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Feedback archive › Feedback 2006

Out of his depth on radioactive dating, but still a match for his chemistry prof.

3 June 2006

This week’s feedback comes from a supporter, a college student studying radiometric dating in his chemistry class. Our correspondent was regularly called on by his prof. to present the creationist position, but he was getting out of his depth on such a specialized subject. This feedback shows the importance of seeking answers, and of strategies for handling a discussion when it advances beyond your level of expertise.

First, his letter in its entirety; then a reply by Dr Tas Walker.

He also updated us on how this exchange progressed in class.


Dear Tas,

I am in my second semester of General Chemistry at my local community college and I have been discussing creationism with my professor. He is a hard believer of evolution with his Ph.D. from [name of university]. Even though he is Jewish, he is very difficult to communicate with concerning any other origin ideas. He has been teaching since the late 60’s and he keeps up on current events in chemistry. He calls on me a great deal in class for a creationist’s perspective on certain subjects, but it is still difficult to get ideas across.

What I am up against right now is nuclear chemistry; specifically radiometric dating techniques. I can’t seem to find the actual answer I have been searching for on [any creationist] websites. So, I’m hoping you could give me a bit of information I can use in my discussion on the dating game.

I’d like to start by showing you a copy of a paragraph that is written in my chem. textbook:

‘It takes 4,500,000,000 years for half of a sample of uranium-238 to decay to lead-206. The age of the rocks containing uranium can therefore be determined by measuring the ratio of lead-206 to uranium-238. If the lead-206 had somehow become incorporated into the rock by normal chemical processes instead of by radioactive decay, the rock would also contain large amounts of the more abundant isotope lead-208. In the absence of large amounts of this “geonormal” isotope of lead, it is assumed that all the lead-206 was at one time uranium-238. The oldest rocks found on Earth are approximately 3,000,000,000 years old. This age indicates that Earth’s crust has been solid for at least this length of time. Scientists estimate that it required 1,000,000,000 to 1,500,000,000 years for Earth to cool and its surface to become solid. This places the age of the Earth at 4,000,000,000 years old.’

My problem is where it states:

‘If the lead-206 had somehow become incorporated into the rock by normal chemical processes instead of by radioactive decay, the rock would also contain large amounts of the more abundant isotope lead-208. In the absence of large amounts of this “geonormal” isotope of lead, it is assumed that all the lead-206 was at one time uranium-238.’

Does this mean that in every sample of rock that is ever tested, there is NO lead-208 in the sample? They only find uranium-238 AND lead-206 in the sample? Even if there was no lead-208, can we really say that the U-238 has decayed into lead-206?

I can see how many lay people will just take the scientist’s word for truth, because not many people, like myself, actually ask those deeper questions. Does lead-206 only come from decayed U-238?

And last question: Could decay rates actually have been altered in the past? Decay rates are not really altered by temperature or pressure changes, but If the radiometric material was saturated with flood water, could this alter the decay rate?

I hope you can give me some brief info on these questions to help me in my discussions.

In His grip,

JK


Hi JK,

Thanks for your email.

The crux of radioactive dating (or any other type of scientific dating) is this:

Image of an EPR spectrometer.

Photo courtesy wikipedia.org
Radiometric dating involves precise measurements of geological samples, but all measurements are made in the present. ‘Dates’ can only be calculated after assuming what was in the sample in the past.

  • It is impossible to measure the age of something scientifically. All measurements are of something other than time (e.g. amounts of U and Pb isotopes) and all these measurements are made in the present (not in the past).
  • Every age is ‘calculated’ based on assumptions.
  • Basically, we have to assume the history of the thing we are trying to date. In other words, we have to assume the answer before we start.
  • You can get any age you like depending on the assumptions that you make.
  • There is no way of checking that your calculated age is correct (i.e. the method cannot be calibrated because we cannot go back in time to check).
  • The calculated age is accepted by geologists only if it agrees with what they already think that the age should be. If it doesn’t, they will change their assumptions about the past to make the ‘story’ consistent with what they already believe.
  • In other words, the calculated number (‘age’) means nothing on its own. All dates have to be interpreted. Every respectable geological paper that reports an age first describes the sampling, testing and calculating methods before giving the result. Then it goes on to ‘interpret’ the date (i.e. develop their assumptions about the past so that the number fits with the other geological evidence).

Check The way it really is: little-known facts about radiometric dating and The earth: how old does it look? as well as the Q&A pages on Radiometric Dating and ‘Young’ age of the Earth & Universe

I would recommend my DVD, The Age of the Earth, which explains the principles behind all scientific dating methods and the only reliable way of knowing the age of something, and the book Refuting Compromise.

More importantly, you need to think about how you handle this issue in class. I think the professor is seeking to trap you by getting you to defend your position. He is much more knowledgeable and experienced than you are, and so he may not have any problem discrediting you and making you seem silly in front of the class.

I suggest that you try turning the ‘burden of proof back’ on the professor. When he asks you for the creationist position on a topic, you could respond by saying that you are interested in understanding his position (after all, your fees are paying his salary, so he is there to teach you!) and clarifying a few points that puzzle you and don’t seem to make sense. Then ask him questions that expose his position (frame the questions according to the principles above). Note that Jesus frequently used questions to teach and to refute critics (sometimes referred to as the Socratic Method).

