(classical music)
- Hey there, and welcome to Brain Stuff.
I'm Josh Clark and this
is the Brain Stuff where
I explain to you how
carbon-14 dating works.
Carbon-14 dating, which we
also just call carbon dating,
is a form of radiometric dating.
And all it is, is measuring the decay
of a certain type of atom
found in a once living organism
to determine when it was last alive.
And all of this starts in the up, up,
upper atmosphere of Earth,
where it's constantly
bombarded by cosmic rays.
And one of the things these cosmic rays do
is knock neutrons off of some atoms
and protons off of others
and before you know it,
the nice, stable,
family-man nitrogen-14 atom
is all wrapped up and gone crazy,
and becomes what's known
as a carbon-14 atom,
which is radioactive.
Now, carbon-14 atoms aren't the only ones
in the upper atmosphere.
There's also carbon-12 atoms.
Carbon-12 atoms are in much more abundance
and they're pretty stable.
Carbon-14 atoms are, again,
radioactive and unstable.
But, they're formed at
a reliable, steady rate
so at any point in time,
we have a pretty good idea
of the ratio of carbon-12
atoms to carbon-14 atoms.
Got that?
It's important.
Now, carbon dioxide is
essential to life here on Earth.
Plants breathe it in,
animals eat the plants,
we eat the animals and the plants,
and these carbon molecules,
carbon-14 and 12 that make
up the carbon dioxide,
get in everything.
What's neat is that the ratio
of carbon-12 to carbon-14
found in all these living
things here on Earth
is pretty much the same as
what's in the atmosphere.
Which means it's predictable.
And some very, very smart
scientists have figured out
that carbon-14 actually
decays at a predictable rate.
Carbon-14, like all radioactive particles,
has what's called a half-life.
Now the half-life is the
amount of time it takes
for the number of radioactive
particles in a sample
to decrease by half.
Carbon-14 has a half-life of 5,730 years.
That means that after 5,730 years,
the amount of carbon-14 atoms in a plant
that you found fossilized will be half
of what it was the last time
that plant took a breath of life.
After 11,460 years,
which is two half-lives,
there'll be a quarter of
the amount of carbon-14
that was originally present.
And then after 17,190
years, you'll have just
an eighth of the number of carbon-14.
And so on, and so on,
until there's none left.
And this is actually
one of the limitations
of carbon-14 dating, that eventually
you're going to go far enough back in time
that all of the carbon-14
atoms have decayed.
And you don't know whether this took place
a day before or 100,000 years before.
Which means the time limit
that you can date things
using carbon-14 is roughly 50,000 years.
But as long as you are
trying to date something
that lived on Earth within
the last 50,000 years,
you can figure out roughly
when it was last alive.
The way that you do that is by measuring
the rate of decay of carbon-14 atoms
compared to the slow and
steady carbon-12 atoms
that are also present in there.
Presto, chango, you've
got some carbon-14 dating,
and all of a sudden you say,
"Oh my God, this wooden axe
handle is 12,000 years old."
Pretty sweet stuff.
And this whole thing gets
even more interesting
when you realize that
future archaeologists
are going to have a lot of trouble
using carbon-14 dating thanks to us,
humans of the present time.
Our industrial activities
have been pumping
C-0-2 into the atmosphere
and really messing
with the ratio of carbon-12 to carbon-14
and, even more astounding,
all of the nuclear bombs that we set off
in the mid-20th century,
well those messed with the
atmospheric ratios, too.
So, good luck with all of that,
future anthropologists and archaeologists.
Sorry.
