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?A swimming race illustrates the simple principles involved in measuring time.This swimmer is competing in a 1,500 metre race and we have an accurate, calibrated wristwatch.Stalagmites are calcium carbonate deposits left behind when carbon dioxide evaporates out of cave seepage water.They found that carbon-14 concentrations were twice their modern level during that period.The result overturned 10 years of hope among Christians that it was real, after the first scientific tests found evidence of blood and serum stains.The earliest documented sighting of the shroud is from 1353, but last week a historian claimed in the Vatican's newspaper that she had found a "missing link" in the Holy See's Secret Archives proving the Knights Templar had safeguarded it during the 13th century.
Beck and colleagues tested slices of a half-metre long stalagmite that grew between 45 000 and 11 000 years ago in a cave in the Bahamas.
There are about two dozen decay pairs used for dating.
Uranium 235 decay to lead has a half-life of 713 million years, so it is well suited to dating the universe.
Carbon dating is a mainstay of geology and archaeology - but an enormous peak discovered in the amount of carbon-14 in the atmosphere between 45 thousand and 11 thousand years ago casts doubt on the biological carbon cycle that underpins the technique.
The study led by physicist Warren Beck of the University of Arizona, US, could also affect estimates of how quickly the Earth can re-absorb the excess carbon dioxide generated by fossil fuels (J W Beck et al 2001 Science to appear).