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Radiometric Dating — Is It Accurate?

Radiometric dating is a much misunderstood phenomenon. Evolutionists often misunderstand the method, assuming it gives a definite age for tested samples. Creationists also often misunderstand it, claiming that the process is inaccurate.

Radiometric Dating Is Not Inaccurate

Perhaps a good place to start this article would be to affirm that radiometric dating is not inaccurate. It is certainly incorrect, and it is certainly based on wrong assumptions, but it is not inaccurate.

What do I mean? How can something be accurate and yet wrong? To understand this point, we need to understand what exactly is being measured during a radiometric dating test. One thing that is not being directly measured is the actual age of the sample.

No “Age-Meter”

There is no “age-meter” that you can plug into a rock, giving an immediate read-out of the rock’s age. It needs to be remembered that observational science can only measure things in the here-and-now, in a manner which can be repeated. Historical science is concerned with trying to work out what may have happened in a one-off event in the past. Historical science is not capable of repetition, checking or peer—˜review. The age of a rock sample falls under the heading of historical science, not observational science. So what do the observational scientists in the radiometric dating lab do?

Radioactive isotopes are unstable and will decay into more stable isotopes of other elements. One common radiometric dating method is the Uranium-Lead method. This involves uranium isotopes with an atomic mass of 238. This is the most common form of uranium. It decays by a 14-step process into lead-206, which is stable. Each step involves the elimination of either an alpha or a beta particle. Therefore the process is:

Uranium Decay Equation

Uranium Decay Equation

Each individual atom has a chance of decaying by this process. If you were able to examine just one atom, you would not know whether or not it would decay. The chance of it decaying is not definite, by human standards, and is similar to the chance of rolling a particular number on a dice. Although we cannot determine what will happen to an individual atom, we can determine what will happen to a few million atoms. This is similar to our dice analogy. We cannot tell what number we will roll in any one shake, but if we rolled 6,000 dice, the chances are very high that 1,000 of them would have landed on a six. One dice is unpredictable. Many dice follow a statistically predictable pattern. In the same way, one U-238 atom is unpredictable, but a sample containing many millions of U-238 atoms will be very predictable.

What happens statistically is that half of the available atoms will have decayed in a given period, specific to each radioactive species, called the half-life. For example, if element Aa had a half-life of 1 day and we had 1,000 lbs. of it on Monday, then we would have 500 lbs. on Tuesday, 250 lbs. (half of 500) on Wednesday and 125 lbs. on Thursday.

Determining Half-Life

By observing how fast U-238 decays into lead-206, we can calculate the half-life of U-238. This is a theoretical calculation, and we can therefore determine that the half-life of U-238 is 4.5 billion years. Remember that the half-life is a statistical measure. Granting that U-238 has a half-life of 4.5 billion years in no way negates the idea that the Earth is only 6,000 years old.

A very common rock that contains U-238 is granite. If we look at some of the very small zircon crystals in granite, we can accurately measure how much U-238 and Pb-206 the crystal contains. In order to calculate the age of the rock, we need three other pieces of information:

  1. We need to know how fast the U-238 turns into Pb-206. The half-life gives us this value, provided the half-life has never altered during the lifetime of the zircon crystal.
  2. We need to know how much Pb-206 there was in the original rock. This is clearly impossible. It is usually assumed, without justification, that the original quantity of Pb-206 in the rock was zero.
  3. We need to be sure that no lead compounds have been added to or taken away from the rock. Given that lead compounds are fairly soluble in water, this is something that we cannot be very sure of.

Using the above assumptions, it is calculated that the zircon crystals have an age of about 1.5 billion years.

Based Upon Assumptions

The radioactive decay process above can be seen to produce 8 alpha-particles for each one atom of U-238. Each α-particle could gain new electrons and become an atom of helium. The rate of diffusion of helium from a zircon crustal can be measured. It turns out that this rate of diffusion of helium is compatible with the crystals being about 5,000 years old, not 1.5 billion years old. Although assumptions 2 and 3 are not provable, they actually seem very likely in this particular example. Therefore, it seems that the first assumption must be wrong1. Remember that we have already said that these experimenters are highly skilled. It is therefore unlikely that the laboratory technicians have made a mistake in their measurements of U-238 or Pb-206. The only possible conclusion, therefore, is that the half-life of U-238 has not been constant throughout the lifetime of the granite and its zircon crystals.

Other radiometric dating methods are based on similar assumptions. If the assumptions cannot be trusted, then the calculations based on them are unsound. It is for this reason that creationists question radiometric dating methods and do not accept their results.

  1. For more on this important work, please see Humphreys, R., Young Helium Diffusion Age of Zircons Supports Accelerated Nuclear Decay, in Vardiman, L., Snelling, A.A., and Chaffin, E.F. (2005), Radioisotopes and the Age of the Earth, Volume 2, (California: Institute for Creation Research), pages 25-100.

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