# How does carbon dating work?

I got into a debate the other day with a friend, and she brought up carbon dating. I thought to myself, "How does carbon dating even work?" So I really did not go into it with her because I have always heard about carbon dating but have been oblivious to what carbon dating is all about.

So I ask the great people of TA. What is carbon dating? How does it work? How do they determine this rock or bone is over 5,000 years old vs a rock or bone that is 5,000,000 years old? How precise is this science? Can there be false positives? What other questions am I missing?

Sure, I could do a google search but I thought other people might have the same exact questions as me about this topic.

Thanks

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### John Major

@Sophie. Nelson has given a full explanation there. It's covered quite well in the Richard Dawkin's book, 'The Greatest Show on Earth.'

Incidentally, the book also explains dendrochronology. This is dating using tree rings! That allows dating to within one year and is possible now going back 11,000 years!

http://en.wikipedia.org/wiki/Dendrochronology
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### Reg The Fronkey Farmer

We know the number of particles that are present when a radioactive element is complete. It decays at a constant rate – that is, it loses half of it energy (mass) over a fixed period of time. We call this the “half-life” of the element. If it has a half-life of 10,000 years then after 5000 years it has half the number of particles left. Then after 2500 years it would have ¼ of the original left and so on. The environment – temperature, soil erosion or any other factor has no effect on the rate of decay. Some elements have very long half-lives and others very short ones. However they can all be seen as types of “atomic clocks” as their decay (loss of energy or mass) is constant and we can work back to see how old something is. Carbon dating is just one form of radioactive decay and is used to date organic material. Other elements date non organic material. Sometimes called “clocks in the rocks.” As mentioned above the Dawkins books has a very good chapter on it.

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### SteveInCO

Good job, Nelson!

Sophie,

One thing to be aware of is that many people use "carbon dating" to mean any sort of radiometric dating, not just the dating of recent organic samples.  (I even heard them make that goof on "Universe" early in the first season.)  This is unfortunately because carbon dating has a lot of very important differences from other variants, not just in terms of how far back one can date, or what it is being dated (plant or animal matter vs. rock), but because carbon dating is the only method that has to be calibrated.  Since the carbon-14 decays to nitrogen-14 which is already butt-common in organic matter, we have to measure the ratio of c-14 to c-12, but just doing that can be misleading without calibration because the starting ratio varies over time.  Calibration is just the process of accounting for that difference.

That's an added extra complication that creationists will try to exploit to make the whole thing sound very iffy.  Which is ironic because most of the creationist disputes are with ages we got with other methods that don't have this issue at all!  Carbon dating of course will never be used to give evidence that the earth is 4.5 billion years old, but other methods are more useful and far simpler to deal with.