May
19
Necessary Conditions for Reliability in Radiometric Dating
“Each timepiece has its own purposes and limitations…. When used outside of its intended purposes or limitations, any dating technique can produce incorrect and unreliable results. When used within its intended purposes and limitations, radiometric dating can and does serve as a reliable and trustworthy tool, just as satellite pictures and Doppler radar do in today’s weather forecasts.” — Dr. Hugh Ross, A Matter of Days, 2nd ed. (2015)
Would you use a yardstick to mark off miles traveled during a cross-country trip? No way!
Would you employ a stopwatch to measure your vacation time? Of course not! (Though, maybe you would right before, if you were actually “counting the seconds” until your vacation began. 🙂 )
Over the past few months, I have intermittently been sharing excerpts from Hugh Ross’s book A Matter of Days, 2nd ed., that address various methods by which scientists measure great distances and ages. In particular, these are areas that Young Earth Creationists (YEC) — and sometimes others, as well — typically misunderstand and/or challenge mainstream science with. It has been several weeks since my last such post, but today I’d like to look at Ross’s discussion about the primary conditions that influence the reliability of dating via radiometric methods.
Take it away, Dr. Ross…
“Just as thermometers, barometers, and radar all have specific applications and limits, so too do the more than 40 different radiometric decay dating methods. One limiting condition concerns the rock sample’s age. The closer that age is to the radiometric half-life of the isotope being measured, the better. Accurate dates can be determined only for samples that are no younger than about one-sixth the half-life (of the isotope) and no older than about six times its half-life — unless one has a very large sample. Thus, carbon-14 (perhaps the most commonly known radiometric dating tool), with its half-life of 5,715 years, provides reliable dates — with rare exceptions — specifically for samples of once-living material between 900 and 35,000 years old.
Attempts to date the Shroud of Turin offer an example of these radiometric dating limitations. Researchers at specialized laboratories in the United States, England, and Switzerland assured the shroud’s protectors that if it were genuinely a 2,000-year-old article, a four-square-inch sample of fabric would suffice to establish that fact. The carbon-14 studies placed the shroud’s origin in the thirteenth century AD, making it only about 800 years old — just outside the 900-year lower boundary for accuracy in carbon-14 dating. Those who wanted passionately to believe in the shroud’s authenticity as the burial cloth of Christ tried to seize this “inaccuracy” as a way to sustain their hope, but the three research teams have expressed certainty that, even considering the error margin, their carbon-14 measurement definitively rules out a first-century AD date, at least for that portion of the shroud. Each lab offered to provide a more precise and comprehensive date if they could test as much as a one-square-foot piece of the shroud and smaller pieces of other parts of the shroud. Their offers were declined.
This case study reveals a second condition for accurate radiometric dating — sample size. The bigger the sample, the better. This condition explains why archaeologists can provide precise dates for large artifacts but typically only crude dates for tiny ones.
Sample purity is a third condition. The more a sample is contaminated by materials of different ages, the less reliable the radiometric date. When dealing with a seriously contaminated sample, investigators often report several different radiometric dates. The first will be a date for the whole sample. The second will be a date for what they hope is the isolated original sample. The third, fourth, and so on will be estimates for each of the contaminants. Typically, the error bars for each of these measurements indicate how successfully the investigator isolated the original sample from the different contaminants.
Proximity of the actual date to the half-life of the radiometric chronometer, adequate sample size, and adequate sample purity are the three most significant conditions for radiometric dating accuracy. The degree to which the sample has been subjected to pressure is much less significant. (The half-life dependence on pressure applies only to electron-capture decays, and the effect is small where applicable.) Heat, cold, gravitational impulses, vacuum conditions, and chemical environment have no significant effect on radiometric decay rates.
Supposed “evidence” against the reliability of radiometric dating focuses on the method’s “flaws” or inaccuracies when applied outside its limitations. For example, uranium-238 radiometric dating, when applied to young samples, yields absurd dates. Why? With a half-life of 4.51 billion years, uranium-238 dating cannot be effective for measuring the age of any sample younger than a few hundred million years old. Similarly, carbon-14 dating will give absurd dates for extremely ancient samples. As noted already, carbon-14 dating ceases to be reliable for samples older than about 35,000 years. Carbon-14 has the added limitation that it is effective for dating organic (once-living) materials only.
Radiometric dating sometimes produces “discordant” dates. Such discrepancies are explained, however, by the nature and degree of contamination and by differences in sample size. Error bars offer helpful indicators for these variances. A piece of wood dated (by the carbon-14 method) as 6,000 +/- 5,000 years old is a sample in which the investigator was unable to isolate the contaminants. A manuscript dated as 1,600 +/- 200 years old is most likely a sample too small for greater precision. However, numerous large samples of uncontaminated charcoal from an ancient city dated to 1412 BC +/- 1 year would yield a secure conclusion that the city burned sometime between 1414 and 1410 BC.
Recognizing that radioisotope dating establishes both a several-billion-year-old Earth and universe, the Institute for Creation Research (ICR) formed the RATE (Radioisotopes and the Age of the Earth) research group in 1997 to search for ways to interpret radiometric decay in a young-earth context. After eight years of research the RATE group acknowledged that if radiometric decay rates are truly constant, then the universe and Earth must be billions of years old. The group argued that radiometric decay rates must have accelerated by at least a factor of several hundred million times either during the first few Genesis creation days, at the time of Adam’s fall in Eden, during the flood of Noah, in the episode shortly after Noah’s flood, or in some combination of these four brief epochs so as to accommodate their young-earth position.
Measuring past decay rates can be indirect and difficult for geophysicists but direct and easy for astronomers. Because they routinely observe objects thousands, millions, and even billions of light-years distant [see previous posts], astronomers directly observe radiometric isotopes as they were some thousands, millions, and billions of years ago. The measured abundances of these isotopes show that radiometric decay rates have not varied through time.”
(Dr. Ross further discusses the RATE Project and the unlikelihood of accelerated radiometric decay in A Matter of Days. For more info, here is an online article by RTB scholar Jeff Zweerink: “Comments on the RATE Project”. In the book, Ross also addresses concerns about questionable decay products, residual carbon-14, and unknown original amounts of parent and/or daughter isotopes.)
This radiometric dating stuff — especially radiocarbon (aka carbon or C14) dating — comes up a lot in “origins” / age of the Earth discussions, so I hope you found this info from Dr. Ross as interesting and helpful as I did. It really is amazing the many ways God has provided for us to gather information about His Creation, and it is always exciting to see the various methods get increasingly refined and used to discover more about the world around us.
P.S. For a more in-depth article on radiometric dating, check out this one by Dr. Roger C. Wiens.
