Wednesday, March 28, 2007

Talking about dating (carbon that is)

Last week we had a question about carbon dating during our class, and it had been a long time since I'd studied carbon dating, so rather than give out misinformation, I decided to write a blog about it. So here goes:

Carbon dating is really a measurement which looks at the ratio between Carbon-12 and Carbon-14 (a radioactive substance created when cosmic rays hit an atom, causing pieces of it to hit nitrogen-14). Carbon-14 then slowly decays, turning back into Nitrogen-14. Just like Carbon-12, Carbon-14 is absorbed into our bodies as we breathe, eat or drink. This process occurs naturally throughout our lifetimes, up until the time we die. At that point we no longer replace the carbon in our bodies, and from then on it decays back into nitrogen. (Don't worry, I'll sum this all up in a minute). Carbon dating works like this: Half of the Carbon-14 in an object turns back into nitrogen in 5,730 years (this is called the half-life) So, in two half-lives, or 11,460 years, only one-quarter of the carbon-14 in living organisms is present, then it has a theoretical age of 11,460 years. I hope this is making sense.

Here is where the problems come in. First of all, if you think rationally about this process, carbon dating cannot be used to date things older than maybe 50,000 years (not that there is anything that old, but we'll get there in a minute) because every 5,730 years the object loses half of the carbon-14 in it. Here are the approximate numbers:

Year 1Starting amount 1001/1 Remaining
Year 5,730Amount left 501/2 Remaining
Year 11,460Amount left 251/4 Remaining
Year 17,190Amount left 12.51/8 Remaining
Year 22,920Amount left 6.251/16 Remaining
Year 28,650Amount left 3.1251/32 Remaining
Year 34,380Amount left 1.56251/64 Remaining
Year 40,110Amount left 0.781251/128 Remaining
Year 45,840Amount left 0.3906251/256 Remaining

It doesn't matter what amount you start out with, divide it by 256 and you get a really small amount. Would it be possible to go beyond 50,000 years, yes, but reasonably, I say no.

There are a couple of other problems with carbon dating besides this. I'm going to try and avoid being overly technical, but there is one other problem that needs to be brought up, so bear with me on this. Around the Earth is a magnetic field, which protects us from a bunch of things (among it's other uses, it really is a marvel of God) including cosmic radiation (remember, that's how carbon-14 is formed). This field has been weakening, allowing more and more radiation in (like UV rays that damage your skin). As this happens the amount of carbon-14 increases. So here's my other problem with carbon-14 dating. How do you know how much was there to start with? If I give you a barrel of apples, and everyday take half, how would you know how many there were to start? This comes down to a math problem where you are trying to solve to different numbers at the same time. One is how many apples, and the other is how much time has passed. Let's say there are 10 apples in the barrel. If I told you that I'd been taking apples for two days, you could tell me there were 40 apples to begin with (40 divided by two is 20 and then divided by two again is 10). Also, using the same math, if you knew how many apples there were to begin with, you could figure out how many days had passed since I started taking apples from the barrel. I know I'm being simple, but I don't know how to figure out this equation not knowing either variable.

Well, those are my concerns. For more information, check out Feel free to leave comments and tell me what you think. I'll be posting more later.


No comments: