Can someone explain carbon-14 dating to me?

Question

I'm reasonably intelligent, but science just isn't my thing. I read the wikipedia article, and a few other websites, but I'm still pretty confused.

I believe in evolution, but many creationists say that carbon-14 dating is circular logic. They present it as:
"How do you know hold that fossil is?"
"It was found in rock that is 100 million years old!"
"How do you know how old the rock is?"
"It was found with a fossil inside that is 100 million years old!"

I'm sure that explanation isn't right, but I'd like someone to tell me a little more about it, in as basic terms as you can. :) Thanks!

Um.. thanks for giving me the link to the wikipedia article, even though I specifically said that I had already read the article and it did not help me.

In other words, thanks for absolutely nothing.

Answer 1

http://en.wikipedia.org/wiki/Carbon-14_dating

Answer 2

Well basically, carbon 14 decays at a predictable rate. Scientists can measure how long the carbon 14 in a certain object has been decaying for, and consequently they can find out how old that object is. The rate of decay slows over time, which makes it less effective for older items. Carbon 14 dating is accurate to somewhere between forty and sixty thousand years. Consequently it is not used to date fossils that are thought to be older than that, which is why this common creationist argument is absolute bullshit. Carbon dating is simply not used to date something millions of years old. Older fossils, as well as rocks, can be dated using other techniques.

Answer 3

carbon-14 is not used to date fossils, its half-life is only about 6000 years, and after a few half-lives there's not enough material to provide an accurate date, so c-14 can do about 60,000 years at best. i could explain it anyway, but are you sure that's the information you're after?

the basic concept of radiometric dating is to figure out when the material was removed from a source of radioactive isotopes. for example in the interior of the earth there's a large amount of uranium that decays into a variety of products. for radiocarbon dating, the source is cosmic rays in the upper atmosphere. one can think of the decay chain sort of like a waterfall - isotopes are pools where water spends some time - maybe a short time for an unstable isotope, or a long time for the more stable ones.

but when the material is removed from the source of isotopes, there's no more water coming into the pool and so it can behave like an hourglass (sorry for the mixed metaphors). to determine a date, you need to know how much sand is in the top of the hourglass (called the parent isotope), how much is in the bottom (called the daughter isotope), and how fast the sand moves (called the half life). all of these can be measured. preferably, the formation of the material you measure excludes the daughter isotope so its initial concetration is zero. there are ways to check if that assumption is valid, but maybe that's getting too technical.

fossils are sometimes used only because the correlation between the radiometric age of the rocks and certain particular fossils has already been well established. they are not declared by fiat.

you may be interested to read this rather more comprehensive account:
http://www.asa3.org/aSA/resources/Wiens.html

Answer 4

in the most basic terms, it's using the half-life of carbon 14.

the half-life is how long it takes for an element to decay half-way.

carbon 14 has a half-life of approximately 6000 years.

so if we look at something, and only 1/2 of the carbon 14 is left, we know it's 6000 years old. from there it increases exponentially (there's a specific formula for it)


when we look at a fossil, we know how much carbon 14 is left because carbon 14, carbon 14, and carbon 12 are present in equal amounts originally. but when something ceases, the carbon 14 decays but the 12 and 13 don't. so we can compare the amount of carbon 14 to the amount of carbon 12 and 13 to figure out how much carbon 14 has decayed. we then use that to get the age of something.

hope that helps.

Answer 5

Scientists do not rely on circular reasoning! They make use of a variety of radio-isotope pairs which have decay rates fast or slow. Carbon-14 is one of the fast ones. It's like having a variety of ancient water-clocks. Once your water runs out in one of those things you can no longer tell the time, but some water-clocks empty fast, others barely get started, so you can accurately measure any period from seconds to months. This is because you will always have water-clocks that are part-way empty but not fully empty or still full.


Radionuclide dating works very well because there is absolutely no chemical change that will affect the nuclear change. No matter what you do to the carbon-14, react or burn it or combine it in a long-chain polymer, it's still a carbon-14 nucleus until it decays at random. But wouldn't all the carbon-14 vanish after 40,000 years or so? No, because a steady stream of it is regenerated in the upper atmosphere from powerful cosmic radiation.

