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When I have asked an audience this question they have looked at me incredulously and said, “Starting time?

” They realize that you cannot know how long the swimmer took unless you knew the time on the wristwatch when the race started.

We note that at the instant the swimmer touches the end of the pool our wristwatch reads and 53 seconds.

How long has the competitor taken to swim the race?

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.

Organisms capture a certain amount of carbon-14 from the atmosphere when they are alive.

The clock was initially calibrated by dating objects of known age such as Egyptian mummies and bread from Pompeii; work that won Willard Libby the 1960 Nobel Prize in Chemistry.

But even he “realized that there probably would be variation”, says Christopher Bronk Ramsey, a geochronologist at the University of Oxford, UK, who led the latest work, published today in Science.We will deal with carbon dating first and then with the other dating methods.Carbon has unique properties that are essential for life on Earth.Various geologic, atmospheric and solar processes can influence atmospheric carbon-14 levels.Since the 1960s, scientists have started accounting for the variations by calibrating the clock against the known ages of tree rings.A "fact" can be defined as something that is the case—that is, a state of affairs.

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