Definition
Carbon dating is a scientific method that measures the radioactive carbon left in an old artifact or bone to work out when the living thing died. It relies on a special kind of carbon that stays at a steady level while an organism is alive, then shrinks at a steady rate after it dies.
Filled up while alive, shrinking after death
While they are alive, plants take in carbon from the carbon dioxide in the air through photosynthesis. Animals take it in by eating those plants or other animals. The air where this carbon starts out holds mostly ordinary carbon (carbon-12), mixed with a tiny amount of radioactive carbon (carbon-14). A living thing keeps taking carbon in and giving it off, so the share of radioactive carbon in its body stays almost the same as the share in the atmosphere.
But when an organism dies, it stops doing photosynthesis or eating. The doorway for new carbon to enter the body closes completely. From that point on, the carbon-14 already inside begins to change into other substances and shrinks at a steady rate.
It's like an hourglass that starts running the moment life stops. The grains of sand dwindle as time passes, so if you measure how many are left, you can work backward to when the hourglass was flipped.
The magic time it takes to halve: the half-life
The speed at which radioactive carbon decays does not change at all, even when the temperature, humidity, or pressure around it changes. Burn it or freeze it solid, and the time it takes for radioactive carbon to drop to half is always about 5,730 years. Scientists call the time it takes for the amount of a substance to fall by half its half-life.
For example, say a piece of wood dug out of an ancient tomb has exactly half the carbon-14 of a living tree. That tree was cut down about 5,730 years ago. If only a quarter is left, two half-lives have passed, so the object is about 11,460 years old.
This makes carbon-14 a dependable natural clock, since its rate of decay isn't swayed by the environment. One catch: if a sample is contaminated with other carbon, the result can be thrown off, so it's important to handle samples cleanly.
To be more precise: it can't measure everything
Carbon dating isn't all-powerful. For one thing, it can't date pottery made from fired clay, or a bronze sword made of metal. This method works only on organic material, such as bone, wood, leather, and paper, that once lived and breathed.
There is also a limit on how far back it can reach. After many half-lives, about 50,000 to 60,000 years, so little carbon-14 is left that even modern technology struggles to measure it accurately. That's why the age of a dinosaur fossil, tens of millions to hundreds of millions of years old, is worked out differently. Scientists measure the volcanic rock above and below the fossil's rock layer, using other radioactive elements with much longer half-lives.
Scientists also have to account for the fact that the share of carbon in the atmosphere was slightly different in different eras. So they run a calibration step, comparing against the true ages of old trees with clear growth rings, to shrink the margin of error.
Common misconceptions
Carbon dating can also tell the age of dinosaur fossils that are tens of millions to hundreds of millions of years old.
Carbon-14 is almost entirely gone after about 50,000 to 60,000 years, so carbon dating can't measure something as ancient as a dinosaur fossil. The rock layers of the dinosaur age are dated with other radioactive elements, such as uranium or potassium, whose half-lives run from hundreds of millions to billions of years.
Where you meet it
It dates the food residue or charcoal stuck to comb-pattern pottery dug up at a prehistoric site.
It reveals the age of wooden coffins found in the Egyptian pyramids and of the linen cloth wrapped around mummies.
In one sentence
It's a scientific technique that finds an artifact's age by using the fact that, after a living thing dies, its radioactive carbon halves at a steady rate (the half-life).