So, while we are alive, we are absorbing carbon from all sources that's a definite fraction of carbon-14 (exception: water-breathing marine life doesn't). But what happens when we die? We are no longer taking in carbon from food or the air, we stop. THEN the carbon-14 concentration goes down with time in a predictable way from radioactive decay, and it can never be affected by chemical change or heat and cold or anything. The absolute rate of decay is proportional to how much carbon-14 you still have left. The "half-life" is 5,500 years for carbon-14. So that means after that time, 1/2 of the original Carbon-14 will disappear. Wait another 5,500 years, 1/2 of THAT will disappear.

If we know the constant background rate of carbon-14 in nature, we can therefore tell how long ago an animal died. But our measuring instruments have limits: it might not tell the difference between 100% and 99.5%, so you can't use it to measure the very recent periods. Creationists like to mess with our heads and squeal that "scientists can't tell the difference between a butterfly dead 80 years ago and one that's still alive". And that's right, you wouldn't be able to with a carbon-14 set-up. But so what? If we want to see when a butterfly specimen died, we just read the date on the specimen board that a human kindly wrote in for us. We want to go FURTHER...before humans could write down dates.

We also can't go much further back than 40,000 years or so, because that's about 7 half-lives and after a while it's hard to measure the radioactivity and the sample becomes indistinguishable from 0% carbon-14. But fine, scientists measure the error in their instruments and the method and report the number of years with a plus-or-minus figure. If you want to measure things much older, use a different radio-nuclide pair than Carbon-14/Carbon-12.

Answer 6

Seriously, asking this question here on R&S is like asking the science section about fairy tales.

I hope you asked this question in the science section too, as this would show your genuine commitment to really understand, otherwise it looks like you're seeking a biased and suitably ill informed creationistic type of answer..!
You wouldn't be doing that would you..?

Firstly, there are very many independant dating methods, many of which greatly overlap, and these results do corroborate with each other in their respective usefull timescales, and there is more than one type of Carbon dating..
Secondly, Carbon dating does NOT date anything as old as 100 million years, and therefore is NOT used to date Dinosaur fossils for instance. There are many other independant, and therfore verifiable methods to do this.

Ask this question in the appropriate section if you REALLY want to know the answer, or else expect scientifically illiterate, ill informed and incorrect answers from any religious fruitcakes here..
Simple.

Answer 7

Carbon dating revolves around the idea that carbon-14 has a half life of around 5000 or so years. All living things have a certain percentage of carbon-14 to carbon-12..... so when the carbon-14 breaks down in its half life, the carbon-12 is still therre as the same....

So scientists basically take the amount of carbon-14 left and use equations to see how long it was decaying since the ratio of carbon-12 to carbon-14 was as it should.

Carbon dating only works for around 60,000 years, though.

Answer 8

Carbon 14 is used to date objects in the several thousand years range because the half life of the radioactive isotopes are in the thousands of years. A half life is a radioactive element that decays into another element over time, specifically, the half life of the object is how long it takes for half of all the remaining isotopes to decay into a non-radioactive state. By measuring that decay, we can estimate the age of the formation of the object. The creationist arguement above is actually against the geo-column and is completely invalid. We know the age of objects in certain layers of the earth by radiometric dating and then apply the idea that layers will reflect similar age as they would have to sediment at the same time. So, when we have dated objects in those layers, we get an estimate (but an accurate one) that is supported by multiple points of reference.

Answer 9

http://science.howstuffworks.com/carbon-14.htm
If you read all the sections on here, it should clear it up for you.
"As soon as a living organism dies, it stops taking in new carbon. The ratio of carbon-12 to carbon-14 at the moment of death is the same as every other living thing, but the carbon-14 decays and is not replaced. The carbon-14 decays with its half-life of 5,700 years, while the amount of carbon-12 remains constant in the sample. By looking at the ratio of carbon-12 to carbon-14 in the sample and comparing it to the ratio in a living organism, it is possible to determine the age of a formerly living thing fairly precisely."

Answer 10

Ok... in really basic terms.... carbon dating is nothing like you just described...

When things like rock are formed, they have many different "bits" to them, like oxygen, carbon, and other things. As they sit around those "bits" start to decompose at a fairly predictable and steady rate.

We can test how old a rock is by it's rate of "decomposition" (so to speak). That is what is measured by carbon 14 dating... it is not perfect, as there are some factors that cause changes... but... overall... it is a fairly reliable dating process, and not "circular" at all.

Answer 11

also, it might be fun to look up carbon dating and just how accurate it is, when a object is saturated in water for some time.