Claim 1 - Radiometric Dating is Inaccurate

Radiometric dating is one of the processes by which the age of the Earth has been determined. One example of which you might have heard is Carbon-14 dating, which is used for dating of biological samples such as bones. The principle for this technique is rather simple. Carbon-14 is produced at a reasonably constant rate in the upper atmosphere. Living animals have a known level of Carbon-14 in their bodies as they breathe it in all the time from the air. They also have other, stable varieties ("isotopes") of carbon, such as Carbon-12, which is far more common. In a living creature, the ratio of abundance of these two substances simply reflects that in the atmosphere, and is therefore well-known.

If this animal then dies and therefore is not able to gather more Carbon-14 then the ratio of the two elements will start to change as the Carbon-14 decays into a more stable form, such as Carbon-12 or Carbon-13. This is known to happen with a half-life of about 5,700 years, which means that after 5,700 years, the amount of Carbon-14 in a sample should have decayed to approximately half its initial value. If we know the original proportions of Carbon-14 and Carbon-12, and we can measure their ratio at a later date then we can work out how much of the Carbon-14 has decayed, and hence we can calculate the age of the sample.

Carbon-14 dating is typically used to date archaeological remains, especially from iron- and bronze-age sites of a few thousand years in age. Results are generally quoted to within 5 or 10 percent, and the ages thus obtained are usually in good agreement with the expected ages of the samples. Carbon-14 dating cannot be used to date remains older than about 50,000 years because the level of Carbon-14 in the sample will then be far too low for an accurate determination.

Carbon-14 dating assumes that the abundance ratio of the unstable form of Carbon to the stable form has been relatively constant over time, and therefore that the relative abundances of these two chemicals are the same now as they were when the animal or plant died. This is speculative, though it is likely to be accurate. We certainly have no reason to believe it might not be the case. Carbon-14 dating has been tested relative to other radiometric dating techniques using different chemicals which are unrelated to Carbon-14, and it has been found to be in agreement. It is extremely difficult to find processes which will alter the abundance ratios of two wildly different radioactive chemicals by the same amount at the same time. Carbon-14 dating has been tested with a very large number of different other methods. The chances of all these different methods being wrong in exactly the same way and by exactly the same amount are negligibly slim!

Furthermore, Carbon-14 dating has been used to date remains of a known age, such as bones from Roman burial sites, and it gives age estimates agreeing very well with historical records.

If the abundance ratio of Carbon-14 were different by an enormous factor in the past, for example suppose the previous ratio of C-14 to C-12 was twice what it is now, this would result in an age error of the half-life of C-14, or 5,700 years. We would find many remains deposited in the last few thousand years which appeared to have negative ages. This is not the case. Furthermore, samples dated to approximately 50,000 years old would be wrong by only about ten percent. In order to make a 5,000 year old sample appear 50,000 years old, the abundance of Carbon-14 in the atmosphere would have to have been 240 times lower than today's value. Carbon-14 is produced at an almost constant rate in the upper atmosphere by cosmic rays. Whereas a large burst of production is potentially feasible, there is no known method by which the rate of production could be lowered by such an enormous factor, and maintained at its low value for an extended period of time.

Many different unstable chemicals can be treated in a similar way. By choosing a chemical with a significantly longer half-life, one can therefore probe much longer timescales, such as those proposed by modern science for the age of the Earth.

The following page is an excellent explanation of the theory of radiometric dating, explaining why it is unquestionably reliable and safe. He emphasises that the technique has been tested in great detail in order to reduce all forms of systematic error.

One of the arguments is that samples of rock cannot possibly expect to have been isolated from the atmosphere, so radiometric dating will give an inaccurate result due to intermediate contamination. A little thought should show that this will serve to make the rock appear younger than it actually is, rather than older. Making a young rock appear old is extremely difficult to do. Furthermore, significant research has been conducted into possible sources of systematic error, such as anomalous initial element abundances, and methods now exist to correct for these rather accurately.

Below is a table for estimated ages for the oldest rocks on the planet. This will give us a lower limit for the age of the Earth. These rocks have been dated using many different methods, which are all in agreement to within a few percent. This table is taken from the above web page.

Technique Age Range (billion years)
uranium-lead 3.60±0.05
lead-lead 3.56±0.10
lead-lead 3.74±0.12
lead-lead 3.62±0.13
rubidium-strontium 3.64±0.06
rubidium-strontium 3.62±0.14
rubidium-strontium 3.67±0.09
rubidium-strontium 3.66±0.10
rubidium-strontium 3.61±0.22
rubidium-strontium 3.56±0.14
lutetium-hafnium 3.55±0.22
samarium-neodymium 3.56±0.20
(compiled from Dalrymple, 1991)

I have also seen some arguments suggesting that the rate of radioactive decay might have been much faster in the past than it is now. Such a theory clearly misunderstands the whole process of radioactive decay and why it happens. Radioactive decay is based on a few fundamental physical laws which we know extremely well, and which have been tested in an enormous number of circumstances. These include all the environments through which any geological sample may have passed, such as extremes of temperature and pressure. Absolutely nothing whatsoever changes the decay rate of radioactive isotopes. The one slight exception to this rule is pressure-enhanced rates of k-capture decay, which are irrelevant in this argument as we are considering only isotopes which spontaneously decay.

Radioactive decay depends only on the substance decaying, and is not affected by the environment in which it decays. It happens because of fundamental quantum-mechanical processes in the nucleus of an atom. There is absolutely no mechanism known to science under which the rate of radioactive decay might have been different in the past, and in the absence of any such process we must conclude that it has not varied at all. To do otherwise would simply be utterly against all scientific method. It would be akin to making a complex theory based on how the Sun might well turn blue tomorrow, despite the fact that there is no known reason why it should do and plenty of reasons why it could not. As soon as anyone proposes any method by which the rate of radioactive decay might have varied in the past then this proposal will cease to be ludicrous.

One argument appeals to the non-scientist by arguing that Newtonian physics was thought to be correct until Einstein came along and fixed it. Well, to be fair, Newtonian physics is correct for almost all situations, and certainly all those that were measurable at the time. We have tested radioactive decay under all the pressures and temperatures that it is ever going to experience on Earth, and it is rigorously unchanged. Maybe in the future we will discover some process by which the decay rate can be altered, but it certainly won't be by a process that occurs naturally on this planet, and therefore to which any rock sample might have been subjected.

Isochron Dating

Many people have argued in the past that radioactive dating is susceptible to the initial concentrations of elements in the rock, or the effects of intermediate processes leaching out certain chemicals from the rocks. Whilst there is some truth in this for the standard radiometric dating techniques, the process of isochron dating completely removes these objections. The process is very well explained on this site. I can't emphasise enough how important it is to understand this process in order to gain complete confidence in radiometric dating.

Is this a fair representation? If not then drop me an email. Address below.

This page maintained by Colin Frayn. Email
Last Update : 17th March, 2004