Radiometric Dating and Estimated Dates— A Scientist Responds

This article is a followup to an exchange I had with “Douglas” in the comments section of my recent posting, The Three Arguments Against Atheism. In that discussion, I offered to refer the question of estimated dates in radiometric dating to an actual scientist and post the result. The scientist I contacted was Dr. Daniel J. Berger, Professor of Chemistry at Bluffton University, Ohio. With his permission, I have published our exchange below.

From: Robert McNally
Sent: Sunday, December 31, 2006 11:46 PM
To: Daniel Berger
Subject: Question on Radiometric Dating

Dr. Berger,

I found your contact information through madsci.org, where I see you answered a similar question to the one I have. I hope you will take the time to clear up an issue that has arisen in an on-line discussion I am having with a young-earth creationist. This gentleman seems to presume that the Earth is less than 12,000 years old, and is therefore skeptical of radiometric dating methods, saying:

As for any of the “dating” methods – On the form used to submit samples for dating, is a space to indicate how old you “think” the sample should be. If this is an objective and reliable method, why is this field there?

Not being an expert in this area, I asked him to produce such forms, which he did:

Some radiometric dating submission forms for you:

http://www.brocku.ca/earthsciences/radiocarbon/
Check the link to the RTF request form on the left. Once you have the file, look for the “probable age” field.

http://www.radiocarbondating.com/submission%20form%202003.pdf
Again look for “Estimate Age” and “age limits” fields on this form.

http://www.gla.ac.uk/centres/surrc/radiocarbon/submission.html
Download the submission form in Word format. Also has the “estimated age” field.

These are just the first 3 forms I found using Google. All 3 ask for an Estimated Age. The last one even supplies some reasons – “this helps the laboratory to select the appropriate instrument for measurement as well as enabling the laboratory to contact the submitter at an early stage if the estimated age is excessively different from the measured age”.

I am sorry but the reasoning to justify this is very weak. I thought the whole point of radiometric dating is that it is supposed to objectively and reliably supply the dates based on radioactive decay rates etc. I think its nuts that you are submitting a sample to “scientifically” determine the age, and at the same time you have to give them your estimate and upper and lower age limits. What if I have a sample and I’ve guessed wrong by several billion years? They are using my guess to calibrate the machines, and the result comes back way outside of my limits. “Oops,” they say, “the machines must have it wrong. Lets repeat the process.”

As for contaminants, if these cannot be reliably detected by the dating lab, how can the process be considered valid? Many huge dating mistakes have after the fact been ascribed to contamination. If they have no way of knowing that a sample is contaminated enough to severely distort the results beforehand, this process is a waste of time.

I have seen a separate Creationist web site take a similar position:

One interesting thing is that on the form for submitting rock samples to dating and testing labs, you have to specify how old you think the sample is. Presumably, this is because the labs know the discrepancies in the dating method, and want to choose the “correct date” from the many “bad dates.”

My questions are: how would you respond to such a position? Why are estimated dates required prior to dating? Is radiometric dating as subjective as this gentleman asserts?

Thanks for your consideration,

Robert McNally

From: Daniel Berger
Sent: Tuesday, January 2, 2007 7:10 AM
To: Robert McNally
Subject: Re: Question on Radiometric Dating

Dear Mr. McNally,

There seem to be a couple of crucial misunderstandings behind your interlocutor’s objection to the request for estimated dates; I hope that this will be enough to answer your question. If not, please ask again.

First, the objections to radiometric dating seem to be demanding a mythical “crucial experiment.” Crucial experiments, in which one experiment, by itself, establishes the truth or falsehood of an hypothesis, are not completely unknown, but their mythical, highly celebrated status stems from their rarity. Most scientific statements are based on a preponderance of the evidence; well-established ones are based on an overwhelming convergence of evidence. But (pace Popper) a few contradictory results do not instantly destroy a theory, nor can we expect a single “crucial experiment” to establish one in the presence of contradictory evidence.

To put it bluntly, people make mistakes; but mistakes tend to cancel. It’s much less likely that a whole bunch of competent people will make a whole bunch of mistakes. It is even less likely that all the mistakes will produce results that are consistent with each other — unless the experimenters are in a conspiracy of lies. The larger and more competitive the group (and there are lots of scientists, who compete fiercely for prestige), the less likely that such a conspiracy can be maintained.

In the dating game, there are other bits of evidence than radiometrics. The evidence, taken as a whole, is expected to converge on a single answer (or a range within which the answer lies). In the few cases that it doesn’t do so… well, everybody has had experiments go bad. See the appendix to Michael Polanyi‘s “Science, Faith and Society.”

Second, there is no single “radiometric dating” method. Instead, the technique examines several different natural radionuclides, of widely-varying half-lives. For example, C-14 has a half-life of 5730 years; K-40 of 1 billion years; and U-238 of 4.5 billion years. There are differences in detailed chemical behavior that will influence the choice of isotopes depending on the sample being dated — you can’t get a C-14 date on rocks, or a U-Pb date on biological samples — but largely the particular radionuclide you look at will be determined by how old you think the thing is. If the object is thought to be more than a hundred thousand years old, there’s no point in looking for C-14 because it’ll all be gone. On the other hand, you probably can’t get a reliable K-Ar date on something much younger than about half-a-million years old; the result would be indistinguishable from background. The most precise results are probably for objects between about 0.5 half-life and 5 half-lives old, though I could be off a bit because this is not my area of practice.

The estimated age is established by (for example) where you find the item: is it in an undisturbed Etruscan tomb (about 2500 ya)? a modern-human stone-age encampment in Europe (at least 4000 ya, perhaps as much as 60,000 ya, but almost certainly no older than 100,000 ya)? a rock stratum containing a triceratops skeleton (between 65 and 75 millian ya)?

