Hi. This is Mr. Andersen and today I'm going to be talking about radiocarbon
dating. Radiocarbon dating is a way that we use carbon and the amount of carbon 14 left
in an object to figure out how old that object is. So if we find for example a piece of wood
that we think was used by ancient humans, we want to figure out how long ago that was,
we can use radiocarbon dating to do that. Before we talk about that we should probably
talk about what carbon 14 is. There are three types of carbon that we have on our planet.
Good old run of the mill carbon 12 has 6 protons, 6 neutrons and that's going to be 99 percent
of the carbon that we have in the atmosphere. One percent of the carbon in the atmosphere
however is going to be carbon 13. And that means that it has 6 protons and so it's going
to have 7 neutrons. Both of these are stable. In other words once they've been constructed
they're not going to change form. But there's another type and that's called carbon 14.
Now carbon 14 actually comes from nitrogen. And what happens is that nitrogen is hit with
cosmic rays from space. And nitrogen is converted to carbon 14. Carbon 14 has 6 protons and
it's going to have 8 neutrons. And it will decay. In other words it has a tendency to
give off beta particles. And as it does that it turns back into nitrogen. And so what we
can do is measure the amount of carbon 14 in something. And it tells us how old it is.
Now where does this carbon go? The carbon eventually goes into the macromolecules that
make us up. And so when you eat food you're taking carbon that was once in a plant. But
before that it was in the atmosphere. And so before we get too into that let me kind
of talk about how food gets into you. And so if we look at the atmosphere, atmosphere
is made up of a bunch carbon dioxide. And so this has two oxygens. And it's going to
have one carbon. And most of this carbon out here is going to be run of the mill carbon
12. And so it's just regular carbon. But occasionally we're going to have carbon dioxide that actually
has, it's carbon 14. So it has those extra neutrons. And so we called this an isotope
of carbon. And it has a tendency to decay. And so how does that actually get into our
body? Well the first thing that happens is that plants are going to take in that carbon
and they do that through a process called photosynthesis. And so what photosynthesis
is doing is it's a way for plants to take in carbon and actually make something out
of it. And so in this case what they're making is sugar. The sugar is used by the plant to
build itself. But it's also used to burn sugar and to do respiration. Now we don't care about
that in plants. What we care about is actually getting that sugar inside our body. And so
what I like to eat is something called Oatmeal Squares for breakfast. It's my cereal of choice
right now. And so that carbon that was in the atmosphere, carbon 14, eventually ends
up in my Oatmeal Squares and eventually ends up in my mouth and eventually ends up inside
my body. And so the amount of carbon 14 that's in the atmosphere is going to be equal to
the amount of carbon 14 that's in my body. As long as I keep eating. And so as long as
I keep eating, that ratio of carbon 14 to regular carbon 12 is going to be the same
over time. But let's say for example that I get swept up by a wind storm and I end up
dying. It's really sad. Well at this point, I'm not eating anymore. And so since I'm not
eating anymore I'm not taking in anymore carbon 14. In other words we've cut off that food
channel. And so the amount of carbon 14 that I have in my body I'm stuck with. And so let's
say that I'm swept away in a dust storm. And I'm covered up by sediment. And years later
a scientist finds a part of me and wants to figure out, well I wonder how long ago it
was that this science teacher met his doom. Well he or she can figure that out using carbon
14 dating. So how does that actually work? First of all you would have to take the bones
of Mr. Andersen into the lab. And then we're going to measure the amount of carbon 14 inside
me. And we could put me in a chamber like that where we're sensing the amount of beta
particles that are given off. But first let's talk a little bit about the math. At time
0 the amount of carbon 14 that I would have would be 100 percent of the amount of carbon
14. But that carbon 14 is going to decay over time. And so carbon 14 looks like this. But
it's going to break down into nitrogen 14. And as it does that, it's going to give off
beta particles. And those beta particles could be measured as hitting the sides of this pretend
sensor that I have here. Now we know a little bit about the amount of time it takes. In
other words for a gram of me, and so let's just take a little bit of me, for a gram of
me we would expect that 15 of these beta particles would be released every minute. And so if
I were to take a newly deceased or newly dead body into my theoretical lab here. And if
I was getting 15 beta particles per minute, we would know that that is 0 years old. But
if we were to look at it 5730 years later, scientists have found that it would give off
7.5 beta particles per minute. In other words it would give off half the amount of carbon
14 particles that were in there before. And that would continue to drop off and drop off.
And so if we look at this in a graph, this is what is going to happen to the amount of
carbon 14 inside my body. 100 percent of it would be given off at time 0. But every 5730
years we're going to give off half that amount. So this drops down to 25 percent. 12.5, 6.25,
3.125. And so you can imagine that I could create, this is going to be tough to get it
good, a best fit line or a trend line of this data. And so what you could do is you could
find, let's say we find a bone. And that bone, we could say that it has, I don't know, something
like 37% of the amount of carbon 14 that it should normally have. Well we could just read
down here. It's going to tell us how old that is. And so that's how carbon 14 dating works.
You could imagine this is kind of a cool math thing. We'll never reach the bottom. It's
kind of like walking half way to the wall and then half way to the wall again. But eventually
you get out like 60,000 years and the amount of carbon 14 that is left is so small that
you can't get accurate measurements for that. And so we have to use a different isotope
to measure it at that point. Another thing to remember is the amount carbon 14 is going
to drop off, but that's going to be converted to nitrogen 14 as well. And so if we have
25 percent carbon 14 then we've got 75 percent nitrogen 14. Now you maybe asking yourself
how do we know that this is accurate? In other words we weren't around there 10,000 years
ago. So how would we make sure that you're data is actually matching up? Well what you
would need to find is find objects that are old but we know exactly how old they are.
And so one of the first things that they showed was they took an Egyptian, like a royal barge,
that was made of wood. And then they handed it to scientists and scientists using radiocarbon
dating figure out how old this is. And so they figured out how old it was. They were
able to then go back in the written record and show that those data are going to match
up. And so you don't just use one sample. You'd use multiple samples and different isotopes
to get a real accurate kind of a measurement. One caveat to that is this. If you look right
here, this was puzzling to me the first time I saw it. And so this is 1945 through 2000.
And this blue line represents the natural level of carbon 14 in the atmosphere. But
what you see is that we see a peak of that coming right after the 1950s. And it's greater
in the northern hemisphere. Let's see. Yeah the northern hemisphere. This would be like
in Austria then it is in the southern hemisphere. It's almost twice the amount in the the northern
hemisphere you know 5 years later than it normally should be. And so the reason why
is that humans started doing nuclear weapons testing. And so as we did that we actually
increased the amount of carbon 14 in the atmosphere. And so one thing you should remember is that
everything is measured with carbon 14 dating before 1950. And the reason why is that we
can't get super accurate measurements within this time because it's kind of screwed up.
So I couldn't for example radiocarbon date something from 1970 because it's going to
have a different amount, or I'm going to have to somehow compensate for that amount. And
so that's radiocarbon dating. It's pretty cool. It's a neat way to figure out how old
the bones are.