The technique hinges on carbon-14, a radioactive isotope of the element that, unlike other more stable forms of carbon, decays away at a steady rate.
That means, in 1.3 billion years, half of a potassium sample with decay to argon gas.
We note that at the instant the swimmer touches the edge of the pool our wristwatch reads and 53 seconds.
How long has the competitor taken to swim the 1,500 metre race?
Familiar to us as the black substance in charred wood, as diamonds, and the graphite in “lead” pencils, carbon comes in several forms, or isotopes.
One rare form has atoms that are 14 times as heavy as hydrogen atoms: carbon-14, or C ratio gets smaller.