Paul, New Zealand asked:
If a carbon atom is in existence at the beginning of the Earth, spends some of its time in bacteria, then in fish and animals and so on. It gets into a tree and into furniture and we try and carbon date the furniture arenít we just carbon dating bacteria? How does it really work?
They key to carbon dating is that the carbon isnít the carbon thatís been on Earth ever since the Earth was formed. The carbon thatís in carbon dating is carbon thatís been newly made. Where that comes from is when cosmic rays - high energy particles from the sun - hit the Earthís atmosphere they interact with atoms and send neutrons flying around. when one of these neutrons hit a Nitrogen-14 (14N) atom, it knocks out a proton, and the 14N becomes Carbon-14 (14C). This then circulates in the atmosphere but because this process is happening roughly at the same rate continuously the amount of carbon thatís in the atmosphere is roughly continuous. Most of it ends up in the atmosphere as carbon dioxide so you have 14C carbon dioxide. Plants then pick that up in their process of photosynthesis and they turn it into sugar. You then eat the plant and all the time that you are alive youíre gaining radioactive carbon in your body which you incorporate into your body. The level in your body will be roughly constant because youíre taking it in at a roughly constant rate from the environment. The ratio of radioactive to non-radioactive carbon should be the same all the time you or a plant are alive.
The guy we have to credit is Willard Libby who discovered carbon dating in the 1940s, got the Nobel Prize for it actually.