Start Radio carbon dating explained

Radio carbon dating explained

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.

As well as the tree ring record, scientists have used the carbon record from corals to calculate C14/C12 levels right back to 50,000 years ago.

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.

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.

Chemically, carbon-14 is no different from non-radioactive carbon atoms, so it ends up in all the usual carbon places — one trillionth of the carbon atoms in air, plants, animals and us are radioactive.

All radioactive atoms eventually decay into something more stable, and carbon-14 decays into nitrogen.

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.