That gives the radiometric workers something to go on. But it also plays into the “convergent evidence” thing. Radiometric dating is not knock-down proof of age (see the continuing controversy over the dating of the Shroud of Turin; the 14th-Century radiocarbon date has been questioned by experts in ancient textiles, for example). But it’s another piece of evidence. If you get a radiometric date that’s wildly different than your estimated date, the evidence is diverging and you need to go back and look at everything again.

For more, see Wikipedia: http://en.wikipedia.org/wiki/Radiometric_dating

The OTHER thing is that a number of Young-Earth Creationist arguments ignore the reality of knowledge within a range. We often can’t know a number specifically; we know it within a certain range. The fact that two radiocarbon dates may place an object in two different centuries (say, the 5th and the 8th AD) doesn’t mean that the object was produced yesterday. In general the fact that we are having difficulty establishing where a number (say, an age) lies within a certain range (say, 15,000 to 50,000) doesn’t mean it’s just as likely to be wildly outside that range (say, 4,000). See, for example, my MadSci answer at http://www.madsci.org/posts/archives/may97/863724333.As.r.html

From: Robert McNally
Sent: Tuesday, January 2, 2007 11:38 PM
To: Daniel Berger
Subject: Re: Question on Radiometric Dating

Dr. Berger,

Thanks for your very clear response. May I have permission to post it to my blog as an article and credit you?

http://ironwolf.dangerousgames.com/blog/

The exchange in which this question came up was in the comments section of this post:

http://ironwolf.dangerousgames.com/blog/archives/196

My interlocutor posted his comments there as “Douglas”, and specifically said he’d like to see a response post from the scientific community.

Thanks again,

Robert

From: Daniel Berger
Sent: Wednesday, January 3, 2007 5:57 AM
To: Robert McNally
Subject: Re: Question on Radiometric Dating

Dear Mr. McNally,

Thank you, I’m flattered. A conservative Christian myself (a member of the ASA, www.asa3.org), I’m of two minds about whether I want to be associated with a blog like yours.

On the other hand, Douglas is calling into question any but the most vapidly general knowledge about the past, which is clearly silly. I’m a Polanyian in epistemology, and his “meal at McDonald’s” example is a denial of the ways of knowing within both science and history. The assumption that he was the one using his credit card is probably the major weakness in the chain of inferences, but it’s not unreasonable, and — were it considered important enough — other corroborating evidence would be brought into play, such as the consistency of this purchase with others he is known or inferred to have made. Arguments based on his purchase records and other papers would probably be used as evidence of whether he was likely to have lent his credit card to someone else. The purchase could not be established with 100% certainty, but why do we demand such?

To a Polanyian, one is free to deny the ways of knowing of a particular community… but one is not free to think that one can know what scientists and historians know if one denies their methods.

As to the assumptions behind science in general and radiometric dating in particular, they are simply that the patterns we observe now also obtained in the past. Within limits, variances in radiometric dates can be accounted for by comparison with samples whose ages are known independently — radiocarbon dates for the past 5000 years or so are calibrated by comparison to tree ring samples — but even if the number were no better than +/- 20%, that excludes values outside that range with a confidence of at least 80%, if I remember my stats correctly.

I would dearly love to know what “huge assumptions about the past” Douglas thinks radiometric dating makes. I’ve other things to do than go delve into YEC sites — classes start Monday, and I’m teaching two new courses that I have to prep for — but the above, and my previous answer, is my best guess at what he was talking about.

You may go ahead and post what I’ve written (it’s not really any different than material I’ve posted on the Mad Scientist Network), but I’d appreciate seeing how it’s redacted.

Daniel J. Berger

From: Robert McNally
Sent: Wednesday, January 3, 2007 5:19 PM
To: Daniel Berger
Subject: Re: Question on Radiometric Dating

Dr. Berger,

I understand your ambivalence. I did not reveal my own philosophical biases when asking my question, because I did not want to unduly bias your answer. But in the question of publishing, I definitely wanted you to know in advance that the general approach I take in my blog is hardly unbiased.

But while I may rail against religion as harmfully dogmatic, I also have many friends who are religious, and I respect them for all that they are. I also believe in people finding common ground. Your common ground with Douglas is that you both believe in God and the Word of God. Your common ground with me, without prejudice to any other possible way of knowing, is that we both respect the modern scientific way of gaining knowledge. This is an important dialog: while I understand that acceptance of science does not imply a refutation of faith in general, many mainstream believers do not— and I believe this misunderstanding harms both the advancement of science and civil society.

So, with your permission, I would be pleased to publish this entire e-mail exchange unredacted except to 1) put the exchange in chronological order, 2) provide contact information only once, 3) put Wikipedia links on terms with which my readers may be unfamiliar, and 4) provide a brief preface directing readers to the original exchange of comments with Douglas.

Regards,

Robert

From: Daniel Berger
Sent: Thursday, January 4, 2007 4:38 AM
To: Robert McNally
Subject: Re: Question on Radiometric Dating

“So, with your permission, I would be pleased to publish this entire e-mail exchange unredacted except to 1) put the exchange in chronological order, 2) provide contact information only once, 3) put Wikipedia links on terms with which my readers may be unfamiliar, and 4) provide a brief preface directing readers to the original exchange of comments with Douglas.”

Sounds good to me. Please go ahead.

Daniel J. Berger
bergerd@bluffton.edu
419-358-3379
www.bluffton.edu/~bergerd

Professor of Chemistry
Bluffton University 87
One University Drive
Bluffton, OH 45817-2104

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