quote:KalvinB:
I have X isotopes left of a substance that started with X0 isotopes with a half life of Y years that has been sitting for Z years
One equation. Two unknowns. You either have to know the original amount or the time that it spent decaying. All that's given is X and Y.
Ah, I see the confusion. You see, X0 is actually not unknown. The carbon-14 isotopes are present in the air at a relatively constant concentration and are absorbed by plants and plant-eating animals. Since we have estimated, using a variety of techniques, the carbon-14 concentration in the atmosphere over the last 50000 years, we can estimate when a creature died (i.e. stopped absorbing carbon).
Also technically the number of C14 atoms isn't important; the C14:C12 ratio is. So it doesn't matter how much carbon the animal/plant absorbed.
If you wish, you can Google and find the calibration techniques they use to verify carbon-dating and the presence of C-14 in the atmosphere.
The only decent counterargument you could bring up here is how the concentration of C-14 in the atmosphere remains constant. Since it decays, obviously new C-14 is being created, so why can't it be created in the bones of corpses? Well, here is a good explanation:
quote:Radioactive carbon, produced when nitrogen 14 is bombarded by cosmic rays in the atmosphere, drifts down to earth and is absorbed from the air by plants. Animals eat the plants and take C14 into their bodies. Humans in turn take carbon 14 into their bodies by eating both plants and animals. When a living organism dies, it stops absorbing C14 and the C14 that is already in the object begins to disintegrate. Scientists can use this fact to measure how much C14 has disintegrated and how much is left in the object. Carbon 14 decays at a slow but steady rate and reverts to nitrogen 14. The rate at which Carbon decays (Half-life) is known: C14 has a half-life of 5730 years. Basically this means that half of the original amount of C14 in organic matter will have disintegrated 5730 years after the organisms death; half of the remaining C14 will have disintegrated after another 5730 years and so forth. After about 50,000 years, the amount of C14 remaining will be so small that the fossil can't be dated reliably.
A buried fossil/pot/whatever wouldn't be exposed to this radiation. Also when I said the C-14 concentration was "relatively constant" it's because they've discovered the sun's energy output hasn't remained constant. But it didn't change very much in 50000 years.
~CGameProgrammer( );
[edited by - CGameProgrammer on May 1, 2003 8:19:09 AM]