Note that the questions you asked me in your email about radioactive dating are ones that you should be asking him (although it is always best for you to be informed on the subject). He is the one who is trying to prove his case, not you. And like any good lawyer, you have to be aware that his answers may try to fob you off, or even give half-truths, so you need to be persistent and insist that your question is answered, or say, ‘I’m sorry, but I can’t quite understand that. ...’

Cartoon of a scientist next to a large high-tech spectrometer machine while saying: 'Now, I'll just put my rocks in here and we'll have millions of years in no time!' By Steve Cardno

Now I’ll answer below some specific questions from your email.

About the quote from your textbook:

‘It takes 4,500,000,000 years for half of a sample of uranium-238 to decay to lead-206. The age of the rocks containing uranium can therefore be determined by measuring the ratio of lead-206 to uranium-238. If the lead-206 had somehow become incorporated into the rock by normal chemical processes instead of by radioactive decay, the rock would also contain large amounts of the more abundant isotope lead-208. In the absence of large amounts of this “geonormal” isotope of lead, it is assumed that all the lead-206 was at one time uranium-238.’

Notice the word ‘assumed’. They also assume that none was gained or lost since the rock crystallized. But if the age does not work out, they then assume that some must have been gained or lost, and discount the ‘date’.

‘The oldest rocks found on Earth are approximately 3,000,000,000 years old. This age indicates that Earth’s crust has been solid for at least this length of time. Scientists estimate that it required 1,000,000,000 to 1,500,000,000 years for Earth to cool and its surface to become solid. This places the age of the Earth at 4,000,000,000 years old.’

My problem is where it states:

‘If the lead-206 had somehow become incorporated into the rock by normal chemical processes instead of by radioactive decay, the rock would also contain large amounts of the more abundant isotope lead-208. In the absence of large amounts of this “geonormal” isotope of lead, it is assumed that all the lead-206 was at one time uranium-238.’

Does this mean that in every sample of rock that is ever tested, there is NO lead-208 in the sample?

large start double quotesEvery age is ‘calculated’ based on assumptions.large end double quotes

No. Every sample contains some, and geologists make assumptions about how much it was. They have models for how lead isotopes varied in the past and they use these models to calculate an age. They call this a ‘model’ age. They sometimes assume that a whole batch of samples had the same composition of daughter isotopes at the beginning, and they call this an isochron age. But they are all based on assumptions (see Refuting Compromise ch. 12).

They only find uranium-238 AND lead-206 in the sample?

No, they measure many different isotopes of lead and uranium and there are hundreds of isotopes of other elements that they do not measure.

Even if there was no lead-208, can we really say that the U-238 has decayed into lead-206?

No. There are many ways the lead-206 could get there. If the age fits with what they would like, they do not think about other ways the lead could get there. But if the age does not agree they think of all sorts of ways of explaining how it could have got there, and so explain the date away.

I can see how many lay people will just take the scientist’s word for truth, because not many people, like myself, actually ask those deeper questions.

large start double quotesIt’s a bit like reading tea leaves. It is controlled by the paradigm, administered by an élite and held in awe by the masses.large end double quotes

Absolutely. It’s a bit like reading tea leaves. It is controlled by the paradigm, administered by an élite and held in awe by the masses. Historians of science agree that the paradigm’s grip prevents challenges from receiving grants or having their papers published. Philosophers of science note that paradigm holders merely tweak auxiliary hypotheses rather than allow the paradigm to be touched. 99.9% of people have no idea about how it is done or what assumptions are made.

Does lead-206 only come from decayed U-238?

For a start, it could come from any of the other isotopes in the decay chain such as uranium-234, thorium-230, radium-226 and lead-210. Why should it have? Lead happens to have many stable isotopes because it has a ‘magic number’ of protons in the nucleus (Z=82) (a nuclear analog for noble gas configurations). The same is true of tin (Z=50), but no one attributes tin isotopes to radioactive decay.

And last question: Could decay rates actually have been altered in the past?

Yes they could. In fact, there seems to be evidence of this—see the work of the RATE (Radioactive Isotopes and the age of the Earth ) group. This includes evidence from helium diffusion (see our summary and Dr Humphreys’ technical paper) and discordant isochrons and also an article by Woodmorappe about a ‘billionfold’ change in decay rate.

Decay rates are not really altered by temperature or pressure changes, but if the radiometric material was saturated with flood water, could this alter the decay rate?

Perhaps. Who knows what may have affected the rates in the past? Read up the articles on the Radiometric dating Q&A to get a feel for it. It is a huge subject but come back to the principle that there is no instrument to measure time and that it is all based on assumptions. The clincher is that they don’t believe the date themselves if it does not fit with what they believe it should be. See The Dating Game.

I hope you can give me some brief info. on these questions to help me in my discussions.

In His grip,

JK

All the best,

Tas Walker, Ph.D.
CMI-Australia


Addendum: News from JK about how he fared in class:

I asked my Chemistry prof. the questions you and I discussed. He said he wasn’t an expert, so he wasn’t sure what the answers might be. Sounds a little bit like a cop-out. One of the guys in my class praised me for being able to stump the prof. … This is a fascinating subject! Thank you for your correspondence!

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