Is carbon-14 dating accurate? (read the whole article or dont answer)?
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We can take a sample of air, count how many 12C atoms there are for every 14C atom, and calculate the 14C/12C ratio. Because 14C is so well mixed up with 12C, we expect to find that this ratio is the same if we sample a leaf from a tree, or a part of your body.
In living things, although 14C atoms are constantly changing back to 14N, they are still exchanging carbon with their surroundings, so the mixture remains about the same as in the atmosphere. However, as soon as a plant or animal dies, the 14C atoms which decay are no longer replaced, so the amount of 14C in that once-living thing decreases as time goes on. In other words, the 14C/12C ratio gets smaller. So, we have a 鈥榗lock鈥?which starts ticking the moment something dies.
Obviously, this works only for things which were once living. It cannot be used to date volcanic rocks, for example.
The rate of decay of 14C is such that half of an amount will convert back to 14N in 5,730 years (plus or minus 40 years). This is the 鈥榟alf-life.鈥?So, in two half-lives, or 11,460 years, only one-quarter will be left. Thus, if the amount of 14C relative to 12C in a sample is one-quarter of that in living organisms at present, then it has a theoretical age of 11,460 years. Anything over about 50,000 years old, should theoretically have no detectable 14C left. That is why radiocarbon dating cannot give millions of years. In fact, if a sample contains 14C, it is good evidence that it is not millions of years old.
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