The Physics of Carbon Dating

When I was a kid, I wanted to be a hockey player. Then a doctor. Then a paleontologist. Then I saw Raiders of the Lost Ark and I wanted to be an archeaologist.

Well one thing led to another, and I ended up getting a Physics degree instead. Luckily for me though, studying physics gave me the tools and experience to understand all kinds of stuff from other scientific fields.

Take archeology and geology for example. One of the primary tools in this field for determining the age of a specimen is radio-carbon dating. But how does it work, and why do we place so much stock in its results?

Lets start at the beginning. Carbon is atomic element #6. The most abundant form of Carbon is Carbon-12, which has 6 protons and 6 neutrons in its nucleus (6 + 6 = Carbon-12).

We are Carbon-based life forms. Carbon has the ability to form huge molecules making our existence possible. (Silicon also has the ability to form big molecules, which Star Trek has taken advantage of to create some crazy looking Silicon-based aliens).

But there is another form of Carbon out there: Carbon-14 (C14 for short), which has 6 protons and 8 neutrons.

C14 is unstable. That extra neutrons cause problems in the nucleus so eventually Carbon-14 will decay into the happy and stable Nitrogen-14 (which actually makes up 78% of the air we breathe).

So how does this help us with dating stuff? Well C14 is naturally created in the atmosphere. So at any given time, there is a certain amount of C14 in the atmosphere. So when plants undergo photosynthesis, which is when plants take in Carbon dioxide and sunlight to make energy, they absorb some C14 at the same time. After this happens, animals will eat these plants and absorb some C14  themselves.

From: HowStuffWorks.com

Long story short, all living things will have a certain amount of C14 in their systems while they are alive.

But when plants or animals die, they stop taking in C14 from the environment. And since C14 is unstable, it will start to decay.

C14 has a half-life of 5730 years. This means that one half of C14 in a sample will decay after one half-life. For example, if you had 10 grams C14 atoms and waited 5730 years, 5 grams will have decayed, and you would have 5 grams of C14 left. If you waited another 5730 years, 2.5 grams will have decayed and you would have 2.5 grams left, and so on.

So scientists can take a fossilized sample of bone or plant, compare the amount of C14 in the sample to what should be in living tissue, and they can calculate how old the sample must be.

Granted, this is a simplified explanation I have given here, and there are other factors to consider. But calibration of the technique is very good, and we can get an accuracy of the age of a sample to with +/-16 years for a sample younger than 6000 years, and within 160 years for samples less than 50,000 years old.

After 50,000 years, C14 dating doesn’t work so well, because there just isn’t enough C14 to make an accurate measurement. But there are other elements with long half-lives that we can use to date much older stuff, and they all work on the same general principle as what I’ve explained here.

Many Intelligent Design proponents and Young Earth theorists will try and disprove these techniques. They may give arguments about how concentration of C14 has not been constant over the past 50,000 years. This is actually true, but we can account for it in our measurement.

So sorry IDers, the Earth was not created 6000 years ago. It was created roughly 4.5 billion years ago which, I think, is much more awesome.