You probably have heard of carbon dating before. It is a method used by scientist to determine how old something is, and it can be very accurate. There are, however, some shortfalls that limit its usefulness. I have heard people make claims like the fake scientist I quoted above, and after I explain carbon dating, you will be better equipped to know when someone is pulling your leg, just like the crazy scientist above.
There are a few terms that need to defined before we can really get rolling with this, or else it might get too confusing. Carbon dating uses the isotope Carbon-14 to determine the age of an object. Carbon-14 has 6 protons and 8 neutrons which makes it unstable. Carbon-14 undergoes beta decay to gain stability. Scientists know the Specific Activity of Carbon-14, which means they know the amount of radioactive Carbon-14 exists per gram (Specific activity is activity per gram). The term Half Life represents how long it takes for half of a radioactive sample to decay. You also may not understand what a Curie is-it is simply a unit used to quantify how radioactive something is, it is akin to decays per unit time. Now that those terms are out of the way, let's get rolling.
All living things take up carbon through natural processes. For example, plants, when they are living, are constantly absorbing carbon from the soil and incorporating it into their structure. Humans do the same. Once the living object dies, however, it is no longer able to take up new carbon. Most of the carbon intake we have is the stable form of carbon, Carbon-12 (6 protons, 6 neutrons). Yet there is a tiny amount that is the radioactive form, Carbon-14. Since scientists know how much radioactive Carbon-14 there is per gram of sample, they are able to determine how long an organism has been dead (Also, they can use this technique on paper and cloths, because those are made of living things-cotton, trees, etc.).
Using the known half-life of Carbon-14, which is around 5700 years, they determine how much activity is present in a sample and compare that value with the known specific activity of Carbon-14. For example, the known specific activity of Carbon-14 is about 4.5 Curies per gram (data from Argonne National Laboratory). If a scientist found an ancient hut made from wood and wanted to see how old it was, they could determine the specific activity in the wood. Originally, the wood will have the same specific activity as living things have today(4.5 Curies per gram), but once the wood is cut, it is no longer taking in any carbon, and the Carbon-14 it has decays away and is not replenished. Let's imagine the sample from the hut had a specific activity of 1.13 Curies per gram, this shows the Carbon-14 has gone through two half lives. Two half-lives is equal to 11,400 years, so the scientist could confidently say the hut is 11,400 years old (I just made that example up, I do not know of any huts that old, especially ones made of wood that would still be standing).
You may be wondering, "That looks like it works pretty well, why wouldn't the crazy scientist be able to say something is 13 million years old?" The answer is this; when you are constantly taking away half of something, what you have remaining becomes very small. Once something has gone through a couple half lives, you can't really accurately tell how old it is. In the case of radioactive decay most scientist say that once a sample has gone through 7-10 half lives, it is essentially done decaying. Technically it is never done decaying, but we will no longer be able to quantify it after 7-10 half lives. In the case of carbon dating, the uppermost limit something could be dated is 57,000 years. Even then if you are suggesting an object is that old, there is most likely significant error in your calculation. However, for objects that are within that time frame, Carbon dating is quite accurate.
A famous example of carbon dating accurately being used is the Shroud of Turin. The Shroud is a piece of cloth with a face imprinted into it. The catholic church claimed the imprint was that of Jesus, however carbon dating revealed the cloth was only as old of the middle ages, much later then Jesus was walking the Earth.
So, if you ever hear someone say they carbon dating a sample and they determined it to be over 57,000 years old, they are either lying or don't know what they are talking about.
An interesting note-the human civilization is currently messing with the specific activity of carbon, so it may be harder for scientist a couple thousand years from now to date items from our time. There are two primary ways we are messing with the ratio of Carbon-14 to carbon. The first, we are burning tons of fossil fuels (coal) for energy. Fossil fuels are extremely old and have very little Carbon-14 in them. Once we burn them, we lower the specific activity because we are spreading a lot of carbon around the atmosphere that has no Carbon-14 in it. On the other hand, the second example in which we are messing with the specific activity is by having performed above ground nuclear explosions. This releases a higher than normal amount of Carbon-14 in the atmosphere. So, in the end we don't really know what we did to the ratio, we just know we messed it up!
Thanks for reading, leave a comment and check out my other blog entries in the Blog Archive to the right!
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