, see the article titled "How Trustworthy Is Carbon Dating?"