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.
(Theme music)
