Like by looking at a certain artefact scientists can determine that it is from 2100 years ago, how are they so sure of the accuracy.
In: Chemistry
Correct me if I’m wrong, but I assume you’re asking “how do we know carbon dating is accurate, when nobody has been around long enough to have measured the time it takes carbon to decay x amount?” Something to note, carbon dating only really works on previously-living organisms, and only within a window of ~5,730 – ~50,000 years of age. This is because an organism must have been alive to absorb carbon in the first place.
The way we know it’s accurate is, well, we don’t. Carbon dating is based on the half-life of carbon, which is an estimate based on carbon levels taken from organisms that had just died, and then a handful of years later. This difference was used to form an experimental curve, extrapolated to cover a longer amount of time. The results were then tested and refined by measuring the levels of radioactive carbon in samples of organic material from archeological sites of known ages. The levels of radioactive material in a house built in 1200 AD could be determined and compared to the extrapolated experimental curve. Again, this is still an estimate.
EDIT: I want to add that carbon dating can only be somewhat accurate under the assumption that the levels of carbon in the atmosphere have remained constant from the time we’re dating an object.
It can’t be just any artifact, it must be organic.
Plants get CO2 from the air and CO2 has a certain level of C-14.. animals eat plants, both die and get incorporated into the soil.
Once the carbon is fixed in organic material this is a moment in time. C-14 decays
All living things absorb both types of carbon; but once it dies, it will stop absorbing. The C-12 is a very stable element and will not change form after being absorbed; however, C-14 is highly unstable and in fact will immediately begin changing after absorption. Specifically, each nucleus will lose an electron, a process which is referred to as decay. This rate of decay, thankfully, is constant, and can be easily measured in terms of ‘half-life’.
Half-life refers to the amount of time it takes for an object to lose exactly half of the amount of carbon (or other element) stored in it. This half-life is very constant and will continue at the same rate forever. The half-life of carbon is 5,730 years, which means that it will take this amount of time for it to reduce from 100g of carbon to 50g – exactly half its original amount. Similarly, it will take another 5,730 years for the amount of carbon to drop to 25g, and so on and so forth. By testing the amount of carbon stored in an object, and comparing to the original amount of carbon believed to have been stored at the time of death, scientists can estimate its age.
Lately we’ve been putting more and more old carbon in the air, so it’s not exactly accurate. it’s more of an estimate and you’d use other data to determine approximate age such as where it’s found, surrounding artifacts, etc.
Just to clear things up… While alive, plants and animals will have the same ratio of C-12 and C-14 as the atmosphere. Plants are getting their carbon from Carbon Dioxide in the atmosphere, and animals are (ultimately) getting their carbon from plants.
Once a plant or animal dies, it is no longer getting any new carbon. The C-12 is stable, and doesn’t change. The C-14, though, is unstable, and slowly changes into C-12 (radioactive decay). Radio carbon dating doesn’t just look at how much C-14 is in a sample, it looks at how much C-14 is in a sample compared to how much C-12 is in the sample. The older the sample is, the less C-14 will be in it relative to the amount of C-12.
One way the dating method has been confirmed reliable (within error margins), and calibrated was to test samples of known age. (Dendrochronology can be used to date wood back 10,000+ years).
The other answers explain how carbon dating works. It assumes the carbon levels have stayed the same so you can also ask, “Is that actually true? And if not, wouldn’t you get a wrong age?” The answers are mostly yes and slightly.
This is where *calibration* comes in. Often there are multiple ways to date things and in principle they should agree with each other. So you can use one method to validate an other. The result is curves like this: [https://i.imgur.com/Harn3TF.jpg](https://i.imgur.com/Harn3TF.jpg). The black line is what it *should* be if the carbon amount had indeed stayed the same throughout history. You can see that it doesn’t, but that the offset with other methods is roughly the same each time. If it didn’t work at all, the data would be all over the place.
All this means is that you can correct for the assumptions in your method. It’s a bit like using a slow clock: you can still use it if you know exactly by how much it is slow.
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