Today, an isotope rewrites history. The University of
Houston's College of Engineering presents this series about the
machines that make our civilization run, and the people whose
ingenuity created them.
Fall, 1974: I trudge through the forest
along the not-so-blue Danube, under a light drizzle. This is Jugoslavia's
side of the river. I'm on a two-mile walk to a 7500-year-old archaeological
site. It's the late Mesolithic village of Lepenski Vir.
We pass a rude peasant hut and an old woman. Outside, a
simple wooden table and chair overlook the river. On the table
are a knife and pieces of apple. I suppose it's where she sits,
looking at the great river, to think about time and mortality.
Half an hour later we reach a small circle of stone huts in the
muddy woods. Lepenski Vir was part of the great fuss kicked up by
carbon dating. Up to now, prehistory dating had been detective
work based on circumstantial evidence. Event B had to fall after
A. Event C happened sometime after A, but before B, and so on.
Chronologies had few hard dates to go by. Archaeologists
had once dated Lepenski Vir in 2500 B.C. Now it retreats to 5500
B.C. In 1974, scholars were at each others throats trying to stem the
damage that carbon dating was doing to their textbooks.
Here's how carbon dating works: The cosmic radiation of
neutrons converts atoms of nitrogen into the heavy isotope of
carbon, called carbon-14. First that isotope combines to form
high-altitude carbon dioxide. By the time plants and animals take
up carbon dioxide, one carbon atom in a trillion is carbon-14.
But carbon-14 is radioactive. Every 5730 years, half of it
decays back to regular nitrogen. Measure the fraction of carbon-14
left in any artifact that once lived -- in bone, ash, shell, or grain
-- and you can tell how long ago it lived.
To make carbon dating work, you have to calibrate it with
known dates. We know dates of artifacts from certain Egyptian
royalty. The rings of old trees give accurate dates for the wood
inside. You might think tree rings couldn't take you back very
far. But some California bristlecone pines have lived five thousand years.
At first, carbon dating had tough sledding. We had to make
corrections. For one thing, Earth's atmosphere has changed. The
amount of carbon-14 wasn't quite the same four thousand years ago.
As the work went on, old dates moved back into time.
Agriculture became two thousand years older than we'd thought. Traditional
history suffered the most right here in these rainy woods along
the Danube. These people lived in villages, made art, and
worshipped the gods of the river long long before the Pharaohs.
On the way out we pass the table again. The woman seems
older this time. She eats her apple and gazes out at the river.
What are three thousand years more or less, as she honors the
gods of the one thing no one can date -- the timeless water
moving relentlessly below her.
I'm John Lienhard, at the University of Houston, where we're
interested in the way inventive minds work.