Thus the less of it that remained in an object, in proportion to normal carbon, the older the object was.
The most common of the radioactive dating techniques currently in use involves the isotope 14 of carbon, the radiocarbon.
It makes no sense at all if man appeared at the end of billions of years.
We will deal with carbon dating first and then with the other dating methods.
Some of the carbon-14 might find its way into living creatures.
After a creature's death the isotope would slowly decay away over millennia at a fixed rate.
Everything from the fibres in the Shroud of Turin to Otzi the Iceman has had their birthday determined the carbon-14 way. There's plenty of hydrogen, oxygen and nitrogen in living things too, but carbon's got something none of them do — a radioactive isotope that can take thousands of years to decay.
These had pitfalls, which could lead to controversy.
An example of the ingenious technical work and hard-fought debates underlying the main story is the use of radioactive carbon-14 to assign dates to the distant past.
One rare form has atoms that are 14 times as heavy as hydrogen atoms: carbon-14, or C ratio gets smaller.
So, we have a “clock” which starts ticking the moment something dies.