[David Kahn]

David Kahn  asked the Naked Scientists:

How does carbon dating work?

I understand that a radioactive element decomposes (not sure if that term is properly used here) at an exponential rate and by that constant function we can derive the age. But what's the starting point?

If you don't know how many atoms were in an object to begin with, how can determining how many are left tell you anything? The only thing I can think of is that you determine the age not by the amount of atoms left, but by the rate at which they are currently decomposing, since rate is a function of time in this exponential function.

But I still don't get it, what is the physical significance of that time = 0 point? What signifies the birth of an inanimate object?

Thank you scientists I love listening to your podcasts!
 
Cheers,
Dave

What do you think?

[rich42]

The start point is the death of a living organism.
While alive, animals and plants are constantly exchanging carbon with the atmosphere: plants through photosynthesis and animals through eating the plants (or eating animals which have eaten the plants). A small fraction of nitrogen in the air reacts to form carbon-14, which then reacts with oxygen to form carbon dioxide which is used in photosynthesis.
Through these processes, a constant fraction of the carbon present in any living organism is carbon-14 which decays, as you said, exponentially but is constantly replaced. As soon as the animal/plant dies, it no longer takes in carbon-14. From that point, the amount of carbon-14 present decreases through radioactive decay. By measuring the percentage of C14 in a sample and comparing it to the percentage found in a living sample, the date of death can be estimated.

Hope that helps answer your question!

Richard