So the proportion of carbon-14 inside living things is the same as the proportion of carbon-14 in the atmosphere at that time.But when we stop eating, or when plants stop photosynthesising, our carbon-14 levels no longer get topped up.Radiocarbon dating is used to work out the age of things that died up to 50,000 years ago. As far as working out the age of long-dead things goes, carbon has got a few things going for it. The proteins, carbohydrates and fats that make up much of our tissues are all based on carbon.
(You can read up on radioactivity and isotopes here).Carbon-14, the radioactive version of carbon, is rare — it only makes up one trillionth of all the carbon in the world.So calculating the age range of a once-living sample involves measuring the 14C/12C ratio, and using this the known half-life to estimate the length of time since the sample died.That age range is then compared with known 14C/12C ratios from the tree ring/marine record to find the best match, and the result is a calibrated age range you can be 95 per cent sure of.The exact age of an unknown sample can never be known for sure, so short of discovering a time machine, 95 per cent accuracy is as good as it gets.
Radiocarbon may not be perfect, but as any single 30-something can attest, no dating method is.From the moment we die the proportion of carbon-14 compared to non-radioactive carbon-12 in what's left of our bodies starts to drop as it gradually turns to nitrogen.And the longer dead things lie around, the lower the carbon-14 levels get.If you know the rate that carbon-14 decays at, and how much of the carbon in a shroud, iceman or piece of old wood or bone is radioactive, you can work out how long ago they stopped breathing or photosynthesising. We know that on average it takes an atom of carbon-14 a little over 8,000 years to decay to nitrogen (although you never know when an individual atom is going to decay — it's completely random). But the value that's used to calculate the age of an object isn't an absolute figure, it's a statistical term called half-life.We even know that in a gram of carbon, 14 carbon-14 atoms turn into nitrogen every minute. The half-life of a radioactive isotope is the amount of time it takes for half of the atoms in a sample to decay. That means that no matter how many carbon-14 atoms were present when something died, after 5,730 years only half of them are left — the rest have decayed to nitrogen.When those speedy protons hit atoms you end up with a few stray neutrons zipping around the place.