THE 100 MOST INFLUENTIAL INVENTORS OF ALL TIME

7 The 100 Most Influential Inventors of All Time 7

traces of carbon-14. After a plant or other organism dies,
no additional carbon-14 should be incorporated into its
tissues, while that which is already present should decay at
a constant rate. The half-life of carbon-14 was determined
by its codiscoverer, chemist Martin D. Kamen, to be 5,730
years, which, compared with the age of the Earth, is a
short time but one long enough for the production and
decay of carbon-14 to reach equilibrium. In his Nobel
presentation speech, Swedish chemist Arne Westgren
summarized Libby’s method: “Because the activity of
the carbon atoms decreases at a known rate, it should be
possible, by measuring the remaining activity, to deter-
mine the time elapsed since death, if this occurred
during the period between approximately 500 and 30,000
years ago.”
Libby verified the accuracy of his method by applying
it to samples of fir and redwood trees whose ages had
already been found by counting their annual rings and to
artifacts, such as wood from the funerary boat of Pharaoh
Sesostris III, whose ages were already known. By measuring
the radioactivity of plant and animal material obtained
globally from the North Pole to the South Pole, he showed
that the carbon-14 produced by cosmic-ray bombardment
varied little with latitude. On March 4, 1947, Libby and
his students obtained the first age determination using the
carbon-14 dating technique. He also dated linen wrappings
from the Dead Sea Scrolls, bread from Pompeii buried
in the eruption of Vesuvius (79 CE), charcoal from a
Stonehenge campsite, and corncobs from a New Mexico
cave, and he showed that the last North American ice age
ended about 10,000 years ago, not 25,000 years ago as
previously believed by geologists. The most publicized
and controversial case of radiocarbon dating is probably
that of the Shroud of Turin, which believers claim once