Radiometric Dating

[callmeMurph]

Hey everyone, I was hoping someone here could explain (or point me to an explanation) on radiometric dating. Specifically I am interested in how they calculate the age of various things. Basically how do they calculate all of the different variables in order to determine the age. If you could also explain how mass spectrometers work that would be cool too.

I trust the science behind it, although people tell me I shouldn't, and I'd like to be able to explain it better to other people. I'm fairly good at calculus and have a decent understanding of physics so I think if I studied it I could do a reasonable job.

Anyways, the reason I ask is because my roommate brought it up and used an example of why he didn't think it was reliable. His company makes aircraft parts and he is in the research and development department. He basically said that for the longest time that scientists could explain drag (or something like that I can't remember exactly) but when they hit the speed of sound the it turned the physics "upside down". He basically used that example to compare to radiometric dating to say that we only have data for so many years so to extrapolate so far back into the past is totally unreliable. I think that was a pretty bad analogy but whatever. He also said that there is no way we can know that the rates of decay have been constant. I'm pretty sure we can be pretty sure they were but how can it be shown?

Sorry for so many questions and thanks for your help.

 

[Pwnzerfaust]

Radiometric dating works by measuring the quantity of the products of radioactive decay of certain isotopes. Radioactive isotopes have unique half-lives, which, so far as we can tell, do not vary. A half-life is how long it takes for exactly half of a sample of a given isotope to decay into its products. For instance, carbon-14, the isotope used in carbon dating, has a half-life of 5730 years. This means that, 5730 years after an organism dies, half of the original carbon-14 content will remain. 5730 years after that, one quarter of the original will remain. 5730 years after that, one eighth, and so forth.

By measuring the amount of the radioactive isotope, and the amount of radioactive decay products, it is possible to determine the age of the sample.

Carbon dating only works for organic things, however, such as non-fossilized animal and plant remains. Also, due to C-14's short half-life, it's only effective up to about 60,000 years before present. For non-organic, or more ancient things, scientists must use another method. For instance, uranium-lead dating uses the decay of Uranium-238 into Lead-206, with a half-life of about 4.47 billion years, and can date a number of ancient rocks.

This is a rather rudimentary explanation, but it touches on the basics.

 

[sfs]

Useful web pages:
A basic introduction.
An outline from a Christian geologist who knows what he's talking about
The talk.origins FAQ on isochron dating.

 

[Cabal]

Would a change in the fine structure constant affect radioactive decay processes?

 

[Targ]

Could someone could explain to me how you know for certain how much daughter product was in a rock that was, say, 30 million years old? I understand how it works for methods measuring under a million years, such as by calibrating the date with isotopes found in different varve layers, but what about when you go further back to tens of millions of years ago, when you don't have things like varve to compare against? I did read the Weins article on ASA (that sfs linked to) a few weeks ago, but I'm still none the wiser.

 

[sfs]

Could someone could explain to me how you know for certain how much daughter product was in a rock that was, say, 30 million years old? I understand how it works for methods measuring under a million years, such as by calibrating the date with isotopes found in different varve layers, but what about when you go further back to tens of millions of years ago, when you don't have things like varve to compare against? I did read the Weins article on ASA (that sfs linked to) a few weeks ago, but I'm still none the wiser.

I assume you're asking how you know how much daughter product was already in the rock when it formed. You do it (usually) by the isochron method, which Wein covers, although I don't think he uses the name (see Figs. 4 and 5 in his article). It's also explained in some detail in the talk.origins FAQ. You measure the amount not only of the daughter product, but of another isotope of the same element, and you make measurements across multiple minerals in the same rock. The different minerals incorporate different ratios of parent and daughter isotopes. With measurements of all three levels (parent, daughter, different isotope of daughter element) in all of the minerals, you can calculate how much of the daughter was originally present, and how much has appeared due to decay of the parent.

 

[callmeMurph]

How do they measure the different isotopes? I've watched some videos and saw that they used a mass spectrometer. I was wondering how that works though. Sorry I haven't had a chance to read the articles if it was included in them.

 

[juvenissun]

Would a change in the fine structure constant affect radioactive decay processes?

I like to know what do you think? Possible or not possible.

 

[juvenissun]

How do they measure the different isotopes? I've watched some videos and saw that they used a mass spectrometer. I was wondering how that works though. Sorry I haven't had a chance to read the articles if it was included in them.

Yes, that is how the measurement is done.

 

[Cabal]

I like to know what do you think? Possible or not possible.

I think it does, I'm just at a loss at the moment for where the fine structure factors into particular radioactive decay.

The reason I brought it up is in a recent thread someone posted an article about the dipole universe, where the fine structure constant varies in a particular area of the universe.

I suspect it's not changed enough to drastically change the results of radiometric dating though.

 

[Agonaces of Susa]

Radioactive isotopes have unique half-lives, which, so far as we can tell, do not vary.

There are many phenomenon that can throw this off and increase or decrease the rate at which the C14 decays.

A half-life is how long it takes for exactly half of a sample of a given isotope to decay into its products. For instance, carbon-14, the isotope used in carbon dating, has a half-life of 5730 years. This means that, 5730 years after an organism dies, half of the original carbon-14 content will remain. 5730 years after that, one quarter of the original will remain. 5730 years after that, one eighth, and so forth.

However, no one has actually lived for 5730 years to verify the assumption.

 

[Cabal]

There are many phenomenon that can throw this off and increase or decrease the rate at which C14 decays.

And you didn't list any....why? Because you just made this up?

However, no one has actually lived for 5730 years to verify the assumption.

And not only that, but if you're alone, what's behind your head disappears! And all criminals convicted on forensic evidence must henceforth be released!

 

[callmeMurph]

Yes, that is how the measurement is done.

How does that work though?

 

[rockaction]

How do they measure the different isotopes? I've watched some videos and saw that they used a mass spectrometer. I was wondering how that works though. Sorry I haven't had a chance to read the articles if it was included in them.

I've only used mass spec in organic chemistry labs, and it's a good way to measure the masses of on something in your mixture of chemicals. I believe you use an electron beam to ionize your sample, and everything gets separated by the mass-charge ratio, and you get quantitative results. For isotope ratio mass spec, it's way more sensitive, and there's a bunch of different types of instruments. The wikipedia article is pretty good:

Mass spectrometry - Wikipedia, the free encyclopedia

I'm not a physical scientist so that's about all I can do. Anyone else know more about mass spec in the context of isotope measurement?

 

[Agonaces of Susa]

And you didn't list any....why? Because you just made this up?

The sun causes variations in the decay rates of isotopes via ionizing radiation emitted from solar flares.

Atmospheric C14 can contaminate a sample.

An atomic blast can add C14 to a sample etc.

Gamma ray bursts could increase the rate of decay etc.

 

[Cabal]

The sun causes variations in the decay rates of isotopes via ionizing radiation emitted from solar flares.

Atmospheric C14 can contaminate a sample.

An atomic blast can add C14 to a sample etc.

Gamma ray bursts could increase the rate of decay etc.

Two of those have nothing to do with altering the decay rate - do you have citations for the other two?

 

[Agonaces of Susa]

Two of those have nothing to do with altering the decay rate - do you have citations for the other two?

Radioactive decay rates are not constant

 

[Cabal]

Radioactive decay rates are not constant

Cheers, have downloaded their citations and will review.

 

[matthewgar]

There has been no way to increase decay rates of things, though many tests have been done, but there are known things that can throw off some readings, usually due to contamination or resovoir effects, like with snails and seals, where they are feeding on sources that contain lots of old carbon.

 

[keith99]

The sun causes variations in the decay rates of isotopes via ionizing radiation emitted from solar flares.

Atmospheric C14 can contaminate a sample.

An atomic blast can add C14 to a sample etc.

Gamma ray bursts could increase the rate of decay etc.

Multiple fail. NONE of these have any bearing on the decay rate. They only have an impact on the amount (percentege) of C14 initially or due to later enrichment.

And you forgot one, proximity to radioactive sources. Put a graveyard in a Uranium mine and you can get results that remains are not dead yet.